RU2633416C1 - Method for manufacturing article formed by stamping and article moulded by stamping - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: sheet blank is produced from steel containing wt %: C from 0.15 to 0.5, Si from 0.2 to 3, Mn from 0.5 to 3, P 0.05 or less, S 0.05 or less, Al from 0.01 to 1, B from 0.0002 to 0.01, N from 0.001 to 0.01 Ti in amount equal to or greater than 3.4 [N]+0.01 and equal to or less than 3.4 [N]+0.1, where [N] is the content (wt %) N in steel, iron and unavoidable impurities is the rest, wherein average diameter of equivalent circle of Ti-containing precipitated inclusions having diameter of equivalent circumference of 30 nm or less, is 6 nm or less, and the amount of Ti in inclusions and the total amount of Ti in the steel to satisfy the prescribed ratio, the blank is heated to temperature from 900 to 1100°C, stamped in press-mould, wherein in the process of moulding it is cooled with average cooling rate of 20°C/s or more to temperature equal to or lower by 100°C of temperature Bs of the beginning of bainite transformation and equal to or greater than the temperature Ms of the beginning of the martensite transformation, and after completion of moulding the resulting product is cooled with average cooling rate below 20°C/s to temperature 200°C or less.

EFFECT: increase in strength and ductility.

3 cl, 1 dwg, 6 tbl

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a stamped product used in the manufacture of an automotive structural component, and to a method for manufacturing such a stamped product. More specifically, the present invention relates to a stamped product made using when a preheated steel sheet (billet) is molded to a predetermined shape, a stamping method to shape, together with a heat treatment to obtain a predetermined strength, and to a method applicable for the manufacture of such a stamped product.

BACKGROUND

[0002] As one of the measures aimed at improving the fuel efficiency of automobile fuels due to global environmental problems, a vehicle body weight is being reduced, and, in turn, the strength of the steel sheet used for automobiles should be increased as much as possible . On the other hand, when the strength of the steel sheet increases, the accuracy of the mold decreases during stamping.

[0003] On this basis, the component is manufactured using a hot stamping method, where the steel sheet is heated to a predetermined temperature (eg, temperature to form an austenitic phase) to reduce strength, and then molded using a mold with a temperature (eg, room temperature) lower than the temperature of the molded steel sheet, and perform rapid heat treatment (quenching) using the temperature difference between them to provide strength after molding. Such a hot stamping method has various names, such as a hot stamping method, a hot stamping method, a hot die forging method, and a stamp hardening method, in addition to a hot stamping method.

FIG. 1 is a schematic explanatory view showing a configuration of a mold for carrying out the above hot stamping method. In FIG. 1 position code number 1 represents the punch, position code number 2 represents the matrix, position code 3 represents the clamp for the workpiece, position code 4 represents the steel sheet (workpiece), “BHF” represents the holding force of the workpiece, “rp” represents the radius of the edge of the punch , "Rd" represents the radius of the edge of the die, and "CL" represents the gap between the punch and the die. Among these parts, the plunger 1 and the die 2 are configured so that channels 1a and 2a are formed in the respective inner regions to allow the flow of the cooling medium (e.g., water), and that the parts cool when the cooling medium passes through the channel.

[0005] When hot stamping (for example, hot deep drawing) is performed using such a mold, molding begins in a state where the steel sheet (preform) 4 softens as a result of heating at a temperature of the two-phase zone (from the transformation point Ac ₁ to the Ac point ₃ transformations) or the temperature of a single-phase zone equal to or greater than the point of Ac ₃ transformations. More specifically, in a state in which the high temperature steel sheet 4 is sandwiched between the die 2 and the work clamp 3, the steel sheet 4 is pressed into the die cavity 2 (between 2 and 2 in FIG. 1) with the punch 1 and is deformed to form, corresponding to the outer profile of the punch 1, while reducing the outer diameter of the steel sheet 4. In addition, heat is removed from the steel sheet 4 to the mold (punch and die) by cooling the punch and die simultaneously with molding, and the material is quenched by and further curing and cooling the steel sheet at the bottom dead center of the molding (at the point where the punch head is located in the deepest part: the state shown in FIG. 1). When carrying out such a molding method, a stamped product of strength class 1500 MPa can be obtained with high dimensional accuracy, and, moreover, the molding force can be reduced in comparison with the molding situation of a component with the same strength class in cold working conditions, so that the required volume stamping press can be small.

[0006] As a steel sheet for hot stamping, which is widely used at present, a steel sheet using steel 22MnB5 as a material is known. This steel sheet has a tensile strength of 1500 MPa and an elongation of approximately 6 to 8%, and is used for impact-resistant parts (parts that undergo as little deformation as possible during a collision and do not collapse). However, its use for a component to be deformed, such as an energy absorbing part, is difficult due to the low elongation (ductility).

[0007] With respect to a hot stamping steel sheet that exhibits good elongation, methods have also been proposed according to, for example, Patent Documents 1-4. In these methods, the carbon content in the steel sheet is set to a value in various ranges to control the base strength class of the respective steel sheets, and elongation is increased by introducing ferrite having high deformability and reducing the average phase diameters of ferrite and martensite. The above methods are effective for increasing the elongation, but in terms of increasing the elongation in accordance with the strength of the steel sheet, this is still not enough. For example, the elongation EL of a steel sheet having a tensile strength of TS 1270 MPa or more is at most about 12.7%, and further improvement is required.

[0008] On the other hand, the automotive component should be connected mainly by spot welding, but in a hot stamped molded product having a microstructure that mainly includes martensite, it is known that the strength in the heat affected zone (HAZ) is significantly reduced, and the welded the joint undergoes a decrease in strength (softening) (e.g. Non-Patent Document 1).

PROTOTYPE

PATENT DOCUMENT

[0009]

Patent Document 1: JP-A-2010-65292

Patent Document 2: JP-A-2010-65293

Patent Document 3: JP-A-2010-65294

Patent Document 4: JP-A-2010-65295

NON-PATENT DOCUMENT

[0010]

Non-Patent Document 1: Hirosue et al., Nippon Steel Technical Report, No. 378, pp. 15-20 (2003)

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] The present invention has been completed in these circumstances, and its purpose is to provide: a method suitable for manufacturing a stamped product that is capable of achieving a high level balance between high strength and elongation and has a good softening resistance characteristic in HAZ; and a stamped product that exhibits the above properties.

TROUBLESHOOTING MEANS

[0012] In a method for manufacturing a stamped product according to the present invention, which can achieve the above purpose, the hot stamping steel sheet is heated at a temperature of 900 ° C or more, and 1100 ° C or less, wherein the hot stamping steel sheet comprises:

C: from 0.15 to 0.5% (wt.%; Hereinafter the same applies to the chemical component composition),

Si: 0.2 to 3%,

Mn: 0.5 to 3%,

P: 0.05% or less (excluding 0%),

S: 0.05% or less (excluding 0%),

Al: from 0.01 to 1%,

B: 0.0002 to 0.01%,

Ti: in an amount equal to or greater than 3.4 [N] + 0.01% and equal to or less than 3.4 [N] + 0.1% (wherein [N] indicates the content (wt.%) N) , and

N: from 0.001 to 0.01%, with the remaining amount of iron and unavoidable impurities, in which the average diameter of the equivalent circle of Ti-containing precipitated inclusions having an equivalent circle diameter of 30 nm or less among Ti-containing precipitated inclusions contained in steel sheet is 6 nm or less, and the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies the ratio according to the following formula (1),

and then stamping is started, and the steel sheet is cooled to a temperature equal to or lower than the temperature 100 ° C lower than the temperature Bs of the onset of bainitic transformation and equal to or higher than the temperature Ms of the beginning of the martensitic transformation, while at the same time providing an average cooling rate of 20 ° C / s or more in the mold during molding and after molding is completed, and then the steel sheet is cooled to a temperature of 200 ° C or less with an average cooling rate of less than 20 ° C / s. Here, “equivalent circle diameter” is the diameter of a circle having the same area as the size (area) of a Ti-containing inclusion (eg, TiC) when the precipitated inclusion is converted to a circle (“average equivalent circle diameter” represents its average value).

