U.S. patent application number 15/435418 was filed with the patent office on 2018-08-23 for mitigating liquid metal embrittlement in zinc-coated press hardened steels.
This patent application is currently assigned to GM Global Technology Operations LLC. The applicant listed for this patent is GM Global Technology Operations LLC. Invention is credited to Charles M. Enloe, Jianfeng Wang.
Application Number | 20180237877 15/435418 |
Document ID | / |
Family ID | 63166417 |
Filed Date | 2018-08-23 |
United States Patent
Application |
20180237877 |
Kind Code |
A1 |
Wang; Jianfeng ; et
al. |
August 23, 2018 |
MITIGATING LIQUID METAL EMBRITTLEMENT IN ZINC-COATED PRESS HARDENED
STEELS
Abstract
Methods of reducing liquid metal embrittlement (LME) in
zinc-coated high-strength steel alloys are provided. In one
variation, the method includes decarburizing an exposed surface of
a high-strength steel alloy to form a decarburized surface layer.
The decarburized surface layer has a thickness of less than or
equal to about 50 micrometers. The decarburized surface layer may
have greater than or equal to about 80 volume % ferrite. The method
also includes applying a zinc-based coating to the decarburized
surface layer. A blank is formed from the high-strength steel
alloy. The method also includes heating and press hardening the
blank to form a press-hardened component having an ultimate tensile
strength of greater than or equal to about 1,100 MPa that is
substantially free of liquid metal embrittlement.
Inventors: |
Wang; Jianfeng; (Nanjing,
CN) ; Enloe; Charles M.; (Grosse Pointe Woods,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM Global Technology Operations LLC |
Detroit |
MI |
US |
|
|
Assignee: |
GM Global Technology Operations
LLC
Detroit
MI
|
Family ID: |
63166417 |
Appl. No.: |
15/435418 |
Filed: |
February 17, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 9/0068 20130101;
C23C 2/02 20130101; C21D 8/005 20130101; C21D 2211/001 20130101;
C23C 2/28 20130101; C22C 38/02 20130101; C23C 2/26 20130101; C22C
38/04 20130101; C23C 2/40 20130101; C21D 2211/008 20130101; C21D
1/06 20130101; C23C 2/06 20130101; C21D 3/04 20130101; C21D
2211/005 20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C23C 2/06 20060101 C23C002/06; C23C 2/02 20060101
C23C002/02; C23C 2/28 20060101 C23C002/28; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 1/06 20060101
C21D001/06; C21D 3/04 20060101 C21D003/04; C21D 8/00 20060101
C21D008/00 |
Claims
1. A method of reducing liquid metal embrittlement (LME) in
zinc-coated high-strength steel, the method comprising:
decarburizing an exposed surface of a high-strength steel alloy to
form a decarburized surface layer having a thickness of less than
or equal to about 50 micrometers and comprising greater than or
equal to about 80 volume % ferrite; applying a zinc-based coating
to the decarburized surface layer; forming a blank from the
high-strength steel alloy; and heating and press hardening the
blank to form a press-hardened component having an ultimate tensile
strength of greater than or equal to about 1,100 MPa that is
substantially free of liquid metal embrittlement.
2. The method of claim 1, the decarburizing occurs at a temperature
of greater than or equal to about 700.degree. C. in an environment
comprising nitrogen and water.
3. The method of claim 1, wherein the zinc-based coating comprises
passing the blank through a zinc galvanization bath at a
temperature of greater than or equal to about 420.degree. C. to
less than or equal to about 480.degree. C.
4. The method of claim 1, wherein the decarburized surface layer
has a thickness of greater than or equal to about 20 micrometers to
less than or equal to about 50 micrometers.
5. The method of claim 1, wherein the decarburized surface layer
comprises greater than or equal to about 90 volume % ferrite.
6. The method of claim 1, wherein the steel alloy comprises carbon
at less than or equal to about 0.4 weight %.
7. The method of claim 1, wherein the steel alloy comprises:
manganese at greater than or equal to about 0.2 weight % to less
than or equal to about 2.0 weight %; carbon at greater than or
equal to about 0.15 weight % to less than or equal to about 0.4
weight %; and silicon at greater than 0.1 weight % to less than or
equal to about 1 weight %.
8. The method of claim 1, wherein the heating occurs at a
temperature of greater than or equal to about 800.degree. C. to
less than or equal to about 950.degree. C. for austenitization of
the high-strength steel alloy.
9. The method of claim 1, further comprising quenching the
press-hardened component to below room temperature after the press
hardening.
10. The method of claim 1, wherein the high-strength steel alloy
has a first side and a second side opposite to the first side,
wherein after the decarburizing, the first side has a first
decarburized surface layer and the second side has a second
decarburized surface layer that sandwich a central region.
11. The method of claim 10, wherein the heating austenitizes the
high-strength steel alloy and the method further comprises
quenching the high-strength steel alloy blank after the heating and
press hardening so that the central region comprises greater than
or equal to about 98 volume % martensite.
12. The method of claim 1, wherein the press-hardened component has
an ultimate tensile strength of greater than or equal to about
1,300 MPa to less than or equal to about 2,000 MPa.
13. A method of reducing liquid metal embrittlement (LME) in
zinc-coated high-strength steel, the method comprising:
decarburizing an exposed surface of a high-strength steel alloy to
form a decarburized surface layer having a thickness of less than
or equal to about 50 micrometers and comprising greater than or
equal to about 80 volume % ferrite; hot dip galvanizing the
high-strength steel alloy in a heated zinc galvanization bath to
form a zinc-based coating over the decarburized surface layer;
forming a blank from the high-strength steel alloy; heating the
blank to austenitize the high-strength steel alloy; and press
hardening the blank to form a press-hardened component having an
ultimate tensile strength of greater than or equal to about 1,300
MPa to less than or equal to about 2,000 MPa that is substantially
free of liquid metal embrittlement.
14. The method of claim 13, wherein the decarburizing occurs at a
temperature of greater than or equal to about 700.degree. C. in an
environment comprising nitrogen and water.
15. The method of claim 13, wherein the zinc-based coating
comprises passing the blank through a zinc galvanization bath at a
temperature of greater than or equal to about 420.degree. C. to
less than or equal to about 480.degree. C.
16. The method of claim 13, wherein the decarburized surface layer
has a thickness of greater than or equal to about 20 micrometers to
less than or equal to about 50 micrometers.
17. The method of claim 13, wherein the decarburized surface layer
comprises greater than or equal to about 90 volume % ferrite.
