JP7518337B2 - Steel sheet for hot stamped parts and its manufacturing method - Google Patents

Description

The present invention relates to a steel sheet for hot stamped parts and a manufacturing method thereof.

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 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 at an upper critical cooling rate or more and at an average cooling rate from the Ms point to 150°C at a rate of 10 to 500°C/sec.

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 describes a steel sheet having a chemical composition 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%, the balance being Fe and unavoidable impurities, and having a number density of inclusions and precipitates with a maximum length of 10 μm or more of 100 pieces/mm The present invention 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 metal structure in which the grain size of prior austenite after hot stamping is 10 ^μm or less and the grain size of prior austenite 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.

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, 1.8 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 D 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 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 are the starting point for fracture of hot stamped parts during collisions can be reduced as much as possible, thereby greatly improving the fracture resistance of hot stamped parts, making it possible to use hot stamped parts with a tensile strength of 1.8 GPa or more, for example, in structural members (skeletal members) of body shells.

The present invention is as follows:

(1) The chemical composition is C: 0.26-0.50%, Si: 0.001-2.00%, Mn: 0.001-3.00%, P: 0.100% or less, S: less than 0.0050%, Nb: 0.0001-0.100%, Cr: 0.001-0.50%, Al: 0.0001-0.1000%, 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 steel sheet for hot stamped parts, having a metal structure which is a mixed structure of ferrite and pearlite, in which, in terms of area percentage, pearlite is 40% or more, and which satisfies the following formulas (i) and (ii):
C _Mn /Mn<1.30...(i)
C _Cr /Cr<1.50...(ii)
In the formula, Mn and Cr represent the contents (% by mass) of the respective elements, and C _Mn and C _Cr represent the Mn concentration (% by mass) and the Cr concentration (% by mass), respectively, in cementite having a grain size of 0.2 μm or more that is present in the ferrite grain boundaries.

(2) The steel sheet for hot stamped parts described in paragraph 1, wherein the chemical composition contains one or more selected from V: 2.00% or less, Ta: 0.50% or less, and W: 3.00% or less.

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

(4) A steel sheet for hot stamped parts according to any one of items 1 to 3, in which the chemical composition contains 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 steel sheet for hot stamped parts according to any one of items 1 to 4,
a rolling step in which a steel ingot or billet having the above chemical composition is heated to 1150 to 1350°C, followed by rough rolling at 1000 to 1150°C and finish rolling at _Ae3 +50°C or higher;
After the rolling step, a cooling step of holding the sheet for 5 seconds or more and then cooling the sheet to 800° C. at an average cooling rate of 30° C./sec or less;
a winding step of winding the sheet at 500 to 600° C. after the cooling step;
A method for producing a steel sheet for hot stamped parts, comprising:

(6) The method for producing a steel sheet for hot stamped parts described in Item 5, which includes a cold rolling process in which descaling is performed after the coiling process and cold rolling is performed.

(7) The method for producing a steel sheet for hot stamped parts described in claim 6, which includes an annealing step in which annealing is performed after the cold rolling step.

(8) The method for producing a steel sheet for hot stamped parts described in Item 5, which includes an annealing process in which descaling is performed and annealing is performed after the coiling process.

The present invention makes it possible to manufacture hot stamped parts that have ultra-high strength (particularly tensile strength of 1.8 GPa or more) and greatly improved fracture resistance, without tempering, by hot stamping and quenching at that time. This makes it possible to use hot stamped parts with tensile strength of 1.8 GPa or more, for example, in structural members (framework members) of body shells.

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: 0.26-0.50%
C is a very important element that enhances the hardenability of steel sheets for hot stamped parts and mainly determines the tensile strength of the hot stamped parts after quenching. In particular, the tensile strength of the hot stamped parts after quenching is as follows: In order to ensure a modulus of 8 GPa or more, the C content is 0.26% or more, preferably 0.28% or more, and more preferably 0.30% or more. If the amount of C in the steel sheet is too large, the tensile strength of the hot stamped part after quenching becomes too high, and the deterioration of deformability becomes significant. Therefore, the C content is set to 0.50% or less, and preferably 0.40% or less. .38% or less is even more preferable.

(1-2) Si: 0.001-2.00%
Silicon is effective in improving the hardenability of steel sheets for hot stamped parts and stably achieving high strength of the hot stamped parts after quenching. In order to obtain this effect, the Si content is set to 0. The Si content is 0.001% or more, preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Si content exceeds 2.00%, the bendability is significantly reduced. 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: 0.001-3.00%
Mn is an element that is extremely effective in improving the hardenability of steel sheets for hot stamped parts and in stably obtaining high strength of the hot stamped parts after quenching. If the Mn content is less than 0.001%, This effect cannot be sufficiently obtained. Therefore, the Mn content is 0.001% or more, preferably 0.50% or more, and more preferably 1.00% or more. If the Mn content exceeds 0.00%, the above effect is saturated, and it becomes difficult to ensure a stable tensile strength. Therefore, the Mn content is set to 3.00% or less and 2.50% or less. is preferable, and 2.00% or less is more preferable.

(1-4) P: 0.100% or less Since P significantly deteriorates the toughness of hot stamped parts after quenching, the lower the P content, the better, but a content of 0.100% is permissible. Therefore, the P content is 0.100% 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: Less than 0.0050% Since the smaller the S content, the more the deformability and fracture resistance improve, the S content is less than 0.0050%. It is preferably less than 0.0010%. However, since it is costly to remove S in order to reduce the S content, it is preferable that the S content is 0.0001% or more. It is more preferable that the S content is 0.0003% or more.

(1-6) Nb: 0.0001-0.100%
Nb suppresses recrystallization when steel sheets for hot stamped parts are heated to Ac ₃ or higher, and forms fine carbides to refine the austenite grains, greatly improving the toughness of the hot stamped parts. In order to obtain this effect, the Nb content is 0.0001% or more, preferably 0.020% or more, and more preferably 0.040% or more. %, the above effect saturates and it becomes difficult to ensure a stable tensile strength. For this reason, the Nb content is 0.100% or less, and 0.080% or less is preferable. It is preferable that the content of C is 0.060% or less, and more preferable that the content of C is 0.060% or less.

(1-7) Cr: 0.001-0.50%
Cr is an element that is extremely effective in improving the hardenability of steel sheets for hot stamped parts and in stably obtaining high strength of the hot stamped parts after quenching. is 0.001% or more, preferably 0.05% or more, and more preferably 0.10% 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 stably ensure tensile strength. Therefore, the Cr content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

(1-8) Al: 0.0001-0.1000%
Al is an element that is effective in deoxidizing molten steel and improving the soundness of the steel sheet. To obtain this effect, the Al content is 0.0001% or more, preferably 0.0100% or more, and 0.0100% or more is preferable. On the other hand, if the Al content exceeds 0.1000%, the alumina becomes coarse and the bendability decreases. For this reason, the Al content is 0.1000% or less. , 0.0600% or less is preferable, and 0.0400% or less is further preferable.

(1-9) Ti: 0.001-0.500%
Ti preferentially bonds with N to form TiN, suppressing the consumption of B due to the formation of BN, and has the effect of allowing B to function effectively. In order to reliably obtain this effect, the Ti content is The Ti content is 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, even if the Ti content exceeds 0.500%, the above effects are saturated and the cost increases. Therefore, the Ti content is 0.500% or less, preferably 0.040% or less, and more preferably 0.030% 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: 0.0006-0.0030%
B is effective in improving the hardenability of steel sheets 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 segregates at the grain boundaries and effectively prevents the formation of tensile strength problems. B is also an important element in terms of increasing the grain boundary strength and improving the toughness of hot stamped parts. Furthermore, B has a high effect of suppressing the growth of austenite grains during heating of steel sheets for hot stamped parts. To obtain this, the B content is 0.0006% or more, preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the B content exceeds 0.0030%, the B carbon The formation of nitrides reduces the amount of solute B, which deteriorates bendability. Therefore, the B content is 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0020% or less. The following is even more preferred:

(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 0.10% 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 The metal structure is a mixed structure of ferrite and pearlite, in which, in terms of area percentage, pearlite is 40% or more, and the following formulas (i) and (ii) are satisfied.
C _Mn /Mn<1.30...(i)
C _Cr /Cr<1.50...(ii)
In the formula, Mn and Cr represent the contents (% by mass) of the respective elements, and C _Mn and C _Cr represent the Mn concentration (% by mass) and the Cr concentration (% by mass), respectively, in cementite having a grain size of 0.2 μm or more that is present in the ferrite grain boundaries.

(2-1) Mixed Structure of Ferrite and Pearlite The steel sheet according to the present invention has a mixed structure of ferrite and pearlite as a metal structure from the viewpoint of workability of the hot stamping blank, formability and hardenability during hot stamping, and the like.

(2-2) Pearlite area ratio: 40% or more Pearlite has a structure in which ferrite and cementite Fe ₃ C are arranged in alternating layers, and therefore has the property of being easily melted. In the heating process in the hot stamping process, the temperature of the steel plate for hot stamped parts can be raised in a short time, and quenching can be easily performed, so that the high strength of the hot stamped parts can be ensured.

On the other hand, carbides such as the composite carbonitride NbTi(C,N) and cementite other than the cementite that constitutes pearlite (cementite outside pearlite) are difficult to dissolve even when heated, and can induce cracks and impair the fracture resistance properties.

For this reason, it is preferable for the carbon in the steel to exist as cementite, which constitutes pearlite, as much as possible. Therefore, the area ratio of pearlite is set to 40% or more.

(2-3) Satisfying the above formulas (i) and (ii) Usually, some of the Fe atoms in cementite Fe ₃ C are replaced by Cr or Mn. Cementite replaced by Cr or Mn is stable, making it more difficult for the cementite to dissolve when heated.

In the present invention, even if cementite is present at the ferrite grain boundaries, it can be sufficiently dissolved during heating in the hot stamping process. For this reason, the Mn concentration/Mn content (concentration) in the cementite present at the ferrite grain boundaries is set to less than 1.30, and the Cr concentration/Cr content (concentration) is set to less than 1.50.

The area ratios of ferrite and pearlite, and the concentrations of Mn and Cr in cementite can be measured, for example, by the method described in the Examples.

(3) Applications The steel sheet according to the present invention is used for hot stamped parts, such as 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 Steel Sheet for Hot Stamped Part According to the Present Invention Next, a manufacturing method of steel sheet for hot stamped part according to the present invention will be described.

(1) Rolling Step A steel ingot or billet having the above-mentioned chemical composition is heated to 1150 to 1350°C, then rough rolling is completed at 1000 to 1150°C, and finish rolling is completed at Ae ₃ +50°C or higher.
MnS and complex carbonitride NbTi(C,N) are dissolved when steel ingots or steel slabs 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 10 μm or more and complex carbonitride NbTi(C,N) with a diameter of 0.1 μm or more.

MnS dissolves at around 1200-1250°C. In addition, in the compound carbonitride NbTi(C,N), Nb dissolves preferentially at 1150°C or higher, and Ti also dissolves when the temperature is further increased. If Nb dissolves preferentially, the compound carbonitride NbTi(C,N) will finely reprecipitate during the subsequent cooling process. For this reason, in order to dissolve both MnS and the compound carbonitride NbTi(C,N) and finely precipitate them, the heating temperature of the steel ingot or steel piece is 1150°C or higher, and preferably 1200°C or higher.

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 1350°C or less, and preferably 1300°C or less.

In order to ensure that the steel ingot or billet is sufficiently uniformly heated, it is preferable to heat it for 40 minutes or more, and more preferably for 1 hour or more. However, since energy costs increase if the heating time of the steel ingot or billet exceeds 2.0 hours, 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 rough rolling completion temperature is less than 1000°C, the completion temperature for the finish rolling described below cannot be met. On the other hand, if the rough rolling completion temperature exceeds 1150°C, recrystallization will proceed, but the crystal grains will become coarser, leading to coarser crystal grains during the finish rolling.

Thereafter, finish rolling is completed at _Ae3 +50°C or higher. The hot rolling completion temperature should not be lower than _Ae3 +50°C. If hot rolling is performed at a temperature lower than _Ae3 +50°C, worked 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 _Ae3 +50°C 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.
Ae ₃ (℃)=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 rolling step, the material is held at this temperature for 5 seconds or more, and then cooled to 800° C. at an average cooling rate of 30° C./second or less.
Finish rolling is completed at _Ae3 +50°C or higher, and primary cooling is started after holding for 5 seconds or more after finish rolling, i.e., a time of 5 seconds or more is ensured between finish rolling and the start of cooling, thereby transforming from recrystallized austenite and coarsening the ferrite grains. The coarsening of the ferrite grain size reduces the area of the ferrite grain boundaries, and as a result, the number density of cementite with a grain size of 0.2 μm or more outside pearlite that exists at the ferrite grain boundaries can be reduced.

It is desirable for cementite not to be present at the ferrite grain boundaries, but its formation is unavoidable. It is considered that minute cementite (grain size less than 0.2 μm) present at the ferrite grain boundaries does not affect the properties, and the material is held for 5 seconds or more to reduce the number density of cementite with a grain size of 0.2 μm or more. In addition, the average cooling rate up to 800°C is controlled to 30°C/second or less to form a mixed structure of ferrite and pearlite with pearlite at 40% or more.

(3) Coiling Step After hot rolling, the steel sheet is wound into a coil at 500 to 600° C. in order to prevent the concentration of Cr and Mn in the cementite present at the ferrite grain boundaries.

In other words, the concentration of Cr and Mn in cementite progresses due to the diffusion of Cr and Mn in the steel. Compared to cementite in pearlite, cementite present on ferrite grain boundaries is concentrated faster due to grain boundary diffusion. In order to suppress concentration, it is necessary to coil the steel in a temperature range where the diffusion rate is small.

From this viewpoint, in the present invention, the coiling temperature is set to 600°C or less. As a result, the cementite present in the ferrite grain boundaries becomes cementite that satisfies C _Mn /Mn < 1.30, _{C Cr} /Cr < 1.50. On the other hand, if the coiling temperature is set to less than 500°C, the diffusion rate is significantly reduced, so Cr and Mn do not concentrate in the cementite in the first place, but manufacturability may be impaired by an increase in the load during cold rolling. Therefore, in the present invention, the coiling temperature is set to 500 to 600°C.

If necessary, the coil (steel strip) may then be unwound and the scale formed on the surface may be removed (descaled) by one or more processes such as pickling, shot blasting, grinding, etc.

(4) Cold rolling process After coiling, cold rolling is performed as necessary. Cold rolling is performed after the above-mentioned descaling is performed. Cold rolling may be performed under well-known and commonly used conditions, and the cold rolling temperature is preferably 10 to 60°C. Cold rolling has the effect of making the crystal grain size fine after hot stamping.

(5) Annealing process After cold rolling, annealing is performed as necessary. After coiling, pickling, etc., and descaling, annealing may be performed directly without performing the cold rolling process. Annealing may be performed at 600°C or higher, and from the viewpoint of preventing the concentration of Cr and Mn in cementite, 700°C or higher at which the austenite phase precipitates is preferable. The concentration of Cr and Mn in cementite is suppressed by promoting the re-dissolution of cementite using the austenite phase. However, when the annealing temperature exceeds 900°C, not only is this effect saturated, but the combustion cost increases industrially.

Annealing can be continuous annealing in the uncoiled state, or box annealing in the coiled state.

The annealing conditions may be any well-known or customary condition. For example, when continuous annealing a cold-rolled steel strip, the strip is heated to 730-900°C, held at that temperature range for at least 10 seconds, then cooled to a temperature range of 300-500°C at an average cooling rate of 1-100°C/s, further held at the temperature range of 300-500°C for 30 seconds to 10 minutes, and then cooled to room temperature at an average cooling rate of 1-50°C/s to perform annealing.

In this way, the steel sheet for hot stamped parts according to the present invention is 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 Table 3, after which rough rolling was started, and rough rolling was completed at the rough rolling completion temperature shown in Table 3. Finish rolling was then completed at the finish rolling completion temperature shown in Table 3 to a plate thickness of 3.2 mm, and the plate was wound into a coil at the winding temperature shown in Table 3.

