JP2010174293A - Steel sheet to be die-quenched superior in hot-punchability - Google Patents

Abstract

Disclosed is a die quench steel sheet having excellent hot punchability.
SOLUTION: In mass%, C: 0.10% or more and less than 0.19%, Si: 0.15-0.5%, Mn: 1.0-1.8%, P: 0.03% or less, S: 0.020-0.050%, Al: 0.015-0.07% , N: 0.005% or less, Ti, Ti: in the range of 0.05 to 0.15%, and Ti * defined by Ti * = Ti-3.4N-1.5S satisfies C-0.25Ti * ≧ 0.10 Thus, a steel sheet having a composition to be contained and having an average diameter of sulfides containing Ti in the steel sheet is 0.10 μm or more. In addition, it is preferable that the sulfide containing Ti having a diameter of 0.10 μm or more has an average of 30 pieces / mm ² or more in the cross section of the steel plate. In addition to the composition described above, one or more selected from Cr, B, Mo, and W may be further contained.
[Selection figure] None

Description

  TECHNICAL FIELD The present invention relates to a high-strength steel sheet for automobile parts and the like, and in particular, a die quench steel sheet suitable for application of die quench (also referred to as hot press) for increasing strength by heating the steel sheet and quenching it while pressing. This relates to improvement of hot punchability.

  Conventionally, many automobile body structural parts, undercarriage parts, and the like have been manufactured by pressing a steel plate having a predetermined strength. In recent years, from the viewpoint of preservation of the global environment, weight reduction of automobile bodies has been eagerly desired, and higher strength of steel plates to be used has been demanded. However, due to a decrease in workability (formability) associated with high strength of the steel sheet, it is often difficult to process the steel sheet into a desired part shape. For example, high strength steel sheets with a tensile strength exceeding 980MPa, such as when pressing into complex shapes, such as when ductility is reduced and fracture occurs in parts with a large amount of processing, or the spring back becomes large and dimensional accuracy decreases. At the time of cold pressing, problems such as press cracking and insufficient shape freezeability are likely to occur, and there are still parts that have not achieved the desired high strength.

  In order to solve such problems, a technique called a die quench method (also referred to as a hot press method) has been put into practical use as one method for increasing the strength of automobile parts and the like. In the die quench method, the material (steel plate) is heated to the austenite single phase region and softened, and then pressed into a desired shape using a mold cooled by water cooling or the like. This is a method of forming a molded product (part) that has the desired dimensional accuracy and high strength by molding into a desired processed shape and quenching and hardening at the same time, achieving both workability (formability) and high strength. Can be made.

  As a steel plate suitable for such a die quench method, for example, Patent Document 1 proposes a hot forming steel plate excellent in the joint strength and hot formability of a spot weld. In the technique described in Patent Document 1, the steel sheet is, in mass%, C: 0.1 to 0.35%, Si: 0.7 to 2.5%, Mn: 0.5 to 5%, Al: 0.01 to 0.5%, Ti: 0.005 to 0.05. %, B: 0.003 to 0.005%, N: 0.001 to 0.01%, and Ti and N have a composition satisfying {(Ti / 47.9) × 14.0−N} ≧ 0. In the technique described in Patent Document 1, a relatively large amount of Si is contained in the steel sheet to improve the strength of the spot welded portion, and B is contained to suppress deterioration of hot formability due to the large amount of Si. A steel plate composition that can prevent cracking and breakage during hot forming, and further suppress the formation of BN by containing Ti in an amount greater than the N content at an atomic concentration, thereby ensuring the effect of B on hardenability. Thus, after hot stamping (die quench molding) of the steel sheet, a molded product having a predetermined high strength (high hardness) and good quality (hot stamped product) can be obtained.

Patent Document 2 proposes a die quench high-strength steel sheet. The steel sheet described in Patent Document 2 is mass%, C: 0.1 to 0.5%, Si: 1% or less, Mn: 0.1 to 2.5%, Ti: 0.01 to 0.05%, N: 0.0010 to 0.0030%, Ce: It has a composition containing 0.0020% to 0.02%, P, S, and Al are regulated to a predetermined amount or less, and contains 4 particles / mm ² containing Ce oxide at a position slightly inside from the surface and having a particle size of 2 μm or less. A steel sheet having a structure including the above. By making Ce particles containing fine particles having a particle size of 2 μm or less near the surface of the steel sheet, softening near the surface during die quenching can be prevented, and fatigue characteristics after die quenching are improved.