18. The method of claim 13, wherein the steel alloy comprises:
manganese at greater than or equal to about 0.2 weight % to less
than or equal to about 2.0 weight %; carbon at greater than or
equal to about 0.15 weight % to less than or equal to about 0.4
weight %; and silicon at greater than 0.1 weight % to less than or
equal to about 1 weight %.
19. The method of claim 13, wherein the heating occurs at a
temperature of greater than or equal to about 800.degree. C. to
less than or equal to about 950.degree. C.
20. The method of claim 13, wherein the high-strength steel alloy
has a first side and a second side opposite to the first side,
wherein after the decarburizing, the first side has a first
decarburized surface layer and the second side has a second
decarburized surface layer that sandwich a central region, wherein
the method further comprises quenching the blank after the heating
and press hardening so that the central region comprises greater
than or equal to about 98 volume % martensite.
Description
INTRODUCTION
[0001] This section provides background information related to the
present disclosure which is not necessarily prior art.
[0002] Press-hardened steel (PHS), also referred to as "hot-stamped
steel" or "boron-steel" (e.g., 22MnB5 alloy), is one of the
strongest steels used for automotive body structural applications,
typically having tensile strength properties on the order of about
1,400 megapascals (MPa) or higher. Such steel has desirable
properties, including forming steel components with significant
increases in strength-to-weight ratios. PHS components have become
ever more prevalent in various industries and applications,
including general manufacturing, construction equipment, automotive
or other transportation industries, home or industrial structures,
and the like.
[0003] For example, when manufacturing vehicles, especially
automobiles, continual improvement in fuel efficiency and
performance is desirable, thus PHS components have been
increasingly used. PHS components are often used for forming
load-bearing components, like door beams, which usually require
high-strength materials. Thus, the finished state of these steels
are designed to have high strength and enough ductility to resist
external forces, for example, to resist intrusion into the
passenger compartment while maintaining load so as to provide
protection to the occupants. Moreover, galvanized PHS components
may provide cathodic protection.
[0004] Typical PHS processes involve austenitization in a furnace
of a sheet steel blank immediately followed by pressing and
quenching of the sheet in dies. There are two main types of PHS
processes: indirect and direct. Austenitization is typically
conducted in the range of about 900.degree. C. Under the direct
method, the PHS component is formed and pressed simultaneously
between dies, which quenches the steel. Under the indirect method,
the PHS component is cold formed to an intermediate partial shape
before austenitization and the subsequent pressing and quenching
steps. The quenching of the PHS component hardens the component by
transforming the microstructure from austenite to martensite. When
the PHS component is uncoated, an oxide layer forms during the
heating of the blank and transfer from the furnace to the dies.
After quenching, therefore, the oxide must be removed from the PHS
component and the dies. The oxide is typically removed by shot
blasting.
[0005] The PHS component may be coated prior to applicable pre-cold
forming (if the indirect process is used) or austenitization. PHS
components may require cathodic protection. The PHS component may
be coated prior to applicable pre-cold forming (if the indirect
process is used) or austenitization. Coating the PHS component
provides a protective layer (e.g., galvanic protection) to the
underlying steel component. Such coatings typically include an
aluminum-silicon alloy and/or zinc. Zinc-based coatings offer
cathodic protection; the coating acts as a sacrificial layer and
corrodes instead of the steel component, even where the steel is
exposed.
[0006] However, liquid metal embrittlement (LME) may occur when a
metallic system is exposed to a liquid metal, such as zinc, during
forming at high temperature, resulting in potential cracking and a
reduction of total elongation or diminished ductility of a
material. LME may also result in decreased ultimate tensile
strength. To avoid LME in conventional PHS processes, numerous
additional processing steps are conducted. Thus, there is an
ongoing need for high-strength hot-formed press-hardened steel
components having necessary hardness and strength levels, while
providing galvanic protection substantially free of LME.
SUMMARY
[0007] This section provides a general summary of the disclosure,
and is not a comprehensive disclosure of its full scope or all of
its features.
[0008] The present disclosure relates to methods of reducing liquid
metal embrittlement (LME) in zinc-coated high-strength steel
alloys. In one variation, the method includes decarburizing an
exposed surface of a high-strength steel alloy to form a
decarburized surface layer. The decarburized surface layer has a
thickness of less than or equal to about 50 micrometers. The
decarburized surface layer may have greater than or equal to about
80 volume % ferrite. The method also includes applying a zinc-based
coating to the decarburized surface layer. A blank is formed from
the high-strength steel alloy. The method also includes heating and
press hardening the blank to form a press-hardened component having
an ultimate tensile strength of greater than or equal to about
1,100 MPa that is substantially free of liquid metal
embrittlement.
[0009] In one aspect, the decarburizing occurs at a temperature of
greater than or equal to about 700.degree. C. in an environment
that is non-oxidizing to iron. The environment may comprise one or
more of nitrogen, hydrogen, carbon monoxide, carbon dioxide, and
water.
[0010] In one aspect, the zinc-based coating process includes
passing the blank through a zinc galvanization bath at a
temperature of greater than or equal to about 420.degree. C. to
less than or equal to about 480.degree. C.
[0011] In one aspect, the decarburized surface layer has a
thickness of greater than or equal to about 20 micrometers to less
than or equal to about 50 micrometers.
[0012] In one aspect, the decarburized surface layer includes
greater than or equal to about 90 volume % ferrite.
[0013] In one aspect, the steel alloy includes carbon at less than
or equal to about 0.4 weight %.
[0014] In one aspect, the steel alloy includes:
[0015] manganese at greater than or equal to about 0.2 weight % to
less than or equal to about 2.0 weight %;
[0016] carbon at greater than or equal to about 0.15 weight % to
less than or equal to about 0.4 weight %; and
[0017] silicon at greater than 0.1 weight % to less than or equal
to about 1 weight %.
[0018] In one aspect, the heating occurs at a temperature of
greater than or equal to about 800.degree. C. to less than or equal
to about 950.degree. C. for austenitization of the high-strength
steel alloy.
[0019] In one aspect, the method further includes quenching the
press-hardened component to room temperature or below after the
press hardening.
[0020] In one aspect, the high-strength steel alloy has a first
side and a second side opposite to the first side. After the
decarburizing, the first side has a first decarburized surface
layer and the second side has a second decarburized surface layer
that sandwich a central region.
[0021] In one aspect, the method further includes quenching the
high-strength steel alloy blank after the heating and press
hardening so that the central region includes greater than or equal
to about 95 volume % martensite and the first decarburized surface
layer and the second decarburized surface layer have less than or
equal to about 20 volume % martensite.
[0022] In one aspect, the press-hardened component has an ultimate
tensile strength of greater than or equal to about 1,300 MPa to
less than or equal to about 2,000 MPa.
