US20160222483A1

US20160222483A1 - Method for manufacturing press-molded article, and press-molded article

Info

Publication number: US20160222483A1
Application number: US14/917,845
Authority: US
Inventors: Toshio Murakami; Junya Naitou; Keisuke Okita; Shushi Ikeda
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2013-09-10
Filing date: 2013-09-10
Publication date: 2016-08-04
Also published as: CA2923582A1; KR101716624B1; WO2015037059A1; MX2016003258A; CN105518162A; RU2633416C1; CA2923582C; EP3045550A4; CN105518162B; KR20160042070A; EP3045550A1

Abstract

A method for manufacturing a press-formed article includes: heating a specific steel sheet for hot-pressing at 900° C. or more and 1,100° C. or less; starting press forming of the steel sheet which has been heated; and then, cooling the steel sheet to a temperature equal to or less than a temperature 100° C. below a bainite transformation starting temperature Bs and equal to or more than a martensite transformation starting temperature Ms, while ensuring an average cooling rate of 20° C./sec or more in a mold during forming and after the completion of forming; and further cooling the steel sheet, which has been cooled, to 200° C. or less at an average cooling rate of less than 20° C./sec.

Description

TECHNICAL FIELD

The present invention relates to a press-formed article to be used when manufacturing an automotive structural component, and a method for manufacturing such a press-formed article. More specifically, the present invention relates to a press-formed article manufactured by applying, when forming a previously heated steel sheet (blank) into a predetermined shape, a press forming method of imparting a shape together with applying a heat treatment to obtain a predetermined strength, and a method useful for the manufacture of such a press-formed article.

BACKGROUND ART

As one of the measures for automotive fuel economy improvement triggered by global environmental problems, weight saving of a vehicle body is proceeding, and in turn, the strength of a steel sheet used for automobiles must be increased as much as possible. On the other hand, when the strength of a steel sheet is increased, the shape accuracy during press forming decreases.
For this reason, a component is manufactured by employing a hot-press forming method where a steel sheet is heated to a given temperature (e.g., a temperature for forming an austenite phase) to lower the strength and then formed with a mold at a temperature (e.g., room temperature) lower than that of the steel sheet to impart a shape and, perform rapid-cooling heat treatment (quenching) by making use of a temperature difference therebetween so as to ensure the strength after forming. Such a hot-press forming method is referred to by various names such as hot forming method, hot stamping method, hot stamp method and die quenching, method, in addition to hot-pressing method.
FIG. 1 is a schematic explanatory view showing the mold configuration for carrying out the above-described hot-press forming. In FIG. 1, 1 is a punch, 2 is a die, 3 is a blank holder, 4 is a steel sheet (blank), BHF is a blank holding force, rp is a punch shoulder radius, rd is a die shoulder radius, and CL is a punch-to-die clearance, Of these parts, the punch 1 and the die 2 are configured such that passages 1 a and 2 a allowing for passing of a cooling medium (e.g., water) are formed in respective insides and the members are cooled by passing a cooling medium through the passage.
When hot-press forming (for example, hot deep drawing) is performed using such a mold, the forming is started in a state where the steel sheet (blank) 4 is softened by heating at a two-phase zone temperature of (Ac₁transformation point to Ac₃transformation point) or a single-phase zone temperature equal to or more than Ac₃transformation point. More specifically, in the state of the steel sheet 4 at a high temperature being sandwiched between the die 2 and the blank holder 3, the steel sheet 4 is pushed into a hole of the die 2 (between 2 and 2 in FIG. 1) by the punch 1 and formed into a shape corresponding to the outer shape of the punch 1 while reducing the outer diameter of the steel sheet 4. In addition, heat is removed from the steel sheet 4 to the mold (the punch and the die) by cooling the punch and the die in parallel with forming, and quenching of the material is carried out by further holding and cooling the steel sheet at the forming bottom dead center (the point when the punch head is positioned at the deepest part: the state shown in FIG. 1). By carrying out such a forming method, a formed article of 1500 MPa class can be obtained with high dimensional accuracy and moreover, the forming load can be reduced as compared with a case of forming a component of the same strength class by cold working, so that the volume required of the pressing machine can be small.
As the steel sheet for hot-pressing which is widely used at present, a steel sheet using 22MnB5 steel as the material is known. This steel sheet has a tensile strength of 1,500 MPa and an elongation of approximately from 6 to 8% and is applied to an impact-resistant member (a member that undergoes as little a deformation as possible at the time of collision and is not fractured). However, its application to a component requiring a deformation, such as energy-absorbing member, is difficult because of low elongation (ductility).
As the steel sheet for hot-pressing which exerts good elongation, the techniques of for example, Patent Documents 1 to 4 have also been proposed. In these techniques, the carbon content in the steel sheet is set in various ranges to adjust the fundamental strength class of respective steel sheets, and the elongation is enhanced by introducing a ferrite having high deformability and reducing the average particle diameters of ferrite and martensite. The techniques above are effective in enhancing the elongation but in view of elongation enhancement according to the strength of the steel sheet, it is still insufficient. For example, the elongation EL of a steel sheet having a tensile strength TS of 1,270 MPa or more is about 12.7% at the maximum, and further improvement is demanded.
On the other hand, an automotive component needs to be joined mainly by spot welding, but in a hot-stamped formed article having, a microstructure mainly including martensite, it is known that strength in the weld heat affected zone (HAZ) is reduced significantly and the welded joint is subject to a strength reduction (softening) (for example, Non-Patent Document 1).

RELATED ART

Patent Document

Patent Document 1: JP-A-2010-65292
Patent Document 2: JP-A-2010-65293
Patent Document 3: JP-A-2010-65294
Patent Document 4: JP-A-2010-65295

Non-Patent Document

Non-Patent Document 1: Hirosue et al. “Nippon Steel Technical Report”, No. 378, pp. 15-20 (2003)

SUMMARY OF THE INVENTION

Problems that the Invention is to Solve

The present invention has been made under these circumstances, and an object thereof is to provide: a method useful for manufacturing a press-formed article which is capable of achieving a high-level balance between high strength and elongation and has good anti-softening property in HAZ; and a press-formed article which exerts the above properties.

Means for Solving the Problems

In the method for manufacturing a press-formed article in the present invention, which can attain the object above, a steel sheet for hot-pressing is heated at 900° C. or more and 1,100° C. or less, the steel sheet for hot-pressing including:
C: from 0.15 to 0.5% (mass %; hereinafter, the same applies to the chemical component composition),
Si: from 0.2 to 3%,
Mn: from 0.5 to 3%,
P: 0.05% or less (exclusive of 0%),
S: 0.05% or less (exclusive of 0%),
Al: from 0.01 to 1%,
B: from 0.0002 to 0.01%,
Ti: equal to or more than 3.4[N]+0.01% and equal to or less than 3.4[N]+0.1% [wherein [N] indicates a content (mass %) of N], and
N: from 0.001 to 0.01%, with the remainder being iron and unavoidable impurities, in which an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet is 6 nm or less, and a precipitated Ti amount and a total Ti amount in a steel satisfy the following formula (1),
and thereafter press forming is started and the steel sheet is cooled to a temperature equal to or less than a temperature 100° C. below a bainite transformation starting temperature Bs and equal to or more than a martensite transformation starting temperature Ms, while ensuring an average cooling rate of 20° C./sec or more in a mold during forming and after the completion of forming, and thereafter the steel sheet is cooled to 200° C. or less at an average cooling rate of less than 20° C./sec. Here, the “equivalent-circle diameter” is the diameter of a circle having the same area as the size (area) of a Ti-containing precipitate (e.g., TiC) when the precipitate is converted to a circle (“the average equivalent-circle diameter” is the average value thereof).
Precipitated Ti amount (mass %)−3.4[N]<0.5×[total Ti amount (mass %)−3.4[N]] (1)
(in the formula (1), [N] indicates the content (mass %) of N in the steel).
In the steel sheet for hot-pressing to be used in the manufacturing method in the present invention, it is also useful to contain, as the other element(s), at least one of the following (a) to (c), if desired. The properties of the press-formed article are further improved according to the kind of the element that is contained according to need.
(a) One or more kinds selected from the group consisting of V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in total
(b) One or more kinds selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less exclusive of 0%) in total
(c) One or more kinds selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in total
In the press-formed article obtained by this manufacturing method, the metal microstructure of the press-formed article includes bainitic ferrite: from 60 to 97 area %, martensite: 37 area % or less, retained austenite: from 3 to 20 area %, and remainder microstructure: 5 area % or less, the average equivalent-circle diameter of Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing, precipitates contained in the press-formed article is 10 nm or less, and a relationship of the formula (1) is satisfied, and thus, a high-level balance between high strength and elongation can be achieved as uniform properties in the formed article.

Advantage of the Invention

According to the present invention, a steel sheet where the chemical component composition is strictly specified, the size of the Ti-containing precipitate is controlled and the precipitation rate of Ti not forming TiN is controlled is used, so that by hot-pressing the steel sheet under predetermined conditions, the strength-elongation balance of the formed article can be made to be a high-level balance and the anti-softening property in HAZ is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A schematic explanatory view showing the mold configuration for carrying out hot-press forming.

MODE FOR CARRYING OUT THE INVENTION

The present inventors have made studies from various aspects to realize a press-formed article which ensures that, in the manufacture of a press-formed article by heating a steel sheet at a predetermined temperature and then hot-press forming the steel sheet, a press-formed article exhibiting good ductility (elongation) is obtained while assuring high strength after press forming.
As a result, it has been found that when the chemical component composition of the steel sheet for hot-pressing is strictly specified and the size of the Ti-containing precipitate as well as the precipitated Ti amount are controlled and when the steel sheet is hot-press formed under predetermined conditions, a predetermined amount of retained austenite is ensured after press forming and a press-formed article having increased intrinsic ductility (residual ductility) and good anti-softening property in HAX is obtained. The present invention has been accomplished based on this finding.
In the steel sheet for hot-pressing to be used in the present invention, the chemical component composition needs to be strictly specified, and the reason for limiting the range of each chemical component is as follows.
(C: from 0.15 to 0.5%)
C is an important element in lowering the bainite transformation starting temperature Bs to refine bainitic ferrite produced in the cooling process, and increasing the dislocation density in bainitic ferrite to enhance the strength. In addition, the amount of fine retained austenite formed between bainitic ferrite laths is increased, and a high-level balance between high strength and elongation can be ensured. If the C content is less than 0.15%, the bainite transformation starting temperature Bs elevates to bring about coarsening of bainitic ferrite and reduction in the dislocation density, and the strength of a hot press-formed article cannot be ensured. If the C content is too large and exceeds 0.5%, the strength is excessively high, and good ductility is not obtained. The lower limit of the C content is preferably 0.18% or more (more preferably 0.20% or more), and the upper limit is preferably 0.45% or less (more preferably 0.40% or less).
(Si: from 0.2 to 3%)
Si exerts an effect of suppressing cementite formation due to decomposition of retained austenite formed between bainitic ferrite laths during cooling of mold quenching, and forming retained austenite thereby. In order to exert such an effect, the Si content must be 0.2% or more. If the Si content is too large and exceeds 3%, ferrite is readily formed, making it difficult to produce a single phase of austenite during heating, and the fraction of a microstructure other than bainitic ferrite and retained austenite in the steel sheet for hot-pressing exceeds 5 area %. The lower limit of the Si content is preferably 0.5% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).
(Mn: from 0.5 to 3%)
Mn is an element effective in enhancing the quenchability and suppressing the formation of a soft microstructure such as ferrite and pearlite during cooling of mold quenching. In addition, this is an important element in lowering the bainite transformation starting temperature Bs to refine bainitic ferrite produced in the cooling process and increasing the dislocation density in bainitic ferrite to enhance the strength. Furthermore, this is an element capable of stabilizing austenite and is an element contributing to an increase in the retained austenite amount. In order to exert such effects, Mn must be contained in an amount of 0.5% or more. In the case of considering only the properties, the Mn content is preferably larger, but since the cost of alloying addition rises, the content is set to 3% or less. The lower limit of the Mn content is preferably 0.7% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).

(P: 0.05% or Less (Exclusive of 0%))

P is an element unavoidably contained in the steel but deteriorates the ductility and therefore, the P content is preferably reduced as much as possible. However, an extreme reduction causes an increase in the steelmaking cost, and it is difficult in Willis of manufacture to reduce the content to 0%. For this reason, the content thereof is set to 0.05% or less (exclusive of 0%). The upper limit of the P content is preferably 0.045% or less (more preferably 0.040% or less).

(S: 0.05% or Less (Exclusive of 0%))

S is an element unavoidably contained in the steel, as with P, and deteriorates the ductility and therefore, the S content is preferably reduced as much as possible. However, an extreme reduction causes an increase in the steelmaking cost, and it is difficult in terms of manufacture to reduce the content to 0%. For this reason, the content thereof is set to 0.05% or less (exclusive of 0%). The upper limit of the S content is preferably 0.045% or less (more preferably 0.040% or less).
(Al: from 0.01 to 1%)
Al is useful as a deoxidizing element and allows the solute N present in the steel to be fixed as AlN, which is useful in enhancing the ductility. In order to effectively exert such an effect, the Al content must be 0.01% or more. However, if the Al content is too large and exceeds 1%, Al₂O₃is excessively produced to deteriorate the ductility. The lower limit of the Al content is preferably 0.02% or more (more preferably 0.03% or more), and the upper limit is preferably 0.8% or less (more preferably 0.6% or less).
(B: from 0.0002 to 0.01%)
B is an element having an action of suppressing ferrite transformation and pearlite transformation, and therefore, contributes to preventing the formation of ferrite, pearlite and bainite during cooling after heating at a two-phase zone temperature of (Ac₁transformation point to Ac₃transformation point), and ensuring retained austenite. In order to exert such effects, B must be contained in an amount of 0.0002% or more, but even when this element is contained excessively over 0.01%, the effects are saturated. The lower limit of the B content is preferably 0.0003% or more (more preferably 0.0005% or more), and the upper limit is preferably 0.008% or less (more preferably 0.005% or less).
(Ti: Equal to or More than 3.4[N]+0.01% and Equal to or Less than 3.4[N]+0.1%: [N] is the Content (mass %) of N)
Ti exerts an effect of improving the quenchability by fixing N and maintaining B in a solid solution state. In order to exert such an effect, it is important to contain this element in an amount larger than the stoichiometric ratio of Ti and N (3.4 times the N content) by 0.01% or more. In addition, when Ti added excessively relative to N is caused to be present in a solid solution state in a hot-stamp formed article and the precipitated compound is finely dispersed, the strength reduction in HAZ can be suppressed by virtue of precipitation strengthening due to formation, as TiC, of Ti dissolved in solid during welding of the hot-stamp formed article or by virtue of an effect such as delaying increase of the dislocation density due to the dislocation movement-preventing effect of TiC. However, if the Ti content is too large and exceeds 3.4[N]+0.1%, the Ti-containing precipitate (e.g., TiN) formed is coarsened to deteriorate the ductility of the steel sheet. The lower limit of the Ti content is more preferably 3.4[N]+0.02% or more (further preferably 3.4[N]+0.05% or more), and the upper limit is more preferably 3.4[N]+0.09% or less (further preferably 3.4[N]+0.08% or less).
(N: from 0.001 to 0.01%)
N decrease the improvement effect of the hardenability during quenching by fixing B as BN, and thus, the content thereof is preferably reduced as much as possible, but the reduction in an actual process is limited and therefore, the lower limit is set to 0.001%. If the N content is too large, the Ti-containing precipitate (e.g., TiN) formed is coarsened, and this precipitate works as a fracture origin to deteriorate the ductility of the steel sheet. For this reason, the upper limit is set to 0.01%. The upper limit of the N content is preferably 0.008% or less (more preferably 0.006% or less).
The basic chemical components in the steel sheet for hot-pressing to be used in the present invention are as described above, and the remainder is iron and unavoidable impurities (e.g., O, H) other than P, S and N. In the steel sheet for hot-pressing to be used in the present invention, it is also useful to further contain, as the other element(s), at least one of the following (a) to (c), if desired. The properties of press-formed article are further improved according to the kind of the element that is contained according to need. In the case of containing such an element, the preferable range and the reason for limitation on the range are as follows.
(a) One or more kinds selected from the group consisting of V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in total
(b) One or more kinds selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (exclusive of 0%) in total
(c) One or more kinds selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in total
(One or More Kinds Selected from the Group Consisting of V, Nb and Zr, in an Amount of 0.1% or Less (Exclusive of 0%) in Total)
V, Nb and Zr have an effect of forming fine carbide and refining the microstructure by a pinning effect. In order to exert such an effect, these elements are preferably contained in an amount of 0.001% or more in total. However, if the content of these elements is too large, coarse carbide is formed and works out to a fracture origin to conversely deteriorate the ductility. For this reason, the content of these elements is preferably 0.1% or less in total. The lower limit of the content of these elements is more preferably 0.005% or more (still more preferably 0.008% or more) in total, and the upper limit is more preferably 0.08% or less (still more preferably 0.06% or less) in total.
(One or More Kinds Selected from the Group Consisting of Cu, Ni, Cr and Mo: 1% or Less (Exclusive of 0%) in Total)
Cu Ni, Cr and Mo suppress ferrite transformation and pearlite transformation, and therefore, effectively act to prevent the formation of ferrite and perlite during cooling after heating and ensure retained austenite. In order to exert such an effect, these are preferably contained in an amount of 0.01% or more in total. In the case of considering only the properties, the content is preferably larger, but since the cost of alloying addition rises, the content is preferably 1% or less in total. In addition, these elements have an action of greatly increasing the strength of austenite and put a large load on hot rolling, making it difficult to manufacture a steel sheet. Therefore, also from the standpoint of manufacturability, the content is preferably 1% or less. The lower limit of the content of these elements is more preferably 0.05% or more (still more preferably 0.06% or more) in total, and the upper limit is more preferably 0.5% or less (still more preferably 0.3% or less) in total.
(One or More Kinds Selected from the Group Consisting of Mg, Ca and REM, in an Amount of 0.01% or Less (Exclusive of 0%) in Total)
These elements refine the inclusion and therefore, effectively act to enhance the ductility. In order to exert such an effect, these elements are preferably contained in an amount of 0.0001% or more in total. In the case of considering only the properties, the content is preferably larger, but since the effect is saturated, the content is preferably 0.01% or less in total. The lower limit of the content of these elements is more preferably 0.0002% or more (still more preferably 0.0005% or more) in total, and the upper limit is more preferably 0,005% or less (still more preferably 0.003% or less) in total.
In the steel sheet for hot-pressing to be used in the present invention, (A) the average equivalent-circle diameter of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet is 6 nm or less, and (B) the relationship of “precipitated Ti amount (mass %)−3.4[N]<0.5×[total Ti amount (mass %)−3.4[N]]” (the relationship of the formula (1)) is satisfied, are also important requirements.
The Ti-containing precipitate and formula (1) is controlled for preventing softening of HAZ and such a control is originally a control required of a formed article, but these values are little changed between before and after hot-press forming. Therefore, the control needs to be already done at the stage before forming (the steel sheet for hot-pressing). When excessive Ti relative to N in the steel sheet before forming is cause to be present in a solid solution state or refined state, the Ti-containing precipitate can be maintained in a solid solution state or refined state during heating of hot pressing. As a result, the amount of Ti precipitated in the press-formed article can be controlled to not more than a predetermined amount, and softening in HAZ can be prevented, whereby the joint properties can be improved.
From such a standpoint, Ti-containing precipitates needs to be finely dispersed and to this end, the average equivalent-circle diameter of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet must be 6 nm or less (requirement of (A) above). The size (average equivalent-circle diameter) of the Ti-containing precipitate is preferably 5 nm or less, more preferably 3 nm or less. Examples of the Ti-containing precipitate targeted in the present invention include TiC, TiN and other Ti-containing precipitates such as TiVC, TiNbC, TiVCN and TiNbCN.
As described later, the average equivalent-circle diameter of Ti-containing precipitates in the press-formed article is specified to be 10 nm or less, whereas that before forming (steel sheet for hot-pressing) is specified to be 6 nm or less. The reason why the size of the precipitate is specified to be larger in the formed article than in the steel sheet is that Ti is present as a fine precipitate or in a solid solution state in the steel sheet and when heated at near 800° C. for 15 minutes or more, the Ti-containing precipitate is slightly coarsened. In order to ensure the properties as a formed article, the average equivalent-circle diameter of Ti-containing precipitates must be 10 nm or less, and for realizing this precipitation state in a hot-stamp formed article, it is necessary that in the state of the steel sheet for hot-stamping, the average equivalent-circle diameter of fine precipitates of 30 nm or less is adjusted to 6 nm or less and many of Ti is caused to be present in a solid solution state.
In addition, in the steel sheet for hot-pressing, the majority of Ti except for Ti to be used for precipitating and fixing N must be caused to be present in a solid solution state or refined state. To this end, the amount of Ti present as a precipitate other than TiN (i.e., precipitated Ti amount−3.4[N]) needs to be an amount smaller than 0.5 times the remainder after deduction of Ti that forms TiN from total Ti (i.e., 0.5×[(total Ti amount)−3.4[N]]) (requirement of (B) above). The “precipitated Ti amount−3.4[N]” is preferably 0.4×[(total Ti amount)−3.4[N]] or less, more preferably 0.3×[(total Ti amount)−3.4[N]] or less.
For manufacturing the above steel sheet (steel sheet for hot-pressing), a slab prepared by melting a steel material having the above-described chemical component composition may be hot-rolled at a heating temperature: 1,100° C. or more (preferably 1,150° C. or more) and 1,300° C. or less (preferably 1,250° C. or less) and a finish rolling temperature of 850° C. or more (preferably 900° C. or more) and 1,000° C. or less (preferably 950° C. or less), and immediately after that, it may be cooled (rapid cooling) at an average cooling rate of 20° C./sec or more(preferably 30° C./sec or more) until 500° C. or less (preferably 450° C. or less) and after that, it may be wound at a temperature of 350° C. or more (preferably 380° C. or more) and 450° C. or less (preferably 430° C. or less).
In the method above, (1) rolling is terminated in a temperature region where a dislocation introduced into austenite by hot rolling remains, (2) rapid cooling is performed immediately thereafter to allow a Ti-containing precipitate such as TiC to be finely formed on the dislocation, and (3) rapid cooling is further performed, followed by winding, whereby bainite transformation or martensite transformation is controlled to occur.
The steel sheet for hot-pressing which has the above-described chemical component composition and Ti-precipitation state may be directly used for the manufacture by hot pressing or may be subjected to cold rolling at a rolling reduction of 10 to 80% (preferably from 20 to 70%) after pickling and then used for the manufacture by hot pressing. The steel sheet for hot-pressing or a cold rolled material thereof may be subjected to a heat treatment including heating at 830° C. or more (preferably 850° C. or more and 900° C. or less), then rapid cooling at a cooling rate of 20° C./sec or more (preferably 30° C./sec or more) until 500° C. or less (preferably 450° C. or less), and then holding at 500° C. or less for 10 seconds or more and 1,000 seconds or less, or tempering at a temperature of 500° C. or less. In addition, the surface of the steel sheet for hot-pressing (the surface of the base steel sheet) in the present invention may be subjected to plating containing one or more kinds of Al, Zn, Mg and Si.
Using the above-described steel sheet for hot-pressing, the steel sheet is heated at a temperature of 900° C. or more and 1,100° C. or less, and after press forming is started, the steel sheet is cooled to a temperature equal to or less than a temperature 100° C. below the bainite transformation starting temperature Bs (Bs-100° C.) and equal to or more than the martensite transformation starting temperature Ms, while ensuring an average cooling rate of 20° C./sec or more in a mold during forming as well as after the completion of forming, and then cooled to 200° C. or less at an average cooling rate of less than 20° C./sec, whereby an optimal microstructure as a formed article with predetermined strength and high ductility (microstructure mainly including bainitic ferrite) can be produced in a press-formed article having a single property. The reason for specifying each requirement in this forming method is as follows.
If the heating temperature of the steel sheet is less than 900° C., a sufficient amount of austenite cannot be obtained during heating, and the martensite fraction is too large in the final microstructure (microstructure of a formed article). If the heating temperature of the steel sheet exceeds 1,100° C., the austenite grain size grows during heating, the martensite transformation starting temperature Ms and martensite transformation finishing temperature Mf are elevated, retained austenite cannot be ensured during quenching, and good formability is not achieved. The heating temperature is preferably 950 or more and 1,050° C. or less. At this time, if the heating time is too long, the Ti-containing precipitate in the steel sheet can be hardly refined, and a Ti-containing, precipitate even in a small amount is formed during heating and coarsened to reduce the effect of improving weldability. For this reason, the heating time is preferably shorter. The heating time is preferably 3,600 seconds or less, and more preferably 20 seconds or less.
For allowing austenite formed in the heating step above to be a desired microstructure (microstructure mainly including bainitic ferrite) while impeding production of a microstructure such as ferrite or pearlite, the average cooling rate during forming as well as after forming and the cooling finishing temperature must be appropriately controlled. From such a standpoint, it is necessary that the average cooling rate during forming is 20° C./sec or more and the cooling finishing temperature is equal to or less than a temperature 100° C. below the bainite transformation starting temperature Bs and equal to or more than martensite transformation starting temperature Ms. The average cooling rate during forming, is preferably 30° C./sec or more (more preferably 40° C./sec or more). When the cooling finishing temperature is equal to or less than a temperature 100° C. below the bainite transformation starting, temperature Bs, austenite present during heating is transformed to bainite while impeding production of a microstructure such as ferrite or pearlite, whereby fine austenite is caused to remain between bainitic ferrite laths and a predetermined amount of retained austenite is assured while ensuring the amount of bainitic ferrite.
If the cooling finishing temperature exceeds the temperature 100° C. below the bainite transformation starting temperature Bs or the average cooling rate is less than 20° C./sec, a microstructure such as ferrite and pearlite is formed, and a predetermined amount of retained austenite cannot be ensured, resulting in deterioration of the elongation (ductility) in a formed article. When the cooling is performed to a temperature less than the martensite transformation starting temperature Ms, the production amount of martensite is increased and the elongation (ductility) of the formed article is deteriorated.
After reaching a temperature equal to or less than a temperature 100° C. below the bainite transformation starting temperature Bs and equal to or more than the martensite transformation starting temperature Ms, rapid cooling is stopped, and cooling to 200° C. or less is performed at an average cooling rate of less than 20° C./sec. By adding such a cooling step, bainitic ferrite transformation is promoted. If the average cooling rate here is 20° C./sec or more, martensite is formed and although the strength may be increased, good elongation is not obtained. The average cooling rate is preferably 15° C./sec or less, more preferably 10° C./sec or less. The reason why the steel sheet is cooled to 200° C. or less in this cooling is that the amount of retained austenite remaining at room temperature is increased by distributing carbon from bainitic ferrite to untransformed austenite.
After performing the above-described two-stage cooling, fundamentally, the average cooling rate need not be controlled, but the steel sheet may be cooled to room temperature at an average cooling rate of, for example, from 1° C./sec or more and 100° C./sec or less. The control of the average cooling rate during press forming as well as after the completion of forming can be achieved by a technique of, for example, (a) controlling the temperature of the forming mold (the cooling medium shown in FIG. 1), or (b) controlling the thermal conductivity of the mold.
In the press-formed article obtained by this manufacturing method, the metal microstructure includes bainitic ferrite: from 60 to 97 area %, martensite: 37 area % or less, retained austenite: from 3 to 20 area %, and remainder microstructure: 5 area % or less, and the amount of carbon in the retained austenite is 0.50% or more, so that a high-level balance between high strength and elongation can be achieved as a uniform property in a formed article. The reason for setting the range of each requirement (the amount of carbon in basic microstructure and retained austenite) in this hot press-formed article is as follows.
When the main microstructure of a press-formed article is high-strength bainitic ferrite rich in ductility, both of high strength and high ductility of a press-formed article can be satisfied. From such a standpoint, the area fraction of bainitic ferrite must be 60 area % or more. However, if this fraction exceeds 97 area %, the retained austenite fraction is insufficient, and the ductility (residual ductility) is reduced. The lower limit of the bainitic ferrite fraction is preferably 65 area % or more (more preferably 70 area % or more), and the upper limit is preferably 95 area % or less (more preferably 90 area % or less).
The strength of a hot press-formed article can be increased by partially incorporating high-strength martensite, but if the amount thereof is large, the ductility (residual ductility) is reduced. From such a standpoint, the area fraction of martensite must be 37 area % or less. The lower limit of the martensite fraction is preferably 5 area % or more (more preferably 10 area % or more), and the upper limit is preferably 30 area % or less (more preferably 25 area % or less).
Retained austenite has an effect of increasing the work hardening ratio (transformation induced plasticity) and enhancing the ductility of the press-formed article by undergoing transformation to martensite during plastic deformation. In order to exert such an effect, the retained austenite fraction must he 3 area % or more. The ductility is more improved as the retained austenite fraction is higher. In the composition to be used for an automotive steel sheet, the assurable retained austenite is limited, and the upper limit is about 20 area %. The lower limit of the retained austenite is preferably 5 area % or more (more preferably 7 area % or more).
As for the microstructure other than those described above, ferrite, pearlite, and the like may be contained as a remainder microstructure, but such a microstructure is inferior to other microstructures in terms of contribution to strength or contribution to ductility, and it is fundamentally preferable not to contain such a microstructure (may be even 0 area %). However, an area fraction up to 5 area % is acceptable. The area fraction of the remainder microstructure is preferably 4 area % or less, more preferably 3 area % or less.
In the press-formed article above, the average equivalent-circle diameter of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the press-formed article (i.e., in the steel sheet constituting the press-formed article) is 10 nm or less. When this requirement is satisfied, a press-formed article capable of achieving a high-level balance between high strength and elongation can be obtained. The average equivalent-circle diameter of the Ti-containing precipitate is preferably 8 nm or less, more preferably 6 nm or less.
In addition, in the press-formed article, the amount of Ti present as a precipitate other than TiN (i.e., precipitated Ti amount−3.4[N]) is smaller than 0.5 times the remainder Ti after deduction of Ti that forms TiN from total Ti (i.e., smaller than 0.5×[total Ti amount (%)−3.4[N]]). When this requirement is satisfied, Ti dissolved in solid during welding is finely precipitated in HAZ or the existing fine Ti-containing precipitate suppresses recovery, etc, of the dislocation, and as a result, softening in HAZ is prevented, and the weldability is improved. The “precipitated Ti amount−3.4[N]” is preferably 0.4×[total Ti amount)−3.4[N]] or less, more preferably 0.3×[total Ti amount)−3.4[N]] or less.
According to the method in the present invention, the properties such as strength and elongation of a formed article can be controlled by appropriately adjusting the press-forming conditions (heating temperature and cooling rate) and moreover, a press-formed article having high ductility (residual ductility) is obtained, making its application possible to a site (e.g., energy absorption member) to which the conventional hot press-formed article can be hardly applied. This is very useful in expanding the application range of a hot press-formed article.
The effects in the present invention are described more specifically below by referring to Examples, but the present invention is not limited to the following Examples, and all design changes made in light of the gist described above or later are included in the technical range in the present invention.

EXAMPLES

Steel materials (Steel Nos. 1 to 31) having the chemical component composition shown in Table 1 below were melted in vacuum to make an experimental slab, then hot-rolled to prepare a steel sheet, followed by cooling and subjecting to a treatment simulating the winding (sheet thickness: 3.0 mm). As to the method for treatment simulating the winding, the sample was cooled to a winding temperature, and put in a furnace heated at the winding temperature, followed by holding for 30 minutes and then cooling in the furnace. The manufacturing conditions of the steel sheets are shown in Table 2 below. Here, in Table 1, Ac₃transformation point, Ms point, and Bs point were determined using the following formulae (2) to (4) (see, for example, The Physical Metallurgy of Steels, Leslie, Maruzen, (1985)). In addition, treatments (1) and (2) shown in Remarks of Table 2 mean that each treatment (rolling, cooling and alloying) described below was performed.
Ac₃transformation point (° C.)=910-203×[C]^1/2+44.7×[Si]−30×[Mn]+700×[P]+400×[Al]+400×[Ti]+104×[V]−11×[Cr]+31.5×[Mo]−20×[Cu]−15.2×[Ni] (2)
Ms point (° C.)=550-361×[C]−39×[Mn]−10×[Cu]−17×[Ni]−20×[Cr]−5×[Mo]+30×[Al] (3)
Bs point (° C.)=830-270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−8:3×[Mo] (4)
wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cl], [Mo], [Cu] and [Ni] represent the contents (mass %) of C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni, respectively. In the case where the element shown in each term of formulae (2) to (4) is not contained, the calculation is done assuming that the term is not present.
Treatment (1): The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 nun), then heated at 800° C. for simulating continuous annealing in a heat treatment simulator, held for 90 seconds, cooled to 500° C. at an average cooling rate of 20° C./sec, and held for 300 seconds.
Treatment (2): The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm), then heated at 860° C. for simulating a continuous hot-dip galvanizing line in a heat treatment simulator, cooled to 400° C. at an average cooling rate of 30° C./sec, held, further held under the conditions of 500° C.×10 seconds for simulating immersion in a plating bath and alloying treatment, and thereafter cooled to room temperature at an average cooling rate of 20° C./sec.

TABLE 1

Steel	Chemical Component Composition* (mass %)

No.	C	Si	Mn	P	S	Al	B	Ti	N	V	Nb	Cu

1	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
2	0.150	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
3	0.220	0.05	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
4	0.220	0.25	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
5	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
6	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
7	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
8	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
9	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
10	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
11	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
12	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
13	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
14	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
15	0.220	2.00	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
16	0.350	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
17	0.720	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
18	0.220	1.20	0.80	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
19	0.220	1.20	2.40	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
20	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.100	0.0040	—	—	—
21	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.200	0.0040	—	—	—
22	0.220	0.50	1.20	0.0050	0.0020	0.40	0.0020	0.044	0.0040	—	—	—
23	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	0.030	—	—
24	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	0.020	—
25	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	0.20
26	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
27	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
28	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
29	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
30	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.0040	—	—	—
31	0.220	1.20	1.20	0.0050	0.0020	0.030	0.0020	0.044	0.040	—	—	—

		Ac₃
Steel	Chemical Component Composition* (mass %)	transformation	Bs-100° C.	Ms Point

No.	Ni	Zr	Ca	Mg	REM	Cr	Mo	point (° C.)	(° C.)	(° C.)

1	—	—	—	—	—	—	—	845	563	425
2	—	—	—	—	—	0.20	—	880	568	446
3	—	—	—	—	—	0.20	—	812	549	421
4	—	—	—	—	—	0.20	—	821	549	421
5	—	—	—	—	—	0.20	—	853	549	421
6	—	—	—	—	—	0.20	—	863	549	421
7	—	—	—	—	—	0.20	—	863	549	421
8	—	—	—	—	—	0.20	—	863	549	421
9	—	—	—	—	—	0.20	—	863	549	421
10	—	—	—	—	—	0.20	—	863	549	421
11	—	—	—	—	—	0.20	—	863	549	421
12	—	—	—	—	—	0.20	—	863	549	421
13	—	—	—	—	—	0.20	—	863	549	421
14	—	—	—	—	—	0.20	—	863	549	421
15	—	—	—	—	—	0.20	—	899	549	421
16	—	—	—	—	—	0.20	—	838	514	374
17	—	—	—	—	—	0.20	—	786	414	240
18	—	—	—	—	—	0.20	—	875	585	436
19	—	—	—	—	—	0.20	—	827	441	374
20	—	—	—	—	—	0.20	—	886	549	421
21	—	—	—	—	—	0.20	—	926	549	421
22	—	—	—	—	—	0.20	—	980	549	432
23	—	—	—	—	—	0.20	—	866	549	421
24	—	—	—	—	—	0.20	—	863	549	421
25	—	—	—	—	—	0.20	—	859	549	419
26	0.20	—	—	—	—	0.20	—	860	541	417
27	—	—	—	—	—	0.20	0.20	869	532	420
28	—	0.015	—	—	—	0.20	—	863	549	421
29	—	—	0.002	—	—	0.20	—	863	549	421
30	—	—	—	0.002	—	0.20	—	863	549	421
31	—	—	—	—	0.002	0.20	—	863	549	421

*Remainder: Iron and unavoidable impurities except for P, S and N.

	TABLE 2

	Steel Sheet Manufacturing Conditions

			Average
			Cooling
			Rate from
	Heating		Finish Rolling
	Tem-	Finish	Temperature to	Tempera-
	pera-	Rolling	Winding	ture
Steel	ture	Temperature	Temperature	Winding
No.	(° C.)	(° C.)	(° C./sec)	(° C.)	Remarks

1	1200	950	20	500	—
2	1200	950	20	500	—
3	1200	950	20	500	—
4	1200	950	20	500	—
5	1200	950	20	500	—
6	1200	950	20	500	—
7	1200	800	20	500	—
8	1200	950	20	500	treatment (1)
9	1200	950	20	500	treatment (2)
10	1200	950	20	500	—
11	1200	950	20	500	—
12	1200	950	20	500	—
13	1200	950	20	500	—
14	1200	950	20	500	—
15	1200	950	20	500	—
16	1200	950	20	500	—
17	1200	950	20	500	—
18	1200	950	20	500	—
19	1200	950	20	500	—
20	1200	950	20	500	—
21	1200	950	20	500	—
22	1200	980	20	500	—
23	1200	950	20	500	—
24	1200	950	20	500	—
25	1200	950	20	500	—
26	1200	950	20	500	—
27	1200	950	20	500	—
28	1200	950	20	500	—
29	1200	950	20	500	—
30	1200	950	20	500	—
31	1200	950	20	500	—

With respect to the steel sheets (steel sheets for press-forming) obtained, analysis of the Ti precipitation state (“precipitated Ti amount−3.4[M]” and average equivalent-circle diameter of Ti-containing precipitates) was performed in the following manner. The results obtained are shown in Table 3 below together with the calculated value of 0.5×[total Ti amount−3.4[N]].

(Analysis of Ti Precipitation State of Steel Sheet)

An extraction replica sample was prepared, and a transmission electron microscope image (magnifications: 100,000 times) of Ti-containing precipitates was photographed using a transmission electron microscope (TEM). At this time, the Ti-containing precipitate (those having an equivalent-circle diameter of 30 nm or less) was identified by the composition analysis of precipitates by means of an energy dispersive X-ray spectrometer (EDX). At least 100 pieces of Ti-containing precipitates were measured for the area by image analysis, the equivalent-circle diameter was determined therefrom, and the average value thereof was defined as the precipitate size (average equivalent-circle diameter of Ti-containing precipitates). As for the “precipitated Ti amount−3.4[N]” (the amount of Ti present as a precipitate), extraction residue analysis was performed using a mesh having a mesh size of 0.1 μm (during extraction treatment, a fine precipitate resulting from aggregation of precipitates could also be measured), and the “precipitated Ti amount−3.4[N]” was determined. In the case where the Ti-containing precipitate partially contained V or Nb, the contents of these precipitates were also measured.

	TABLE 3

	Steel Sheet for Press-Forming

			Average
			Equivalent-Circle
	Precipitated Ti		Diameter of
Steel	Amount − 3.4[N]	0.5 × [Total Ti Amount −	Ti-Containing
No.	(mass %)	3.4[N] (mass %)	Precipitates (nm)

1	0.009	0.015	4.0
2	0.008	0.015	2.5
3	0.008	0.015	2.3
4	0.006	0.015	3.3
5	0.001	0.003	3.0
6	0.008	0.015	3.2
7	0.018	0.015	9.2
8	0.008	0.015	3.4
9	0.010	0.015	3.2
10	0.008	0.015	2.8
11	0.008	0.015	2.8
12	0.008	0.015	2.8
13	0.008	0.015	2.8
14	0.008	0.015	2.8
15	0.009	0.015	3.9
16	0.009	0.015	3.7
17	0.008	0.015	3.2
18	0.008	0.015	2.7
19	0.006	0.015	2.1
20	0.025	0.043	3.1
21	0.128	0.093	10.8
22	0.008	0.015	2.7
23	0.009	0.015	3.1
24	0.006	0.015	3.6
25	0.007	0.015	3.6
26	0.008	0.015	3.6
27	0.008	0.015	2.7
28	0.006	0.015	3.0
29	0.007	0.015	3.3
30	0.006	0.015	3.2
31	0.006	0.015	3.0

Each of the steel sheets above (1.6 mm^t×150 mm×200 mm) (the thickness t of those other than the treatment (1) and (2) was adjusted to 1.6 mm by hot rolling) was heated at a predetermined temperature in a heating furnace, followed by subjecting to press forming and cooling treatment using a hat-shaped mold (FIG. 1) to obtain a formed article. The press forming conditions (heating temperature, heating time average cooling rate, and rapid cooling finishing temperature during press forming) are shown in Table 4 below.

	TABLE 4

	Press-Forming Conditions

					Cooling
					Rate
					After
				Rapid Cooling	Finishing
	Heating	Heating	Average	Finishing	Rapid
Steel	Temperature	time	Cooling Rate	Temperature	Cooling
No.	(° C.)	(sec)	(° C./sec)	(° C.)	(° C./sec)

1	900	600	40	450	3
2	900	15	40	450	3
3	900	15	40	450	3
4	900	15	40	450	3
5	900	15	40	450	3
6	900	15	40	450	3
7	900	15	40	450	3
8	900	15	40	450	3
9	900	15	40	450	3
10	900	15	40	450	3
11	900	15	40	450	25
12	800	15	40	300	3
13	900	15	5	450	3
14	900	15	40	600	3
15	900	15	40	450	3
16	900	15	40	450	3
17	900	15	40	400	3
18	900	15	40	450	3
19	900	15	40	400	3
20	900	15	40	450	3
21	900	15	40	450	3
22	900	15	40	450	3
23	900	15	40	450	3
24	900	15	40	450	3
25	900	15	40	450	3
26	900	15	40	450	3
27	900	15	40	450	3
28	900	15	40	450	3
29	900	15	40	450	3
30	900	15	40	450	3
31	900	15	40	450	3

With respect to the press-formed articles obtained, the tensile strength (TS), elongation (total elongation EL), observation of metal microstructure (fraction of each microstructure), and hardness reduction amount after heat treatment were measured by the following methods, and the Ti precipitation state was analyzed by the method described above.

(Measurement of Tensile Strength (TS) and Elongation (Total Elongation EL))

A tensile test was performed using a JIS No. 5 test piece, and the tensile strength (TS) and elongation (EL) were measured. At this time, the strain rate in the tensile test was set to 10 mm/sec. In the present invention, the test piece was rated “passed” when a tensile strength (TS) of 1,180 MPa or more and an elongation (EL) of 12.0% or more were satisfied and the strength-elongation balance (TS×EL) was 16,000 (MPa. %) or more.

(Observation of Metal Microstructure (Fraction of Each Microstructure))

(1) With respect to the microstructures of bainitic ferrite, martensite and ferrite in the formed article, the steel sheet was corroded with nital and after distinguishing, bainitic ferrite, martensite and ferrite from each other by SEM observation (magnifications: 1,000 times or 2,000 times), the fraction (area ratio) of each microstructure was determined.
(2) The retained austenite fraction in the formed article was measured by X-ray diffraction method after the steel sheet was ground to ¼ thickness and then subjected to chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7, P. 776).

(Hardness Reduction Amount After Heat Treatment)

As the heat history based on spot welding, the hardness reduction amount (ΔHv) relative to the original hardness (Vickers hardness) was measured after heating to 700° C. at an average heating rate of 50° C./sec in a heat treatment simulator and then cooling at an average cooling rate of 50° C./sec. The anti-softening property in HAZ was judged as good when the hardness reduction amount (ΔHv) was 50 Hv or less.
The observation results (fraction of each microstructure, Ti precipitation state, and precipitated Ti amount−34[N]) of the metal microstructure are shown in Table 5 below. In addition, the mechanical properties (tensile strength TS, elongation EL, TS×EL, and hardness reduction amount ΔHv) of the press-formed article are shown in Table 6 below. Here, the value of “precipitated Ti amount−34[N]” in the press-formed article is slightly different from the value of “precipitated Ti amount−3.4[N]” in the steel sheet for press-forming, but this is a measurement error.

	TABLE 5

	Metal Microstructure of Press-Formed Article

	Bainitic Ferrite	Martensite			Average Equivalent-Circle	Precipitated Ti
Steel	Fraction	Fraction	Ferrite Fraction	Retained Austenite	Diameter of Ti-Containing	Amount-3.4[N]
No.	(area %)	(area %)	(area %)	Fraction (area %)	Precipitates (nm)	(mass %)

1	87	8	0	5	2.7	0.010
2	81	8	5	6	2.5	0.010
3	94	6	0	0	3.0	0.010
4	89	7	0	4	2.5	0.009
5	86	6	0	8	3.5	0.000
6	86	7	0	7	3.3	0.011
7	85	8	0	7	8.0	0.023
8	86	7	0	7	2.9	0.010
9	87	6	0	7	2.1	0.011
10	88	5	0	7	3.0	0.011
11	6	88	0	6	3.4	0.011
12	0	92	0	8	3.9	0.011
13	26	5	48	6	3.2	0.011
14	55	6	33	6	3.4	0.012
15	91	0	0	9	2.4	0.013
16	86	7	0	7	2.3	0.012
17	72	8	8	12	2.8	0.013
18	88	5	0	7	2.4	0.008
19	87	6	0	7	3.0	0.012
20	88	6	0	6	3.8	0.021
21	87	6	0	7	13.8	0.180
22	85	7	0	8	2.1	0.012
23	88	6	0	6	3.1	0.011
24	86	7	0	7	3.6	0.009
25	85	7	0	8	3.0	0.012
26	89	5	0	6	2.0	0.013
27	87	7	0	6	2.0	0.011
28	85	8	0	7	3.6	0.008
39	84	9	0	7	3.0	0.010
30	86	7	0	7	3.3	0.009
31	85	7	0	8	3.0	0.011

	TABLE 6

	Mechanical Properties of
	Press-Formed Article

	Tensile
Steel	Strength TS	Elongation EL	TS × EL	Hardness Reduction
No.	(MPa)	(%)	(MPa · %)	Amount ΔHv (Hv)

1	1217	13.3	16151	42
2	1189	15.5	16188	38
3	1239	10.1	12551	38
4	1210	13.3	16115	35
5	1254	13.0	16296	35
6	1263	12.8	16220	39
7	1241	11.9	14786	68
8	1213	13.6	16488	44
9	1218	13.4	16263	36
10	1206	13.3	16057	42
11	1495	10.6	15847	44
12	1532	10.3	15735	40
13	890	18.1	16096	40
14	955	15.2	14516	43
15	1260	13.0	16329	42
16	1320	12.3	16187	38
17	1992	3.5	6972	40
18	1217	13.4	16311	41
19	1245	13.2	16481	38
20	1215	13.3	16160	44
21	1253	12.9	16174	98
22	1266	12.9	16275	37
23	1221	13.3	16256	40
24	1220	13.2	16063	40
25	1224	13.2	16136	42
26	1244	13.1	16271	41
27	1232	13.2	16236	40
28	1233	13.5	16646	39
29	1250	13.2	16479	42
30	1244	13.1	16271	40
31	1229	13.2	16196	41

These results allow for the following consideration. It is found that in the case of Steel Nos. 1, 2, 4 to 6, 8 to 10, 15, 16, 18 to 20, and 22 to 31, which are Examples satisfying the requirements specified in the present invention, a formed article having a good strength-ductility balance and a good anti-softening property is obtained.
On the other hand, in the case of Steel Nos. 3, 7, 11 to 14, 17 and 21, which are Comparative Examples failing in satisfying any of the requirements specified in the present invention, any of the properties is deteriorated. More specifically, in the case of Steel No. 3 where a steel sheet having a small Si content is used, the retained austenite fraction is not ensured in the press-formed article and since only low elongation EL is obtained, the strength-elongation balance (TS×EL) is deteriorated. In the case of Steel No. 7 where the finish rolling temperature in the manufacture of a steel sheet is low, the relationship of the formula (1) is not satisfied, and not only the Ti-containing precipitate is coarsened to reduce the strength-elongation balance (TS×EL) but also the anti-softening property is deteriorated.
In the case of Steel No. 11 where the cooling rate after rapid cooling during press forming is high, martensite is excessively produced and not only the strength is too high, resulting in obtaining only low EL, but also the strength-elongation balance (TS×EL) is deteriorated. In the case of Steel No. 12 where the rapid cooling finishing temperature during press forming is low, martensite is excessively produced and not only the strength is too high, resulting in obtaining only low EL, but also the strength-elongation balance (TS×EL) is deteriorated.
In the case of Steel No. 13 where the average cooling rate during press forming is low, the area ratio of bainitic ferrite cannot be ensured and not only the strength is too low but also the strength-elongation balance (TS×EL) is deteriorated. In the case of Steel No. 14 where the rapid cooling finishing temperature during press forming is high, the area ratio of bainitic ferrite cannot be ensured due to production of ferrite and not only the strength is too low but also the strength-elongation balance (TS×EL) is deteriorated.
In the case of Steel No. 17 where a steel sheet having an excessive C content is used, the strength of a formed article is high, but only low elongation EL is obtained. In the case of Steel No. 21 where a steel sheet having an excessive Ti content is used, a press-formed article does not satisfy the relationship of the formula (1), and not only the Ti-containing precipitate in the press-formed article is coarsened but also the anti-softening, property is deteriorated.

INDUSTRIAL APPLICABILITY

In the present invention, a steel sheet for hot-pressing which has a predetermined chemical component composition, where the equivalent-circle diameter of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet is 6 nm or less and the precipitated Ti amount and the total Ti amount in the steel satisfy a predetermined relationship, is heated at a temperature of 900° C. or more and 1,100° C. or less, and after press forming is started, the steel sheet is cooled to a temperature equal to or less than a temperature 100° C. below the bainite transformation starting temperature Bs and equal to or more than the martensite transformation starting temperature Ms, while ensuring an average cooling rate of 20° C./sec or more in a mold during forming as well as after the completion of forming, and then cooled to 200° C. or less at an average cooling rate of less than 20° C./sec, whereby a press-formed article capable achieving a high-level balance between high strength and elongation can be obtained and moreover, a press-formed article having good anti-softening property in HAZ can be realized.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1: Punch
2: Die
3: Blank holder
4: Steel sheet (blank)

Claims

1. A method for manufacturing a press-formed article, the method comprising:

heating a steel sheet for hot-pressing at 900° C. or more and 1,100° C. or less, to obtain a heated steel sheet;

beginning press forming of the heated steel sheet;

cooling the steel sheet to a temperature equal to or less than a temperature 100° C. below a bainite transformation starting temperature (Bs) and equal to or more than a martensite transformation starting temperature (Ms), while ensuring an average cooling rate of 20° C./sec or more in a mold during the press forming and after completion of the press forming to obtain a cooled steel sheet; and

further cooling the cooled steel sheet to 200° C. or less at the average cooling rate of less than 20° C./sec,

wherein

the steel sheet comprises

C: from 0.15 to 0.5% by mass,

Si: from 0.2 to 3% by mass,

Mn: from 0.5 to 3% by mass,

P: 0.05% by mass or less (exclusive of 0%),

S: 0.05% by mass or less (exclusive of 0%),

Al: from 0.01 to 1% by mass,

B: from 0.0002 to 0.01% by mass.

Ti: equal to or more than 3.4[N]+0.01% by mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N] indicates a content (mass %) of N,

N: from 0.001 to 0.01%, iron, and

unavoidable impurities;

an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet is 6 nm or less; and

a precipitated Ti amount and a total Ti amount in a steel of the steel sheet satisfies formula (1):

Precipitated Ti amount (mass %)−3.4[N]<0.5×[total Ti amount (mass %)−3.4[N]] (1)

2. The method according to claim 1, wherein the steel sheet for hot-pressing further comprises, as the other element(s), at least one of the following (a) to (c):

(a) at least one selected from the group consisting of V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in total;

(b) at least one selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (exclusive of 0%) in total; and

(c) at least one selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in total.

3. A press-formed article of a steel sheet comprising:

C: from 0.15 to 0.5% by mass;

Si: from 0.2 to 3% by mass;

Mn: from 0.5 to 3% by mass;

P: 0.05% by mass or less (exclusive of 0%);

S: 0.05% by mass or less (exclusive of 0%);

Al: from 0.01 to 1% by mass;

B: from 0.0002 to 0.01% by mass;

Ti: equal to or more than 3.4[N]+0.01% by mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N] indicates a content (mass %) of N;

from 0.001 to 0.01%;

iron; and

unavoidable impurities,

wherein:

a metal microstructure of the press-formed article includes

bainitic ferrite: from 60 to 97 area %,

martensite: 37 area % or less,

retained austenite: from 3 to 20 area %, and

remainder microstructure: 5 area % or less;

an average equivalent-circle diameter of Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the press-formed article is 10 nm or less; and

4. A press-formed article of a steel sheet comprising: