U.S. patent application number 12/998035 was filed with the patent office on 2012-01-12 for carbon steel sheet having excellent carburization properties, and method for producing same.
This patent application is currently assigned to NIPPON STEEL CORPORATION. Invention is credited to Masayuki Abe, Kengo Takeda, Hisayoshi Yatoh.
Application Number | 20120006451 12/998035 |
Document ID | / |
Family ID | 42780478 |
Filed Date | 2012-01-12 |
United States Patent
Application |
20120006451 |
Kind Code |
A1 |
Abe; Masayuki ; et
al. |
January 12, 2012 |
CARBON STEEL SHEET HAVING EXCELLENT CARBURIZATION PROPERTIES, AND
METHOD FOR PRODUCING SAME
Abstract
The invention provides a carbon steel sheet including C: 0.20%
to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by
mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al:
0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to
0.01% by mass and N: 0.001% to 0.01% by mass, in which a K value
that can be obtained from 3C+Mn+0.5Si is greater than or equal to
2.0; surface hardness is less than or equal to 77 on the Rockwell B
Scale; and the average content of N in a zone from the surface to a
depth of 100 .mu.m is less than or equal to 100 ppm. This carbon
steel sheet is configured to be carburized in a carburization
atmosphere with a carbon potential of 0.6 or less.
Inventors: |
Abe; Masayuki; (Tokyo,
JP) ; Takeda; Kengo; (Tokyo, JP) ; Yatoh;
Hisayoshi; (Tokyo, JP) |
Assignee: |
NIPPON STEEL CORPORATION
Tokyo
JP
|
Family ID: |
42780478 |
Appl. No.: |
12/998035 |
Filed: |
March 3, 2010 |
PCT Filed: |
March 3, 2010 |
PCT NO: |
PCT/JP2010/001456 |
371 Date: |
March 8, 2011 |
Current U.S.
Class: |
148/603 ;
148/330; 148/602; 420/120; 420/121 |
Current CPC
Class: |
C21D 1/06 20130101; C21D
8/0226 20130101; C22C 38/001 20130101; C21D 8/0236 20130101; C22C
38/04 20130101; C22C 38/14 20130101; C21D 8/0263 20130101; C22C
38/32 20130101; C21D 9/46 20130101; C22C 38/06 20130101; C21D
8/0463 20130101; C22C 38/18 20130101; C21D 8/0426 20130101; C22C
38/02 20130101; C21D 8/0468 20130101; C23C 8/22 20130101; C21D
9/561 20130101; C22C 38/40 20130101; C21D 8/0268 20130101; C21D
8/0436 20130101; C22C 38/42 20130101 |
Class at
Publication: |
148/603 ;
420/120; 420/121; 148/602; 148/330 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2009 |
JP |
2009-079959 |
Claims
1. A carbon steel sheet configured to be carburized in a
carburization atmosphere with a carbon potential of 0.6 or less,
comprising: C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass,
Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001%
to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by
mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass
with a balance including Fe and inevitable impurities, wherein K
value that can be obtained from 3C+Mn+0.5Si is greater than or
equal to 2.0; surface hardness is less than or equal to 77 on
Rockwell B Scale; and an average content of N in a zone from a
surface to a depth of 100 .mu.m is less than or equal to 100
ppm.
2. The carbon steel sheet according to claim 1, further comprising
one or more components selected from Nb: 0.01% to 0.5% by mass, V:
0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01% to 0.5%
by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03% by mass,
and As: 0.003% to 0.03% by mass.
3. A method for producing the carbon steel sheet according to claim
1, comprising: heating a slab to less than or equal to 1200.degree.
C.; hot-rolling the slab at a final rolling temperature of
800.degree. C. to 940.degree. C. so as to obtain a steel sheet;
cooling the steel sheet at a cooling rate of 20.degree. C./second
or more until a temperature of the steel sheet becomes less than or
equal to 650.degree. C., as a first cooling; cooling the steel
sheet at a cooling rate of 20.degree. C./second or less, as a
second cooling subsequent to the first cooling; coiling the steel
sheet at a temperature of 400.degree. C. to 650.degree. C.;
pickling the steel sheet; and annealing the steel sheet for 10
hours or more at a temperature of 660.degree. C. or more in an
atmosphere with a hydrogen content of 95% or more and a dew point
of less than or equal to -20.degree. C. at a temperature of less
than 400.degree. C. and of less than or equal to -40.degree. C. at
a temperature of more than or equal to 400.degree. C., as a first
annealing.
4. The method for producing the carbon steel sheet according to
claim 3, wherein the first annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
first annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
5. The method for producing the carbon steel sheet according to
claim 4, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the pickling, as a first
cold-rolling.
6. The method for producing the carbon steel sheet according to
claim 5, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the first annealing, as a second
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the second cold-rolling, as a second
annealing.
7. The method for producing the carbon steel sheet according to
claim 6, wherein the second annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
second annealing, a cooling rate is set to 5.degree. C./hour or
less until a temperature becomes Ac1-30.degree. C. after
annealing.
8. The method for producing the carbon steel sheet according to
claim 7, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
9. The method for producing the carbon steel sheet according to
claim 8, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
10. The method for producing the carbon steel sheet according to
claim 6, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
11. The method for producing the carbon steel sheet according to
claim 10, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
12. The method for producing the carbon steel sheet according to
claim 4, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the first annealing, as a second
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the second cold-rolling, as a second
annealing.
13. The method for producing the carbon steel sheet according to
claim 12, wherein the second annealing is performed in an
atmosphere with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
second annealing, a cooling rate is set to 5.degree. C./hour or
less until a temperature becomes Ac1-30.degree. C. after
annealing.
14. The method for producing the carbon steel sheet according to
claim 13, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
15. The method for producing the carbon steel sheet according to
claim 14, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
16. The method for producing the carbon steel sheet according to
claim 12, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
17. The method for producing the carbon steel sheet according to
claim 16, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
18. The method for producing the carbon steel sheet according to
claim 3, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the pickling, as a first
cold-rolling.
19. The method for producing the carbon steel sheet according to
claim 18, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the first annealing, as a second
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the second cold-rolling, as a second
annealing.
20. The method for producing the carbon steel sheet according to
claim 19, wherein the second annealing is performed in an
atmosphere with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
second annealing, a cooling rate is set to 5.degree. C./hour or
less until a temperature becomes Ac1-30.degree. C. after
annealing.
21. The method for producing the carbon steel sheet according to
claim 20, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
22. The method for producing the carbon steel sheet according to
claim 21, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
23. The method for producing the carbon steel sheet according to
claim 19, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
24. The method for producing the carbon steel sheet according to
claim 23, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
25. The method for producing the carbon steel sheet according to
claim 3, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the first annealing, as a second
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the second cold-rolling, as a second
annealing.
26. The method for producing the carbon steel sheet according to
claim 25, wherein the second annealing is performed in an
atmosphere with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
second annealing, a cooling rate is set to 5.degree. C./hour or
less until a temperature becomes Ac1-30.degree. C. after
annealing.
27. The method for producing the carbon steel sheet according to
claim 26, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
28. The method for producing the carbon steel sheet according to
claim 27, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
29. The method for producing the carbon steel sheet according to
claim 25, further comprising: cold-rolling the steel sheet with a
rolling ratio of 5% to 60% after the second annealing, as a third
cold-rolling; and annealing the steel sheet at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C. after the third cold-rolling, as a third
annealing.
30. The method for producing the carbon steel sheet according to
claim 29, wherein the third annealing is performed in an atmosphere
with a hydrogen content of 95% or more with an annealing
temperature range from Ac1 to Ac1+50.degree. C., and after the
third annealing, a cooling rate is set to 5.degree. C./hour or less
until a temperature becomes Ac1-30.degree. C. after annealing.
31. A carbon steel sheet configured to be carburized in a
carburization atmosphere with a carbon potential of 0.6 or less,
comprising C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass,
Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001%
to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by
mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass,
and further comprising one or more components selected from: Cr:
0.01% to 2.0% by mass, Ni: 0.01% to 1.0% by mass, Cu: 0.005% to
0.5% by mass and Mo: 0.01% to 1.0% by mass with a balance including
Fe and inevitable impurities, wherein K' value that can be obtained
from 3C+Mn+0.5Si+Cr+Ni+Mo+Cu is greater than or equal to 2.0;
surface hardness is less than or equal to 77 on Rockwell B Scale;
and an average content of N in a zone from a surface to a depth of
100 .mu.M is less than or equal to 100 ppm.
32. The carbon steel sheet according to claim 31, further
comprising one or more components selected from: Nb: 0.01% to 0.5%
by mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W:
0.01% to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to
0.03% by mass, and As: 0.003% to 0.03% by mass.
33. The method for producing the carbon steel sheet according to
claim 31, comprising: heating a slab to less than or equal to
1200.degree. C.; hot-rolling the slab at a final rolling
temperature of 800.degree. C. to 940.degree. C. so as to obtain a
steel sheet; cooling the steel sheet at a cooling rate of
20.degree. C./second or more until a temperature of the steel sheet
becomes less than or equal to 650.degree. C., as a first cooling;
cooling the steel sheet at a cooling rate of less than or equal to
20.degree. C./second, as a second cooling subsequent to the first
cooling; coiling the steel sheet at a temperature of 400.degree. C.
to 650.degree. C.; pickling the steel sheet; and annealing the
steel sheet for more than or equal to 10 hours at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C., as a first annealing.
Description
TECHNICAL FIELD
[0001] The present invention relates to a carbon steel sheet having
excellent carburization properties and a method for producing the
same.
[0002] The present application claims priority based on Japanese
Patent Application No. 2009-079959 filed in Japan on Mar. 27, 2009,
and its content is incorporated by reference herein.
BACKGROUND ART
[0003] In the past, industrial machine parts or automobile parts,
such as chains, gears, or clutches, were produced by hardening the
surfaces thereof with a thermal treatment, such as quenching, after
a forming process.
[0004] However, in recent years, as the forms of parts have become
more complex, abrasion resistance, fatigue characteristics, or the
like have been in demand. Therefore, materials are required to
satisfy not only workability that can withstand complicated
processes while processing parts but also hardenability for surface
hardening. The hardenability and workability of materials are
opposing characteristics in terms of material design. In general,
softening of materials is effective for the improvement of
workability, but most of elements for enhancing hardenability
increase the hardness of the steel sheet, and thus sacrifice
workability.
[0005] On the other hand, if hardenability of parts after the
processing is not adequate, abnormal layers in which structures
such as perlite, sorbite or troostite are mixed are generated in
the products.
[0006] In order to manufacture steel sheets having excellent
workability and hardenability at a low cost, it is effective to add
B in the steel sheets. However, due to the reactivity of B,
changes, such as oxidation, deboronization, or nitrogenization,
occur at the surfaces of the steel sheet, therefore it is difficult
to secure hardenability at a surface layer portion.
[0007] In addition, in steel sheets in which B has been added
(hereafter referred to as B-added steel sheets) and carburizing has
been performed at a carbon potential (Cp) of about 0.8 (which is
commonly used), carburized carbon increases hardenability and thus
it becomes difficult for quenched abnormal layers to occur at a
surface layer portion after quenching, therefore no serious problem
occurs. However, in weakly carburized zones with a low carbon
potential (for example, Cp.ltoreq.0.6), B degrades hardenability
due to the above reaction, and furthermore, hardenability by carbon
(C) also cannot be secured, therefore the B-added steel sheets are
not widely used.