[0023] In another variation, the present disclosure further
provides a method of reducing liquid metal embrittlement (LME) in
zinc-coated high-strength steel. The method includes decarburizing
an exposed surface of a high-strength steel alloy to form a
decarburized surface layer. The decarburized surface layer has a
thickness of less than or equal to about 50 micrometers. The
decarburized surface layer includes greater than or equal to about
80 volume % ferrite. The high-strength steel alloy is then hot-dip
galvanized in a heated zinc galvanization bath to form a zinc-based
coating over the decarburized surface layer. A blank is formed from
the high-strength steel alloy. The blank is heated for
austenitization and press hardened to form a press-hardened
component having an ultimate tensile strength of greater than or
equal to about 1,300 MPa to less than or equal to about 2,000 MPa.
The press-hardened component is substantially free of liquid metal
embrittlement.
[0024] In one aspect, the decarburizing occurs at a temperature of
greater than or equal to about 700.degree. C. in an environment
that is non-oxidizing to iron. The environment may comprise one or
more of nitrogen, hydrogen, carbon monoxide, carbon dioxide, and
water.
[0025] In one aspect, the zinc-based coating includes passing the
blank through a zinc galvanization bath at a temperature of greater
than or equal to about 420.degree. C. to less than or equal to
about 480.degree. C.
[0026] In one aspect, the decarburized surface layer has a
thickness of greater than or equal to about 20 micrometers to less
than or equal to about 50 micrometers.
[0027] In one aspect, the decarburized surface layer includes
greater than or equal to about 90 volume % ferrite.
[0028] In one aspect, the steel alloy includes:
[0029] manganese at greater than or equal to about 0.2 weight % to
less than or equal to about 2.0 weight %;
[0030] carbon at greater than or equal to about 0.15 weight % to
less than or equal to about 0.4 weight %; and
[0031] silicon at greater than 0.1 weight % to less than or equal
to about 1.0 weight %.
[0032] In one aspect, the heating occurs at a temperature of
greater than or equal to about 800.degree. C. to less than or equal
to about 950.degree. C. for austenitization of the high-strength
steel alloy.
[0033] In one aspect, the high-strength steel alloy has a first
side and a second side opposite to the first side, wherein after
the decarburizing, the first side has a first decarburized surface
layer and the second side has a second decarburized surface layer
that sandwich a central region. The central region predominantly
includes ferrite and pearlite before the heating and press
hardening. After heating, press hardening, and cooling, the blank
has a central region that includes greater than or equal to about
95 volume % martensite. The first decarburized surface layer and
the second decarburized surface layer have less than or equal to
about 20 volume % martensite.
[0034] Further areas of applicability will become apparent from the
description provided herein. The description and specific examples
in this summary are intended for purposes of illustration only and
are not intended to limit the scope of the present disclosure.
DRAWINGS
[0035] The drawings described herein are for illustrative purposes
only of selected embodiments and not all possible implementations,
and are not intended to limit the scope of the present
disclosure.
[0036] FIG. 1 shows a cross section of a high-strength steel alloy
blank formed according to certain methods of the present disclosure
having decarburized surface layers and zinc-based coating
layers;
[0037] FIG. 2 shows a representative press-hardening process for a
high-strength steel including a continuous decarburization process
according to certain aspects of the present disclosure; and
[0038] FIG. 3 shows another representative press-hardening process
for a high-strength steel including a batch decarburization process
according to certain aspects of the present disclosure.
[0039] Corresponding reference numerals indicate corresponding
parts throughout the several views of the drawings.
DETAILED DESCRIPTION
[0040] Example embodiments are provided so that this disclosure
will be thorough, and will fully convey the scope to those who are
skilled in the art. Numerous specific details are set forth such as
examples of specific compositions, components, devices, and
methods, to provide a thorough understanding of embodiments of the
present disclosure. It will be apparent to those skilled in the art
that specific details need not be employed, that example
embodiments may be embodied in many different forms and that
neither should be construed to limit the scope of the disclosure.
In some example embodiments, well-known processes, well-known
device structures, and well-known technologies are not described in
detail.
[0041] The terminology used herein is for the purpose of describing
particular example embodiments only and is not intended to be
limiting. As used herein, the singular forms "a," "an," and "the"
may be intended to include the plural forms as well, unless the
context clearly indicates otherwise. The terms "comprises,"
"comprising," "including," and "having," are inclusive and
therefore specify the presence of stated features, elements,
compositions, steps, integers, operations, and/or components, but
do not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or
groups thereof. Although the open-ended term "comprising," is to be
understood as a non-restrictive term used to describe and claim
various embodiments set forth herein, in certain aspects, the term
may alternatively be understood to instead be a more limiting and
restrictive term, such as "consisting of" or "consisting
essentially of." Thus, for any given embodiment reciting
compositions, materials, components, elements, features, integers,
operations, and/or process steps, the present disclosure also
specifically includes embodiments consisting of, or consisting
essentially of, such recited compositions, materials, components,
elements, features, integers, operations, and/or process steps. In
the case of "consisting of," the alternative embodiment excludes
any additional compositions, materials, components, elements,
features, integers, operations, and/or process steps, while in the
case of "consisting essentially of," any additional compositions,
materials, components, elements, features, integers, operations,
and/or process steps that materially affect the basic and novel
characteristics are excluded from such an embodiment, but any
compositions, materials, components, elements, features, integers,
operations, and/or process steps that do not materially affect the
basic and novel characteristics can be included in the
embodiment.
[0042] Any method steps, processes, and operations described herein
are not to be construed as necessarily requiring their performance
in the particular order discussed or illustrated, unless
specifically identified as an order of performance. It is also to
be understood that additional or alternative steps may be employed,
unless otherwise indicated.
[0043] When a component, element, or layer is referred to as being
"on," "engaged to," "connected to," or "coupled to" another element
or layer, it may be directly on, engaged, connected or coupled to
the other component, element, or layer, or intervening elements or
layers may be present. In contrast, when an element is referred to
as being "directly on," "directly engaged to," "directly connected
to," or "directly coupled to" another element or layer, there may
be no intervening elements or layers present. Other words used to
describe the relationship between elements should be interpreted in
a like fashion (e.g., "between" versus "directly between,"
"adjacent" versus "directly adjacent," etc.). As used herein, the
term "and/or" includes any and all combinations of one or more of
the associated listed items.
