JP7477750B2 - Hot stamped parts and manufacturing method thereof - Google Patents

Description

The present invention relates to hot stamped parts and their manufacturing methods.

In recent years, in order to improve fuel efficiency, vigorous efforts have been made to reduce the weight of automobiles by increasing the strength of the steel materials used. As a result, in the manufacture of parts (hereinafter referred to as "press-formed parts") manufactured by cold press forming thin steel sheets, which are widely used in automobiles, it has become difficult to manufacture press-formed parts with complex shapes due to the decrease in press formability that accompanies the increase in the strength of the steel sheets.

Specifically, problems such as breakage at highly processed areas of press-molded parts and reduced dimensional accuracy of press-molded parts due to increased springback and wall warping have frequently occurred due to a decrease in the ductility of steel sheets. In particular, it is not easy to manufacture press-molded parts made of high-strength steel sheets with a tensile strength of 780 MPa or more.

By using roll forming rather than cold press forming, roll-formed parts made of high-strength steel sheets can be easily manufactured. However, roll forming can only produce roll-formed parts that have a constant cross-section in the longitudinal direction, and press-formed parts with complex cross-sectional shapes cannot be manufactured.

In contrast, in the hot stamping method (also known as hot press forming), in which heated steel sheet is press-formed, the hot steel sheet becomes soft and highly ductile during forming, so press-formed parts with complex shapes can be formed with good dimensional accuracy without forming defects such as breakage, springback, or wall warping.

Furthermore, with hot stamping, the steel sheet is heated to a temperature in the austenite single phase region before being press-formed, and the formed product is then rapidly cooled and quenched inside the die used for press forming, thereby simultaneously forming the steel sheet and increasing the strength of the press-formed part through martensitic transformation. In this way, the hot stamping method is an excellent technology suitable for manufacturing high-strength press-formed parts. In the following explanation, press-formed parts manufactured by the hot stamping method will be referred to as "hot stamped parts".

Currently, bumper reinforcements, which have a relatively simple shape, are known as an example of hot stamped parts. Hot stamped parts with a tensile strength of 1.5 GPa are already widely used in structural members (framework members) of body shells, such as bumper reinforcements, for example B-pillar reinforcements.

In recent years, there has been research into the production of hot-stamped parts with higher strength, particularly those with a tensile strength of 1.8 GPa or more. For example, there is consideration of using hot-stamped parts with a tensile strength of 1.8 GPa or more for structural members (framework members) of the body shell, such as B-pillar reinforcements, which bear the majority of the impact load during a collision.

However, when the tensile strength of a hot stamped part reaches an ultra-high strength of 1.8 GPa or more, the deformability of the hot stamped part becomes insufficient, and the fracture resistance characteristics (e.g., bendability) of the hot stamped part decrease, and the hot stamped part's absorbed energy decreases during a collision, and the hot stamped part itself is more likely to break. For this reason, the only hot stamped parts with a tensile strength of 1.8 GPa or more that have been put into practical use are 1.8 GPa-class bumper reinforcements, and in fact, no examples of the manufacture of hot stamped parts with high deformability and a tensile strength of 1.8 GPa or more have been reported to date.

For this reason, in order to manufacture hot stamped parts with a tensile strength of 1.8 GPa or more, it is necessary to further temper the hot stamped parts to increase their deformability. However, adding a tempering process to the hot stamping process significantly increases the manufacturing costs of the hot stamped parts due to reduced work efficiency and increased equipment costs.

Patent Document 1 discloses a hot stamped part having a steel structure constituted by automatically tempered martensite with an average prior austenite grain size of 10 μm or less, excellent toughness and a tensile strength of 1.8 GPa or more in an as-quenched state, and a steel sheet for this hot stamped part, which is obtained by holding a steel sheet containing C: 0.25 to 0.45% (in this specification, "%" relating to a chemical composition or concentration means "mass %" unless otherwise specified), Mn + Cr: _0.5 to 3.0%, and further containing one or more of Si: _0.5 % or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less in a temperature range of Ac 3 point or more (Ac 3 point + 100°C) or less for 5 minutes or less, press forming, and then cooling at a cooling rate to the Ms point that is equal to or higher than the upper critical cooling rate and at an average cooling rate from the Ms point to 150°C that is 10 to 500°C/s.

Patent Documents 2 and 3 disclose hot stamped parts having a fine structure including automatically tempered martensite with a prior austenite grain size of 10 μm or less, and excellent toughness in an as-quenched state, and a tensile strength of 1.8 GPa or more, by holding a steel plate having a chemical composition including C: _0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 1.0%, Ti in an amount satisfying 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5, and one or more of Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less in a temperature range of Ac ₃ point or more (Ac 3 point + 100 ° C.) or less for 5 minutes or less, press forming, and then cooling at a cooling rate to the Ms point equal to or higher than the upper critical cooling rate and at an average cooling rate from the Ms point to 150 ° C. of 10 to 500 ° C./sec.

Patent Document 4 discloses a hot stamped steel plate member having a tensile strength of 1.8 to 2.5 GPa and excellent toughness, which is obtained by hot stamping a steel plate having a chemical composition consisting of C: 0.25 to 0.40%, Si: 0.05% or more and less than 0.5%, Mn: 1.0 to 1.7%, P: 0.020% or less, S: less than 0.0010%, Al: 0.002 to 0.06%, N: 0.006% or less, Cr: 0.02 to 0.6%, B: 0.00010 to 0.0040%, Ti: 0.005 to ^0.04 %, Nb: 0.03 to 0.12%, and the balance: Fe and unavoidable impurities, and having a number density of inclusions and precipitates having a maximum length of 10 μm or more of 100 pieces/mm 2 or less and a metal structure in which the prior austenite grain size after hot stamping is 10 μm or less.

JP 2006-152427 A International Publication No. 2007/129676 JP 2012-180594 A JP 2017-43825 A

The inventions disclosed in Patent Documents 1 to 4 certainly provide hot stamped parts with a tensile strength of 1.8 GPa or more in the as-quenched state.

However, as mentioned above, in order to use hot stamped parts with a tensile strength of 1.8 GPa or more in the structural members (skeletal members) of the body shell, the hot stamped parts not only need to be strengthened (tensile strength of 1.8 GPa or more), but also need to have improved fracture resistance (e.g., bendability) through further improvements in deformability. From this perspective, there is still room for improvement in the deformability and fracture resistance of the hot stamped parts disclosed in Patent Documents 1 to 4.

In addition, when using hot stamped parts, it is undesirable for the impact properties to vary depending on the orientation of the hot stamped part.

The present invention was made in consideration of the problems with conventional technology, and aims to provide a technology for manufacturing hot stamped parts that have excellent deformability and fracture resistance during impact, and have a tensile strength of, for example, 1.5 GPa or more, without tempering after quenching.

Generally, as the tensile strength of a hot stamped part increases, it becomes more difficult to ensure the bendability of the hot stamped part. As a result of extensive research focused on improving the fracture resistance of hot stamped parts during collisions, the inventors obtained the findings A to E listed below and completed the present invention.

(A) When a steel sheet for hot stamped parts was subjected to three-point bending, elongated MnS was found in the cracked area, and many dimples caused by this MnS were confirmed on the fracture surface. In other words, the elongated MnS becomes the starting point of fracture. For this reason, it is necessary to reduce the amount of elongated MnS in steel sheets for hot stamped parts.

(B) When the hot stamped parts were examined, it was found that, even though the hot stamping method heats the steel plate for hot stamped parts sufficiently to the austenite single phase region, there was a large amount of undissolved fine cementite that caused low strength and low deformability. For this reason, it is necessary to promote the dissolution of cementite.

(C) If Nb is added to avoid a decrease in deformability, Nb-based carbides are formed, which become the starting point for the formation of voids. In order to suppress the formation of voids, it is necessary to optimize the Nb content.

(D) In other words, in the manufacturing process of steel plates for hot stamped parts and hot stamped parts, impurities such as inclusions and carbides in the steel that can be the starting point of fracture in hot stamped parts during collisions can be reduced as much as possible, thereby greatly improving the fracture resistance of the hot stamped parts, and thus making it possible to provide hot stamped parts that can also be used, for example, as structural members (skeletal members) of a body shell.

(E) On the other hand, inclusions and carbides in steel cannot be completely eliminated. On the contrary, they can be utilized to increase the strength of hot stamped parts.

The present invention is as follows:

(1) The chemical composition is C: 0.18-0.50%, Si: 0.001-2.00%, Mn: 0.001-3.00%, P: 0.10% or less, S: 0.0015% or less, Nb: 0.010-0.100%, Cr: 0.001-0.50%, Al: 0.0001-0.100%, Ti: 0.001-0.500%, O: less than 0.0030%, B: 0.0006-0.0030%, N: 0.0100% or less, and the balance: Fe and impurities;
A hot stamped part having a tensile strength of 1.5 GPa or more, in which the metal structure is martensite, the area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the plate thickness is 0.0100% or less, and the number ratio of carbides (cementite and NbTi complex carbonitrides) having a particle size of 1.2 μm or more among carbides (cementite and NbTi complex carbonitrides) having a particle size of 0.5 μm or more present in the steel is 10.0% or less.

(2) The hot stamped part according to claim 1, wherein the chemical composition includes one or more selected from V: 2.00% or less, Ta: 0.50% or less, and W: 3.00% or less.

(3) The hot stamped part according to claim 1 or 2, wherein the chemical composition includes one or more selected from Ni: 5.00% or less, Cu: 3.00% or less, and Mo: 0.50% or less.

(4) A hot stamped part according to any one of items 1 to 3, in which the chemical composition includes one or more selected from Mg: 0.0030% or less, Ca: 0.0030% or less, La: 0.030% or less, and Ce: 0.030% or less.

(5) A method for producing a hot stamped part according to any one of (1) to (4),
a hot stamping process for hot stamping a steel sheet having a chemical composition of C: 0.18-0.50%, Si: 0.001-2.00%, Mn: 0.001-3.00%, P: 0.10%, S: 0.0015%, Nb: 0.010-0.100%, Cr: 0.001-0.50%, Al: 0.0001-0.100%, Ti: 0.001-0.500%, O: less than 0.0030%, B: 0.0006-0.0030%, N: 0.0100% or less, and the balance: Fe and impurities, and a metal structure is a mixed structure of ferrite and pearlite, and an area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the sheet thickness is 0.0100% or less;
and a heat treatment step of subjecting the obtained hot stamped part to a heat treatment at 500 to 100° C. that satisfies the condition of the following formula (i).
log(t)≦(625−T)/125 (i)
In formula (i), t is the heat treatment time (seconds), and T is the heat treatment temperature (° C.).

(6) The method for manufacturing a hot stamped part according to claim 5, wherein the chemical composition includes one or more selected from V: 2.00% or less, Ta: 0.50% or less, and W: 3.00% or less.

(7) The method for manufacturing a hot stamped part according to claim 5 or 6, wherein the chemical composition includes one or more selected from Ni: 5.00% or less, Cu: 3.00% or less, and Mo: 0.50% or less.

(8) A method for manufacturing a hot stamped part according to any one of items 5 to 7, in which the chemical composition includes one or more selected from Mg: 0.0030% or less, Ca: 0.0030% or less, La: 0.030% or less, and Ce: 0.030% or less.

(9) A rolling step of heating a steel ingot or steel slab having a chemical composition according to any one of items 1 to 4 at a temperature of T (°C) or higher represented by formula (ii) for 30 minutes or more, completing rough rolling at 1000 to 1150°C, and completing finish rolling at Ae ₃ or higher;
A cooling step of cooling at an average cooling rate of 10 to 200 ° C. / sec.
A winding process at 680 to 560°C;
A pickling process for removing scale by pickling;
A cold rolling process in which the reduction ratio is 10 to 60%;
An annealing step of heating to 730 to 900 ° C.;
a hot stamping process for hot stamping the steel sheet that has been annealed;
and a heat treatment step of subjecting the obtained hot stamped part to a heat treatment at 500 to 100° C. that satisfies the following condition (i).
log(t)≦(625−T)/125 (i)
T (° C.)=[0.020+(mass% Mn)(mass% S)]/(1.9×10 ⁻⁵ ) (ii)
In formula (i), t is the heat treatment time (seconds), and T is the heat treatment temperature (° C.). In formula (ii), mass % Mn and mass % S represent the contents of Mn and S, respectively.

The present invention makes it possible to manufacture hot stamped parts that have excellent fracture resistance, ultra-high strength (particularly tensile strength of 1.5 GPa or more) and greatly improved fracture resistance, without tempering, and that are hot stamped and quenched at the time. This makes it possible to use hot stamped parts with a tensile strength of 1.5 GPa or more for structural members (framework members) of body shells, for example, and prevents an increase in the manufacturing costs of hot stamped parts with a tensile strength of 1.5 GPa or more.

Explain the present invention.

1. Steel Sheet for Hot Stamped Parts According to the Present Invention (1) Chemical Composition First, essential elements will be described.

(1-1) C: 0.18 to 0.50%
C is a very important element that enhances the hardenability of the steel plate for hot stamped parts and mainly determines the tensile strength of the hot stamped parts after quenching. In particular, in order to ensure that the tensile strength of the hot stamped parts after quenching is 1.5 GPa or more, the C content is 0.18% or more, preferably 0.20% or more, and more preferably 0.24% or more. On the other hand, if the C content exceeds 0.50%, the tensile strength of the hot stamped parts after quenching becomes too high, so that the toughness deteriorates significantly. For this reason, the C content is 0.50% or less, preferably 0.40% or less, and more preferably 0.38% or less.

(1-2) Si: 0.001 to 2.00%
Si is effective in improving the hardenability of steel sheets for hot stamped parts and stably achieving high strength of hot stamped parts after quenching. To obtain this effect, the Si content is 0.001% or more, preferably 0.050% or more, and more preferably 0.100% or more. On the other hand, even if the Si content exceeds 2.00%, the above effect is saturated and costs increase. For this reason, the Si content is 2.00% or less, preferably 1.50% or less, and more preferably 1.00% or less.

(1-3) Mn: 0.001 to 3.00%
Mn is an element that is very effective in improving the hardenability of steel sheets for hot stamped parts and stably obtaining high strength of hot stamped parts after quenching. If the Mn content is less than 0.001%, this effect cannot be obtained sufficiently. Therefore, the Mn content is 0.001% or more, preferably 0.50% or more, and more preferably 1.00% or more. On the other hand, even if the Mn content exceeds 3.00%, the above effect is saturated, and conversely, it becomes difficult to ensure a stable tensile strength. Therefore, the Mn content is 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

(1-4) P: 0.10% or less P significantly deteriorates the toughness of hot stamped parts after quenching, so the lower the P content, the better, but a content of 0.10% is permissible. Therefore, the P content is 0.10% or less. However, reducing the P content to less than 0.001% inevitably increases the steelmaking cost. For this reason, the P content is preferably 0.001% or more.

(1-5) S: 0.0015% or less Since the smaller the S content, the more the deformability and fracture resistance improve, the S content is set to 0.0015% or less. It is preferably less than 0.0010%. However, since reducing the S content requires desulfurization costs, the S content is preferably set to 0.0001% or more. The S content is more preferably 0.0003% or more.

(1-6) Nb: 0.010 to 0.100%
Nb has the effect of suppressing recrystallization and forming fine carbides to refine austenite grains when a steel sheet for hot stamped parts is heated to Ac ₃ or higher, and thus greatly improving the toughness of the hot stamped parts. To obtain this effect, the Nb content is 0.010% or more, preferably 0.020% or more, and more preferably 0.040% or more. On the other hand, as described below, if the Nb content exceeds 0.100%, the Nb carbonitrides in the steel ingot or steel billet become coarse. For this reason, the Nb content is 0.100% or less, preferably 0.080% or less, and more preferably 0.06% or less.

(1-7) Cr: 0.001 to 0.50%
Cr is an element that is very effective in improving the hardenability of steel sheets for hot stamped parts and stably obtaining high strength of hot stamped parts after quenching. To obtain this effect, the Cr content is 0.001% or more, preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, even if the Cr content exceeds 0.50%, the above effect is saturated, and conversely, it becomes difficult to ensure a stable tensile strength. For this reason, the Cr content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

(1-8) Al: 0.0001 to 0.100%
Al is an element that is effective in improving the hardenability of a steel sheet for hot stamped parts and stably ensuring the tensile strength of the hot stamped parts after quenching. To obtain this effect, the Al content is 0.0001% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, even if the Al content exceeds 0.100%, the above effect is saturated and the cost only increases. For this reason, the Al content is 0.100% or less, preferably 0.060% or less, and more preferably 0.040% or less.

(1-9) Ti: 0.001 to 0.500%
Ti has the effect of suppressing recrystallization and forming fine carbides to refine austenite grains when a steel sheet for hot stamped parts is heated to Ac ₃ or higher, thereby greatly improving the toughness of the hot stamped parts. In order to reliably obtain this effect, the Ti content is 0.001% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, even if the Ti content exceeds 0.500%, the above effect is saturated and the cost only increases. For this reason, the Ti content is 0.500% or less, preferably 0.100% or less, and more preferably 0.050% or less.

(1-10) O: Less than 0.0030% O exists in steel as an impurity. If O exists, it forms oxides and reduces the fracture resistance properties, so it is preferable that the O content is small, but a content of about 0.0030% is permissible. For this reason, the O content is less than 0.0030%.

(1-11) B: 0.0006 to 0.0030%
B is effective in improving the hardenability of the steel sheet for hot stamped parts and in enhancing the effect of stably securing the tensile strength of the hot stamped parts after quenching. In addition, B is an important element in that it segregates at the grain boundaries to increase the grain boundary strength and improve the toughness of the hot stamped parts. Furthermore, B has a high effect of suppressing the growth of austenite grains during heating of the steel sheet for hot stamped parts. In order to obtain this effect, the B content is 0.0006% or more, preferably 0.001% or more, and more preferably 0.0015% or more. On the other hand, even if the B content exceeds 0.0030%, the above effect is saturated and the cost is only increased. For this reason, the B content is 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0020% or less.

(1-12) N: 0.0100% or less N exists in steel as an impurity. When N exists, it bonds with B to form BN, which reduces the effect of B, so a small N content is preferable, but a content of about 0.0100% is permissible. For this reason, the N content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less.

Next, we will explain optional elements.

(1-13) V: 2.00% or less V is an element that is effective in improving the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after quenching. However, even if the V content exceeds 2.00%, the above effect is saturated and the cost only increases. For this reason, the V content is 2.00% or less, preferably 1.50% or less, and more preferably 1.00% or less. In order to reliably obtain the above effect, the V content is 0.001% or more, preferably 0.10% or more, and more preferably 0.20% or more.

(1-14) Ta: 0.50% or less Ta is an element that is effective in improving the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after quenching. However, if the Ta content exceeds 0.50%, the above effect is saturated and the cost only increases. For this reason, the Ta content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less. In order to reliably obtain the above effect, the Ta content is 0.001% or more, preferably 0.005% or more, and more preferably 0.10% or more.

(1-15) W: 3.00% or less W is an element that is effective in improving the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after quenching. However, when the W content exceeds 3.00%, the above effect is saturated and the cost only increases. For this reason, the W content is 3.00% or less, preferably 2.00% or less, and more preferably 1.00% or less. In order to reliably obtain the above effect, the W content is 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more.

(1-16) Ni: 5.00% or less Ni is an element that is effective in improving the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after quenching. However, when the Ni content exceeds 5.00%, the above effect is saturated and the cost only increases. For this reason, the Ni content is 5.00% or less, preferably 3.00% or less, and more preferably 2.00% or less. In order to reliably obtain the above effect, the Ni content is 0.01% or more, preferably 0.10% or more, and more preferably 0.20% or more.

(1-17) Cu: 3.00% or less Cu is an element that is effective in improving the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after quenching. However, even if the Cu content exceeds 3.00%, the above effect is saturated and the cost only increases. For this reason, the Cu content is 3.00% or less, preferably 2.00% or less, and more preferably 1.00% or less. In order to reliably obtain the above effect, the Cu content is 0.01% or more, preferably 0.20% or more, and more preferably 0.50% or more.

(1-18) Mo: 0.50% or less Mo is an element that is effective in improving the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after quenching. However, even if the Mo content exceeds 0.50%, the above effect is saturated and the cost only increases. For this reason, the Mo content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less. In order to reliably obtain the above effect, the Mo content is 0.005% or more, preferably 0.10% or more, and more preferably 0.20% or more.

(1-19) Mg: 0.0030% or less Mg has the effect of refining inclusions in steel and improving the toughness of hot stamped parts after quenching. However, when the Mg content exceeds 0.0030%, this effect saturates and the cost increases. For this reason, the Mg content is 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0020% or less. In order to reliably obtain the above effect, the Mg content is 0.0005% or more, preferably 0.0010% or more, and more preferably 0.0015% or more.

(1-20) Ca: 0.0030% or less Ca has the effect of refining inclusions in steel and improving the toughness of hot stamped parts after quenching. However, when the Ca content exceeds 0.0030%, this effect saturates and the cost increases. For this reason, the Ca content is 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0020% or less. In order to reliably obtain the above effect, the Ca content is 0.0005% or more, preferably 0.0010% or more, and more preferably 0.0015% or more.

(1-21) La: 0.030% or less La has the effect of refining inclusions in steel and improving the toughness of hot stamped parts after quenching. However, when the La content exceeds 0.030%, this effect saturates and the cost increases. For this reason, the La content is 0.030% or less, preferably 0.020% or less, and more preferably 0.010% or less. In order to reliably obtain the above effect, the La content is 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.

(1-22) Ce: 0.030% or less Ce has the effect of refining inclusions in steel and improving the toughness of hot stamped parts after quenching. However, when the Ce content exceeds 0.030%, this effect saturates and the cost increases. Therefore, the Ce content is 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less. In order to reliably obtain the above effect, the Ce content is 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more.

The remainder is Fe and impurities. Examples of impurities include those contained in raw materials such as ore and scrap, and those contained during the manufacturing process.

(2) Metal structure (2-1) Martensite structure The hot stamped part according to the present invention is quenched by hot stamping, and therefore the metal structure is a martensite structure. The martensite structure includes the automatically tempered martensite disclosed in Patent Documents 1 to 3. The automatically tempered martensite is tempered martensite that is formed during cooling at the time of quenching without performing heat treatment for tempering, and is distinguished from complete martensite by the precipitation of fine cementite inside the laths.

(2-2) Area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the plate thickness: 0.0100% or less Segregation is increased in the center of a slab produced by continuous casting. Even when the slab is rolled into a thin plate, the segregation is continued, and there is a lot of segregation in the center of the thin plate. For this reason, in the present invention, attention is paid to the position of the center of the plate thickness (1/4 to 3/4t).

It is known that MnS can be the starting point of cracks. In the present invention, the formation of MnS is suppressed by reducing the S content in the steel. However, if the anisotropy is large depending on the direction of the steel sheet for hot stamped parts, not only will it be difficult for hot stamp manufacturers to carry out manufacturing operations, but there is also a risk of problems occurring in the characteristics of the hot stamped parts.

The inventors conducted a detailed investigation into the cause of this anisotropy and discovered that MnS is the cause of the deterioration of the anisotropy of toughness. MnS is an inclusion that is easily stretched. When the length of MnS is 100 μm or more, the anisotropy associated with the stretching of MnS becomes large. However, even if MnS of 100 μm or more is present in steel, there is no particular problem of anisotropy as long as its area ratio is 0.0100% or less.

Therefore, in the present invention, the area ratio of MnS with a length of 100 μm or more is specified to be 0.0100% or less. Note that there is no particular lower limit for the area ratio of MnS, but considering the contents of Mn and S present in the steel, there will be 0.0001% or more of MnS, including MnS with a length of 100 μm or less.

(2-3) Proportion of the number of carbides (cementite and NbTi composite carbonitrides) with a grain size of 1.2 μm or more among carbides with a grain size of 0.5 μm or more present in the steel: 10.0% or less In the present invention, since Nb and Ti are contained as essential elements, a composite carbonitride of Nb and Ti, NbTi(C,N), is generated. The composite carbonitride NbTi(C,N) at the slab stage is coarse, and if it remains in the steel plate after rolling, it will cause a decrease in the toughness of the steel plate for hot stamped parts or the hot stamped parts.

Furthermore, if cementite (Fe ₃ C) remains undissolved in the steel sheet for hot stamped parts, it dissolves in austenite during heating of the steel sheet for hot stamped parts, increasing the concentration of C in the austenite. If heating in the hot stamping process is insufficient, for example, if heating is performed quickly for a short period of time to improve manufacturing efficiency, cementite will not dissolve sufficiently, leading to a decrease in the toughness of the hot stamped part.

In this way, the presence of compound carbonitride NbTi(C,N) and cementite (hereinafter collectively referred to as "carbides") reduces toughness. On the other hand, fine compound carbonitride NbTi(C,N) has the effect of increasing the strength of the steel plate by precipitation strengthening due to its pinning action. For this reason, it is necessary to maintain a certain amount of fine compound carbonitride NbTi(C,N).

The inventors conducted a detailed investigation into the relationship between carbides and toughness and found that, regardless of the type of carbide, the presence of a large amount of some coarse carbides reduces the toughness of the steel plate for hot stamped parts or the hot stamped parts. In other words, the toughness of the steel plate for hot stamped parts or the hot stamped parts can be ensured when the proportion of carbides with a grain size of 1.2 μm or more among the carbides with a grain size of 0.5 μm or more present in the steel is 10.0% or less.

In addition, NbTi(C,N) composite carbonitrides with a grain size of less than 1.2 μm contribute to improving the strength of the steel plate. On the other hand, cementite with a grain size of less than 1.2 μm can be easily dissolved in the hot stamping process, and the toughness of the hot stamped part does not decrease. In other words, rather than simply reducing the amount of carbides, if the proportion of relatively fine carbides with a grain size of 0.5 μm or more and less than 1.2 μm in the steel plate is made to exceed 90.0%, the strength and toughness of the hot stamped part can be achieved at the same time.

The reason why the number percentage of carbides with a grain size of 1.2 μm or more is specified as the number percentage excluding fine carbides with a grain size of less than 0.5 μm is that fine carbides may not be observed by tissue observation.

The area ratio of martensite, the area ratio of MnS, and the number ratio of carbides can be measured, for example, by the method described in the examples.

(3) Applications Examples of hot stamped parts include bumper reinforcement and structural members of automobile body shells (e.g., A-pillar reinforcement, B-pillar reinforcement, front side members, rear side members, roof rails, various cross members, etc.).

2. Manufacturing Method of Hot Stamped Component According to the Present Invention Next, a manufacturing method of a hot stamped component according to the present invention will be described.

(1) Rolling step A steel ingot or slab having the above-mentioned chemical composition is heated to a temperature T (°C) or higher represented by the formula (ii): T (°C) = [0.020 + (mass % Mn) (mass % S)] / (1.9 × ^10-5 ) (mass % Mn and mass % S indicate the Mn and S contents, respectively), for 30 minutes or longer, and then rough rolling is completed at 1000 to 1150°C.

MnS and NbTi (C, N) are dissolved when steel ingots or steel pieces are heated. If cooled, they will precipitate again, but if they are reprecipitated after dissolving, these inclusions will be finely dispersed, and it will be difficult to produce MnS with a length of 100 μm or more, or NbTi (C, N) with a diameter of 1.2 μm or more.

MnS dissolves at around 1200 to 1250°C. In addition, NbTi(C,N) dissolves preferentially at 1150°C or higher, and Ti also dissolves when the temperature is further increased. If Nb dissolves preferentially, NbTi(C,N) will finely reprecipitate during the subsequent cooling process. For this reason, in order to dissolve both MnS and NbTi(C,N) and cause them to finely precipitate, the heating temperature of the steel ingot or steel slab must be equal to or higher than the temperature T°C defined by formula (ii), and preferably equal to or higher than 1200°C.

On the other hand, if the heating temperature of the steel ingot or steel billet is too high, the energy costs will increase, so the heating temperature of the steel ingot or steel billet is preferably 1350°C or less, and more preferably 1300°C or less.

In order to sufficiently uniformly heat the steel ingot or billet, it is effective to heat it for 30 minutes or more. However, if the heating time of the steel ingot or billet exceeds 2.0 hours, the energy cost increases, so it is preferable that the heating time of the steel ingot or billet is 2.0 hours or less.

After the steel ingot or billet is heated as described above, rough rolling begins and is completed at 1000-1150°C. If the end temperature of rough rolling is less than 1000°C, the end temperature of finish rolling, described below, cannot be met. On the other hand, if the end temperature of rough rolling exceeds 1150°C, recrystallization will proceed, but the crystal grains will become coarser, leading to coarser crystal grains during finish rolling.

After this, the finish rolling is completed at Ae ₃ or higher. If hot rolling is performed at a temperature lower than Ae ₃ , processed ferrite remains and ductility is significantly deteriorated. In the chemical composition system of the present invention, these problems do not occur if the hot rolling completion temperature is Ae ₃ or higher. On the other hand, if the hot rolling completion temperature exceeds 1050 ° C, surface defects such as scale bite may occur. Therefore, it is preferable that the hot rolling completion temperature is 1050 ° C or lower.

The _Ae3 point is calculated according to the following formula.
_Ae3 (℃) = 937 - 477C + 56Si - 20Mn - 16Cu - 15Ni - 5Cr + 38Mo + 136Ti - 19Nb + 198Al + 3315B
In the above formula, the element symbols represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

(2) Cooling step After the finish rolling is completed, the steel sheet is cooled at an average cooling rate of 10 to 200°C/sec. This makes it possible to suppress the precipitation and growth of the above-mentioned precipitates after hot rolling. The lower limit of the average cooling rate is preferably 20°C/sec, and more preferably 30°C/sec. On the other hand, the upper limit of the average cooling rate is preferably 150°C/sec, and more preferably 100°C/sec.

(3) Winding process After hot rolling, the steel sheet is wound into a coil at 680 to 560°C. By setting the winding temperature at 680°C or less, it is possible to prevent the above-mentioned precipitates from becoming coarse. On the other hand, if the winding temperature is less than 560°C, the structure will be mainly composed of bainite, and the rolling load in the cold rolling performed subsequent to the hot rolling will increase, resulting in a deterioration in cold rolling properties. For this reason, the winding temperature is 560°C or more, and preferably 600°C or more.

(4) Pickling Step The coil (steel strip) wound into a coil after hot rolling is once uncoiled and then pickled to remove scale formed on the surface (descaling).

(5) Cold rolling process After pickling, cold rolling is performed at a rolling reduction of 10 to 60%. Cold rolling may be performed under well-known and commonly used conditions. If the rolling reduction is less than 10%, the structure after annealing tends to be mixed grain, which causes variation in hardenability and tends to cause variation in hardness. On the other hand, if the rolling reduction exceeds 60%, the load during cold rolling becomes large, making it difficult and cost-effective to perform rolling at an industrial level.

(6) Annealing process After cold rolling, annealing is performed by heating to 730 to 900°C. From the viewpoint of preventing the concentration of Cr and Mn in cementite, the annealing after cold rolling is performed at 730°C or higher at which the austenite phase precipitates, and the austenite phase is utilized to promote the re-dissolution of cementite and suppress the concentration of Cr and Mn in cementite. However, if the annealing temperature exceeds 900°C, not only does this effect saturate, but the combustion cost also increases industrially. For this reason, the heating temperature for annealing is set to 730 to 900°C.

In this way, the chemical composition is: C: 0.18-0.50%, Si: 0.001-2.00%, Mn: 0.001-3.00%, P: 0.10% or less, S: 0.0015% or less, Nb: 0.010-0.100%, Cr: 0.001-0.50%, Al: 0.0001-0.100%, Ti: 0.001-0.500%, O: less than 0.0030%, B: 0.0006-0.0030%, N: 0.0100% or less, V: 2.00% or less, Ta: 0.50% or less, W: 3.00% or less, Ni: 5.00% or less, Cu: 3.00% or less, Mo: 0.50% or less, Mg: 0.0030% or less. , Ca: 0.0030% or less, La: 0.030% or less, Ce: 0.030% or less, and the balance: Fe and impurities, the metal structure is a mixed structure of ferrite and pearlite, the area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the plate thickness is 0.0100% or less, and the number ratio of carbides (cementite and NbTi complex carbonitrides) having a grain size of 1.2 μm or more among carbides (cementite and NbTi complex carbonitrides) having a grain size of 0.5 μm or more present in the steel is 10.0% or less.

(7) Hot stamping process The steel sheet for hot stamped parts is subjected to hot stamp forming. The hot stamp forming may be performed under well-known and conventional conditions.

In hot stamp forming, the hot part steel sheet is soft and highly ductile at high temperatures during forming, so press-formed parts with complex shapes can be formed with good dimensional accuracy without forming defects such as breakage, springback, or wall warping. In addition, by heating the hot stamp part steel sheet to a temperature in the austenite single phase region before press forming, and then rapidly cooling and quenching the formed product inside the die used for press forming, the hot stamped parts can be strengthened by martensitic transformation while the hot stamp part steel sheet is being formed.

(8) Heat Treatment Step The obtained hot stamped part is subjected to a heat treatment at 500 to 100°C that satisfies the condition (i): log(t)≦(625−T)/125 (t: heat treatment time (seconds), T: heat treatment temperature (°C)). The hot stamped part of the present invention has fine precipitated carbides, and therefore the strength can be improved. Furthermore, the fine carbides function as hydrogen trapping sites, and can also contribute to improving the hydrogen embrittlement resistance of the hot stamped part. In order to obtain this effect, the hot stamped part is subjected to the heat treatment step.

Formula (i) specifies that by subjecting hot stamped parts to heat treatment, dissolved carbide-forming elements are precipitated and used as hydrogen trapping sites. However, if the heat treatment is held at a specified temperature for a relatively long time, the carbides that form will coarsen and their ability to trap hydrogen will decrease. Therefore, formula (i) specifies the relationship between the heat treatment temperature and heat treatment time that will function as a hydrogen trapping site while still providing the strength required for hot stamping.

In this manner, the hot stamped parts according to the present invention are manufactured.

The present invention will be specifically explained with reference to examples.

A slab having the chemical composition shown in Tables 1 and 2 (the remainder other than that shown in Tables 1 and 2 is Fe and impurities) was heated at the slab heating temperature and slab heating time shown in Tables 3 and 4, and then rough rolling was started. Rough rolling was completed at the rough rolling end temperature shown in Tables 3 and 4. Finish rolling was then completed at the finish rolling end temperature shown in Tables 3 and 4 to a plate thickness of 3.2 mm. The slab was cooled while controlling the cooling temperature, and then wound into a coil at the winding temperature shown in Tables 3 and 4.

After rewinding, the coil was pickled and descaled, then cold rolled to a thickness of 1.6 mm, and annealed at the annealing temperatures shown in Tables 3 and 4. The underlines in Tables 2 and 4 indicate values outside the scope of the present invention. Steels that satisfy formula (i) were judged as ◯, and steels that do not satisfy formula (i) were judged as ×.

The manufactured coils were subjected to hot stamping and heat treatment, and the structure of the hot stamped parts after heat treatment was observed and evaluated for their properties as steel sheets for hot stamped parts. Specifically, hot stamped steel sheets No. 1 to 34 and x1 to x14 were manufactured, and the structure observations shown in (1) to (3) below were performed.

Note that No. 1 and No. x1, No. 2 and No. x2, No. x3 and No. x4, and No. x5 and No. x6 are examples of hot stamped parts manufactured under different heat treatment conditions after the hot stamping process using hot stamped part steel sheets manufactured under the same manufacturing conditions from slabs with the same chemical composition.

(1) Area ratio of martensite A test piece was cut out from a part of a steel sheet, the cross section of the sample L was mirror-polished, and the metal structure was corroded using a nital solution. Then, the ¼t region of the sheet thickness was observed with an optical microscope at a magnification of 500 times, and it was confirmed that the structure was martensite.

(2) Area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the sheet thickness A sample piece was cut out from a part of the hot stamped part, and the sample L cross section was mirror polished. The size of MnS was identified and its area ratio was calculated by image analysis of an optical microscope photograph. Here, the measurement area was the 1/4 to 3/4t region of the sheet thickness, that is, a 20 mm wide region (0.8 mm x 20 mm = 16 mm ² ) in the range of 0.8 mm in the center of the sheet thickness of 1.6 mmt.

(3) The number ratio of carbides (cementite, NbTi complex carbonitrides) with a grain size of 1.2 μm or more among carbides with a grain size of 0.5 μm or more present in steel A sample piece was cut out from a part of a hot stamped part, the sample L cross section was mirror polished, and the number ratio of carbides (cementite and NbTi complex carbonitrides) was measured using a scanning electron microscope. Here, the 1/4 to 3/4t region of the plate thickness, that is, in the case of a plate thickness of 1.6 mmt, a 5 mm wide region (0.8 mm x 5 mm = 4 mm ² ) in the range of 0.8 mm in the center was used as the measurement region, and composition analysis was performed using EDX to confirm whether it was a carbide, and carbides with a grain size of 0.5 μm or more were distinguished into carbides with a grain size of 1.2 μm or more and carbides with a grain size of less than 1.2 μm were counted, and the number of carbides with a grain size of 1.2 μm or more was divided by the number of carbides with a grain size of 0.5 μm or more to calculate the number ratio of carbides with a grain size of 1.2 μm or more.

On the other hand, the steel sheet properties were evaluated using the methods shown below in (4) to (7).

(4) Strength (TS, YS), elongation EL
No. 5 test pieces as specified in JIS Z 2201 were taken from the steel plates, and tensile tests were carried out in accordance with JIS Z 2241 to evaluate the strengths (TS, YS) and elongations (EL).

(5) Limit bending radius
Regarding the bendability evaluation, a 30 mm x 100 mm test piece was taken from the steel plate, and a 90-degree bending test was performed using a punch with a tip R of 2.0 to 5.0 to determine the maximum R at which cracks occurred in the bent portion. Since the maximum R at which cracks occurred also depends on the plate thickness t, the obtained maximum R was divided by the plate thickness t (1.6 mm) to obtain the limit R/t, and a R/t of 2.2 or less was evaluated as having good bendability.

(6) vTrs and vE in the L and C directions
For the hot stamped parts after hot stamping and heat treatment, an impact test was performed to confirm the anisotropy of the parts with respect to the fracture transition temperature (vTrs) and absorbed energy (vE). Steel pieces with a thickness of 1.6 mm were cut out from the hot stamped parts so that their long sides were parallel (L direction) or perpendicular (C direction) to the rolling direction, and six steel pieces in the same direction were stacked and screwed together to prepare V-notch test pieces, which were then subjected to a Charpy impact test.

Here, specimens were evaluated as ○ (good) if the fracture appearance transition temperatures (vTrs) parallel to the rolling direction (L direction) and perpendicular to the rolling direction (C direction) were both −40° C. or lower, and the absorbed energy (vE) at room temperature was 50 J/cm ² or higher.

(7) Hydrogen Resistance Evaluation For the hydrogen resistance evaluation, test pieces 68 mm long and 6 mm wide were cut out from the hot stamped parts after the heat treatment with the rolling direction as the longitudinal direction, and the test pieces were immersed in hydrochloric acid of pH 3 while a load equivalent to the yield stress was applied to the surface of the test pieces by four-point bending. Test pieces that did not crack even after 100 hours were rated as ○ (good), and those that cracked within 100 hours were rated as × (bad). Note that for those that were evaluated for bendability as described above and had a limit R/t of more than 2.2, hydrogen resistance evaluation was not performed because they were not suitable for hot stamped parts in the first place (indicated as "-" in Table 6).

Table 5 shows the results of the microstructure observation. Table 6 shows the results of the mechanical properties of the hot stamped parts. The underlines in Table 5 indicate values outside the scope of the present invention, and the underlines in Table 6 indicate values where the mechanical properties are poor.

No. 1 to No. 34 in Table 6 are examples of the present invention that satisfy all of the provisions of the present invention, and No. x1 to x14 are comparative examples that do not satisfy the provisions of the present invention.

As shown in Table 6, No. 1 to No. In the example of the present invention, no. 34, a steel sheet for hot press parts is obtained having an area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the plate thickness: 0.0041 to 0.0090 (%), and a number ratio of carbides having a grain size of 1.2 μm or more among carbides having a grain size of 0.5 μm or more: 3.1 to 9.7 (%). It can be seen that hot stamp forming and heat treatment of the formed part without tempering can produce a hot stamp part having mechanical properties of TS: 1545 to 2510 (MPa), YS: 989 to 1605 (MPa), EL: 7.8 to 10.7 (%), limit R / t: 1.56 to 2.19, vTrs in L direction and C direction: -40 ° C. or less, vE: 50 J / cm ² or more, and good hydrogen resistance properties. It can be seen that a hot stamped part having high strength (especially tensile strength of 1.5 GPa or more) and greatly improved fracture resistance can be manufactured.

In contrast, No. x1 was a hot stamped part manufactured using the same steel plate for hot stamped parts as No. 1, as described above. No. x1 was manufactured using a heat treatment process that did not satisfy formula (i), and therefore had a poor hydrogen resistance evaluation.

As mentioned above, No. x2 was a hot stamped part manufactured using the same steel plate for hot stamped parts as No. 2. No. x2 was manufactured using a heat treatment process that did not satisfy formula (i), and therefore had a poor hydrogen resistance evaluation.

In No. x3, the S content exceeded the upper limit of the range of the present invention, and T calculated by formula (ii) became large, resulting in the slab heating temperature being lower than T. As a result, the area ratio of MnS and the number ratio of carbides exceeded the upper limit of the range of the present invention, resulting in a steel plate with poor fracture transition temperature (vTrs) and absorbed energy (vE).

In No. x4, the S content exceeded the upper limit of the range of the present invention, and T calculated by formula (ii) became large, resulting in a slab heating temperature lower than T. Therefore, the area ratio of MnS and the number ratio of carbides exceeded the upper limit of the range of the present invention, resulting in a steel plate with poor fracture transition temperature (vTrs) and absorbed energy (vE). As described above, No. x4 is a hot stamped part manufactured using the same hot stamped part steel plate as No. x3. While the hydrogen resistance evaluation of No. x3 was good, the hydrogen resistance evaluation of No. x4 was poor because the hot stamped part was manufactured using a heat treatment process that did not satisfy formula (i).

In No. x5, the S content and Nb content exceeded the upper limit of the range of the present invention, and the number ratio of the above carbides exceeded the upper limit of the range of the present invention, resulting in a steel plate with poor fracture transition temperature (vTrs) and absorbed energy (vE).

In No. x6, the S content and Nb content exceeded the upper limit of the range of the present invention, and the number ratio of the above-mentioned carbides exceeded the upper limit of the range of the present invention, so the fracture transition temperature (vTrs) and absorbed energy (vE) were poor values. As described above, No. x6 is a hot stamped part manufactured using the same hot stamped part steel plate as No. x5. While the hydrogen resistance evaluation of No. x5 was good, the hydrogen resistance evaluation of No. x6 was poor because the hot stamped part was manufactured using a heat treatment process that did not satisfy formula (i).

In No. x7, the S content and Nb content exceeded the upper limit of the range of the present invention, and the slab heating temperature was below the lower limit of the range of the present invention, resulting in a steel plate in which the area ratio of MnS and the number ratio of carbides exceeded the upper limit of the range of the present invention, and the fracture transition temperature (vTrs) and absorbed energy (vE) were poor.

For No. x8, the finish rolling completion temperature was below the lower limit of the range of the present invention, so the tensile strength (TS) was less than 1.5 MPa, which is an unsatisfactory value, and the limit R/t was also an unsatisfactory value.

For No. x9, the average cooling rate after finish rolling was below the lower limit of the range of the present invention, so the number ratio of the above carbides exceeded the upper limit of the range of the present invention, the critical R/t was an unfavorable value, and the fracture transition temperature (vTrs) and absorbed energy (vE) were also unfavorable values.

For No. x10, the coiling temperature exceeded the upper limit of the range of the present invention, so the number ratio of the above carbides exceeded the upper limit of the range of the present invention, the limit R/t was an unfavorable value, and the fracture transition temperature (vTrs) and absorbed energy (vE) were also unfavorable values.

For No. x11, the annealing temperature exceeded the upper limit of the range of the present invention, so the number ratio of the above carbides exceeded the upper limit of the range of the present invention, the limit R/t was an unfavorable value, and the fracture transition temperature (vTrs) and absorbed energy (vE) were also unfavorable values.

For No. x12, the Mn content exceeded the upper limit of the range of the present invention, so the area ratio of the MnS exceeded the upper limit of the present invention, the critical R/t was an unsatisfactory value, and the fracture transition temperature (vTrs) and absorbed energy (vE) were also unsatisfactory values.

No. x13 had an Nb content exceeding the upper limit of the range of the present invention, so the number ratio of the above carbides exceeded the upper limit of the range of the present invention, the critical R/t was an unfavorable value, and the fracture transition temperature (vTrs) and absorbed energy (vE) were also unfavorable values.

Furthermore, in No. x14, the Ti content exceeded the upper limit of the range of the present invention, so the number ratio of the above carbides exceeded the upper limit of the range of the present invention, the critical R/t was an unfavorable value, and the fracture transition temperature (vTrs) and absorbed energy (vE) were also unfavorable values.

Claims (9)

The chemical composition, in mass%, is
C: 0.18 to 0.50%,
Si: 0.001 to 2.00%,
Mn: 0.001 to 3.00%,
P: 0.10% or less,
S: 0.0015% or less,
Nb: 0.010 to 0.100%,
Cr: 0.001 to 0.50%,
Al: 0.0001 to 0.100%,
Ti: 0.001 to 0.500%,
O: less than 0.0030%,
B: 0.0006 to 0.0030%,
N: 0.0100% or less, and the balance: Fe and impurities;
The metal structure is martensite,
The area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the plate thickness is 0.0100% or less,
A hot stamped part having a tensile strength of 1.5 GPa or more, in which the number ratio of carbides having a grain size of 1.2 μm or more among carbides having a grain size of 0.5 μm or more present in steel is 10.0% or less. The chemical composition, in mass%,
V: 2.00% or less,
The hot stamped part according to claim 1 , comprising one or more selected from the group consisting of Ta: 0.50% or less, and W: 3.00% or less. The chemical composition, in mass%,
Ni: 5.00% or less,
The hot stamped part according to claim 1 or 2, comprising one or more selected from the group consisting of Cu: 3.00% or less, and Mo: 0.50% or less. The chemical composition, in mass%,
Mg: 0.0030% or less,
Ca: 0.0030% or less,
The hot stamped part according to any one of claims 1 to 3, comprising one or more selected from La: 0.030% or less, and Ce: 0.030% or less. A method for producing a hot stamped component according to any one of claims 1 to 4, comprising the steps of:
The chemical composition, in mass%, is
C: 0.18 to 0.50%,
Si: 0.001 to 2.00%,
Mn: 0.001 to 3.00%,
P: 0.10% or less,
S: 0.0015% or less,
Nb: 0.010 to 0.100%,
Cr: 0.001 to 0.50%,
Al: 0.0001 to 0.100%,
Ti: 0.001 to 0.500%,
O: less than 0.0030%,
B: 0.0006 to 0.0030%,
N: 0.0100% or less, and the balance: Fe and impurities;
A hot stamping process of hot stamping a steel sheet having a metal structure which is a mixed structure of ferrite and pearlite and an area ratio of MnS having a length of 100 μm or more at a depth position of 1/4 to 3/4 of the sheet thickness is 0.0100% or less;
and a heat treatment step of subjecting the obtained hot stamped part to a heat treatment at 500 to 100° C. that satisfies the condition of the following formula (i).
log(t)≦(625−T)/125 (i)
In formula (i), t is the heat treatment time (seconds), and T is the heat treatment temperature (° C.). The chemical composition, in mass%,
V: 2.00% or less,
The method for producing a hot stamped part according to claim 5, further comprising the step of: Ta: 0.50% or less; and W: 3.00% or less. The chemical composition, in mass%,
Ni: 5.00% or less,
The method for producing a hot stamped part according to claim 5 or 6, further comprising the step of: adding at least one selected from the group consisting of Cu: 3.00% or less, and Mo: 0.50% or less. The chemical composition, in mass%,
Mg: 0.0030% or less,
Ca: 0.0030% or less,
The method for producing a hot stamped part according to any one of claims 5 to 7, comprising one or more selected from La: 0.030% or less, and Ce: 0.030% or less. A method for producing a hot stamped component according to any one of claims 1 to 4, comprising the steps of:
a rolling step of heating a steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 at a temperature equal to or higher than T (°C) represented by the formula (ii) for a time period of 30 minutes or more, completing rough rolling at 1000 to 1150°C, and completing finish rolling at an Ae point of ₃ or higher;
A cooling step of cooling at an average cooling rate of 10 to 200 ° C. / sec.
A winding process at 680 to 560°C;
A pickling process for removing scale by pickling;
A cold rolling process in which the reduction ratio is 10 to 60%;
An annealing step of heating to 730 to 900 ° C.;
a hot stamping process for hot stamping the steel sheet that has been annealed;
and a heat treatment step of subjecting the obtained hot stamped part to a heat treatment at 500 to 100° C. that satisfies the condition of the following formula (i).
log(t)≦(625−T)/125 (i)
T (° C.)=[0.020+(mass% Mn)(mass% S)]/(1.9×10 ⁻⁵ ) (ii)
In formula (i), t is the heat treatment time (seconds), and T is the heat treatment temperature (° C.). In formula (ii), mass % Mn and mass % S represent the contents of Mn and S, respectively.