KR20160090865A - Hot-formed steel sheet member, method for producing same, and steel sheet for hot forming - Google Patents

Abstract

The steel sheet according to the present invention is characterized in that it contains 0.100% to 0.340% of C, 0.50% to 2.00% of Si, 1.00% to 3.00% of Mn, 0.050% By mass, tempering martensite and / or tempering bainite in a total amount of 20% to 70%, and a total amount of Fe and impurities, A method for manufacturing a hot-formed steel sheet member having a steel structure comprising martensite: 25% to 75%, a total of 90% or more thereof, and residual austenite: 0% to 5% .

Description

TECHNICAL FIELD [0001] The present invention relates to a hot-formed steel sheet member, a method of manufacturing the same, and a steel sheet for hot forming. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

TECHNICAL FIELD The present invention relates to a hot-formed steel plate member, a method of manufacturing the same, and a steel sheet for hot forming, which are used, for example, in mechanical structural parts such as body structural parts of automobiles. Specifically, the present invention relates to a hot-formed steel sheet member having a high tensile strength and excellent ductility and bendability, a method of manufacturing the same, and a steel sheet for hot forming to obtain the same.

In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel used for the vehicle body and to reduce the weight of the vehicle. In the thin steel sheet widely used for automobiles, as the steel sheet strength is increased, the press formability is lowered, making it difficult to produce a member having a complicated shape. Concretely, there arises a problem that ductility is lowered, fracture occurs at a portion having a high degree of machining, or springback or wall warpage is increased, and dimensional accuracy is deteriorated. Therefore, it is not easy to produce such a member by press molding using a steel sheet having a high strength, particularly a tensile strength of 980 MPa or more. Roll forming, rather than press forming, can process high-strength steel sheets, but can not be applied to members having a uniform cross section in the longitudinal direction.

On the other hand, as disclosed in Patent Document 1, in a method called a hot press for press-forming a heated steel sheet, since the steel sheet is high in temperature and soft and highly ductile, it is possible to mold a member having a complicated shape with high dimensional accuracy Do. Further, by heating the steel sheet to the austenite single-phase region and quenching it in the mold, the strength of the member due to the martensitic transformation can be simultaneously achieved. Therefore, such a hot pressing method is an excellent molding method capable of simultaneously securing the strength of the member and the moldability of the steel sheet.

Further, Patent Document 2 discloses a pre-press quenching method which achieves high strength of a member by heating the member into a predetermined shape at room temperature and then heating it to an austenite region and quenching in the mold. The preliminary press quenching method, which is one form of such hot pressing, is an excellent molding method which can secure the strength of members and high dimensional accuracy at the same time because the members can be restrained by the metal mold to suppress deformation due to thermal distortion.

However, recently, there has been a demand for ductility in the hot-pressed steel plate members, and a problem that the conventional technique represented by Patent Document 1 or Patent Document 2, in which the steel structure is substantially a martensite single phase, .

In view of the above background, Patent Document 3 discloses a method in which a steel sheet is heated in a two-phase temperature region of ferrite and austenite, pressed while keeping the two-phase structure, There is disclosed a hot pressed steel plate member which is considered to have high strength and high ductility due to the structure. However, under such two-phase heating conditions, the steel structure tends to become uneven, so that the bending property and toughness of the hot pressed steel plate member are deteriorated, and the impact absorption characteristic thereof is remarkably lowered.

On the other hand, Patent Document 4 discloses that a steel sheet having a steel structure having 80% by volume or more of martensite or bainite is heated at an Ac ₁ transformation point or higher and quenched in a mold to obtain a retained austenite having a structure of 3 to 20% A hot pressed steel plate member having a high strength and a high ductility and containing 30 to 97% by volume of tempering martensite or tempered bainite and 0 to 67% by volume of martensite.

In addition, Patent Document 5 discloses that the area ratio of martensite to the whole steel sheet structure is 10% or more and 85% or less, 25% or more of martensite is tempering martensite, the amount of retained austenite is 5% The area ratio of the bainitic ferrite in the knit to the entire steel sheet structure is 5% or more, the sum of the area ratio of martensite, the area ratio of retained austenite, and the area ratio of bainitic ferrite in bainite Of 65% or more is satisfied.

Patent Document 6 discloses a hot-press steel sheet having a total fraction of bainite and martensite of 80% or more by area.

Patent Document 7 discloses a hot-press steel sheet in which the fraction of ferrite is 30% by area or more.

British Patent No. 1490535 Japanese Unexamined Patent Application Publication No. 10-96031 Japanese Patent Application Laid-Open No. 2010-65292 Japanese Patent Application Laid-Open Publication No. 2012-237066 International Publication No. WO2011 / 111333 Japanese Patent Application Laid-Open No. 2013-185243 Japanese Patent Application Laid-Open No. 2013-185248

For example, as described in Patent Document 4, when the steel structure of the hot-press steel sheet is made of bainite or martensite, not only the ductility of the hot-press steel sheet member but also the inventors of the present invention, . However, the deterioration of the bendability can not be solved even by the structural control of such a member, and bending breakage of the member occurring in the buckling portion at the time of impact deformation can not be prevented. This problem becomes present when the tensile strength of the steel becomes high (for example, 980 MPa or more). As described above, for a hot-pressed steel plate member having a high tensile strength (for example, a tensile strength of 980 MPa or more) and excellent ductility as well as ductility, manufacturing techniques have not been established, It is a situation.

Similarly, in the case of a hot-formed steel plate member having a high tensile strength (for example, a tensile strength of 980 MPa or more) and excellent bendability in addition to ductility in addition to the hot-pressed steel plate member, In addition to establishing technology, such products have not yet been proposed.

A specific object of the present invention is to provide a hot pressed steel sheet member having excellent tensile strength and excellent ductility and bendability after hot pressing, which is not present in the prior art as described above, and a method for producing the same, and a hot press steel sheet for obtaining the same . If generalized, the present invention can be applied to hot forming, which includes means for cooling a steel sheet immediately after or immediately after molding as in the case of a hot press. Accordingly, a specific object of the present invention is to provide a hot-formed steel sheet member having high tensile strength and excellent ductility and bendability after hot forming, a method of manufacturing the same, and a steel sheet for hot forming to obtain the same.