[0044] Although the terms first, second, third, etc. may be used
herein to describe various steps, elements, components, regions,
layers and/or sections, these steps, elements, components, regions,
layers and/or sections should not be limited by these terms, unless
otherwise indicated. These terms may be only used to distinguish
one step, element, component, region, layer or section from another
step, element, component, region, layer or section. Terms such as
"first," "second," and other numerical terms when used herein do
not imply a sequence or order unless clearly indicated by the
context. Thus, a first step, element, component, region, layer or
section discussed below could be termed a second step, element,
component, region, layer or section without departing from the
teachings of the example embodiments.
[0045] Spatially or temporally relative terms, such as "before,"
"after," "inner," "outer," "beneath," "below," "lower," "above,"
"upper," and the like, may be used herein for ease of description
to describe one element or feature's relationship to another
element(s) or feature(s) as illustrated in the figures. Spatially
or temporally relative terms may be intended to encompass different
orientations of the device or system in use or operation in
addition to the orientation depicted in the figures.
[0046] Throughout this disclosure, the numerical values represent
approximate measures or limits to ranges to encompass minor
deviations from the given values and embodiments having about the
value mentioned as well as those having exactly the value
mentioned. Other than in the working examples provided at the end
of the detailed description, all numerical values of parameters
(e.g., of quantities or conditions) in this specification,
including the appended claims, are to be understood as being
modified in all instances by the term "about" whether or not
"about" actually appears before the numerical value. "About"
indicates that the stated numerical value allows some slight
imprecision (with some approach to exactness in the value;
approximately or reasonably close to the value; nearly). If the
imprecision provided by "about" is not otherwise understood in the
art with this ordinary meaning, then "about" as used herein
indicates at least variations that may arise from ordinary methods
of measuring and using such parameters. For example, "about" may
comprise a variation of less than or equal to 5%, optionally less
than or equal to 4%, optionally less than or equal to 3%,
optionally less than or equal to 2%, optionally less than or equal
to 1%, optionally less than or equal to 0.5%, and in certain
aspects, optionally less than or equal to 0.1%.
[0047] As used herein, all amounts are weight % (or mass %), unless
otherwise indicated.
[0048] In addition, disclosure of ranges includes disclosure of all
values and further divided ranges within the entire range,
including endpoints and sub-ranges given for the ranges.
[0049] As referred to herein, the word "substantially," when
applied to a characteristic of a composition or method of this
disclosure, indicates that there may be variation in the
characteristic without having a substantial effect on the chemical
or physical attributes of the composition or method.
[0050] Example embodiments will now be described more fully with
reference to the accompanying drawings.
[0051] The present disclosure provides methods of press-hardening a
galvanized, pre-treated steel alloy to form a press-hardened
component having high strength and significantly reduced
susceptibility to liquid metal embrittlement during press forming.
Press hardened structural components comprising a galvanic coating
comprising zinc can be formed from a galvanized steel alloy blank
prepared in accordance with the present technology. In certain
variations, the steel may be galvanized by hot-dipping the blank in
a galvanization bath. In certain variations, such a press-hardened
steel component comprises a galvanic coating comprising zinc that
is formed from a hot-dipped galvanized steel alloy blank.
[0052] Such a high-strength three-dimensional component may be
incorporated into a device, such as a vehicle. While the
high-strength structures are particularly suitable for use in
components of an automobile or other vehicles (e.g., motorcycles,
boats, tractors, buses, motorcycles, mobile homes, campers, and
tanks), they may also be used in a variety of other industries and
applications, including aerospace components, consumer goods,
office equipment and furniture, industrial and construction
equipment and machinery, farm equipment, or heavy machinery, by way
of non-limiting example. Non-limiting examples of components and
vehicles that can be manufactured by the current technology include
automobiles, tractors, buses, motorcycles, boats, mobile homes,
campers, and tanks. Other exemplary structures that have frames
that can be manufactured by the current technology include
construction and buildings, such as houses, offices, bridges,
sheds, warehouses, and devices. It will be appreciated by those
skilled in the art that numerous other components may be fabricated
by the methods of the present invention, and that such additional
components are deemed to be within the scope of the present
disclosure.
[0053] Structural components capable of being formed of galvanized
steel alloy include those used in vehicles, like automotive
applications including, but not limited to, rocker rails, engine
rails, structural pillars, A-pillars, B-pillars, C-pillars,
D-pillars, bumper, hinge pillars, cross-members, body panels,
panels, including structural panels, door panels, and door
components, interior floors, floor pans, roofs, hoods, exterior
surfaces, underbody shields, wheels, storage areas, including glove
boxes, console boxes, trunk lids, trunks, trunk floors, truck beds,
lamp pockets and other components, shock tower cap, control arms
and other suspension, undercarriage or drive train components, and
the like. While exemplary components are illustrated and described
throughout the specification, it is understood that the inventive
concepts in the present disclosure may also be applied to any
structural component capable of being formed of galvanized steel
alloy. Specifically, the present disclosure is particularly
suitable for any piece of hardware subject to loads or impact
(e.g., load bearing) or requiring cathodic protection.
[0054] As mentioned above, the use of such galvanic coatings on
press-hardened steel has a number of advantages over uncoated
steel. Such a galvanic coating (e.g., comprising zinc) provides
cathodic protection to the underlying steel. In addition to
providing an additional measure of corrosion-resistant benefits as
a barrier coating, subsequent cleaning operations following press
hardening to remove scale from the die surfaces and parts may be
optional. For example, strong post-forming cleaning processes, such
as shot blasting, may not be needed. Nonetheless, some cleaning may
be desirable for subsequent welding and painting processes.
[0055] Initially, a blank is formed of a high-strength steel. A
high-strength steel for automotive body structure applications is
one that has an ultimate tensile strength of greater than or equal
to about 1,000 megapascals (MPa), for example, optionally greater
than or equal to about 1,100 MPa, optionally greater than or equal
to about 1,200 MPa, optionally greater than or equal to about 1,300
MPa, optionally greater than or equal to about 1,400 MPa,
optionally greater than or equal to about 1,500 MPa, optionally
greater than or equal to about 1,700 MPa, and in certain
variations, optionally greater than or equal to about 2,000 MPa. In
certain aspects, a high-strength steel has an ultimate tensile
strength greater than or equal to about 1,100 MPa to less than or
equal to about 2,200 MPa, optionally greater than or equal to about
1,300 MPa to less than or equal to about 2,200 MPa, optionally
greater than or equal to about 1,400 MPa to less than or equal to
about 2,100 MPa.
[0056] In various aspects, particularly suitable, non-limiting
steel compositions may include a high-strength steel alloy
composition comprising carbon at greater than or equal to 0.15
weight %. For example, carbon may be present at optionally greater
than or equal to about 0.15 weight % to less than or equal to about
0.4 weight % carbon, or optionally greater than or equal to about
0.2 weight % to less than or equal to about 0.4 weight %. In
certain aspects, suitable high-strength steel alloys may have
manganese at greater than 0.2 weight % to less than or equal to
about 2 weight % and optionally at greater than or equal to about
0.2 weight % to less than or equal to about 1.5 weight %. Silicon
may be present at greater than or equal to about 0.1 weight % to
less than or equal to about 1 weight %, optionally greater than or
equal to about 0.2 weight % to less than or equal to about 0.5
weight %. Aluminum is optionally present at less than or equal to
about 0.01 weight %. One or more other alloying elements and/or
impurities in the steel alloy are cumulatively present at less than
or equal to about 1 weight % and optionally at less than or equal
to about 0.5 weight %. Other elements, such as chromium (Cr) and
molybdenum (Mo), can also be present (typically at less than about
0.5 weight %) for hardenability, while others like titanium (Ti),
niobium (Nb) and vanadium (V) (typically at less than about 0.5
weight %) may be added for grain refinement purpose and hence
better performance. A balance of such a steel composition is
iron.
[0057] Non-limiting exemplary high-strength steels include 22MnB5,
which comprises about 0.21% to about 0.25% carbon (C), about 1.1%
to about 1.35% manganese (Mn), about 0.15% to about 0.4% silicon
(Si), about 0.0015% to about 0.004 boron (B), a maximum of 0.16%
chromium (Cr), a maximum of 0.01% sulfur (S), a maximum of 0.023%
phosphorus (P), impurities cumulatively less than about 0.1%, and a
remainder iron (Fe). Other high-strength steel alloys include
30MnB5 and 35MnB5. 30MnB5 comprises about 0.27% to about 0.33%
carbon (C), about 1.15 to about 1.45% manganese (Mn), a maximum of
0.4% silicon (Si), about 0.0004 to about 0.005% boron (B), a
maximum of 0.035% sulfur (S), a maximum of 0.025% phosphorus (P),
impurities cumulatively less than about 0.1%, and a remainder iron
(Fe). 35MnB5 comprises about 0.32% to about 0.4% carbon (C), about
1.2 to about 1.5% manganese (Mn), a maximum of 0.5% silicon (Si),
about 0.0008 to about 0.005% boron (B), a maximum of 0.035% sulfur
(S), a maximum of 0.035% phosphorus (P), impurities cumulatively
less than about 0.1%, and a remainder iron (Fe).
[0058] In certain aspects, the present disclosure contemplates
modifying conventional steel alloy compositions so that they may
have zinc galvanic coatings, yet can be processed via press
hardening (PHS) to form components with high strength and
negligible liquid metal (e.g., zinc) embrittlement (LME). Zinc
melts around 420.degree. C. and reacts to form an intermetallic
compound with iron around 782.degree. C. Thus, zinc-based coatings
may cause LME when exposed to high temperatures, especially at
temperatures of greater than 782.degree. C. However, PHS components
formed from conventional steel alloys processed with a
decarburizing step in accordance with the present disclosure are
able to be galvanized and subsequently heated to temperatures of
greater than or equal to about 782.degree. C. during press
hardening, while successfully minimizing or avoiding LME formation
and exhibiting good strength.
[0059] In various aspects, the present disclosure thus provides a
method of reducing liquid metal embrittlement (LME) in zinc-coated
high-strength steel. First, the method may include decarburizing an
exposed surface of a high-strength steel alloy to form a
decarburized surface layer. Next, the high-strength steel alloy may
be galvanized. Thus, after the decarburizing step, a zinc-based
coating may be applied to the decarburized surface layer. The
high-strength steel alloy may be formed into a blank. Notably, the
high-strength steel alloy may be formed into a blank before the
decarburizing and galvanizing step to apply zinc-based coating or
after these steps. A blank of the high-strength steel alloy may be
heated and press hardened to form a press-hardened component having
an ultimate tensile strength of greater than or equal to about
1,100 MPa or any of the ultimate tensile strength levels described
previously above.
[0060] In accordance with various aspects of the present
disclosure, press hardened steel (PHS) components formed by such
processes are substantially free of liquid metal embrittlement
(LME). The term "substantially free of LME" as referred to herein
means that the LME microstructures and defects are absent to the
extent that undesirable physical properties and limitations
attendant with their presence are avoided (e.g., cracking, loss of
ductility, and/or loss of strength). In certain embodiments, a PHS
component that is "substantially free of LME" defects exhibits LME
cracking with an average depth of less than or equal to about 20
micrometers (.mu.m) following press forming, optionally with an
average depth of less than or equal to about 12 .mu.m, optionally
with an average depth of less than or equal to about 9 .mu.m,
optionally with an average of less than or equal to about 6 .mu.m,
and in certain variations, optionally with an average depth of less
than or equal to about 3 .mu.m.
[0061] The method includes decarburizing the blank. The
decarburizing may include disposing the blank in an environment
that is non-oxidizing to iron. For example, the environment may
comprise one or more of the following: nitrogen (N.sub.2), hydrogen
(H.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), and
water (H.sub.2O). In one variation, a suitable wet non-oxidizing
atmosphere includes nitrogen and water and has a dew point of
greater than about -5.degree. C. In one variation, the
decarburizing occurs at a temperature of greater than or equal to
about 700.degree. C. in an environment comprising nitrogen and
water. The decarburizing may include heating the blank with peak
metal temperatures above about 700.degree. C. in a wet
non-oxidizing (to Fe) atmosphere (e.g.,
N.sub.2--H.sub.2--CO--CO.sub.2--H.sub.2O; dew point greater than
about -5 .degree. C.) to produce a 10 .mu.m to 20 .mu.m thick
decarburized layer on a hot-dip coated steel intended for
subsequent press hardening. In other variations, the decarburizing
may also occur coupled with a batch or inline annealing method of
decarburization in a wet non-oxidizing (to Fe) atmosphere (having a
dew point greater than about -5.degree. C.) and heat treatment with
peak metal temperatures above 700.degree. C. to produce a 10 .mu.m
to 20 .mu.m thick decarburized layer on a bare steel surface
intended for subsequent hot-dip coating or electro-coating and
subsequent press hardening.
[0062] Previously, decarburizing of high-strength steels was
avoided due to loss of strength that occurs when carbon is removed
from the alloy. However, in the context of the present disclosure,
only a thin decarburized surface layer is formed on one or more
exposed surfaces of the high-strength steel blank. In certain
aspects, the decarburized surface layer has a thickness of less
than or equal to about 50 micrometers, for example, greater than or
equal to about 20 micrometers to less than or equal to about 50
micrometers, optionally greater than or equal to about 10
micrometers to less than or equal to about 20 micrometers. Thus, a
controlled decarburization process creates a thin surface layer
having a reduced carbon content compared to a bulk carbon content
in the body of the press hardenable steel blank. The decarburized
thin surface layer is optionally formed on the steel blank prior to
zinc-based coating or the hot forming process. The decarburized
surface layer has a larger grain size and hence less grain boundary
area for liquid metal embrittlement to occur during the hot
stamping process.
[0063] After decarburizing, the steel blank may have an oxide layer
that forms on the surface. Such an oxide may be removed in a
cleaning step prior to applying the galvanic zinc-based coating. In
certain variations, the oxide is removed by a pickling process
known in the art that uses acid to dissolve oxide from the surface,
which is then neutralized and rinsed.
[0064] In one variation, the steel alloy blank may optionally be
annealed prior to the zinc-based coating being applied. The
zinc-based coating may be applied by conventional methods, such as
hot dip galvanizing of the blank. The zinc-based coating may be
applied by passing the blank through a zinc galvanization bath.
Continuous hot-dip galvanizing may be used to coat the steel alloy.
A degree of heating occurs for hot dipping to bring the steel alloy
to a zinc bath temperature. The coating is applied by passing the
uncoiled steel alloy through the zinc galvanizing bath held above
about 420.degree. C., optionally at a temperature of greater than
or equal to about 420.degree. C. up to about 480.degree. C.,
followed by cooling to solidify the zinc into a surface coating.
Continuous hot-dip galvanizing provides a relatively pure
zinc-based coating with high cathodic corrosion resistance.
Alternatively, aluminum may be added to the zinc galvanizing bath,
promoting formation of a layer that prevents extensive diffusion
between the zinc and iron. In other aspects, the zinc-based coating
may include iron or other constituents. In certain aspects, a
zinc-based coating comprises zinc or an alloy of zinc, where the
coating predominantly comprises zinc at greater than about 90%. A
zinc-based coating may be applied on one or both exposed sides of
the blank. In certain aspects, the process may further include
galvannealing the decarburized steel alloy. The steel alloy may be
galvannealed following the hot-dipped galvanizing by heating the
hot-dipped galvanized steel alloy to greater than or equal to about
500.degree. C. to less than or equal to about 565.degree. C. and
holding for a few seconds.
[0065] Example embodiments will now be described more fully with
reference to the accompanying drawings.
[0066] FIG. 1 shows a cross-sectional view of a sheet blank 50 that
may be formed from a metal stock or coil in a blanking operation,
for example, by cutting. The sheet blank 50 includes a main body 52
formed of a high-strength steel like the compositions previously
discussed above. After decarburization, a first decarburized layer
54 may be formed on a first side 56 of the main body 52, while a
second decarburized layer 58 may be formed on a second opposite
side 60 of the main body 52.
[0067] Prior to the decarburizing step, the sheet blank 50 formed
of high-strength steel may have a microstructure comprising
predominantly pearlite and ferrite, for example, cumulatively
greater than or equal to about 95 volume % of these phases,
optionally greater than or equal to about 96 volume %, optionally
greater than or equal to about 97 volume %, optionally greater than
or equal to about 98 volume %, optionally greater than or equal to
about 99 volume % and up to about 100 volume % of ferrite and
austenite. The decarburizing step that forms the first decarburized
layer 54 and the second decarburized layer 58 removes carbon and
thus changes the microstructure within the decarburized surface
region.
[0068] After heating and press hardening, but prior to a quenching
process, the central region or main body 52 comprises greater than
or equal to about 95 volume % austenite, optionally greater than or
equal to about 96 volume % austenite, optionally greater than or
equal to about 97 volume % austenite, optionally greater than or
equal to about 98 volume % austenite, optionally greater than or
equal to about 99 volume % austenite and up to about 100 volume %
austenite. When the core or main body 52 of the blank 50 is fully
austenitized, the surface will have some ferrite phases due to its
lower carbon level. Thus, each decarburized surface layer may
comprise greater than or equal to about 80 volume % ferrite or
optionally greater than or equal to about 90 volume % ferrite.
After quenching, the central region or main body 52 comprises
greater than or equal to about 95 volume % martensite, optionally
greater than or equal to about 96 volume % martensite, optionally
greater than or equal to about 97 volume % martensite, optionally
greater than or equal to about 98 volume % martensite, optionally
greater than or equal to about 99 volume % martensite and up to
about 100 volume % martensite. However, the levels of martensite in
the decarburized surface layers are minimized. In this manner, the
sheet blank 50 is capable of maintaining its high strength
properties in the bulk body with highly retained levels of
austenite that transform to martensite, while removing reactive
carbon from the surface regions on which a zinc-based coating will
be applied.
[0069] Zinc has a melting temperature of 420.degree. C. and, at
782.degree. C., begins to react with iron via a eutectoid reaction
and forms a brittle phase that results in liquid metal
embrittlement (LME). Where temperatures are favorable (e.g., above
782.degree. C. for certain high-strength steels) and the zinc is a
liquid metal, during deformation processes, the zinc can wet
freshly exposed grain boundaries (of the phase in the substrate)
and cause de-cohesion/separation along the grain boundary. The zinc
thus attacks grain boundaries, especially where austenite is
present, which can undesirably form cracks associated with LME. As
such, three factors together lead to LME, namely tensile stress,
liquid zinc, and grain boundary area. In removing the relatively
high levels of carbon in the surface regions after decarburizing,
the microstructure transforms to having low levels of austenite so
that the sheet blank 50 has a significantly reduced grain boundary
area and reduced grain boundary energy. In this manner, the sheet
blank 50 can be galvanized and later heated to relatively high
temperatures during press hardening, while avoiding LME.
[0070] As shown in FIG. 1, a first zinc-based coating 62 is applied
over the first decarburized layer 54 on the first side 56. A second
zinc-based coating 64 is applied over the second decarburized layer
58 on the second side 60. The first zinc-based coating 62 and the
second zinc-based coating 64 comprise zinc; for example, such
coatings may be zinc or an alloy of zinc and thus predominantly
comprise zinc at greater than about 90%. It should be appreciated,
however, that the composition of the first zinc-based coating 62
and the second zinc-based coating 64 is not limited to comprising
zinc, but may further include additional elements as discussed
previously above. The sheet blank 50 thus undergoes the hot forming
process to provide a three-dimensionally formed component.
[0071] During hot forming, the sheet blank may be heated, for
example, by being introduced into a furnace or other heat source.
The amount of heat applied to the sheet blank heats and soaks the
sheet blank to a temperature of at least the austenitization
temperature. In certain aspects, the high-strength steel has an
austenitization temperature (T.sub.1) of greater than or equal to
about 800.degree. C. to less than or equal to about 950.degree. C.
In this manner, pearlite can be transformed to austenite while the
high-strength steel can be hot formed/stamped. In one aspect, the
heating for austenitization occurs at a temperature of greater than
or equal to about 800.degree. C. for a specified time to fully
austenitize the steel through its thickness. The sheet blank is
soaked for a period long enough to austenitize the high-strength
steel to a desired level. However, due to the decarburized surface
layer(s), despite heating the blank to temperatures in excess of
800.degree. C., LME described above can be significantly reduced or
eliminated. Full austenitization of the core thus may occur, while
the surface is only partially austenitized due to lower initial
pearlite or carbon content. As such, an increased zinc
concentration on the hot formed component results in improved
corrosion protection, while maintaining strength.
[0072] The high-strength steel alloy can then be hot formed (e.g.,
stamped in a die) and then cooled, for example, by a quenching
process having a high rate of cooling to facilitate transformation
of the austenite formed during heating to martensite. Thus, after
exiting the furnace, the sheet blank can be transferred into a
stamping press. The stamping press may include a die having a
cooling system or mechanism. For example, the die(s) may have a
liquid-cooling system, which are well known in the art. The die is
designed to form a desired final three-dimensional shape of the
component from the austenitized sheet blank. The die may include a
first forming die and a second forming die that are brought
together to form the desired final shape of the three-dimensional
component therebetween.
[0073] The cooled dies thus may quench the formed sheet blank in a
controlled manner across surfaces of the formed component to cause
a phase transformation from austenite to martensite (e.g., within
the central body region shown as main body 52 in FIG. 1).
Therefore, the first and second die may cooperate to function as a
heat sink to draw heat from, and otherwise quench, the formed
component. In certain aspects, the press-hardened component is
quenched to a temperature at or below a martensite finish
temperature and allowed to cool in air to room temperature (e.g.,
21.degree. C.) after press hardening.
[0074] Referring to FIG. 2, a flowchart showing the steps of a
press-hardening process 100 is shown, where the decarburizing is
conducted continuously. For brevity, the process conditions and
steps described previously above will not be repeated herein. A
rolled coil 110 of a steel alloy is unwound and annealed in an
annealing chamber 120 or furnace at a temperature of at least about
680.degree. C. The annealed steel alloy is then passed into a
decarburizing chamber 122 having a decarburizing environment. The
annealing chamber 120 and decarburizing chamber 122 may be
partitioned parts or chambers of the same furnace or may be
distinct furnaces. The steel alloy thus has a carburized surface
layer formed on at least one exposed surface. An optional cleaning
process (not shown) may be used to remove surface residue and/or
oxides from the carburized surface layer on the steel alloy.
However, a cleaning process can be omitted where the decarbonizing
atmosphere is not oxidizing to iron. Thus, as shown in FIG. 2, the
steel alloy passes from the decarburizing chamber 122 directly into
a zinc galvanizing bath 130. The zinc galvanizing bath 130 may be a
hot-dipping process.
[0075] In certain variations, while not shown, a galvannealing
furnace (e.g., induction furnace) may be used after the zinc
galvanizing bath 130 for galvannealing the galvanic coating.
Notably, annealing (e.g., as shown in the annealing furnace 120)
prior to hot-dip galvanizing in the zinc galvanizing bath 130 is
not required.
[0076] An annealed, decarburized, hot-dipped galvanized steel alloy
coil 140 is thus collected. The steel alloy may then be optionally
coiled again into coil 140 for easier transportability. When
coiled, the steel alloy coil 140 is then subsequently uncoiled and
sheared to form a blank 150 by shearing sections of the steel
alloy. The blank 150 may be sheared with trim dies, alligator
shears, bench shears, guillotines, power shears, throatless shears,
or the like.
[0077] The preformed blank 150 is then heated in an oven 170 (e.g.,
the heating may austenitize the blank, so that the oven has a
temperature of about 800-950.degree. C. for a predetermined period,
such as about 300 to about 1,000 seconds). For example,
representative conditions could be heating in an oven at about
850.degree. C. for about 400 seconds, heating at about 930.degree.
C. for about 360 seconds, or heating at about 950.degree. C. for
about 300 seconds. The preformed blank is then press hardened
between dies 180 and 190 to form and simultaneously quench the PHS
component 195. The PHS component 195 may be quenched in the dies
180 and 190, for example, quenched at an exemplary rate of more
than 27.degree. C./s to transform the austenite into martensite.
However, the quench rates may be greater or less depending on the
specific alloy composition.
[0078] The zinc-based coating protects PHS component 195 from
oxidation that would otherwise occur between the austenitizing and
press-hardening steps. There is, therefore, little to no need for
surface cleaning of PHS component 195 after the press hardening. If
necessary, the PHS component 195 may then then cleaned, for
example, with shot blasting 200 or acids, to remove scale.
[0079] Referring to FIG. 3, a flowchart showing the steps of a
press-hardening process 202 is shown where the decarburizing is
conducted in a batch process. For brevity, the process conditions
and steps described previously above will not be repeated herein. A
rolled coil 210 of a steel alloy is first placed in a decarburizing
chamber 220. A plurality of spacers 222 is disposed between
respective layers of the rolled alloy steel in the coil to ensure
exposure of the surfaces to the gases in the decarburizing chamber
220. Decarburizing gases may be introduced into the chamber and
exhaust gases exit as effluent during the batch operation. After
the batch carburization, the spacers 222 may be removed from the
rolled coil 210 of the steel alloy.
[0080] The rolled coil 210 is then unwound and annealed in an
annealing chamber 230 or furnace at a temperature of at least about
680.degree. C. It should be noted that the annealing process may be
conducted prior to the batch decarburization conducted in the
decarburizing chamber 220; however, such a step is optional. An
optional cleaning process (not shown) may be used to remove surface
residue and/or oxides from the carburized surface layer on the
steel alloy. However, a cleaning process can be omitted where the
decarbonizing atmosphere is not oxidizing to iron. Thus, the steel
alloy passes from the annealing chamber 230 into a zinc galvanizing
bath 232. The zinc galvanizing bath 232 may be a hot-dipping
process.
[0081] In certain variations, while not shown, a galvannealing
furnace (e.g., induction furnace) may be used after the zinc
galvanizing bath 232 for galvannealing the galvanic coating.
Notably, annealing (e.g., as shown in the annealing furnace 230)
prior to hot-dip galvanizing in the zinc galvanizing bath 230 is
not required. In such a variation, the decarburized steel on the
rolled coil 210 may be unrolled and passed directly to a zinc
galvanizing bath 232.
