U.S. patent application number 13/620355 was filed with the patent office on 2013-01-17 for quenchable steel sheet having high hot press workability and method of manufacturing the same.
This patent application is currently assigned to HYUNDAI STEEL COMPANY. The applicant listed for this patent is Seongju KIM, Taekjoon KIM, Seungha LEE. Invention is credited to Seongju KIM, Taekjoon KIM, Seungha LEE.
Application Number | 20130014555 13/620355 |
Document ID | / |
Family ID | 44187937 |
Filed Date | 2013-01-17 |
United States Patent
Application |
20130014555 |
Kind Code |
A1 |
KIM; Taekjoon ; et
al. |
January 17, 2013 |
QUENCHABLE STEEL SHEET HAVING HIGH HOT PRESS WORKABILITY AND METHOD
OF MANUFACTURING THE SAME
Abstract
The quenchable steel sheet has an alloy composition including
carbon (C) in an amount of 0.15 to 0.30 wt %, silicon (Si) in an
amount of 0.05 to 0.5 wt %, manganese (Mn) in an amount of 1.0 to
2.0 wt %, boron (B) in an amount of 0.0005 to 0.0040 wt %, sulfur
(S) in an amount of 0.003 wt % or less, phosphorus (P) in an amount
of 0.012 wt % or less, one or more selected from among calcium (Ca)
in an amount of 0.0010 to 0.0040 wt % and copper (Cu) in an amount
of 0.05 to 1.0 wt %, two or more selected from among cobalt (Co),
zirconium (Zr) and antimony (Sb), and iron (Fe). Alloy elements are
controlled to increasing hot ductility and enabling pressing at 600
to 900.degree. C. so that a tensile strength of 1400 MPa or more
and an elongation of 8% or more are obtained after pressing.
Inventors: |
KIM; Taekjoon; (Seosan-si,
KR) ; LEE; Seungha; (Dangjin-gun, KR) ; KIM;
Seongju; (Yongin-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KIM; Taekjoon
LEE; Seungha
KIM; Seongju |
Seosan-si
Dangjin-gun
Yongin-si |
|
KR
KR
KR |
|
|
Assignee: |
HYUNDAI STEEL COMPANY
Incheon
KR
|
Family ID: |
44187937 |
Appl. No.: |
13/620355 |
Filed: |
September 14, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13029634 |
Feb 17, 2011 |
8293379 |
|
|
13620355 |
|
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PCT/KR2009/007996 |
Dec 30, 2009 |
|
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13029634 |
|
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Current U.S.
Class: |
72/200 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/04 20130101; Y10T 428/12757 20150115; C22C 38/10 20130101;
C22C 38/16 20130101 |
Class at
Publication: |
72/200 |
International
Class: |
B21B 27/06 20060101
B21B027/06 |
Claims
1. A method of making an article using a quenchable steel sheet,
comprising: (a) making a hot-rolled steel sheet, which comprises:
heating a steel slab to a temperature of at 1100.degree. C. or
higher, the steel slab comprising carbon (C) in an amount of 0.15
to 0.3 wt %, silicon (Si) in an amount of 0.05 to 0.5 wt %,
manganese (Mn) in an amount of 1.0 to 2.0 wt %, boron (B) in an
amount of 0.0005 to 0.004 wt %, sulfur (S) in an amount of 0.003 wt
% or less, phosphorus (P) in an amount of 0.012 wt % or less, iron
(Fe) and other inevitable impurities, hot-rolling the steel slab at
a temperature of Ar.sub.3 to Ar.sub.3+50.degree. C.; and winding
the hot-rolled steel sheet into a roll; (b) washing the hot-rolled
steel sheet using acid, cold-rolling the steel sheet, and plating
the steel sheet to inhibit oxide scale from being generated during
hot-pressing process; (c) hot pressing the cold-rolled, plated
steel sheet at a temperature of at 600 to 900.degree. C. to make an
article having a desired shape; and (d) cooling the hot pressed
article, wherein that the article has a tensile strength of 1400
MPa or more and an elongation of 8% or more.
