JP2012237048A - Hot stamping steel plate excellent in hot composite molding property and delayed fracture resistance in punched part thereof, method for manufacturing the same and method for smelting the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a part, especially an automobile part that has a strength of 1,470 MPa or more and also is excellent in delayed fracture resistance and toughness of the processed part thereof, by a hot stamp technology.SOLUTION: A hot stamping steel plate contains, by mass, 0.001-0.005% S, 0.005-0.03% REM (or 0.005-0.03% Mg), and 0.003-0.007% O, wherein a spherical inclusion containing two or more of S, O and REM and having a diameter of 0.1 μm or less is dispersed, thereby excellent in hot composite molding property and delayed fracture resistance in the punched part thereof.

Description

  The present invention relates to a hot stamping steel plate that is excellent in hot composite formability and delayed fracture resistance of a punched portion and can be processed into a component having a complicated shape, and a manufacturing method and a melting method thereof.

  In recent years, it has been an urgent task to reduce the weight of automobile bodies from the viewpoint of protecting the global environment. On the other hand, since the study of applying high-strength steel sheets has been actively conducted, the strength required of steel materials Is getting higher and higher. However, as steel plate strength increases, workability deteriorates and consideration for shape freezeability is required. On the other hand, in the press work that is normally used, the molding load increases more and more, and the improvement of the press capability is also a big issue for practical use.

  On the other hand, the hot stamping technique is to perform press forming after heating a steel plate to a high temperature in the austenite region. For this reason, the molding load is greatly reduced as compared with the normal pressing performed at room temperature.

  Moreover, since the hot stamping technique performs a so-called quenching process in the mold simultaneously with the press working, it is possible to obtain a strength corresponding to the C amount of steel. Therefore, hot stamping technology is attracting attention as a technology that achieves both shape freezing properties and strength.

  Patent Document 1 defines the average particle size and distribution density of oxides containing Mg as a steel plate utilizing hot stamping technology, and includes delayed fracture characteristics in addition to strength, cutability and punchability after hot stamping. And steel plates with improved spot weldability are disclosed. However, the steel sheet of Patent Document 1 is not intended to improve the formability at the time of hot stamping and the punchability that enables punching performed at the same time.

  Patent Document 2 discloses a technique related to a hot forming steel sheet and a method for manufacturing the same. In the above technology, the steel sheet before hot forming is subjected to a predetermined heat treatment and then heated again to perform hot press forming. Formability in stretch flange forming during hot stamping In addition, delayed fracture characteristics in the hot punched part are not taken into consideration.

JP 2006-009116 A JP 2006-283064 A

  Parts for automobiles, especially parts such as frames, members, and reinforcements, due to their roles, parts that efficiently absorb energy at the time of collision, and ensure strength, transmit energy at the time of collision without deformation Classified into parts.

  In particular, the required strength for reinforcement is increasing, and cold press forming requires a strength of 1470 MPa because the press machine capacity is insufficient and the shape freezeability of the steel sheet deteriorates. The number of parts to which hot stamp materials are applied is increasing.

  Moreover, in order to realize further weight reduction of the automobile body, a member having a strength of 1770 MPa or more is required. Therefore, in order to expand the number of parts to which the hot stamp technology is applied, it is necessary to consider the delayed fracture characteristics of the molded part after being molded into the part as well as improving the hot press formability.

  In view of the above situation in the prior art, the present inventors have an object of manufacturing a part having a strength of 1470 MPa or more and excellent in delayed fracture resistance and toughness of a processed part by a hot stamping technique.

  The present inventors diligently studied a method for achieving the above object. As a result, it was found that when composite sulfur oxides containing two or more of S, O, and REM are dispersed in the steel sheet as spherical inclusions, good toughness and delayed fracture resistance can be secured in the processed part. .

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  (1) In a steel sheet for hot stamping containing, in mass%, S: 0.001 to 0.005%, REM: 0.005 to 0.03%, and O: 0.003 to 0.007%, For hot stamping excellent in hot composite moldability and delayed fracture resistance of the punched portion, characterized in that spherical inclusions having a diameter of 0.1 μm or less, including two or more of, O, and REM are dispersed steel sheet.