Some of the coils were rewound, descaled, and annealed. Others were rewound, pickled, descaled, and then cold-rolled to a thickness of 1.6 mm.

Some of the cold-rolled pieces were further annealed. For the pieces that were cold-rolled and annealed, the cold-rolling temperature and annealing temperature are shown in Table 3. Note that RT in Table 3 stands for room temperature. The underlines in Tables 2 and 3 indicate that the temperature is outside the scope of the present invention.

製造したコイルについては、組織観察を行うとともに、ホットスタンプ処理を施し、ホットスタンプ部品用鋼板としての特性を有するか評価した。具体的には、Ｎｏ．１～３２、ｘ１～ｘ１４のホットスタンプ部品用鋼板を製造するとともに、下記（１）～（２）に示す組織観察を行った。

The produced coils were subjected to structural observation and hot stamping treatment to evaluate whether they have properties as steel sheets for hot stamped parts. Specifically, steel sheets for hot stamped parts Nos. 1 to 32 and x1 to x14 were produced and subjected to structural observation as shown in (1) and (2) below.

(1) Metal structure The cross section of the sample L was mirror-polished, then etched with nital, and the structure was observed at 1/4 of the plate thickness using an optical microscope. The area ratio of ferrite and the area ratio of pearlite were determined by image analysis from optical microscope photographs at a magnification of 500 times.

(2) Mn concentration (mass%) and Cr concentration (mass%) in cementite with a grain size of 0.2 μm or more present at ferrite grain boundaries relative to the Mn content and Cr content in steel
A sample piece was cut out from a part of the steel sheet, and the sample L cross section was mirror-polished, then nital etched and observed in a scanning microscope. Here, the structure observation was performed in a 100 μm × 100 μm region at the 1/4t position of the sheet thickness from the viewpoint of investigating the number density of cementite at an average position, and cementite with a grain size of 0.2 μm or more present at the ferrite grain boundary was subjected to composition analysis by EDX, and the Cr concentration C _Cr and the Mn concentration C _Mn were measured. The Cr concentration and Mn concentration were divided by the Cr concentration Cr and the Mn concentration Mn of the slab to obtain C _Cr /Cr and C _Mn /Mn.

On the other hand, the properties of the hot stamped parts were evaluated by the following methods (3) to (4).

(3) Strength (TS, YS), elongation EL
No. 5 test pieces as specified in JIS Z 2201 were taken from the hot stamped parts , and tensile tests were carried out in accordance with JIS Z 2241 to evaluate the tensile strength TS, yield strength YS, and elongation EL.

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

The results of the structure observation are shown in Table 4. The results of the plate thickness and mechanical properties of the hot stamped parts are shown in Table 5. The underlines in Table 4 indicate values outside the range of the present invention, and the underlines in Table 5 indicate values with poor mechanical properties.

No. 1 to No. 32 in Table 5 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 5, the present invention examples No. 1 to No. 32 have a mixed structure of ferrite and pearlite, and the Mn concentration (mass%) and Cr concentration (mass%) in the cementite with a grain size of 0.2 μm or more present at the ferrite grain boundary are 40% or more by area%, and the Mn concentration (mass%) and Cr concentration (mass%) satisfy C _Mn /Mn < 1.3 and _{C Cr} /Cr < 1.5. It can be seen that a hot stamped part having mechanical properties of TS: 1834 to 2509 (MPa), YS: 1265 to 1660 (MPa), EL: 8.3 to 10.4 (%), and limit R / t: 1.88 to 2.19 can be manufactured without tempering, by hot stamp forming and quenching at that time. The hot stamped part has ultra-high strength (especially tensile strength of 1.8 GPa or more, and particularly excellent tensile strength of 2.0 GPa or more) and has greatly improved fracture resistance.

In contrast, No. x1 and No. x2 had poor critical R/t values because the S content exceeded the upper limit of the range of the present invention.

In No. x3, the holding time after the completion of finish rolling was below the lower limit of the range of the present invention, so the Mn concentration (C _Mn /Mn) and Cr concentration (C _Cr /Cr) in the cementite were also high, and the limit R/t was an unfavorable value.

In No. x4, the average cooling rate up to 800°C exceeded the upper limit of the range of the present invention, resulting in a low pearlite area ratio and an unfavorable limit R/t value.

No. x5 had a low tensile strength Ts because the winding temperature exceeded the upper limit of the range of the present invention.

No. x6 had an unfavorable limit R/t value because the Si content exceeded the upper limit of the range of the present invention.

In No. x7, the Mn content exceeded the upper limit of the range of the present invention, and therefore the Mn concentration (C _Mn /Mn) was high and the limit R/t was an unsatisfactory value.

In No. x8, the Cr content exceeded the upper limit of the range of the present invention, and therefore the Cr concentration (C _Cr /Cr) was high and the limit R/t was an unsatisfactory value.

No. x9 had an unfavorable limit R/t value because the Nb content exceeded the upper limit of the range of the present invention.

No. x10 had an unfavorable limit R/t value because the Ti content exceeded the upper limit of the range of the present invention.

No. x11 had a poor limit R/t value because the B content exceeded the upper limit of the range of the present invention.

No. x12 had an unfavorable limit R/t value because the Al content exceeded the upper limit of the range of the present invention.

No. x13 had an unfavorable limit R/t value because the N content exceeded the upper limit of the range of the present invention.

No. x14 had an unfavorable limit R/t value because the O content exceeded the upper limit of the range of the present invention.

Claims (8)

The chemical composition, in mass%, is
C: 0.26-0.50%,
Si: 0.001 to 2.00%,
Mn: 0.001 to 3.00%,
P: 0.100% or less,
S: less than 0.0050%,
Nb: 0.0001 to 0.100%,
Cr: 0.001-0.50%,
Al: 0.0001 to 0.1000%,
Ti: 0.001 to 0.500%,
O: less than 0.0030%,
B: 0.0006-0.0030%,
N: 0.0100% or less, and
The balance is Fe and impurities.
The metal structure is a mixed structure of ferrite and pearlite, and is represented by the area percentage:
Pearlite: 40% or more, and
A steel sheet for hot stamped parts, which satisfies the following formulas (i) and (ii):
C _Mn /Mn<1.30...(i)
C _Cr /Cr<1.50...(ii)
In the formula, Mn and Cr represent the contents (% by mass) of the respective elements, and C _Mn and C _Cr represent the Mn concentration (% by mass) and the Cr concentration (% by mass), respectively, in cementite having a grain size of 0.2 μm or more that is present in the ferrite grain boundaries. The chemical composition, in mass%,
V: 2.00% or less,
Ta: 0.50% or less, and
W: 3.00% or less,
The steel sheet for hot stamped parts according to claim 1, comprising one or more selected from the following: The chemical composition, in mass%,
Ni: 5.00% or less,
Cu: 3.00% or less, and
Mo: 0.50% or less,
The steel sheet for hot stamped parts according to claim 1 or claim 2, comprising one or more selected from the following: The chemical composition, in mass%,
Mg: 0.0030% or less,
Ca: 0.0030% or less,
La: 0.030% or less, and
Ce: 0.030% or less,
The steel sheet for hot stamped parts according to any one of claims 1 to 3, comprising one or more selected from the following: A method for producing a steel sheet for hot stamped parts according to any one of claims 1 to 4,
a rolling step in which a steel ingot or billet having the above chemical composition is heated to 1150 to 1350°C, followed by rough rolling at 1000 to 1150°C and finish rolling at _Ae3 +50°C or higher;
After the rolling step, a cooling step of holding the sheet for 5 seconds or more and then cooling the sheet to 800° C. at an average cooling rate of 30° C./sec or less;
a winding step of winding the sheet at 500 to 600° C. after the cooling step;
A method for producing a steel sheet for hot stamped parts, comprising: The method for producing a steel sheet for hot stamped parts according to claim 5, further comprising a cold rolling step in which descaling is performed after the coiling step and cold rolling is performed. The method for producing a steel sheet for hot stamped parts according to claim 6, further comprising an annealing step in which annealing is performed after the cold rolling step. The method for producing a steel sheet for hot stamped parts according to claim 5, further comprising an annealing step in which descaling is performed and annealing is performed after the coiling step.