JP2021155791A - Steel plate for hot-stamping component and method for manufacturing the same - Google Patents

Abstract

To manufacture a hot-stamping component that is excellent in fracture resistance, has tensile strength of 1.5 GPa or more and is excellent in the fracture resistance, while remained hot-stamp molded and quenched thereafter.SOLUTION: A steel plate for a hot-stamping component comprises: 0.18-0.50% of C; 0.001-2.00% of Si; 0.001-3.00% of Mn; 0.100% or less of P; 0.0015% or less of S; 0.010-0.100% of Nb; 0.001-0.50% of Cr; 0.0001-0.100% of Al; 0.001-0.500% of Ti; less than 0.0030% of O; 0.0006-0.0030% of B; and 0.0100% or less of N, with the balance comprising Fe and impurities, wherein: formulae (i) to (iii) are satisfied; a metallic structure comprises ferrite having a ferrite grain size of 30.0 μm or less and 15% or more of perlite, wherein a total area of the ferrite and perlite is 95% or more, and a balance structure contains bainite and one or more of residuals γ and θ; an area rate of MnS having a length of 100 μm or more at a depth position of 1/4-3/4 of plate thickness is 0.0100% or less; and out of carbides having a grain size of 0.5 μm or more existing in the steel, a number percentage of carbides having the grain size of 1.2 μm or more is 10.0% or less.SELECTED DRAWING: None

Description

The present invention relates to a steel sheet for hot stamping parts and a method for manufacturing the same.

In recent years, efforts to reduce the weight of automobiles by increasing the strength of the steel materials used have been strongly made in order to improve the fuel efficiency of automobiles. As a result, in the production of parts manufactured by cold press molding of thin steel plates widely used in automobiles (hereinafter referred to as "press molded parts"), it is possible to manufacture press molded parts having a complicated shape. , It becomes difficult due to the decrease in press formability due to the increase in the strength of the steel sheet.

Specifically, due to the decrease in ductility of the steel sheet, there are problems such as fracture at a highly processed part of the press-formed part, so-called springback and wall warpage, and deterioration of the dimensional accuracy of the press-formed part. Is occurring frequently. In particular, it is not easy to manufacture a press-molded part made of a high-strength steel plate having a tensile strength of 780 MPa or more.

By roll forming rather than cold press forming, roll-formed parts made of high-strength steel sheets can be easily manufactured. However, in roll molding, only roll-molded parts having a constant cross-sectional shape in the longitudinal direction can be manufactured, and press-molded parts having a complicated cross-sectional shape cannot be manufactured.

On the other hand, in the hot stamping method (also called hot press forming method) in which a heated steel sheet is press-formed, the high-temperature steel sheet at the time of forming is soft and highly ductile, so that the press-formed part has a complicated shape. Can be molded with high dimensional accuracy without causing molding defects such as breakage, springback, and wall warpage.

Moreover, according to the hot stamping method, the steel sheet is heated to a temperature in the austenite single-phase region and then press-molded, and the molded product is rapidly cooled and baked inside the mold used for press-molding. At the same time as molding, it is possible to increase the strength of press-molded parts by martensitic transformation. As described above, the hot stamping method is an excellent technique suitable for manufacturing high-strength press-molded parts. In the following description, the press-molded parts manufactured by the hot stamping method will be referred to as "hot stamping parts".

Currently, as an example of a hot stamping component, a bumper reinforcement having a relatively simple shape is known. Hot stamping parts having a tensile strength of 1.5 GPa class have already been widely used for structural members (skeleton members) of body shells such as bumper reinforcements, for example, B-pillar reinforcements.

In recent years, it has been studied to manufacture hot stamped parts having higher strength, particularly tensile strength of 1.8 GPa or more, and mainly bears an impact load in the event of a collision such as B-pillar reinforcement. It is being studied to use hot stamped parts having a tensile strength of 1.8 GPa or more for the structural members (skeleton members) of the body shell.

However, when the tensile strength of the hot stamped component reaches an ultra-high strength of 1.8 GPa or more, the deformability of the hot stamped component is insufficient and the fracture resistance (for example, bendability) of the hot stamped component deteriorates. There is an increased risk that the absorbed energy of the hot stamping component will decrease and that the hot stamping component itself will break. For this reason, only 1.8 GPa class bumper reinforcements have been put into practical use as hot stamping parts with a tensile strength of 1.8 GPa or more, and in fact, they have a high deformability and a tensile strength of 1.8 GPa or more. No examples of manufacturing hot stamping parts have been reported so far.

Therefore, in order to manufacture a hot stamped part having a tensile strength of 1.8 GPa or more, it is necessary to further temper the hot stamped part to enhance its deformability. However, adding a tempering process to the hot stamping process significantly increases the manufacturing cost of hot stamping parts due to a decrease in work efficiency and an increase in equipment cost.

In Patent Document 1, C: 0.25 to 0.45% (in the present specification, "%" regarding the chemical composition or concentration means "mass%" unless otherwise specified), Mn + Cr: 0.5 to Ac. _{Press molding is performed after holding in a temperature range of 3} points or more (Ac ₃ points + 100 ° C.) for 5 minutes or less, and then the cooling rate to the Ms point is equal to or higher than the upper critical cooling rate, and the average from the Ms point to 150 ° C. It has a steel structure composed of automatically tempered martensite with an average austenite particle size of 10 μm or less by cooling at a cooling rate of 10 to 500 ° C./sec, and has excellent tensile strength as it is hardened. A hot stamped component having a temperature of 1.8 GPa or more and a steel plate for this hot stamped component are disclosed.

Patent Documents 2 and 3 describe C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 1.0%, 3.42N + 0.001 ≦ Ti ≦ 3. An amount of Ti that satisfies .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. The steel plate having the chemical composition contained is _{held in a temperature range of Acc 3} points or more (Ac ₃ points + 100 ° C.) or less for 5 minutes or less, and then press-molded, and then the cooling rate to the Ms point is equal to or higher than the upper critical cooling rate. Moreover, it has a microstructure containing automatic tempered martensite having an old austenite particle size of 10 μm or less by cooling at an average cooling rate of 10 to 500 ° C./sec from the Ms point to 150 ° C., and has toughness as it is hardened. A hot stamped component having an excellent tensile strength of 1.8 GPa or more is disclosed.

Patent Document 4 describes 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: It has a chemical composition of 0.005 to 0.04%, Nb: 0.03 to 0.12%, balance: Fe and unavoidable impurities, and the number density of inclusions and precipitates having a maximum length of 10 μm or more is 100. It has a tensile strength of 1.8 to 2.5 GPa obtained by hot stamping a steel plate having a metallographic structure having a metal structure of / mm ^{2 or less and an old austenite particle size of 10 μm or less after hot stamping.} A hot stamped steel plate member having excellent toughness is disclosed.

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

According to the inventions disclosed in Patent Documents 1 to 4, it is true that a hot stamped component having a tensile strength of 1.8 GPa or more as it is hardened is provided.

However, as described above, in order to use a hot stamping component having a tensile strength of 1.8 GPa or more for the structural member (skeleton member) of the body shell, the hot stamping component has a high strength (tensile strength of 1.8 GPa). In addition to the above), it is necessary to improve the fracture resistance (for example, bendability) by further improving the deformability. From this point of view, 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 particular, in hot stamping parts having a tensile strength of 1.8 GPa or more, the plate thickness and weight can be reduced by improving the fracture resistance characteristics by further improving the deformability.

Further, when a hot stamped component is used, it is not desirable that the impact characteristics differ depending on the direction of the hot stamped component.

The present invention has been made in view of this problem of the prior art, and is excellent in deformability, fracture resistance at the time of collision and impact characteristics without tempering after quenching, and is anisotropic of impact characteristics. It is an object of the present invention to provide a technique for manufacturing a hot stamped part having a small amount and a tensile strength of, for example, 1.8 GPa or more.

Generally, when the tensile strength of a hot stamped component is high, it becomes difficult to secure the bendability of the hot stamped component. As a result of intensive studies focusing on the improvement of the fracture resistance of hot stamped parts at the time of collision, the present inventors have obtained the findings A to E listed below and completed the present invention.

(A) When the steel sheet for hot stamping parts was bent at three points, stretched MnS was present in the cracked portion, and many dimples caused by this MnS were confirmed on the fracture surface. That is, the stretched MnS becomes the starting point of fracture. Therefore, it is necessary to reduce the stretched MnS in the steel sheet for hot stamping parts.

(B) When the hot stamping parts were confirmed, in the hot stamping method, although the steel sheet for the hot stamping parts was sufficiently heated to the austenite single-phase region, undissolved fine particles causing low strength and low deformation ability. There were many cementites. Therefore, it is necessary to promote the dissolution of cementite.

(C) When Nb is added in order to avoid a decrease in deformability, Nb-based carbides are generated and serve as a starting point for void formation. In order to suppress the formation of voids, it is necessary to optimize the Nb content.

(D) That is, in the manufacturing process of steel sheets for hot stamping parts and hot stamping parts, impurities such as inclusions and carbides in the steel, which are the starting points of destruction of the hot stamping parts at the time of collision, are reduced as much as possible to make the hot stamping parts hot. The fracture resistance of the stamped component can be greatly improved, and this makes it possible to provide a hot stamped component that can be used, for example, as a structural member (skeleton member) of a body shell.

(E) On the other hand, inclusions and carbides in steel cannot be completely eliminated. On the contrary, if this is used, the strength of the hot stamping component can be increased.

The present invention is as listed below.

(1) The chemical composition 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, which are the following formulas (i), (ii) and (iii). Satisfy the formula,
The metal structure is mainly composed of ferrite and pearlite, the ferrite particle size is 30.0 μm or less, the pearlite area ratio is 15% or more, the total area ratio of ferrite and pearlite is 95% or more, and the balance. The structure contains one or more of bainite, retained austenite and cementite, and the area ratio of MnS having a length of 100 μm or more at a depth of 1/4 to 3/4 of the plate thickness is 0.0100% or less, and the area ratio is 0.0100% or less. Steel plate for hot stamping parts in which the number ratio of carbides (cementite and NbTi composite carbide) having a particle size of 1.2 μm or more is 10.0% or less among the carbides having a particle size of 0.5 μm or more in steel. ..
3.0 ° C / sec ≤ Vc90 ≤ 30 ° C / sec ... (i)
log (Vc90) = 2.94-0.75A ... (ii)
A = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ... (iii)
However, in equation (iii), the element symbol indicates the content (mass%) of each element.

(2) The hot stamping component according to item 1, wherein the chemical composition has one or more selected from V: 2.00% or less, Ta: 0.50% or less, and W: 3.00% or less. Steel plate for.

(3) The hot according to item 1 or 2, wherein the chemical composition has at least one selected from Ni: 5.00% or less, Cu: 3.00% or less, and Mo: 0.50% or less. Steel plate for stamp parts.

(4) The chemical composition has 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. The steel plate for hot stamping parts according to any one of Items 1 to 3.

(5) The method for manufacturing a steel sheet for hot stamping parts according to any one of Items 1 to 4.
After heating the ingot or steel piece having the chemical composition at T (° C.) or higher represented by the formula (iv) for 30 minutes or longer, rough rolling is completed at 1000 ° C. to 1150 ° C. at ₃ or more Ae points. The rolling process that completes the finish rolling and
A cooling process that cools at an average cooling rate of 10 to 200 ° C / sec,
The winding process of winding at 450-600 ° C and
Cold rolling process performed at a reduction rate of 10 to 60% and
A method for manufacturing a steel sheet for hot stamping parts, which comprises an annealing step of annealing in a temperature range of 750 ° C. or higher and then cooling at a cooling rate of 3 to 40 ° C./sec at 750 to 550 ° C. after annealing.
T (° C.) = [0.020+ (mass% Mn) (mass% S)] / (1.9 × ^10-5 ) ... (iv)
However, in equation (iv), mass% Mn and mass% S indicate the contents (mass%) of Mn and S, respectively.

According to the present invention, a hot stamped component having ultra-high strength (particularly a tensile strength of 1.8 GPa or more) and greatly improved fracture resistance characteristics can be obtained by hot stamping and quenching at that time without tempering. It becomes possible to manufacture. This makes it possible to use hot stamped parts with a tensile strength of 1.8 GPa or more for, for example, structural members (skeleton members) of body shells, and the manufacturing cost of hot stamped parts with a tensile strength of 1.8 GPa or more. Can be prevented from rising.

The present invention will be described.

1. 1. Steel Sheet for Hot Stamping 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 sheet for hot stamping parts and mainly determines the tensile strength of the hot stamping parts after quenching. In particular, in order to secure a tensile strength of 1.8 GPa or more for the hot stamped parts after quenching, 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 is significantly deteriorated. Therefore, 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 stamping parts and stably achieving high strength of hot stamping parts after quenching. In order 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 the cost is increased. Therefore, the Si content is 2.00% or less, preferably 1.50% or less, and more preferably 1.0% 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 stamping parts and stably obtaining high strength of hot stamping 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. On the other hand, even if the Mn content exceeds 3.00%, the above effect is saturated, and conversely, it becomes difficult to stably secure the tensile strength. Therefore, the Mn content is 3.00% or less, preferably 2.50% or less, and even more preferably 2.00% or less.

(1-4) P: 0.10% or less Since P significantly deteriorates the toughness of the hot stamped parts after quenching, the smaller the P content is, the more preferable, but 0.10% is allowed. Therefore, the P content is 0.10% or less. However, in order to reduce the P content to less than 0.001%, an increase in steelmaking cost is unavoidable. Therefore, the P content is preferably 0.001% or more.

(1-5) S: 0.0015% or less Since the smaller the amount of S, the better the deformability and fracture resistance, the S content should be 0.0015% or less. It is preferably less than 0.001%. However, since it takes a cost to remove S to reduce the S content, the S content is preferably 0.0001% or more. The S content is more preferably 0.0003% or more.

(1-6) Nb: 0.010 to 0.100%
Nb greatly improves the toughness of hot stamping parts because it suppresses recrystallization and forms fine carbides to make austenite grains finer when the steel sheet for hot stamping parts _{is heated to 3 points or more.} Has the effect of In order 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 will be described later, when the Nb content exceeds 0.100%, the Nb carbonitride of the ingot or steel piece becomes coarse. Therefore, the Nb content is 0.100% or less, preferably 0.080% or less, and even more preferably 0.060% 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 stamping parts and stably obtaining high strength of hot stamping parts after quenching. In order 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 stably secure the 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 to 0.100%
Al is an element that is effective in improving the hardenability of the steel sheet for hot stamping parts and stably securing the tensile strength of the hot stamping parts after quenching. In order 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 is increased. Therefore, 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 greatly improving the toughness of hot stamping parts in order to suppress recrystallization and form fine carbides to make austenite grains finer when the steel sheet for hot stamping parts _{is heated to 3 points or more.} Play. In order to surely 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 is increased. Therefore, 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 is present in steel as an impurity. In the presence of O, a small O content is preferable in order to form an oxide and lower the fracture resistance characteristics, but a content of about 0.0030% is allowed. Therefore, the O content is less than 0.0030%.

(1-11) B: 0.0006 to 0.0030%
B is effective in enhancing the hardenability of the steel sheet for hot stamping parts and enhancing the effect of stably securing the tensile strength of the hot stamping parts after quenching. B is also 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. Further, B also has a high effect of suppressing the growth of austenite grains during heating of the steel plate for hot stamping parts. In order to obtain this effect, the B content is 0.0006% or more, preferably 0.001% or more, and even 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 increased. Therefore, the B content is 0.0030% or less, preferably 0.0025% or less, and even more preferably 0.0020% or less.

(1-12) N: 0.0100% or less N is present in steel as an impurity. When N is present, it binds to B to generate BN and reduces the effect of B. Therefore, a small N content is preferable, but a content of about 0.010% is allowed. Therefore, the N content is 0.0100% or less, preferably 0.0080% or less, and even more preferably 0.0060% or less.

Next, arbitrary elements will be described.

(1-13) V: 2.00% or less V is an element that is effective in improving the hardenability of steel sheets for hot stamping parts and stably ensuring the tensile strength of hot stamping parts after quenching. .. However, even if the V content exceeds 2.00%, the above effect is saturated and the cost is increased. Therefore, the V content is 2.00% or less, preferably 1.50% or less, and even more preferably 1.00% or less. In order to surely 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 stamping parts and stably securing the tensile strength of hot stamping parts after quenching. .. However, if the Ta content exceeds 0.50%, the above effect is saturated and the cost is increased. Therefore, the Ta content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less. In order to surely obtain the above effect, the Ta content is 0.001% or more, preferably 0.005% or more, and further preferably 0.1% 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 stamping parts and stably securing the tensile strength of hot stamping parts after quenching. .. However, when the W content exceeds 3.00%, the above effect is saturated and the cost is increased. Therefore, the W content is 3.00% or less, preferably 2.00% or less, and even more preferably 1.00% or less. In order to surely 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 stamping parts and stably securing the tensile strength of hot stamping parts after quenching. .. However, if the Ni content exceeds 5.00%, the above effect is saturated and the cost is increased. Therefore, the Ni content is 5.00% or less, preferably 3.00% or less, and more preferably 2.00% or less. In order to surely 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 stamping parts and stably securing the tensile strength of hot stamping parts after quenching. .. However, even if the Cu content exceeds 3.00%, the above effect is saturated and the cost is increased. Therefore, the Cu content is 3.00% or less, preferably 2.00% or less, and even more preferably 1.00% or less. In order to surely obtain the above effect, the Cu content is 0.01% or more, preferably 0.20% or more, and more preferably 0.5% 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 stamping parts and stably securing the tensile strength of hot stamping parts after quenching. .. However, even if the Mo content exceeds 0.50%, the above effect is saturated and the cost is increased. Therefore, the Mo content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less. In order to surely 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, if the Mg content exceeds 0.0030%, this effect is saturated and the cost increases. Therefore, the Mg content is 0.0030% or less, preferably 0.0025% or less, and even more preferably 0.0020% or less. In order to surely 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, if the Ca content exceeds 0.0030%, this effect is saturated and the cost increases. Therefore, the Ca content is 0.0030% or less, preferably 0.0025% or less, and even more preferably 0.0020% or less. In order to surely obtain the above effect, the Ca content is 0.0005% or more, preferably 0.0010% or more, and further 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, if the La content exceeds 0.030%, this effect is saturated and the cost increases. Therefore, the La content is 0.030% or less, preferably 0.020% or less, and more preferably 0.010% or less. In order to surely obtain the above effect, the La content is 0.001% or more, preferably 0.003% or more, and further 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, if the Ce content exceeds 0.030%, this effect is saturated 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 surely obtain the above effect, the Ce content is 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more.

(1-23) The following equations (i), (ii) and (iii) are satisfied.
3.0 ° C / sec ≤ Vc90 ≤ 30 ° C / sec ... (i)
log (Vc90) = 2.94-0.75A ... (ii)
A = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ... (iii)
However, in equation (iii), the element symbol indicates the content (mass%) of each element.
Vc90 is a critical cooling rate at which a 90% martensite structure can be obtained, and is a parameter that is an index of hardenability. When Vc90 is less than 3.0 ° C./sec, hardenability is excellent, but cracks and the like are likely to occur when the coil is rewound and cold rolled, and the toughness is lowered. On the other hand, when Vc90 exceeds 30 ° C./sec, the variation in hardness at the corners and corners of the hot stamping component becomes large, and the collision characteristics cannot be ensured. Therefore, Vc90 is set to 3.0 to 30 ° C./sec.

The rest other than the above is Fe and impurities. Examples of impurities include those contained in raw materials such as ore and scrap, and those contained in the manufacturing process.

(2) Metallic structure (2-1) Mixed structure of ferrite and pearlite The steel plate according to the present invention is a metal thereof from the viewpoint of workability of a blank for hot stamping, formability during hot stamping, and hardenability. The structure is mainly ferrite and pearlite.

(2-2) Ferrite grain size: 30.0 μm or less When the ferrite grains become coarse, the spacing between the pearlite structures existing at the same time becomes large, so that the distribution of carbon in the austenite phase during heating of the hot stamping varies. It causes the hardness after quenching to vary. Due to the variation in hardness, a portion having a hardness higher than necessary is formed, which adversely affects the impact resistance and toughness. Therefore, the ferrite particle size is preferably 30.0 μm or less in order to uniformly disperse the pearlite structure in advance as the structure of the steel sheet for hot stamping parts so that the hardness does not vary when the hot stamp is heated. Is 25.0 μm or less.

(2-3) Pearlite area ratio: 15% or more Pearlite is a structure in which ferrite and cementite are alternately arranged in layers. Therefore, it is easily dissolved by rapid heating. If the temperature of the steel plate for hot stamping parts can be raised in a short period of time, it becomes easier to harden and the high strength of the hot stamping parts can be ensured. As a steel sheet for hot stamping parts used by rapid heating, the area ratio of pearlite is 15% or more, preferably 20% or more, and more preferably 25% or more.

(2-4) Total area ratio of ferrite and pearlite: 95% or more It may contain inevitably formed structures such as bainite, retained austenite, and cementite. If the total area ratio of ferrite and pearlite is 95% or more, there is no characteristic problem in using it as a steel sheet for hot stamping parts to be used by rapid heating. The residual tissue is, for example, bainite, retained austenite, and cementite. If the total area ratio of the remaining structure is 5% or less, there is no problem in terms of characteristics.

(2-5) 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 The central part of the slab manufactured by continuous casting has a large segregation. Become. Even if the slab is rolled into a thin plate, the segregation is inherited, and the segregation increases in the central portion of the thin plate. Therefore, in the present invention, attention is paid to the position of the central portion (1/4 to 3/4 t) of the plate thickness.

It is known that MnS is the starting point of cracking. 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 plate for hot stamping parts, not only the manufacturing operation by the hot stamping maker becomes difficult, but also the characteristics of the hot stamping parts may be defective.

The present inventors investigated the cause of this anisotropy in detail, and found that MnS is a cause of exacerbating 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 the steel, if the area ratio is 0.0100% or less, the problem of anisotropy does not occur.

Therefore, in the present invention, the area ratio of MnS having a length of 100 μm or more is defined as 0.0100% or less. The lower limit of the area ratio of MnS is not particularly specified, but considering the contents of Mn and S present in the steel, 0.0001% or more of MnS including MnS of 100 μm or less exists.

(2-6) Of the carbides having a particle size of 0.5 μm or more present in steel, the number ratio of carbides (cementite and NbTi composite carbonitride) having a particle size of 1.2 μm or more: 10.0% or less In the present invention. , Nb and Ti are contained as essential elements, so that a composite carbide nitride NbTi (C, N) of Nb and Ti is produced. The composite carbonitride NbTi (C, N) at the slab stage is coarse, and if it remains on the rolled steel sheet, it causes a decrease in the toughness of the hot stamped steel sheet or the hot stamped part.

When cementite (Fe ₃ C) remains undissolved in the steel sheet for hot stamping parts, the steel sheet for hot stamping parts is dissolved in austenite during heating to increase the C concentration in austenite. If the heating in the hot stamping process is insufficient, for example, if the cementite is heated rapidly for a short time in order to improve the manufacturing efficiency, the cementite is not sufficiently dissolved, which leads to a decrease in the toughness of the hot stamping part. ..

The presence of the composite carbonitrides NbTi (C, N) and cementite (hereinafter collectively referred to as "carbides") causes a decrease in toughness. On the other hand, the fine composite carbonitride NbTi (C, N) has the effect of increasing the strength of the steel sheet by strengthening precipitation by the pinning action. For this reason, it is necessary to have a certain amount of the fine composite carbonitride NbTi (C, N) present.

When the present inventors investigated the relationship between carbides and toughness in detail, the toughness of the steel plate for hot stamping parts or the toughness of hot stamping parts was lowered when a large amount of some coarse carbides was present regardless of the type of carbides. Was found. That is, the toughness of the steel plate for hot stamping parts or the toughness of hot stamping parts when the number ratio of the carbides having a particle size of 1.2 μm or more in the steel is 10.0% or less. Can be secured.

Further, the composite carbonitride NbTi (C, N) having a particle size of less than 1.2 μm contributes to the improvement of the strength of the steel sheet. On the other hand, cementite having a particle size of less than 1.2 μm can be easily dissolved in the hot stamping step, and the toughness of the hot stamping component is not lowered. In other words, instead of simply reducing carbides, if the number ratio of relatively fine carbides with a particle size of 0.5 μm or more and less than 1.2 μm present in the steel sheet is more than 90.0%, the hot stamping component It is possible to achieve both strength and toughness.

The number ratio of carbides having a particle size of 1.2 μm or more is specified by the number ratio excluding fine carbides having a particle size of less than 0.5 μm because fine carbides may not be observed by microstructure observation.

The area ratio of ferrite and pearlite, the ferrite grain size, the area ratio of MnS, and the number ratio of carbides can be measured by, for example, the method described in Examples.

(3) Applications The steel sheet according to the present invention is used for hot stamping parts. Target hot stamp parts include bumper reinforcements and structural members of automobile body shells (for example, A-pillar reinforcement, B-pillar reinforcement, front side members, rear side members, roof rails, various cross members, etc.). Is exemplified.

2. Method for Manufacturing Steel Sheet for Hot Stamping Parts According to the Present Invention Next, a method for manufacturing a steel sheet for hot stamping parts according to the present invention will be described.

(2-1) Rolling Step A steel ingot or steel piece having the above-mentioned chemical composition is subjected to the formula (iv): T (° C.) = [0.020+ (mass% Mn) (mass% S)] / (1.9). × ^10-5 ) (Mass% Mn and mass% S indicate the contents of Mn and S, respectively.) After heating for 30 minutes or more at a temperature T (° C.) or higher, rough rolling is completed at 1000 to 1150 ° C. do. In the component system having the above-mentioned chemical composition, the content of low S is basically low, and MnS is reduced from the viewpoint of components. In addition to this, the heating temperature is optimized to promote the solution of MnS.

Limiting the heating temperature of the ingot or piece of steel, specifically, promoting solution formation by setting the temperature T (° C.) or higher represented by the formula (iv) or higher, preferably 1200 ° C. or higher for 1 hour or longer. Thereby, the influence of MnS present on the ingot or the steel piece can be reduced.

Further, the cementite is set to a temperature T (° C.) or higher represented by the formula (iv) in order to prevent coarse carbides remaining on the steel plate for hot stamping parts and to dissolve the coarse cementite generated in the slab.

Further, the Nb-based carbonitride Nb (C, N) promotes solutionization of the coarse Nb-based carbonitride present in the ingot or the steel piece. From the viewpoint of preventing the coarsening of the Nb carbonitride of the ingot or steel piece, the upper limit of Nb is 0.100%, and the heating temperature of the steel ingot or steel piece is the temperature T (° C.) or more represented by the equation (iv). , Desirably, the influence of Nb can be reduced by setting the temperature to 1200 ° C. or higher.

After heating the ingot or piece of steel as described above, rough rolling is started, and rough rolling is completed at 1000 to 1150 ° C. If the end temperature of rough rolling is less than 1000 ° C., recrystallization does not proceed and the anisotropy due to rolling becomes strong. On the other hand, when the end temperature of the rough rolling exceeds 1150 ° C., although recrystallization proceeds, the coarsening of the crystal grains progresses, which leads to the coarsening of the crystal grains in the finish rolling.

After that, the finish rolling is completed at _{3 or more Ae points.} This is because if the finish rolling is performed at a temperature lower than _{3 points of Ae, the processed ferrite remains and the ductility is significantly deteriorated.} In the component system having the chemical composition specified in the present invention, these problems do not occur if _{the finish rolling completion temperature is Ae 3 points or more.} On the other hand, if the finish rolling completion temperature exceeds 1050 ° C., surface defects such as scale biting may occur. Therefore, the completion temperature of finish rolling is preferably 1050 ° C. or lower.

The Ae ₃ points are calculated according to the following formula.
Ae ₃ (° C.) = 937-477C + 56Si-20Mn-16Cu-15Ni-5Cr + 38Mo + 136Ti-19Nb + 198Al + 3315B
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.

(2-2) Cooling step After finishing rolling, the product 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, more preferably 30 ° C./sec. On the other hand, the upper limit of the average cooling rate is preferably 150 ° C./sec, more preferably 100 ° C./sec.

(2-3) Winding step After hot rolling, the coil is wound at 600 to 450 ° C. When the winding temperature is 600 ° C. or lower, a sufficient amount of pearlite structure can be obtained, and coarsening of the precipitate can be prevented. On the other hand, when the winding temperature is less than 450 ° C., the structure is mainly bainite, the rolling load in the cold rolling performed after the hot rolling becomes high, and the cold rollability deteriorates. Therefore, the winding temperature is 450 ° C. or higher, preferably 500 ° C. or higher.

(2-4) Pickling step If necessary, the coil (steel strip) wound around the coil after hot rolling is once rewound and then pickled to remove scale generated on the surface (descale). )I do.

(2-5) Cold Rolling Step After winding or pickling, cold rolling is performed at a rolling reduction of 10 to 60%. Cold rolling may be carried out under well-known and customary conditions. If the reduction rate is less than 10%, the structure after annealing tends to be mixed and the hardenability varies, and the hardness tends to vary. On the other hand, if the rolling reduction ratio exceeds 60%, the load during cold rolling becomes large and it becomes difficult to perform rolling at the industrial level in terms of cost.

(2-6) Annealing process After cold rolling, annealing is performed in a temperature range of 750 ° C. or higher in order to promote the redissolution of cementite by utilizing the austenite phase and prevent the concentration of Cr and Mn in cementite. .. By annealing in this temperature range, it is possible to appropriately prevent the concentration of Cr and Mn in cementite. Further, in order to make the metal structure after cooling mainly a mixed structure of ferrite and pearlite, the metal structure is cooled at a cooling rate of 3 to 40 ° C./sec at 750 to 550 ° C. after annealing.

The annealing of the cold-rolled steel strip may be continuous annealing performed in an uncoiled state, or box annealing performed by winding on a coil.

In this way, the steel sheet for hot stamping parts according to the present invention is manufactured.

The present invention will be specifically described with reference to Examples.

After heating a slab having the chemical composition shown in Table 1 (the rest other than that shown in Table 1 is Fe and impurities) at the slab heating temperature and slab heating time shown in Table 2, rough rolling is started, and the rough rolling shown in Table 2 is started. Rough rolling is completed at the rolling end temperature, and finish rolling is completed at the finish rolling end temperature shown in Table 2 to a plate thickness of 1.12 to 2.56 mm. After cooling while controlling the cooling temperature, Table 2 The coil was wound at the winding temperature shown in. The wound coil was unwound, pickled, descaled, cold-rolled, annealed at the annealing temperature shown in Table 2, and after annealing, cooled while controlling the cooling rate. Underlines in Tables 1 and 2 indicate that they are outside the scope of the present invention.

The manufactured coil was subjected to hot stamping treatment, and the structure of the hot stamped parts after the hot stamping treatment was observed, and whether or not the coil had characteristics as a steel plate for hot stamping parts was evaluated. Specifically, No. Steel sheets for hot stamping parts of 1 to 21 and x1 to x13 were manufactured, and the microstructures shown in the following (1) to (3) were observed.

(1) Metallic structure After mirror polishing the cross section of the sample L, nightal etching was performed, and the structure was observed with an optical microscope at a plate thickness of 1/4. The ferrite particle size, the pearlite area ratio, and the total area ratio of ferrite and pearlite were determined by image analysis from an optical micrograph at a magnification of 500 times.

(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 A sample piece is cut out from a part of a steel plate, the cross section of the sample L is mirror-polished, and an optical microscope photograph is taken. The size of MnS was specified by image analysis from, and the area ratio was calculated. ^{Here, a region having a width of 20 mm (0.8 mm × 20 mm = 16 mm 2} ) is defined as a measurement region in a region of 1/4 to 3/4 t of the plate thickness, that is, a region of 0.8 mm in the center of the plate thickness of 1.6 mmt. bottom.

(3) Number ratio of carbides (cementite, NbTi composite carbide nitride) with a particle size of 1.2 μm or more among the carbides with a particle size of 0.5 μm or more present in steel A sample piece is cut out from a part of a steel plate and sampled. The L cross section was mirror-polished, and the number ratio of carbides (cementite and NbTi composite carbonitride) was measured with a scanning electron microscope. Here, in the case of a 1/4 to 3/4 t region of the plate thickness, that is, in the case of a plate thickness of 1.6 mmt, a region having a width of 5 mm (0.8 mm × 5 mm = 4 mm ² ) is measured in a range of 0.8 mm in the central portion. As a region, composition analysis was performed with EDX to confirm whether it was a carbide, and for carbides with a particle size of 0.5 μm or more, carbides of 1.2 μm or more and less than 1.2 μm were distinguished and the number was counted. The number ratio of carbides having a particle size of 1.2 μm or more was calculated by dividing the number of carbides having a particle size of 2 μm or more by the number of carbides having a particle size of 0.5 μm or more.
On the other hand, the steel sheet characteristics were evaluated by the methods shown in (4) to (6) below.

(4) Strength (TS, YS), elongation EL
The No. 5 test piece specified in JIS Z 2201 was sampled from a steel plate and subjected to a tensile test in accordance with JIS Z 2241 to evaluate the strength (TS, YS) and elongation EL.

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

(6) vTrs and vE in the L and C directions
The hot stamped parts after hot stamping and heat treatment were subjected to an impact test, and the anisotropy of the parts was confirmed with respect to the fracture surface transition temperature (vTrs) and the absorbed energy (vE). A steel piece with a thickness of 1.6 mm cut out from a hot stamped part is sampled so that its long side is parallel (L direction) or perpendicular (C direction) to the rolling direction, and six steel pieces in the same direction are laminated. After screwing, a V-notch test piece was prepared and a Charpy impact test was performed.

Here, the fracture surface transition temperature (vTrs) parallel (L direction) and perpendicular (C direction) to the rolling direction are both -40 ° C or less, and the absorbed energy (vE) at room temperature is both 50 J / cm ² or more. Was evaluated as ○ (good).

Table 3 shows the results of tissue observation. Table 4 shows the results of the mechanical properties of the steel sheet. The underline in Table 3 indicates that it is outside the scope of the present invention, and the underline in Table 4 indicates that the mechanical properties are not good.

No. in Table 4 1-No. Reference numeral 21 denotes an example of the present invention that satisfies all the provisions of the present invention. x1 to x13 are comparative examples that do not satisfy the provisions of the present invention.

As shown in Table 4, No. 1-No. In the 21st example of the present invention, ferrite and pearlite are the main structures, the ferrite particle size is 19.9 μm or less, the pearlite area ratio is 16% or more, the total area ratio of ferrite and pearlite is 95% or more, and the MnS area ratio is 100 μm or more. Is 0.0096% or less, and the carbide ratio of 1.2 μm or more is 9.7% or less. : 1075 to 1615 (MPa), EL: 8.7 to 10.8 (%), limit R / t: 1.25 to 2.19, vTrs in the L and C directions: -40 ° C or less, vE: 50J It is possible to manufacture hot stamped parts that have mechanical properties of / cm ² or more and good hydrogen resistance characteristics, have ultra-high strength (particularly tensile strength of 1.8 GPa or more), and have greatly improved fracture resistance characteristics. I understand.

On the other hand, No. In x1, the winding temperature exceeded the upper limit of the range of the present invention, and the average cooling rate of 750 to 550 ° C. after annealing was also lower than the lower limit of the range of the present invention, so that pearlite could not be sufficiently produced. .. Therefore, the hardenability at the time of hot stamping deteriorates, and a sufficient tensile strength TS cannot be obtained.

No. For x2, the average cooling rate of 750 to 550 ° C. after annealing was also below the lower limit of the range of the present invention, so that pearlite could not be sufficiently produced. Therefore, the hardenability at the time of hot stamping deteriorates, and a sufficient tensile strength TS cannot be obtained.

No. For x3, the average cooling rate after annealing exceeded the upper limit of the range of the present invention, so that pearlite could not be sufficiently produced. Therefore, the hardenability at the time of hot stamping deteriorates, and a sufficient tensile strength TS cannot be obtained.

No. With x4, pearlite could not be sufficiently produced because the winding temperature exceeded the upper limit of the range of the present invention. Therefore, the hardenability at the time of hot stamping deteriorates, and a sufficient tensile strength TS cannot be obtained.

No. Since the slab heating temperature is below the lower limit of the range of the present invention, x5 is a value in which the area ratio of MnS and the ratio of the carbides both exceed the upper limit of the range of the present invention and the limit R / t is not good. The fracture surface transition temperature (vTrs) and absorbed energy (vE) were also unfavorable values.

No. Since x6 is below the lower limit of the range of the present invention, the area ratio of MnS and the ratio of the carbides both exceed the upper limit of the range of the present invention, and the limit R / t is not good. The fracture surface transition temperature (vTrs) and absorbed energy (vE) were also unfavorable values.

No. In x7, since the average cooling rate after finish rolling was below the lower limit of the range of the present invention, the proportions of the above-mentioned carbides all exceeded the upper limit of the range of the present invention, and the limit R / t was a value that was not favorable.

No. In x8, since the annealing temperature after cold rolling was below the lower limit of the range of the present invention, the proportions of the above-mentioned carbides all exceeded the upper limit of the range of the present invention, and the limit R / t was a value that was not favorable.

No. In x9, the Cr content exceeds the upper limit of the range of the present invention, and the Vc90 value also exceeds the upper limit of the range of the present invention. vTrs) and absorbed energy (vE) were also unfavorable values.

No. In x10, since the Vc90 value was below the lower limit of the range of the present invention, the fracture surface transition temperature (vTrs) and the absorbed energy (vE) were unfavorable values.

No. In x11, since the Vc90 value exceeds the upper limit of the range of the present invention, the hardenability at the time of hot stamping deteriorates, the tensile strength TS becomes small, and the fracture surface transition temperature (vTrs) and the absorbed energy (vE) are also unfavorable values. there were.

No. For x12, the Nb content was below the lower limit of the range of the present invention, and the winding temperature exceeded the upper limit of the range of the present invention, so that pearlite could not be sufficiently produced. Therefore, the tensile strength TS became small, and the fracture surface transition temperature (vTrs) and the absorbed energy (vE) were unfavorable values.

Furthermore, No. x13 is a value at which the limit R / t is not favorable because the Nb content exceeds the upper limit of the range of the present invention and the proportion of carbides of 1.2 μm or more exceeds 10%, and the fracture surface transition temperature (vTrs). And the absorbed energy (vE) was also an unfavorable value.

Claims (5)

The chemical composition is mass%,
C: 0.18 to 0.50%,
Si: 0.001-2.00%,
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.
Satisfy the following equations (i), (ii) and (iii),
The metal structure is mainly composed of ferrite and pearlite, the ferrite particle size is 30.0 μm or less, the pearlite area ratio is 15% or more, the total area ratio of ferrite and pearlite is 95% or more, and the balance. The tissue contains one or more of bainite, retained austenite and cementite,
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 area ratio is 0.0100% or less.
A steel sheet for hot stamping parts in which the number ratio of carbides having a particle size of 1.2 μm or more is 10.0% or less among the carbides having a particle size of 0.5 μm or more present in steel.
3.0 ° C / sec ≤ Vc90 ≤ 30 ° C / sec ... (i)
log (Vc90) = 2.94-0.75A ... (ii)
A = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ... (iii)
However, in equation (iii), the element symbol indicates the content (mass%) of each element. When the chemical composition is mass%,
V: 2.00% or less,
The steel sheet for hot stamping parts according to claim 1, which has at least one selected from Ta: 0.50% or less and W: 3.00% or less. When the chemical composition is mass%,
Ni: 5.00% or less,
The steel sheet for hot stamping parts according to claim 1 or 2, which has at least one selected from Cu: 3.00% or less and Mo: 0.50% or less. When the chemical composition is mass%,
Mg: 0.0030% or less,
Ca: 0.0030% or less,
The steel sheet for hot stamping parts according to any one of claims 1 to 3, which has one or more selected from La: 0.030% or less and Ce: 0.030% or less. The method for manufacturing a steel sheet for hot stamping parts according to any one of claims 1 to 4.
After heating the ingot or steel piece having the chemical composition at T (° C.) or higher represented by the formula (iv) for 30 minutes or longer, rough rolling is completed at 1000 ° C. to 1150 ° C. at ₃ or more Ae points. The rolling process that completes the finish rolling and
A cooling process that cools at an average cooling rate of 10 to 200 ° C / sec,
The winding process of winding at 450-600 ° C and
Cold rolling process performed at a reduction rate of 10 to 60% and
A method for manufacturing a steel sheet for hot stamping parts, which comprises an annealing step of annealing in a temperature range of 750 ° C. or higher and then cooling at a cooling rate of 3 to 40 ° C./sec at 750 to 550 ° C. after annealing.
T (° C.) = [0.020+ (mass% Mn) (mass% S)] / (1.9 × ^10-5 ) ... (iv)
However, in equation (iv), mass% Mn and mass% S indicate the contents (mass%) of Mn and S, respectively.