2. The method of claim 1, wherein the plated steel sheet comprises
an Al--Si plated layer.
3. The method of claim 1, wherein the plated steel sheet comprises
a plated layer with a thickness of 10 .mu.m to 30 .mu.m.
4. The method of claim 1, wherein, in the step (b), the
cold-rolling is performed at a reduction ratio of about 50%.
5. The method of claim 1, wherein, in the step (c), the
hot-pressing comprises: heating the plated steel sheet to a
temperature of 700.degree. C. or higher and hot-pressing the plated
steel, and cooling the hot-pressed article within a mold.
Description
RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
application Ser. No. 13/029,634 filed Feb. 17, 2011, now pending,
which is a continuation application under 35 U.S.C. .sctn.365(c) of
International Application No. PCT/KR2009/007996, filed Dec. 30,
2009 designating the United States. This application incorporates
herein by reference U.S. application Ser. No. 13/029,634 and
International Application No. PCT/KR2009/007996 in their
entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to a quenchable steel sheet
having high hot press workability and a method of manufacturing the
same.
BACKGROUND ART
[0003] In the current state of the automobile industry, the
application of ultrahigh strength steel sheets is increasing in
order to cope with the requirements of safety and lightness.
However, it is difficult to use ultrahigh strength steel sheets to
produce automobile parts having complicated shapes because they
have low formability. Thus the demand for quenchable steel sheets
ensuring high strength by heating, hot pressing and then quenching
them.
[0004] This section is to provide general background information,
and does not constitute an admission of prior art.
SUMMARY
[0005] An aspect of the present invention is to provide a
quenchable steel sheet having high hot press workability and
enhanced hot ductility so as to facilitate hot pressing, and a
method of manufacturing the same.
[0006] Another aspect of the present invention is to provide a
quenchable steel sheet and a method of manufacturing the same, the
quenchable steel sheet having high hot press workability so that
the quenchable steel sheet can be pressed even at a low
temperature, such as 600.degree. C., thereby minimizing the
generation of oxide scales when the steel sheet is a non-plated
steel sheet and preventing the surface of the sheet from being
damaged when the sheet is a plated steel sheet.
[0007] In an aspect of the invention, a quenchable steel sheet
having an alloy composition comprises carbon (C) in an amount of
0.15 to 0.30 wt %, silicon (Si) in an amount of 0.05 to 0.5 wt %,
manganese (Mn) in an amount of 1.0 to 2.0 wt %, boron (B) in an
amount of 0.0005 to 0.0040 wt %, sulfur (S) in an amount of 0.003
wt % or less, phosphorus (P) in an amount of 0.012 wt % or less,
one or more selected from among calcium (Ca) in an amount of 0.0010
to 0.0040 wt % and copper (Cu) in an amount of 0.05 to 1.0 wt %,
two or more selected from among cobalt (Co), zirconium (Zr) and
antimony (Sb), and iron (Fe) and other inevitable impurities.
[0008] The method of manufacturing a quenchable steel sheet,
comprises hot pressing a plated steel sheet at 600 to 900.degree.
C., thus exhibiting a tensile strength of 1400 MPa or more and an
elongation of 8% or more, wherein the plated steel sheet having an
alloy composition comprising carbon (C) in an amount of 0.15 to
0.30 wt %, silicon (Si) in an amount of 0.05 to 0.5 wt %, manganese
(Mn) in an amount of 1.0 to 2.0 wt %, boron (B) in an amount of
0.0005 to 0.0040 wt %, sulfur (S) in an amount of 0.003 wt % or
less, phosphorus (P) in an amount of 0.012 wt % or less, one or
more selected from among calcium (Ca) in an amount of 0.0010 to
0.0040 wt % and copper (Cu) in an amount of 0.05 to 1.0 wt %, two
or more selected from among cobalt (Co), zirconium (Zr) and
antimony (Sb), andiron (Fe) and other inevitable impurities.
[0009] The zirconium (Zr) may be contained in an amount of 0.0005
to 0.1 wt %.