  (2) In a steel sheet for hot stamping containing, in mass%, S: 0.001 to 0.005%, Mg: 0.005 to 0.03%, and O: 0.003 to 0.007%, For hot stamping excellent in hot composite moldability and delayed fracture resistance of the punched portion, characterized in that spherical inclusions having a diameter of 0.1 μm or less containing two or more of, O and Mg are dispersed steel sheet.

  (3) The steel sheet for hot stamping is mass%, C: 0.20 to 0.35%, Si: 0.1 to 0.5%, Mn + Cr: 1 to 3%, Ti: 0.005 to 0 .1% and Nb: 0.005 to 0.1%, the balance being Fe and inevitable impurities, the hot composite formability and punching according to (1) or (2) above Steel sheet for hot stamping with excellent delayed fracture resistance.

  (4) The steel sheet for hot stamping is further mass%, B: 0.0005 to 0.002%, V: 0.005 to 0.1%, and Mo: 0.05 to 0.5%. The steel sheet for hot stamping which is excellent in hot composite formability and delayed fracture resistance of the punched portion according to (3) above, comprising one or more of the above.

  (5) The hot stamping excellent in hot composite formability and delayed fracture resistance of the punched portion according to any one of (1) to (4), wherein the spherical inclusion is a composite sulfate Steel plate.

  (6) The slab having the composition described in the above (3) or (4) is heated to a temperature range of 1250 ° C. or less and subjected to hot rolling, and in the temperature range of Ar3 or higher, the last stand and the previous stand Finishing the finish rolling with the total reduction amount at the stand being 60% or more, starting the cooling within 1 second after finishing the finish rolling, and winding up at a temperature of 600 ° C. or less, A method for manufacturing a steel sheet for hot stamping with excellent delayed fracture resistance at the punched portion.

  (7) The molten steel having the composition described in (3) or (4) is pre-deoxidized with Al to reduce free oxygen in the molten steel to 10 to 50 ppm, and then with Si or Ti and REM or Mg A method for producing a steel sheet for hot stamping, which is excellent in hot composite formability and delayed fracture resistance of a punched portion, characterized by performing composite deoxidation.

  According to the present invention, a steel sheet for hot stamping that is excellent in composite formability and punchability at the time of hot stamping, has strength of 1470 MPa or more after hot stamping, and has excellent delayed fracture resistance and toughness of a processed part. Can be provided.

It is a figure explaining evaluation of stretch flange formability in hot. (A) shows the structure of the metal mold | die used for an evaluation test, (b) shows the shape of the blank used for an evaluation test. It is a figure which shows the presence or absence of the crack in the relationship between the addition amount (mass%) of REM or Mg, and the flange height H (mm). It is a figure which shows the cross-sectional aspect of a hot punching test apparatus.

  First, the experiment that led to the completion of the present invention will be described.

  The present inventors, in mass%, 0.28% C-0.5% Si-1% Mn-0.5% Cr-0.003% S-0.05% Ti-0.05% Nb- Based on 0.0041% O steel, the blank (molded material) taken from a cold-rolled / annealed plate with a thickness of 1.6 mm added by changing the amount of REM or Mg was heated to 900 ° C. Thereafter, it was press-molded with a mold shown in FIG. 1A, and immediately after molding, it was cooled to room temperature at 50 ° C./second.

  As shown in FIG. 1A, press molding was performed such that a blank D was sandwiched between a punch A and a pad C, and at the same time, a die B was moved in the direction of an arrow to raise a flange. FIG. 1B shows the shape of the blank D used. The blank width W is 140 mm, the radius of curvature R of the corner portion is 10 mm, and the opening angle θ of the corner portion is 120 °.

The flange height H of the stretch flange formed part x is defined by the following equation.
Flange height H = Total length H _total of the blank central portion _−Non -molded portion y (on punch and pad)
The length H _flat of the center of the blank flat part)

  About the molding material after cooling, while investigating the presence or absence of the crack in the stretch flange molding part x, the hardness of the non-molding part y was measured by Vickers hardness (load: 10 kgf).

  In FIG. 2, the presence or absence of the crack in the relationship between the addition amount (mass%) of REM or Mg and the flange height H (mm) is shown. From FIG. 2, it is necessary to add REM or Mg in an amount of 0.005% by mass or more in order to realize the formation of a stretch-free flange portion without cracks, and a higher formability (flange height H ≧ 30 mm). It can be seen that it is necessary to add 0.01% by mass or more in order to ensure.

  In any of the molding materials, the hardness of the non-molded portion y was Hv ≧ 540 corresponding to a tensile strength of 1770 MPa or more.

  Further, a 150 mm square steel plate collected from the cold-rolled / annealed plate was heated to 900 ° C., and then a hole with a diameter of 20 mm was punched by a hot punching test apparatus shown in FIG.

  In the hot punching test apparatus shown in FIG. 3, a stripper 4 for holding a punch holding table 1 a having a 20φ punch 1 by a spring 5 is arranged on an upper part of a base 9 having a 20.3φ die 2 in the center. ing. The stripper 4 is provided with a spacer 3 on the upper surface to ensure a distance from the punch holder 1a.

  The punch holding base 1a and the stripper 4 are lowered, and the punch holding base 1a is further lowered. The punch 1 and the die 2 cooperate to hold the punch 9 on the base 9 with the cushion pin 7 and to position with the positioning pin 6. A hole having a diameter of 20 mm can be punched into the steel plate 8 that has been formed.

  Since the punch 1 and the die 2 have a water cooling structure, the steel plate 8 can be cooled from 900 ° C. to room temperature simultaneously with the punching of holes.

The steel plate with punched holes was immersed in an aqueous solution containing 0.3% ammonium thiocyanate and 3% NaCl at room temperature, and a current was passed through the steel plate at a current density of 0.1 mA / cm ² for 48 hours. The presence or absence of cracks on the punched fracture surface was investigated. As a result, it was found that when REM or Mg is added within the scope of the present invention, delayed fracture due to hydrogen does not occur on the punched fracture surface. The present inventors have completed the present invention based on these experimental facts.

  Hereinafter, the limitation requirement of this invention is demonstrated. First, the reason for limiting the component composition of steel will be described. In addition,% concerning a component composition means the mass%.

  S is an element added to form a composite sulfate with REM or Mg. Therefore, 0.001% or more is added, but if it exceeds 0.005%, composite sulfate is excessively generated, hot workability deterioration in hot stamping, and delayed fracture characteristics deterioration in the punched part. Therefore, S is made 0.001 to 0.005%. Preferably it is 0.002 to 0.004%.

  REM and Mg are elements added to form a composite sulfate with S and O. Therefore, 0.005% or more is added, but if it exceeds 0.03%, composite sulfate is excessively generated, hot workability deterioration in hot stamping, and delayed fracture characteristics deterioration in the punched part. Therefore, both REM and Mg are made 0.005 to 0.03%. Preferably, both REM and Mg are 0.008 to 0.02%.

  O is an element that is positively added to form a composite sulfate with REM, Mg, and S. If it is less than 0.003%, the amount of composite sulfate produced is small, and hot workability with hot stamping cannot be sufficiently obtained. On the other hand, if it exceeds 0.007%, composite sulfate is excessively generated, and not only the hot workability is deteriorated, but also the delayed fracture characteristics at the punched portion are deteriorated. Therefore, O is 0.003 to 0.007%. Preferably it is 0.004 to 0.006%.

  In the present invention, the composite sulfur oxide formed of S, O, and REM or Mg has a diameter of 0.1 μm or less and is dispersed as spherical inclusions in the steel sheet.

  When the size of the spherical inclusions exceeds 0.1 μm in diameter, not only the hot workability is deteriorated but also the number of spherical inclusions to be generated is reduced. Since the spherical inclusion functions as a site for trapping hydrogen that causes delayed fracture, a decrease in the number of spherical inclusions leads to deterioration of delayed fracture resistance. Therefore, the size of the spherical inclusion is set to 0.1 μm or less in diameter.

  C is an element necessary for ensuring the strength after quenching. In order to obtain a strength of 1470 MPa or more after quenching, 0.20% or more is necessary. However, if it exceeds 0.35%, breakage is likely to occur during impact deformation, weldability deteriorates, and the strength of the welded portion increases. Since it falls, C is made 0.20 to 0.35%. Preferably it is 0.23 to 0.31%.

  Si is an element that suppresses precipitation of cementite as well as a solid solution strengthening element. To obtain the effect of addition, 0.1% or more is added, but if it exceeds 0.5%, the plating property deteriorates, so Si is made 0.1 to 0.5%. Preferably it is 0.2 to 0.4%.

  Mn and Cr are important elements necessary for ensuring hardenability. In order to ensure hardenability when performing hot stamping, the total of Mn and Cr is required to be 1% or more. However, if it exceeds 3%, the improvement of hardenability is saturated, so the total of Mn and Cr Is 1 to 3%. Preferably it is 1.4 to 2.6%.

  Although the lower limit of Mn and Cr is not particularly limited, Mn is preferably 0.01% or more and Cr is preferably 0.01% or more for ensuring hardenability.

  Ti and Nb are elements that generate fine carbides, refine the prior austenite grain size after hot stamping, and ensure toughness. In order to make the prior austenite particle size after hot stamping an average value, preferably 6 μm or less, 0.005% or more is added respectively. However, if it exceeds 0.1%, the effect of addition is saturated. And Nb is made 0.005 to 0.1%. Preferably, Ti is 0.01 to 0.06% and Nb is 0.01 to 0.06%.

  B is an element effective for ensuring hardenability. If it is less than 0.0005%, the effect of addition does not appear, and if it exceeds 0.002%, the effect of addition is saturated, so B is made 0.0005 to 0.002%. Preferably it is 0.0007 to 0.0015%.

  V is an element added to refine the structure and ensure toughness. When the steel plate is heated to the Ac3 point or higher, fine V carbides are generated, which suppresses recrystallization and grain growth to make austenite grains fine and improve toughness. If it is less than 0.005%, the effect of addition cannot be obtained, and if it exceeds 0.1%, the effect of addition is saturated and the manufacturing cost increases, so V is set to 0.005 to 0.1%. Preferably it is 0.01 to 0.06%.

  Mo, like Ti, Nb, and V, when steel plate is heated to Ac3 point or higher, produces fine carbides, suppresses recrystallization and grain growth, refines austenite grains, and improves toughness. . If it is less than 0.05%, the effect of addition cannot be obtained, and if it exceeds 0.5%, the effect of addition is saturated and the manufacturing cost increases, so Mo is made 0.05 to 0.5%. Preferably it is 0.08 to 0.3%.

  P is a solid solution strengthening element and can increase the strength of the steel sheet relatively inexpensively, but is easily segregated at the grain boundary and causes low temperature embrittlement when the strength is high. In the present invention, the amount of P is not limited, but is preferably 0.015% or less. The smaller the amount of P, the better. However, a reduction from 0.001% leads to an increase in de-P cost, so 0.001% or more is preferable.

  Al is an element usually added for deoxidation. In the present invention, the amount of Al is not limited, but if it is less than 0.005%, deoxidation becomes insufficient, a large amount of oxide remains in the steel, and local deformability deteriorates, so 0.005% or more Is preferred. On the other hand, if it exceeds 0.05%, a large amount of oxide mainly composed of alumina remains in the steel and local deformability deteriorates, so 0.05% or less is preferable.

  N is an inevitable impurity, and the smaller the number, the more preferable element. In the present invention, the amount of N is not limited, but a reduction to less than 0.001% increases the refining cost, so 0.001% or more is preferable. On the other hand, if it exceeds 0.003%, precipitates are generated and the toughness after quenching deteriorates, so 0.003% or less is preferable.

  In addition, the steel plate of this invention may contain Cu, Cr, Sn, Ni, Mo etc. which mix from scraps etc. in the steelmaking stage in the range which does not impair the effect of this invention. Moreover, you may contain REM containing Ca used as a deoxidation element, Ce, etc. in the range which does not impair the effect of this invention.