[0008] The carbon potential mentioned herein refers to a value
indicating the carburizing capability of atmospheres when
carburizing steel materials. The carbon potential is equivalent to
the carbon concentration at steel surfaces when reaching an
equilibrium with a gaseous atmosphere at a carburizing
temperature.
[0009] Therefore, in B-added steel sheets, material optimization is
required throughout all the processes from materials to parts
processing, such as the establishment of production conditions in
which the effects of the addition of B can be sufficiently obtained
and the securement of workability for severe processes, such as
profile forming, and treatability of surface hardening, such as
carburizing.
[0010] Regarding production conditions of B-added steel sheets,
Patent Document 1 discloses annealing under a hydrogen atmosphere
with nitrogen content suppressed to 10% or less by volume or an
argon atmosphere, but nothing is described regarding the prior or
subsequent processes. In addition, there is no technology disclosed
regarding a carburizing treatment at a low carbon potential which
is the subject of the invention.
RELATED ART DOCUMENT
Patent Document
[0011] [Patent Document 1] Japanese Unexamined Patent Application,
First Publication No. H5-331534
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0012] An object of the invention is to provide a B-added steel
sheet having excellent hardenability even in a carburization with a
low carbon potential condition and furthermore equipped with
workability, and to optimize a method for producing thereof, in
order to solve the above-described problems.
Means for Solving the Problems
[0013] The invention adopts the following measures to solve the
above described problems.
[0014] (1) A first aspect of the invention is a carbon steel sheet
configured to be carburized in a carburization atmosphere with a
carbon potential of 0.6 or less, including: C, 0.20% to 0.45% by
mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P:
0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to
0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by
mass and N: 0.001% to 0.01% by mass with a balance including Fe and
inevitable impurities, wherein K value that can be obtained from
3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is
less than or equal to 77 on Rockwell B Scale; and an average
content of N in a zone from a surface to a depth of 100 .mu.m is
less than or equal to 100 ppm.
[0015] (2) The carbon steel sheet in the above (1) may further
include one or more components selected from Nb: 0.01% to 0.5% by
mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01%
to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03%
by mass, and As: 0.003% to 0.03% by mass.
[0016] (3) A second aspect of the invention is a method for
producing the carbon steel sheet according to claim 1 or 2,
including: heating a slab to less than or equal to 1200.degree. C.;
hot-rolling the slab at a final rolling temperature of 800.degree.
C. to 940.degree. C. so as to obtain a steel sheet; cooling the
steel sheet at a cooling rate of 20.degree. C./second or more until
a temperature of the steel sheet becomes less than or equal to
650.degree. C., as a first cooling; cooling the steel sheet at a
cooling rate of 20.degree. C./second or less, as a second cooling
subsequent to the first cooling; coiling the steel sheet at a
temperature of 400.degree. C. to 650.degree. C.; pickling the steel
sheet; and annealing the steel sheet for 10 hours or more at a
temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C., as a first annealing.
[0017] (4) In the method for producing the carbon steel sheet
described in the above (3), the first annealing may be performed in
an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the first annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0018] (5) The method for producing the carbon steel sheet
described in the above (4) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the pickling,
as a first cold-rolling.
[0019] (6) The method for producing the carbon steel sheet
described in the above (5) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the first
annealing, as a second cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the second cold-rolling,
as a second annealing.
[0020] (7) In the method for producing the carbon steel sheet
described in the above (6), the second annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the second annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0021] (8) The method for producing the carbon steel sheet
described in the above (7) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0022] (9) In the method for producing the carbon steel sheet
described in the above (8), the third annealing may be performed in
an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0023] (10) The method for producing the carbon steel sheet
described in the above (6) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0024] (11) In the method for producing the carbon steel sheet
described in the above (10), the third annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0025] (12) The method for producing the carbon steel sheet
described in the above (4) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the first
annealing, as a second cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the second cold-rolling,
as a second annealing.
[0026] (13) In the method for producing the carbon steel sheet
described in the above (12), the second annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the second annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0027] (14) The method for producing the carbon steel sheet
described in the above (13) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0028] (15) In the method for producing the carbon steel sheet
described in the above (14), the third annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Act1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0029] (16) The method for producing the carbon steel sheet
described in the above (12) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0030] (17) In the method for producing the carbon steel sheet
described in the above (16), the third annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0031] (18) The method for producing the carbon steel sheet
described in the above (3) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the pickling,
as a first cold-rolling.
[0032] (19) The method for producing the carbon steel sheet
described in the above (18) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the first
annealing, as a second cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the second cold-rolling,
as a second annealing.
[0033] (20) In the method for producing the carbon steel sheet
described in the above (19), the second annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the second annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0034] (21) The method for producing the carbon steel sheet
described in the above (20) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0035] (22) In the method for producing the carbon steel sheet
described in the above (21), the third annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Act1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0036] (23) The method for producing the carbon steel sheet
described in the above (19) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0037] (24) In the method for producing the carbon steel sheet
described in the above (23), the third annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0038] (25) The method for producing the carbon steel sheet
described in the above (3) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the first
annealing, as a second cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the second cold-rolling,
as a second annealing.
[0039] (26) In the method for producing the carbon steel sheet
described in the above (25), the second annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the second annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0040] (27) The method for producing the carbon steel sheet
described in the above (26) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0041] (28) In the method for producing the carbon steel sheet
described in the above (27), the third annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0042] (29) The method for producing the carbon steel sheet
described in the above (25) may further include cold-rolling the
steel sheet with a rolling ratio of 5% to 60% after the second
annealing, as a third cold-rolling; and annealing the steel sheet
at a temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of less than 400.degree.
C. and of less than or equal to -40.degree. C. at a temperature of
more than or equal to 400.degree. C. after the third cold-rolling,
as a third annealing.
[0043] (30) In the method for producing the carbon steel sheet
described in the above (29), the third annealing may be performed
in an atmosphere with a hydrogen content of 95% or more with an
annealing temperature range from Ac1 to Ac1+50.degree. C., and
after the third annealing, a cooling rate may be set to 5.degree.
C./hour or less until a temperature becomes Ac1-30.degree. C. after
annealing.
[0044] (31) A third aspect of the invention is a' carbon steel
sheet configured to be carburized in a carburization atmosphere
with a carbon potential of 0.6 or less, including C, 0.20% to 0.45%
by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P:
0.001% to 0.04% by mass, 0.0001% to 0.006% by mass, Al: 0.01% to
0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by
mass and N: 0.001% to 0.01% by mass, and further including one or
more components selected from: Cr: 0.01% to 2.0% by mass, Ni: 0.01%
to 1.0% by mass, Cu: 0.005% to 0.5% by mass and Mo: 0.01% to 1.0%
by mass with a balance including Fe and inevitable impurities,
wherein K' value that can be obtained from 3C+Mn+0.5Si+Cr+Ni+Mo+Cu
is greater than or equal to 2.0; surface hardness is less than or
equal to 77 on Rockwell B Scale; and an average content of N in a
zone from a surface to a depth of 100 .mu.m is less than or equal
to 100 ppm.
[0045] (32) The carbon steel sheet in the above (31) may further
include one or more components selected from Nb: 0.01% to 0.5% by
mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01%
to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03%
by mass, and As: 0.003% to 0.03% by mass.
[0046] (33) The carbon steel sheet described in the above (31) or
(32) is the method for producing the carbon steel sheet described
in the above (31) or (32) including: heating a slab to less than or
equal to 1200.degree. C.; hot-rolling the slab at a final rolling
temperature of 800.degree. C. to 940.degree. C. so as to obtain a
steel sheet; cooling the steel sheet at a cooling rate of
20.degree. C./second or more until a temperature of the steel sheet
becomes less than or equal to 650.degree. C., as a first cooling;
cooling the steel sheet at a cooling rate of less than or equal to
20.degree. C./second, as a second cooling subsequent to the first
cooling; coiling the steel sheet at a temperature of 400.degree. C.
to 650.degree. C.; pickling the steel sheet; and annealing the
steel sheet for more than or equal to 10 hours at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of more than or equal to
400.degree. C., as a first annealing.
Effects of the Invention
[0047] In the configurations described in the above (1) and (31),
since it is defined that the K value and the K' value are greater
than or equal to 2.0, and the average amount of N in surface layers
is less than or equal to 100 ppm, it is possible to develop high
hardenability even in a carburization with a low carbon potential
condition and thus obtain a B-added carbon steel sheet equipped
with high workability.
[0048] According to the configurations described in the above (2)
and (32), it is possible to obtain an effect of stabilizing
precipitates or improving toughness or an effect of suppressing
component variations in the surface layer portion of a steel
sheet.
[0049] According to the methods described in the above (3) and
(33), it is possible to stably produce a carbon steel sheet having
excellent workability and post-processing carburization
treatability.
[0050] According to the methods described in the above (4) to (30),
it is possible to further improve the workability or softening of a
carbon steel sheet.
[0051] As described above, according to the invention, it is
possible to produce a steel material having not only excellent
carburization properties so as to prevent the generation of
abnormal layers due to inferior hardenability while carburizing a
B-added steel but also excellent workability for producing parts or
the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] FIG. 1 is a diagram showing a relationship between the K
value or the K' value and the average amount of N in a surface
layer relating to the generation of abnormal layers during
carburization.
[0053] FIG. 2 is a diagram showing a relationship between a crack
in the profile formed portions and material hardness during the
profile forming process.
[0054] FIG. 3 is a flowchart to explain the production method.
EMBODIMENTS OF THE INVENTION
[0055] The inventors conducted hardness variation or structure
investigation at a surface layer portion during carburized
quenching with a variety of changes in the components of B-added
steel sheets or production conditions during production processes,
and clarified the relationship between the structures and
components of the surface layers which affect hardenability of the
surface layers. As a result, it was found that there were cases in
which, instead of martensite, structures which are more softened
than martensite, such as pearlite, sorbite, troostite, were
generated, and, particularly, such structures were often observed
in the outermost surface layers from the surface to a depth of
about 100 .mu.m.
[0056] FIG. 1 shows the generation of abnormal layers in a 0.22%
C-based material which has been carburized quenched at a carbon
potential of 0.3. It was found that the abnormal layers have a
significant relationship with the content of nitrogen (N) present
in a surface layer of the steel sheet to a depth of 100 .mu.m (the
average amount of N in the surface layer) and the K values (or the
K' values) obtained by steel sheet components.
[0057] Here, the average amount of N in the surface layer refers to
a value obtained by analyzing the content of nitrogen (N) in
shavings taken after planing the surface of the steel sheet to a
depth of 100 .mu.m before carburization.
[0058] In order to observe the effect of steel sheet components,
the K value represented by Formula (1) and the K' value represented
by Formula (2) were introduced.
K value=3C+Mn+0.5Si (1)
[0059] Wherein C, Mn and Si represent the content of each element
(% by mass).