Here, in general, a molded product (part) often requires a hole for attaching another part. However, in the steel plate subjected to this die quench method, the strength after die quench greatly exceeds 980 MPa, so it is difficult to make a hole in the molded product, or even if it can be made, the residual stress increases. Therefore, there are concerns about the occurrence of delayed fracture.
For such a problem, for example, Patent Document 3 proposes a steel sheet for hot pressing. The steel sheet described in Patent Document 3 is mass%, C: 0.15-0.3%, Si: 0.005-1.0%, Mn: 0.01-3.0%, P: 0.005-0.1%, S: 0.02% or less, Al: A composition containing 0.01 to 3.0%, N: 0.01% or less, Cr: 0.02 to 0.5%, V: 0.002 to 0.5%, B: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, and an average particle size of 0.01 to 5.0 Mg oxide in the range of μm, sulfide, composite crystallized product, and composite precipitates, 1 × 10 ² to 1 × 10 ² per square mm It is a steel plate characterized by including ^seven pieces, and is said to be a steel plate excellent in delayed fracture characteristics of the punched portion after hot press quenching and excellent in spot weldability. In the technique described in Patent Document 3, by adding Mg, cracks generated in the punched section can be made fine and uniform, and oxides present in the steel sheet and composite crystallized materials / precipitates using these as nuclei. They are uniformly dispersed to suppress the generation of coarse cracks during punching, and these oxides and the like serve as hydrogen trap sites to suppress delayed fracture.

JP 2007-169679 JP 2007-247001 Japanese Unexamined Patent Publication No. 2006-9116

  By the way, instead of processing holes for mounting other parts into molded products with increased strength after die quenching, the material (steel plate) is punched before being die quenched after heating the material (steel plate) It is conceivable to form a predetermined hole by performing die quenching after that. However, in this method, since a material (steel plate) softened by heating is punched, large burrs are often generated on the punched end surface. This burr remains even after die quenching, deteriorates the part shape, and hardens at the time of die quenching, so it can easily become caught when transporting molded parts (parts), and in some cases damages the press die. There is.

Moreover, the steel sheet obtained by the techniques described in Patent Documents 1 to 3 described above has a problem that it does not have an excellent hot punching property sufficient to avoid such a problem.
For example, if a steel plate described in Patent Document 1 is used as a raw material and heated and punched to form a predetermined hole, burrs are likely to occur during punching. This is because the steel sheet described in Patent Document 1 contains a relatively large amount of Si, has a high temperature viscosity, and has excellent hot formability. Moreover, even if the steel plate described in Patent Document 2 is used as a raw material, fine particles containing Ce oxide dispersed in the steel plate and having a particle size of 2 μm or less are for preventing surface softening after die quenching, and before die quenching. It does not affect the hot punching property during heating. Moreover, when the steel plate described in Patent Document 3 is used as a raw material, the base structure becomes an austenite structure having a different crystal system by heating at the time of hot punching, so that a fine Mg oxide dispersed in a large number in the steel plate. It is considered that any of sulfide, composite crystallized product, and composite precipitate has no effect until the punchability is improved.

  The present invention solves the problems of the prior art, is a steel plate to which the die quench method is applied, is excellent in hot punching, and has a tensile strength after die quench of 1200 MPa or higher, preferably less than 1500 MPa, It aims at providing the steel plate for die quenches. The term “excellent in hot punching” as used herein means the highest height of burrs after the material (steel plate) is heated to a temperature range where it becomes an austenite single phase, punched with a punch with a diameter of 10 mm, and allowed to cool to room temperature. When the length is 40 μm or less.

In order to achieve the above-described object, the present inventors have intensively studied various factors affecting hot punching properties. As a result, in order to improve the hot punching ability, it has been conceived that it is effective to actively utilize precipitates that are stable at high temperatures.
Conventionally, when cold-pressing high-strength steel sheets, inclusions and precipitates such as coarse sulfides are likely to become crack initiation points and crack propagation paths at the end face, and it is thought that they should be reduced as much as possible. It was. However, unlike cold pressing, the die quench method forms a steel sheet softened by heating, which reduces the occurrence of cracks and cracks, and the negative effects of coarse inclusions and precipitates are almost negligible. It came.

In die quenching, the steel sheet is usually heated to the temperature of the austenite single-phase region, so that it has become important to use stable precipitates in the austenite region in order to improve the hot punching property.
And, since the present inventors, for example, TiS and TiMnS precipitate at a high temperature and grow to a certain size, pay attention to sulfides containing Ti, and utilize sulfides containing Ti that are stable in the austenite region. I thought of it. A precipitate that precipitates in a ferrite region such as TiC and easily re-dissolves in an austenite region is inappropriate in the present invention. And after making S content into an appropriate range, Ti content is adjusted to an appropriate range in relation to S, N, and C contents, and the appropriate size that the particle size (diameter) becomes 0.10 μm or more in the steel sheet. By dispersing the sulfide containing Ti, when the steel plate is heated by die quenching and punched out hot, the sulfide containing Ti becomes the optimum size and amount, and the optimum dispersion state. The present inventors have newly found that the hot punching property is remarkably improved.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.10% or more and less than 0.19%, Si: 0.15-0.5%, Mn: 1.0-1.8%, P: 0.03% or less, S: 0.020-0.050%, Al: 0.015-0.07%, N: Including 0.005% or less, Ti in the range of Ti: 0.05-0.15%, and the following formula (1)
Ti * = Ti-3.4N-1.5S (1)
(Here, Ti, N, S: content of each element (mass%))
Ti * defined by the following formula (2)
C-0.25Ti * ≧ 0.10 (2)
(Here, Ti *: value defined by formula (1), C: content of carbon C (mass%))
Is a steel plate having a composition consisting of the balance Fe and inevitable impurities, wherein the average diameter of sulfides containing Ti in the steel plate is 0.10 μm or more. Steel plate for die quenching with excellent punchability.
(2) A steel plate for die quenching according to (1), comprising 30 or more sulfides containing Ti having a diameter of 0.10 μm or more per 1 mm ^{2 in} cross section of the steel plate.
(3) In (1) or (2), a sulfide containing Ti having a diameter of 0.10 μm or more is parallel to the plate surface from the outermost surface to the depth direction of 100 μm in the steel plate thickness direction cross section. A die quench steel sheet having an average of 10 or more in a 5 mm length range.
(4) In any one of (1) to (3), in addition to the above composition, Cr: 0.15-1%, B: 0.0008-0.0030%, Mo: 0.1-0.5%, W: 0.05 A steel plate for die quench characterized by having a composition containing one or more selected from ˜1%.

  According to the present invention, it is possible to easily and inexpensively manufacture a die quench steel sheet that is excellent in hot punching ability and can obtain high strength in a range of tensile strength of 1200 MPa or more, preferably less than 1500 MPa after die quenching. Play. In addition, according to the present invention, since the generation of burrs can be suppressed during hot punching, it becomes possible to perform desired hole processing by hot punching after heating in the die quench method and before die quenching, and a high-strength holed molded product is inexpensive. And there is an effect that it can be manufactured as it is.

The die quench steel sheet of the present invention is preferably a hot rolled steel sheet or a cold rolled steel sheet, and is not particularly limited.
First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.10% or more and less than 0.19% C is the most important element in the present invention that contributes to the improvement of hardenability and is effective in improving the strength after die quenching (work hardening). In order to make the strength of the molded product (product) after die quenching high target strength of TS: 1200 MPa or more using such effects, the content of 0.10% or more is required. On the other hand, when it contains 0.19% or more, the strength after die quenching is too high exceeding the target strength range. For this reason, C was limited to the range of 0.10% or more and less than 0.19%.

Si: 0.15-0.5%
Si is an element that acts as a deoxidizer, facilitates precipitation of carbides, and enhances temper softening resistance. In the present invention, it is necessary to contain 0.15% or more. When the Si content is less than 0.15%, the temper softening resistance is low, the martensite phase generated at the time of die quenching is easily tempered, and the resulting carbide is coarse and the toughness tends to be lowered. On the other hand, if the content exceeds 0.5%, the ferrite phase starts to precipitate during cooling, and the desired high strength cannot be ensured. For this reason, Si was limited to the range of 0.15 to 0.5%.

