U.S. patent application number 15/173200 was filed with the patent office on 2016-09-29 for heat-hardened steel with excellent crashworthiness and method for manufacturing heat-hardenable parts using same.
The applicant listed for this patent is HYUNDAI STEEL COMPANY. Invention is credited to Hee-Joong IM, Dong-Eun KIM, Young-Jin KIM, Bo-Ryong LEE, Seung-Ha LEE, Man-Been MOON, Seung-Man NAM.
Application Number | 20160281190 15/173200 |
Document ID | / |
Family ID | 45614663 |
Filed Date | 2016-09-29 |
United States Patent
Application |
20160281190 |
Kind Code |
A1 |
NAM; Seung-Man ; et
al. |
September 29, 2016 |
HEAT-HARDENED STEEL WITH EXCELLENT CRASHWORTHINESS AND METHOD FOR
MANUFACTURING HEAT-HARDENABLE PARTS USING SAME
Abstract
Disclosed are heat-hardened steel with excellent crashworthiness
and a method for manufacturing heat-hardenable parts using the
same. The heat-hardened steel according to the invention comprises,
based on wt %; C: 0.12-0.8%; Cr: 0.01-2%; Mo: 0.2% or less; B:
0.0005-0.08%; Ca: 0.01 or less; Sb: 1.0% or less; and Ti and/or Nb:
0.2%; and the reminder being Fe and inevitable impurities. In
addition, the heat-treatment hardening steel satisfies anyone of
following conditions i)-iv), wherein condition i) comprises Si:
0.5-3%; Mn: 1-10% and Al: 0.05-2%; condition ii) comprises Si: 1%
or less; Mn: 0.5-5%; Al: 0.1-2.5%; and Ni: 0.01-8%; condition iii)
comprises Si: 0.5-3%; Mn: 1-10%; Al: 0.1% or less; and Ni: 0.01-8%;
and condition iv) comprises Si: 0.5-3%; Mn: 1-10%; Al: 0.1-2.5%;
and Ni: 0.01-8%.
Inventors: |
NAM; Seung-Man; (Seoul,
KR) ; IM; Hee-Joong; (Pyeongtaek-si, KR) ;
LEE; Seung-Ha; (Dangjin-gun, KR) ; KIM; Dong-Eun;
(Pyeongtaek-si, KR) ; LEE; Bo-Ryong; (Incheon,
KR) ; KIM; Young-Jin; (Anyang-si, KR) ; MOON;
Man-Been; (Guri-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HYUNDAI STEEL COMPANY |
INCHEON |
|
KR |
|
|
Family ID: |
45614663 |
Appl. No.: |
15/173200 |
Filed: |
June 3, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14115516 |
Nov 4, 2013 |
|
|
|
PCT/KR2011/004785 |
Jun 30, 2011 |
|
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15173200 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B32B 15/012 20130101;
C21D 8/0247 20130101; C22C 38/06 20130101; C22C 38/02 20130101;
C22C 38/22 20130101; C22C 38/26 20130101; C21D 6/008 20130101; C22C
38/32 20130101; C21D 6/005 20130101; C23C 2/12 20130101; C21D 1/673
20130101; C23C 2/02 20130101; C22C 38/50 20130101; C21D 8/005
20130101; C21D 6/002 20130101; Y10T 428/31678 20150401; C21D 6/004
20130101; Y10T 428/12799 20150115; C21D 7/02 20130101; C22C 38/58
20130101; B32B 15/013 20130101; C22C 38/28 20130101; C21D 8/00
20130101; C22C 38/48 20130101; C22C 38/38 20130101; C22C 38/04
20130101; C21D 2211/008 20130101; C22C 38/34 20130101; C22C 38/54
20130101; Y10T 428/12757 20150115; C21D 9/46 20130101; C22C 38/44
20130101; C23C 2/06 20130101; C22C 38/60 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 7/02 20060101 C21D007/02; C21D 6/00 20060101
C21D006/00; C21D 1/673 20060101 C21D001/673; C22C 38/58 20060101
C22C038/58; C22C 38/02 20060101 C22C038/02; C22C 38/60 20060101
C22C038/60; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/44 20060101 C22C038/44; C22C 38/06 20060101
C22C038/06; C21D 8/02 20060101 C21D008/02; C22C 38/54 20060101