K' value=3C+Mn+0.5Si+Cr+Ni+Mo+Cu (2)
[0060] Wherein C, Mn, Si, Cr, Ni, Mo and Cu represent the content
of each element (% by mass). Further, they will be zero if the
corresponding elements are not included.
[0061] As shown in FIG. 1, it was found that abnormal layers are
not observed in a zone where the K value (the K' value in a case in
which Cr, Ni, Mo and Cu are included) is greater than or equal to
2.0 and the average amount of N in the surface layer is less than
or equal to 100 ppm so that carburization properties are excellent.
The reason why such a favorable zone as above can be obtained is
because, as the average amount of N in the surface layer increases,
the amount of nitrogen (N) precipitated as nitrides during the
production processes is increased, and the growth of autenite
grains are delayed during carburization, thereby degrading
hardenability. In particular, it is considered that, since B is
nitrogenized by N so as to become BN, the amount of solute B in the
steel is decreased and thereby, the hardenability of the steel
sheet is impaired.
[0062] In addition, in terms of steel sheet hardenability, the
steel sheet needs to include alloy elements to a certain level, and
the hardenability could be clarified by indicating the amount of
the alloy elements with the K value (or K' value) shown above. A
higher K value (or K' value) is advantageous to secure higher
hardenability, but, if the K value (or K' value) is too high, there
are cases in which problems occur, such as increased steel sheet
hardness that degrades workability, or the occurrence of hardening
cracks during quenching depending on the form of parts. The upper
limit of the K value (or K' value in a case in which Cr, Ni, Mo and
Cu are included) is not particularly specified, but, if the value
exceeds 3.6, hardenability becomes too high, and thus defects, such
as the above hardening crack or the like, will occur, therefore the
value is desirably less than or equal to 3.6.
[0063] In terms of steel sheet workability, the invention defined
the surface hardness of the steel sheet as less than or equal to 77
on the Rockwell B Scale (HRB). The steel material according to the
invention, which may be used for automobile parts or the like, may
subject to severe processes such as tooth-shape forming (profile
forming) of gear parts. Therefore, workability that can withstand
such severe processes is required.
[0064] In the invention, as the evaluation of workability, it was
investigated whether a crack was formed in an area in the base
portion of a profile formed portion where shear deformation occurs
after conducting a processing test which simulated profile forming
process. Using a 0.22% C-based steel material, 3 mm-thick steel
sheets were produced under varied conditions of hot-rolling,
cold-rolling and annealing so as to prepare test specimens. As to
the shape of the profile formed object, a rack-shaped die was
produced at a module of 1.5 mm defined by JIS-B1703, and 3 mm-thick
steel sheets were 2 mm-pressed, and then whether or not cracks
occurred in the profile formed portions was evaluated.
[0065] FIG. 2 shows the results. Crack occurrence from severe
processes, such as profile forming, shows a favorable
correspondence with surface hardness, and it was found that it is
effective to achieve softening with a surface hardness of HRB 77 or
less to produce a material that can withstand profile forming.
[0066] Meanwhile, in the invention, in terms of securing
hardenability as described above, the lower limit of the K value
(or K' value) is defined. A higher K makes a material harder and
thus is advantageous for hardness during quenching, but degrades
workability, therefore problems, such as the formation of cracks,
occur during processes. As a result, it is necessary to carry out
the production method defined in the invention and to carry out
softening of steel sheets while controlling the annealing
atmosphere.
[0067] Hereinafter, steel sheet components and production
conditions will be described.
[0068] C is a basic element necessary to obtain the strength of a
steel sheet. With a carbon content of less than 0.20%, it is not
possible to obtain the strength demanded to produce products, and
hardenability is also degraded at the core portion of the parts so
that desired characteristics cannot be obtained. However, since if
a large amount of C exceeding 0.45% is included, it is difficult to
secure toughness and formability after thermal treatments, the
content of C is specified in a range from 0.20% by mass to 0.45% by
mass (hereinafter, unless otherwise described, contents will have a
unit of % by mass). A more preferable range is from 0.20% to
0.40%.
[0069] Si is used as a deoxidizing agent of steel and is also
effective in terms of hardenability. It is necessary to include
0.05% or more of Si. However, since as the content of Si increases,
degradation of surface texture occurs due to scale defects or the
like during hot-rolling, the upper limit was defined as 0.80%. A
more preferable range is 0.05% to 0.50%.
[0070] Mn is used as a deoxidizing agent and is also effective in
terms of hardenability. In terms of securing hardenability during
carburization carried out at a low Cp, addition of 0.85% or more of
Mn is required in the invention. An excessive content of Mn results
in degradation or scattering (variation) of impact characteristics
caused by segregation-induced structural variation after quenching
and tempering, therefore the upper limit is defined as 2.0%. A more
preferable range is from 0.90% to 1.80%.
[0071] In the steel of the invention, P is a harmful element in
terms of toughness or workability, therefore a lower content of P
is desirable and the upper limit is defined as 0.04%. In addition,
the lower limit is desirably lower, but a decrease in the content
below 0.001% significantly raises industrial costs, therefore the
lower limit is defined as 0.001%. A more preferable range is 0.003%
to 0.025%.
[0072] S accelerates the generation of non-metallic inclusions in
steel so as to degrade forming workability, toughness after thermal
treatments, or the like. As a result, a lower content of S is
desirable, and the upper limit thereof is defined as 0.006%. The
lower limit is desirably lower, but a decrease in the content below
0.0001% significantly raises industrial costs, therefore the lower
limit is defined as 0.0001%. A more preferable range is from
0.0001% to 0.003%.
[0073] Al is used as a deoxidizing agent of steel, and therefore
0.01% or more of Al is required. However, even when more than 0.10%
of Al is added, the effect is saturated, and scale defects are
likely to occur. In addition, Al is also effectively bonded with N
and accelerates nitrogen absorption during steel sheet production.
However, if the content exceeds 0.10%, Al nitrides are stabilized
so as to hinder grain growth during carburization thermal
treatments and degrade hardenability. As a result, the content of
Al is defined in a range from 0.01% to 0.10%. A more preferable
range is from 0.01% to 0.06%.
[0074] Ti is effective as a deoxidizing agent of steel. In
addition, Ti effectively bonds with N. Therefore, it is necessary
to add 0.005% or more of Ti from the relationship with the amount
of N. However, even when more than 0.30% of Ti is added, the effect
is saturated, and the cost also rises. Furthermore, since the
amount of precipitates induced by nitrogen absorption during
production processes is increased, grain growth is hindered during
carburization and hardenability is degraded. As a result, the
content of Ti is defined in a range from 0.01% to 0.30%. A more
preferable range is 0.01% to 0.10%.
[0075] B is an effective element to improve the hardenability of
steel, and such an effect can be achieved with an extremely small
amount. In order to obtain the effect of hardenability improvement,
it is necessary to add 0.0005% or more of B. However, if a large
amount of B exceeding 0.01% is included, castability is degraded
and cracks occur during slab casting. Furthermore, B-based
compounds are generated in steel so as to cause adverse effects,
such as a decrease in toughness. As a result, the content of B is
defined in a range from 0.0005% to 0.01%. A more preferable range
is 0.0005% to 0.005%.
[0076] N is bonded with B so as to generate nitrides and degrades
the hardenability improvement effect of B. Therefore, a lower
content of N is preferable, but a decrease in the content below
0.001% leads to an increase in costs. In addition, if the content
of N exceeds 0.01% as an average composition of steel, a large
amount of elements that bond with N, such as Al or Ti, are
required, and precipitates, such as AlN or TiN, hinder grain growth
during carburization so as to degrade hardenability, which not only
results in generation of abnormal layers but also degrades
mechanical characteristics, such as toughness. As a result, the
upper limit of N content is defined as 0.01%. A more preferable
range is 0.001% to 0.006%.
[0077] In addition, N is likely to intrude into steel during
production processes and is introduced from the atmosphere during
hot-rolling and heating or annealing, and, in particular, is likely
to be concentrated in the surface layer portion, therefore it is
necessary to suppress such effects in order to prevent the
degradation of hardenability of parts in the surface layer portion.
If the amount of nitrogen intruded from atmosphere during heating
or annealing exceeds 100 ppm, the amount of precipitated nitrides
becomes large during coiling or annealing, and grain growth is
delayed during heating before quenching, thereby degrading
hardenability. As a result, it is important to define the content
of N particularly in the surface layer portion (a zone from the
surface to a depth of 100 .mu.m) (the average amount of N in the
surface layer) as less than or equal to 100 ppm. The amount of N in
the surface layer portion is further preferably less than or equal
to 70 ppm.
[0078] Cr is an effective element that can be added in terms of the
hardenability of steel, and the effect becomes remarkable with a
content of 0.01% or more, but even when more than 2% of Cr is
added, the effect is saturated, and the cost also rises. As a
result, the content is defined in a range from 0.01% to 2.0%. A
more preferable range is 0.05% to 0.50%.
[0079] Ni is an effective element in terms of improvement in the
hardenability or toughness of steel, and addition of 0.01% or more
is effective, but addition of more than 1% of Ni merely results in
an increase in costs and rarely changes the effect, therefore the
content is defined in a range from 0.02% to 1.0%. A more preferable
range is 0.05% to 0.50%.
[0080] Cu is an effective element in terms of improvement in the
hardenability or toughness of steels, and addition of 0.01% or more
is effective, but addition of more than 0.5% of Cu merely results
in an increase in costs and rarely changes the effect, therefore
the content is defined in a range from 0.005% to 0.5%. A more
preferable range is 0.02% to 0.35%.
[0081] Mo is an effective element that improves the hardenability
of steel and an effective element to increase resistance against
softening by tempering. In order to obtain such effects, addition
of 0.01% or more is required. However, even when more than 1.0% of
Mo is included, the effect is saturated, and the cost also rises,
therefore the content is defined in a range from 0.01% to 1.0%. A
more preferable range is 0.01% to 0.40%.
[0082] 0.01% or more of Nb has effects of forming carbonitrides,
stabilizing precipitates or improving toughness, but addition of
more than 0.5% of Nb merely results in an increase in costs and a
decrease in hardenability by the formation of carbides, therefore
the content is defined in a range from 0.01% to 0.5%. A more
preferable range is 0.01% to 0.20%.
[0083] Similarly to Nb, 0.01% or more of V has effects of forming
carbonitrides, stabilizing precipitates or improving toughness, but
addition of more than 0.5% of V merely results in an increase in
costs and rarely changes the effect, and also lowers hardenability
by the formation of carbides. Therefore, the content is defined in
a range from 0.01% to 0.5%. A more preferable range is 0.01% to
0.20%.
[0084] Similarly to Nb and V, 0.01% or more of Ta has effects of
forming carbonitrides, stabilizing precipitates or improving
toughness, but addition of more than 0.5% of Ta merely results in
an increase in costs and rarely changes the effect, and also lowers
hardenability by the formation of carbides. Therefore, the content
is defined in a range from 0.01% to 0.5%. A more preferable range
is 0.01% to 0.30%.
[0085] Similarly to Nb, V and Ta, 0.01% or more of W has effects of
forming carbonitrides, stabilizing precipitates or improving
toughness, but addition of more than 0.5% of W merely results in an
increase in costs and rarely changes the effect, and also lowers
hardenability by the formation of carbides. Therefore, the content
is defined in a range from 0.01% to 0.5%. A more preferable range
is 0.01% to 0.20%.