  The method for producing a steel slab for hot rolling is not limited to the continuous casting method. A normal continuous casting method or a method of casting a thin slab having a thickness of 100 mm or less can be employed.

  Here, in order to make the amount of free oxygen in the molten steel 10 to 50 ppm, preliminary deoxidation is performed with Al, and composite deoxidation is performed with Si or Ti and REM or Mg. At this time, if it is less than 10 ppm, the number of sulfur oxides functioning as hydrogen trap sites is not sufficiently ensured. On the other hand, if it exceeds 50 ppm, the number becomes too large, so 50 ppm is made the upper limit.

  In the present invention, the prior austenite grain size obtained after hot stamping needs to be made as small as possible from the viewpoint of ensuring the required delayed fracture resistance and toughness. In order to make the prior austenite grain size as small as possible, the setting of hot rolling conditions is important.

  In order to refine the structure of the hot-rolled plate stage, it is preferable that the heating temperature during hot rolling is low. Therefore, heating temperature shall be 1250 degrees C or less. The finishing temperature is preferably as low as possible. However, considering the rolling property, the finishing temperature is set to Ar3 or higher. Then, at the time of finish rolling, the total reduction amount at the last stand and the previous stand is set to 60% or more, and further, cooling is started within one second immediately after finish rolling.

  After finishing rolling, cooling is started within 1 second, and the hot-rolled sheet is wound in a temperature range of 600 ° C. or lower. By this series of cooling and winding, the prior austenite grain size after hot stamping can be made 6 μm or less.

  However, since the hot-rolled sheet strength becomes too high when wound at less than 400 ° C., the winding temperature is preferably 400 ° C. or more. However, after winding at less than 400 ° C., it is 750 ° C. or less for the purpose of softening. You may reheat at temperature.

  In the present invention, conditions for cold rolling, annealing, and plating following hot rolling are not particularly defined, and may be ordinary conditions. Cold rolling may be performed in a range of a normal cold rolling reduction ratio, for example, 40 to 80%. Plating is performed after hot rolling, after cold rolling, or after recrystallization annealing, but heating conditions and cooling conditions are not particularly defined.

  Plating is mainly preferably Zn plating or Al plating. The Zn plating may or may not be alloyed. As for Al plating, even if Si is contained in the plating, the present invention is not affected.

  The temper rolling of the hot-rolled plate, cold-rolled plate, and plated plate may be appropriately performed at a rolling reduction of 0.2 to 3%, for example, in order to appropriately adjust the shape.

  When carrying out hot stamping, the hot-rolled steel sheet, cold-rolled steel sheet, or plated steel sheet is heated to Ac3 point or higher. When the heating temperature is less than the Ac3 point, a region that does not partially austenite is formed. In this region, since martensite is not generated, sufficient strength cannot be obtained on the assumption of the steel sheet.

  However, when the heating rate is less than 5 ° C./second or when the heating temperature exceeds 900 ° C., the prior austenite grains are coarsened, and the prior austenite grain size of 6 μm or less cannot be obtained. The heating temperature is preferably 900 ° C. or less.

  In the present invention, the heat retention time is not particularly defined, but is preferably 60 seconds or less. When the cooling rate in the temperature range of Ar3 to 300 ° C. is less than 50 ° C./second, the strength is likely to fluctuate in the member. Therefore, the cooling rate in the temperature range of Ar3 to 300 ° C. is preferably 50 ° C./second or more. .

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
After the molten steel having the composition shown in Table 1 is removed from the converter and made into a slab, the hot rolling conditions of the present invention (heating temperature: 1220 ° C., finishing temperature: 870 ° C., the final stand and the previous stand are used. Hot rolling was performed at a total rolling amount of 65%, the time from finish finish rolling to the start of cooling: 0.5 seconds, coiling temperature: 600 ° C. to obtain a hot-rolled steel sheet having a thickness of 3 mm.

