JP2017043825A - Steel sheet for hot stamp and production method therefor, and hot stamp steel sheet member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot stamp steel sheet member having a tensile strength of 1.8 to 2.5 GPa and nonetheless having excellent toughness, a steel sheet for hot stamp which is a raw material of the hot stamp steel sheet member and suitable as e.g. a thin sheet steel sheet for an automobile, and a production method for the steel sheet for hot stamp.SOLUTION: The steel sheet for hot stamp is provided that has a chemical composition containing, C:0.25 to 0.40%, Si:0.05% or more and less than 0.5%, Mn:1.0 to 1.7%, P:0.020% or less, S:less than 0.0010%, Al:0.002 to 0.06%, N:0.006% or less, Cr:0.02 to 0.6%, B:0.00010 to 0.0040%, Ti:0.005 to 0.04%, Nb:0.03 to 0.12% and the balance Fe with inevitable impurities, and that has a metallographic structure in which number density of inclusions and deposition with a maximum length of 10 μm or more is 100/mmor less and prior austenite grain diameter after hot stamp is 10 μm or less and that has a mechanical characteristics, namely, a tensile strength after hot stamp of 1.8 to 2.5 GPa and an impact absorption energy at -40°C in a V notch Charpy test of 30 J/cmor more.SELECTED DRAWING: None

Description

  TECHNICAL FIELD The present invention relates to a steel sheet for hot stamping, a manufacturing method thereof, and a hot stamping steel sheet member. Specifically, the hot stamping steel sheet member having a tensile strength of 1.8 GPa to 2.5 GPa and a material thereof, for example, an automobile TECHNICAL FIELD The present invention relates to a hot stamping steel plate suitable as a thin steel plate and a manufacturing method thereof.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel used for the vehicle body and reduce the weight used. Thin steel plates that are widely used in automobiles have reduced press formability with increasing strength, making it difficult to produce complex shapes. Specifically, there arises a problem that ductility is reduced and fracture occurs at a portion with a high degree of processing, or that dimensional accuracy is deteriorated due to large springback and wall warpage. Therefore, it is not easy to produce a part by press molding using a steel plate having high strength, particularly a tensile strength (hereinafter also referred to as “TS”) of 780 MPa class or higher.

  On the other hand, as disclosed in Patent Document 1, in the hot stamping method (also referred to as hot pressing method) in which a hot stamped steel plate member is manufactured by press forming a heated steel plate, the steel plate at the time of forming is hot, soft and Since it has high ductility, it is possible to form a hot stamped steel plate member having a complicated shape with high dimensional accuracy. Further, according to the hot stamping method, the steel sheet is rapidly cooled (quenched) in the mold after being heated to the austenite single phase region, thereby simultaneously achieving high strength of the hot stamped steel sheet member by martensitic transformation. Therefore, the hot stamp method is an excellent forming method capable of simultaneously achieving high strength and formability of the hot stamped steel plate member.

  Patent Document 2 discloses a pre-press quench method that achieves high strength of a hot stamped steel sheet member by forming it in a predetermined shape at room temperature, heating it to an austenite region, and rapidly cooling it in a mold. Has been. This pre-press quench method, which is one aspect of the hot stamping method, can restrain deformation due to thermal strain by constraining the material to be molded by a mold, so that high strength and high dimensional accuracy of the hot stamped steel plate member are achieved. Is an excellent molding method capable of ensuring the same at the same time.

  As described above, the hot stamping method is an excellent molding method, and in recent years, its application has expanded and research on the hot stamping method has been further promoted. Conventional hot stamping steel plates are mainly used to provide hot stamping steel plate members having a tensile strength of 1470 MPa, but in recent years, further weight reduction of hot stamping steel plate members has been demanded.

  For this reason, for example, in Patent Document 3, in mass%, C: 0.26 to 0.35%, Mn: 1.5 to 2.0%, Nb: 0.01 to 1.0%, B: 0 0.0001 to 0.01%, Cr: 0.5% or less, P: 0.05% or less, S: 0.03% or less, Si: 0.5% or less, Cu: 1% or less, V: 1% or less, Mo: 1% or less, Al: 1% or less, and N: 0.01% or less, or one or more selected from the group consisting of 0.01% or less, and the formula (1): Containing Ti that satisfies 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5, with the balance being a chemical composition composed of Fe and impurities, and having a steel structure with a prior austenite average particle size of 10 μm or less, A hot-pressed steel sheet member having mechanical properties having a tensile strength of 1.8 GPa or more and 2.0 GPa or less is disclosed. ing. This is because by optimizing the chemical composition of the hot stamping steel plate (reducing reduction in Mn content) and suppressing the variation in tensile strength, the tensile strength of 1.8 GPa or more and 2.0 GPa or less is excellent in toughness and delayed fracture resistance. It is a hot stamped steel plate member and has already been mass-produced as a bumper reinforcement with a tensile strength of 1.8 GPa.

British Patent No. 1490535 Japanese Patent Laid-Open No. 10-96031 JP 2014-15638 A

  However, hot stamped steel plate members with a tensile strength of over 1.8 GPa have a significant deterioration in toughness. Therefore, frame members (such as various pillars and members) of automobile bodies and undercarriage parts (suspension arms and suspension members) of automobiles. Etc.) is not used yet. For this reason, further improvement in toughness and weldability is strongly demanded for hot stamping steel plates, which are materials for hot stamping steel plate members having a tensile strength exceeding 1.8 GPa.

  The present invention has been made in view of such problems of the prior art, and has a tensile strength of 1.8 to 2.5 GPa and is excellent in toughness, and a material thereof, for example, an automobile. An object of the present invention is to provide a hot stamping steel plate suitable as a thin steel plate and a manufacturing method thereof.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies, and (a) the Mn content of a hot stamping steel plate for improving the toughness of a hot stamping steel plate member having a tensile strength of 1.8 to 2.5 GPa. (B) by optimizing the number density of inclusions and precipitates in the steel sheet for hot stamping, and (c) by optimizing the number density of carbide in the hot stamped steel sheet member after hot stamping. The toughness of a hot stamped steel plate member having a tensile strength of 5 GPa can be remarkably improved, whereby a hot stamped steel plate member having a tensile strength of 1.8 to 2.5 GPa Etc.) and undercarriage parts of automobiles (suspension arms, suspension members, etc.).

Specifically, the present inventors obtained the knowledge A to D listed below and completed the present invention.
(A) By making Mn content of the steel sheet for hot stamping 1.0% or more and 1.7%, deterioration of toughness due to segregation of Mn can be avoided while securing the hardenability of the steel sheet for hot stamping.

(B) In addition to reducing the Mn content of the steel sheet for hot stamping described in item A and S: content of the steel sheet for hot stamping to less than 0.0010%, optimization of casting conditions (increase in solidification speed) And by optimizing the hot rolling conditions (high temperature heating), the number density of inclusions and precipitates having a maximum length of 10 μm or more can be suppressed to 100 pieces / mm ² or less, and the toughness can be further improved.

(C) By optimizing the Nb content of the steel sheet for hot stamping and optimizing the hot stamping conditions, the grain size of the prior austenite after hot stamping can be suppressed to 10 μm or less, and the average equivalent grain diameter is 0 The number density of carbides of 5 μm or more can be reduced to 20 pieces / mm ² or less.

  (D) By these, the hot stamped steel plate member which combines the super high strength of the tensile strength of 1770 MPa or more and the excellent toughness can be provided.

The present invention is listed below.
(1) By mass%, 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: 0.002-0.06%, N: 0.006% or less, Cr: 0.02-0.6%, B: 0.00010-0.0040%, Ti : 0.005-0.04%, Nb: 0.03-0.12%, having a chemical composition consisting of the balance Fe and inevitable impurities,
The number density of inclusions and precipitates with a maximum length of 10 μm or more is 100 pieces / mm ² or less, and the prior austenite grain size after hot stamping has a metal structure of 10 μm or less,
For hot stamping, characterized by having a tensile strength after hot stamping of 1.8 to 2.5 GPa and an impact absorption energy at −40 ° C. in a V-notch Charpy test of 30 J / cm ² or more. steel sheet.

  (2) The chemical composition further includes Cu: 2.0% or less, Ni: 2.0% or less, Mo: 2.0% or less, V: 0.2% or less, and W: 0.2% or less. The steel sheet for hot stamping described in 1 item | term which has 1 type or 2 types or more selected from the group which consists of.

  (3) The chemical composition further includes Ca: 0.1% or less, Mg: 0.1% or less, REM: 0.1% or less, Zr: 0.1% or less, Nd: 0.1% or less, and Sb: A steel sheet for hot stamping described in 1 or 2 having one or more selected from the group consisting of 0.1% or less.

(4) The metal structure is described in any one of items 1 to 3, wherein the number density of carbides having a circle-equivalent average particle diameter of 0.5 μm or more after hot stamping is 20 pieces / mm ² or less. Steel plate for hot stamping.

  (5) Producing a slab at a solidification rate of 0.2 ° C./sec or higher during continuous casting, heating the slab to 1200 ° C. or higher, holding it for 90 minutes or more, and rough rolling to a rough rolling outlet temperature of 950 ° C. or higher. 1. Rough rolling for 3 minutes or longer, hot rolling at a finish rolling temperature of 850 ° C. or higher, winding at a winding temperature of 700 ° C. or lower, and then cold rolling. The manufacturing method of the steel sheet for hot stamps described in any 1 item | term from 1 to 4.

  (6) The method for producing a steel sheet for hot stamping according to item 5, wherein the steel sheet is further annealed at an annealing temperature of 750 ° C. or higher after the cold rolling.

  (7) A hot stamped steel plate member obtained by hot stamping a hot stamped steel plate described in any one of items 1 to 4.

  According to the present invention, a hot stamped steel sheet member having excellent toughness while having a tensile strength of 1.8 to 2.5 GPa exceeding 1.5 GPa class, and a hot stamping sheet suitable for, for example, a sheet steel sheet for automobiles. Steel sheet and its manufacturing method can be provided. For this reason, according to the present invention, a hot stamped steel plate member having a tensile strength of 1.8 to 2.5 GPa is used as a skeleton member (for example, various pillars and members) of an automobile body or an automobile undercarriage component (suspension arm, suspension). Member).

  The present invention will be described. In the following description, “%” related to chemical composition means “% by mass” unless otherwise specified.

1. Hot Stamp Steel Sheet According to the Present Invention The chemical composition of the hot stamp steel sheet will be described. First, the essential elements are explained.

(1-1) C: 0.25 to 0.40%
C is a very important element that enhances the hardenability of the steel sheet for hot stamping and mainly determines the strength after hot stamping. In particular, the C content is at least 0.25% so that the tensile strength TS after hot stamping is 1.8 GPa or more. From the same viewpoint, the C content is preferably 0.28% or more.

  On the other hand, if the C content exceeds 0.40%, the tensile strength TS after hot stamping becomes too high, and the toughness deterioration becomes remarkable. Therefore, the C content is set to 0.40% or less. From the same viewpoint, the C content is preferably 0.35% or less.

(1-2) Si: 0.05% or more and less than 0.5% Si contains 0.05% or more, thereby improving the hardenability of steel and ensuring the stability of strength after hot stamping. . Therefore, the Si content is 0.05% or more. Preferably, the Si content is 0.10% or more. However, when the Si content is 0.5% or more, the effect is small and the cost is increased unnecessarily. For this reason, Si content shall be less than 0.5%. Preferably, the Si content is 0.40% or less.

(1-3) Mn: 1.0 to 1.7%
Mn is an element that is very effective for enhancing the hardenability of the steel sheet and stably securing the tensile strength after hot stamping. However, if the Mn content is less than 1.0%, the effect is not sufficient. Therefore, the Mn content is 1.0% or more. Preferably, the Mn content is 1.15% or more. On the other hand, if the Mn content exceeds 1.7%, the toughness after hot stamping is significantly deteriorated. Therefore, the Mn content is 1.7% or less. From the same viewpoint, the Mn content is preferably 1.5% or less. More preferably, the Mn content is 1.40% or less.

(1-4) P: 0.020% or less P is an element that enhances the hardenability of steel and has an effect of ensuring the stability of strength after hot stamping. Therefore, P is contained. However, if the P content exceeds 0.020%, segregation occurs at the grain boundaries, and the toughness after hot stamping is significantly deteriorated. Therefore, the P content is within the above range. Preferably, the P content is 0.015% or less. In addition, in order to acquire the said effect more reliably, it is preferable that P content is 0.0001% or more.

(1-5) S: less than 0.0010% S is present as an unavoidable impurity and combines with Mn to form MnS. Since precipitation of MnS significantly deteriorates toughness, the S content is set to less than 0.0010%. From the same viewpoint, the S content is preferably less than 0.0008%.

(1-6) Al: 0.002 to 0.06%
By containing 0.002% or more, Al is an element that enhances the hardenability of steel and is effective in ensuring the stability of strength after hot stamping. Therefore, the Al content is 0.002% or more. Preferably, the Al content is 0.01% or more. However, even if the Al content exceeds 0.06%, the effect is small and the cost is unnecessarily increased. Therefore, the Al content is set to 0.06% or less. Preferably, the Al content is 0.05% or less.

(1-7) N: 0.006% or less N is an element that enhances the hardenability of steel and has an effect of ensuring the stability of strength after hot stamping. Therefore, N is contained. However, even if the N content exceeds 0.006%, the effect is small and the cost is unnecessarily increased. Therefore, the N content is set in the above range. Preferably, the N content is 0.004% or less. In addition, in order to acquire the said effect more reliably, it is preferable that N content is 0.0005% or more.

(1-8) Cr: 0.02 to 0.6%
Cr is an element that is effective in enhancing the hardenability of steel and ensuring the stability of strength after hot stamping by containing 0.02% or more. Therefore, the Cr content is 0.02% or more. Preferably, the Cr content is 0.05% or more. However, even if the Cr content exceeds 0.6%, the effect is small and the cost is increased unnecessarily, so the Cr content is set to 0.6% or less. Preferably, the Cr content is 0.5% or less.

(1-9) B: 0.00010 to 0.0040%
B is an element effective for enhancing the hardenability of steel and further enhancing the effect of ensuring the stability of strength after hot stamping. It is also an important element in that it segregates at grain boundaries to increase grain boundary strength and improve toughness and delayed fracture resistance. Furthermore, it has the effect | action which improves toughness by suppressing the grain growth of the austenite in the heating process at the time of using for a hot stamp. If the B content is less than 0.00010%, it is difficult to obtain the effect of the above action. Therefore, the B content is 0.00010% or more. Preferably, the B content is 0.00015% or more. On the other hand, if the B content exceeds 0.0040%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the B content is 0.0040% or less. Preferably, the B content is 0.0030% or less.

  The grain boundary segregation amount of B is affected by the austenite grain size when subjected to hot stamping. That is, the smaller the austenite grain size, the more B segregation sites increase, so that more B can segregate. On the other hand, as the austenite grain size increases, the segregation sites of B decrease, so that the segregable amount of B decreases. Therefore, since the upper limit of the B content is determined according to the austenite grain size when subjected to hot stamping, that is, the prior austenite grain size in the hot stamped steel plate member, the effect of B segregation can be obtained efficiently. preferable. Specifically, it is preferable to satisfy B content (ppm) ≦ exp (4.57−0.571 × ln (r)). Here, r is the average section length (μm) of the prior austenite grains. In order to satisfy this equation, the relationship between the chemical composition and the austenite grain size when subjected to hot stamping is determined empirically, and the chemical composition and hot stamping conditions may be adjusted.

(1-10) Ti: 0.005 to 0.04%
Ti has the effect | action which improves the effect by B by fixing N in steel. When the Ti content is less than 0.005%, it is difficult to obtain the effect by the above action. Therefore, the Ti content is set to 0.005% or more. Preferably, the Ti content is 0.01% or more. On the other hand, if the Ti content exceeds 0.04%, a large amount of Ti-based precipitates are generated, and the toughness is deteriorated. Therefore, the Ti content is 0.04% or less. Preferably, the Ti content is 0.03% or less.

(1-11) Nb: 0.03 to 0.12%
Nb has the effect of greatly improving the toughness after hot stamping because it suppresses recrystallization and forms fine carbides to make austenite grains fine when the steel sheet is heated to Ac ₃ points or more. However, if the Nb content is less than 0.03%, it is difficult to obtain the effect by the above action. Therefore, the Nb content is 0.03% or more. From the same viewpoint, the Nb content is preferably 0.04% or more, and more preferably 0.06% or more. On the other hand, when the Nb content exceeds 0.12%, the effect of the above action is saturated, and the cost is unnecessarily increased. Therefore, the Nb content is 0.12% or less. From the same viewpoint, the Nb content is preferably 0.10% or less.

  Next, an optional additive element will be described. One kind selected from the group consisting of Cu, Ni, Mo, V, and W may be contained alone or in combination of two or more, if necessary.

(1-12) Cu: 2.0% or less Cu is an element that enhances the hardenability of steel and is effective in ensuring the stability of strength after hot stamping. Therefore, Cu may be contained. However, even if the Cu content exceeds 2.0%, the effect is small and the cost is unnecessarily increased, so the Cu content is set to 2.0% or less. Preferably, the Cu content is 1.0% or less. In addition, in order to acquire the said effect more reliably, it is preferable that Cu content is 0.01% or more.

(1-13) Ni: 2.0% or less Ni is a very effective element for enhancing the hardenability of the steel sheet and stably ensuring the strength after hot stamping. Furthermore, it has the effect of increasing the cleavage fracture strength and greatly improving toughness. Furthermore, it has the effect of improving delayed fracture resistance. Therefore, Ni may be included. However, when the Ni content exceeds 2.0%, the effect is saturated and the cost is increased. Therefore, the Ni content is 2.0% or less. Preferably it is 1.0% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable that Ni content shall be 0.01% or more, and it is more preferable to set it as 0.1% or more.

(1-14) Mo: 2.0% or less Mo is an element that enhances the hardenability of steel and is effective in ensuring the stability of strength after hot stamping. Therefore, you may contain Mo. However, even if the Mo content exceeds 2.0%, the effect is small and the cost is unnecessarily increased. Therefore, the Mo content is set to 2.0% or less. Preferably, the Mo content is 1.0% or less. In addition, in order to acquire the said effect more reliably, it is preferable that Mo content is 0.01% or more.

(1-15) V: 0.2% or less V is an element that enhances the hardenability of steel and is effective in ensuring the stability of strength after hot stamping. Therefore, V may be contained. However, even if the V content exceeds 0.2%, the effect is small and the cost is unnecessarily increased. Therefore, the V content is set to 0.2% or less. Preferably, the V content is 0.15% or less. In addition, in order to acquire the said effect more reliably, it is preferable that V content is 0.005% or more.

(1-16) W: 0.2% or less W is an element that enhances the hardenability of steel and is effective in ensuring the stability of strength after hot stamping. Therefore, W may be contained. However, even if the W content exceeds 0.2%, the effect is small and the cost is unnecessarily increased. Therefore, the W content is set to 0.2% or less. Preferably, the W content is 0.15% or less. In addition, in order to acquire the said effect more reliably, it is preferable that W content is 0.005% or more.

  Furthermore, you may contain 1 type chosen from the group which consists of Ca, Mg, REM, Zr, Nd, and Sb individually or in combination of 2 or more types as needed.

(1-17) Ca: 0.1% or less Ca is an element that contributes to inclusion control during steelmaking, in particular, fine dispersion of inclusions, and has an effect of increasing toughness after hot stamping. Therefore, Ca may be contained. However, when the Ca content exceeds 0.1%, the deterioration of the surface properties may become obvious. Therefore, the Ca content is 0.1% or less. Preferably, the Ca content is 0.010% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable that Ca content is 0.0002% or more.

(1-18) Mg: 0.1% or less Mg is an element that contributes to inclusion control during steelmaking, particularly fine dispersion of inclusions, and has an effect of increasing toughness after hot stamping. Therefore, Mg may be included. However, when the Mg content exceeds 0.1%, surface property deterioration may become apparent. Therefore, the Mg content is 0.1% or less. Preferably, the Mg content is 0.010% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make Mg content 0.0002% or more.

(1-19) REM: 0.1% or less REM is an element that contributes to inclusion control during steelmaking, particularly fine dispersion of inclusions, and has an effect of increasing toughness after hot stamping. Therefore, REM may be included. However, when the REM content exceeds 0.1%, the deterioration of the surface properties may become obvious. Therefore, the REM content is 0.1% or less. Preferably, the REM content is 0.010% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable to make REM content 0.0002% or more.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

(1-20) Zr: 0.1% or less Zr is an element that contributes to inclusion control during steelmaking, particularly fine dispersion of inclusions, and has an effect of increasing toughness after hot stamping. Therefore, Zr may be contained. However, when the Zr content exceeds 0.1%, the deterioration of the surface properties may become obvious. Therefore, the Zr content is 0.1% or less. Preferably, the Zr content is 0.010% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make Zr content 0.0002% or more.

(1-21) Nd: 0.1% or less Nd is an element that contributes to inclusion control during steelmaking, particularly fine dispersion of inclusions, and has an effect of increasing toughness after hot stamping. Therefore, Zr may be contained. However, when the Zr content exceeds 0.1%, the deterioration of the surface properties may become obvious. Therefore, the Nd content is 0.1% or less. Preferably, the Nd content is 0.010% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make Nd content 0.0002% or more.

(1-22) Sb: 0.1% or less Sb is an element that contributes to inclusion control during steelmaking, particularly fine dispersion of inclusions, and has an effect of increasing toughness after hot stamping. Therefore, Sb may be included. However, when the Sb content exceeds 0.1%, deterioration of the surface properties may become obvious. Therefore, the Sb content is 0.1% or less. Preferably, the Sb content is 0.010% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make Sb content 0.0002% or more.

The balance other than the above is Fe and inevitable impurities.
Next, the metal structure of the hot stamping steel plate will be described.

(2-1) Number density of inclusions and precipitates having a maximum length of 10 μm or more: 100 pieces / mm ² or less Hot stamping is achieved because the number density of inclusions and precipitates having a maximum length of 10 μm or more is 100 pieces / mm ² or less. The toughness of the steel sheet member is improved.

(2-2) Old austenite particle size after hot stamping: 10 μm or less The old austenite particle size after hot stamping is 10 μm or less, so that even if stress is applied, the slip surface that is resistant to slippage is different. Since the number of points increases and cracks hardly progress, the toughness of the hot stamped steel sheet member is improved.

(2-3) The number density of carbides having a circle equivalent average particle diameter of 0.5 μm or more after hot stamping is 20 pieces / mm ² or less In addition to the prior austenite particle diameter after heat treatment by hot stamping being 10 μm or less. By setting the number density of carbides having an equivalent circle average particle size of 0.5 μm or more to 20 pieces / mm ² or less, the occurrence of cracks between the carbides and the metal structure can be suppressed, and Toughness is improved. For this reason, it is preferable that the number density of carbides having an equivalent circle average particle diameter of 0.5 μm or more is 20 pieces / mm ² or less.

Next, mechanical properties of the hot stamping steel plate will be described.
(3-1) Tensile strength after hot stamping: 1.8 GPa or more and 2.5 GPa or less According to the present invention, the tensile strength after hot stamping is 1.8 GPa to 2.5 GPa exceeding 1.5 GPa class.

(3-2) Toughness The steel sheet for hot stamping according to the present invention has excellent toughness with an impact absorption energy at −40 ° C. in a V-notch Charpy test of 30 J / cm ² or more.

2. Manufacturing method of steel sheet for hot stamping according to the present invention A slab having the above-described chemical composition is manufactured by setting a solidification rate during continuous casting to 0.2 ° C./sec or more, and after heating the slab to 1200 ° C. or more, Hold for 90 minutes or more, rough rolling time is 3 minutes or more, rough rolling is performed at a rough rolling outlet temperature of 950 ° C. or higher, hot rolling is performed at a finishing rolling temperature of 850 ° C. or higher, and a winding temperature is 700 ° C. or lower. The steel sheet for hot stamping described above can be manufactured by winding the sheet with, followed by cold rolling.

  Here, the rough rolling time is the last (downstream) rough rolling from the time when the tip of the slab is inserted into the first (upstream) roughing mill in a plurality of rough rolling mills on the production line. Time until the rear end of the slab is discharged from the machine. The rough rolling exit temperature refers to the surface temperature of the slab after being discharged from the last rough rolling mill.

The slab does not have the above-described chemical composition, the solidification rate during continuous casting is less than 0.2 ° C / sec, or the slab heating temperature before hot rolling is below 1200 ° C, or the slab is heated When the time is less than 90 minutes, the rough rolling time is less than 3 minutes, and the rough rolling exit temperature is less than 950 ° C., the inclusions and precipitates crystallized and precipitated during casting cannot be finely dispersed, and the maximum length is 10 μm or more. The number density of inclusions and precipitates exceeds 100 pieces / mm ² . Therefore, when a stress is applied, cracks generated from the interface between inclusions and precipitates and the base material are easily connected, and the toughness is extremely deteriorated.

  Also, if the finish rolling temperature is less than 850 ° C. or the winding temperature exceeds 700 ° C., the pinning effect of austenite grain growth due to fine precipitates due to (Ti, Nb) (C, N) during heating before hot stamping , The prior austenite grain size after hot stamping cannot be made 10 μm or less, and the toughness is extremely deteriorated.

Since the steel plate obtained as described above is further annealed at an annealing temperature of 750 ° C. or higher, the number density of carbides having a circle-equivalent average particle size of 0.5 μm or more can be reduced to 20 pieces / mm ² or less after hot stamping. ,preferable. Thereby, the progress of cracks generated from the carbide / matrix interface of 0.5 μm or more can be suppressed, and the toughness is further improved.

3. Hot stamped steel plate member according to the present invention Hot stamped steel plate member according to the present invention is manufactured by performing hot stamping using the steel sheet for hot stamping according to the present invention manufactured as described above as a raw material by well-known and conventional means. .

  The hot stamped steel plate member has a higher Mn content and an appropriate number density of inclusions and precipitates as compared with the hot stamped steel plate disclosed in Patent Document 3, for example. By adjusting the number density, the toughness of a hot stamped steel plate member having a tensile strength of 1.8 to 2.7 GPa can be remarkably improved, whereby a hot stamped steel plate member having a tensile strength of 1.8 to 2.7 GPa can be obtained. It can be used for a framework member of an automobile body (for example, various pillars, members, etc.) and an automobile underbody part (suspension arm, suspension member, etc.).

Examples of the present invention will be described.
A slab melted in a laboratory having the chemical composition shown in Table 1 was heated under the conditions shown in Table 2 and hot-rolled to obtain a steel plate having a thickness of 4 mm. During slab casting, the solidification rate is changed, and before hot rolling, the slab heating temperature, slab heating holding time, rough rolling time, and rough rolling outlet temperature are changed to change the maximum length of inclusions and precipitates. It was.

  Hot rolling and winding after hot rolling were performed under the conditions shown in Table 2. The coiling temperature was adjusted by the amount of water sprayed. The obtained hot rolled sheet was made to have a thickness of 2.6 mm by cold rolling after removing the scale by pickling. Some steel plates were annealed after cold rolling to change the number density of carbides having an equivalent circle average particle diameter of 0.5 μm or more.

  The hot stamping steel plate thus obtained was heated at 900 ° C. for 4 minutes in a gas furnace set to an air-fuel ratio of 0.9, then removed from the heating furnace, and a flat steel plate was made within 7 seconds. Hot stamping was performed using a mold.

  The production conditions are summarized in Table 2.

  About the obtained hot stamped steel plate member, it used for the prior austenite particle size measurement (average slice length measurement) by a cutting method, a tensile test (JIS No. 5 test piece), and a Charpy impact test.

In the Charpy impact test, 1 / 4-size (plate thickness: 2.5 mm) V-notch Charpy test pieces were prepared and evaluated. As toughness evaluation, when the impact value at −40 ° C. was 30 J / cm ² or more and 60 J / cm ² or less, it was regarded as acceptable. When it exceeded 60 J / cm ² , the toughness was very excellent.

  The results are summarized in Table 3.

  As shown in Table 3, according to the present invention, a hot stamping steel plate material having excellent toughness while having a tensile strength of 1.8 to 2.5 GPa, such as a hot steel plate suitable for a thin steel plate for automobiles, for example. It turns out that the steel plate for stamps can be provided.

Claims (7)

In mass%, 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: 0 Less than .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: 0.0. 005 to 0.04%, Nb: 0.03 to 0.12%, having a chemical composition consisting of the balance Fe and inevitable impurities,
The number density of inclusions and precipitates with a maximum length of 10 μm or more is 100 pieces / mm ² or less, and the prior austenite grain size after hot stamping has a metal structure of 10 μm or less,
For hot stamping, characterized by having a tensile strength after hot stamping of 1.8 to 2.5 GPa and an impact absorption energy at −40 ° C. in a V-notch Charpy test of 30 J / cm ² or more. steel sheet.   The chemical composition further includes a group consisting of Cu: 2.0% or less, Ni: 2.0% or less, Mo: 2.0% or less, V: 0.2% or less, and W: 0.2% or less. The steel sheet for hot stamping according to claim 1, which has one kind or two or more kinds selected.   The chemical composition further includes Ca: 0.1% or less, Mg: 0.1% or less, REM: 0.1% or less, Zr: 0.1% or less, Nd: 0.1% or less, and Sb: 0 The steel sheet for hot stamping according to claim 1 or 2, wherein the steel sheet has one or more selected from the group consisting of 1% or less. 4. The metal structure according to claim 1, wherein the number density of carbides having a circle-equivalent average particle diameter of 0.5 μm or more after hot stamping is 20 pieces / mm ² or less. 5. Steel sheet for hot stamping.   A slab was produced at a solidification rate of 0.2 ° C./sec or higher during continuous casting, and the slab was heated to 1200 ° C. or higher and held for 90 minutes or longer. The rough rolling outlet temperature was 950 ° C. or higher and the rough rolling time was 3 The method of claim 1, wherein rough rolling is performed for at least minutes, hot rolling is performed at a finish rolling temperature of 850 ° C. or higher, winding is performed at a winding temperature of 700 ° C. or lower, and then cold rolling is performed. Item 5. A method for producing a hot stamping steel plate according to any one of Items 1 to 4.   6. The method for producing a hot stamping steel plate according to claim 5, further comprising annealing at an annealing temperature of 750 [deg.] C. or higher after the cold rolling.   A hot stamped steel plate member obtained by performing hot stamping on the hot stamped steel plate according to any one of claims 1 to 4.