[0086] Furthermore, in addition to the above, in the invention, in
order to suppress component variation in the surface layer portion
of steel sheets, a certain amount of one or more components
selected from Sn, Sb and As may be added.
[0087] Sn, Sb, As: 0.003% to 0.03%
[0088] Sn, Sb and As are elements having a high tendency of
segregating at interfaces, surfaces or the like and a function that
suppresses surface reaction, such as nitrogen absorption or
decarburization, during production processes. Therefore, the
addition thereof has an effect of preventing remarkable component
variation by suppressing the reaction of elements which are liable
to induce component variation, such as nitrogen or carbon, even in
a state in which steel materials are exposed to a high-temperature
atmosphere during heating or annealing in a hot-rolling process.
Therefore, Sn, Sb and As may be optionally added. With regard to
the each added amount, if the amount is less than 0.003%, the
effect is small, and, addition of a large amount exceeding 0.03%
not only saturates the effect but also results in a decrease in
toughness and an increase in costs by extending carburization time.
As a result, it is desirable to be added in a range from 0.003% to
0.03%.
[0089] In the steel sheet according to the invention, the content
of oxygen (O) is not defined, but, if oxides are agglomerated and
thus coarsened, ductility is lowered, therefore the content of
oxygen is preferably less than or equal to 0.025%. A lower content
of oxygen is preferable, but a content of less than 0.0001% is
technically difficult to achieve, therefore the content is
preferably more than or equal to 0.0001%.
[0090] In addition, the carbon steel sheet according to the
invention may include impurities inevitably mixed during production
processes in addition to the above elements, but it is preferable
to prevent impurities from being mixed therein as much as
possible.
[0091] Next, production conditions will be described with reference
to the flowchart in FIG. 3.
[0092] In the invention considering the consistent optimization of
steel material components and an annealing process thereafter,
hot-rolling is important, and it is important to intensively
suppress component variation in the surface layer portion of steel
sheets, that is, the intrusion of N into or decarburization in the
surface layer portion. Therefore, heating is conducted at
1200.degree. C. or less without applying high-temperature heating
which is commonly used and conducted at a temperature of more than
1200.degree. C. (S1). Furthermore, in this case, as soaking time is
extended, nitrogen intrusion into the surface layer portion is also
increased, and hardening characteristics of the products are
affected, therefore it is important not to conduct heating for a
long time. Specifically, it is preferable to conduct heating for a
retention time not exceeding 60 minutes at 1200.degree. C. and 90
minutes at 1100.degree. C.
[0093] Next, hot-rolling is conducted at a final rolling
temperature of 800.degree. C. to 940.degree. C. (S2). If the final
rolling temperature is lower than 800.degree. C., many
burn-in-induced defects occur, and, if the final rolling
temperature is higher than 940.degree. C., the generation frequency
of scale-induced defects is increased, and thus the product yield
ratio is decreased, thereby increasing costs.
[0094] After finishing the final hot-rolling, cooling is conducted
to 650.degree. C. or less at a cooling rate of 20.degree. C./second
or more (S3, first cooling). If the cooling to 650.degree. C. after
finishing the rolling is conducted at a rate slower than 20.degree.
C./second, structural variations called pearlite bands resulting
from segregation occur, which leads to degradation of workability.
Therefore, the cooling rate is controlled at 20.degree. C./second
or more to a temperature of 650.degree. C. or less after finishing
the rolling, and then at 20.degree. C./second or less to a coiling
temperature for slow cooling which is supposed to be conducted on
homogeneous pearlite transformation, pearlite+bainite structure,
bainite structure or the like (S4, second cooling). Thereby, it is
possible to suppress the occurrence of structural heterogeneity in
the coils. In addition, with respect to the coiling temperature, it
is possible to reduce structural variation in the coils by
conducting coiling at a temperature of 400.degree. C. to
650.degree. C. which is to achieve structural homogeneity as
described above (S5). The hot-rolled steel sheets produced by the
above processes are pickled (S6). After pickling, annealing or
cold-rolling is conducted as necessary depending on product sheet
thickness or necessary levels of softening, but the following is
important as production conditions in this case.
[0095] With respect to annealing, since the steel sheet according
to the invention has a high carbon content, it is not possible to
obtain the characteristics by a continuous annealing process that
is used for soft steel sheets. Basically, a process in which coils
are annealed as they are, such as batch annealing or box annealing,
is applied (S7, first annealing).
[0096] In this case, in terms of preventing nitrogen concentration
in the surface layer portion, an annealing atmosphere majorly
includes hydrogen and has a hydrogen concentration of 95% or more.
In addition, in the case of performing annealing in a hydrogen
atmosphere, in terms of safety, the inside of an annealing furnace
is firstly substituted with nitrogen at room temperature so as to
form a nitrogen atmosphere, and then substituted with hydrogen. In
this case, it is desirable to raise the temperature after
substituting with hydrogen in terms of preventing nitrogenization,
but the atmosphere may be substituted with hydrogen while raising
temperature from a nitrogen atmosphere, and it is necessary to have
a hydrogen concentration of 95% or more at a possible low
temperature. In addition, in terms of preventing component
variation in the surface layer portion, it is important to have,
particularly, a dew point of more than or equal to -20.degree. C.
up to 400.degree. C. and a dew point of less than or equal to
-40.degree. C. during retention at a temperature of more than or
equal to 400.degree. C. (retention time depends on materials, but
10 hours or more of retention at a temperature of 660.degree. C. or
more is desirable to soften the steel sheet according to the
invention), and, if a dew point is high, deboronization,
decarburization or the like occurs, and poorly-quenched abnormal
layers are generated in a ease of performing carburization at a low
carbon potential. By completing the above series of processes
(hot-rolling+thermal treatments), the steel sheet according to the
invention having excellent workability and, furthermore, excellent
carburization properties during a carburization treatment after
processing can be obtained.
[0097] In terms of softening, high-temperature annealing at a
temperature of Ac1 or more is also effective. It is preferable to
conduct annealing in a temperature range of "Ac1" to
"Ac1+50.degree. C.", and then set a cooling rate of 5.degree.
C./hour so as to cool it to "Ac1-30.degree. C." after the
annealing. Thereby, ferrite phases generated during cooling with a
cooling rate of 5.degree. C./hour or less are likely to be
coarsened and softening is accelerated by austenite phases
generated at Ac1 or more due to scavenging action by the fine
carbides. If annealing is conducted at a temperature greater than
"Ac1+50.degree. C.", in the components of the steel according to
the invention, the phase ratio of austenite phases becomes too high
and pearlite is generated at some places during cooling which
hardens the steel, therefore, the temperature of high-temperature
annealing in the present invention is preferably less than or equal
to "Ac1+50.degree. C.". In addition, in the steel according to the
invention, even when slow cooling is conducted after the
temperature reached "Ac1-30.degree. C.", the effect is saturated
and an extended annealing time results in an increase in costs,
therefore the end-point temperature of slow cooling is preferably
"Ac1-30.degree. C.".
[0098] Here, Ac1 represents a temperature at which austenite phases
appear in the temperature-raising process, and, in the invention,
A1 transformation points were obtained by taking samples from
hot-rolled steel sheets and measuring expansion curves with a
Formaster tester when raising the temperature at 0.3.degree.
C./second. In addition, written references also disclose simpler
methods obtaining Ac1 from components, and an example thereof is
Ac1 (.degree. C.)=723-10.7.times.% Mn-16.9.times.% Ni+29.1.times.%
Si+16.9.times.% Cr+290.times.% As+6.38.times.% W disclosed in "The
Physical metallurgy of Steel" written by William C. Leslie, and
such empirical formulae can be used.
[0099] Furthermore, the cold-rolling process is used to complete
sheet product thickness with a high accuracy and to efficiently
conduct softening in combination with annealing. Therefore, in the
above series of processes, cold-rolling (S6-2, first cold-rolling)
may be conducted after conducting the hot-rolling and coiling (S5)
and the pickling (S6). Particularly, by cold-rolling with a rolling
ratio of 5% or more, carbides are accelerated to be spherical, and
recrystallization not accompanied by nuclei generation or softening
in which grain diameters are relatively large when completing
recrystallization and grain growth-induced coarsening is likely to
occur is accelerated.
[0100] The upper limit is not particularly specified, but, if
rolling is conducted with a rolling ratio exceeding 60%,
homogeneity of the metallic structure of the steel sheet is further
increased by cold-rolling, but a higher cold-rolling ratio makes
grains recrystallized during annealing smaller, and thus annealing
time needs to be extended for softening, therefore, the
cold-rolling ratio can be determined in terms of costs and product
homogenization.
[0101] In the production method according to the invention, it is
possible to conduct another cold-rolling with a rolling ratio of 5%
or more (S7-2, second cold-rolling) on the steel sheet and then
conduct annealing in an atmosphere including 95% or more of
hydrogen (S7-3, second annealing) after the above annealing. By
undergoing the processes of the cold-rolling (S7-2, second
cold-rolling) and an annealing (S7-3, second cold-rolling) after
the above annealing (S7-1, first annealing), structural
homogenization or crystal grain coarsening can be achieved, and it
is possible to further proceed with workability improvement or
softening.
[0102] In the production method according to the invention, it is
possible to conduct additional cold-rolling with a rolling ratio of
5% or more (S7-4, third cold-rolling) on the steel sheet and then
conduct annealing in an atmosphere including 95% of hydrogen (S7-5,
third annealing) after the above annealing (S7-3, second
annealing), and the annealing conditions for this case are as
described above.
[0103] In addition, in the production method according to the
invention, in terms of softening, it is possible to conduct the
above annealing process in combination with cold-rolling more than
three times, and, even in this case, the process needs to be
carried out within the above production conditions.
[0104] The carbon steel sheet according to the first embodiment of
the invention can be described in the following manner, that is, a
carbon steel sheet which includes, by mass %, C: more than or equal
to 0.20% and less than or equal to 0.45%, Si: more than or equal to
0.05% and less than or equal to 0.8%, Mn: more than or equal to
0.85% and less than or equal to 2.0%, P: more than or equal to
0.001% and less than or equal to 0.04%, S: more than or equal to
0.0001% and less than or equal to 0.006%, Al: more than or equal to
0.01% and less than or equal to 0.1%, Ti: more than or equal to
0.005% and less than or equal to 0.3%, B: more than or equal to
0.0005% and less than or equal to 0.01% and N: more than or equal
to 0.001% and less than or equal to 0.01% with a balance including
Fe and inevitable impurities; has a value represented by
3C+Mn+0.5Si+Cr+Ni+Mo+Cu of 2.0 or more and surface hardness of the
steel sheet of less than or equal to 77 on the Rockwell B Scale
(HRB); has an average content of nitrogen (N) in a zone from the
surface to a depth of 100 .mu.m of 100 ppm or less; is used in a
weak carburization atmosphere with a carbon potential (Cp) of 0.6
or less; and has excellent carburization properties. Here, C, Mn,
Si, Cr, N, Mo and Cu represent the content of each element (% by
mass) and are zero when the corresponding elements are not
included.