The amount of Ti (wt.%) In the inclusions -3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] ... (1)

(in the formula (1) [N] denotes the content (wt.%) N in steel).

[0013] In the steel sheet for hot stamping used in the manufacturing method according to the present invention, if desired, the content, as other (s) element (s), of at least one of the following (a) to ( from). The properties of the stamped product are further improved in accordance with the type of element that is contained as necessary.

(a) One or more selected from the group consisting of V, Nb and Zr in an amount of 0.1% or less (excluding 0%) in total

(b) One or more selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (excluding 0%) in total

(c) One or more selected from the group consisting of Mg, Ca and REM (rare earth metals), in an amount of 0.01% or less (excluding 0%) in total

[0014] In the molded product obtained by this manufacturing method, the metallographic microstructure of the molded product includes bainitic ferrite: from 60 to 97% by area, martensite: 37% by area or less, residual austenite: from 3 to 20% by area, and the rest microstructure: 5% by area or less, the average diameter of the equivalent circle of the Ti-containing precipitated phase having an equivalent circle diameter of 30 nm or less, among the Ti-containing precipitated phases contained in the stamped product, is 10 nm or less, and satisfactory ryaetsya ratio according to formula (1), and thereby a molded article can be achieved by a high-level balance between a high strength and elongation as a uniform characteristics.

Advantage of the Invention

[0015] According to the present invention, a steel sheet is used with a strictly defined chemical component composition, the size of Ti-containing inclusions is controlled, and the degree of Ti release into non-TiN forming inclusions is controlled, so that by hot pressing the steel sheet under predetermined conditions, it can be A strength-elongation balance has been achieved as a high-level balance, and the softening resistance characteristic in HAZ improves.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]

[FIG. 1] A schematic explanatory view showing the configuration of a mold for hot stamping.

MODE FOR CARRYING OUT THE INVENTION

[0017] The inventors of the present invention have studied various aspects to create a stamped product that ensures that when a product is manufactured by heating a steel sheet at a predetermined temperature and then hot-stamped the steel sheet, a stamped product exhibits good ductility (elongation), providing together with high strength after stamping.

[0018] As a result, it was found that when the chemical component composition of the hot stamping steel sheet is strictly predetermined, the size of the Ti-containing precipitates and the amount of Ti in the inclusions are regulated, and when the steel sheet is hot stamped under predetermined conditions , after stamping, a predetermined amount of residual austenite is provided, and a stamped product is obtained having increased intrinsic ductility (residual ductility) and good character softening resistance history in HAZ. The present invention has been completed based on these findings.

[0019] In the steel sheet for hot stamping used in the present invention, the chemical component composition must be strictly defined, and the rationale for limiting the range of each chemical component is as follows.

[0020]

(C: 0.15 to 0.5%)

Carbon (C) is an important element for lowering the temperature Bs of the onset of bainitic transformation for grinding bainitic ferrite formed during cooling and increasing the dislocation density in bainitic ferrite to increase strength. In addition, the amount of finely divided residual austenite formed between the bainitic ferrite slats increases, and a high level balance between high strength and elongation can be achieved. If the C content is less than 0.15%, the temperature Bs of the onset of bainitic transformation increases, leading to the enlargement of bainitic ferrite and a decrease in the density of dislocations, and the strength of the hot stamped product cannot be ensured. If the C content is too high and exceeds 0.5%, the strength is excessively high and good ductility is not achieved. The lower limit of the C content is preferably 0.18% or more (more preferably 0.20% or more), and the upper limit is preferably 0.45% or less (more preferably 0.40% or less).

[0021]

(Si: 0.2 to 3%)

Silicon (Si) has the effect of suppressing the formation of cementite due to the decomposition of residual austenite formed between the bainite ferrite rods during cooling during quenching in a die, and thereby promoting the formation of residual austenite. To exhibit such an effect, the Si content should be 0.2% or more. If the Si content is too high and exceeds 3%, ferrite is easily formed, making it difficult to create a single austenite phase during heating, and the proportion of a different microstructure than bainitic ferrite and residual austenite in the steel sheet for hot stamping exceeds 5% in area. The lower limit of the Si content is preferably 0.5% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).

[0022]

(Mn: 0.5 to 3%)

Manganese (Mn) is an element that effectively increases hardenability and suppresses the formation of a soft microstructure, such as ferrite and perlite, during cooling during hardening in a die. In addition, it is an important element for lowering the temperature Bs of the onset of bainitic transformation in order to grind the bainitic ferrite formed during the cooling process and increase the dislocation density in bainitic ferrite to increase strength. In addition, it is an element capable of stabilizing austenite, and an element that contributes to an increase in the amount of residual austenite. To exhibit such effects, Mn must be present in an amount of 0.5% or more. If only characteristics are taken into account, then the Mn content is preferably higher, but since the cost of the dopant increases, the upper limit is adjusted to 3% or less. The lower limit of the Mn content is preferably 0.7% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).

[0023]

(P: 0.05% or less (excluding 0%))

Phosphorus (P) is an element inevitably contained in steel, but it impairs ductility, and therefore, the content of P is preferably reduced as much as possible. However, the marginal decrease leads to an increase in the cost of steel production, and is difficult in terms of manufacturing with a decrease in the content to 0%. On this basis, its content is regulated by 0.05% or less (with the exception of 0%). The upper limit of the content of P is preferably 0.045% or less (more preferably 0.040% or less).

[0024]

(S: 0.05% or less (excluding 0%))

Sulfur (S), like P, is an element that is inevitably contained in steel and impairs ductility, and therefore, the content of S is preferably reduced as much as possible. However, excessive removal causes an increase in the cost of steel production, and is difficult in terms of manufacturing with a decrease in the content to 0%. For this reason, its content is adjusted to 0.05% or less (with the exception of 0%). The upper limit of the S content is preferably 0.045% or less (more preferably 0.040% or less).

[0025]

(Al: 0.01 to 1%)

Aluminum (Al) is applicable as a deoxidizing element and allows to bind dissolved N in the form of AlN present in steel, which is useful for increasing ductility. For the effective manifestation of such an action, the Al content should be 0.01% or more. However, if the Al content is too high and exceeds 1%, Al ₂ O _{3 is} excessively formed, impairing ductility. The lower limit of the Al content is preferably 0.02% or more (more preferably 0.03% or more), and the upper limit is preferably 0.8% or less (more preferably 0.6% or less).

[0026]

(B: 0.0002 to 0.01%)

Boron (B) is an element whose action is to suppress ferrite transformation and pearlite transformation, and therefore helps to prevent the formation of ferrite, perlite and bainite during cooling after heating at a temperature of the two-phase zone (from the point of AC ₁ transformation to the point AC ₃ transformation) , and provides the formation of residual austenite. For the manifestation of such effects, B must be contained in an amount of 0.0002% or more, but even if this element is contained in excess in excess of 0.01%, the effects are saturated. The lower limit of B content is preferably 0.0003% or more (more preferably 0.0005% or more), and the upper limit is preferably 0.008% or less (more preferably 0.005% or less).

[0027]

(Ti: in an amount equal to or greater than 3.4 [N] + 0.01% and equal to or less than 3.4 [N] + 0.1%: [N] represents the content (wt.%) N)

Titanium (Ti) exhibits an action consisting in improving hardenability by binding N and maintaining B in a solid solution state. For the manifestation of such an effect, it is important that this element is contained in a greater amount than the stoichiometric ratio of Ti and N (3.4-fold content of N), by 0.01% or more. In addition, when Ti is added in excess with respect to N, this causes it to be present in the state of solid solution in the hot stamped product, and the precipitated inclusions are finely dispersed, the decrease in strength in HAZ due to dispersion hardening due to the formation, in the form of TiC dissolved in solid, can be suppressed Ti material during welding of a hot-stamped product, or due to an effect such as a delayed increase in the dislocation density, due to the action of TiС, which prevents dislocation room. However, if the Ti content is too high and exceeds 3.4 [N] + 0.1%, the resulting Ti-containing inclusions a (for example, TiN) become coarse-grained, impairing the ductility of the steel sheet. The lower limit of the Ti content is preferably 3.4 [N] + 0.02% or more (more preferably 3.4 [N] + 0.05% or more), and the upper limit is preferably 3.4 [N] +0 09% or less (more preferably 3.4 [N] + 0.08% or less).

[0028]

(N: 0.001 to 0.01%)

Nitrogen (N) attenuates the effect of improving hardenability during quenching due to B binding in the form of BN, and therefore its content is preferably reduced as much as possible, but the decrease in content in the real process is limited, and therefore the lower limit is adjusted to 0.001%. If the N content is too large, the released Ti-containing inclusions (e.g., TiN) are coarsened, and these inclusions act as a fracture source, impairing the ductility of the steel sheet. For this reason, the upper limit is adjusted to 0.01%. The upper limit of the N content is preferably 0.008% or less (more preferably 0.006% or less).

[0029] The main chemical components in the hot stamping steel sheet used in the present invention are as described above, and the remainder is iron and unavoidable impurities (eg, O, H) other than P, S and N In the steel sheet for hot stamping used in the present invention, additional content is also useful, as the other (s) element (s), at least one element from the following (a) to (c), if desired. The properties of the stamped product are further improved in accordance with the type of element that is contained as necessary. In the case of the content of such an element, the preferred range and rationale for limiting the range are as follows.

(a) One or more selected from the group consisting of V, Nb and Zr in an amount of 0.1% or less (excluding 0%) as a whole.

(b) One or more selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (excluding 0%) as a whole.

(c) One or more selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (excluding 0%) as a whole.

[0030]

(One or more selected from the group consisting of V, Nb and Zr in an amount of 0.1% or less (excluding 0%) in total)

Vanadium (V), niobium (Nb) and zirconium (Zr) exhibit an action consisting in the formation of fine-grained carbide and grinding of the microstructure as a result of the pinning effect. To exhibit such an effect, these elements are preferably contained in an amount of 0.001% or more in total. However, if the content of these elements is too high, coarse carbide is formed and acts as a source of destruction, impairing ductility. On this basis, the content of these elements is preferably 0.1% or less in total. The lower limit of the content of these elements is more preferably 0.005% or more (even more preferably 0.008% or more) in total, and the upper limit is more preferably 0.08% or less (even more preferably 0.06% or less) in general.

[0031]

(One or more types selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (excluding 0%) in total)

Copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo) inhibit ferrite and pearlite transformations, and therefore act effectively in preventing the formation of ferrite and perlite during cooling after heating, and provide the formation of residual austenite. To exhibit such an effect, they are preferably contained in an amount of 0.01% or more in total. If only properties are taken into account, the content is preferably higher, but since the cost of the dopant increases, the content is preferably 1% or less in total. In addition, these elements act so as to significantly increase the strength of austenite and cause an increased load during hot rolling, making it difficult to manufacture a steel sheet. Therefore, also for manufacturing reasons, the content is preferably 1% or less. The lower limit of the content of these elements is more preferably 0.05% or more (even more preferably 0.06% or more) in general, and the upper limit is more preferably 0.5% or less (even more preferably 0.3% or less ) generally.

[0032]

(One or more types selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (excluding 0%) in total)

These elements grind inclusions and therefore act effectively to increase ductility. To exhibit such an effect, these elements are preferably contained in an amount of 0.0001% or more in total. When only properties are taken into account, the content is preferably higher, but since the effect is saturated, the content is preferably 0.01% or less in total. The lower limit of the content of these elements is more preferably 0.0002% or more (even more preferably 0.0005% or more) in general, and the upper limit is more preferably 0.005% or less, preferably 0.0003% or less.

[0033] The following requirements are also important in the hot stamping steel sheet used in the present invention: (A) the average diameter of the equivalent circle of Ti-containing precipitated phases having an equivalent circle diameter of 30 nm or less among Ti-containing inclusions contained in the steel sheet is 6 nm or less, and (B) the ratio “amount of Ti (wt.%) in inclusions -3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3 is satisfied , 4 [N]] ”(ratio according to the formula (1)).

[0034] Ti-containing inclusions and the ratio in formula (1) are controlled to prevent softening of the HAZ, and such regulation is initially the control required in the molded product, but these values do not change much between the states before and after hot stamping. Therefore, regulation should be carried out already at the stage before molding (in a steel sheet for hot stamping). When an excess amount of Ti relative to N in the steel sheet prior to molding causes it to be in a solid solution or in a ground state, the Ti-containing inclusions can be maintained in a solid solution or in a finely divided state during heating by hot pressing. As a result of this, the amount of Ti released in the inclusion in the stamped product can be adjusted to a value no more than a predetermined amount, and softening in the HAZ can be prevented, whereby the properties of the welded joint can be improved.

[0035] For these reasons, Ti-containing inclusions should be finely dispersed, and for this purpose, the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less, among Ti-containing inclusions contained in the steel sheet should be 6 nm or less (above requirement (A)). The size (average equivalent circle diameter) of the Ti-containing inclusions is preferably 5 nm or less, more preferably 3 nm or less. Examples of Ti-containing inclusions in the present invention include TiC, TiN and other Ti-containing inclusions such as TiVC, TiNbC, TiVCN and TiNbCN.

[0036] As described below, the average diameter of the equivalent circle of Ti-containing inclusions in the stamped product is set to 10 nm or less, while before molding (steel sheet for hot stamping) is set to 6 nm or less. The rationale for why the size of the precipitated inclusions is set larger in the molded product than in the steel sheet is that Ti is present in the steel sheet in the form of a finely divided precipitated phase or in the state of a solid solution, and being heated at a temperature of about 800 ° C for 15 minutes or more, Ti-containing inclusions slightly enlarged. To ensure properties similar to those in a molded product, the average diameter of the equivalent circumference of Ti-containing inclusions should be 10 nm or less, and to achieve this state of inclusions in a hot-stamped product, it is necessary that, in the state of a hot-stamped steel sheet, the average diameter of the equivalent circumference of finely divided inclusions 30 nm or less has been adjusted to 6 nm or less, and most Ti must be present in the solid solution state.