The inventors of the present invention have made extensive studies in order to improve ductility and bendability of a hot-formed steel sheet member having a high tensile strength. As a result, the following new knowledge was obtained. That is, a steel sheet for hot forming having a chemical composition in which Si is positively contained in a specific amount of C and Mn and a steel structure containing at least one of ferrite, martensite and bainite is used. The heat treatment conditions for hot forming that are optimal for the steel sheet for hot forming are also applied. As a result, unlike a conventional hot-formed steel plate member, the steel structure is made to have an area ratio of not more than 5% even if it does not contain or retain retained austenite, and at least one of ferrite and tempering martensite and tempering bainite Martensite is made into a garnet having a predetermined area ratio. By the above-mentioned chemical composition and the steel structure, it is possible to produce a hot-formed steel sheet member having high tensile strength and excellent ductility and bending property.

The present invention based on the above findings is as follows.

(1) A steel sheet comprising: 0.100% to 0.340% of C, 0.50% to 2.00% of Si, 1.00% to 3.00% of Mn, 0.050% 1.000% and N: 0.0100% or less, and has a chemical composition consisting of the remainder Fe and impurities,

Ferrites, at least one of tempering martensite and tempering bainite, and a martensite-containing steel structure, the ferrites being 5% to 50%, the tempering martensite and the tempering bainite being 20% to 70% A steel sheet member having a steel structure of 90% or more in total, and 0% to 5% in retained austenite: martensite: 25% to 75%, ferrite, tempering martensite, tempering bainite and martensite.

(2) The steel sheet according to any one of the above items (1) to (3), wherein the above chemical composition contains, in mass%, 0.200% or less of Ti, 0.200% or less of Nb, 0.200% or less of V, 1.000% or less, and Ni: 1.000% or less. The hot-formed steel sheet member according to (1)

(3) The hot-formed steel plate member according to the item (1) or (2), wherein the chemical composition contains 0.0025% or less by mass of B instead of a part of Fe.

(4) The steel sheet according to any one of the above items (1) to (3), wherein the chemical composition is one or two selected from the group consisting of 0.0100% or less of Ca, 0.0100% or less of Mg, 0.0100% or less of REM, (1) to (3), wherein the heat-resistant steel sheet member contains at least one member selected from the group consisting of iron, iron and zirconium.

(5) The hot-formed steel sheet member according to any one of (1) to (4), wherein the chemical composition contains 0.0% or less of Bi by mass% instead of a part of Fe.

(6) A ferritic stainless steel comprising: 0.100 to 0.340% of C, 0.50 to 2.00% of Si, 1.00 to 3.00% of Mn, 0.050% of P or less, 0.0100% or less of S, 1.000% and N: 0.0100% or less, and has a chemical composition consisting of the remainder Fe and impurities,

Ferrites having an aspect ratio of 2.0 or less and ferrites containing at least one of martensite and bainite and having an area percentage of 5% to 50%, martensite and bainite: 45% to 90% in total, ferrites, Martensite and bainite: a steel sheet having a steel structure of 90% or more in total.

(7) The steel sheet according to any one of the above items (1) to (5), wherein the above chemical composition contains, in mass%, Ti: not more than 0.200%, Nb: not more than 0.200%, V: not more than 0.200%, Cr: not more than 1.000% 1.000% or less, and Ni: 1.000% or less. The steel sheet for hot-forming according to (6)

(8) The hot-for-molding steel sheet according to item (6) or (7), wherein the chemical composition contains 0.0025% or less by mass of B instead of a part of Fe.

(9) The steel sheet according to any one of the above items (1) to (3), wherein the chemical composition is one or two selected from the group consisting of 0.0100% or less of Ca, 0.0100% or less of Mg, 0.0100% or less of REM, The steel sheet for hot forming according to any one of (6) to (8), wherein the steel sheet contains at least one species.

(10) The hot-formed steel sheet according to any one of (6) to (9), wherein the chemical composition contains 0.0100% or less of Bi by mass% instead of a part of Fe.

11 (6) to (10), the hot forming steel sheet described in any one of items, and heated to a temperature range of less than 720 ℃ Ac ₃ point, for from the end of the heating to initiation of hot forming of steel sheet by the time of exposure to air cooling for three seconds to about 20 seconds subjected to hot molding, 10 ℃ / sec to 500 ℃ / sec in average cooling rate for cooling from a temperature range of more than M _S point, hot forming steel member Gt;

The present invention achieves a technically advantageous effect that the hot-formed steel sheet member having a high tensile strength, excellent ductility, and excellent bendability can be put into practical use for the first time while being hot-formed. The hot-formed steel sheet member according to the present invention exhibits a very excellent impact property that the impact can be absorbed by bending deformation even in a collision where extreme plastic deformation occurs. Therefore, the hot-formed steel sheet member according to the present invention is particularly suitable for manufacturing a body structural component of an automobile, but can also be applied to other applications such as mechanical structural components.

1 is a photograph showing an example of the steel structure of the present invention.

Next, the reason why the present invention is limited to each range will be described. In the following description, a hot press, which is a specific form for hot forming, will be described as an example. The numerical range indicated by " to " indicates a range in which the numerical values described before and after the numerical value are the minimum value and the maximum value, respectively.

1. Chemical composition

First, the reason why the chemical composition of the hot-formed steel sheet member according to the present invention (hereinafter simply referred to as " steel sheet member ") and the hot-formed steel sheet (hereinafter simply referred to as " steel sheet ") is defined as described above will be described. In the following description, "% " representing the content of each alloy element means "% by mass " unless otherwise specified.

(C: 0.100% to 0.340%)

C is a very important element that increases the hardenability of steel and mainly determines the strength after hot pressing (after quenching). When the C content is less than 0.100%, it becomes difficult to secure the tensile strength after hot pressing (quenching) (for example, tensile strength of 980 MPa or more). Therefore, the C content is 0.100% or more, preferably 0.120% or more. On the other hand, if the C content is more than 0.340%, the martensite after hot pressing (after quenching) becomes hard, not only the deterioration of the bendability becomes significant, but also the ductility is lowered. Therefore, the C content is 0.340% or less. From the viewpoint of weldability, the C content is preferably 0.300% or less, and more preferably 0.280% or less.

(Si: 0.50% to 2.00%)

Si is a very effective element for improving the ductility of a steel heated to a two-phase temperature region of ferrite and austenite and for stably securing strength after hot pressing (after quenching). When the Si content is less than 0.50%, it is difficult to obtain the above action. Therefore, the Si content should be 0.50% or more. From the viewpoint of improving the weldability, the Si content is preferably 0.70% or more, more preferably 1.10% or more. On the other hand, if the Si content is more than 2.00%, the effect of the above action becomes saturated and becomes economically disadvantageous, and deterioration of the plating wettability becomes remarkable, resulting in frequent plating. Therefore, the Si content should be 2.00% or less. From the viewpoint of suppressing surface defects of the hot-formed steel plate member, the Si content is preferably 1.80% or less, and more preferably 1.50% or less.

(Mn: 1.00% to 3.00%)

Mn is a very effective element for enhancing the quenching of steel and securing strength after hot pressing (after quenching). However, when the Mn content is less than 1.00%, securing of the tensile strength after hot pressing (quenching) (for example, tensile strength of 980 MPa or more) becomes very difficult, and bendability may be lowered. Therefore, the Mn content should be 1.00% or more. In order to more reliably obtain the above action, the Mn content is preferably 1.10% or more, and more preferably 1.20% or more. On the other hand, if the Mn content is more than 3.00%, the steel structure after hot pressing (after quenching) becomes a remarkable band shape due to Mn segregation, resulting in a decrease in toughness and deterioration in collision characteristics. Therefore, the Mn content should be 3.00% or less. From the viewpoint of productivity in hot rolling and cold rolling, the Mn content is preferably 2.50% or less, and more preferably 2.40% or less.

By defining C, Si and Mn within the above-mentioned range, it becomes possible to make the steel structure of the hot-formed steel sheet into a multi-phase steel structure including at least one of ferrite, martensite and bainite, The steel structure of the hot-formed steel plate member becomes a steel structure having a desired appearance.

(P: 0.050% or less)

P is an impurity contained in the steel in general, but it may actively increase the strength of the steel sheet by solid solution strengthening. However, if the P content exceeds 0.050%, the deterioration of the weldability becomes remarkable. Therefore, the content of P is 0.050% or less. The P content is preferably 0.018% or less. In order to more reliably obtain the effect of the above action, the P content is preferably 0.003% or more.

(S: 0.0100% or less)

S is an impurity contained in the steel, and it is preferable from the viewpoint of weldability. If the S content is more than 0.0100%, the weldability deteriorates remarkably. Therefore, the S content is 0.0100% or less. The S content is preferably 0.0030% or less, more preferably 0.0015% or less. From the viewpoint of the desulfurization cost, the S content is preferably 0.0006% or more.

(sol.Al (soluble Al): 0.001% to 1.000%).

Al is an element that acts to deoxidize steel and to repair the steel. If the sol.Al content is less than 0.001%, it becomes difficult to obtain the above-mentioned action. Therefore, the sol.Al content is set to 0.001% or more, preferably 0.015% or more. On the other hand, if the content of sol.Al is more than 1.000%, the deterioration of the weldability becomes remarkable, and the oxide inclusions increase and the deterioration of the surface property becomes remarkable. Therefore, the content of sol.Al is 1.000% or less, preferably 0.080% or less. The term sol.Al means an acid-soluble Al that is not an oxide such as Al ₂ O ₃ and is soluble in an acid.

(N: 0.0100% or less)

N is an impurity contained in the steel, and is preferably as small as possible from the viewpoint of weldability. When the N content is more than 0.0100%, the weldability deteriorates remarkably. Therefore, the N content is set to 0.0100% or less, preferably 0.0060% or less. From the viewpoint of the denitrification cost, the N content is preferably 0.0020% or more.

〔impurities〕

The impurity refers to a component contained in the raw material or a component incorporated in the manufacturing process and intentionally not contained in the steel sheet member or the steel sheet for hot forming.

The chemical composition of the steel sheet member and the hot-for-molding steel sheet according to the present invention may contain at least one kind of element as described below.

(Ti: not more than 0.200%, Nb: not more than 0.200%, V: not more than 0.200%, Cr: not more than 1.000%, Mo: not more than 1.000%, Cu: not more than 1.000% Two or more species)

All of these elements are effective elements for stably securing the strength after hot pressing (after quenching). Therefore, one or more of these elements may be contained. However, when Ti, Nb and V are contained in an amount exceeding 0.200%, there are cases where hot rolling and cold rolling become difficult, and conversely, it may be difficult to obtain a stable strength. Therefore, the Ti content, the Nb content, and the V content are preferably 0.200% or less, respectively. With respect to Cr, if it exceeds 1.000%, it may become difficult to secure a stable strength. Therefore, the Cr content is preferably 1.000% or less. Further, when Mo is contained in an amount exceeding 1.000%, hot rolling and cold rolling may become difficult. Therefore, the Mo content is preferably 1.000% or less. Even when Cu and Ni are contained in an amount exceeding 1.000%, the effect due to the above action tends to be saturated, which may be economically disadvantageous, and it may be difficult to perform hot rolling or cold rolling. Therefore, the Cu content and the Ni content are preferably set to 1.000% or less, respectively.

In order to more reliably obtain the effect of the above action, it is preferable to use an alloy containing at least 0.003% of Ti, at least 0.003% of Nb, at least 0.003% of V, at least 0.005% of Cr, at least 0.005% of Mo, And Ni: 0.005% or more.

That is, the lower limit of the Ti content is preferably 0.003%. The lower limit of the Nb content is preferably 0.003%. The lower limit of the V content is preferably 0.003%. The lower limit of the Cr content is preferably 0.005%. The lower limit of the Mo content is preferably 0.005%. The lower limit of the Cu content is preferably 0.005%. The lower limit of the Ni content is preferably 0.005%.

(B: 0.0025% or less)

B is an element having an action of increasing the toughness of the steel. Therefore, B may be added. However, if B is contained in an amount exceeding 0.0025%, the steel structure for hot-molding may not contain ferrite in some cases, and the ductility and bendability of the hot-formed steel sheet member may deteriorate in some cases. Therefore, the B content is preferably 0.0025% or less. In order to more reliably obtain the effect of the above action, the B content is preferably 0.0003% or more.

(Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less and Zr: 0.0100% or less)

These elements all contribute to the control of inclusions, particularly the fine dispersion of inclusions, and are elements having an action of increasing toughness. Therefore, one or more of these elements may be contained. However, if any element is contained in an amount exceeding 0.0100%, the deterioration of the surface property sometimes becomes present. Therefore, the content of each element is preferably 0.0100% or less. In order to more reliably obtain the effect of the above action, the content of at least one of these elements is preferably 0.0003% or more. That is, the lower limit of the Ca content, Mg content, REM content and Zr content is preferably 0.0003%.

Here, REM refers to a total of 17 elements of Sc, Y and lanthanoids, and at least one species. The content of the REM means the total content of at least one of these elements. In the case of lanthanoids, it is industrially added in the form of mischmetal.

(Bi: 0.0100% or less)

Bi is an element having a function of making the structure uniform and increasing the bending property. Therefore, Bi may be added. However, when Bi is contained in an amount exceeding 0.0100%, the hot workability is deteriorated and hot rolling may become difficult. Therefore, the Bi content is preferably 0.0100% or less. In order to more reliably obtain the effect of the above action, the Bi content is preferably 0.0003% or more.

2. Steel structure of hot-formed steel plate member

Next, the steel structure of the hot-formed steel plate member according to the present invention will be described.

The hot-formed steel sheet member according to the present invention has a steel structure containing ferrite, at least one of tempering martensite and tempering bainite, and martensite at the following specific area ratio. That is, the steel structure may contain only one of the tempering martensite and the tempering bainite, or both of them. Incidentally, this steel structure does not contain the retained austenite or includes the area ratio of 5% or less.

Here, FIG. 1 shows an example of the steel structure of the present invention. The steel structure shown in Fig. 1 shows a steel structure containing ferrite, tempering martensite, and martensite and not containing retained austenite.

(Area ratio of ferrite: 5% to 50%)

If the area ratio of the ferrite is less than 5%, the ductility and the bending property are deteriorated. Therefore, the area ratio of the ferrite is set to 5% or more, preferably 15% or more. On the other hand, if the area ratio of the ferrite exceeds 50%, the bending property is lowered. Therefore, the area ratio of the ferrite is set to 50% or less, preferably 40% or less.

The aspect ratio of the ferrite is preferably 2.0 or less in view of suppressing lowering of the bending property. When the aspect ratio of the ferrite exceeds 2.0, the anisotropy of the ferrite (crystal grains of the ferrite) is increased, which is a starting point of stress concentration, and the bendability may be lowered. Therefore, the aspect ratio of the ferrite is preferably 2.0 or less, and more preferably 1.8 or less. On the other hand, as the aspect ratio of the ferrite is closer to 1.0, the anisotropy of the ferrite (the crystal grain of the ferrite) is reduced, so that the lower limit of the aspect ratio of the ferrite is preferably 1.0. However, from the viewpoint of increasing the yield strength of the steel sheet member after hot pressing, the lower limit of the aspect ratio of the ferrite is preferably 1.2.

The aspect ratio of ferrite is a value measured by a method described in detail in the following embodiments.

(Total area ratio of tempered martensite and tempered bainite: 20% to 70%)

If the total area ratio of the tempering martensite and the tempering bainite is less than 20%, the bendability decreases. Therefore, the total area ratio of the tempering martensite and the tempering bainite is 20% or more, preferably 30% or more. On the other hand, if the total area ratio of the tempered martensite and the tempered martensite exceeds 70%, the ductility is lowered. Therefore, the total area ratio of the tempered martensite and the tempered martensite is set to 70% or less, preferably 50% or less.

(Area ratio of martensite: 25% to 75%)

By forming the martensite in the steel, the strength after hot pressing (after quenching) can be increased. If the area ratio of the martensite is less than 25%, it becomes difficult to secure the tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching). Therefore, the area ratio of martensite should be 25% or more. On the other hand, if the area ratio of martensite exceeds 75%, the ductility is lowered. Therefore, the area ratio of martensite is set to 75% or less, preferably 50% or less.

Here, " martensite " means both martensite as quenched and martensite as age-hardened as the quenched martensite is age-hardened. In other words, the "area ratio of martensite" means the total area ratio of martensite as quenched and martensite as quenched after age-hardening.

(Total area ratio of ferrite, tempered martensite, tempered bainite and martensite: 90% or more)

The hot-formed steel sheet member according to the present invention is based on having a structure composed of ferrite, tempering martensite, tempering bainite and martensite. However, depending on the production conditions, one or more of bainite, retained austenite, cementite and pearlite may be incorporated as other phases or structures. In this case, if the phase or the structure other than the ferrite, the tempering martensite, the tempering bainite and the martensite exceeds 10%, the desired characteristics may not be obtained due to the influence of these phases or the structure. Therefore, the incorporation of phases or structures other than ferrite, tempered martensite, tempered bainite and martensite is 10% or less, preferably 5% or less. That is, the total area ratio of ferrite, tempered martensite, tempered bainite and martensite is 90% or more, preferably 95% or more. The upper limit value of the total area ratio of ferrite, tempered martensite, tempered bainite and martensite is 100%.

(Area ratio of retained austenite: 0% to 5%)

When the residual austenite is mixed (remained) in an area ratio exceeding 5%, particularly in an image or a structure other than ferrite, tempered martensite, tempered bainite and martensite, the bendability decreases. Therefore, the area ratio of the retained austenite is 5% or less, preferably 3% or less, even if the retained austenite is not contained or included. The area ratio of the retained austenite is most preferably 0%.

The area ratios of the phases and the textures in the steel structure of the above-mentioned hot-formed steel plate members are values measured by the method described in detail in the following embodiments.

The steel plate member according to the present invention means a member formed by hot forming from a steel plate, for example, a steel plate member formed by hot press forming. Typically, there is a door guard bar used in automobile body structural parts. In addition, for automobiles, bumper reinforcements are also available. For machine structural parts, there is also a hot-formed steel pipe for a structural structure made of a steel sheet.

3. Mechanical properties

The hot-formed steel sheet member according to the present invention preferably has a tensile strength (TS) of 980 MPa or more as sufficient strength to contribute to weight reduction of an automobile.

4. Manufacturing Method

Next, a preferable manufacturing method of the hot-formed steel sheet member according to the present invention having the above-described characteristics will be described.

In the hot-formed steel sheet member according to the present invention, in order to obtain ductility and bendability while having a high tensile strength (for example, tensile strength of 980 MPa or more), the steel structure after hot pressing (after quenching) The total area ratio of the tempering martensite and the tempering bainite is 20% to 70%, the area ratio of the martensite is 25% to 75%, and the area ratio of the ferrite is 5% , Ferrite, tempering martensite, tempering bainite and martensite of 90% or more and area percent of retained austenite of 0% to 5%.

In order to obtain the steel structure of the hot-formed steel plate member according to the present invention, a steel sheet (hot-formed steel sheet) as a hot-forming material is required to have a composition of ferrite having the chemical composition described above and having an aspect ratio of 2.0 or less and martensite and bainite And the total area ratio of ferrite, martensite and bainite is 90% or more, the total area ratio of ferrite, martensite and bainite is 90% It is preferable to use a steel plate having a steel structure (augmentation structure) of not less than 10 mm. Then, by heating the steel sheet (for hot-forming the steel sheet) with a temperature range of less than 720 ℃ Ac ₃ point, and subsequently, to 3, the time at which the steel sheet is exposed to the air cooling in from the end of heating to the start of the hot press seconds 20 seconds to may be subjected to hot press, it cooled to a temperature range of less than M _S point to 10 ℃ / sec average cooling rate to 500 ℃ / sec.

The steel sheet for hot forming having the chemical composition and the steel structure is subjected to hot pressing under the above conditions to have a desired steel structure and a high tensile strength (for example, a tensile strength of 980 MPa or more) , A hot-formed steel sheet member having excellent ductility and bendability can be obtained.

(Steel structure of steel sheet for hot forming)

- Aspect ratio of ferrite: 2.0 or less -

If the aspect ratio of the ferrite is more than 2.0, the aspect ratio of the ferrite in the steel structure of the steel sheet member after hot pressing may be more than 2.0, and the ferrite may excessively transform into austenite during heating, The ferrite area ratio of the member may be less than 5%. If the ferrite aspect ratio of the steel sheet member exceeds 2.0, the anisotropy of the ferrite (crystal grains of the ferrite) becomes high, which is a starting point of stress concentration, and the bendability may be lowered. Therefore, the aspect ratio of the ferrite is 2.0 or less, preferably 1.8 or less. On the other hand, the closer the aspect ratio of ferrite to 1.0, the lower the anisotropy of ferrite (crystal grains of ferrite), so that the lower limit of the aspect ratio of ferrite is preferably 1.0. However, from the viewpoint of increasing the yield strength of the steel sheet member after hot pressing, the lower limit value of the aspect ratio of the ferrite is preferably 1.2.

The aspect ratio of ferrite is a value measured by a method described in detail in the following embodiments.

- area ratio of ferrite: 5% to 50%

If the area ratio of the ferrite is less than 5%, the area ratio of the ferrite in the steel structure of the steel sheet member after hot pressing may be less than 5%. Therefore, the area ratio of the ferrite is set to 5% or more, preferably 15% or more. Similarly, if the area ratio of the ferrite exceeds 50%, the area ratio of ferrite in the steel structure of the steel sheet member after hot pressing may also exceed 50%. Therefore, the area ratio of the ferrite is set to 50% or less, preferably 45% or less.

- the total area ratio of martensite and bainite: 45% to 90%

If the total area ratio of martensite and bainite is less than 45%, the total area ratio of the tempering martensite and the tempering bainite in the steel structure of the steel sheet member after hot pressing may be less than 20%. In addition, the area ratio of martensite in the steel structure of the steel sheet member after hot pressing may be less than 25%. Therefore, the total area ratio of martensite and bainite is 45% or more, preferably 50% or more. Similarly, if the total area ratio of martensite and bainite exceeds 90%, the total area ratio of tempered martensite and tempered bainite in the steel structure of the steel sheet member after hot pressing may exceed 70%. Further, the area ratio of martensite in the steel structure of the steel sheet member after hot pressing may exceed 75%. Therefore, the total area ratio of martensite and bainite is 90% or less, preferably 80% or less.

- Total area ratio of ferrite, martensite and bainite: 90% or more -

If the total area ratio of ferrite, martensite and bainite is less than 90%, the incorporation of phases or structures other than ferrite, tempering martensite, tempering bainite and martensite in the steel structure of the steel sheet member after hot pressing is 10 %. ≪ / RTI > In particular, the area ratio of the retained austenite may exceed 5%. Therefore, the total area ratio of ferrite, martensite and bainite is 90% or more, preferably 93% or more. The upper limit value of the total area ratio of ferrite, martensite and bainite is 100%.

The area ratios of the phases and the textures in the steel structure of the above hot-forming steel sheet are the values measured by the method described in detail in the following embodiments.

(Production of steel sheet for hot forming)

The hot-formed steel sheet may be a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet. Examples of the coated steel sheet include an aluminum-based coated steel sheet and a zinc-based coated steel sheet.

Since the hot-rolled steel sheet having the steel structure has C, Si and Mn within the above-described range with respect to its chemical composition, finish rolling is completed at 850 캜 to 930 캜, and the steel sheet is heated to a temperature of 740 캜 to 660 캜 for 3 seconds or longer And then winding it in a temperature range of 450 DEG C or less. The cold-rolled steel sheet having the steel structure may be produced by an annealing process in which after cold rolling, the steel sheet is heated at 780 캜 to 900 캜 and cooled at an average cooling rate of 10 캜 / second or more. Further, the coated steel sheet having the steel structure can be produced by subjecting the surface of the hot-rolled steel sheet or the cold-rolled steel sheet to a well-known plating treatment after manufacturing the hot-rolled steel sheet or the cold-rolled steel sheet.

(Heating of the steel sheet for hot forming: heating to a temperature range of less than 720 ℃ Ac ₃ point)

Heating the steel sheet for hot forming is carried out by more than 720 ℃ Ac ₃ point temperature of less than. Wherein, Ac ₃ point (℃) is to a temperature less than the Ac ₃ point (℃) which the austenite phase is defined by the empirical formula (i).

30? Mn-11? Cr-20? Cu + 700? P + 400? Sol.Al + Ac? ₃ = 910-203 (C ^0.5 ) -15.2 x Ni + 44.7 x Si + 104 x V + 31.5 x Mo- 50 x Ti ... ... (i)

Here, the symbol of the element in the formula (i) represents the content (unit: mass%) of each element in the chemical composition of the steel sheet. In addition, the formula (i) is calculated by setting 0 (0 mass%) as an element not included in the steel sheet.

If the heating temperature is lower than 720 占 폚, austenitization becomes insufficient, martensite is not contained in the hot-pressed steel sheet, and a high tensile strength (for example, tensile strength of 980 MPa or more) is secured after hot pressing It becomes difficult. Therefore, the heating temperature is set to 720 DEG C or higher, preferably 750 DEG C or higher. On the other hand, if the heating temperature is equal to or higher than the Ac ₃ point, even when exposed to air cooling after that, the area ratio of martensite exceeds 75% in the steel structure after hot pressing (quenching), and the deterioration of ductility becomes remarkable . Therefore, the heating temperature is set to be not more than Ac ₃ point, and preferably not more than Ac ₃ point -30 ° C.

In this case, the heating rate up to 720 占 폚 and the heating time to be maintained in the temperature region are not particularly limited, but are preferably set within the following ranges.

The average heating rate during heating up to 720 占 폚 is preferably 0.2 占 폚 / sec to 100 占 sec / sec. By setting the average heating rate to 0.2 DEG C / second or more, higher productivity can be ensured. Further, by setting the average heating rate to be 100 DEG C / second or less, it becomes easy to control the heating temperature in the case of heating using a normal furnace.

The heating time in the temperature region of less than 720 ℃ Ac ₃ point is preferably for 2 minutes to 10 minutes. Here, the heating time is a time from when the temperature of the steel sheet reaches 720 占 폚 to when the heating ends. Specifically, the heating end time is when the steel sheet is taken out from the heating furnace in the case of the furnace heating, and when the energization is finished in the case of the conduction heating or induction heating. By setting the heating time to 2 minutes or more, the strength after hot pressing (after quenching) becomes more stable. Further, by setting the holding time to 10 minutes or less, the structure of the steel plate member can be finer, and the toughness of the steel plate member is further improved.

(Time during which the steel sheet is exposed to air cooling from the end of the heating to the start of the hot press: 3 seconds to 20 seconds)

Generally, the hot-formed steel sheet is heated in a heating furnace and then conveyed to the hot press apparatus. At this time, the steel sheet may be partially exposed to air cooling, for example, at the time of extraction from a heating furnace, at the time of transportation to a hot press apparatus, or at the time of charging. During such air cooling, since the ferrite is newly produced or grows, the time during which the steel is exposed to air cooling affects the tensile strength. Therefore, in order to stably secure the strength after hot pressing (after quenching), it is preferable to shorten such air cooling. In particular, if the time for which the steel sheet is exposed to air cooling during the period from the end of the heating to the start of the hot press is more than 20 seconds, the tensile strength of the steel sheet member after hot pressing (after quenching) , A tensile strength of 980 MPa or more is secured), the carbon concentration of the austenite becomes remarkable and the martensitic transformation part becomes fragile and the bendability is lowered. Therefore, the time for exposing the steel sheet to air cooling from the end of the heating to the start of the hot press is within 20 seconds, preferably within 16 seconds. On the other hand, the austenite formed during heating precipitates in the form of needles. A part of the precipitated austenite undergoes ferrite transformation during cooling and the shape of the austenite gradually changes from a needle shape to a spherical shape and therefore the time for which the steel sheet is exposed to air cooling from the end of heating to the start of hot pressing is less than 3 seconds, When hot pressing (quenching) and martensitic transformation is performed, the needle-shaped martensitic transformation portion becomes a starting point of stress concentration, not only the bendability is lowered, but also the retained austenite is easily produced. Therefore, the time for which the steel sheet is exposed to air cooling from the end of the heating to the start of the hot press is 3 seconds or more, preferably 7 seconds or more, and more preferably 10 seconds or more.

Here, the adjustment of the time for exposure to air cooling can be performed by adjusting the transport time to the press mold, which is normally exposed to air cooling after being taken out from the heating furnace.

(Average cooling rate up to the temperature range of M _S point or less: 10 캜 / sec to 500 캜 / sec)

When the hot-formed steel sheet is hot-pressed and cooled to a temperature range lower than the M _S point (temperature at which the M _S point = martensitic transformation starts) at an average cooling rate of 10 ° C / sec to 500 ° C / It becomes difficult for transformation to occur. If the average cooling rate is less than 10 ° C / second, bainite transformation proceeds excessively. Alternatively, the pearlite transformation may occur and the area ratio of the martensite as the strengthened phase can not be ensured, making it difficult to secure a high tensile strength (for example, a tensile strength of 980 MPa or more) after hot pressing (after quenching). Alternatively, the austenite is stabilized and the bendability is lowered. Therefore, the average cooling rate in the temperature range is 10 DEG C / second or more, preferably 30 DEG C / second or more. On the other hand, if the average cooling rate exceeds 500 deg. C / second, it becomes very difficult to maintain cracks in the steel plate member, and the strength becomes unstable. Therefore, the average cooling rate is set to 500 ° C / second or less, preferably 200 ° C / second or less.

Here, the average cooling rate is a value obtained by dividing the difference between the temperature (° C.) and the Ms point (° C.) at which the hot press is performed by the time from the temperature (° C.) to the hot press (Ms point (° C.).

Further, since the heat generation due to the phase transformation is very large after reaching 400 캜 at the time of cooling, a sufficient cooling rate can not be ensured by the same cooling method as that in the temperature region of 400 캜 or more. Thus, from 400 ℃ than cooling to 400 ℃ it is necessary to strongly perform the cooling to the M _S point, specifically, it is preferably as described below. In the hot pressing method, cooling is usually achieved by a forced mold at ordinary temperature or several tens of degrees Celsius. Therefore, in order to change the cooling rate, the size of the mold may be changed to change the heat capacity. It is also possible to change the cooling rate by changing the mold material to a different metal (for example, copper). If the mold dimensions can not be changed, the cooling rate can also be changed by changing the cooling water using a water-cooled mold. It is also possible to change the cooling rate by changing the cooling rate by passing water through the grooves in the press during the press, raising the press during the press, and flowing water therebetween. Also, the cooling rate can be changed by changing the mold clearance and changing the contact area with the steel plate. For example, as means for changing the cooling rate at about 400 캜, the following means can be considered.

(1) Immediately after reaching 400 ° C, the mold is moved to a mold having a different heat capacity or a mold at room temperature to change the cooling rate;

(2) In the case of a water-cooling mold, the flow rate in the mold is changed immediately after reaching 400 캜, and the cooling rate is changed;

(3) Immediately after reaching 400 캜, water is flowed between the mold and the member, and the cooling rate is changed by changing the quantity.

In the present invention, the form of molding in the hot pressing method is not particularly limited. Examples include bending, drawing molding, stretch forming, hole expanding molding, and flange molding. It may be suitably selected in accordance with the type of the hot-formed steel plate member to be used. Typical examples of the hot-formed steel plate members include door guard bars, bumper reinforcements, and the like, which are automotive reinforcing parts as described above.

The hot-formed steel sheet member according to the present invention is characterized by being excellent in ductility and bending property. It is preferable that the total elongation of the tensile test is 12% or more as ductility to withstand practical use at that time. More preferably, the total elongation is 14% or more. As the bending property, it is preferable that the limit bending radius of the V-bending test in which the tip angle is 90 DEG is 5t or less.

The hot-formed steel sheet member after hot pressing may be shot blasted for the purpose of scaling down. The shot blasting process has an advantage of introducing a compressive stress to the surface, thereby preventing delayed fracture and improving fatigue strength.

In the above description, a hot press as a concrete form has been described as an example of hot forming. However, the present invention is also applicable to hot forming including means for cooling the steel plate immediately after or immediately after the forming, It is also applicable to molding.

(Example)

An embodiment of the present invention will be described. However, the present invention is not limited to the embodiments.

A steel sheet having the chemical composition shown in Table 1 was used as a sealant. These slabs were heated in the laboratory at 1250 ° C for 30 minutes and then subjected to finish rolling in the range of 880 ° C to 910 ° C except for No. 6 and No. 22, Lt; 0 > C for 5 seconds to obtain a hot-rolled steel sheet having a thickness of 2.6 mm. After the hot rolling, the steel sheet is cooled to a temperature of 420 DEG C or less and then slowly cooled to room temperature at 20 DEG C / hour to simulate a hot rolling process in which the steel sheet is rolled in a temperature range of 420 DEG C or less.

[Table 1]

The hot-rolled steel sheet thus obtained was mainly a composite structure of ferrite and martensite or ferrite and bainite.

On the other hand, the hot rolling conditions of the specimens No. 6 and No. 22 are different from the above-mentioned conditions. The blank No. 6 was held in the range of 740 ° C to 660 ° C for 2 seconds and cooled to room temperature by water spray cooling to simulate a hot rolling step of winding at room temperature. The disclosure material No. 22 simulates a hot-rolled winding process at 670 ° C by cooling to water at 670 ° C followed by gradual cooling to room temperature at 20 ° C / hour.

A part of the thus obtained hot-rolled steel sheet was subjected to cold-rolling so as to have a plate thickness of 1.6 mm after removing the scale by pickling, then heated at 780 ° C or higher and 900 ° C or lower and cooled at an average cooling rate of 30 ° C / Lt; / RTI > However, the disclosure material No. 27 was annealed under the conditions of heating at 920 占 폚 and cooling at an average cooling rate of 30 占 sec.

The area ratios of the ferrite and martensite and bainite of the steel plates provided for these hot presses were measured by EBSP (Electron Back Scatter Pattern) method. Concretely, a section in a rolling direction and in both directions perpendicular to the rolling direction are cut out from a steel sheet provided in a hot press. Each of these cut-out cross-sections was subjected to polishing and etching away. Subsequently, an IQ image (image quality: magnification: 2000 times) of EBSP of each section cut out was obtained by EBSP analysis using a scanning electron microscope (SEM) "Semanta 200 (manufactured by FEI)" with a scanning electron microscope (SEM) equipped with an EBSP detector . The area ratios of the ferrite, martensite and bainite are measured based on the IQ values of the respective EBSPs of the cross sections in the rolling direction and the both directions perpendicular to the rolling direction, Respectively. The EBSP analysis was performed under conditions of an acceleration voltage of 25 kV, a work distance of 15 mm, and a measurement step of 0.2 mu m.

The ferrite aspect ratio of the steel sheet to be provided to the hot press was measured as follows. Concretely, a section in a rolling direction and in both directions perpendicular to the rolling direction are cut out from a steel sheet provided in a hot press. Each of these cut-out cross-sections was subjected to polishing and etching away. Subsequently, an IQ image (image quality: magnification: 2000 times) of EBSP of each section cut out was obtained by EBSP analysis using a scanning electron microscope (SEM) "Semanta 200 (manufactured by FEI)" with a scanning electron microscope (SEM) equipped with an EBSP detector . The aspect ratios of the ferrite grains were measured as the average value of the 50 aspect ratios of the ferrite grains based on the IQ values of the EBSPs of the cross sections in both directions in the rolling direction and in the both directions perpendicular to the rolling direction. The EBSP analysis was performed under conditions of an acceleration voltage of 25 kV, a work distance of 15 mm, and a measurement step of 0.2 mu m.

Table 2 shows the steel structure of the steel sheet provided in the hot press.

[Table 2]

The obtained steel sheet was heated in a gas furnace under the conditions shown in Table 3 at an air / fuel ratio of 0.85. Subsequently, the heated steel sheet is taken out of the heating furnace, and the steel sheet is subjected to air cooling at the time from the air cooling time until the hot press to the time it is put into the mold, that is, from the end of heating until the start of hot forming Exposed time) was changed in accordance with the time shown in Table 3, and the hot press was performed using the flat mandrel. Subsequently, by after hot pressing, by cooling at an average cooling rate shown in while in contact with the steel sheet and the die table 3 to 150 ℃ than that M _S, and taken out from the mold was allowed to cool, was prepared as a steel sheet for various Disclosure (hereinafter, this disclosure Quot; hot-pressed steel sheet ").

The cooling was carried out by 1) cooling the periphery of the mold with cooling water, 2) cooling the mold at room temperature, or 3) cooling the mold with a cooling mold and cooling the periphery of the mold with cooling water. The average cooling rate up to 150 占 폚 was obtained by attaching a thermocouple to the end of the steel sheet provided for the hot press and measuring the temperature. The heating time is the time from when the temperature reached 720 占 폚 after charging into the furnace to when the furnace was taken out of the furnace. Here, in Examples 6, 18 and 25, various types of steel plates for public use were prepared by gas cooling at a predetermined cooling rate after a predetermined air cooling time in order to simulate the hot press conditions of changing the cooling rate with the grooved mold.

The area ratio of ferrite, tempered martensite, tempering bainite and martensite of a hot-pressed steel sheet is determined by EBSP (Electron Back Scatter Pattern: EBST) as well as the area ratio of ferrite, martensite and bainite, Electron beam backscattering pattern) method. These results are shown in Table 4.

The ferrite aspect ratio of the hot pressed steel sheet was measured in the same manner as the ferrite aspect ratio of the steel sheet provided in the hot press.

The mechanical properties of the hot pressed steel sheet were investigated as follows. These measurement results are also shown in Table 4.

First, JIS No. 5 tensile test specimens were taken from each steel sheet in the direction perpendicular to the rolling direction, and subjected to a tensile test to measure TS (tensile strength) and El (total elongation).

A rectangular specimen was taken from each steel sheet so that the bending ridgeline was perpendicular to the rolling direction and the one side was mechanically polished to prepare a bend test piece having a thickness of 1 mm, a width of 30 mm and a length of 60 mm. 90 °, and a tip radius of 5 mm, 4 mm, and 3 mm, respectively, to evaluate the bendability. Further, at the time of the test, the ground surface was made to be the inside of the bend. The surface of the bent portion after the test was visually observed and evaluated by the following evaluation criteria.

- Evaluation criteria of bendability -

A: After the V-bend test with a tip radius of 4 mm, no cracks are seen.

B: After the V-bending test with a tip radius of 4 mm, fine cracking or necking is observed.

C: After the V-bending test with a tip radius of 4 mm, cracks are seen.

D: After the V-bending test with a tip radius of 5 mm, cracks are seen.

The steel sheet produced in this example is not subjected to hot pressing by a metal mold but is subjected to the same thermal history as that of the hot pressed steel sheet member so that the mechanical properties of the steel sheet are substantially the same as those of hot pressed steel sheet members having the same thermal history Do.

In addition, numerical values underlined in Tables 1 to 4 indicate that the contents, conditions, or mechanical characteristics indicated by the numerical values are outside the scope of the present invention.

[Table 3]

[Table 4]

The specimens No. 1, 3, 5, 6, 9, 10, 11, 13, 15, 17, 19, 21, 22, 24, 27, 28, 29, 31, The steel plate member of the present invention satisfying all the conditions of the present invention, that is, the hot pressed steel plate member. All of the hot-pressed steel sheet members of the present invention examples are high in tensile strength of 980 MPa or more, excellent in ductility, and excellent in bending property while being hot-formed.

On the other hand, in No. 2, since the heating temperature of the steel sheet exceeds the upper limit of the range specified in the present invention, a desired structure can not be obtained and ductility and bendability are bad.

The disclosure No. 4 had poor ductility because the Si content was below the lower limit of the range specified in the present invention.

In the disclosure item No. 7, the steel sheet and the hot-pressed steel sheet member to be provided in the hot press had no structure specified in the present invention, so that ductility and bendability were bad.

Specification No. 8 can not obtain a desired structure for a steel sheet and a hot-pressed steel sheet member to be provided in a hot press, resulting in poor ductility and bendability.

In the case of Specified Substance No. 12, the steel sheet and the hot-pressed steel plate member provided in the hot press have a C content exceeding the upper limit of the range specified in the present invention and do not have the structure specified in the present invention, It was bad.

In Publication No. 14, a desired structure could not be obtained for a steel sheet and a hot-pressed steel sheet member provided in a hot press, and ductility and bendability were bad.

Since the air-cooling time, the heating temperature, and the average cooling rate are out of the ranges specified in the present invention, the desired materials can not be obtained for the hot-pressed steel plate members, I could not get the strength.

Since the average cooling rate deviates from the range specified in the present invention, the disclosure material No. 18 can not obtain a desired structure for the hot-pressed steel plate member, and has poor bendability.

Since the Mn content is lower than the lower limit of the range specified in the present invention, and the steel sheet and the hot-pressed steel sheet member provided in the hot press have no structure specified in the present invention, the target tensile strength And the bending property was bad.

In Publication No. 26, the steel sheet and the hot-pressed steel sheet member provided in the hot press had no structure specified in the present invention, so that the bendability was bad.

As for the disclosure material No. 30, the target tensile strength could not be obtained because the C content fell below the lower limit of the range specified in the present invention.

Since the air cooling time is outside the range specified in the present invention, the disclosure material No. 32 can not obtain a desired structure for the hot-pressed steel plate member, and has poor bendability.

In addition, the disclosure material No. 34 has a low tensile strength and poor ductility because the steel sheet and the hot-pressed steel sheet member provided in the hot press have no structure prescribed in the present invention.

The disclosure of Japanese Patent Application No. 2013-247814 is also incorporated herein by reference in its entirety.

All publications, patent applications, and technical specifications described in this specification are herein incorporated by reference to the same extent as if each individual document, patent application, and technical specification were specifically and individually stated to be incorporated by reference.

Claims (11)

The steel sheet contains 0.100% to 0.340% of C, 0.50% to 2.00% of Si, 1.00% to 3.00% of Mn, 0.050% or less of P, 0.0100% or less of S and 0.001% to 1.000% of sol. N: 0.0100% or less, and having a chemical composition consisting of the remaining Fe and impurities,
Ferrites, at least one of tempering martensite and tempering bainite, and a martensite-containing steel structure, the ferrites being 5% to 50%, the tempering martensite and the tempering bainite being 20% to 70% A steel sheet member having a steel structure of 90% or more in total, and 0% to 5% in retained austenite: martensite: 25% to 75%, ferrite, tempering martensite, tempering bainite and martensite. The method according to claim 1,
Wherein the above chemical composition is not more than 1.00% by mass, not more than 1.000% by mass, not more than 1.000% by mass, not more than 0.000% by mass, And Ni: 1.000% or less. 3. The method according to claim 1 or 2,
Wherein the chemical composition contains, in mass%, B: not more than 0.0025%, instead of a part of Fe. 4. The method according to any one of claims 1 to 3,
Wherein the above chemical composition is at least one selected from the group consisting of 0.0100% or less of Ca, 0.0100% or less of Mg, 0.0100% or less of REM and 0.0100% or less of Zr in mass% Wherein the hot-formed steel sheet member comprises: 5. The method according to any one of claims 1 to 4,
Wherein the chemical composition contains 0.0% or less of Bi by mass%, instead of a part of Fe. The steel sheet contains 0.100% to 0.340% of C, 0.50% to 2.00% of Si, 1.00% to 3.00% of Mn, 0.050% or less of P, 0.0100% or less of S and 0.001% to 1.000% of sol. N: 0.0100% or less, and having a chemical composition consisting of the remaining Fe and impurities,
Ferrites having an aspect ratio of not more than 2.0 and ferrites having at least one of martensite and bainite and having an area percentage of 5% to 50%, martensite and bainite: 45% to 90% Martensite and bainite: a steel sheet having a steel structure of 90% or more in total. The method according to claim 6,
Wherein the above chemical composition is not more than 1.00% by mass, not more than 1.000% by mass, not more than 1.000% by mass, not more than 0.000% by mass, And Ni: 1.000% or less, based on the total weight of the hot-rolled steel sheet. 8. The method according to claim 6 or 7,
Wherein the above chemical composition contains, in mass%, B: not more than 0.0025%, instead of a part of Fe. 9. The method according to any one of claims 6 to 8,
Wherein the above chemical composition is at least one selected from the group consisting of 0.0100% or less of Ca, 0.0100% or less of Mg, 0.0100% or less of REM and 0.0100% or less of Zr in mass% A steel sheet for hot forming. 10. The method according to any one of claims 6 to 9,
Wherein said chemical composition contains 0.0100% or less of Bi by mass%, instead of a part of Fe. Claim 6 to claim 10, wherein either the one of the preceding for hot forming steel sheet according to, and heated to a temperature range of less than 720 ℃ Ac ₃ point, the steel sheet is exposed to the air cooling in from the end of the heating to the start of the hot forming of the by the time for three seconds to about 20 seconds and subjected to hot molding, 10 ℃ / sec to 500 ℃ / sec average cooling rate to a method of producing a hot-formed steel plate member for cooling to a temperature range of more than M _S point.

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