[0105] The above carbon steel sheet may further include, by mass %,
one or more components selected from Cr: 0.01% to 2.0%, Ni: 0.01%
to 1.0%, Cu: 0.005% to 0.5% and Mo: 0.01% to 1.0%; and has a value
represented by 3C+Mn+0.5Si+Cr+Ni+Mo+Cu which is greater than or
equal to 2.0.
[0106] The above carbon steel sheet may further include, by mass %,
one kind or two or more kinds of Nb: from 0.01% to 0.5%, V: from
0.01% to 0.5%, Ta: from 0.01% to 0.5% and W: from 0.01% to
0.5%.
[0107] The above carbon steel sheet may further include, by mass %,
one kind or two or more kinds of Sn: from 0.003% to 0.03%, Sb: from
0.003% to 0.03%, and As: from 0.003% to 0.03%.
[0108] When hot-rolling a slab including the above components, a
carbon steel sheet having excellent carburization properties may be
produced by conducting heating at less than or equal to
1200.degree. C.; having a final rolling temperature of hot-rolling
of 800.degree. C. to 940.degree. C.; after completion of the final
rolling, conducting cooling at a cooling rate of 20.degree.
C./second or more to 650.degree. C.; subsequently, conducting
cooling at a cooling rate of 20.degree. C./second or less;
conducting coiling at a coiling temperature of 400.degree. C. to
650.degree. C.; then conducting pickling; and then conducting
annealing for more than or equal to 10 hours at a temperature of
660.degree. C. or more in an atmosphere with a hydrogen content of
95% or more and a dew point of less than or equal to -20.degree. C.
at a temperature of less than 400.degree. C. and of less than or
equal to -40.degree. C. at a temperature of 400.degree. C. or
more.
[0109] It is also possible to conduct the above annealing after
conducting cold-rolling with a rolling ratio of 5% to 60% after the
above pickling.
[0110] It is also possible to conduct another annealing at a
temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of up to 400.degree. C.
and of less than or equal to -40.degree. C. at a temperature of
400.degree. C. or more after conducting cold-rolling with a rolling
ratio of 5% to 60% after the above annealing.
[0111] It is also possible to conduct another annealing at a
temperature of 660.degree. C. or more in an atmosphere with a
hydrogen content of 95% or more and a dew point of less than or
equal to -20.degree. C. at a temperature of up to 400.degree. C.
and of less than or equal to -40.degree. C. at a temperature of
400.degree. C. or more after conducting cold-rolling with a rolling
ratio of 5% to 60% after the above second annealing.
[0112] With respect to annealing conducted on the above hot-rolled
steel sheet or cold-rolled steel sheet, it is possible to conduct
annealing in an atmosphere having a hydrogen content of 95% or more
and at an annealing temperature in a range from "Ac1" to
"Ac1+50.degree. C." and to conduct slow cooling at a cooling rate
of 5.degree. C./hour or less to "Ac1-30.degree. C." after the
annealing.
Examples
[0113] The invention will be described based on examples.
[0114] Steel specimens obtained by casting steel including
components shown in Tables 1 to 6 into 50 kg steel ingots by vacuum
melting were hot-rolled under the conditions described in Tables 7
to 12. Heating for hot-rolling was conducted in the air atmosphere,
and the thickness of hot-rolled steel sheets was 3 mm in the case
of conducting no cold-rolling. In the case of conducting
cold-rolling, the thickness of the hot-rolled steel sheets were
controlled so that the cold-rolled steel sheets will become 3 mm.
The hot-rolled steel sheets were pickled by hydrochloric acid and
then subjected to annealing or cold-rolling so as to produce 3
mm-thick steel sheets for evaluation. The details on the production
conditions and evaluation results are shown in Tables 7 to 12.
After that, under the conditions described in Tables 7 to 12, a
single annealing case, a cold-rolling and annealing case, a case in
which a first annealing was followed by cold-rolling and then
annealing again (annealing twice), and those repetition case
(annealing three times) were carried out as shown in Tables 7 to 12
according to each treatment condition. With respect to the
annealing atmosphere, the inside of a furnace was first substituted
with nitrogen at room temperature, and then hydrogen was introduced
until a predetermined amount of hydrogen was attained, and then the
temperature was raised. In addition, dew points were measured using
a dew-point meter with a thin film aluminum oxide moisture
sensor.
[0115] Surface hardness of the obtained steel sheets were measured
on the Rockwell B Scale (HRB), and the average amount of N in the
surface layer was obtained by analyzing the content of nitrogen (N)
in shavings taken by planing the surface layer portion of the steel
sheet at a depth of 100 .mu.m before carburization. Then, specimens
which had been subjected to a profile forming process were
carburized, and whether abnormal layers were present on the surface
was investigated.
[0116] Meanwhile, the carburization treatment was conducted by the
gas carburization method, and carbon potentials were measured by
the CO.sub.2 amount controlling method using an infrared gas
analyzer.
[0117] The numeric parts in the "No." column in Tables 7 to 12 are
equivalent to the "No." in Tables 1 to 6, therefore it is possible
to distinguish materials with which components have been subjected
to which conditions.
[0118] As shown in Tables 7 to 12, in conditions departing from the
conditions of the invention (underlined) and in the comparative
steel, lack of product hardness, cracks during the profiling
forming process or abnormal layers in the surface layer portion
during carburization were observed, which clarified the effects of
the invention.
[0119] [Table 1]
[0120] [Table 2]
[0121] [Table 3]
[0122] [Table 4]
[0123] [Table 5]
[0124] [Table 6]
[0125] [Table 7]
[0126] [Table 8]
[0127] [Table 9]
[0128] [Table 10]
[0129] [Table 11]
[0130] [Table 12]
[0131] In general, since workability is degraded with an increase
in surface hardness, it is preferable to maintain the surface
hardness of steel sheets before a carburization treatment at less
than or equal to a certain value in terms of securing product
workability. The surface hardness HRB (Rockwell B Scale) of the
steel sheets produced according to the conditions of the invention
were all less than or equal to HRB 77, and it was confirmed from
the results of the profile forming test (Tables 7 to 12) that, if
the HRB is less than or equal to HRB 77, no cracks occur. That is,
it was confirmed that the steel sheet according to the invention
was excellent in terms of workability.
[0132] In addition, from the results shown in Tables 7 to 12, it
was confirmed that the steel sheets according to the invention show
sufficient performance even at a low carbon potential
(Cp.ltoreq.0.6), thereby being excellent in terms of not only
carburization properties but also workability.
[0133] From the evaluation results of carburization properties, it
was confirmed that none of the steel sheets produced according to
the conditions of the invention included abnormal layers. That is,
it was confirmed that the steel sheet according to the invention
was excellent in terms of carburization properties.
INDUSTRIAL APPLICABILITY
[0134] As described above, according to the invention, it is
possible to obtain a steel sheet which has excellent workability
and can secure hardenability at the surface layer portion during
carburization and a production method thereof. Since this steel
sheet can be applied not only to automobile parts or a variety of
industrial machine parts, but also to a wide range of tools or
blades, the steel has a broad range of applications and can be used
throughout many industries, therefore it is needless to say that
this steel sheet is highly valuable in an industrial sense.
TABLE-US-00001 TABLE 1 Chemical composition (%) No C Si Mn P S Al
Ti B N Cr Ni Cu Mo 1 0.24 0.12 1.26 0.023 0.0022 0.022 0.015 0.0035
0.0024 2 0.28 0.2 1.08 0.035 0.0055 0.032 0.022 0.0023 0.0035 3
0.35 0.24 0.87 0.018 0.0032 0.034 0.023 0.0039 0.0028 4 0.34 0.25
0.85 0.025 0.0022 0.057 0.014 0.0017 0.0033 0.15 5 0.22 0.33 1.13
0.013 0.0041 0.044 0.034 0.0033 0.0022 0.15 0.02 Chemical
composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K' value)
(.degree. C.) Note 1 2.04 713 Invention steel 2 2.02 717 Invention
steel 3 2.04 721 Invention steel 4 (2.15) 724 Invention steel 5
(2.11) 720 Invention steel
TABLE-US-00002 TABLE 2 Chemical composition (%) No C Si Mn P S Al
Ti B N Cr Ni Cu Mo 6 0.21 0.05 1.22 0.009 0.0013 0.023 0.012 0.0019
0.0029 0.03 0.5 7 0.24 0.45 1.34 0.015 0.0025 0.046 0.023 0.0036
0.0035 0.12 0.06 0.08 8 0.22 0.22 1.25 0.022 0.0015 0.032 0.021
0.0028 0.0036 0.21 0.015 0.01 9 0.28 0.22 1.44 0.018 0.0054 0.026
0.028 0.0028 0.0019 10 0.33 0.42 0.88 0.027 0.005 0.033 0.033
0.0033 0.0032 Chemical composition (%) K value Ac1 No Nb V Ta W Sn
Sb As (K' value) (.degree. C.) Note 6 (2.41) 703 Invention steel 7
(2.49) 720 Invention steel 8 (2.25) 719 Invention steel 9 0.03 2.39
719 Invention steel 10 0.21 2.08 726 Invention steel