  This hot-rolled steel sheet was made into a cold-rolled steel sheet having a thickness of 1.4 mm by cold rolling, and then subjected to plating under the conditions shown in Table 2 after continuous annealing or annealing and annealing. The plating treatment was hot dip galvanization (GI (no alloying treatment) / GA (with alloying treatment)) or hot dip aluminum plating (Al) containing 10% of Si.

  About the steel plate which gave the said process, after heating at 900 degreeC with a heating furnace, the water supply port which ejects water on the surface of the metal mold | die (Dice B and the pad C) shown to Fig.1 (a), and the drain port which sucks the water Is placed between the steel plate in the shape of the blank D and at the same time, the pad C is moved in the direction of the arrow to raise the flange, and immediately after that water is sprayed to cool the steel plate to room temperature. This simulated the thermal history of hot stamping.

  About the steel plate after shaping | molding and heat processing, while investigating the presence or absence of the crack in a stretch flange shaping | molding part (x part), the hardness of the non-forming part (y part) was measured by Vickers hardness Hv (load: 10 kgf). The results are also shown in Table 2.

  The delayed fracture resistance was evaluated by using the processed specimen as it was, immersing it in an aqueous solution in which 3 g / l of ammonium thiocyanate was dissolved in 3% saline at room temperature for 24 hours, and determining whether or not the flange portion had fractured. (No break: ○, with break: x).

Low temperature brittleness was evaluated by collecting a test piece from a non-molded part (y part) and conducting a Charpy test at -40 ° C. The test result was evaluated by converting to a plate thickness of 10 mm, and a case showing an energy absorption amount of 100 J / cm ² or more was regarded as acceptable (◯).

  Invented steels (A steel to K steel) according to the present invention have Hv: 470 to 670, a tensile strength equivalent to 1470 to 1970 MPa is obtained, and there is no problem in delayed fracture resistance and toughness.

  The L steel with a low C content and outside the scope of the present invention does not reach the strength of Hv: 450 (strength: equivalent to 1470 MPa). In steel M, which has a high C content and is out of the scope of the present invention, Hv is 731 and the strength is too high, so there is a concern about delayed fracture resistance and deterioration of toughness.

  In N steel, O steel, R steel, S steel, and T steel to which no Ti or Nb is added, the prior austenite grain size is over 6 μm and the toughness is poor. In P steel to which REM and Mg are not added, and in Q steel that is low in O and out of the scope of the present invention, a composite sulfur oxide of 0.1 μm or less that functions as a hydrogen trap site is not sufficiently formed. Destructive properties are not sufficient.

  In U steel and W steel, in which O and REM deviate from the scope of the present invention, a large amount of composite sulfur oxide was generated, so not only cracking occurred in the hot stretch flange formed part, but also toughness was insufficient. ing.

  In the V steel in which Mn + Cr is out of the range of the present invention, the amount of C is too high for the strength, so there is much precipitation of cementite and there is a problem in toughness. In steel X where Si deviates from the scope of the present invention, there is an unplated portion and the plating property is poor. In Y steel in which S deviates significantly from the scope of the present invention, inclusions containing S are excessively formed and not only cracks occur in the hot flange-formed part, but also the toughness is poor.

(Example 2)
D steel and E steel shown in Table 1 were produced in a converter and slabs were obtained by continuous casting. At that time, in the deoxidation step, preliminary deoxidation was performed with Al to reduce the free oxygen in the molten steel to 40 ppm, and then composite deoxidation was performed with Si or Ti and REM or Mg.

  Hot rolling conditions within the scope of the present invention (heating temperature: 1250 ° C., finishing temperature: 880 ° C., total rolling reduction at the last stand and the previous stand: 60%, time after finishing rolling and starting cooling: 0 8 seconds, coiling temperature: 550 ° C.), a hot-rolled steel sheet having a thickness of 2 mm, pickled, heated to 880 ° C. as it is, and then heated in the same mold as the mold used in Example 1. Stamp molding and thermal history were simulated.

  Further, after pickling, zinc (GI, GA) plating or molten aluminum plating containing 10% Si was performed, and then a similar hot stamping test was performed.