[0082] As shown in FIG. 3, a decarburized, hot-dipped galvanized
steel alloy coil 240 is thus collected. The steel alloy may then be
optionally coiled again into coil 240 for easier transportability.
When coiled, the steel alloy coil 240 is then subsequently uncoiled
and sheared to form a blank 250 by shearing sections of the steel
alloy. The blank 250 may be sheared with trim dies, alligator
shears, bench shears, guillotines, power shears, throatless shears,
or the like.
[0083] The preformed blank 250 is then heated in an oven 270 (e.g.,
the heating may austenitize the blank, so that the oven has a
temperature of about 950.degree. C. for a predetermined period,
such as about 300 seconds or any of the representative temperatures
and times discussed previously above). The preformed blank is then
press hardened between dies 280 and 290 to form and simultaneously
quench the PHS component 295. The PHS component 295 may be quenched
in the dies 280 and 290, for example, quenched at a non-limiting
rate of more than 27.degree. C./s to transform the austenite into
martensite. The quenching rate may be more or less depending on the
specific alloy composition.
[0084] The zinc-based coating protects PHS component 295 from
oxidation that would otherwise occur during or between the
austenitizing and press-hardening steps. There is, therefore,
little to no need for surface cleaning of PHS component 295 after
the press hardening. If necessary, the PHS component 295 may then
then cleaned, for example, with shot blasting 300 or acid
treatment, to remove scale.
[0085] A method of press-hardening a high-strength steel alloy is
thus provided that comprises creating a high-strength steel alloy
blank having a zinc-coated decarburized surface layer. The
decarburized surface layer has a thickness of less than or equal to
about 50 micrometers and comprises greater than or equal to about
80 volume % ferrite. The blank is then heated, press hardened, and
quenched in a die to form a press-hardened component with an
ultimate tensile strength of greater than or equal to about 1,100
MPa that is substantially free of liquid metal embrittlement.
Before the heating and press hardening, a central body region
predominantly comprises pearlite and ferrite, for example, at
greater than or equal to about 95 volume % cumulatively. During the
heating process, pearlite is transformed to austenite. The method
further comprises quenching the blank after the heating and press
hardening so that austenite is transformed to martensite in the
central region. The central region may comprises greater than or
equal to about 95 volume % martensite, optionally greater than or
equal to about 98 volume % martensite. Notably, after the
press-hardening and quenching, the decarburized surface layer may
have less than or equal to about 20 volume % martensite, optionally
less than or equal to about 10 volume % martensite, optionally less
than or equal to about 5 volume % martensite, and in certain
variations, optionally less than or equal to about 1 volume %
martensite.
[0086] In another variation, a press-hardened high-strength steel
component is thus provided. After heating to austenitize,
press-hardening, and quenching, the press-hardened high-strength
steel component has a decarburized surface layer with a thickness
of less than or equal to about 50 micrometers. The decarburized
surface layer may have greater than or equal to about 80 volume %
ferrite and less than or equal to about 20 volume % martensite. The
central region includes greater than or equal to about 90 volume %
martensite, optionally greater than or equal to about 95 volume %
martensite, optionally greater than or equal to about 98 volume %
martensite, after heating, press hardening, and quenching. The
press-hardened component has an ultimate tensile strength of
greater than or equal to about 1,000 MPa or any of the strengths
discussed previously above. Further, the press-hardened component
is substantially free of liquid metal embrittlement.
[0087] The mechanical performance of the hot stamped component is
significantly improved. For example, after processing, the
decarburized surface layer forms a soft decarburization zone that
can significantly enhance sheet bendability, which can provide
enhanced crash performance for vehicle. For example, structural
pillars, like a B-pillar, should have extreme strength in certain
sections, but a balance of strength, ductility, and bendability in
other sections. The combination of these different properties
promotes buckling at a desired location when a force or impact is
applied to the B-pillar, which may correspond to seat level within
the interior of the vehicle to protect the occupant(s) after the
force or impact is applied.
[0088] Thus, press-hardened components formed according to the
methods of the present disclosure provide a greater bending angle
that enhances bendability. The bending angle is dependent on
thickness. In certain aspects, a blank formed according to certain
aspects of the present disclosure that has the soft decarburization
surface zone can have a bending angle that is at least about 1.2
times greater than a bending angle of a comparative blank, such as
a blank formed from the same processes with the same conditions,
but lacking a decarburizing step. In certain variations, depending
on the blank thickness, a bending angle may optionally be at least
about 1.5 times greater of a bending angle, optionally at least
about 1.6 times greater of a bending angle, optionally at least
about 1.7 times greater of a bending angle, optionally at least
about 1.8 times greater of a bending angle, optionally at least
about 1.9 times greater of a bending angle, and in certain
variations, at least about 2 times greater of a bending angle of a
comparative blank formed from the same process and conditions
except for the decarburizing process. In one variation, the bending
angle of the press-hardened component formed according to certain
aspects of the present disclosure may be greater than or equal to
about 85.degree., optionally greater than or equal to about
90.degree. and in certain variations, optionally greater than or
equal to about 93.degree.. Such a bending angle can be measured by
the Verband der Automobilindustrie (VDA) 238-100 test procedure
"Plate Bending Test for Metallic Materials."
[0089] In various aspects, the present disclosure significantly
reduces or eliminates the liquid metal embrittlement which tends to
happen when hot forming a zinc-coated press hardenable steel blank
by using a controlled decarburization process (prior to the hot
forming process) to create a thin surface layer having reduced
carbon content. While decarburization is typically undesirable for
use with high-strength steel, by using a controlled process, a thin
decarburized surface layer is formed having a larger grain size and
reduced grain boundary energy. The larger grain size results in
less grain boundary area for liquid metal embrittlement to occur
during the hot stamping process and the reduced carbon results in
reduced boundary energy that limits zinc intrusion. Additionally,
lower carbon content in the surface layer reduces transformational
stresses associated with martensite formation from austenite. In
this manner, an in-situ "layered" steel microstructure
(decarburized zone/steel core) helps to prevent LME.
[0090] The foregoing description of the embodiments has been
provided for purposes of illustration and description. It is not
intended to be exhaustive or to limit the disclosure. Individual
elements or features of a particular embodiment are generally not
limited to that particular embodiment, but, where applicable, are
interchangeable and can be used in a selected embodiment, even if
not specifically shown or described. The same may also be varied in
many ways. Such variations are not to be regarded as a departure
from the disclosure, and all such modifications are intended to be
included within the scope of the disclosure.
* * * * *