[0037] In addition, in the steel sheet for hot stamping, the main amount of Ti, with the exception of Ti, which has been converted to N and bonded, must be present in a solid solution state or in a finely divided state. On this basis, the amount of Ti present in the form of inclusions other than TiN (that is, the amount of Ti in the inclusions) -3.4 [N]) should be less than half the residue after subtracting Ti, which forms TiN, from the total amount of Ti (ie, 0.5 × [(total amount of Ti) -3.4 [N]] (above requirement (B)). The value "amount of Ti in inclusions) -3.4 [N]" is preferably 0.4 × [(total Ti) -3.4 [N]] or less, more preferably 0.3 × [(total Ti) -3.4 [N]] or less.

[0038] For the manufacture of the above steel sheet (hot stamping steel sheet), a slab obtained by melting a steel material having the above chemical component composition can be hot rolled at a heating temperature of 1100 ° C. or more (preferably 1150 ° C. or more) and 1300 ° C or less (preferably 1250 ° C or less), and finish rolling at a temperature of 850 ° C or more (preferably 900 ° C or more), and 1000 ° C or less (preferably 950 ° C or less), and immediately after that he can be chilled n (rapid cooling) at an average cooling rate of 20 ° C / s or more (preferably 30 ° C / s or more) until a temperature of 500 ° C or less (preferably 450 ° C or less) is reached, and then it can be wound into a roll at a temperature of 350 ° C or more (preferably 380 ° C or more), and 450 ° C or less (preferably 430 ° C or less).

[0039] In the above method (1), rolling is stopped in the temperature range where the dislocation embedded in austenite during hot rolling remains, (2) rapid cooling is carried out immediately afterwards to ensure that a Ti-containing precipitated phase, such as finely divided, forms on the dislocation TiC, and (3) then perform rapid cooling, followed by winding into a roll, than regulate the course of bainitic transformation or martensitic transformation.

[0040] A hot stamping steel sheet that has the above chemical component composition and state of Ti inclusions can be directly used for hot pressing, or can be cold rolled with a reduction ratio of 10 to 80% during rolling (preferably 20 to 80%) 70%) after decapitation, and then used for manufacturing by hot pressing. Hot stamping steel sheet or cold rolled material can be heat treated, including heating at a temperature of 830 ° C or more (preferably 850 ° C or more and 900 ° C or less), then rapid cooling at a cooling rate of 20 ° C / s or more (preferably 30 ° C / s or more) until a temperature of 500 ° C or less (preferably 450 ° C or less) is reached, and then held at a temperature of 500 ° C or less for 10 s or more, and 1000 s or less, or tempered at a temperature of 500 ° C. or less. In addition, the surface of the hot stamping steel sheet (surface of the base steel sheet) of the present invention can be clad to form a coating containing one or more elements of Al, Zn, Mg and Si.

[0041] Using the above hot stamping steel sheet, the steel sheet is heated at a temperature of 900 ° C or more and 1100 ° C or less, and after the stamping is started, the steel sheet is cooled to a temperature equal to or lower than 100 ° C. below the temperature Bs of the onset of bainitic transformation (Bs-100 ° C) and equal to or greater than the temperature Ms of the onset of martensitic transformation, while at the same time providing an average cooling rate of 20 ° C / s or more in the mold during molding, and also after completion of molding, and then cooling give up to a temperature of 200 ° C or less at an average cooling rate of less than 20 ° C / s, as a result of which an optimal microstructure, as in a molded product, with a predetermined strength and high ductility (a microstructure including mainly bainitic ferrite) can be created in a stamped product having a uniform property. The rationale for setting each requirement in this molding method is as follows.

[0042] If the heating temperature of the steel sheet is less than 900 ° C, a sufficient amount of austenite cannot be obtained during heating, and the proportion of martensite is too large in the final microstructure (microstructure of the molded product). If the heating temperature of the steel sheet exceeds 1100 ° C, the size of the austenitic grains increases during heating, the temperature Ms of the beginning of the martensitic transformation and the temperature Mf of the end of the martensitic transformation increase, residual austenite cannot be provided during quenching, and good formability is not achieved. The heating temperature of the steel sheet is preferably 950 or more, and 1050 ° C. or less. At this time, if the heating time is too long, Ti-containing inclusions in the steel sheet can hardly be crushed, and Ti-containing inclusions even in small quantities are formed during heating and coarsened, weakening the effect of improving weldability. For this reason, the duration of the heating is preferably shorter. The duration of the heating is preferably 3600 s or less, and more preferably 20 s or less.

[0043] In order to ensure that the austenite formed in the above heating step is converted to a desired microstructure (a microstructure including mainly bainitic ferrite), while preventing the formation of a microstructure such as ferrite or perlite, the average cooling rate should be adequately controlled during molding, as well as after molding, and the temperature of the end of cooling. For such reasons, it is necessary that the average cooling rate during molding is 20 ° C / s or more, and the temperature of the end of cooling is equal to or lower than the temperature 100 ° C lower than the temperature Bs of the onset of bainitic transformation and equal to or higher than the temperature Ms the beginning of the martensitic transformation. The average cooling rate during molding is preferably 30 ° C / s or more (more preferably 40 ° C / s or more). When the temperature of the end of cooling is equal to or lower than the temperature 100 ° C lower than the temperature Bs of the onset of bainitic transformation, the austenite present during heating turns into bainite, while at the same time preventing the formation of such a microstructure as ferrite or perlite, which ensures that finely dispersed austenite is retained between the bays of bainitic ferrite, and a predetermined amount of residual austenite is provided, while guaranteeing the amount of bainitic ferrite.

[0044] If the temperature of the end of cooling exceeds a temperature that should be 100 ° C lower than the temperature Bs of the onset of bainitic transformation, or the average cooling rate is less than 20 ° C / s, a microstructure such as ferrite and perlite is formed and cannot a predetermined amount of residual austenite will be provided, leading to a deterioration in elongation (ductility) in the molded product. When cooling is carried out to a temperature lower than the temperature Ms of the onset of martensitic transformation, the amount of martensite formed increases and the elongation (ductility) of the molded product deteriorates.

[0045] After reaching a temperature equal to or lower than the temperature 100 ° C lower than the temperature Bs of the onset of bainitic transformation and equal to or higher than the temperature Ms of the onset of martensitic transformation, rapid cooling is stopped, and cooling to a temperature of 200 ° C or less is performed with average cooling rate less than 20 ° C / s. By adding this cooling step, the conversion of bainitic ferrite is stimulated. If the average cooling rate here is 20 ° C / s or more, martensite is formed, and although strength can be increased, good elongation is not achieved. The average cooling rate is preferably 15 ° C / s or less, more preferably 10 ° C / s or less. The rationale for why the steel sheet is cooled to a temperature of 200 ° C. or less in this cooling cycle is that the amount of residual austenite remaining at room temperature increases as a result of the distribution of carbon from bainitic ferrite to unreformed austenite.

[0046] After performing the above two-stage cooling, the regulation of the average cooling rate is in principle not required, but the steel sheet can be cooled to room temperature with an average cooling rate of, for example, 1 ° C / s or more, and 100 ° C / s or less. Monitoring the average cooling rate during molding and also after molding is completed can be achieved by, for example, (a) controlling the temperature of the molding mold (using the cooling medium shown in FIG. 1), or (b) controlling the thermal conductivity Press forms.

[0047] In a stamped product obtained by the manufacturing method described above, the metallographic microstructure includes bainitic ferrite: from 60 to 97% by area, martensite: 37% by area or less, residual austenite: from 3 to 20% by area, and the rest of the microstructure: 5% by area or less, and the amount of carbon in the residual austenite is 0.50% or more, so that a high level balance between high strength and elongation as uniform characteristics can be achieved in the molded product. The rationale for regulating the range of each requirement (the amount of carbon in the base microstructure and residual austenite) in such a hot stamped product is as follows.

[0048] When the basic microstructure of the stamped article is high strength bainitic ferrite having high ductility, both high strength and high ductility of the stamped article can be satisfactory. From this point of view, the proportion of the area of bainitic ferrite should be 60% by area or more. However, if this fraction exceeds 97% by area, the fraction of residual austenite is insufficient, and ductility (residual ductility) is reduced. The lower limit of the proportion of bainitic ferrite is preferably 65% by area or more (more preferably 70% by area or more), and the upper limit is preferably 95% by area or less (more preferably 90% by area or less).

[0049] The strength of a hot-stamped product can be increased by the partial introduction of high-strength martensite, but if its amount becomes large, ductility (residual ductility) is reduced. For these reasons, the martensite area should be 37% or less. The lower limit of the martensite fraction is preferably 5% by area or more (more preferably 10% by area or more), and the upper limit is preferably 30% by area or less (more preferably 25% by area or less).

[0050] Residual austenite has an effect consisting in increasing the degree of strain hardening (induced by the plasticity transformation) and enhancing the plasticity of the stamped product as a result of transformation into martensite occurring during plastic deformation. To exhibit this effect, the fraction of residual austenite should be 3% or more. Plasticity improves even more as the proportion of residual austenite increases. In the composition used for automotive steel sheet, the provided residual austenite is limited, and the upper limit is about 20% by area. The lower limit of residual austenite is preferably 5% by area or more (more preferably 7% by area).

[0051] Regarding a different microstructure than those described above, ferrite, perlite and the like may be contained as the rest of the microstructure, but such a microstructure is worse than other microstructures in terms of contributing to strength or promoting ductility, and it is in principle preferred that such the microstructure was not contained (maybe even with 0% by area). However, an area ratio of up to 5% by area is permissible. The area fraction of the remaining microstructure is preferably 4% by area or less, more preferably 3% by area or less.

[0052] In the above stamped product, the average diameter of the equivalent circle of Ti-containing precipitated phases having an equivalent circle diameter of 30 nm or less among Ti-containing precipitated phases present in the stamped product (that is, in the steel sheet forming the stamped product), is 10 nm or less. When this requirement is met, a stamped product capable of achieving a high level balance between high strength and elongation can be obtained. The average equivalent diameter of the Ti-containing precipitated phase is preferably 8 nm or less, more preferably 6 nm or less.

[0053] In addition, in the stamped product, the amount of Ti present as a different precipitated phase than TiN (that is, the amount of Ti-3.4 [N] precipitated phase) is less than half the residue after subtracting Ti, which forms TiN, from the total Ti (that is, less than 0.5 × [total Ti (%) - 3.4 [N]]). When this requirement is met, Ti dissolved in the solid material during welding forms a finely divided precipitated phase in the HAZ, or an existing finely divided Ti-containing precipitated phase suppresses reduction, etc. dislocation, and as a result, softening in HAZ is prevented, and weldability is improved. The value “amount of Ti in inclusions of -3.4 [N]” is preferably 0.4 × [(total amount of Ti) -3.4 [N]] or less, more preferably 0.3 × [(total amount of Ti) - 3.4 [N]] or less.

[0054] According to the method of the present invention, characteristics such as strength and elongation of the molded product can be adjusted by appropriately adjusting the stamping conditions (heating temperature and cooling rate), and furthermore, a stamped product having high ductility is obtained ( residual ductility), ensuring its use in place (for example, for an energy-absorbing part), for which a traditional hot-stamped product can hardly be applied. This is very useful for expanding the range of use of the hot stamped product.

[0055] The effects of the present invention are more specifically described below with reference to Examples, but the present invention is not limited to the following Examples, and all configuration changes made within the scope of the above described invention or later are included in the technical field of the present invention.

EXAMPLES

[0056]

Steel materials (Steels Nos. 1-31) having the chemical component composition shown in Table 1 below were melted in a vacuum to obtain an experimental slab, then hot rolled to obtain a steel sheet, followed by cooling and subjected to processing simulating winding into a roll (sheet thickness: 3.0 mm). As for the method simulating winding into a processing roll, the sample was cooled to the winding temperature and placed in a furnace heated to the winding temperature, followed by aging for 30 minutes and then cooling in the oven. The steel sheet manufacturing conditions are shown in Table 2 below. Here, in Table 1, the transformation point Ac ₃ , the Ms point and the Bs point were determined using the following formulas (2) - (4) (see, for example, The Physical Metallurgy of Steels ”, authors Leslie, Maruzen, (1985)). In addition, treatments (1) and (2) shown in the Notes to Table 2 mean that each treatment described below (rolling, cooling, and alloying) was performed.

[0057]

Transformation point Ac ₃ (° C) = 910-203 × [C] ^1/2 + 44.7 × [Si] -30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti ] + 104 × [V] -11 × [Cr] + 31.5 × [Mo] -20 × [Cu] -15.2 × [Ni] ..... (2)

Point Ms (° C) = 550-361 × [C] -39 × [Mn] -10 × [Cu] -17 × [Ni] -20 × [Cr] -5 × [Mo] + 30 × [Al] ..... (3)

Point Bs (° C) = 830-270 × [C] -90 × [Mn] -37 × [Ni] -70 × [Cr] -83 × [Mo] ...... (4)

moreover, [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni] represent levels of content (wt. %) of the elements C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu, and Ni, respectively. In the case where the element shown in each term of formulas (2) - (4) is not contained, the calculation is performed taking into account that the term is not present.

[0058] Processing (1): a hot-rolled steel sheet was cold rolled (sheet thickness: 1.6 mm), then heated at a temperature of 800 ° C to simulate continuous annealing in a heat treatment simulator, held for 90 s, cooled to a temperature of 500 ° C at an average cooling rate of 20 ° C / s, and held for 300 s.

Processing (2): hot-rolled steel sheet was cold rolled (sheet thickness: 1.6 mm), then heated at a temperature of 860 ° C to simulate a continuous dip galvanizing production line in a heat treatment simulator, cooled to a temperature of 400 ° C at an average cooling speed 30 ° C / s, kept, additionally kept at a temperature of 500 ° C for 10 s to simulate immersion in a clad bath and fusion treatment, and then cooled to room temperature with an average cooling rate of 2 0 ° C / s.

[0059]

Table 1 Steel No. Chemical component composition * (wt.%) C Si Mn P S Al B Ti N V Nb Cu one 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 2 0.150 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 3 0.220 0.05 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - four 0.220 0.25 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 5 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0.024 0.0040 - - - 6 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 7 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 8 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 9 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 10 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - eleven 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 12 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 13 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - fourteen 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - fifteen 0.220 2.00 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 16 0.350 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - * The rest: iron and inevitable impurities, with the exception of P, S and N.

Table 1 (continued) Steel No. Chemical component composition * (wt.%) Ac ₃ conversion point (ºC) Bs-100ºC (ºC) Point Ms (ºC) Ni Zr Mg Ca Rem Cr Mo one - - - - - - - 845 563 425 2 - - - - - 0.20 - 880 568 446 3 - - - - - 0.20 - 812 549 421 four - - - - - 0.20 - 821 549 421 5 - - - - - 0.20 - 853 549 421 6 - - - - - 0.20 - 863 549 421 7 - - - - - 0.20 - 863 549 421 8 - - - - - 0.20 - 863 549 421 9 - - - - - 0.20 - 863 549 421 10 - - - - - 0.20 - 863 549 421 eleven - - - - - 0.20 - 863 549 421 12 - - - - - 0.20 - 863 549 421 13 - - - - - 0.20 - 863 549 421 fourteen - - - - - 0.20 - 863 549 421 fifteen - - - - - 0.20 - 899 549 421 16 - - - - - 0.20 - 838 514 374 * The rest: iron and inevitable impurities, with the exception of P, S and N.

Table 1 (continued) Steel No. Chemical component composition (wt.%) C Si Mn P S Al B Ti N V Nb Cu 17 0.720 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - eighteen 0.220 1.20 0.80 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 19 0.220 1.20 2.40 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - twenty 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,100 0.0040 - - - 21 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,200 0.0040 - - - 22 0.220 0.50 1.20 0.0050 0.0020 0.40 0.0020 0,044 0.0040 - - - 23 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 0,030 - - 24 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - 0,020 - 25 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - 0.20 26 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 27 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 28 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 29th 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - thirty 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0.0040 - - - 31 0.220 1.20 1.20 0.0050 0.0020 0,030 0.0020 0,044 0,040 - - - * The rest: iron and inevitable impurities, with the exception of P, S and N.

[Table 1] (continued) Steel No. Chemical component composition (wt.%) Ac ₃ conversion point (ºC) Bs-100 ° C (° C) Point Ms (° C) Ni Zr Mg Ca Rem Cr Mo 17 - - - - - 0.20 - 786 414 240 eighteen - - - - - 0.20 - 875 585 436 19 - - - - - 0.20 - 827 441 374 twenty - - - - - 0.20 - 886 549 421 21 - - - - - 0.20 - 926 549 421 22 - - - - - 0.20 - 980 549 432 23 - - - - - 0.20 - 866 549 421 24 - - - - - 0.20 - 863 549 421 25 - - - - - 0.20 - 859 549 419 26 0.20 - - - - 0.20 - 860 541 417 27 - - - - - 0.20 0.20 869 532 420 28 - 0.015 - - - 0.20 - 863 549 421 29th - - 0.002 - - 0.20 - 863 549 421 thirty - - - 0.002 - 0.20 - 863 549 421 31 - - - - 0.002 0.20 - 863 549 421 * The rest: iron and inevitable impurities, with the exception of P, S and N.

[0060]

[Table 2] Steel No. Steel sheet manufacturing conditions Heating temperature (° C) Finish rolling temperature (° C) Average cooling rate from the finish rolling temperature to the winding temperature (° C / s) Winding Temperature (° C) Notes one 1200 950 twenty 500 - 2 1200 950 twenty 500 - 3 1200 950 twenty 500 - four 1200 950 twenty 500 - 5 1200 950 twenty 500 - 6 1200 950 twenty 500 - 7 1200 800 twenty 500 - 8 1200 950 twenty 500 Processing (1) 9 1200 950 twenty 500 Processing (2) 10 1200 950 twenty 500 - eleven 1200 950 twenty 500 - 12 1200 950 twenty 500 - 13 1200 950 twenty 500 - fourteen 1200 950 twenty 500 - fifteen 1200 950 twenty 500 - 16 1200 950 twenty 500 -

[Table 2] (continued) Steel No. Steel sheet manufacturing conditions Temperature
heating
(° C) Finish rolling temperature (° C) Average cooling rate from the finish rolling temperature to the winding temperature (° C / s) Winding Temperature (° C) Notes 17 1200 950 twenty 500 - eighteen 1200 950 twenty 500 - 19 1200 950 twenty 500 - twenty 1200 950 twenty 500 - 21 1200 950 twenty 500 - 22 1200 980 twenty 500 - 23 1200 950 twenty 500 - 24 1200 950 twenty 500 - 25 1200 950 twenty 500 - 26 1200 950 twenty 500 - 27 1200 950 twenty 500 - 28 1200 950 twenty 500 - 29th 1200 950 twenty 500 - thirty 1200 950 twenty 500 - 31 1200 950 twenty 500 -

[0061] Regarding the obtained steel sheets (steel sheets for hot stamping), the analysis of the state of Ti inclusions (“the amount of Ti in inclusions is 3.4 [N]” and the average diameter of the equivalent circle of Ti-containing inclusions) was performed as follows. The results are shown below in Table 3 together with a calculated value of 0.5 × [total amount of Ti-3.4 [N]].

[0062]

(Analysis of the state of Ti inclusions in a steel sheet)

A sample of the extraction replica was prepared, and a TEM image of the Ti-containing inclusions was photographed using a transmission electron microscope (100,000x magnifications). At this time, Ti-containing inclusions (having an equivalent circle diameter of 30 nm or less) were identified by composition analysis inclusions using an energy dispersive x-ray spectrometer (EDX). At least 100 sections of Ti-containing inclusions were measured by area using image analysis, the diameter of the equivalent circle was determined from it, and its average value was derived as the size of the released inclusions (average diameter of the equivalent circle of Ti-containing inclusions). Regarding the value “the amount of Ti in the inclusions is 3.4 [N]]” (the amount of Ti present in the form of inclusions), the extraction residue was analyzed using a mesh having a mesh size of 0.1 μm (during extraction processing it could be the finely divided precipitated phase formed by the aggregation of the precipitated phases was also measured), and the “amount of Ti in inclusions — 3.4 [N]” (indicated in Tables 5 and 6 as “The amount of Ti in inclusions — 3.4 [N]”) was determined. In the case where the Ti-containing inclusions partially contained V or Nb, the levels of these precipitated phases were also measured.

[0063]

Table 3 Steel No. Stamping steel sheet The amount of Ti in the inclusions -3.4 [N] (wt.%) 0.5 × [total amount of Ti-3.4 [N] (wt.%) The average diameter of the equivalent circle of Ti-containing inclusions (nm) one 0.009 0.015 4.0 2 0.008 0.015 2.5 3 0.008 0.015 2,3 four 0.006 0.015 3.3 5 0.001 0.003 3.0 6 0.008 0.015 3.2 7 0.018 0.015 9.2 8 0.008 0.015 3.4 9 0.010 0.015 3.2 10 0.008 0.015 2,8 eleven 0.008 0.015 2,8 12 0.008 0.015 2,8 13 0.008 0.015 2,8 fourteen 0.008 0.015 2,8 fifteen 0.009 0.015 3.9 16 0.009 0.015 3,7 17 0.008 0.015 3.2 eighteen 0.008 0.015 2.7 19 0.006 0.015 2.1 twenty 0,025 0,043 3,1 21 0.128 0,093 10.8 22 0.008 0.015 2.7 23 0.009 0.015 3,1 24 0.006 0.015 3.6 25 0.007 0.015 3.6 26 0.008 0.015 3.6 27 0.008 0.015 2.7 28 0.006 0.015 3.0 29th 0.007 0.015 3.3 thirty 0.006 0.015 3.2 31 0.006 0.015 3.0

[0064] Each of the above steel sheets (1.6 mm (thickness t) × 150 mm × 200 mm) (sheet thickness t, other than for processing (1) and (2), was adjusted to 1.6 mm by hot rolling ) was heated at a predetermined temperature in a heating furnace, followed by stamping and cooling using a hat-shaped mold (FIG. 1) to obtain a molded product. The stamping conditions (heating temperature, duration of heating, average cooling rate and temperature of the end of rapid cooling during stamping) are shown below in Table 4.

[0065]

Table 4 Steel No. Stamping conditions Heating Temperature (ºC) Duration of heating (s) Average cooling rate (ºC / s) Quick Cooling End Temperature (ºC) Cooling rate after the end of rapid cooling (ºC / s) one 900 600 40 450 3 2 900 fifteen 40 450 3 3 900 fifteen 40 450 3 four 900 fifteen 40 450 3 5 900 fifteen 40 450 3 6 900 fifteen 40 450 3 7 900 fifteen 40 450 3 8 900 fifteen 40 450 3 9 900 fifteen 40 450 3 10 900 fifteen 40 450 3 eleven 900 fifteen 40 450 25 12 800 fifteen 40 300 3 13 900 fifteen 5 450 3 fourteen 900 fifteen 40 600 3 fifteen 900 fifteen 40 450 3 16 900 fifteen 40 450 3 17 900 fifteen 40 400 3 eighteen 900 fifteen 40 450 3 19 900 fifteen 40 400 3 twenty 900 fifteen 40 450 3 21 900 fifteen 40 450 3 22 900 fifteen 40 450 3 23 900 fifteen 40 450 3 24 900 fifteen 40 450 3 25 900 fifteen 40 450 3 26 900 fifteen 40 450 3 27 900 fifteen 40 450 3 28 900 fifteen 40 450 3 29th 900 fifteen 40 450 3 thirty 900 fifteen 40 450 3 31 900 fifteen 40 450 3

[0066] In relation to the obtained stamped products, measurement of tensile strength (TS), elongation (total elongation EL), examination of the metallographic microstructure (fraction of each microstructure), and measurement of the degree of hardness reduction after heat treatment were carried out by the following methods, and the condition precipitated Ti phases were analyzed by the method described above.

[0067]

(Measurement of ultimate tensile strength (TS) and elongation (total elongation EL)

Tensile testing was performed using test sample JIS No. 5, and tensile strength (TS) and elongation (EL) were measured. At this time, the strain rate in the tensile test was set to 10 mm / s. In the present invention, the test piece was evaluated as “passed the test” when the tensile strength (TS) of 1180 MPa or more and the elongation (EL) of 12.0% or more and the strength-elongation balance (TS) were satisfied × EL) was 16,000 (MPa⋅%) or more.

[0068]

(Inspection of the metallographic microstructure (share of each microstructure))

(1) With respect to the microstructure of bainitic ferrite, martensite and ferrite in the molded product, the steel sheet was etched with nital, and after distinguishing between bainitic ferrite, martensite and ferrite from each other, a fraction was determined by SEM examination (magnification: 1000 times or 2000 times) ( fraction of area) of each microstructure.

(2) The fraction of residual austenite in the molded product was measured by x-ray diffraction after the steel sheet was ground to 1/4 thickness and then chemically polished (for example, according to ISJJ Int., Volume 33, (1933), No. 7 , p. 776).

[0069]

(Degree of hardness reduction after heat treatment)

As a thermal history based on spot welding, we measured the degree of decrease in hardness (ΔHV) relative to the initial hardness (Vickers hardness) after heating to a temperature of 700 ° C with an average heating rate of 50 ° C / s in a heat treatment simulator, and then cooling with an average cooling rate of 50 ° C /from. The softening resistance characteristic in HAZ was rated good when the degree of hardness reduction (ΔHv) was 50 HV or less.

[0070] The results of the examination (the proportion of each microstructure, the state of the Ti inclusions, and the amount of Ti in the inclusions is 3.4 [N]) of the metallographic microstructure are shown below in Table 5. In addition, mechanical characteristics are shown in Table 6 below (tensile strength TS, elongation EL, TS × EL, and degree of reduction in hardness ∆HV) of the molded product. Here, the value of “amount of Ti in inclusions - 3.4 [N]” in the molded product is slightly different from the value of “amount of Ti in inclusions - 3.4 [N]” in the steel sheet for stamping, but this is a measurement error.

[0071]

Table 5 Steel No. The metallographic microstructure of the stamped product The proportion of bainitic ferrite
(% by area) Martensite share (% by area) Share of ferrite (% by area) The proportion of residual austenite (% by area) The average diameter of the equivalent circle of Ti-containing inclusions (nm) The amount of Ti in the inclusions -3.4 [N] (wt.%) one 87 8 0 5 2.7 0.010 2 81 8 5 6 2.5 0.010 3 94 6 0 0 3.0 0.010 four 89 7 0 four 2.5 0.009 5 86 6 0 8 3,5 0,000 6 86 7 0 7 3.3 0.011 7 85 8 0 7 8.0 0,023 8 86 7 0 7 2.9 0.010 9 87 6 0 7 2.1 0.011 10 88 5 0 7 3.0 0.011 eleven 6 88 0 6 3.4 0.011 12 0 92 0 8 3.9 0.011 13 26 5 48 6 3.2 0.011 fourteen 55 6 33 6 3.4 0.012 fifteen 91 0 0 9 2,4 0.013 16 86 7 0 7 2,3 0.012

Table 5 (continued) Steel No. The metallographic microstructure of the stamped product The proportion of bainitic ferrite
(% by area) Martensite share (% by area) Share of ferrite (% by area) The proportion of residual austenite (% by area) The average diameter of the equivalent circle of Ti-containing inclusions (nm) The amount of Ti in the inclusions -3.4 [N] (wt.%) 17 72 8 8 12 2,8 0.013 eighteen 88 5 0 7 2,4 0.008 19 87 6 0 7 3.0 0.012 twenty 88 6 0 6 3.8 0,021 21 87 6 0 7 13.8 0.180 22 85 7 0 8 2.1 0.012 23 88 6 0 6 3,1 0.011 24 86 7 0 7 3.6 0.009 25 85 7 0 8 3.0 0.012 26 89 5 0 6 2.0 0.013 27 87 7 0 6 2.0 0.011 28 85 8 0 7 3.6 0.008 29th 84 9 0 7 3.0 0.010 thirty 86 7 0 7 3.3 0.009 31 85 7 0 8 3.0 0.011

[0072]

Table 6 Steel No. The mechanical characteristics of the stamped product Tensile Strength TS (MPa) Elongation EL (%) TS × EL (MPa⋅%) The degree of decrease in hardness ΔHv (Hv) one 1217 13.3 16151 42 2 1189 15,5 16188 38 3 1239 10.1 12551 38 four 1210 13.3 16115 35 5 1254 13.0 16296 35 6 1263 12.8 16220 39 7 1241 11.9 14786 68 8 1213 13.6 16488 44 9 1218 13,4 16263 36 10 1206 13.3 16057 42 eleven 1495 10.6 15847 44 12 1532 10.3 15735 40 13 890 18.1 16096 40 fourteen 955 15,2 14516 43 fifteen 1260 13.0 16329 42 16 1320 12.3 16187 38 17 1992 3,5 6972 40 eighteen 1217 13,4 16311 41 19 1245 13,2 16481 38 twenty 1215 13.3 16160 44 21 1253 12.9 16174 98 22 1266 12.9 16275 37 23 1221 13.3 16256 40 24 1220 13,2 16063 40 25 1224 13,2 16136 42 26 1244 13.1 16271 41 27 1232 13,2 16236 40 28 1233 13.5 16646 39 29th 1250 13,2 16479 42 thirty 1244 13.1 16271 40 31 1229 13,2 16196 41

[0073] These results allow us to state the following consideration. It was found that in the case of Steels No. 1, 2, 4-6, 8-10, 15, 16, 18-20, and 22-31, which are Examples that satisfy the requirements specified in the present invention, a molded product having good balance "strength-ductility" and a good characteristic of resistance to softening.

[0074] On the other hand, in the case of Steels No. 3, 7, 11-14, 17 and 21, which are Comparative Examples that do not satisfy any of the requirements specified in the present invention, any of the properties is deteriorated. More specifically, in the case of Steel No. 3, where a steel sheet having a low Si content is used, a fraction of residual austenite is not provided in the stamped product, and since only a low elongation EL is obtained, the strength-elongation balance (TS × EL ) In the case of Steel No. 7, where the finish temperature is low in the manufacture of the steel sheet, the ratio according to formula (1) is not satisfied, and not only the Ti-containing precipitated phase is enlarged with a reduction in the strength-elongation balance (TS × EL), but and the softening resistance characteristic deteriorates.

[0075] In the case of Steel No. 11, where the cooling rate is high after rapid cooling during stamping, martensite is formed in an excessive amount, and not only is the strength too high, resulting in only low EL, but the strength-elongation balance also worsens. "(TS × EL). In the case of Steel No. 12, where the temperature of the end of rapid cooling during stamping is low, martensite is formed in an excessive amount, and not only is the strength too high, resulting in only a low EL, but also the balance “strength-elongation” (TS × EL).

[0076] In the case of Steel No. 13, where the average cooling rate during stamping is low, the proportion of the bainitic ferrite area cannot be ensured, and not only is the strength too low, but the strength-elongation balance (TS × EL) also worsens. . In the case of Steel No. 14, where the temperature of the end of rapid cooling during stamping is high, the proportion of the bainitic ferrite area cannot be ensured due to the formation of ferrite, and not only is the strength too low, but the balance “strength-elongation” also worsens (TS × EL )

[0077] In the case of Steel No. 17, where a steel sheet having an excessively high C content is used, the strength of the molded product is high, but only a low elongation EL is obtained. In the case of Steel No. 21, where a steel sheet having an excessively high Ti content is used, the stamped product does not satisfy the ratio according to formula (1), and not only the Ti-containing precipitated phase in the stamped product is enlarged, but also the softening resistance characteristic deteriorates.

INDUSTRIAL APPLICABILITY

[0078] In the present invention, a hot stamping steel sheet that has a predetermined chemical component composition, wherein the diameter of the equivalent circle of Ti-containing precipitated phases having an equivalent circle diameter of 30 nm or less among Ti-containing precipitated phases contained in the steel sheet is 6 nm or less, and the amount of Ti phases released and the total amount of Ti in the steel satisfy a predetermined ratio, are heated at a temperature of 900 ° C or more, and 1100 ° C or less, and after stamping steel sheet is cooled to a temperature equal to or lower than the temperature 100 ° C lower than the temperature Bs of the onset of bainitic transformation and equal to or higher than the temperature Ms of the beginning of martensitic transformation, while at the same time providing an average cooling rate of 20 ° C / s or more in the mold during molding, and also after molding, and then cooled to a temperature of 200 ° C or less at an average cooling rate of less than 20 ° C / s, whereby a stamped product capable of reaching a high level can be obtained a balance between high strength and elongation, and moreover, a stamped article having a good softening resistance characteristic in HAZ can be created.

DESCRIPTION OF CODE NUMBERS OF POSITIONS AND DESIGNATIONS

[0079]

1: Punch

2: Matrix

3: Workpiece clamp

4: Steel sheet (blank)

Claims (23)

1. A method of manufacturing a stamped product from a steel sheet, including obtaining a sheet blank for hot stamping from steel containing in wt.%: C 0.15 to 0.5 Si from 0.2 to 3, Mn from 0.5 to 3, P 0.05 or less S 0.05 or less Al from 0.01 to 1, B from 0.0002 to 0.01, N from 0.001 to 0.01, Ti in an amount equal to or more than 3.4 [N] +0.01 and equal to or less than 3.4 [N] +0.1, where [N] is the content (wt.%) N in steel, iron and unavoidable impurities - the rest, moreover, the average diameter of the equivalent circle of Ti-containing precipitated inclusions having an equivalent circle diameter of 30 nm or less is 6 nm or less, and the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies the ratio according to the formula (1) the amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5⋅ [(total amount of Ti (wt.%)) - 3.4 [N]], where [N] is the content (wt. %) N in steel, heating the sheet stock to a temperature of 900 ° C or more and 1100 ° C or less, stamping in the mold, wherein the sheet blank during molding in the mold is cooled with an average cooling rate of 20 ° C / s or more to a temperature equal to or lower by 100 ° C than the temperature Bs of the onset of bainitic transformation and equal to or higher than the temperature Ms of the beginning of martensitic transformation, and after completion of molding the resulting product is cooled with an average cooling rate of less than 20 ° C / s to a temperature of 200 ° C or less. 2. The method according to claim 1, in which the steel sheet for hot stamping further comprises at least one of the following (a) to (c), wt.%: (a) one or more selected from the group: V, Nb and Zr, in an amount of 0.1 or less (excluding 0%) in total, (b) one or more selected from the group: Cu, Ni, Cr and Mo, in an amount of 1% or less (except 0%) in total, (c) one or more selected from the group: Mg, Ca and REM (rare earth metals), in an amount of 0.01 or less (excluding 0%) in total. 3. A stamped steel sheet product manufactured by the method according to claim 1 or 2 and having a metallographic microstructure comprising bainitic ferrite from 60 to 97% by area, martensite 37% by area or less, residual austenite from 3 to 20% by area and the rest of the microstructure is 5% by area or less, the average diameter of the equivalent circle of Ti-containing precipitated inclusions having an equivalent circle diameter of 30 nm or less, among Ti-containing released inclusions contained in the stamped product, is 10 nm silt and less and satisfies the ratio according to the formula (1) the amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5⋅ [(total amount of Ti (wt.%)) - 3.4 [N]], where [N] is the content (wt. %) N in steel.

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