  On the other hand, the hot rolling conditions within the scope of the present invention (heating temperature: 1250 ° C., finishing temperature: 890 ° C., total rolling reduction at the final stand and the previous stand: 65%, time from the end of finish rolling to the start of cooling : 0.5 seconds, coiling temperature: 500 ° C.), a hot rolled steel sheet having a thickness of 3.2 mm, and after pickling, a cold rolled steel sheet having a cold rolling rate of 50% and 1.6 mm. After this cold-rolled sheet was heated to 900 ° C. in a heating furnace, molding and cooling by hot stamping were performed using the same mold as that used in Example 1. About the obtained molded product, the material characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 3. Since both are steels according to the present invention, there are no problems in hot stretch flange formability, delayed fracture resistance, and low temperature toughness, and desired characteristics are obtained.

  As described above, according to the present invention, the composite moldability and punchability at the time of hot stamping are excellent, and after hot stamping, it has a strength of 1470 MPa or more, and is excellent in delayed fracture resistance and toughness of a processed part. A hot stamping steel plate can be provided.

  Therefore, according to the present invention, it is possible to process parts including stretch flange molding and hole expansion processing with the hot stamping technique, and the degree of freedom of the shape of the parts that can be processed is significantly widened. Therefore, the present invention has high applicability in the machine component manufacturing industry.

A Punch B Dies C Pad D Blank x Stretched flange molding part y Non-molding part W Blank width H _{total Overall} length of blank center part H Flat length of blank flat part sandwiched between punch and pad H Flange height R Corner part Curvature radius θ Corner opening angle 1 Punch 1a Punch holding base 2 Die 3 Spacer 4 Stripper 5 Spring 6 Positioning pin 7 Cushion pin 8 Steel plate 9 Base

Claims (7)

  In a steel sheet for hot stamping containing S: 0.001 to 0.005%, REM: 0.005 to 0.03%, and O: 0.003 to 0.007% by mass%, S, O, A hot stamping steel sheet excellent in hot composite formability and delayed fracture resistance at the punched portion, characterized in that spherical inclusions having a diameter of 0.1 μm or less including two or more types of REM are dispersed.   In the steel sheet for hot stamping containing S: 0.001 to 0.005%, Mg: 0.005 to 0.03%, and O: 0.003 to 0.007% by mass%, S, O, And a steel sheet for hot stamping excellent in hot composite formability and delayed fracture resistance of the punched portion, characterized in that spherical inclusions having a diameter of 0.1 μm or less containing two or more kinds of Mg are dispersed.   The steel sheet for hot stamping is mass%, C: 0.20 to 0.35%, Si: 0.1 to 0.5%, Mn + Cr: 1 to 3%, Ti: 0.005 to 0.1%. And Nb: 0.005 to 0.1%, comprising the balance Fe and unavoidable impurities, and hot composite formability and delayed fracture resistance of the punched portion according to claim 1, Excellent steel sheet for hot stamping.   The steel sheet for hot stamping is one type of B: 0.0005-0.002%, V: 0.005-0.1%, and Mo: 0.05-0.5%. Or a steel sheet for hot stamping excellent in hot composite formability and delayed fracture resistance of a punched portion according to claim 3, comprising two or more kinds.   The steel sheet for hot stamping according to any one of claims 1 to 4, wherein the spherical inclusion is a composite sulfate, and is excellent in hot composite formability and delayed fracture resistance of a punched portion.   The slab having the component composition according to claim 3 or 4 is heated to a temperature range of 1250 ° C. or less and subjected to hot rolling, and the total reduction amount at the final stand and the previous stand in a temperature range of Ar3 or higher. Finish rolling is finished with 60% or more, and after finishing rolling, cooling is started within 1 second, and it is wound at a temperature of 600 ° C. or lower, and hot composite formability and delayed fracture resistance of the punched part A method of manufacturing hot stamping steel plates with excellent characteristics.   The molten steel having the component composition according to claim 3 or 4 is pre-deoxidized with Al to reduce free oxygen in the molten steel to 10 to 50 ppm, and then combined deoxidation with Si or Ti and REM or Mg. A method for melting a hot stamping steel sheet having excellent hot composite formability and delayed fracture resistance of a punched portion.

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