Mn: 1.0-1.8%
Mn is an element that improves the strength of steel through improvement of the hardenability of steel, and such an effect becomes remarkable when the content is 1.0% or more. If the content is less than 1.0%, pearlite is generated during quenching, and the tensile strength is significantly reduced. On the other hand, when the content exceeds 1.8%, segregation is likely to be formed in the central portion of the plate thickness, and defects due to segregation are likely to occur during press working. For this reason, Mn was limited to the range of 1.0 to 1.8%.

P: 0.03% or less P is an element that has the effect of increasing the strength by dissolving in a normal high-strength steel sheet. However, if contained in a large amount exceeding 0.03%, the strength of the welded part is reduced during welding after die quenching. Make it easier to run out of problems. For this reason, in the present invention, P is limited to 0.03% or less.
S: 0.020 to 0.050%
S is one of the most important elements in the present invention, which forms sulfides and contributes effectively to the improvement of hot punchability. In the present invention, S needs to be contained in an amount of 0.020% or more in order to precipitate a sufficient amount of a sulfide having a proper size in the austenite region and improve the hot punching property. On the other hand, a large content exceeding 0.050% not only saturates the effect, but also greatly reduces the hot ductility in the steel sheet manufacturing process and tends to cause defects on the steel sheet surface due to hot cracking. For this reason, S was limited to the range of 0.020 to 0.050%.

Al: 0.015-0.07%
Al acts as a deoxidizing agent and is an element necessary for deoxidizing molten steel. In order to obtain such an effect, a content of 0.015% or more is required. On the other hand, even if it is contained in a large amount exceeding 0.07%, the above-described effect is saturated, and the amount of oxide increases and the cleanliness decreases. For this reason, Al was limited to the range of 0.015 to 0.07%.

N: 0.005% or less N is bonded to Ti at a high temperature to form TiN and reduce the amount of Ti that becomes sulfide. Therefore, it is desirable to reduce it as much as possible in the present invention, but up to 0.005% is acceptable. For this reason, N was limited to 0.005% or less.
Ti: 0.05-0.15%
Ti is an element having a function of improving the hot punchability by forming a sulfide, and, like S, is one of the most important elements in the present invention. In the present invention, Ti is contained in an amount of 0.05% or more in order to deposit a sufficient amount of a sulfide that is stable in the austenite region in an appropriate size and to improve the hot punching property. Thereby, at the time of the die quench heating, a sulfide having an appropriate size and an appropriate dispersion density can be present, and the hot punching property is remarkably improved. On the other hand, if it exceeds 0.15%, the effect is not only saturated, but excess Ti is precipitated as carbide. These carbides of Ti may not be sufficiently re-dissolved during the heating of die quenching. For this reason, solid solution C in the steel is reduced, and the strength after die quenching is lowered. For these reasons, Ti is limited to the range of 0.05 to 0.15%.

Furthermore, Ti is within the above-mentioned content range, and the following formula (1)
Ti * = Ti-3.4N-1.5S (1)
Here, Ti, N, S: Content of each element (% by mass)
Ti * defined by the following equation (2) in relation to the C content:
C-0.25Ti * ≧ 0.10 (2)
Here, C: Carbon C content (% by mass)
The content is adjusted so as to satisfy. Even if Ti is within the above-mentioned content range (0.05 to 0.15%), if the formula (2) is not satisfied, the Ti content becomes excessive with respect to the C content, and the solute C content decreases and die quench. Later strength decreases.

Although the above-described components have a basic composition, in addition to the basic composition, one or more selected from Cr, B, Mo, and W can be further contained as selective elements.
One or more selected from Cr: 0.15 to 1%, B: 0.0008 to 0.0030%, Mo: 0.1 to 0.5%, W: 0.05 to 1%
Cr, B, Mo, and W are all elements that contribute effectively to improving the hardenability, and can be selected as necessary to contain one or more. In order to obtain such an effect, it is desirable to contain Cr: 0.15% or more, B: 0.0008% or more, Mo: 0.1% or more, W: 0.05% or more, but Cr: 1%, B: Even if the content exceeds 0.0030%, Mo: 0.5%, and W: 1%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, when it contains, it is preferable to limit to Cr: 0.15-1%, B: 0.0008-0.0030%, Mo: 0.1-0.5%, W: 0.05-1%, respectively.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include, for example, O: 0.01% or less, Sb: 0.02% or less, and Sn: 0.01% or less.
The steel sheet of the present invention is a steel sheet having the above-described composition, and sulfides containing Ti are dispersed in the steel sheet. The average diameter of the sulfides containing Ti is 0.10 μm or more, preferably 1.0 μm or less.

Specific examples of sulfides containing Ti include TiS, TiMnS, Ti ₄ C ₂ S _2, etc., but since they are heated to the austenite region at the time of die quench heating, the sulfide should be stable even in the austenite region. preferable. Ti ₄ C ₂ S ₂ precipitates in the ferrite and easily re-dissolves in the austenite region. For this reason, in the present invention, the sulfide containing Ti is preferably TiS or TiMnS. Note that Ti ₄ C ₂ S ₂ having a sufficient size that does not re-dissolve when heated by die quenching, for example, Ti ₄ C ₂ S ₂ having a diameter of 0.10 μm or more can sufficiently contribute to improvement of hot punching property.

  If the size of the sulfide containing Ti is 0.10 μm or more in diameter, it will not dissolve and disappear even if heated under normal die quench heating conditions, that is, in the temperature range where it becomes an austenite single phase. , Excellent hot punchability can be secured stably. If the size of the sulfide containing Ti is less than 0.10 μm in diameter, even if it is under normal die quenching heating conditions, a part of it will re-dissolve and become smaller in size, and hot punching by the sulfide containing Ti will be reduced. Improvement effect is reduced. Therefore, the average diameter of sulfides containing Ti dispersed in the steel is set to 0.10 μm or more. In the steel sheet of the present invention, sulfides containing Ti are present in a dispersed state. However, when the average diameter of sulfides containing Ti is less than 0.10 μm, it contains Ti that dissolves again and disappears during die quench heating. The amount of sulfide increases, making it difficult to stably obtain excellent hot punchability. The upper limit of the average diameter of sulfides containing Ti is not particularly limited, but if it becomes too large, the number of sulfides will decrease, and the hot punchability will be improved by sulfides containing the desired Ti. It becomes difficult to obtain the effect. For this reason, it is preferable that the average diameter of the sulfide containing Ti is 1.0 μm or less.

In the steel sheet of the present invention, it is preferable to disperse 30 or more sulfides containing Ti having a diameter of 0.10 μm or more on an average per 1 mm ² of the steel sheet cross section. If the number of sulfides containing Ti dispersed is less than 30 on average per 1 mm ^{2 of the} cross section of the steel sheet, the burr height of the fractured end surface generated during hot punching tends to be uneven and the heat generated by sulfide containing Ti There are cases where the effect of improving the punching property becomes insufficient. For this reason, it is preferable that the number of dispersed sulfides containing Ti is 30 or more per 1 mm ^{2 of the} cross section of the steel sheet in average in the cross section in the steel sheet rolling direction.

  Further, in the steel sheet of the present invention, it is preferable that a sulfide containing Ti is present in the vicinity of the outermost layer of the steel sheet in order to cause breakage to proceed uniformly and reduce the burr height at the time of hot punching. On one side (single side), in the cross section in the thickness direction of the steel plate, from the outermost surface of the steel plate (plate surface) to 100 μm in the depth direction and within a range of 5 mm length parallel to the plate surface, a diameter of 0.10 μm or more It is preferable that 10 or more sulfides containing Ti are present on average. Needless to say, it is more preferable to ensure the dispersion state of the sulfide containing Ti in the outermost layer on both surfaces of the steel sheet.

The production method of the steel sheet of the present invention having the above-described composition and the above-described sulfide containing Ti is not particularly limited, and any of the commonly known methods for producing hot-rolled steel sheets or the commonly known methods for producing cold-rolled steel sheets can be applied. However, a more preferable production method will be described below.
The production method of the steel sheet of the present invention having the above-described composition and the above-described sulfide containing Ti is not particularly limited, and any of the commonly known methods for producing hot-rolled steel sheets or the commonly known methods for producing cold-rolled steel sheets can be applied. However, a more preferable production method will be described below.

It is preferable that the molten steel having the above composition is melted by a conventional melting method such as a converter and is made into a slab or the like by a conventional casting method such as a continuous casting method. Then, these slabs are hot-rolled to obtain hot-rolled steel sheets, or, further, pickled hot-rolled steel sheets, then cold-rolled, and further annealed as necessary to obtain cold-rolled steel sheets. Good.
In hot rolling, the slab is preferably heated to 1050 to 1300 ° C, or continuously after casting without heating, the total rolling reduction: 95% or more, and the finish rolling finish temperature: 850 to 950 ° C After hot rolling is completed, if necessary, accelerated cooling is preferably performed at a cooling rate of 20 ° C./s or more, and then winding is performed at a winding temperature of 200 to 700 ° C. to obtain a desired thickness. It is preferable to use a hot rolled steel sheet. In addition, it is preferable to adjust suitably the cooling rate of accelerated cooling, and coiling temperature in consideration of the load etc. to a subsequent pickling process and a cold rolling process. To reduce the load of the pickling process, lower the coiling temperature to increase the cooling speed of accelerated cooling, and to reduce the load of the cold rolling process, lower the coiling temperature to lower the cooling speed of accelerated cooling. It is preferable to set it higher.

In the case of a cold-rolled steel sheet, the hot-rolled steel sheet that has undergone the pickling process is preferably subjected to cold rolling at a reduction rate of 40% or more, and is preferably a cold-rolled steel sheet having a desired thickness, or It is preferable to subject the cold-rolled steel sheet to an annealing process at an annealing temperature of 550 to 900 ° C.
Preferably, the steel sheet of the present invention obtained by the above-described production method is processed into a die quench molded product having a predetermined size and shape by applying a die quench method to obtain a product. In addition, when this invention steel plate is a hot-rolled steel plate, it cannot be overemphasized that it is preferable to generally remove the scale formed in the surface layer by the pickling process etc.

Below, based on an Example, this invention is demonstrated in detail.
Molten steel having the composition shown in Table 1 is melted and made into a slab (slab: wall thickness 260 mm) by a continuous casting method, and then the slab is heated at a heating temperature of 1200 ° C., and then finish rolling finish temperature: 850 ° C. Were subjected to hot rolling, then cooled to form a hot rolled sheet, and wound at a winding temperature of 600 ° C. The obtained hot-rolled sheet (thickness: 2.8 mm) was pickled and then cold-rolled to obtain a cold-rolled sheet having a thickness of 1.4 mm.

About the obtained steel plate, the hot punching test was implemented and hot punching property was evaluated. Moreover, the precipitate was observed about the obtained steel plate. Moreover, the die quench test was implemented about the obtained steel plate, and the intensity | strength of the molded article after die quench was measured. The test method was as follows.
(1) Hot punching test A test plate (size: plate thickness x 100 mm x 100 mm) is taken from the obtained steel plate, heated and held in the atmosphere (900 ° C x 15 min), and then punched with a 10 mm diameter punch. A hot punching test was carried out. The punching temperature was controlled between 750 ° C and 500 ° C. The blanking clearance was 12% of the plate thickness. After punching, it was allowed to cool to room temperature and the burr height was measured. The burr height was determined based on the surface at a position 2 mm away from the punched hole, and the height of the highest vertex of the burr was defined as the burr height of the steel plate. In some cases, the heating and holding was replaced with an air atmosphere, and an Ar gas atmosphere was used.

(2) Precipitate observation A specimen for observation was collected from the obtained steel sheet, the cross section in the rolling direction was polished, and the precipitate was observed using a scanning electron microscope (magnification: 5000 times). The observation positions were the surface layer (up to 100 μm in the depth direction from the outermost surface and within a range of 5 mm length parallel to the plate surface), and the plate thickness center position.
Ten fields of view were observed at each position, the tissue was imaged, and the size (diameter) and the number of dispersions of sulfides containing Ti were calculated using an image analyzer. In addition, it was confirmed that each precipitate was qualitatively analyzed by an energy dispersive X-ray analyzer (EDX) of a scanning electron microscope for each precipitate. The size (diameter) of the sulfide containing Ti was obtained by calculating the area of each particle (precipitate) and converting it to an equivalent circle diameter. Furthermore, the average value was calculated | required about the diameter of each particle | grain (precipitate), and it was set as the magnitude | size (average of a diameter) of the sulfide containing Ti of the steel plate.