[0010] The cobalt (Co) and antimony (Sb) are present in amounts
satisfying 0.0005 wt %.ltoreq.(CO+Sb).ltoreq.0.5 wt %.
[0011] The weight ratio of Ca/S may fall in the range of 0.5 to
3.0.
[0012] The hot pressing process may be performed by heating the
plated steel sheet to 700.degree. C. or higher, placing the heated
steel sheet into a die, and performing pressing at 600 to
900.degree. C. and cooling in the die.
[0013] The plated steel sheet may be an Al--Si plated steel
sheet.
[0014] According to embodiments of the present invention, to ensure
hot ductility at least two selected from among cobalt (Co),
antimony (Sb) and zirconium (Zr) are used, instead of titanium
(Ti), niobium (Nb), molybdenum (Mo) or chromium (Cr) that causes
cracks on a steel sheet during hot pressing. Because pressing is
possible at a low temperature, the energy consumption can be
reduced, and in the case of a plated steel sheet, a plating layer
can be protected, and in the case of a non-plated steel sheet, the
occurrence of oxide scales can be prevented.
[0015] Even when the plating layer is formed to have a thickness of
10 .mu.m to 30 .mu.m, scales are not formed, and the generation of
cracks and pores on the plating layer can be reduced; thus
corrosion resistance increases.
[0016] Also according to embodiments of the present invention, in
lieu of aluminum, silicon which is inexpensive is used as a
deoxidizer during steel making; thus economic benefits are
maximized.
[0017] Also, according to embodiments of the present invention,
calcium (Ca) is added to control the shape of inclusions in a
manner of spheroidizing sulfur (S) inclusions. This enhances
toughness of quenchable steel sheets.
[0018] Also, according to embodiments of the present invention,
copper (Cu) is added to minimize hydrogen delayed fracture in steel
or welding portions. Thus it is possible to manufacture quenchable
steel sheets having enhanced resistance to hydrogen delayed
fracture without additional processing incurring additional
costs.
[0019] Therefore, quenchable steel sheets, which have superior
press workability and satisfy a tensile strength of 1400 MPa or
more and an elongation of 8% or more after pressing can be
manufactured at comparatively low cost.
[0020] Such quenchable steel sheets can be variously applied to
automobile parts at lower costs, in particular, can be reliably
employed in automobile parts that are sensitive to hydrogen
embrittlement.
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 schematically shows a hydrogen delayed fracture
caused by moisture attached to the surface of a steel sheet which
does not contain Cu;
[0022] FIG. 2 schematically shows the principle of how Cu increases
resistance to hydrogen delayed fracture;
[0023] FIG. 3 shows scanning electron microscope (SEM) images of
plating layers after hot pressing in (a) Comparative Example 1 and
(b) Inventive Example 1;
[0024] FIG. 4 shows the glow discharge spectrometry (GDS) profile
of an element distribution in a depth direction from a surface of
the steel sheet in (a) Comparative Example 1 after hot pressing;
and
[0025] FIG. 5 shows the GDS profile of an element distribution in a
depth direction from a surface of the steel sheet in (b) Inventive
Example 1 after hot pressing.
DETAILED DESCRIPTION OF EMBODIMENTS
[0026] Hereinafter, a detailed description will be given of a
quenchable steel sheet having high hot press workability and a
method of manufacturing the same according to embodiments of the
present invention.
[0027] According to embodiments of the present invention, a
quenchable steel sheet has an alloy composition comprising iron
(Fe), carbon (C) in an amount of 0.15 to 0.30 wt %, silicon (Si) in
an amount of 0.05 to 0.5 wt %, manganese (Mn) in an amount of 1.0
to 2.0 wt %, boron (B) in an amount of 0.0005 to 0.0040 wt %,
sulfur (S) in an amount of 0.003 wt % or less, phosphorus (P) in an
amount of 0.012 wt % or less, one or both selected from among
calcium (Ca) in an amount of 0.0010 to 0.0040 wt % and copper (Cu)
in an amount of 0.05 to 1.0 wt %, two or more selected from among
cobalt (Co), zirconium (Zr) and antimony (Sb), and other inevitable
impurities.