TABLE-US-00003 TABLE 3 Chemical composition (%) No C Si Mn P S Al
Ti B N Cr Ni Cu Mo 11 0.23 0.28 1.25 0.027 0.0023 0.031 0.015
0.0023 0.0037 12 0.29 0.23 1.11 0.017 0.0044 0.028 0.033 0.0028
0.0022 13 0.31 0.23 0.96 0.025 0.0033 0.045 0.027 0.0042 0.0027 14
0.22 0.42 1.82 0.033 0.0048 0.019 0.027 0.0028 0.0031 15 0.34 0.5
1.55 0.031 0.0021 0.033 0.049 0.0038 0.0028 Chemical composition
(%) K value Ac1 No Nb V Ta W Sn Sb As (K' value) (.degree. C.) Note
11 0.28 2.08 719 Invention steel 12 0.08 2.10 718 Invention steel
13 0.015 0.3 2.01 719 Invention steel 14 0.03 0.28 2.69 716
Invention steel 15 0.023 0.08 2.82 721 Invention steel
TABLE-US-00004 TABLE 4 Chemical composition (%) No C Si Mn P S Al
Ti B N Cr Ni Cu Mo 16 0.44 0.23 0.87 0.029 0.0025 0.048 0.029
0.0042 0.0025 17 0.22 0.21 1.18 0.023 0.0046 0.048 0.022 0.0048
0.0034 0.05 0.05 18 0.24 0.77 1.11 0.022 0.0028 0.076 0.028 0.0013
0.0036 0.35 19 0.29 0.13 1.05 0.008 0.0018 0.056 0.019 0.0017
0.0023 0.12 0.06 20 0.35 0.08 1.28 0.006 0.002 0.034 0.033 0.0022
0.0045 0.08 0.04 Chemical composition (%) K value Ac1 No Nb V Ta W
Sn Sb As (K' value) (.degree. C.) Note 16 0.08 0.02 0.012 2.31 723
Invention steel 17 0.03 (2.05) 717 Invention steel 18 0.012 (2.57)
739 Invention steel 19 0.02 0.013 (2.17) 717 Invention steel 20
0.015 (2.45) 710 Invention steel
TABLE-US-00005 TABLE 5 Chemical composition (%) No C Si Mn P S Al
Ti B N Cr Ni Cu Mo 21 0.44 0.23 1.47 0.012 0.0034 0.029 0.041
0.0016 0.0039 0.014 0.02 0.12 22 0.28 0.35 1.82 0.013 0.0029 0.041
0.015 0.0027 0.0022 0.08 23 0.33 0.18 1.56 0.026 0.0043 0.019 0.06
0.0037 0.0037 0.023 0.015 0.011 24 0.23 0.15 0.94 0.029 0.0038 0.09
0.21 0.0051 0.0018 0.45 0.73 0.021 25 0.34 0.21 0.87 0.037 0.0029
0.017 0.011 0.0044 0.0015 0.34 0.035 0.025 26 0.28 0.24 0.87 0.026
0.0039 0.047 0.023 0.0023 0.0024 0.12 0.024 0.125 27 0.23 0.15 1.38
0.014 0.0022 0.036 0.022 0.0036 0.0033 28 0.29 0.35 1.28 0.027
0.0027 0.028 0.024 0.0029 0.0025 0.12 0.06 0.11 29 0.35 0.34 1.34
0.019 0.0015 0.042 0.029 0.0026 0.0024 0.22 0.12 0.23 Chemical
composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K' value)
(.degree. C.) Note 21 0.04 0.11 (3.06) 714 Invention steel 22
(2.84) 714 Invention steel 23 0.016 (2.69) 712 Invention steel 24
0.011 (2.89) 719 Invention steel 25 0.025 0.023 (2.37) 727
Invention steel 26 (2.08) 721 Invention steel 27 0.012 2.15 713
Invention steel 28 0.015 (2.56) 722 Invention steel 29 0.035 0.008
0.005 (3.13) 720 Invention steel
TABLE-US-00006 TABLE 6 Chemical composition (%) No C Si Mn P S Al
Ti B N Cr Ni Cu Mo 30 0.23 0.1 0.59 0.025 0.0045 0.033 0.015 0.0022
0.0029 0.2 0.014 0.02 31 0.28 0.09 0.64 0.024 0.0033 0.043 0.021
0.0029 0.0032 0.23 0.014 0.02 32 0.35 0.07 0.52 0.022 0.0023 0.042
0.018 0.0022 0.0026 0.09 0.033 33 0.43 0.05 0.47 0.019 0.0034 0.029
0.017 0.0033 0.0035 0.09 0.04 34 0.24 0.83 1.25 0.025 0.0045 0.033
0.015 0.0025 0.0034 0.45 0.25 0.02 35 0.29 0.35 2.31 0.024 0.0057
0.037 0.028 0.0039 0.0033 0.23 0.014 36 0.44 0.19 2.13 0.019 0.0033
0.028 0.041 0.0018 0.0037 0.09 0.04 37 0.22 0.21 1.23 0.025 0.0034
0.035 0.022 0.0003 0.0035 0.21 0.014 Chemical composition (%) K
value Ac1 No Nb V Ta W Sn Sb As (K' value) (.degree. C.) Note 30
(1.54) 723 Comparative Example 31 (1.77) 722 Comparative Example 32
(1.70) 721 Comparative Example 33 0.015 (1.92) 721 Comparative
Example 34 (3.09) 737 Comparative Example 35 0.013 (3.60) 712
Comparative Example 36 0.015 (3.68) 707 Comparative Example 37
(2.22) 719 Comparative Example
TABLE-US-00007 TABLE 7 Cold rolling and annealing processes Hot
rolling conditions (First) annealing Cooling Cooling Dew Dew
Cooling Heating Reten- Final rate rate Coiling Cold point point at
rate when temper- tion temper- to before temper- rolling thru
400.degree. C. or annealing ature time ature 650.degree. C. coiling
ature ratio Atmo- 400.degree. C. higher Annealing to Ac1 or No
(.degree. C.) (min) (.degree. C.) (.degree. C./s) (.degree. C./s)
(.degree. C.) (%) sphere (.degree. C.) (.degree. C.) condition
higher 1A 1150 30 860 40 15 620 -- Hydrogen -30 -45 700.degree. C.
.times. -- 100% 36 h 1B 1150 30 860 40 15 620 -- Hydrogen -35 -50
700.degree. C. .times. -- 100% 36 h 1C 1150 30 860 40 15 620 --
Hydrogen -30 -60 700.degree. C. .times. -- 100% 24 h 2A 1180 30 900
35 20 630 -- Hydrogen -40 -60 710.degree. C. .times. -- 95% + 36 h
nitrogen 5% 2B 1180 30 900 35 20 630 -- Hydrogen -45 -60
640.degree. C. .times. -- 95% + 10 h nitrogen 5% 2C 1180 30 900 35
20 630 -- Hydrogen -10 -40 710.degree. C. .times. -- 95% + 36 h
nitrogen 5% 3A 1200 35 860 40 10 600 -- Hydrogen -30 -60
710.degree. C. .times. -- 100% 48 h 3B 1280 40 900 30 15 620 --
Hydrogen -35 -55 710.degree. C. .times. -- 100% 48 h 3C 1200 60 860
40 10 580 -- Hydrogen -35 -60 660.degree. C. .times. -- 100% 6 h 4A
1150 40 880 25 10 600 -- Hydrogen -40 -65 710.degree. C. .times. --
95% + 48 h nitrogen 5% 4B 1150 40 880 25 10 600 -- Hydrogen -30 -35
710.degree. C. .times. -- 95% + 48 h nitrogen 5% 5A 1150 40 880 20
5 580 -- Hydrogen -30 -50 700.degree. C. .times. -- 100% 48 h 5B
1150 40 880 20 5 580 30 Hydrogen -25 -55 700.degree. C. .times. --
100% 36 h Cold rolling and annealing processes (Second) annealing
(Third) annealing Dew Dew Cooling Dew Dew Cold point point at rate
when Cold point point at rolling thru 400.degree. C. or annealing
rolling thru 400.degree. C. or ratio Atmo- 400.degree. C. higher
Annealing to Ac1 or ratio Atmo- 400.degree. C. higher No No. (%)
sphere (.degree. C.) (.degree. C.) condition higher (%) sphere
(.degree. C.) (.degree. C.) 1A 1A -- -- -- -- -- -- -- -- -- -- 1B
1B -- -- -- -- -- -- -- -- -- -- 1C 1C 20 Hydrogen -35 -55
725.degree. C. .times. 1.degree. C./hr -- -- -- -- 100% 10 h 2A 2A
-- -- -- -- -- -- -- -- -- -- 2B 2B -- -- -- -- -- -- -- -- -- --
2C 2C -- -- -- -- -- -- -- -- -- -- 3A 3A -- -- -- -- -- -- -- --
-- -- 3B 3B -- -- -- -- -- -- -- -- -- -- 3C 3C -- -- -- -- -- --
-- -- -- -- 4A 4A -- -- -- -- -- -- -- -- -- -- 4B 4B -- -- -- --
-- -- -- -- -- -- 5A 5A -- -- -- -- -- -- -- -- -- -- 5B 5B -- --
-- -- -- -- -- -- -- -- Cold rolling and annealing processes
(Third) annealing Cooling Sheet product characteristics rate when
Amount of N in Product crack Carburized annealing sheet products
hard- during material Annealing to Ac1 or thru a depth of ness
profile abnormal No condition higher 100 .mu.m (ppm) (HRB) forming
Cp layers Note 1A -- -- 51 71 -- 0.3 Not Present present invention
1B -- -- 43 71 -- 0.38 Not Present present invention 1C -- -- 74 68
-- 0.45 Not Present present invention 2A -- -- 67 74 -- 0.3 Not
Present present invention 2B -- -- 58 82 Crack 0.3 Not Comparative
occurred present Example 2C -- -- 104 73 -- 0.35 Present
Comparative Example 3A -- -- 39 74 -- 0.45 Not Present present
invention 3B -- -- 120 75 -- 0.45 Present Comparative Example 3C --
-- 36 83 Crack 0.45 Not Comparative occurred present Example 4A --
-- 84 75 -- 0.45 Not Present present invention 4B -- -- 144 74 --
0.45 Present Comparative Example 5A -- -- 35 75 -- 0.3 Not Present
present invention 5B -- -- 33 72 -- 0.3 Not Present present
invention
TABLE-US-00008 TABLE 8 Cold rolling and annealing processes Hot
rolling conditions (First) annealing Cooling Cooling Dew Dew
Cooling Heating Reten- Final rate rate Coiling Cold point point at
rate when temper- tion temper- to before temper- rolling thru
400.degree. C. or annealing ature time ature 650.degree. C. coiling
ature ratio Atmo- 400.degree. C. higher Annealing to Ac1 or No
(.degree. C.) (min) (.degree. C.) (.degree. C./s) (.degree. C./s)
(.degree. C.) (%) sphere (.degree. C.) (.degree. C.) condition
higher 6A 1180 40 880 35 20 550 -- Hydrogen -40 -60 690.degree. C.
.times. -- 100% 36 h 6B 1180 40 880 35 20 550 -- Hydrogen -30 -60
690.degree. C. .times. -- 90% + 48 h Nitrogen 10% 7A 1180 30 840 40
15 620 -- Hydrogen -30 -50 700.degree. C. .times. -- 100% 36 h 7B
1180 30 840 40 15 620 -- Hydrogen -30 -50 700.degree. C. .times. --
100% 36 h 8A 1180 30 840 40 15 630 -- Hydrogen -25 -45 690.degree.
C. .times. -- 100% 24 h 8B 1260 30 850 30 20 600 -- Hydrogen -35
-60 690.degree. C. .times. -- 100% 48 h 9A 1180 30 830 35 20 580 --
Hydrogen -50 -60 710.degree. C. .times. -- 100% 36 h 9B 1200 40 860
30 15 640 -- Hydrogen -35 -60 735.degree. C. .times. 10.degree.
C./hr 100% 12 h 9C 1200 60 860 30 15 640 -- Hydrogen -40 -60
690.degree. C. .times. -- 90% + 48 h Nitrogen 10% 9D 1200 90 840 30
15 620 -- Hydrogen -35 -60 690.degree. C. .times. -- 95% + 36 h
nitrogen 5% 10A 1100 60 840 40 20 580 -- Hydrogen -30 -55
710.degree. C. .times. -- 100% 36 h 10B 1100 60 840 40 20 580 --
Hydrogen -15 -45 710.degree. C. .times. -- 100% 48 h Cold rolling
and annealing processes (Second) annealing (Third) annealing Dew
Dew Cooling Dew Dew Cold point point at rate when Cold point point
at rolling thru 400.degree. C. or annealing rolling thru
400.degree. C. or ratio Atmo- 400.degree. C. higher Annealing to
Ac1 or ratio Atmo- 400.degree. C. higher No No. (%) sphere
(.degree. C.) (.degree. C.) condition higher (%) sphere (.degree.
C.) (.degree. C.) 6A 6A -- -- -- -- -- -- -- -- -- -- 6B 6B -- --
-- -- -- -- -- -- -- -- 7A 7A 15 Hydrogen -40 -60 700.degree. C.
.times. -- -- -- -- -- 100% 36 h 7B 7B 15 Hydrogen -20 -35
700.degree. C. .times. -- -- -- -- -- 100% 36 h 8A 8A 20 Hydrogen
-40 -60 690.degree. C. .times. -- -- -- -- -- 100% 24 h 8B 8B -- --
-- -- -- -- -- -- -- -- 9A 9A -- -- -- -- -- -- -- -- -- -- 9B 9B
-- -- -- -- -- -- -- -- -- -- 9C 9C -- -- -- -- -- -- -- -- -- --
9D 9D -- -- -- -- -- -- -- -- -- -- 10A 10A -- -- -- -- -- -- -- --
-- -- 10B 10B -- -- -- -- -- -- -- -- -- -- Cold rolling and
annealing processes (Third) annealing Cooling Sheet product
characteristics rate when Amount of N in Product crack Carburized
annealing sheet products hard- during material Annealing to Ac1 or
thru a depth of ness profile abnormal No condition higher 100 .mu.m
(ppm) (HRB) forming Cp layers Note 6A -- -- 48 75 -- 0.3 Not
Present present invention 6B -- -- 108 74 -- 0.3 Present
Comparative Example 7A -- -- 66 74 -- 0.3 Not Present present
invention 7B -- -- 66 74 -- 0.3 Present Comparative Example 8A --
-- 89 73 -- 0.3 Not Present present invention 8B -- -- 143 74 --
0.3 Present Comparative Example 9A -- -- 29 74 -- 0.3 Not Present
present invention 9B -- -- 36 80 Crack 0.3 Not Comparative occurred
present Example 9C -- -- 110 74 -- 0.3 Present Comparative Example
9D -- -- 103 75 -- 0.3 Present Comparative Example 10A -- -- 47 74
-- 0.45 Not Present present invention 10B -- -- 58 72 -- 0.45
Present Comparative Example
TABLE-US-00009 TABLE 9 Cold rolling and annealing processes Hot
rolling conditions (First) annealing Cooling Cooling Dew Dew
Cooling Heating Reten- Final rate rate Coiling Cold point point at
rate when temper- tion temper- to before temper- rolling thru
400.degree. C. or annealing ature time ature 650.degree. C. coiling
ature ratio Atmo- 400.degree. C. higher Annealing to Ac1 or No
(.degree. C.) (min) (.degree. C.) (.degree. C./s) (.degree. C./s)
(.degree. C.) (%) sphere (.degree. C.) (.degree. C.) condition
higher 11A 1100 50 880 25 15 600 -- Hydrogen -35 -50 690.degree. C.
.times. -- 100% 36 h 11B 1100 50 880 25 15 600 -- Hydrogen 5% + -30
-45 690.degree. C. .times. -- Nitrogen 95% 24 h 11C 1100 50 880 25
15 600 -- Hydrogen -40 -65 650.degree. C. .times. -- 100% 12 h 11D
1100 150 880 25 15 600 -- Hydrogen -40 -65 690.degree. C. .times.
-- 100% 36 h 12A 1100 45 900 30 10 570 -- Hydrogen -40 -60
710.degree. C. .times. -- 100% 36 h 12B 1100 45 900 30 10 570 --
Hydrogen -40 -65 630.degree. C. .times. -- 100% 10 h 12C 1100 45
900 30 10 570 -- Hydrogen -35 -60 730.degree. C. .times. 7.degree.
C./hr 100% 10 h 12D 1100 120 880 25 10 600 -- Hydrogen -35 -45
890.degree. C. .times. -- 100% 36 h 13A 1200 30 820 30 10 540 --
Hydrogen -45 -60 710.degree. C. .times. -- 100% 36 h 13B 1200 30
820 30 10 540 30 Hydrogen -40 -60 710.degree. C. .times. -- 100% 36
h 14A 1200 30 800 30 8 590 -- Hydrogen -35 -55 690.degree. C.
.times. -- 100% 24 h 14B 1200 30 800 30 8 590 -- Hydrogen -35 -55
690.degree. C. .times. -- 100% 24 h 15A 1200 30 840 35 13 600 --
Hydrogen -30 -50 710.degree. C. .times. -- 100% 36 h 15B 1200 30
840 35 13 600 20 Hydrogen -30 -50 690.degree. C. .times. -- 100% 12
h 15C 1200 30 840 35 13 600 20 Hydrogen -30 -50 690.degree. C.
.times. -- 100% 12 h Cold rolling and annealing processes (Second)
annealing (Third) annealing Dew Dew Cooling Dew Dew Cold point
point at rate when Cold point point at rolling thru 400.degree. C.
or annealing rolling thru 400.degree. C. or ratio Atmo- 400.degree.
C. higher Annealing to Ac1 or ratio Atmo- 400.degree. C. higher No
No. (%) sphere (.degree. C.) (.degree. C.) condition higher (%)
sphere (.degree. C.) (.degree. C.) 11A 11A -- -- -- -- -- -- -- --
-- -- 11B 11B -- -- -- -- -- -- -- -- -- -- 11C 11C -- -- -- -- --
-- -- -- -- 11D 11C -- -- -- -- -- -- -- -- -- 12A 12A -- -- -- --
-- -- -- -- -- -- 12B 12B -- -- -- -- -- -- -- -- -- -- 12C 12C --
-- -- -- -- -- -- -- -- -- 12D 12B -- -- -- -- -- -- -- -- -- --
13A 13A -- -- -- -- -- -- -- -- -- -- 13B 13B -- -- -- -- -- -- --
-- -- -- 14A 14A 25 Hydrogen -35 -55 690.degree. C. .times. -- 30
Hydrogen -35 -55 100% 36 h 100% 14B 14B 25 Hydrogen -35 -55
690.degree. C. .times. -- 30 Hydrogen -35 -55 100% 36 h 80% +
Nitrogen 20% 15A 15A 20 Hydrogen -20 -50 710.degree. C. .times. --
25 Hydrogen -45 -60 100% 24 h 100% 15B 15B 25 -20 -50 680.degree.
C. .times. -- -- -- -- -- 6 h 15C 25 Hydrogen -30 -60 690.degree.
C. .times. -- -- -- -- -- 92% + 36 h Nitrogen Cold rolling and
annealing processes (Third) annealing Cooling Sheet product
characteristics rate when Amount of N in Product crack Carburized
annealing sheet products hard- during material Annealing to Ac1 or
thru a depth of ness profile abnormal No condition higher 100 .mu.m
(ppm) (HRB) forming Cp layers Note 11A -- -- 52 73 -- 0.45 Not
Present present invention 11B -- -- 323 75 -- 0.45 Present
Comparative Example 11C -- -- 45 83 Crack 0.45 Not Comparative
occurred present Example 11D -- -- 125 74 -- 0.3 Present
Comparative Example 12A -- -- 41 75 -- 0.33 Not Present present
invention 12B -- -- 34 81 Crack 0.33 Not Comparative occurred
present Example 12C -- -- 46 79 Crack 0.33 Not Comparative occurred
present Example 12D -- -- 108 76 -- 0.33 Present Comparative
Example 13A -- -- 45 77 -- 0.45 Not Present present invention 13B
-- -- 40 73 -- 0.45 Not Present present invention 14A 680.degree.
C. .times. -- 61 69 -- 0.6 Not Present 12 h present invention 14B
680.degree. C. .times. -- 129 69 -- 0.3 Present Comparative 12 h
Example 15A 680.degree. C. .times. -- 67 71 -- 0.35 Not Present 12
h present invention 15B -- -- 60 79 Crack 0.35 Not Comparative
occurred present Example 15C -- -- 108 72 -- 0.35 Present
Comparative Example
TABLE-US-00010 TABLE 10 Cold rolling and annealing processes Hot
rolling conditions (First) annealing Cooling Cooling Dew Dew
Cooling Heating Reten- Final rate rate Coiling Cold point point at
rate when temper- tion temper- to before temper- rolling thru
400.degree. C. or annealing ature time ature 650.degree. C. coiling
ature ratio Atmo- 400.degree. C. higher Annealing to Ac1 or No
(.degree. C.) (min) (.degree. C.) (.degree. C./s) (.degree. C./s)
(.degree. C.) (%) sphere (.degree. C.) (.degree. C.) condition
higher 16A 1180 40 860 25 20 550 -- Hydrogen -35 -60 710.degree. C.
.times. -- 100% 36 h 16B 1180 40 860 25 20 550 -- Hydrogen -20 -35
700.degree. C. .times. -- 100% 48 h 16C 1180 40 860 25 20 550 --
Hydrogen -35 -60 700.degree. C. .times. -- 100% 12 h 17A 1180 50
860 30 20 500 -- Hydrogen -30 -50 690.degree. C. .times. -- 100% 36
h 17B 1180 50 860 30 20 500 20 Hydrogen -30 -55 690.degree. C.
.times. -- 100% 48 h 17C 1220 80 840 40 15 600 -- Hydrogen -25 -55
690.degree. C. .times. -- 100% 24 h 17D 1180 50 860 30 20 500 --
Hydrogen -25 -55 690.degree. C. .times. -- 100% 24 h 18A 1180 40
880 20 10 500 -- Hydrogen -20 -45 710.degree. C. .times. -- 100% 48
h 18B 1180 40 880 20 10 500 50 Hydrogen -25 -40 710.degree. C.
.times. -- 100% 36 h 18C 1180 40 880 20 10 500 50 Hydrogen -25 -40
710.degree. C. .times. -- 100% 36 h 19A 1180 40 880 40 15 560 --
Hydrogen -30 -45 710.degree. C. .times. -- 100% 36 h 19B 1180 40
880 40 15 560 40 Hydrogen -30 -40 730.degree. C. .times. 2.degree.
C./hr 100% 10 h 20A 1170 30 850 35 10 480 -- Hydrogen -40 -60
700.degree. C. .times. -- 100% 12 h 20B 1170 30 850 35 10 480 --
Hydrogen -40 -60 700.degree. C. .times. -- 100% 12 h Cold rolling
and annealing processes (Second) annealing (Third) annealing Dew
Dew Cooling Dew Dew Cold point point at rate when Cold point point
at rolling thru 400.degree. C. or annealing rolling thru
400.degree. C. or ratio Atmo- 400.degree. C. higher Annealing to
Ac1 or ratio Atmo- 400.degree. C. higher No No. (%) sphere
(.degree. C.) (.degree. C.) condition higher (%) sphere (.degree.
C.) (.degree. C.) 16A 16A -- -- -- -- -- -- -- -- -- -- 16B 16B --
-- -- -- -- -- -- -- -- -- 16C 16C 15 Hydrogen -25 -60 700.degree.
C. .times. -- 5 Hydrogen -35 -55 100% 36 h 100% 17A 17A -- -- -- --
-- -- -- -- -- -- 17B 17B -- -- -- -- -- -- -- -- -- -- 17C 17C 20
Hydrogen -40 -60 700.degree. C. .times. -- -- -- -- -- 100% 24 h
17D 17D 20 Hydrogen -15 -45 700.degree. C. .times. -- -- -- -- --
100% 36 h 18A 18A 25 Hydrogen -35 -60 700.degree. C. .times. -- 30
Hydrogen -40 -65 100% 36 h 100% 18B 18B 10 Hydrogen -20 -50
700.degree. C. .times. -- -- -- -- -- 100% 24 h 18C 18C 10 Hydrogen
-10 -40 700.degree. C. .times. -- -- -- -- -- 100% 36 h 19A 19A --
-- -- -- -- -- -- -- -- -- 19B 19B -- -- -- -- -- -- -- -- -- --
20A 20A 30 Hydrogen -30 -60 725.degree. C. .times. 2.degree. C./hr
-- -- -- -- 100% 12 h 20B 20B 30 -30 -60 690.degree. C. .times. --
-- -- -- -- 36 h Cold rolling and annealing processes (Third)
annealing Cooling Sheet product characteristics rate when Amount of
N in Product crack Carburized annealing sheet products hard- during
material Annealing to Ac1 or thru a depth of ness profile abnormal
No condition higher 100 .mu.m (ppm) (HRB) forming Cp layers Note
16A -- -- 43 74 -- 0.6 Not Present present invention 16B -- -- 49
73 -- 0.48 Present Comparative Example 16C 730.degree. C. .times.
1.degree. C./hr 51 71 -- 0.48 Not Present 12 h present invention
17A -- -- 51 71 -- 0.3 Not Present present invention 17B -- -- 58
69 -- 0.3 Not Present present invention 17C -- -- 135 71 -- 0.3
Present Comparative Example 17D -- -- 65 70 -- 0.3 Present
Comparative Example 18A 690.degree. C. .times. -- 78 74 -- 0.3 Not
Present 12 h present invention 18B -- -- 59 73 -- 0.3 Not Present
present invention 18C -- -- 64 72 -- 0.3 Present Comparative
Example 19A -- -- 73 74 -- 0.45 Not Present present invention 19B
-- -- 92 73 -- 0.45 Not Present present invention 20A -- -- 63 71
-- 0.45 Not Present present invention 20B -- -- 111 73 -- 0.4
Present Comparative Example
TABLE-US-00011 TABLE 11 Cold rolling and annealing processes Hot
rolling conditions (First) annealing Cooling Cooling Dew Dew
Cooling Heating Reten- Final rate rate Coiling Cold point point at
rate when temper- tion temper- to before temper- rolling thru
400.degree. C. or annealing ature time ature 650.degree. C. coiling
ature ratio Atmo- 400.degree. C. higher Annealing to Ac1 or No
(.degree. C.) (min) (.degree. C.) (.degree. C./s) (.degree. C./s)
(.degree. C.) (%) sphere (.degree. C.) (.degree. C.) condition
higher 21A 1170 30 890 45 8 520 -- Hydrogen -35 -55 710.degree. C.
.times. -- 100% 36 h 21B 1170 30 890 45 8 520 30 Hydrogen -30 -65
730.degree. C. .times. 1.degree. C./hr 100% 10 h 21C 1170 30 890 45
8 520 -- Hydrogen -40 -65 700.degree. C. .times. -- 100% 36 h 21D
1180 40 880 35 10 580 -- Hydrogen -40 -65 700.degree. C. .times. --
100% 36 h 22A 1120 30 910 50 10 550 -- Hydrogen -35 -60 710.degree.
C. .times. -- 100% 36 h 22B 1120 30 910 50 10 550 -- Hydrogen -15
-35 710.degree. C. .times. -- 100% 36 h 23 1120 50 910 35 15 580 --
Hydrogen -30 -50 700.degree. C. .times. -- 100% 36 h 24 1100 60 830
20 15 600 25 Hydrogen -30 -45 710.degree. C. .times. -- 100% 24 h
25 1100 60 880 25 20 600 -- Hydrogen -40 -60 710.degree. C. .times.
-- 100% 36 h 26 1100 60 860 35 15 540 -- Hydrogen -35 -55
710.degree. C. .times. -- 100% 36 h 27 1180 40 870 30 10 600 --
Hydrogen -30 -60 690.degree. C. .times. -- 95% + 72 h nitrogen 5%
28 1180 40 870 25 20 600 -- Hydrogen -25 -50 690.degree. C. .times.
-- 95% + 72 h nitrogen 5% 29 1180 40 860 30 15 600 30 Hydrogen -40
-55 690.degree. C. .times. -- 95% + 36 h nitrogen 5% Cold rolling
and annealing processes (Second) annealing (Third) annealing Dew
Dew Cooling Dew Dew Cold point point at rate when Cold point point
at rolling thru 400.degree. C. or annealing rolling thru
400.degree. C. or ratio Atmo- 400.degree. C. higher Annealing to
Ac1 or ratio Atmo- 400.degree. C. higher No No. (%) sphere
(.degree. C.) (.degree. C.) condition higher (%) sphere (.degree.
C.) (.degree. C.) 21A 21A 25 Hydrogen -35 -50 710.degree. C.
.times. -- -- -- -- -- 100% 48 h 21B 21B -- -- -- -- -- -- -- -- --
-- 21C 21C 30 Hydrogen -30 -55 730.degree. C. .times. 10.degree.
C./hr -- -- -- -- 100% 10 h 21D 21D 10 Hydrogen -30 -55 700.degree.
C. .times. 5 Hydrogen -35 -55 100% 12 h 100% 22A 22A 15 Hydrogen
-25 -60 710.degree. C. .times. -- 15 Hydrogen -35 -55 100% 36 h
100% 22B 22B 15 Hydrogen -40 -65 710.degree. C. .times. -- 15
Hydrogen -40 -60 100% 36 h 100% 23 23 30 Hydrogen -45 -60
700.degree. C. .times. -- 20 Hydrogen -40 -55 100% 24 h 100% 24 24
-- -- -- -- -- -- -- -- -- -- 25 25 -- -- -- -- -- -- -- -- -- --
26 26 -- -- -- -- -- -- -- -- -- -- 27 27 -- -- -- -- -- -- -- --
-- -- 28 28 -- -- -- -- -- -- -- -- -- -- 29 29 5 Hydrogen -40 -55
690.degree. C. .times. -- -- -- -- -- 95% + 12 h nitrogen 5% Cold
rolling and annealing processes (Third) annealing Cooling Sheet
product characteristics rate when Amount of N in Product crack
Carburized annealing sheet products hard- during material Annealing
to Ac1 or thru a depth of ness profile abnormal No condition higher
100 .mu.m (ppm) (HRB) forming Cp layers Note 21A -- -- 76 72 -- 0.6
Not Present present invention 21B -- -- 55 72 -- 0.6 Not Present
present invention 21C -- -- 64 79 Crack 0.6 Not Comparative
occurred present Example 21D 730.degree. C. .times. 2.degree. C./hr
64 69 -- 0.6 Not Present 12 h present invention 22A 700.degree. C.
.times. -- 48 68 -- 0.3 Not Present 24 h present invention 22B
700.degree. C. .times. -- 116 68 -- 0.3 Present Comparative 24 h
Example 23 690.degree. C. .times. -- 72 70 -- 0.45 Not Present 12 h
present invention 24 -- -- 44 71 -- 0.3 Not Present present
invention 25 -- -- 32 74 -- 0.45 Not Present present invention 26
-- -- 56 72 -- 0.3 Not Present present invention 27 -- -- 61 73 --
0.3 Not Present present invention 28 -- -- 53 74 -- 0.35 Not
Present present invention 29 -- -- 48 74 -- 0.4 Not Present present
invention
TABLE-US-00012 TABLE 12 Cold rolling and annealing processes Hot
rolling conditions (First) annealing Cooling Cooling Dew Dew
Cooling Heating Reten- Final rate rate Coiling Cold point point at
rate when temper- tion temper- to before temper- rolling thru
400.degree. C. or annealing ature time ature 650.degree. C. coiling
ature ratio Atmo- 400.degree. C. higher Annealing to Ac1 or No
(.degree. C.) (min) (.degree. C.) (.degree. C./s) (.degree. C./s)
(.degree. C.) (%) sphere (.degree. C.) (.degree. C.) condition
higher 30 1200 40 860 25 15 550 -- Hydrogen -30 -45 710.degree. C.
.times. -- 100% 24 h 31 1200 40 880 30 10 600 -- Hydrogen -40 -55
710.degree. C. .times. -- 100% 36 h 32 1180 30 670 35 10 620 --
Hydrogen -40 -60 710.degree. C. .times. -- 100% 36 h 33 1180 30 830
30 20 590 -- Hydrogen -35 -55 710.degree. C. .times. -- 100% 36 h
34 1150 60 840 40 20 540 -- Hydrogen -35 -50 710.degree. C. .times.
-- 100% 24 h 35 1150 60 900 25 15 580 -- Hydrogen -25 -45
710.degree. C. .times. -- 100% 36 h 36 1150 40 870 30 5 450 --
Hydrogen -30 -55 700.degree. C. .times. -- 100% 36 h 37 1180 30 850
30 15 600 -- Hydrogen -25 -40 690.degree. C. .times. -- 100% 24 h
Cold rolling and annealing processes (Second) annealing (Third)
annealing Dew Dew Cooling Dew Dew Cold point point at rate when
Cold point point at rolling thru 400.degree. C. or annealing
rolling thru 400.degree. C. or ratio Atmo- 400.degree. C. higher
Annealing to Ac1 or ratio Atmo- 400.degree. C. higher No No. (%)
sphere (.degree. C.) (.degree. C.) condition higher (%) sphere
(.degree. C.) (.degree. C.) 30 30 -- -- -- -- -- -- -- -- -- -- 31
31 -- -- -- -- -- -- -- -- -- -- 32 32 -- -- -- -- -- -- -- -- --
-- 33 33 15 Hydrogen -35 -50 710.degree. C. .times. -- -- -- -- --
100% 12 h 34 34 20 Hydrogen -40 -55 710.degree. C. .times. -- 25
Hydrogen -30 -45 100% 12 h 100% 35 35 15 Hydrogen -30 -50
710.degree. C. .times. -- 20 Hydrogen -35 -55 100% 24 h 100% 36 36
20 Hydrogen -30 -50 700.degree. C. .times. -- 15 Hydrogen -45 -55
100% 12 h 100% 37 37 20 Hydrogen -45 -60 690.degree. C. .times. --
-- -- -- -- 100% 24 h Cold rolling and annealing processes (Third)
annealing Cooling Sheet product characteristics rate when Amount of
N in Product crack Carburized annealing sheet products hard- during
material Annealing to Ac1 or thru a depth of ness profile abnormal
No condition higher 100 .mu.m (ppm) (HRB) forming Cp layers Note 30
-- -- 39 73 -- 0.3 Present Comparative Example 31 -- -- 46 72 --
0.3 Present Comparative Example 32 -- -- 39 71 -- 0.45 Present
Comparative Example 33 -- -- 48 69 -- 0.45 Present Comparative
Example 34 680.degree. C. .times. -- 51 80 Crack 0.3 Not
Comparative 12 h occurred present Example 35 680.degree. C. .times.
-- 49 81 Crack 0.45 Not Comparative 12 h occurred present Example
36 680.degree. C. .times. -- 53 82 Crack 0.6 Not Comparative 12 h
occurred present Example 37 -- -- 70 69 -- 0.3 Present Comparative
Example
* * * * *