U.S. patent application number 14/361293 was filed with the patent office on 2015-09-24 for martensitic steels with 1700-2200 mpa tensile strength.
The applicant listed for this patent is ARCELORMITTAL LNVESTIGACION Y DESARROLLO S.L.. Invention is credited to Narayan S. Pottore, Rongjie Song.
Application Number | 20150267281 14/361293 |
Document ID | / |
Family ID | 48536024 |
Filed Date | 2015-09-24 |
United States Patent
Application |
20150267281 |
Kind Code |
A1 |
Song; Rongjie ; et
al. |
September 24, 2015 |
MARTENSITIC STEELS WITH 1700-2200 MPA TENSILE STRENGTH
Abstract
Martensitic steel compositions and methods of production
thereof. More specifically, the martensitic steels have tensile
strengths ranging from 1700 to 2200 MPa. Most specifically, the
invention relates to thin gage (thickness of .ltoreq.1 mm) ultra
high strength steel with an ultimate tensile strength of 1700-2200
MPa and methods of production thereof.
Inventors: |
Song; Rongjie; (Chicago,
IL) ; Pottore; Narayan S.; (Munster, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ARCELORMITTAL LNVESTIGACION Y DESARROLLO S.L. |
ES-48910 Sestao, Biskaia |
|
ES |
|
|
Family ID: |
48536024 |
Appl. No.: |
14/361293 |
Filed: |
November 28, 2012 |
PCT Filed: |
November 28, 2012 |
PCT NO: |
PCT/US12/66895 |
371 Date: |
May 28, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61629762 |
Nov 28, 2011 |
|
|
|
Current U.S.
Class: |
420/120 ;
420/121; 420/126; 420/128 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/002 20130101; C22C 38/14 20130101; C21D 6/005 20130101;
C22C 38/001 20130101; C22C 38/02 20130101; C21D 2211/008 20130101;
C22C 38/04 20130101; C22C 38/12 20130101 |
International
Class: |
C22C 38/14 20060101
C22C038/14; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/00 20060101 C22C038/00; C22C 38/06 20060101
C22C038/06 |
Claims
1. A martensitic steel alloy, said alloy having an ultimate tensile
strength of at least 1700 MPa.
2. The martensitic steel alloy of claim 1, wherein said alloy has
an ultimate tensile strength of at least 1800 MPa.
3. The martensitic steel alloy of claim 2, wherein said alloy has
an ultimate tensile strength of at least 1900 MPa.
4. The martensitic steel alloy of claim 3, wherein said alloy has
an ultimate tensile strength of at least 2000 MPa.
5. The martensitic steel alloy of claim 4, wherein said alloy has
an ultimate tensile strength of at least 2100 MPa.
6. The martensitic steel alloy of claim 1, wherein said alloy has
an ultimate tensile strength between 1700 and 2200 MPa.
7. The martensitic steel alloy of claim 1, wherein said alloy has a
total elongation of at least 3.5%.
8. The martensitic steel alloy of claim 7, wherein said alloy has a
total elongation of at least 5%.
9. The martensitic steel alloy of claim 1, wherein said alloy is in
the form of a cold rolled sheet, band or coil.
10. The martensitic steel alloy of claim 9, wherein said a cold
rolled sheet, band or coil has a thickness of less than or equal to
1 mm.
11. The martensitic steel alloy of claim 1, wherein said alloy has
a carbon equivalent of less than 0.44 using the formula:
C.sub.eq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 where C.sub.eq is the carbon
equivalent, C, Mn, Cr, Mo, V, Ni, and Cu are in wt. % of the
elements in the alloy.
12. The martensitic steel alloy of claim 1, wherein said alloy
contains between 0.22 and 0.36 wt. % carbon.
13. The martensitic steel alloy of claim 12, wherein said alloy
contains between 0.22 and 0.28 wt. % carbon.
14. The martensitic steel alloy of claim 12, wherein said alloy
contains between 0.28 and 0.36 wt. % carbon.
15. The martensitic steel alloy of claim 12, wherein said alloy
contains between 0.5 and 2.0 wt. % manganese.
16. The martensitic steel alloy of claim 15, wherein said alloy
contains about 0.2 wt. % silicon.
17. The martensitic steel alloy of claim 15, wherein said alloy
further contains one or more of Nb, Ti, B, Al, N, S, P.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. National Stage Application
(filed under 35 U.S.C. 371) of prior International Application No.
PCT/US12/66895, filed Nov. 28, 2012, and published on Jun. 6, 2013
as WO 2013/082188, which claims the benefit of/priority to U.S.
Provisional Application No. 61/629,762 filed Nov. 28, 2011.
FIELD OF THE INVENTION
[0002] The present invention relates to martensitic steel
compositions and methods of production thereof. More specifically,
the martensitic steels have tensile strengths ranging from 1700 to
2200 MPa. Most specifically, the invention relates to thin gage
(thickness of .ltoreq.1 mm) ultra high strength steel with an
ultimate tensile strength of 1700-2200 MPa and methods of
production thereof.
BACKGROUND OF THE INVENTION
[0003] Low-carbon steels with martensitic microstructure constitute
a class of Advanced High Strength Steels (AHSS) with the highest
strengths attainable in sheet steels. By varying the carbon content
in the steel, ArcelorMittal has been producing martensitic steels
with tensile strength ranging from 900 to 1500 MPa for two decades.
Martensitic steels are increasingly being used in applications that
require high strength for side impact and roll over vehicle
protection, and have long been used for applications such as
bumpers that can readily be rolled formed.
[0004] Currently, thin gage (thickness of .ltoreq.1 mm) ultra high
strength steel with ultimate tensile strength of 1700-2200 MPa with
good roll formability, weldability, punchability and delayed
fracture resistance is in demand for the manufacture of hang on
automotive parts such as bumper beams. Light gauge, high strength
steels are required to fend off competitive challenges from
alternative materials, such as lightweight 7xxx series of aluminum
alloys. Carbon content has been the most important factor in
determining the ultimate tensile strength of martensitic steels.
The steel has to have sufficient hardenability so as to fully
transform to martensite when quenched from a supercritical
annealing temperature.
SUMMARY OF THE INVENTION
[0005] The present invention comprises a martensitic steel alloy
that has an ultimate tensile strength of at least 1700 MPa.
Preferably, the alloy may have an ultimate tensile strength of at
least 1800 MPa, at least 1900 MPa, at least 2000 MPa or even at
least 2100 MPa. The martensitic steel alloy may have an ultimate
tensile strength between 1700 and 2200 MPa. The martensitic steel
alloy may have a total elongation of at least 3.5% and more
preferably at least 5%.
[0006] The martensitic steel alloy may be in the form of a cold
rolled sheet, band or coil and may have a thickness of less than or
equal to 1 mm. The martensitic steel alloy may have a carbon
equivalent of less than 0.44 using the formula
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15, where Ceq is the carbon
equivalent, and C, Mn, Cr, Mo, V, Ni, and Cu are in wt. % of the
elements in the alloy.
[0007] The martensitic steel alloy may contain between 0.22 and
0.36 wt. % carbon. More specifically, the alloy may contain between
0.22 and 0.28 wt. % carbon or in the alternative the alloy may
contain between 0.28 and 0.36 wt. % carbon. The martensitic steel
alloy may further contain between 0.5 and 2.0 wt. % manganese. The
alloy may also contain about 0.2 wt. % silicon. The optionally may
contain one or more of Nb, Ti, B, Al, N, S, P.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIGS. 1a and 1b are schematic illustrations of annealing
procedures useful in producing the alloys of the present
invention;
[0009] FIGS. 2a, 2b and 2c are SEM micrographs of experimental
steels with 2.0% Mn-0.2% Si and various carbon contents (2a has
0.22% C; 2b has 0.25% C; and 2c has 0.28% C) after hot rolling and
simulated coiling at 580.degree. C.;
[0010] FIG. 3 is a plot of the tensile properties at room
temperature of experimental steel hot bands useful in producing
alloys of the present invention;
[0011] FIGS. 4a-4b are SEM micrographs of experimental steels with
0.22% C-0.2% Si-0.02% Nb and two different Mn contents (4a has
1.48% and 4b has 2.0%) after hot rolling and simulated coiling at
580.degree. C.;
[0012] FIG. 5 is a plot of the tensile properties at room
temperature of another experimental steel hot bands useful in
producing alloys of the present invention;
[0013] FIGS. 6a-6b are SEM micrographs of experimental steels with
0.22% C-2.0% Mn-0.2% Si and different Nb contents (6a has 0% and 6b
has 0.018%) after hot rolling and simulated coiling at 580.degree.
C.;
[0014] FIG. 7 is a plot of the tensile properties at room
temperature of yet another experimental steel hot bands useful in
producing alloys of the present invention;
[0015] FIGS. 8a-8f illustrate the effects of soaking temperature
(830, 850 and 870.degree. C.) and steel composition (FIGS. 8a &
8b show varied C, 8c & 8d show varied Mn and 8e & 8f show
varied Nb) on the tensile properties of steels of the present
invention;
[0016] FIGS. 9a-9f show the effects of quenching temperature
(780,810 and 840.degree. C.) and steel composition (FIGS. 9a &
9b show varied C, 9c & 9d show varied Mn and 9e & 9f show
varied Nb) on tensile properties of additional steels of the
present invention;
[0017] FIGS. 10a and 10b are schematic depictions of the additional
anneal cycles useful in producing alloys of the present
invention;
[0018] FIGS. 11a and 11 b plot the tensile properties at room
temperature of hot bands useful in producing steels of the present
invention, after hot rolling and simulated coiling at 580.degree.
C.;
[0019] FIGS. 12a-12d are SEM micrographs at 1000.times. of the
microstructure of hot band steels after hot rolling and simulated
coiling at 660.degree. C.;
[0020] FIGS. 13a-13b plot the tensile properties of experimental
hot band steels at room temperature;
[0021] FIGS. 14a-14d represent the effects of soaking temperature
(830.degree. C., 850.degree. C. and 870.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (Ti, B and Nb additions to the base steel) on the
tensile properties of the steels after anneal simulation;
[0022] FIGS. 15a-15d show the effects of quenching temperature
(780.degree. C., 810.degree. C. and 840.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (Ti, B and Nb additions to the base steel) on the
tensile properties of the steels after anneal simulation;
[0023] FIGS. 16a-16c are even more schematic depictions of anneal
cycles useful in producing the alloys of the present invention;
[0024] FIGS. 17a to 17e are SEM micrographs at 1,000.times. of hot
rolled steels (0.28 to 0.36% C) after hot rolling and simulated
coiling at 580.degree. C.;
[0025] FIGS. 18a and 18b plot the corresponding tensile properties
of the hot rolled steels of FIGS. 17a-17e, at room temperature
(after hot rolling and simulated coiling at 580.degree. C.);
[0026] FIGS. 19a-19e are SEM micrographs at 1,000.times. of hot
rolled steels (0.28 to 0.36% C) after hot rolling and simulated
coiling at 660.degree. C.;
[0027] FIGS. 20a and 20b plot the corresponding tensile properties
of the hot rolled steels of FIGS. 19a-19e, at room temperature
(after hot rolling and simulated coiling at 660.degree. C.);
[0028] FIGS. 21a-21d represents the effects of soaking temperature
(830.degree. C., 850.degree. C. and 870.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (C content and B addition to the base steel) on the
tensile properties of the steels after annealing simulation;
[0029] FIGS. 22a-22d show the effects of quenching temperature
(780.degree. C., 810.degree. C. and 840.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (C content and B addition to the base steel) on the
tensile properties of the steels after annealing simulation;
[0030] FIGS. 23a-23d illustrates the effect of composition and
annealing cycle on (23a-23b) tensile strength and (23c-23d)
ductility;
[0031] FIGS. 24a-24l are micrographs of four alloys which were
annealed using various soak/quenching temperature pairs; and
[0032] FIGS. 25a-25d show the tensile properties of the steels with
0.5% to 2.0% Mn after coiling at 580.degree. C., cold rolling (50%
cold rolling reduction for the steel with 0.5 and 1.0% Mn and 75%
cold rolling reduction for the steel with 2.0% Mn) and various
annealing cycles.
DETAILED DESCRIPTION OF THE INVENTION
[0033] The present invention is a family of martensitic steels with
tensile strength ranging from 1700 to 2200 MPa. The steel may be
thin gauge (thickness of less than or equal to 1 mm) sheet steel.
The present invention also includes the process for producing the
very high tensile strength martensitic steels. Examples and
embodiments of the present invention are presented below.
Example 1
Materials and Experimental Procedures
[0034] Table 1 shows the chemical compositions of some steels
within the present invention, which includes a range of carbon
content from 0.22 to 0.28 wt % (steels 2, 4 and 5), manganese
content from 1.5 to 2.0 wt % (steels 1 and 3) and niobium content
from 0 to 0.02 wt % (alloys 2 and 3). The remainder of the steel
composition is iron and inevitable impurities.
TABLE-US-00001 TABLE 1 ID Steel C Mn Si Nb Al N S P 1
0.22C--1.5Mn--0.018Nb 0.22 1.48 0.198 0.019 0.036 0.0043 0.002
0.006 2 0.22C--2.0Mn 0.22 2.00 0.199 -- 0.027 0.0049 0.002 0.006 3
0.22C--2.0Mn--0.018Nb 0.22 2.00 0.197 0.018 0.033 0.0045 0.002
0.006 4 0.25C--2.0Mn 0.25 1.99 0.201 -- 0.025 0.005 0.003 0.009 5
0.28C--2.0Mn 0.28 2.01 0.202 -- 0.032 0.0045 0.003 0.007
[0035] Five 45 Kg slabs were cast in the laboratory. After
reheating and austenitization at 1230.degree. C. for 3 hours, the
slabs were hot rolled from 63 mm to 20 mm in thickness on a
laboratory mill. The finishing temperature was about 900.degree. C.
The plates were air cooled after hot rolling.
[0036] After shearing and reheating the pre-rolled 20 mm thick
plates to 1230.degree. C. for 2 hours, the plates were hot rolled
from a thickness of 20 mm to 3.5 mm. The finish rolling temperature
was about 900.degree. C. After controlled cooling at an average
cooling rate of about 45.degree. C./s, the hot bands of each
composition were held in a furnace at 580.degree. C. for 1 hour,
followed by a 24-hour furnace cooling to simulate the industrial
coiling process.
[0037] Three JIS-T standard specimens were prepared from each hot
band for room temperature tensile test. Microstructure
characterization of hot bands was carried out by Scanning Electron
Microscopy (SEM) at the quarter thickness location in the
longitudinal cross-sections.
[0038] Both surfaces of the hot rolled bands were ground to remove
any decarburized layer. They were then subjected to 75% lab cold
rolling to obtain full hard steels with final thickness of 0.6 mm
for further annealing simulations.
[0039] Annealing simulation was performed using two salt pots and
one oil bath. The effects of soaking and quenching temperatures
were analyzed for all of the steels. A schematic illustration of
the heat treatment is shown in FIGS. 1(a) and 1(b). FIG. 1(a)
illustrates the annealing processes with different soaking
temperatures from 830.degree. C. to 870.degree. C. FIG. 1(b)
illustrates the annealing processes with different quenching
temperatures from 780.degree. C. to 840.degree. C.
[0040] To study the effect of soaking temperature, the annealing
process included reheating the cold rolled strips (0.6 mm thick) to
870.degree. C., 850.degree. C. and 830.degree. C. respectively
followed by isothermal holding for 60 seconds. The samples were
immediately transferred to the second salt pot maintained at a
temperature of 810.degree. C. and isothermally held for 25 s. This
was followed by a water quench. The samples were then reheated to
200.degree. C. for 60 s in an oil bath, followed by air cooling to
room temperature to simulate overage treatment. The holding times
at soaking, quenching and overaging temperatures were chosen to
closely approximate industrial conditions for this gauge.
[0041] To study the effect of quenching temperature, the analysis
includes reheating of cold rolled strips to 870.degree. C. for 60
seconds, followed by immediate cooling to 840.degree. C.,
810.degree. C. and 780.degree. C. After a 25 second isothermal hold
at the quenching temperature, the specimens were quenched in water.
The steels were then reheated to 200.degree. C. for 60 seconds
followed by air cooling to simulate the overage treatment. Three
ASTM-T standard specimens were prepared from each annealed blank
for tensile testing at room temperature.
[0042] The samples processed at 870.degree. C. soaking temperature
and quenched from 810.degree. C. were selected for bend testing. A
90.degree. free V-bend with the bending axis in the rolling
direction was employed for bendability characterization. A
dedicated Instron mechanical testing system with 90.degree. die
block and punches was utilized for this test. A series of
interchangeable punches with different die radius facilitated the
determination of minimum die radius at which the samples could be
bent without microcracks. The test was run at a constant stroke of
15 mm/sec until the sample was bent by 90.degree.. A 80 KN force
and 5 second dwell time was deployed at the maximum bend angle
after which the load was released and the specimen was allowed to
spring back. In the present test, the range of die radius varied
from 1.75 to 2.75 mm with 0.25 mm incremental increase. The sample
surface after bend testing was observed under 10.times.
magnification. A crack length on the sample bending surface that is
smaller than 0.5 mm is considered to be a "micro crack", and any
that is larger than 0.5 mm is recognized as a crack and the test
marked as a failure. Samples with no visible crack are identified
as "passed test".
Microstructure and Tensile Properties of Hot Rolled Bands
Effect of Composition on Microstructure and Tensile Properties of
Hot Rolled Steels
[0043] FIGS. 2a, 2b and 2c are SEM micrographs of experimental
steels with 2.0% Mn-0.2% Si and various carbon contents (2a has
0.22% C; 2b has 0.25% C; and 2c has 0.28% C) after hot rolling and
simulated coiling at 580.degree. C.
[0044] The increase in carbon content resulted in an increase in
the volume fraction and the colony size of pearlite. The
corresponding tensile properties at room temperature of the
experimental steels are plotted in FIG. 3, where strength in MPa
(top half of the graph) and ductility in percentage (bottom half of
the graph) are plotted against carbon content. In FIG. 3 and
herein, UTS means ultimate tensile strength, YS means yield
strength, TE means total elongation, UE means uniform elongation.
As shown, the increase in carbon content from 0.22 to 0.28% led to
a slight increase in ultimate tensile strength from 609 to 632 MPa,
a slight decrease in yield strength from 440 to 426 MPa but little
change in ductility (average TE and UE are about 16% and 11%
respectively).
[0045] FIGS. 4a-4b are SEM micrographs of experimental steels with
0.22% C-0.2% Si-0.02% Nb and two different Mn contents (4a has
1.48% and 4b has 2.0%) after hot rolling and simulated coiling at
580.degree. C. An increase in the Mn content resulted in an
increase in the volume fraction and in size of pearlite colony. The
large grain size in the higher Mn steel can be attributed to grain
coarsening during finish rolling and subsequent cooling. The hot
rolling finish temperature was about 900.degree. C., which is in
the austenite region for both of the experimental steels but it is
much higher than the Ar.sub.a temperature for the higher Mn steel.
Thus, during and after finish rolling, the austenite in the higher
Mn steel had a greater opportunity to coarsen, resulting in a
coarser ferrite-pearlite microstructure after phase
transformation.
[0046] The corresponding tensile properties of the experimental
steels with 0.22% C-2.0% Mn at room temperature are plotted in FIG.
5, where strength in MPa (top half of the graph) and ductility in
percentage (bottom half of the graph) are plotted against manganese
content. As shown, an increase in the Mn content from 1.48 to 2.0%
led to a small increase in the ultimate tensile strength from 655
to 680 MPa, a marked decrease in yield strength from 540 to 416 MPa
and a slight decrease in ductility from 22 to 18% for TE and from
12 to 11% for UE. The corresponding yield ratio (YR) dropped from
0.8 to 0.6 and yield point elongation (YPE) decreased from 3.1 to
0.3% with the increase in Mn content. The tremendous decrease in
YS, YR and YPE in spite of solid solution strengthening by Mn may
be attributed to the formation of martensite in the higher Mn
steel. A small amount of martensite (even less than 5%) can create
free dislocations surrounding ferrite to facilitate initial plastic
deformation, as is well known for DP steels. In addition, higher
hardenability of the higher Mn steel may also result in coarse
austenite grain size.
[0047] FIGS. 6a-6b are SEM micrographs of experimental steels with
0.22% C-2.0% Mn-0.2% Si and different Nb contents (6a has 0% and 6b
has 0.018%) after hot rolling and simulated coiling at 580.degree.
C. An increase in the Nb content resulted in an increase in the
volume fraction and colony size of pearlite, which can be explained
by higher hardenability of the steel with Nb and lower temperature
of pearlite formation.
[0048] The corresponding tensile properties of the compared steels
with 0.22% C-2.0% Mn are illustrated in FIG. 7, where strength in
MPa (top half of the graph) and ductility in percentage (bottom
half of the graph) are plotted against niobium content. As shown,
the addition of 0.018% Nb led to an increase in the ultimate
tensile strength (UTS) from 609 to 680 MPa, a small decrease in
yield strength (YS) from 440 to 416 MPa and a slight increase in
average TE from 16.8 to 18.0% with UE decreasing from 11.8 to
10.8%. The corresponding yield ratio (YR) dropped from 0.72 to 0.61
and yield point elongation (YPE) decreased from 2.3 to 0.3% with
the increase in Nb content.
Tensile Properties of the Investigated Steels after Cold Rolling
and Annealing Simulation
[0049] FIGS. 8a-8f illustrate the effects of soaking temperature
(830, 850 and 870.degree. C.) and steel composition (FIGS. 8a &
8b show varied C, 8c & 8d show varied Mn and 8e & 8f show
varied Nb) on the tensile properties of steels. The decrease in
soaking temperature from 870 to 850.degree. C. resulted in an
increase of 28-76 MPa in yield strength (YS) and 30-103 MPa in
ultimate tensile strength (UTS), which may be attributed to the
smaller grain size at lower soaking temperature. A further decrease
in soaking temperature from 850 to 830.degree. C. did not lead to a
significant change in UTS. There is no effect of soaking
temperature on ductility and the uniform/total elongation ranges
from 3 to 4.75% in all the experimental steels. It should be
stressed that UTS exceeding 2000 MPa and uniform/total elongation
of .about.3.5-4.5% were achieved in the steel with 0.28% C-2.0%
Mn-0.2% Si (see FIGS. 8a-8b).
[0050] FIGS. 9a-9f show the effects of quenching temperature (780,
810 and 840.degree. C.) and steel composition (FIGS. 9a & 9b
show varied C, 9c & 9d show varied Mn and 9e & 9f show
varied Nb) on tensile properties of the investigated steels. There
is no significant effect of quenching temperature on strength and
ductility when 100% martensite is obtained. The uniform/total
elongation ranges from 2.75 to 5.5% in all the experimental steels.
The data suggests that a wide process window is feasible during
anneal.
[0051] FIGS. 8a, 8b, 9a, and 9b show that an increase in the C
content resulted in a significant increase in tensile strength but
had little effect on ductility. Taking the annealing cycle of
830.degree. C. (soaking temperature)-810.degree. C. (quenching
temperature) as an example, the increase in YS and UTS is 163 and
233 MPa, respectively, when C content is increased from 0.22 to
0.28 wt %. The increase in Mn content from 1.5 to 2.0 wt % has
barely any effect on strength and ductility (see FIGS. 8c, 8d, 9c
and 9d). The addition of Nb (about 0.02 wt %) led to an increase in
YS up to 94 MPa with almost no effect on UTS but a decrease in
total elongation of 2.4% (see FIGS. 8e, 8f, 9e and 9f).
Bendability of the Investigated Steels
[0052] Table 2 summarizes the effects of C, Mn and Nb on tensile
properties and bendability of the experimental steels after 75%
cold rolling and annealing. The annealing cycle included: heating
the cold rolled bands (about 0.6 mm thick) to 870.degree. C.,
isothermal hold for 60 seconds at soaking temperature, immediate
cooling to 810.degree. C., 25 seconds isothermal holding at that
temperature, followed by rapid water quench. The panels were then
reheated to 200.degree. C. in an oil bath and held for 60 seconds,
followed by air cooling to simulate overage treatment. The data
shows that carbon has the strongest effect on strength and a slight
effect on bendability. The addition of Nb increases yield strength
and improves bendability. The improvement in bendability is
achieved in spite of marginally inferior elongation. An increase in
the Mn content from 1.5 to 2.0% in the Nb bearing steel has no
significant effect on tensile properties but results in a big
improvement in bendability.
TABLE-US-00002 TABLE 2 T.sub.soak T.sub.GJC T.sub.O\ Gauge YS TS
Bendability Bendability micro Steel .degree. C. .degree. C.
.degree. C. mm MPa MPa YS/TS UE % TE % YPE % pass crack < 0.5 mm
0.22C--1.5Mn--0.018Nb 870 810 200 0.69 1518 1737 0.87 3.6 4 0 4.0t
2.9t 0.22C--2.0Mn--0.018Nb 870 810 200 0.69 1518 1766 0.86 3.8 3.7
0 2.9t 2.5t 0.22C--2.0Mn 870 810 200 0.66 1465 1760 0.83 4.1 4.2 0
3.7t 2.2t 0.25C--2.0Mn 870 810 200 0.68 1533 1858 0.83 4 4.8 0 3.7t
2.6t 0.28C--2.0Mn 870 810 200 0.68 1581 1927 0.82 4.3 4.2 0 4.0t
3.2t
Example 2
[0053] In order to reduce carbon equivalent, thus to improve the
weldability of the steels of Example 1, steels containing 0.28 wt %
carbon and reduced manganese content (about 1.0 wt % vs. 2.0 wt %
of Example 1) along with were produced. The alloys were cast into
slabs, hot rolled, cold rolled, annealed (simulated) and over age
treated. In addition, the
effect of Mn content (1.0 and 2.0% Mn) on the properties of hot
rolled bands and annealed products are described in detail.
Heat Preparation
[0054] Table 3 shows the chemical compositions of investigated
steels. The alloy design analyzed the effects of incorporated Ti
(steels 1 and 2), B (steels 2 and 3) and Nb (alloys 3 and 4).
TABLE-US-00003 TABLE 3 ID Steel C Mn Si S P N Al Ti B Nb 1 Base
0.28 0.98 0.204 0.003 0.007 0.0049 0.035 2 Base-Ti 0.28 0.98 0.198
0.003 0.005 0.0047 0.04 0.024 3 Base-Ti--B 0.28 0.98 0.204 0.003
0.005 0.0047 0.04 0.024 0.0018 4 Base-Ti--B--Nb 0.28 0.97 0.202
0.003 0.006 0.0048 0.037 0.024 0.0017 0.029
[0055] Four 45 Kg slabs (one of each alloy) were cast in the
laboratory. After reheating and austenitization at 1230.degree. C.
for 3 hours, the slabs were hot rolled from 63 mm to 20 mm in
thickness on a laboratory mill. The finishing temperature was about
900.degree. C. The plates were air cooled after hot rolling.
Hot Rolling and Microstructure/Tensile Property Investigation
[0056] After shearing and reheating the pre-rolled 20 mm thick
plates to 1230.degree. C. for 2 hours, the plates were hot rolled
from a thickness of 20 mm to 3.5 mm. The finish rolling temperature
was about 900.degree. C. After controlled cooling at an average
cooling rate of about 45.degree. C./s, the hot bands of each
composition were held in a furnace at 580.degree. C. and
660.degree. C. respectively for 1 hour, followed by a 24-hour
furnace cooling to simulate the industrial coiling process. The use
of two different coiling temperatures was designed to understand
the available process window during hot rolling for the manufacture
of this product.
[0057] A recheck of hot band compositions was performed by
inductively coupled plasma (ICP). In comparison with ingot derived
data, a carbon loss is generally observed in the hot bands. Three
JIS-T standard specimens were prepared from each hot band for room
temperature tensile tests. Microstructure characterization of hot
bands was carried out by Scanning Electron Microscopy (SEM) at the
quarter thickness location of longitudinal cross-sections.
Cold Rolling
[0058] After grinding both surfaces of the hot rolled bands to
remove any decarburized layer, the steels were cold rolled in the
laboratory by 50% to obtain full hard steels with final thickness
of 1.0 mm for further annealing simulations.
Annealing Simulation
[0059] The effects of soaking and quenching temperatures during
annealing on the mechanical properties of the steels were
investigated for all of the experimental steels. A schematic of the
anneal cycles is shown in FIGS. 10a and 10b. FIG. 10a illustrates
the annealing processes with different soaking temperatures from
830.degree. C. to 870.degree. C. FIG. 10b illustrates the annealing
processes with different quenching temperatures from 780.degree. C.
to 840.degree. C.
[0060] The annealing process includes reheating the cold band
(about 1.0 mm thick) to 870.degree. C., 850.degree. C. and
830.degree. C. for 100 s, respectively, to investigate the effect
of soaking temperature on final properties. After immediate cooling
to 810.degree. C. and isothermal holding for 40 s, water quench was
applied. The steels were then reheated to 200.degree. C. for 100 s,
and followed by air cooling to simulate overaging treatment.
[0061] The annealing process includes reheating the cold band to
870.degree. C. for 100 s and immediate cooling to 840.degree. C.,
810.degree. C. and 780.degree. C. respectively to investigate the
effect of quenching temperature on the mechanical properties of the
steels. Water quench was employed after 40 s isothermal hold at the
quenching temperature. The steels were then reheated to 200.degree.
C. for 100 s, and followed by air cooling to simulate the overaging
treatment.
Tensile Property and Bendability of Annealed Steels
[0062] Three ASTM-T standard tensile specimens were prepared from
each annealed band for room temperature tensile test. Samples
processed by one annealing cycle were selected for bend testing.
This annealing cycle involved the reheating of the cold band (about
1.0 mm thick) to 850.degree. C. for 100 s, immediate cooling to
810.degree. C., 40 s isothermal hold at quench temperature,
followed by water quench. The steels were then reheated to
200.degree. C. for 100 s, and followed by air cooling to simulate
the overaging treatment. A 90.degree. free V-bend testing along the
rolling direction was employed for bendability characterization. In
the present study, the range of die radius varied from 2.75 to 4.00
mm at 0.25 mm increments. The sample surface after bend testing was
observed under 10.times. magnification. When the crack length on
the sample at the outer bend surface is smaller than 0.5 mm the
crack is deemed a "micro crack". A crack larger than 0.5 mm is
recognized as a failure. Samples without any visible crack are
identified as "passed test".
Chemical Analysis of the Hot Bands
[0063] Table 4 shows the chemical compositions of the steels with
different Ti, B and Nb contents after hot rolling. Compared with
the compositions of ingots (Table 3), there was about 0.03% carbon
and 0.001% B loss after hot rolling.
TABLE-US-00004 TABLE 4 ID Steel C Mn Si S P N Al Ti B Nb 1 Base
(0.25C--1.0Mn--0.2Si) 0.249 0.985 0.204 0.003 0.007 0.0047 0.034 2
Base-0.025Ti 0.247 0.981 0.197 0.003 0.005 0.005 0.038 0.024 3
Base-0.025Ti--0.001B 0.254 0.996 0.201 0.003 0.005 0.0044 0.039
0.024 0.001 4 Base-0.025Ti--0.001B--0.03Nb 0.251 0.988 0.201 0.003
0.005 0.0044 0.038 0.024 0.001 0.028
Microstructure and Tensile Properties of Hot Bands
[0064] FIGS. 11a and 11b show the tensile properties (JIS-T
standard) of experimental steels (of Table 4) at room temperature,
after hot rolling and simulated coiling at 580.degree. C. The base
composition consists of 0.28% C-1.0% Mn-0.2% Si. FIG. 11a
graphically depicts the strength of the four alloys, while FIG. 11b
plots their ductility. It can be seen that the addition of Ti, B
and Nb led to significant increases in the ultimate tensile
strength from 571 to 688 MPa yield strength from 375 to 544 MPa,
and a decrease in total and uniform elongations (TE: from 32 to
13%; UE: from 17 to 11%). The addition of Nb to the Ti--B steel
resulted in a pronounced drop in total elongation from 28 to
13%.
[0065] As shown in FIGS. 12a-12d, the microstructure of steels
after hot rolling and simulated coiling at 660.degree. C. consist
of ferrite and pearlite for each laboratory processed experimental
steel. FIGS. 12a-12d are SEM micrographs at 1000.times. of the base
alloy, base alloy+Ti, base alloy+Ti & B, and base alloy+Ti, B
and Nb, respectively. The addition of B seems to result in slightly
larger sized pearlite islands (FIG. 12c). The ferrite-pearlite
microstructure is elongated along the rolling direction in the Nb
added steel (FIG. 12d), which may be attributed to the Nb addition
retarding austenite recrystallization during hot rolling. Thus, the
finish rolling occurred in the austenite non-recrystallization
region, and the elongated ferrite-pearlite microstructure was
transformed directly from the deformed austenite.
[0066] The corresponding tensile properties of the experimental
steels at room temperature are shown in FIGS. 13a-13b. FIG. 13a
graphically depicts the strength of the four alloys, while FIG. 13b
plots their ductility. It can be seen that the addition of Nb
(0.03%) led to significant increases in ultimate tensile strength
from 535 to 588 MPa and yield strength from 383 to 452 MPa, and
slight decreases in total elongation from 31.3 to 29.0% and uniform
elongation from 17.8 to 16.4%.
Effect of Coiling Temperature on Tensile Properties
[0067] Comparing the tensile properties in FIGS. 11 and 13, the
increase in coiling temperature from 580.degree. C. to 660.degree.
C. led to a decrease in strength and an increase in ductility,
attributes favorable for increased cold reduction possibility and
enhanced gauge-width capability. The additions of Ti, B and Nb to
the base steel have less of an effect on the tensile properties of
the steels at the higher coiling temperature of 660.degree. C. in
comparison to 580.degree. C. The purpose of studying the effect of
coiling at 660.degree. C. in the laboratory was to understand the
effect of coiling temperature on both, hot band strength and the
strength of the cold rolled and annealed martensitic steels.
Tensile Properties of the Steels after Annealing Simulation
[0068] FIGS. 14a-14d represent the effects of soaking temperature
(830.degree. C., 850.degree. C. and 870.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (Ti, B and Nb additions to the base steel) on the
tensile properties of the steels after anneal simulation. FIGS. 14a
and 14b plot the strengths of the four alloys at different soaking
temperatures and at coiling temperatures of 580.degree. C. and
660.degree. C., respectively. FIGS. 14c and 14d plot the
ductilities of the four alloys at different soaking temperatures
and at coiling temperatures of 580.degree. C. and 660.degree. C.,
respectively. It can be seen that a decrease in the soaking
temperature from 870.degree. C. to 830.degree. C. resulted in
increases in yield strength of 41 MPa and ultimate tensile strength
of 56 MPa for Ti--B steel after hot rolling and simulated coiling
at 580.degree. C. (FIG. 14a). For Ti--B--Nb steel, after simulated
coiling at the same temperature (FIG. 14a), the highest strength
was represented at the soaking temperature of 850.degree. C. (YS:
1702 MPa and UTS: 1981 MPa). Further increase or decrease of
soaking temperature will not improve the strength of Ti--B--Nb
steel. The soaking temperature had no obvious effect on the
strength for Ti--B of Ti--B--Nb steels after simulated coiling at
660.degree. C. It also had no significant effect on strength for
the base and Ti steels at both coiling temperatures, and no effect
on ductility for all of the experimental steels.
[0069] FIGS. 15a-15d show the effects of quenching temperature
(780.degree. C., 810.degree. C. and 840.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (Ti, B and Nb additions to the base steel) on the
tensile properties of the steels after anneal simulation. FIGS. 15a
and 15b plot the strengths of the four alloys at different
quenching temperatures and at coiling temperatures of 580.degree.
C. and 660.degree. C., respectively. FIGS. 15c and 15d plot the
ductilities of the four alloys at different quenching temperatures
and at coiling temperatures of 580.degree. C. and 660.degree. C.,
respectively. A decrease in the quenching temperature from
840.degree. C. to 780.degree. C. resulted in increases in both
yield and ultimate tensile strengths of about 50-60 MPa in the base
and Ti steels after hot rolling and simulated coiling at
580.degree. C. (FIG. 15a). The quenching temperature had no obvious
effect on the strength of base and Ti steels after simulated
coiling at 660.degree. C. It also had no significant effect on the
strength of Ti--B and Ti--B--Nb steels at both coiling
temperatures, and on ductility for all of the experimental
steels.
Effect of Coiling Temperature (580.degree. C. and 660.degree.
c.)
[0070] Comparing FIGS. 14a and 15a with FIGS. 14b and 15b, the
increase in coiling temperature from 580.degree. C. to 660.degree.
C. did not lead to a significant change in the tensile strength,
but resulted in a slight decrease in the yield strength of about 50
MPa on average for all of the experimental steels at various
annealing conditions. Increasing coiling temperature did not have a
measurable effect on ductility in the Ti and Ti--B steels, but
slightly reduced by about 0.5%, the ductility of the base and
Ti--B--Nb steels. These small changes are, however, within the
range of test deviation and therefore, not very significant.
Effect of Composition (Ti, B and Nb)
[0071] As shown in FIGS. 14a-14d and 15a-15d, the addition of Ti
and B in 0.28% C-1.0% Mn-0.2% Si steel did not have a significant
effect on strength at both coiling temperatures of 580.degree. C.
and 660.degree. C. The addition of Nb resulted in increases in
yield strength of 45-103 MPa and tensile strength of 26-85 MPa at a
coiling temperature of 580.degree. C. (FIG. 14a), but not for
660.degree. C. (FIG. 14b). Except for the Ti added steel which
displayed a slightly better ductility at 660.degree. C. coiling
temperature (FIGS. 14d and 15d), alloy additions generally led to a
slight decrease in ductility (<1%).
Bendability of the Steels after Anneal Simulation
[0072] Table 5 summarizes the effect of Ti, B and Nb on the tensile
properties and bendability of the steels after 50% cold rolling and
annealing after simulated coiling at 580.degree. C. The annealing
process consisted of reheating the cold band (about 1.0 mm thick)
to 850.degree. C. for 100 seconds, immediate cooling to 810.degree.
C., 40 seconds isothermal hold at "quench" temperature, followed by
water quench. The steels were then reheated to 200.degree. C. for
100 seconds followed by air cooling to simulate overaging treatment
(OA). As shown, it was possible to produce steels with ultimate
tensile strength between 1850 and 2000 MPa by varying alloy
composition. The steel with only C, Mn and Si demonstrated the best
bendability. The addition of Nb increased strength with a slight
deterioration of bendability. Bendability pass defined as "micro
crack length smaller than 0.5 mm at 10.times. magnification.
TABLE-US-00005 TABLE 5 T.sub.soak T.sub.quench T.sub.OA Gauge YS
UTS Bendability ID Steel .degree. C. .degree. C. .degree. C. mm YPE
% MPa MPa YS/UTS UE % TE % pass 1 Base (0.28C--1.0Mn--0.2Si) 850
810 200 1.03 0 1599 1896 0.84 4.3 5.7 3.5t 2 Base-0.025Ti 850 810
200 0.99 0 1597 1901 0.84 4 4.8 >4.0t 3 Base-0.025Ti--0.001B 850
810 200 1 0 1578 1886 0.84 3.5 4.9 3.75t 4
Base-0.025Ti--0.001B--0.03Nb 850 810 200 0.99 0 1702 1981 0.86 3.4
4.4 >4.0t
Comparison with Example 1
Effect of Manganese
[0073] The steel with 0.28% C-2.0% Mn-0.2% Si was presented in
Example 1 above. We can compare its behavior with the steel of
Example 2 containing 0.28% C-1.0% Mn-0.2% Si to investigate the
effect of Mn (1.0 and 2.0%) on tensile properties. The detailed
chemical compositions of both steels are shown in Table 6.
TABLE-US-00006 TABLE 6 Steel C Mn Si S P N Al Example 1
(0.28C--1.0Mn--0.2Si) 0.249 0.985 0.204 0.003 0.007 0.0047 0.034
Example 2 (0.28C--2.0Mn--0.2Si) 0.25 2.01 0.202 0.003 0.007 0.0045
0.032
Tensile Properties of Hot Rolled Bands with 1.0 and 2.0% Mn
[0074] Table 7 displays the tensile properties of the steels with
1.0% and 2.0% Mn respectively after hot rolling and simulated
coiling at 580.degree. C. For the tensile properties of hot rolled
bands, the steel with the lower Mn content showed a lower strength
than the steel with the higher Mn content (51 MPa lower in YS and
61 MPa lower in UTS). This may facilitate a higher extent of cold
rolling for the low Mn steel.
TABLE-US-00007 TABLE 7 Gauge, YS, UTS, Steel mm YPE, % MPa MPa
YS/UTS UE, % TE, % 0.28C--1.0Mn--0.2Si 3.44 1.68 375 571 0.66 17.6
32.2 0.28C--2.0Mn--0.2Si 3.67 1.82 426 632 0.67 11.3 15.8
[0075] Table 8 shows the tensile properties of the steels with 1.0%
and 2.0% Mn respectively after cold rolling (50% cold rolling
reduction for the steel with 1.0% Mn and 75% cold rolling reduction
for the steel with 2.0% Mn) and various annealing cycles. It can be
seen that at the same annealing treatment of 870.degree. C.
(soaking), 840.degree. C. (quench) and 200.degree. C. (overaging),
Mn content had no significant effect on strength. At the same
quenching temperature of 810.degree. C., the decrease in soaking
temperature from 870 to 830.degree. C. did not affect the strength
of the steel with 1.0% Mn, but significantly increased the strength
of the steel with 2.0% Mn by about 90 MPa. This indicates that the
steel with 1.0% Mn is quite stable in strength regardless soaking
temperature (870 to 830.degree. C.), and the steel with 2.0% Mn is
more sensitive to the soaking temperature, perhaps due to grain
coarsening at higher anneal temperatures. The steel with 1.0% Mn
will be relatively easier to process during manufacturing due to
the wider process windows.
TABLE-US-00008 TABLE 8 Gauge T.sub.Soak .degree. C. T.sub.Quench
.degree. C. T.sub.OA .degree. C. YS TS Steel mm 100 s 60 s 40 s 25
s 100 s 60 s YPE % MPa MPa YS/UTS UE % TE % 0.28 C 1.03 870 840 200
0 1593 1888 0.84 4.2 6 1.0 Mn 1.03 870 810 200 0 1597 1882 0.85 4.1
5.5 0.2 Si 0.95 870 780 200 0 1652 1945 0.85 4 5.5 1.03 850 810 200
0 1599 1896 0.84 4.3 5.7 1.03 830 810 200 0 1606 1896 0.85 4.3 5.5
0.28 C 0.68 870 840 200 0 1589 1891 0.84 3.8 3.8 2.0 Mn 0.68 870
810 200 0 1581 1927 0.82 4.3 4.3 0.2 Si 0.68 870 780 200 0 1558
1907 0.82 4.5 5.4 0.69 850 810 200 0 1657 2023 0.82 3.6 3.6 0.69
830 810 200 0 1656 2019 0.82 3.4 4.4
Bendability of Annealed Steels with 1.0 and 2.0% Mn
[0076] Table 9 lists the tensile properties and bendability of the
steels with 1.0% and 2.0% Mn after anneal simulation. The steel
with 1.0% Mn demonstrated a better bendability (3.5t compared to
4.0t) at a comparable strength level. Bendability pass is defined
as micro crack length smaller than 0.5 mm at 10.times.
magnification.
TABLE-US-00009 TABLE 9 Gauge T.sub.Soak T.sub.GJC T.sub.OA YS TS
Bendability Steel mm .degree. C. .degree. C. .degree. C. YPE % MPa
MPa YS/UTS UE % TE % pass 0.28C--1.0Mn--0.2Si 1.03 850 810 200 0
1599 1896 0.84 4.3 5.7 3.5t 0.28C--2.0Mn--0.2Si 0.68 870 810 200 0
1581 1927 0.82 4.3 4.3 4.0t
Example 3
[0077] To ensure good weldability of the steels, the carbon
equivalent (C.sub.eq) should be less than 0.44. The carbon
equivalent for the present steels is defined as:
C.sub.eq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15.
Thus, at a C content of 0.28 wt % and Mn content of 1 or 2 wt %,
the weld integrity is determined to be unacceptable. The present
examples are designed to reduce the Ceq and still meet the strength
and ductility needs. High carbon content is beneficial for
increasing strength but deteriorates weldability. According to the
carbon equivalent formula, Mn is another element which deteriorates
weldability. Thus, the motivation is to maintain a certain amount
of carbon content (at least 0.28%) to achieve sufficient ultra-high
strength and to study the effect of Mn content on UTS. The
inventors look to reduce Mn content to improve the weldability but
at maintain an ultra-high strength level.
Heat Preparation
[0078] Table 10 shows the chemical compositions of investigated
steels in Example 3. The alloy design incorporated the
understanding of the effect of C content and B addition on tensile
properties in the final annealed products.
TABLE-US-00010 TABLE 10 No. ID C Mn Si Ti B Al N S P C.sub.eg 1 28C
0.282 0.577 0.199 0.021 0.02 0.004 0.005 0.004 0.38 2 28C--2B 0.281
0.58 0.197 0.022 0.0016 0.022 0.0042 0.004 0.004 0.38 3 32C 0.321
0.578 0.195 0.021 0.021 0.0044 0.004 0.004 0.42 4 32C--2B 0.323
0.578 0.196 0.022 0.0017 0.032 0.0053 0.004 0.005 0.42 5 36C 0.363
0.58 0.196 0.022 0.025 0.0044 0.004 0.004 0.46
[0079] Five 45 Kg slabs (one of each alloy) were cast in the
laboratory. After reheating and austenitization at 1230.degree. C.
for 3 hours, the slabs were hot rolled from 63 mm to 20 mm in
thickness on a laboratory mill. The finishing temperature was about
900.degree. C. The plates were air cooled after hot rolling.
Hot Rolling and Microstructure/Tensile Property Investigation
[0080] After shearing and reheating the pre-rolled 20 mm thick
plates to 1230.degree. C. for 2 hours, the plates were hot rolled
from a thickness of 20 mm to 3.5 mm. The finish rolling temperature
was about 900.degree. C. After controlled cooling at an average
cooling rate of about 45.degree. C./s, the hot bands of each
composition were held in a furnace at 580.degree. C. and
660.degree. C. respectively for 1 hour, followed by a 24-hour
furnace cooling to simulate industrial coiling process. The use of
two different coiling temperatures was designed to understand the
available process window during hot rolling for the manufacture of
this product.
[0081] Three JIS-T standard specimens were prepared from each hot
rolled steel (also known as a "hot band") for room temperature
tensile tests. Microstructure characterization of hot bands was
carried out by Scanning Electron Microscopy (SEM) at the quarter
thickness location of longitudinal cross-sections.
Cold Rolling and Annealing Simulation
[0082] After grinding both surfaces of the hot rolled bands to
remove any decarburized layer, the steels were cold rolled in the
laboratory by 50% to obtain full hard steels with final thickness
of 1.0 mm for further annealing simulations.
[0083] The effects of soaking, quenching temperatures and a
comparison of different combination of soaking and quenching
temperatures during annealing on the mechanical properties of the
steels were investigated for all of the experimental steels. A
schematic of the anneal cycles is shown in FIGS. 16a-16c. FIG. 16a
depicts the anneal cycle with varied soaking temperature from
830.degree. C. to 870.degree. C. FIG. 16b depicts the anneal cycle
with varied quenching temperature from 780.degree. C. to
840.degree. C. FIG. 16c depicts the anneal cycle with varied
combinations of soaking and quenching temperatures.
Effect of Soaking Temperature
[0084] The annealing process includes reheating the cold band
(about 1.0 mm thick) to 870.degree. C., 850.degree. C. and
830.degree. C. for 100 seconds, respectively, to investigate the
effect of soaking temperature on the final properties. After
immediate cooling to 810.degree. C. and isothermal holding for 40
seconds, water quench was applied. The steels were then reheated to
200.degree. C. for 100 seconds, followed by air cooling to simulate
overaging treatment.
Effect of Quenching Temperature
[0085] The annealing process includes reheating the cold band to
870.degree. C. for 100 seconds and immediate cooling to 840.degree.
C., 810.degree. C. and 780.degree. C. respectively to investigate
the effect of quenching temperature on the mechanical properties of
the steels. Water quench was employed after 40 seconds of
isothermal hold at the quenching temperature. The steels were then
reheated to 200.degree. C. for 100 seconds, followed by air cooling
to simulate overaging treatment.
Effect of the Different Combination of Annealing Cycle
[0086] The annealing cycle includes reheating the cold rolled
steels to 790.degree. C., 810.degree. C. and 830.degree. C. for 100
seconds respectively, immediate cooling to various quench
temperatures (770.degree. C., 790.degree. C. and 810.degree. C.
respectively), isothermal holding for 40 seconds, followed by water
quench. The steels were then reheated to 200.degree. C. for 100
seconds, followed by air cooling to simulate overaging
treatment.
Tensile Property and Bendability of Annealed Steels
[0087] ASTM-T standard tensile specimens were prepared from each
annealed band for room temperature tensile test. The samples
processed by one annealing cycle were selected for bend testing.
This annealing cycle involved the reheating of the cold band (about
1.0 mm thick) to 850.degree. C. for 100 seconds, immediate cooling
to 810.degree. C., 40 seconds isothermal hold at the quench
temperature, followed by water quench. The steels were then
reheated to 200.degree. C. for 100 seconds, followed by air cooling
to simulate overaging treatment. A 90.degree. free V-bend test
along the rolling direction was employed for bendability
characterization. In the present study, the range of die radius
varied from 2.75 to 4.00 mm at 0.25 mm increments. The sample
surface after bend testing was observed under 10.times.
magnification. A crack length on the sample at the outer bend
surface that is smaller than 0.5 mm is considered to be a "micro
crack", and a crack larger than 0.5 mm is recognized as a failure.
A sample without any length of visible crack is identified as
"passed the test".
Microstructure and Tensile Properties of Hot Bands
[0088] FIGS. 17a to 17e are SEM micrographs at 1,000.times. of hot
rolled steels (0.28 to 0.36% C) after hot rolling and simulated
coiling at 580.degree. C. The increase in carbon content and the
addition of boron led to an increase in martensite volume fraction,
which can be attributed to the role of C and B in increasing
hardenability. FIG. 17a is an SEM of the steel with 0.28C. FIG. 17b
is an SEM of the steel with 0.28C-0.002B. FIG. 17c is an SEM of the
steel with 0.32C. FIG. 17d is an SEM of the steel with
0.32C-0.002B. FIG. 17e is an SEM of the steel with 0.36C.
[0089] The corresponding tensile properties of the experimental
steels at room temperature (after hot rolling and simulated coiling
at 580.degree. C.) are shown in FIGS. 18a and 18b. FIG. 18a plots
the strength of the alloys versus carbon content, with and without
boron. FIG. 18b plots the ductility of the alloys versus carbon
content, with and without boron. The increase in carbon content
from 0.28% to 0.36% led to an increase in ultimate tensile strength
from 529 to 615 MPa and yield strength from 374 to 417 MPa. Total
and uniform elongations remained similar at 29% and 15%,
respectively. The addition of 0.002% boron in 0.28 and 0.32% C
steels resulted in an increase in UTS of about 40 MPa.
[0090] FIGS. 19a-19e are SEM micrographs at 1,000.times. of hot
rolled steels (0.28 to 0.36% C) after hot rolling and simulated
coiling at 660.degree. C. FIG. 19a is an SEM of the steel with
0.28C. FIG. 19b is an SEM of the steel with 0.28C-0.002B. FIG. 19c
is an SEM of the steel with 0.32C. FIG. 19d is an SEM of the steel
with 0.32C-0.002B. FIG. 19e is an SEM of the steel with 0.36C. The
addition of boron led to a slight grain coarsening, which may be
attributed to B retarding phase transformation during cooling.
Thus, the finish rolling occurred in the austenite region with
relatively coarse austenite grain size for the B added steels, and
the coarse austenite transformed directly to a coarse
ferrite-pearlite microstructure.
[0091] The corresponding tensile properties at room temperature
(after hot rolling and simulated coiling at 660.degree. C.) are
represented in FIGS. 20a and 20b. FIG. 20a plots the strength of
the alloys versus carbon content, with and without boron. FIG. 20b
plots the ductility of the alloys versus carbon content, with and
without boron. The increase in carbon content from 0.28% to 0.36%
did not significantly impact tensile properties. The addition of
0.002% boron in 0.28 and 0.32% C steels resulted in a slight
decrease in strength which may be due to grain coarsening. Based on
the observed strength levels, the steels should be easily cold
rolled to light gauges without any difficulty.
Effect of Coiling Temperature on Tensile Properties
[0092] Comparing the tensile properties in FIGS. 18a-18b and FIGS.
20a-20b, the increase in coiling temperature from 580.degree. C. to
660.degree. C. led to a decrease in strength and an increase in
ductility, which attributes favorable the possibility of increased
cold reduction and enhanced gauge-width capability. The increase in
C content from 0.28% to 0.36% and the addition of B to the base
steel have less effect on the tensile properties of the steels at
the higher coiling temperature of 660.degree. C. in comparison with
580.degree. C. The purpose of studying the effect of coiling at
660.degree. C. in the laboratory was to understand the effect of
coiling temperature on both, hot band strength and the strength of
the cold rolled and annealed martensitic steels.
Tensile Properties of the Steels after Annealing Simulation
Effect of Soaking Temperature (830.degree. C., 850.degree. C. and
870.degree. C.)
[0093] FIGS. 21a-21d represents the effects of soaking temperature
(830.degree. C., 850.degree. C. and 870.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (C content and B addition to the base steel) on the
tensile properties of the steels after annealing simulation. FIGS.
21a and 21b plot the strengths of the five alloys at different
soaking temperatures and at coiling temperatures of 580.degree. C.
and 660.degree. C., respectively. FIGS. 21c and 21d plot the
ductilities of the five alloys at different soaking temperatures
and at coiling temperatures of 580.degree. C. and 660.degree. C.,
respectively. It can be seen that martensitic steels with UTS level
of 2000 to greater than 2100 MPa and TE of 3.5-5.0% can be obtained
in the laboratory using the 0.32 and 0.36% C steel compositions at
soak temperatures of 830 and 850.degree. C. A decrease in the
soaking temperature from 870.degree. C. to 850.degree. C. resulted
in a slightly increase in strength for most of the steels. The
increase in coiling temperature had no significant effect on
strength but slightly improved ductility in most of cases. The
increase in C content from 0.28 to 0.36% resulted in an increase in
UTS of approximately 200 MPa. The addition of 0.002% B to the base
steel led to a decrease in strength for the lower coiling
temperature of 580.degree. C. but not for the coiling temperature
of 660.degree. C. There was no significant effect of B addition on
ductility regardless of coiling temperature.
Effect of Quenching Temperature (780.degree. C., 810.degree. C. and
840.degree. C.)
[0094] FIGS. 22a-22d show the effects of quenching temperature
(780.degree. C., 810.degree. C. and 840.degree. C.), coiling
temperature (580.degree. C. and 660.degree. C.), and alloy
composition (C content and B addition to the base steel) on the
tensile properties of the steels after annealing simulation. FIGS.
22a and 22b plot the strengths of the five alloys at different
quenching temperatures and at coiling temperatures of 580.degree.
C. and 660.degree. C., respectively. FIGS. 22c and 22d plot the
ductilities of the five alloys at different quenching temperatures
and at coiling temperatures of 580.degree. C. and 660.degree. C.,
respectively. It can be seen that martensitic steels with a UTS
close to or exceeding 2100 MPa and a TE of 3.5-5.0% can be obtained
in the laboratory using the steel with 0.36% C at the soaking
temperature of 870.degree. C. and various quench temperatures. In
comparison with the results in FIGS. 21a and 21b, the steels with
not only 0.36% C but also 0.32% C could be heat treated to obtain a
UTS level of 2000-2100 MPa and a TE of 3.5-5.0% at soaking
temperatures of 830 and 850.degree. C. Thus, a soak temperature of
about 850.degree. C. can help to achieve optimal mechanical
properties. A decrease in the quenching temperature from
840.degree. C. to 780.degree. C. had no major effect on tensile
properties for the steels with 0.32 and 0.36% C regardless of the
addition of B and coiling temperature. However, a decrease in the
quenching temperature from 840.degree. C. to 780.degree. C. for the
steels with 0.28% C (coiling temperature of 580.degree. C.) led to
an decrease in strength by 100 MPa when there was no B addition,
and this effect became less obvious when there was B addition, i.e.
only 40 MPa increase. It demonstrates that B addition is beneficial
for the stabilization of tensile properties, especially for the
steels with a relatively low C content. The increase in C content
from 0.28 to 0.36% resulted in an increase in UTS of approximately
200-300 MPa with no obvious change in ductility especially for the
higher coiling temperature of 660.degree. C. Overall, compared to
the steels after coiling at 580.degree. C., the tensile properties
of the steels coiled at 660.degree. C. had less sensitivity to the
quench temperatures.
[0095] FIGS. 23a-23d illustrates the effect of composition and
annealing cycle on (23a-23b) tensile strength and (23c-23d)
ductility. FIGS. 22a and 22b plot the strengths of the five alloys
at three different soak/quenching temperature pairs (790.degree.
C./770.degree. C., 810.degree. C./790.degree. C., and 830.degree.
C./810.degree. C.) and at coiling temperatures of 580.degree. C.
and 660.degree. C., respectively. FIGS. 22c and 22d plot the
ductilities of the five alloys at the three different
soak/quenching temperature pairs and at coiling temperatures of
580.degree. C. and 660.degree. C., respectively. The steels
processed at a soak temperature of 790.degree. C. and a quench
temperature of 770.degree. C. demonstrated the lowest strength,
which can be attributed to the incomplete austenitization at
790.degree. C. soaking temperature. FIGS. 24a-24d are micrographs
of four of the five alloys which were coiled at 660.degree. C.,
cold rolled and annealed using the soak/quenching temperature pair
790.degree. C./770.degree. C. As can be seen, ferrite formed after
the annealing cycle for all four of the steel compositions.
Similarly, FIGS. 24e-24h are micrographs of four of the five alloys
which were annealed using the soak/quenching temperature pair
810.degree. C./790.degree. C. Ferrite formation can still be
observed for the steels with 0.28% C and 0.32% C. The increase in C
content resulted in an increase in hardenability so that less
ferrite is formed at the same annealing cycle. Finally, FIGS.
24i-24l are micrographs of four of the five alloys which were
annealed using the soak/quenching temperature pair 830.degree.
C./810.degree. C. Most of the steels show the highest strength
after annealing at these temperatures, which may be due to the
almost fully martensitic microstructure obtained.
Bendability of the Steels after Anneal Simulation
[0096] Table 11 summarizes the effects of C and B on the tensile
properties and bendability of the steels after 50% cold rolling and
annealing after simulated coiling at 580.degree. C. The annealing
process consisted of reheating the cold band (about 1.0 mm thick)
to 850.degree. C. for 100 seconds, immediate cooling to 810.degree.
C., 40 seconds isothermal hold at "quench" temperature, followed by
water quench. The steels were then reheated to 200.degree. C. for
100 seconds, followed by air cooling to simulate overaging
treatment (OA). As shown in Table 11, it was possible to produce
steels with ultimate tensile strength between 1830 and 2080 MPa by
varying alloy composition.
TABLE-US-00011 TABLE 11 T.sub.Soak T.sub.Quench T.sub.OA Gauge YS
UTS Bendability ID Steel .degree. C. .degree. C. .degree. C. mm YPE
% MPa MPa YS/UTS UE % TE % pass 1 28C 850 810 200 0.93 0 1593 1908
0.83 3.5 4 3.5t 2 28C--B 850 810 200 1.06 0 1540 1838 0.84 3.2 3.2
3.75t 3 32C 850 810 200 0.99 0 1644 2005 0.82 4.1 4.5 4.0t 4
32C--2B 850 810 200 0.99 0 1569 1922 0.82 4 4.9 3.5t 5 36C 850 810
200 0.97 0 1688 2080 0.81 3.5 3.5 4.0t
Comparison with Examples 1 and 2--Effect of Manganese for the
Steels with 0.28% C
[0097] The steels with 0.28% C and 1.0%/2.0% Mn were presented
above in Examples 1 and 2. We now compare those steels with the
steel containing 0.28% C and 0.5% Mn to investigate the effect of
Mn (0.5% to 2.0%) on tensile properties. The detailed chemical
compositions of the steels are shown in Table 12.
TABLE-US-00012 TABLE 12 No. ID C Mn Si Ti B Al N S P Ceq 1
28C--0.5Mn--Ti 0.282 0.577 0.199 0.021 0.02 0.004 0.005 0.004 0.38
2 28C--0.5Mn--Ti--B 0.281 0.58 0.197 0.022 0.0016 0.022 0.0042
0.004 0.004 0.38 3 28C--1.0Mn--Ti 0.28 0.98 0.198 0.024 0.04 0.0047
0.003 0.005 0.44 4 28C--1.0Mn--Ti--B 0.29 0.98 0.204 0.024 0.0018
0.04 0.0047 0.003 0.005 0.45 5 28C--1.0Mn 0.29 0.98 0.204 0.035
0.0049 0.003 0.007 0.45 6 28C--2.0Mn 0.28 2.01 0.201 0.034 0.005
0.003 0.006 0.62
[0098] Table 13 displays the tensile properties of the steels with
0.5% to 2.0% Mn and the additions of Ti and B after hot rolling and
simulated coiling at 580.degree. C. For the steels with Ti
addition, the increase in Mn content from 0.5% to 1.0% led to an
increase in both yield and tensile strengths and yield ratio but no
significant effect on ductility. The addition of B in Ti added
steels with 0.5% to 1.0% Mn resulted in an increase in strength.
Compared to the steel "28C-1.0Mn", the addition of Ti was
beneficial for increasing both strength and yield ratio, which may
be attributed to the effect of Ti precipitation hardening. The
steels with the lower Mn content showed a lower strength than the
steel with the higher Mn content. This may facilitate a higher
extent of cold rolling for the low Mn steel.
TABLE-US-00013 TABLE 13 Gauge, YS, UTS, Steel mm YPE, % MPa MPa
YS/UTS UE, % TE, % 28C--0.5Mn--Ti 3.89 2.15 374 529 0.71 16.4 29.3
28C--0.5Mn--Ti--B 3.77 1.7 390 567 0.69 15.3 32 28C--1.0Mn--Ti 3.49
3.86 448 612 0.73 15.5 29.6 28C--1.0Mn--Ti--B 3.61 3.93 491 655
0.75 13.7 27.5 28C--1.0Mn 3.44 1.68 375 571 0.66 17.6 32.2
28C--2.0Mn 3.64 1.82 426 632 0.67 11.3 15.8
[0099] FIGS. 25a-25d show the tensile properties of the steels with
0.5% to 2.0% Mn after coiling at 580.degree. C., cold rolling (50%
cold rolling reduction for the steel with 0.5 and 1.0% Mn and 75%
cold rolling reduction for the steel with 2.0% Mn) and various
annealing cycles. The X-axis of FIGS. 25a-25d indicates soak and
quench temperature, i.e., 870/840 means soaking at 870.degree. C.
and quenching at 840.degree. C. It can be seen that at the same
annealing treatment of 850.degree. C.-810.degree. C.
(soaking-quenching temperature) and 200.degree. C. (overaging), the
increase in Mn content from 0.5% to 1.0% had no significant effect
on strength for the steel with Ti, but resulted in an increase in
strength for the steel with both Ti and B additions and an increase
in ductility. The further increase in Mn content to 2.0% led to a
pronounced increase in UTS of over 100 MPa, YS of over 50 MPa and a
decrease in ductility. This effect was not applicable for high
soaking temperature of 870.degree. C., at which the steels with
2.0% Mn did not show an increase in strength. This indicates that
the steel with 2.0% Mn is more sensitive to the soaking
temperature, which may be due to grain coarsening at higher anneal
temperatures. At the soaking temperature of 870.degree. C., the
increase in Mn from 0.5% to 1.0% resulted in increases in both
strength and ductility for 810.degree. C. and 780.degree. C.
quenching temperatures. The steel with 0.5 to 1.0% Mn will be
relatively easier to process during manufacturing due to the wider
process windows.
Bendability of Annealed Steels with 0.5 to 2.0% Mn (0.28% C)
[0100] Table 14 lists the tensile properties and bendability of the
steels with 0.5% to 2.0% Mn after anneal simulation, which were
previously coiled at 580.degree. C. The steel "28C-0.5Mn--Ti"
demonstrated a better bendability than the steel "28C-1.0Mn--Ti"
(3.5t compared to 4.0t) at a comparable UTS level of 1900 MPa.
TABLE-US-00014 TABLE 14 T.sub.Soak T.sub.Quench T.sub.OA Gauge YS
UTS Bendability Steel .degree. C. .degree. C. .degree. C. mm YPE %
MPa MPa YS/UTS UE % TE % pass 28C--0.5Mn--Ti 850 810 200 0.93 0
1593 1908 0.83 3.5 4 3.5t 28C--0.5Mn--Ti--B 850 810 200 1.06 0 1540
1838 0.84 3.2 3.2 3.75t 28C--1.0Mn--Ti 850 810 200 0.99 0 1597 1901
0.84 4 4.8 >4.0t 28C--1.0Mn--Ti--B 850 810 200 1 0 1578 1886
0.84 3.5 4.9 3.75t 28C--1.0Mn 850 810 200 1.03 0 1599 1896 0.84 4.3
5.7 3.5t 28C--2.0Mn 0.68 870 810 200 0 1581 1927 0.82 4.3 4.3
4.0t
[0101] It is to be understood that the disclosure set forth herein
is presented in the form of detailed embodiments described for the
purpose of making a full and complete disclosure of the present
invention, and that such details are not to be interpreted as
limiting the true scope of this invention as set forth and defined
in the appended claims.
* * * * *