C22C038/54 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2011 |
KR |
10-2011-0064159 |
Claims
1. A method for manufacturing a heat-treatment hardening component,
comprising: (a) preparing a blank formed of heat-treatment
hardening steel, the heat-treatment hardening steel comprising: by
wt %, C: 0.12.about.0.8%, Cr: 0.01.about.2%, Mo: 0.2% or less, B:
0.0005.about.0.08%, Ca: 0.01 or less, Sb: 1.0% or less, at least
one of Ti and Nb: 0.2% or less, components satisfying anyone of the
following compositions i) to iv), and the balance of Fe and
unavoidable impurities; By wt %, i) Si: 0.5.about.3%; Mn:
1.about.10% and Al: 0.05.about.2% ii) Si: 1% or less; Mn:
0.5.about.5%; Al: 0.1.about.2.5% and Ni: 0.01.about.8% iii) Si:
0.5.about.3%; Mn: 1.about.10%; Al: 0.1% or less and Ni:
0.01.about.8% iv) Si: 0.5.about.3%; Mn: 1.about.10%; Al:
0.1.about.2.5% and Ni: 0.01.about.8%, (b) heating the blank; (c)
hot-forming and quenching the heated blank in dies; and (d)
performing post-treatment of a formed body formed in the (c)
hot-forming and quenching.
2. The method according to claim 1, wherein the (b) heating is
performed by heating the blank to a temperature of 700.degree. C.
to 1100.degree. C.
3. The method according to claim 1, wherein, in the (c) hot-forming
and quenching, quenching is performed by cooling the heated blank
in the dies at a rate of 10.degree. C./sec to 300.degree. C./sec to
a martensite transformation start temperature or less of the
heat-treatment hardening steel.
4. The method according to claim 1, wherein the heat-treatment
hardening steel has at least one layer selected from an Al--Si
based coating layer, a galvanized layer and a high temperature
oxidation resistant coating layer on a surface thereof.
5. A method for manufacturing a heat-treatment hardening component,
comprising: (a) preparing a blank formed of heat-treatment
hardening steel, the heat-treatment hardening steel comprising:, by
wt %, C: 0.12.about.0.8%, Cr: 0.01.about.2%, Mo: 0.2% or less, B:
0.0005.about.0.08%, Ca: 0.01 or less, Sb: 1.0% or less, at least
one of Ti and Nb: 0.2% or less, components satisfying anyone of the
following compositions i) to iv), and the balance of Fe and
unavoidable impurities; By wt %, i) Si: 0.5.about.3%; Mn:
1.about.10% and Al: 0.05.about.2% ii) Si: 1% or less; Mn:
0.5.about.5%; Al: 0.1.about.2.5% and Ni: 0.01.about.8% iii) Si:
0.5.about.3%; Mn: 1.about.10%; Al: 0.1% or less and Ni:
0.01.about.8% iv) Si: 0.5.about.3%; Mn: 1.about.10%; Al:
0.1.about.2.5% and Ni: 0.01.about.8%, (a') performing
primary-forming of the blank through cold working; (b) heating a
primary formed body formed in the (a') performing primary forming;
(c) performing secondary-forming and quenching of the heated
primary formed body in dies; and (d) performing post-treatment of a
secondary formed body formed in the (c) performing
secondary-forming and quenching.
6. The method according to claim 5, wherein the (b) heating is
performed by heating the blank to a temperature of 700.degree. C.
to 1100.degree. C.
7. The method according to claim 5, wherein, in the (c) performing
secondary-forming and quenching, quenching is performed by cooling
the heated blank in the dies at a rate of 10.degree. C./sec to
300.degree. C./sec to a martensite transformation start temperature
or less of the heat-treatment hardening steel.
8. The method according to claim 5, wherein the heat-treatment
hardening steel has at least one layer selected from an Al--Si
based coating layer, a galvanized layer and a high temperature
oxidation resistant coating layer on a surface thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a Divisional Application of U.S.
Ser. No. 14/115,516 filed Nov. 4, 2013, which is a National Phase
application of No. PCT/KR2011/004785 filed on Jun. 30, 2011 and
also which claims the benefit under 35 U.S.C. .sctn.119 of Korean
Patent Application No. 10-2011-0064159 filed on Jun. 30, 2011 in
the Korean Intellectual Property Office, the entirety of which
disclosure is incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a technology for
manufacturing a high strength component using heat-treatment
hardening steel, and more particularly, to heat-treatment hardening
steel having high strength and crashworthiness after heat treatment
and a method for manufacturing a heat-treatment hardening component
using the same.
BACKGROUND ART
[0003] Recently, automobile components have been developed to be
light in weight and to have high strength for improvement of fuel
efficiency.
[0004] Recently, with the development of techniques for
manufacturing automobile components, hot stamping has been
developed. Hot stamping is a process of manufacturing a high
strength component by quenching to form a martensite microstructure
as soon as a material having a tensile strength of about 500 MPa
and heated to about 900.degree. C. is formed into a desired shape.
Hot stamping may be used to produce high strength components having
a tensile strength of 1000 MPa or more.
[0005] Steel for hot stamping comprises, in terms of % by weight
(wt %), C: 0.23%; Si: 0.24%; Mn: 1.2%; Cr: 0.18%; Mo: 0.0025%; Al:
0.03%; Ti 0.035%; B: 0.002% and the balance of Fe and unavoidable
impurities.
[0006] Steel having such a composition may exhibit a tensile
strength of 490 MPa to 590 MPa and an elongation of 20% to 30%
depending on process conditions. When the steel is heated to about
900.degree. C., the steel may exhibit a tensile strength of 100 MPa
to 200 MPa and an elongation of 50% to 60%, allowing easy forming.
Then, when the steel is subjected to forming in dies and quenching,
the formed steel has microstructures approaching full martensite,
whereby a finished component has an ultra-high tensile strength of
about 1470 MPa. The prepared component may have ultra-high strength
and thus does not require a separate reinforcing material to
enhance strength.
[0007] As such, hot stamping can facilitate weight reduction and
reduce the number of welds through elimination of components such
as a reinforcing material, thereby improving productivity while
reducing manufacturing costs.
[0008] However, components manufactured by this process have a
drawback in that such components have a low elongation of 6% to 7%
due to microstructures approaching full martensite, which is
advantageous for securing high strength.
[0009] Such low elongation causes brittleness failure of a
component due to insufficient absorption of impact when external
impact is applied thereto.
DISCLOSURE
Technical Problem
[0010] An aspect of the present invention is to provide
heat-treatment hardening steel that exhibits high ductility and
toughness together with high strength through adjustment of alloy
components after heat treatment, thereby providing improved
crashworthiness.
[0011] Another aspect of the present invention is to provide a
method for manufacturing a heat-treatment hardening component using
the heat-treatment hardening steel.
Technical Solution
[0012] In accordance with one aspect of the present invention,
heat-treatment hardening steel comprises, in terms of % by weight
(wt %), C: 0.12.about.0.8%; Cr: 0.01.about.2%; Mo: 0.2% or less; B:
0.0005.about.0.08%; Ca: 0.01 or less; Sb: 1.0% or less; at least
one of Ti and Nb: 0.2% or less; components satisfying any one of
the following compositions i) to iv); and the balance of Fe and
unavoidable impurities.
[0013] By wt %,
[0014] i) Si: 0.5.about.3%; Mn: 1.about.10%; and Al:
0.05.about.2%
[0015] ii) Si: 1% or less; Mn: 0.5.about.5%; Al: 0.1.about.2.5%;
and Ni: 0.01.about.8%
[0016] iii) Si: 0.5.about.3%; Mn: 1.about.10%; Al: 0.1% or less;
and Ni: 0.01.about.8%
[0017] iv) Si: 0.5.about.3%; Mn: 1.about.10%; Al: 0.1.about.2.5%;
and Ni: 0.01.about.8%
[0018] The steel may have a layer selected from among an Al--Si
plated layer, a galvanized layer, and a high temperature oxidation
resistant coating layer on a surface thereof.
[0019] In accordance with another aspect of the present invention,
a method for manufacturing a heat-treatment hardening component
includes: (a) preparing a blank formed of the heat-treatment
hardening steel as described above; (b) heating the blank; (c)
hot-forming and quenching the heated blank in dies; and (d)
performing post-treatment of a formed body formed in the (c)
hot-forming and quenching.
[0020] In accordance with a further aspect of the present
invention, a method for manufacturing a heat-treatment hardening
component includes: (a) preparing a blank formed of the
heat-treatment hardening steel as described above; (a') performing
primary-forming of the blank through cold working; (b) heating a
primary formed body formed in the (a') performing primary forming;
(c) performing secondary-forming and quenching of the heated
primary formed body in dies; and (d) performing post-treatment of a
secondary formed body formed in the (c) performing
secondary-forming and quenching.
Advantageous Effects
[0021] The heat-treatment hardening steel according to the present
invention may provide a high strength, highly tough and highly
ductile component having a tensile strength of 1000 MPa or more, a
yield strength of 800 MPa or more, and an elongation of 10% or more
through hot stamping. Accordingly, the component manufactured by
the method according to the present invention may exhibit improved
crashworthiness through high strength and excellent impact
absorption capabilities.
DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a schematic flowchart of a method for
manufacturing a heat-treatment hardening component in accordance
with one embodiment of the present invention.
[0023] FIG. 2 is a schematic flowchart of a method for
manufacturing a heat-treatment hardening component in accordance
with another embodiment of the present invention.
[0024] FIG. 3 shows a microstructure of a specimen prepared in
Comparative Example 1.
[0025] FIG. 4 shows a microstructure of a specimen prepared in
Example 1.
BEST MODE
[0026] The above and other aspects, features, and advantages of the
present invention will become apparent from the detailed
description of the following embodiments in conjunction with the
accompanying drawings.
[0027] It should be understood that the present invention is not
limited to the following embodiments and may be embodied in
different ways, and that the embodiments are provided for complete
disclosure and thorough understanding of the invention by those
skilled in the art. The scope of the present invention will be
defined only by the claims.
[0028] Hereinafter, heat-treatment hardening steel with excellent
crashworthiness and a method for manufacturing a heat-treatment
hardening component using the same according to the present
invention will be described in detail.
[0029] Heat-Treatment Hardening Steel
[0030] Heat-treatment hardening steel according to the present
invention comprises, by wt %, C: 0.12.about.0.8%; Cr:
0.01.about.2%; Mo: 0.2% or less; at least one of titanium (Ti) and
niobium (Nb): 0.2% or less; B: 0.0005.about.0.08%; and Sb: 1.0% or
less.
[0031] In addition, the heat-treatment hardening steel satisfies at
least one of the following compositions i) to iv):
[0032] By wt %,
[0033] i) Si: 0.5.about.3%; Mn: 1.about.10% and Al:
0.05.about.2%;
[0034] ii) Si: 1% or less; Mn: 0.5.about.5%; Al: 0.1.about.2.5% and
Ni: 0.01.about.8%;
[0035] iii) Si: 0.5.about.3%; Mn: 1.about.10%; Al: 0.1% or less and
Ni: 0.01.about.8%; and
[0036] iv) Si: 0.5.about.3%; Mn: 1.about.10%; Al: 0.1.about.2.5%
and Ni: 0.01.about.8%.
[0037] The heat-treatment hardening steel also comprises the
balance of Fe and unavoidable impurities.
[0038] Now, the amounts and functions of the respective components
included in the heat-treatment hardening steel according to the
present invention will be described in more detail.
[0039] Carbon (C)
[0040] Carbon (C) is added to secure strength of steel. In
addition, carbon serves to stabilize an austenite phase according
to the amount of carbon enriched in the austenite phase.
[0041] Preferably, carbon is present in an amount of 0.12 wt % to
0.8 wt % based on the total weight of the steel. If the carbon
content is less than 0.12 wt %, it is difficult to secure
sufficient strength. On the contrary, if the carbon content exceeds
0.8 wt %, the steel can suffer from significant deterioration in
toughness and weldability despite increase of strength.
[0042] Chromium (Cr)
[0043] Chromium (Cr) improves elongation through stabilization of
ferrite crystal grains, and increases strength through
stabilization of austenite by increasing the amount of carbon
enriched in the austenite phase.
[0044] Preferably, chromium is present in an amount of 0.01 wt % to
2 wt % based on the total weight of the steel. If the chromium
content is less than 0.01 wt %, the added chromium does not provide
sufficient functions thereof. On the contrary, a chromium content
of greater than 2 wt % makes it difficult to secure sufficient
yield strength after heat treatment, and deteriorates
wettability.
[0045] Molybdenum (Mo)
[0046] Molybdenum (Mo) is an effective element for enhancing
strength of steel through precipitation strengthening and
solid-solution strengthening. However, if the molybdenum content
exceeds 0.2 wt %, the steel can suffer from deterioration in
processibility.
[0047] Therefore, molybdenum is preferably present in an amount of
0.2 wt % or less based on the total weight of the steel.
[0048] Titanium (Ti), Niobium (Nb)
[0049] Titanium (Ti) and niobium (Nb) are carbonitride forming
elements and sever to enhance strength of steel. However, if the
total amount of titanium and niobium exceeds 0.2 wt %, the steel
can suffer from deterioration in toughness. Therefore, titanium or
niobium is preferably present in a total amount of 0.2 wt % or less
based on the total weight of the steel.
[0050] Boron (B)
[0051] Boron (B) enhances strength of steel through quenching
ability. Preferably, boron is present in an amount of 0.0005 wt %
to 0.08 wt % based on the total weight of the steel. If the boron
content is less than 0.0005 wt %, boron does not provide functions
thereof. On the contrary, if the boron content exceeds 0.08 wt %,
the steel can suffer from significant deterioration in toughness
due to excessive increase in quenching ability.
[0052] Antimony (Sb)
[0053] Antimony (Sb) enhances coating properties of steel by
preventing enrichment of silicon and manganese in grain boundaries.
However, if the antimony content exceeds 1%, the steel can suffer
from cracking and secondary work embrittlement.
[0054] Therefore, antimony is preferably used in an amount of 1% or
less based on the total weight of the steel.
[0055] Silicon (Si), Manganese (Mn), Aluminum (Al), Nickel (Ni)
[0056] Through studies for long duration, the inventors of the
present invention have found that silicon, manganese, aluminum and
nickel enhance tensile strength, yield strength and elongation
after heat treatment while satisfying at least one of the following
compositions i) to iv).
[0057] By wt %,
[0058] i) Si: 0.5.about.3%; Mn: 1.about.10% and Al:
0.05.about.2%
[0059] ii) Si: 1% or less; Mn: 0.5.about.5%; Al: 0.1.about.2.5% and
Ni: 0.01.about.8%
[0060] iii) Si: 0.5.about.3%; Mn: 1.about.10%; Al: 0.1% or less and
Ni: 0.01.about.8%
[0061] iv) Si: 0.5.about.3%; Mn: 1.about.10%; Al: 0.1.about.2.5%
and Ni: 0.01.about.8%
[0062] In compositions i) to iv), silicon (Si) acts as a deoxidizer
and enhances strength of steel through solid-solution
strengthening. If the silicon content exceeds the range provided by
each of compositions i) to iv), the steel can suffer from
deterioration in weldability and coating properties. In addition,
in the case of the compositions i), iii) and iv), if the silicon
content is less than the proposed range, the steel can suffer from
deterioration in weldability.
[0063] In compositions i) to iv), manganese (Mn) enhances strength
of steel through austenite stabilization. If the manganese content
is less than the proposed range in each of i).about.iv), the effect
of stabilizing the austenite phase becomes insufficient. On the
contrary, if the manganese content exceeds the range provided by
each of compositions i).about.iv), there are problems of
deterioration in weldability and toughness.
[0064] In compositions i) to iv), aluminum (Al) serves to prevent
hydrogen embrittlement. If the aluminum content is less than the
proposed range in each of i).about.iv), the effect provided by
addition of aluminum can become insufficient. On the contrary, if
the aluminum content exceeds the range provided by each of
compositions i) to iv), aluminum forms excess inclusions, thereby
deteriorating ductility and toughness of the steel.
[0065] In compositions ii) to iv), nickel (Ni) is advantageous in
securing strength and toughness of steel. If the nickel content is
less than the proposed range in each of compositions ii) to iv),
the effect provided by addition of nickel can become insufficient.
Conversely, if the nickel content exceeds the range provided by
each of compositions ii) to iv), the effects provided by addition
of nickel can become saturated, thereby significantly increasing
manufacturing costs.
[0066] The heat-treatment hardening steel having the above
composition according to the invention may be produced in forms of
hot-rolled steel sheets, hot-rolled plated steel sheets,
cold-rolled steel sheets, cold-rolled plated steel sheets, high
temperature oxidation resistant coated steel sheets, and the like.
Here, the heat-treatment hardening steel according to the present
invention may have an Al--Si based coating layer, galvanized layer
or high temperature oxidation resistant coating layer on a surface
thereof in order to prevent decarburization and oxidation in a hot
stamping process for fabrication of components described below. The
Al--Si based coating layer and the galvanized layer are generally
applied to cold-rolled plated steel sheets, without being limited
thereto. In addition, the galvanized layer may be formed by various
methods such as hot-dip galvanizing, hot-dip galvannealing,
electro-galvanizing, and the like.
[0067] Here, when the heat-treatment hardening steel according to
the present invention is a cold-rolled plated steel sheet,
annealing may be performed at a temperature ranging from
650.degree. C. to 850.degree. C. If the annealing temperature is
less than 650.degree. C., it is difficult to achieve desired
effects such as ductility improvement and the like even by
annealing. Conversely, if annealing temperature exceeds 850.degree.
C., there is a high possibility of enrichment of silicon,
manganese, and the like in grain boundaries even by addition of
antimony, thereby causing deterioration in coating properties.
[0068] On the other hand, the heat-treatment hardening steel having
the above composition according to the invention may have a tensile
strength of 490 MPa to 980 MPa, a yield strength of 370 MPa to 600
MPa, and an elongation of 20% to 50% according to process
conditions, that is, hot rolling, cold rolling, annealing, and the
like. Although the heat-treatment hardening steel does not need to
have these mechanical properties, heat-treatment hardening steel
having these mechanical properties is advantageous in forming
through hot stamping for fabrication of components.
[0069] In addition, the heat-treatment hardening steel having the
above composition and mechanical properties according to the
present invention may have a composite microstructure including
martensite and retained austenite after heat treatment.
[0070] Further, the heat-treatment hardening steel having the above
composition and mechanical properties according to the present
invention may have a tensile strength of 1000 MPa or more, a yield
strength of 800 MPa or more, and an elongation of 10% or more after
heat treatment, since the retained austenite structure is included
in the microstructure even after hot stamping.
[0071] Method of Manufacturing Heat-Treatment Hardening
Component
[0072] FIG. 1 is a schematic flowchart of a method for
manufacturing a heat-treatment hardening component in accordance
with one embodiment of the invention.
[0073] Herein, the term "component" may refer to collision members
of automobiles, without being limited thereto.
[0074] Referring to FIG. 1, the method for manufacturing a
heat-treatment hardening component includes preparing a blank
(S110), heating the blank (S120), forming/quenching (S130), and
post-treatment (S140).
[0075] In operation of preparing a blank (S110), a blank is
prepared from the heat-treatment hardening steel having the
composition according to the present invention.
[0076] As described above, the heat-treatment hardening steel may
have a tensile strength of 490 MPa to 980 MPa, a yield strength of
370 MPa to 600 MPa, and an elongation of 20% to 50%. In addition,
considering blank heating (S120) and forming/quenching (S130)
described hereinafter, the steel may have an Al--Si based coating
layer, a galvanized layer, a high temperature oxidation resistant
coating layer or the like formed on the surface thereof.
[0077] Next, in operation of heating the blank (S120), the blank is
heated to a temperature suitable for hot stamping. Heating may be
performed outside dies which will be used for hot stamping, that
is, forming/quenching, and may be performed inside the dies after
heating is performed to a predetermined temperature outside the
dies.
[0078] The heating temperature may range from 700.degree. C. to
1100.degree. C. If the heating temperature is less than 700.degree.
C., austenite formation becomes insufficient, thereby causing
insufficient strength after the operation of forming/quenching
(S130). Conversely, if the heating temperature exceeds 1100.degree.
C., it is difficult to secure high ductility due to an insufficient
fraction of the retained austenite after the operation of
forming/quenching (S130), thereby causing deterioration of
crashworthiness.
[0079] Next, in the operation of forming/quenching (S130), the
blank heated in the dies is formed into a formed body having a
predetermined shape, which in turn is subjected to quenching inside
the dies to secure desired properties.
[0080] Quenching may be performed to a martensite transformation
start temperature or less, for example, to a temperature ranging
from about 80.degree. C. to about 500.degree. C., in order to
secure the martensite fraction. In addition, quenching may be
performed at a cooling rate of 10.degree. C./sec to 300.degree.
C./sec. If the cooling rate is less than 10.degree. C./sec, it is
difficult to secure sufficient strength. Conversely, if the
quenching rate exceeds 300.degree. C./sec, it is difficult to
secure toughness and ductility.
[0081] After forming/quenching, the formed body may have a
composite microstructure comprising martensite and retained
austenite. As a result, the formed body formed through
forming/quenching may have a tensile strength of 1000 MPa or more,
a tensile strength of 800 MPa or more, and an elongation of 10% or
more.
[0082] In post treatment (S140), the formed body formed through
forming/quenching is subjected to laser processing to perform
trimming, piercing, and the like.
[0083] FIG. 2 is a schematic flowchart of a method for
manufacturing a heat-treatment hardening component in accordance
with another embodiment.
[0084] Referring to FIG. 2, the method for manufacturing a
heat-treatment hardening component includes blank preparation
(S210), cold working (S215), heating the blank (S220),
forming/quenching (S230), and post treatment (S240).
[0085] In the embodiment shown in FIG. 2, the method further
includes cold rolling (S215). In operation of cold rolling (S215),
the blank is subjected to primary forming through cold working. In
this case, during primary forming through cold working, a primary
formed body is prepared through forming, trimming, piercing, and
the like. Thus, in post treatment (S240), laser processing is
performed on a portion of a secondary formed body, which is
subjected to secondary forming (S230) through forming/quenching
within dies.
EXAMPLES
[0086] Next, the present invention will be described in more detail
with reference to examples. Here, the following examples are
provided for illustration only and should not be construed in any
way as limiting the present invention.
[0087] Descriptions of details apparent to those skilled in the art
will be omitted.
[0088] 1. Preparation of Specimen
[0089] In order to observe heat-treatment hardening properties of
steel according to alloy compositions, specimens of Examples 1 to 4
and Comparative Example 1 having compositions as listed in Table 1
and mechanical properties before heat treatment as listed in Table
2 were heated to 900.degree. C., left for 5 minutes, and cooled to
100.degree. C. at an average cooling rate of 50.degree. C./sec.
TABLE-US-00001 TABLE 1 (Unit: wt %) C Si Mn Cr Mo Al Ti Nb B Ni Sb
Comparative 0.229 0.238 1.19 0.183 0.0025 0.03 0.036 -- 0.002 -- --
Example 1 Example 1 0.3 1.0 7.5 0.3 0.01 1.5 0.05 -- 0.003 -- 0.8
Example 2 0.3 0.4 3.0 0.3 0.01 2.0 0.05 0.01 0.005 2.0 0.8 Example
3 0.4 1.5 5.5 0.2 0.01 0.05 0.05 0.05 0.003 3.0 0.8 Example 4 0.4
1.7 6.0 0.2 0.01 2.0 -- 0.10 0.002 3.0 0.8
[0090] 2. Mechanical Properties
[0091] Table 2 shows mechanical properties of the specimens of
Examples 1 to 4 and Comparative Example 1 before and after heat
treatment.
TABLE-US-00002 TABLE 2 Before heat treatment After heat treatment
Tensile Yield Tensile Yield Fraction of strength strength
Elongation strength strength Elongation retained (MPa) (MPa) (%)
(MPa) (MPa) (%) austenite (%) Comparative 510 380 25 1470 840 6.0
<1% Example 1 Example 1 515 383 26 1291 1136 15.0 3~5% Example 2
520 382 24 1302 1124 14.8 5~15% Example 3 710 491 22 1884 1138 15.1
10~40% Example 4 723 490 21 1817 1054 14.7 30~60%
[0092] Referring to Table 2, the specimens of Examples 1 to 4 and
Comparative Example 1 exhibited similar mechanical properties
before heat treatment.
[0093] However, after heat treatment, the specimen of Comparative
Example 1 had a low elongation of 6% despite very high tensile
strength. On the contrary, although the specimens of Examples 1 to
4 had slightly lower tensile strength than the specimens of
Comparative Example 1, these specimens had an elongation of about
15% and exhibited relatively high yield strength.
[0094] Accordingly, upon application of external impact, the
specimen of Comparative Example 1 can suffer from brittleness
failure due to low yield strength and elongation as compared with
tensile strength, whereas the specimens of Examples 1 to 4 can
sufficiently absorb the impact due to relatively high yield
strength and elongation.
[0095] In addition, in order to measure the fraction of retained
austenite, various tests such as microscopic observation, magnetic
measurement, X-ray diffraction analysis, and the like were
performed. As a result, although the specimens of Examples 1 to 4
have different values according to the measurement methods, these
specimens include retained austenite in an area fraction of at
least 1% or more.
[0096] However, the specimen of Comparative Example 1 included
retained martensite in an area fraction of less than 1% even by any
measurement methods, and thus had a full martensite
microstructure.
[0097] Difference in physical properties after heat treatment
between the specimens of Examples 1 to 4 and Comparative Example 1
can be confirmed through difference in final microstructure.
[0098] FIG. 3 shows the microstructure of a specimen prepared in
Comparative Example 1, and FIG. 4 shows the microstructure of a
specimen prepared in Example 1.
[0099] Referring to FIG. 3, the specimen of Comparative Example 1
had a microstructure approaching full martensite. On the other
hand, referring to FIG. 4, it can be seen that the specimen of
Example 1 includes retained austenite (y) in addition to
martensite.
[0100] By such microstructures, the specimen of Comparative Example
1 can have a very low elongation despite very high yield strength,
whereas the specimen of Example 1 can have high elongation.
[0101] Although some embodiments have been disclosed herein, it
should be understood that these embodiments are provided for
illustration only and various modifications, changes, alterations
and equivalent embodiments can be made without departing from the
scope of the present invention. Therefore, the scope and sprit of
the invention should be defined only by the accompanying claims and
equivalents thereof.
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