[0028] According embodiments of to the present invention, a
manufacturing method includes heating a plated steel sheet having
the above alloy composition to 700.degree. C. or higher, placing
the plated steel sheet into a die, and performing pressing at 600
to 900.degree. C. and cooling in the die. The plated steel sheet is
an Al--Si plated steel sheet.
[0029] Specifically, titanium (Ti), niobium (Nb), molybdenum (Mo),
and chromium (Cr) are not added as they cause cracks to be formed
on the steel sheet during the hot pressing. Instead, at least two
selected from among Co, Sb and Zr are added to manufacture the
quenchable steel sheet having hot ductility. Titanium (Ti), niobium
(Nb), molybdenum (Mo), and chromium (Cr) inhibit the production of
a second phase such as perlite or bainite and also delay
transformation to obtain martensite texture, but they may bind with
C and N of steel to form a deposit, thereby undesirably decreasing
hot ductility of the steel sheet.
[0030] Cobalt (Co) in an amount of 0.0005 to 0.5 wt %, zirconium
(Zr) in an amount of 0.0005 to 0.1 wt % and antimony (Sb) in an
amount of 0.0005 to 0.5 wt % may be added. In the selected two
elements, Co and Sb are present in total amount to satisfy 0.0005
wt %.ltoreq.(Co+Sb).ltoreq.0.5 wt %.
[0031] This is to increase stability of the strength of the steel
sheet after hot pressing. If Co and Sb are present in a total
amount less than 0.0005 wt %, there is no stability of the
strength. In contrast, if a total amount of Co and Sb exceeds 0.5
wt %, it is difficult to control a steel making process, and the
resulting steel sheet may deteriorate.
[0032] Zr and Co have a higher affinity for N, S, C and H than that
of Ti, and are thus adapted to fix such elements. Zr may react with
N like Ti to form ZrN, thereby preventing the formation of B into
BN. When B is formed into BN, it is intergranularly precipitated
and quenching properties may decrease.
[0033] Zr and Co may suppress intergranular corrosion while showing
a good surface appearance; thus corrosion resistance increases.
[0034] Specifically, Zr and Co may be dispersed in the plating
layer after plating the steel sheet, and form numerous nuclei. Such
nuclei may cause intergranular interference in the course of
coagulating the plating material; thus the growth of crystal grains
is controlled. When the growth of crystal grains is controlled in
this way, a good surface appearance may be obtained and
intergranular corrosion may be suppressed; thus corrosion
resistance is enhanced.
[0035] In particular, numerous nuclei dispersed in the plating
layer are able to form a multilayered alloy plating that functions
to inhibit and block the permeation of various elements of the
external environment, for example, hydrogen.
[0036] The multilayered alloy plating may prevent the reaction
between aluminum (Al) and iron (Fe); thus the growth of the alloy
layer is inhibited, and a plating layer having high workability is
formed. Even when the plating layer is formed to have a thickness
of 10 .mu.m to 30 .mu.m, scales are not produced, and cracks and
pores on the plating layer after hot pressing can be minimized.
[0037] When cracks and pores formed on the plating layer are
minimized, corrosion resistance of the quenchable steel sheet may
be increased, and the desired shape of parts may be freely formed.
For reference, when an alloy layer of Al and Fe is formed upon
plating, the plating layer may become brittle.
[0038] Furthermore, Co may inhibit Si or Mn forming an oxide on the
surface of the steel sheet; thus plating wettability increases.
Before hot pressing, the steel sheet is plated with Al--Si in order
to prevent the generation of oxide scales at high temperatures. If
an oxide of Si or Mn is formed on the surface of the steel sheet,
the portion where the oxide is formed cannot be plated.
[0039] Also, the amounts of impurities, that is, the elements that
decrease hot workability, such as P and S, are controlled to be
present in a very small amount to improve hot workability.
[0040] In embodiments of the present invention, the amount and
ratio of Co, Zr, Sb, P and S are controlled to improve hot press
workability; thus hot pressing at a temperature of 600 to
900.degree. C. can be performed without causing cracks.
[0041] The final microstructure according to embodiments of the
present invention is martensite such that the final product has a
tensile strength of 1400 MPa or more and an elongation of 20% or
more even at a high temperature of 600 to 900.degree. C.
[0042] Below, the alloy elements according to embodiments of the
present invention are specified in terms of function and
amount.
[0043] C: 0.15 to 0.30 wt %
[0044] C is an element essential to the high strength steel sheet.
However, in order to increase the hardness of the quenchable steel
sheet, the amount of C should be appropriately adjusted. If the
amount of C is present in amount less than 0.15 wt %, the
hardenability of the steel may decrease; thus after heat treatment
it is difficult to obtain sufficient martensite structure which
would ensure high tensile strength.
[0045] In contrast, if the amount of C exceeds 0.30 wt %, such
hardenability may increase to ensure sufficient tensile strength.
However, the strength of the steel before heat treatment may
increase undesirably and it would be difficult to form a
product.
[0046] Si: 0.05 to 0.5 wt %
[0047] Si is added as a deoxidizer for removing oxygen from steel
in the steel making process. Also Si functions to enhance quenching
properties. However, if too much amount of Si is added, an oxide
may form on the surface of the steel sheet, and undesirably degrade
plating properties. And, the viscosity of molten metal may
increase, and thus, in a trimming step of the part manufacturing
process, undesirable problems on the cut surface of the steel sheet
can be caused. So, the upper limit of Si is set to 0.5 wt %. If the
amount of Si is less than 0.05 wt %, desired effects cannot be
obtained.
[0048] Mn: 1.0 to 2.0 wt %
[0049] Mn inhibits the production of perlite structure and promotes
the formation of austenite and concentration of carbon in steel,
and thus contributes to form residual austenite, and also functions
to increase the quenching properties of the steel sheet and
reliably ensure the strength of the steel sheet after quenching. Mn
is added in an amount of 1.0 wt % or more so as to ensure a tensile
strength of 1400 MPa or more.
[0050] However, if the amount thereof exceeds 2.0 wt %, corrosion
resistance and weldability may decrease. So, this element is added
in an amount not exceeding 2.0 wt %.
[0051] B: 0.0005 to 0.0040 wt %
[0052] B is added to delay the transformation of austenite into
ferrite so as to increase the quenching properties of the steel
sheet. Thus after quenching a product may have high tensile
strength. B should be added in an amount of 0.0005 wt % or more in
order to increase the quenching properties of the steel sheet.
However, if the amount thereof exceeds 0.0040 wt %, it is difficult
to control a steel making process, and undesirable quality
variations in the material after heat treatment are caused. So,
this element is added in an amount not exceeding 0.0040 wt %.
[0053] Ca: 0.0010 to 0.0040 wt %
[0054] Ca may be added to enhance toughness of the steel sheet. Ca
may spheroidize an S inclusion (MnS) to increase toughness. Even
when the amount of S is controlled to be very small, if the S
inclusion is present in a linear shape, impact resistance and
toughness may decrease.
[0055] Ca is added after desulfurization in the steel making
process.
[0056] If the amount of Ca is less than 0.0010 wt %, the effect
thereof becomes insignificant. In contrast, if the amount thereof
exceeds 0.0040 wt %, the effect cannot be maximized and it is
difficult to control the steel making process.
[0057] In particular, in order to maximize the toughness of the
steel sheet after hot pressing, the weight ratio of Ca/S should
fall in the range of 0.5 to 3.0. If the weight ratio of Ca/S falls
in the range of 0.5 to 3.0, the spheroidization effect of the S
inclusion (MnS) may increase.
[0058] If the weight ratio of Ca/S is less than 0.5, the effect of
maximizing toughness may become insignificant. In contrast, if the
weight ratio thereof exceeds 3.0, such an effect cannot be
maximized and it is difficult to control the steel making
process.
[0059] Cu: 0.05 to 1.0 wt %
[0060] Cu may be added to prevent cathodic reaction of sulfide and
intergranular hydrogen delayed fracture in steel or welding
portions.
[0061] Cu may increase the quenching properties of the steel sheet
and the stability of strength after quenching, and also may inhibit
the cathodic reaction of sulfide and intergranular hydrogen
permeation in steel or welding portions.
[0062] As shown in FIG. 1, when the steel sheet is exposed to
moisture environments, moisture causes a reduction reaction,
2H.sup.++2e.sup.-- - - - - .fwdarw.H.sub.2, by movement of
electrons emitted from Fe which is a base metal. As such, H.sub.2
produced by the reduction reaction may intergranularly diffuse into
the base metal at a fast rate even at low temperatures; thus
intergranular bondability is weakened.
[0063] When H.sub.2 meets with sulfide in steel, intergranular
bondability may be further weakened and cracks may be generated.
Accordingly, sudden fracture may take place after a lapse of a
predetermined period of time.
[0064] As shown in FIG. 2, when Cu is added, Cu is positioned at
the intergranules, and thus may inhibit internal permeation of
H.sub.2 and may surround the outer surface of sulfide to thus
prevent the contact between H.sub.2 and sulfide. Thus the cathodic
reaction of sulfide by H.sub.2 present in steel may be
inhibited.
[0065] If the amount of Cu is less than 0.05 wt %, it is difficult
to reduce hydrogen delayed fracture. In contrast, if the amount
thereof exceeds 1.0 wt %, intergranular permeation of Cu may occur
upon re-heating of a slab; thus cracks may be generated upon hot
pressing.
[0066] Thus the amount of Cu is set to the range of 0.05 to 1.0 wt
%.
[0067] S: 0.003 wt % or less
[0068] S is contained in an amount of about 0.015 wt % in molten
steel after a typical desulfurization process. However, S may
decrease hot workability of steel at high temperatures just as P
does, and thus the amount thereof should be controlled to be
minimized in order to enhance hot workability. A longside the
recent development of steel making techniques, the amount of S may
be controlled to 0.003 wt % or less.
[0069] In particular, as the amount of S becomes low, impact
absorption energy is increased after heat treatment. When the
amount of S is controlled to 0.003 wt % or less compared to steel
containing 0.010 wt % of S, impact absorption energy may be at
least doubled.
[0070] The experimental results show that the steel sheet has
impact absorption energy of 35 J when the amount of S is 0.010 wt
%, but the impact absorption energy is doubled to 70 J when the
amount of S is controlled to 0.003 wt %.
[0071] P: 0.012 wt % or less
[0072] P is contained in an amount of about 0.020 wt % in molten
steel after a typical dephosphorization process. However, P may
decrease hot workability of steel at high temperatures, and the
amount thereof should be controlled to be very small in order to
increase hot workability. Alongside the recent development of steel
making techniques, P may be controlled to 0.012 wt % or less, which
is set to the maximum value.
[0073] Zr: 0.0005 to 0.1 wt %
[0074] Zr may be added to remove N. N inevitably exists in the
steel during the steel making process. N present in steel may bind
with B and thus may precipitate as a BN compound, which may
deteriorate quenching properties. In order to maximally prohibit
the existence of N in steel, Zr is added to form a compound with N
at high temperatures. When Zr is added in an amount of 0.0005 wt %
or more, the desired effects can be expected. If the amount of Zr
exceeds 0.1 wt %, there is no industrial value.
[0075] Co, Sb: 0.0005 to 0.5 wt %
[0076] These elements may increase the quenching properties of a
steel sheet and stabilize the strength of the steel sheet after hot
pressing. Thus these elements are added to ensure oxidation
resistance at high temperatures and increase elongation.
[0077] When the total amount of Co and Sb is 0.0005 wt % or more,
desired effects may be obtained. If the total amount thereof
exceeds 0.5 wt %, it is difficult to control a steel making process
and the steel sheet may deteriorate. Even when either of Co and Sb
is added, it may be added in the above range for the same
reasons.
[0078] The steel sheet according to embodiments of the present
invention includes the above components, iron (Fe) and the elements
that are inevitably present. Inevitable impurities such as N or O
may be contained in trace amounts depending on conditions such as
feeds, materials and manufacturing equipment.
[0079] The steel slab having the above composition is manufactured
by using a steel casting process including providing molten steel
and then forming an ingot or performing a continuous casting
process. A hot-rolled or cold-rolled steel sheet is plated and then
hot pressed; thus a quenchable steel sheet as below produced.
[0080] [Method of Manufacturing Quenchable Steel Sheet]
[0081] The steel slab according to embodiments of the present
invention is manufactured by performing a steel making process
including providing molten steel, and then forming an ingot or
being subjected to a continuous casting process. In order to
dissolve the components segregated when casting, the slab is
re-heated in a furnace at 1100.degree. C. or higher, and hot-rolled
at a temperature of Ar3 to Ar3+50; thus a single-phase hot-rolled
coil is produced. Winding is carried out at a coiling temperature
(CT) of 400 to 700.degree. C. in order to facilitate cold-rolling.
The surface of the steel sheet is pickled to remove an oxide.
[0082] Subsequently, cold-rolling is carried out. This cold-rolling
is performed at a reduction ratio of about 50%, and the cold-rolled
steel sheet may be used in a without plating or may be plated in
order to prevent oxidation.
[0083] Al--Si plating is performed to inhibit oxide scales from
being formed during hot pressing. The hot-rolled steel sheet may be
used in a state of not having been plated or may be plated to
prevent oxidation and be subjected to Al--Si plating.
[0084] Subsequently, hot pressing is performed to produce a final
product having the desired shape. The hot pressing includes heating
to 700.degree. C. or higher which is a temperature of Ar3 or more,
and then pressing at 600 to 900.degree. C. to manufacture the final
product. Cooling is performed at the same time as pressing is being
conducted.
[0085] As such, even when the steel sheet is heated to have a
temperature of 600 to 900.degree. C. which is lower than a typical
heating temperature, an elongation of 20% or more may be ensured at
this temperature by controlling the amount and ratio conditions of
Co, Zr, Sb, P, and S.
[0086] The component ratio of the above alloy elements is
controlled so that hot pressing is performed in the range of 600 to
900.degree. C. In the case of a plated steel sheet, stripping of
the plating at the high temperature may be prevented. In the case
of a non-plated steel sheet, the production of oxide scales on the
surface of the steel sheet at the high temperature may be
prevented. If the hot pressing process is carried out at a
temperature lower than 600.degree. C., it is difficult to ensure
the desired press workability.
[0087] Below, examples of quenchable steel sheets having high hot
press workability and the method of manufacturing the same are
discussed.
EXAMPLES
[0088] A steel slab having each of alloy compositions shown in
Table 1 was heated to 1100.degree. C. or higher for 2 hours,
finish-rolled at about 900.degree. C., wound at 400 to 700.degree.
C. for 1 hour, and furnace-cooled to room temperature and
cold-rolled to be a cold-rolled steel sheet. The cold rolled steel
sheet was heated to 700.degree. C. or higher, hot pressed at 600 to
900.degree. C. and cooled in a die.
[0089] The alloy compositions of the comparative example and
inventive Examples are shown in Table 1, and the mechanical
properties of steel sheet products manufactured using the alloy
compositions of Table 1 at high temperatures and room temperature
(RT) are shown in Table 2 below.
TABLE-US-00001 TABLE 1 (Final Alloy Composition of Steel Sheet wt
%: remainder Fe) C Si Mn P S Cu Ca Al Ti Cr Co Zr Sb B Note C. Ex.
1 0.20 0.3 1.2 0.018 0.006 -- -- 0.02 0.035 0.2 -- -- -- 0.002 Al
Deoxidizer Inv. 0.23 0.3 1.2 0.005 0.001 0.05 -- -- -- -- 0.10 0.03
0.02 0.002 Si Deoxidizer Ex. 1 Inv. 0.23 0.3 1.5 0.007 0.002 0.05
-- -- -- -- 0.05 -- 0.03 0.002 Si Deoxidizer Ex. 2 Inv. 0.23 0.3
1.5 0.012 0.003 0.05 -- -- -- -- 0.20 0.05 -- 0.002 Si Deoxidizer
Ex. 3 Inv. 0.23 0.3 1.5 0.012 0.003 0.05 0.0030 -- -- -- 0.20 0.05
-- 0.002 Si Deoxidizer Ex. 4 Inv. 0.23 0.3 1.5 0.012 0.003 --
0.0030 -- -- -- 0.20 0.05 -- 0.002 Si Deoxidizer Ex. 5
TABLE-US-00002 TABLE 2 Temp. 600.degree. C. 700.degree. C.
900.degree. C. RT Part Tensile Tensile Tensile Tensile Strength EL
Strength EL Strength EL Strength EL C. Ex. 1 228 16 132 17 104 22
1520 6 Inv. 223 22 153 24 106 28 1550 10 Ex. 1 Inv. 232 20 169 23
118 26 1507 9 Ex. 2 Inv. 201 20 128 21 98 23 1560 8 Ex. 3 Inv. 203
20 129 21 99 24 1560 10 Ex. 4 Inv. 202 20 127 21 98 24 1559 10 Ex.
5 [MPa: tensile strength, EL (wt %): elongation]
[0090] As is apparent from Tables 1 and 2, when two or more
selected from among Co, Sb and Zr are added instead of Al, Ti and
Cr, it can be guaranteed that the elongation of the steel be 20% or
more at a high temperature of 600 to 900.degree. C.
[0091] In the case of parts resulting from hot pressing the steel
sheet having the elongation of 20% or more at high temperatures, it
can be seen that a tensile strength of 1400 MPa and an elongation
of 8% or more at a room temperature are obtained after cooling in
the die.
[0092] When Ca is added in the range of weight ratio of Ca/S of 0.5
to 3.0, the elongation is further improved (see Inventive Examples
3 to 5). The hot pressing process as above may be applied to an
Al--Si plated steel sheet.
[0093] The quenchable steel sheet manufactured as above enables hot
pressing at 600 to 900.degree. C., and thus a plating layer is
protected, the generation of oxide scales is prevented, and high
tensile strength is ensured.
[0094] FIG. 3 shows SEM images of the plating layer after hot
pressing in (a)
[0095] Comparative Example 1 and (b) Inventive Example 1. FIG. 4
shows the GDS profile of element distribution in a depth direction
from the surface layer of the steel sheet of (a) Comparative
Example 1. FIG. 5 shows the GDS profile of element distribution in
a depth direction from the surface layer of the steel sheet of (b)
Inventive Example 1.
[0096] As shown in FIG. 3, in the case of (a) Comparative Example
1, there are cracks and pores generated on the plating layer, and
in the case of (b) Inventive Example 1, neither cracks nor pores
can be seen on the plating layer.
[0097] As shown in FIGS. 4 and 5, in the case of (a) Comparative
Example 1, the amount of Fe is remarkably increased at a position
of 40 .mu.m downward from the surface of the plating layer, whereas
in the case of (b) Inventive Example 1 the amount of Fe is
considerably increased at a position of 25 .mu.m downward from the
surface of the plating layer. The drastic increase in the amount of
Fe indicates that the plating layer comes to an end, and thereby
the thickness of the plating layer may be estimated.
[0098] Even when the plating layer is formed as thin as 10 to 30
scales do not form and the generation of cracks and pores on the
plating layer is reduced; thus corrosion resistance increases.
[0099] Although the embodiments of the present invention have been
disclosed for illustrative purposes, those skilled in the art will
appreciate that various modifications, additions and substitutions
are possible, without departing from the scope and spirit of the
invention as disclosed in the accompanying claims.
* * * * *