  In addition, the dispersion density of the sulfide containing Ti is measured at each observed position, the number of particles of sulfide containing Ti having a diameter of 0.10 μm or more in each field of view, the average number of those particles, the position, The dispersion density of the field of view (number per unit area) was used, and the average value of the dispersion density of each field of view was the dispersion density at each position of the steel sheet. The number of surface layers in the cross section in the plate thickness direction is the number within the range of 5 mm length from the outermost surface to the depth direction of 100 μm and parallel to the plate surface.

(3) Die quench test A test plate (size: plate thickness x 200 mm x 300 mm) was collected from the obtained steel plate, heated and held in the atmosphere (900 ° C x 15 min), and then flat-plate water-cooled gold Die quenching was performed by sandwiching with a mold and cooling to obtain a test product. JIS No. 5 test specimens are collected from the obtained test products in accordance with the provisions of JIS Z 2201, and tensile tests are performed in accordance with the provisions of JIS Z 2241 to determine the tensile strength TS after die quenching. It was. In some cases, the heating and holding was replaced with an air atmosphere, and an Ar gas atmosphere was used.

  The obtained results are also shown in Table 2.

  In all of the inventive examples, the burr height is 40 μm or less and the steel sheet is excellent in hot punching, and all of the inventive examples have a tensile strength TS of 1200 MPa or more and less than 1500 MPa after die quenching. It can be seen that this is a high strength test product. Note that by making the die quenching heating atmosphere an Ar gas atmosphere, the oxidation of the steel sheet surface is suppressed, and the surface state is further improved.

  On the other hand, in the comparative example that is out of the scope of the present invention, the hot punching property is lowered, or the strength after die quenching is lowered. In comparative examples (steel plates Nos. 13 to 15) in which the content of one or both of Ti and S deviates from the scope of the present invention, the burr height is larger than 40 μm, and the hot punching property is lowered. . Moreover, the comparative example (steel plate No. 16) in which Ti and S deviate from the scope of the present invention has a surface defect. Further, in the comparative example (steel plate No. 17) not satisfying the formula (2), a part of C was precipitated as carbide, the tensile strength after die quenching was lower than the desired tensile strength, and the strength was insufficient. .

Claims (4)

% By mass
C: 0.10% or more and less than 0.19%, Si: 0.15-0.5%,
Mn: 1.0 to 1.8%, P: 0.03% or less,
S: 0.020 to 0.050%, Al: 0.015 to 0.07%,
N: 0.005% or less, further containing Ti so that Ti: in the range of 0.05 to 0.15% and Ti * defined by the following formula (1) satisfies the following formula (2), and the balance A steel plate for die quenching excellent in hot punching characteristics, characterized in that the steel plate has a composition comprising Fe and inevitable impurities, and the average diameter of sulfides containing Ti in the steel plate is 0.10 µm or more.
Record
Ti * = Ti-3.4N-1.5S (1)
C-0.25Ti * ≧ 0.10 (2)
Here, Ti, N, S, C: Content of each element (mass%) ^2. The die quench steel sheet according to claim 1, having an average of 30 or more sulfides containing Ti having a diameter of 0.10 μm or more per 1 mm ^{2 of the} cross section of the steel sheet.   Ten or more sulfides containing Ti with a diameter of 0.10μm or more on average within a range of 5mm length parallel to the plate surface from the outermost surface to the depth direction of 100μm in the thickness direction cross section of the steel plate The die quenching steel sheet according to claim 1, wherein the die quenching steel sheet is present.   In addition to the above composition, one or more selected from Cr: 0.15 to 1%, B: 0.0008 to 0.0030%, Mo: 0.1 to 0.5%, W: 0.05 to 1% The die quenching steel sheet according to any one of claims 1 to 3, wherein the die quenching steel sheet has a composition containing: