RU2625374C1 - Hot-molded component of steel sheet and method of its manufacture and steel sheet for hot moulding - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: sheet has the following chemical composition, wt %: 0.100 to 0.340 carbon, 0.50 to 2.00 silicon, 1.00 to 3.00 manganese, 0.050 or less phosphorus, 0.0100 or less sulfur, 0.001 to 1.000 soluble aluminium, 0.0100 or less nitrogen, the rest is iron and impurities. The steel structure of the obtained hot-stamped component contains ferrite, at least one of the phases - tempered martensite or tempered bainite, and martensite. The share of the ferrite area is from 5 to 50%, the total share of the tempered martensite and tempered bainite is from 20 to 70%, the share of martensite area is from 25 to 75%, and the total share of the area of the ferrite, tempered martensite, tempered bainite and martensite is 90% or more, and the share of residual austenite is from 0 to 5%.

EFFECT: produced hot-stamped component has a high time resistance, high plasticity and flexibility after hot stamping.

5 cl, 1 dwg, 4 tbl

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a hot-formed component of a steel sheet intended for use, for example, as a structural member of a machine, such as a structural member of a car body, and to a method for manufacturing this component, as well as to a steel sheet for hot molding. In particular, the present invention relates to a hot-formed component of a steel sheet having excellent ductility and flexibility with high temporary resistance, and to a method for manufacturing this component, as well as to a steel sheet for hot forming of this component.

BACKGROUND

[0002] In recent years, in order to reduce vehicle weight, efforts have been made to increase the strength of the steel used for the body so as to reduce the design weight. As applied to sheet steel, which is widely used in automobiles, with an increase in the strength of this steel, its stampability decreases so that the production of a component of complex shape becomes difficult. In particular, a problem arises, for example, in that ductility decreases, causing fracture in the region of a high degree of processing, or in that the elastic aftereffect or deformation of the wall becomes significant, reducing dimensional accuracy. Therefore, it is not easy to manufacture such a component by stamping using a high strength steel sheet, especially having a temporary resistance in the range of 980 MPa or more. Although a high-strength steel sheet can be machined not by stamping, but by roll forming, however, this method is applicable only to a component having the same cross-sectional shape in the length direction.

[0003] On the other hand, by using a method known as “hot stamping”, by means of which a heated steel sheet is stamped, as disclosed in Patent Document 1, complex shaped components can be formed with high dimensional accuracy since the steel sheet becomes soft and highly plastic at high temperature. Additionally, if the steel sheet is heated to a single-phase austenitic region with subsequent quenching (hardening) in the stamp, it is possible to simultaneously increase the strength of the component due to martensitic transformation. Therefore, such a hot stamping method is an excellent molding method that achieves high component strength and at the same time excellent formability of the steel sheet.

[0004] Additionally, Patent Document 2 discloses a pre-stamping method followed by hardening, by which a steel sheet is pre-stamped at room temperature to give a predetermined shape, then heated to an austenitic region and then quenched in a die, achieving high component strength. Since such a method, which is an embodiment of hot stamping, can suppress deformation caused by thermal stress due to component restriction on the die side, it is an excellent method that guarantees increased component strength and at the same time high dimensional accuracy.

[0005] However, in recent years, plasticity has also been found to be in demand for hot stamped steel components, and conventional methods, such as those presented in Patent Document 1 or Patent Document 2, in which the steel structure is almost purely martensitic, have encountered the problem of not being able to satisfy such a requirement.

[0006] In this regard, Patent Document 3 discloses a hot stamped component of a steel sheet, which is said to have high strength and ductility due to the two-phase structure of ferrite and martensite, for which the steel sheet is heated to a temperature range of the two-phase region of ferrite and austenite, stamped, preserving the two-phase structure, and hardened in a stamp, as disclosed in Patent Document 3. However, under such a condition of heating the two-phase region, the steel structure can become heterogeneous, and the flexibility the toughness of the hot-pressed steel sheet components may deteriorate and the impact energy absorption characteristic may deteriorate sharply.

[0007] At the same time, Patent Document 4 discloses a hot stamped component from a steel sheet, which is made by heating a steel sheet having a structure containing 80% vol. or more martensite or bainite at the point of transformation of Ac ₁ or higher, followed by hardening of the component in the stamp, forming a structure containing 3-20% vol. residual austenite, 30-97% vol. tempered martensite or tempered bainite and 0-67% vol. martensite. It is claimed that this component has high strength and ductility.

[0008] In addition, Patent Document 5 discloses a high-strength stamped component that satisfies the conditions: the proportion of martensite area relative to the entire structure of the steel sheet is 10-85%; 25% or more of martensite is tempered martensite; the content of residual austenite is 5-40%; the proportion of the area of bainitic ferrite in bainite relative to the entire structure of the steel sheet is 5% or more; the total area fraction of martensite, the area area of residual austenite and the area area of bainitic ferrite in bainite is 65% or more relative to the entire structure of the steel sheet.

Patent Document 6 discloses a hot stamping steel sheet in which the total proportion of bainite and martensite is 80% or more over an area.

Additionally, Patent Document 7 discloses a hot stamping steel sheet in which the ferrite area fraction is 30% or more.

List of Contrasted Materials

Patent Literature

[0009]

Patent Document 1: UK Patent No. 1490535

Patent Document 2: Japanese Patent Application Laid-Open

(JP-A) No. H10-96031

Patent Document 3: JP-A No. 2010-65292

Patent Document 4: JP-A No. 2012-237066

Patent Document 5: International Publication No. WO2011 / 111333

Patent Document 6: JP-A No. 2013-185243

Patent Document 7: JP-A No. 2013-185248

SUMMARY OF THE INVENTION

Technical problem

[0010] As a result of research by the present inventors, it has become clear that the formation of a hot stamping steel sheet structure, which mainly contains bainite or martensite, not only increases the ductility of the hot stamped steel sheet component, as described, for example, in Patent Document 4, but also improves toughness. However, even with this adjustment of the structure of the component, it is not possible to avoid a deterioration in flexibility, and it is impossible to prevent a crack during bending of the component that appears in the warping region during the impact deformation. This disadvantage becomes apparent with a high temporary resistance of steel (for example, 980 MPa or more). Regarding the hot-stamped component of a steel sheet having a high temporary resistance (for example, a temporary resistance of 980 MPa or more) and excellent flexibility in addition to ductility, such a product itself has not yet been proposed, not to mention the development of production technology.

[0011] Similarly, even with a general overview of the generally hot-formed components from a steel sheet, including not only hot-stamped components, but also hot-formed components, it turns out that with respect to a hot-formed component from a steel sheet with high temporal resistance (for example, a temporary resistance of 980 MPa or more ) and excellent flexibility in addition to ductility, the product itself has not yet been proposed, not to mention the development of production technology.

[0012] A specific objective of the present invention is to provide a hot stamped component from a steel sheet that has high tensile strength as well as excellent ductility and flexibility after hot stamping, not available for the conventional prior art methods described above, and to provide a method for manufacturing such a component as well as a steel sheet for hot stamping of this component. In general, the present invention is also applicable to hot forming provided with means for cooling a steel sheet simultaneously during or immediately after molding in the same way as in the case of hot stamping. Therefore, the specific objective of the present invention is to provide a hot-formed component of a steel sheet having excellent ductility and flexibility with high temporary resistance after hot forming, and a method for manufacturing such a component, as well as a steel sheet for hot forming of this component.

Solution

[0013] The inventors have thoroughly investigated the improvement in ductility and flexibility of a hot-formed component from a steel sheet with high temporary resistance. As a result, the following new knowledge was obtained. Namely, it is necessary to use a steel sheet for hot molding, including a chemical composition in which silicon is effectively added to a certain amount of carbon and manganese, and also including a structure containing ferrite and at least one of the phases — martensite or bainite. Additionally, heat treatment conditions for hot forming, optimal steel sheet for hot forming, should be applied. By the means described above, unlike a conventional hot-formed component from a steel sheet, it is possible to form a steel structure comprising two phases that does not contain or contains no more than 5% (by area) of residual austenite, and also contains ferrite, at least one of tempered martensite or tempered bainite, and martensite in predetermined fractions of the area. As a result, new knowledge was gained that it is possible to make a hot-formed component from a steel sheet, which has excellent ductility and flexibility in the presence of high temporary resistance, if the specified chemical composition and structure of steel take place.

[0014] The present invention, based on this knowledge, is as follows.

[1] A hot-formed steel sheet component, comprising:

chemical composition containing (wt.%) from 0.100% to 0.340% carbon, from 0.50% to 2.00% silicon, from 1.00% to 3.00% manganese, 0.050% or less phosphorus, 0.0100 % or less sulfur, from 0.001% to 1,000% soluble aluminum, and 0.0100% or less nitrogen, the rest is iron and impurities; and

a steel structure containing ferrite, at least one of tempered martensite or tempered bainite, and martensite, with a ferrite area fraction of 5% to 50%, the total fraction of tempered martensite and tempered bainite from 20% to 70%, martensite area from 25% to 75%, the total area fraction of ferrite, tempered martensite, tempered bainite and martensite is 90% or more, and the fraction of the area of residual austenite is from 0% to 5%.

[0015]

[2] The hot-formed steel sheet component according to [1], wherein the chemical composition contains one element or two or more elements selected from the group consisting of (wt.%) 0.200% or less titanium, 0.200% or less niobium , 0.200% or less of vanadium, 1,000% or less of chromium, 1,000% or less of molybdenum, 1,000% or less of copper, and 1,000% or less of nickel, instead of part of the iron.

[0016]

[3] A hot-formed component from a steel sheet according to [1] or according to p. [2], in which the chemical composition contains 0.0025 wt.% or less of boron, instead of part of the iron.

[0017]

[4] The hot-formed component from a steel sheet according to any one of [1] to [3], wherein the chemical composition contains one element or two or more elements selected from the group consisting of (wt.%) 0.0100% or less calcium, 0.0100% or less magnesium, 0.0100% or less rare earth, and 0.0100% or less zirconium, instead of part of iron.

[0018]

[5] The hot-formed component of a steel sheet according to any one of [1] to [4], wherein the chemical composition contains 0.0100 wt.% Bismuth instead of a portion of iron.

[0019]

[6] A steel sheet for hot forming, comprising:

chemical composition containing (wt.%) from 0.100% to 0.340% carbon, from 0.50% to 2.00% silicon, from 1.00% to 3.00% manganese, 0.050% or less phosphorus, 0.0100 % or less sulfur, from 0.001% to 1,000% soluble aluminum, and 0.0100% or less nitrogen, the rest is iron and impurities; and

a steel structure containing ferrite with an aspect ratio of 2.0 or less, and at least one of the phases is martensite or bainite, where the fraction of the ferrite area is from 5% to 50%, the total fraction of the area of martensite and bainite is from 45% to 90% , and the total area fraction of ferrite, martensite, and bainite is 90% or more.

[0020]

[7] The hot forming steel sheet according to [6], wherein the chemical composition contains one element or two or more elements selected from the group consisting of (wt.%) 0.200% or less titanium, 0.200% or less niobium , 0.200% or less of vanadium, 1,000% or less of chromium, 1,000% or less of molybdenum, 1,000% or less of copper, and 1,000% or less of nickel, instead of part of the iron.

[0021]

[8] Steel sheet for hot forming according to [6] or p. [7], in which the chemical composition contains 0.0025 wt.% or less of boron instead of part of the iron.

[0022]

[9] A hot forming steel sheet according to any one of [6] to [8], wherein the chemical composition contains one element or two or more elements selected from the group consisting of (wt.%) 0.0100% or less calcium, 0.0100% or less magnesium, 0.0100% or less rare earth, and 0.0100% or less zirconium, instead of part of iron.

[0023]

[10] A hot-formed steel sheet according to any one of [6] to [9], wherein the chemical composition contains 0.0100 wt.% Bismuth instead of a portion of iron.

[0024]

[11] A method of manufacturing a hot-formed component from a steel sheet, comprising: heating a steel sheet for hot molding, corresponding to any one of [6] to [10], to a temperature in the range of 720 ° C. or higher, but below the Ac ₃ point; performing hot forming in a period of time from 3 s to 20 s, during which the steel sheet is subjected to air cooling from the end of heating to the start of hot forming; and cooling to a temperature not higher than the Ms point at an average rate of 10 ° C / s to 500 ° C / s.

Beneficial effects of the invention

[0025] Thanks to the present invention, a technologically valuable effect is achieved in that a hot-formed component of a steel sheet having, after hot molding, high tensile strength and excellent ductility, as well as flexibility, can finally be put into practical use. The hot-formed steel sheet component in accordance with the present invention exhibits excellent collision characteristics so that this component can absorb shock due to bending deformation even in a collision causing the most severe plastic deformation. Therefore, the hot-formed component from a steel sheet in accordance with the present invention is particularly suitable for manufacturing a component of a car body structure, but, of course, this component can be used for other applications, for example, as a component of a machine structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]

[Fig. 1] Fig. 1 is a photograph showing an example of a steel structure in accordance with the present invention.

DESCRIPTION OF EMBODIMENTS

[0027] Next, reasons for limiting each range introduced in accordance with the present invention will be described. In this case, hot molding will be described below using hot stamping as an example, which is a specific embodiment of hot molding. The numerical range expressed as “from x to y” includes the x and y values in this range as the minimum and maximum values, respectively.

[0028] 1. Chemical composition

First of all, the reasons for the aforementioned chemical composition of a hot-formed component from a steel sheet in accordance with the present invention (hereinafter also referred to simply as “a component from a steel sheet”) and a hot-formed steel sheet in accordance with the present invention (hereinafter also described) will be described. simply referred to as "steel sheet"). In the following description, “%”, representing the content of each element of the alloy, means “% by weight” unless otherwise indicated.

[0029] (Carbon from 0.100% to 0.340%)

Carbon is a very important element that increases the hardenability of steel and mainly determines the strength after hot stamping (after hardening). With a carbon content of less than 0.100%, it becomes difficult to guarantee high tensile strength (for example, tensile strength of 980 MPa or more) after hot stamping (after quenching). Therefore, the carbon content should be 0.100% or more, and preferably 0.120% or more. At the same time, when the carbon content is more than 0.340%, martensite after hot stamping (after hardening) becomes hard so that not only does the flexibility become noticeable, but ductility also decreases. Therefore, the carbon content should be 0.340% or less. From a weldability point of view, the carbon content is preferably 0.300% or less, and even more preferably 0.280% or less.

[0030] (Silicon from 0.50% to 2.00%)

Silicon is a very effective element for increasing the ductility of steel heated to the temperature range of two phases - ferrite and austenite, and guaranteeing stable strength after hot stamping (after quenching). With a silicon content of less than 0.50%, these effects are difficult to obtain. Therefore, the silicon content should be 0.50% or more. From the viewpoint of improving weldability, the silicon content is preferably 0.70% or more, and even more preferably 1.10% or more. At the same time, when the silicon content is more than 2.00%, the above effect is saturated, which is economically disadvantageous and, moreover, often defects in the coating occur due to a noticeable decrease in the wettability of the coating. Therefore, the silicon content should be 2.00% or less. From the point of view of suppressing surface defects in a hot-formed component from a steel sheet, the silicon content is preferably 1.80% or less, and even more preferably 1.50% or less.

[0031] (Manganese from 1.00% to 3.00%)

Manganese is a very effective element for improving the hardenability of steel and guaranteeing strength after hot stamping (after hardening). However, when the manganese content is less than 1.00%, it not only becomes difficult to guarantee high tensile strength (for example, temporary resistance of 980 MPa or more) after hot stamping (after quenching), but also the flexibility may decrease. Therefore, the manganese content should be 1.00% or more. To guarantee a more stable specified effect, the manganese content is preferably 1.10% or more, and even more preferably 1.20% or more. Moreover, when the manganese content exceeds 3.00%, an explicit zone appears in the structure of the steel after hot stamping (after quenching), caused by segregation of manganese, which reduces the toughness and significantly impairs the collision characteristics. Therefore, the manganese content should be 3.00% or less. From the point of view of the performance of hot rolling and cold rolling, the manganese content is preferably 2.50% or less, and even more preferably 2.40% or less.

[0032] By setting the carbon, silicon and manganese values in the above ranges, the hot-formed steel sheet structure can be formed into a two-phase structure containing ferrite and at least one of the phases — martensite or bainite. Additionally, by setting the heating mode during the hot stamping process in accordance with the present invention, the structure of the hot-formed component from the steel sheet can be formed into the desired two-phase structure.

[0033] (Phosphorus 0.050% or less)

Although phosphorus is usually an impurity contained in steel, it acts so as to increase the strength of the steel sheet by hardening the solid solution; therefore, it can be added in a noticeable amount. However, when the phosphorus content exceeds 0.050%, a deterioration in weldability becomes noticeable. Therefore, the phosphorus content should be 0.050% or less. The phosphorus content is preferably 0.018% or less. To guarantee the above effect, the phosphorus content is preferably 0.003% or more.

[0034] (Sulfur 0.0100% or less)

Sulfur is an impurity contained in steel, and its content, from the point of view of weldability, is preferably as low as possible. When the sulfur content exceeds 0.0100%, a deterioration in weldability becomes noticeable. Therefore, the sulfur content should be 0.0100% or less. The sulfur content is preferably 0.0030% or less, and even more preferably 0.0015% or less. In terms of desulfurization costs, the sulfur content is preferably 0.0006% or more.

[0035] (Soluble aluminum from 0.001% to 1,000%)

Aluminum is an element that improves the quality of the strength of steel due to deoxidation. When the soluble aluminum content is less than 0.001%, it becomes difficult to obtain this effect. Therefore, the soluble aluminum content should be 0.001% or more, and preferably 0.05% or more. At the same time, when the soluble aluminum content exceeds 1,000%, a deterioration in weldability becomes noticeable, and the number of inclusions of the oxide type also increases, so that a deterioration in surface quality also becomes noticeable. Therefore, the soluble aluminum content should be 1,000% or less, and preferably 0.080% or less. In this case, “soluble aluminum” means “acid-soluble aluminum,” which does not exist in the form of an oxide, such as Al ₂ O ₃ , and is soluble in acid.

[0036] (Nitrogen 0.0100% or less)

Nitrogen is an impurity contained in steel, and its content, from the point of view of weldability, is preferably as low as possible. When the nitrogen content exceeds 0.0100%, a deterioration in weldability becomes noticeable. Therefore, the nitrogen content should be 0.0100% or less, and more preferably 0.0060% or less. From the point of view of the costs of de-nitration, the nitrogen content is preferably 0.0020% or more.

[0037] [Impurities]

Impurities relate to ingredients contained in the starting material, or to ingredients that enter steel during the manufacturing process. These ingredients are not intentionally incorporated into a component from a steel sheet or into a hot formed steel sheet.

[0038] The chemical compositions of the steel sheet component and the hot formed steel sheet in accordance with the present invention may further comprise at least one of the elements described below.

[0039] (One element or two or more elements selected from the group consisting of 0.200% or less titanium, 0.200% or less niobium, 0.200% or less vanadium, 1,000% or less chromium, 1,000% or less molybdenum, 1,000% or less copper, and 1,000% or less nickel)

Each of these elements has the effect of guaranteeing consistently high strength after hot stamping (after hardening). Therefore, one element or two or more of these elements can be added to steel. However, with respect to titanium, niobium, and vanadium, when any of these elements are higher than 0.200%, not only is hot rolling and cold rolling difficult, but it can also be difficult to guarantee consistently high strength. Therefore, preferably, the titanium content, the niobium content and the vanadium content are 0.200% or less, respectively. With respect to chromium, when its content exceeds 1,000%, it can be difficult to guarantee high strength. Therefore, the chromium content is preferably 1,000% or less. Regarding molybdenum, when its content exceeds 1,000%, hot rolling and cold rolling can become difficult. Therefore, the molybdenum content is preferably 1,000% or less. Regarding copper and nickel, when the content of any of them exceeds 1,000%, the effect of increasing the strength reaches saturation, which is economically disadvantageous and additionally, hot rolling and cold rolling may become difficult. Therefore, the copper content and the nickel content are 1,000% or less, respectively.

[0040] To guarantee the effect of the action, preferably the content of at least one element from the group: 0.003% or more titanium, 0.003% or more niobium, 0.003% or more vanadium, 0.005% or more chromium, 0.005% or more molybdenum, 0.005% or more copper, 0.005% or more nickel. In other words, the lower limit of the titanium content is preferably 0.003%. The lower limit of the niobium content is preferably 0.003%. The lower limit of the vanadium content is preferably 0.003%. The lower limit of the chromium content is preferably 0.005%. The lower limit of the molybdenum content is preferably 0.005%. The lower limit of the copper content is preferably 0.005%. The lower limit of the nickel content is preferably 0.005%.

[0041] (Boron 0.0025% or less)

Boron is an element with the effect of increasing the toughness of steel. Therefore, boron can be added to steel. However, when boron is added in an amount in excess of 0.0025%, this may adversely affect the ferrite content in the structure of the hot-formed steel sheet, and the ductility and flexibility of the hot-formed component from the steel sheet may deteriorate. Therefore, the boron content should be 0.0025% or less. Additionally, to guarantee an effect, the boron content is preferably 0.0003% or more.

[0042] (One element or two or more elements selected from the group consisting of 0.0100% or less calcium, 0.0100% or less magnesium, 0.0100% or less rare earth, and 0.0100% or less zirconium)

Any of these elements has the effect of increasing toughness due to the contribution of inclusion control, especially in the microdispersion of inclusions. Therefore, you can enter one or two or more of these elements. However, when the content of any element exceeds 0.0100%, deterioration in surface quality may become noticeable. Therefore, the content of each of these elements is preferably 0.0100% or less. In order to obtain a more reliable effect, the content of at least one of these elements should preferably be 0.0003% or more. Namely, it is preferable that the lower limits of calcium, magnesium, rare earth, and zirconium are 0.0003%, respectively.

Moreover, the rare earth element represents at least one of the 17 elements, including scandium, yttrium and lanthanides. The content of the rare earth element means the total content of at least one of these elements. In industry, lanthanide is introduced in the form of mischmetal.

[0043] (Bismuth 0.0100% or less)

Bismuth is an element that has the effect of forming a homogeneous structure and increasing flexibility. Consequently, bismuth may be contained in steel. However, when bismuth is added in an amount of more than 0.0100%, the hot forming ability may be impaired so that hot rolling becomes difficult. Therefore, the bismuth content should preferably be 0.0100% or less. To obtain a more reliable effect, the bismuth content should preferably be 0.0003% or more.

[0044] 2. The steel structure of the hot-formed component of a steel sheet

Next, the steel structure of the hot-formed component of a steel sheet in accordance with the present invention will be described. The hot-formed steel sheet component in accordance with the present invention comprises a steel structure consisting of ferrite, at least one of tempered martensite or tempered bainite, and martensite in the following predefined area fractions. In other words, the steel structure may contain tempered martensite or tempered bainite, or both. Additionally, this structure does not contain residual austenite or contains no more than 5% of the area fraction.

[0045] Figure 1 shows an example of a steel structure in accordance with the present invention. The steel structure shown in FIG. 1 contains ferrite, tempered martensite, and martensite, but does not contain residual austenite.

[0046] (The proportion of the ferrite area from 5% to 50%)

When the ferrite area fraction is less than 5%, ductility and flexibility are reduced. Therefore, the area fraction of ferrite should be 5% or more, and preferably 15% or more. Moreover, when the fraction of the ferrite area exceeds 50%, flexibility decreases. Therefore, the area fraction of ferrite should be 50% or less, and preferably 40% or less.

[0047] The aspect ratio of ferrite is preferably 2.0 or less in terms of suppressing a decrease in flexibility. When the aspect ratio of ferrite exceeds 2.0, the anisotropy of ferrite (crystalline grain of ferrite) increases, and such ferrite may be a source of stress concentration and flexibility may decrease. Therefore, the aspect ratio of ferrite is preferably 2.0 or less, and more preferably 1.8 or less. Moreover, when the aspect ratio of ferrite approaches 1.0, the anisotropy of ferrite (crystalline grain of ferrite) decreases, therefore, the lower limit of this ratio for ferrite is preferably 1.0. However, from the point of view of increasing the yield strength of the component from the steel sheet after hot stamping, the lower limit of the aspect ratio of ferrite is preferably 1.2. The aspect ratio of ferrite is a value that was measured by the method described in detail in the Example below.

[0048] (The total fraction of the area of tempered martensite and tempered bainite is from 20% to 70%)

When the total area fraction of tempered martensite and tempered bainite is less than 20%, flexibility decreases. Therefore, this proportion should be 20% or more, and preferably 30% or more. Meanwhile, when the indicated fraction of the area of tempered martensite and tempered bainite exceeds 70%, ductility decreases. Therefore, the total area fraction of tempered martensite and tempered bainite should be 70% or less, and preferably 50% or less.

[0049] (The proportion of martensite area from 25% to 75%)

Due to the formation of martensite in steel, the strength after hot stamping (after quenching) can increase. When the martensite area fraction is less than 25%, it becomes difficult to guarantee high tensile strength (for example, tensile strength of 980 MPa or more) after hot stamping (after quenching). Therefore, the martensite area fraction should be 25% or more. Moreover, when this proportion exceeds 75%, ductility decreases. Therefore, the martensite area fraction should be 75% or less, and preferably 50% or less.

[0050] In this case, "martensite" means both martensite in the state after quenching and martensite after hardening during aging, which was formed during the hardening during aging of martensite in the state after quenching. Namely, “fraction of the area of martensite” means the total fraction of the area of martensite in the state after quenching and martensite after hardening during aging formed during the hardening during aging of martensite in the state after quenching.

[0051] (The total fraction of the area of ferrite, tempered martensite, tempered bainite, and martensite 90% or more)

The substantially hot formed steel sheet component of the present invention has a structure comprising ferrite, tempered martensite, tempered bainite, and martensite. However, depending on the production conditions, one or two or more of the group — bainite, residual austenite, cementite, or perlite — may be mixed in as a phase or structure different from the above. In this case, when the percentage of such a phase or structure other than ferrite, tempered martensite, tempered bainite, and martensite exceeds 10%, the intended characteristics may not be obtained due to the influence of this phase or structure. Therefore, a mixture of a phase or structure other than ferrite, tempered martensite, tempered bainite, and martensite should be 10% or less, and preferably 5% or less. Namely, the total area ratio of ferrite, tempered martensite, tempered bainite, and martensite should be 90% or more, and preferably 95% or more. The upper limit of the total area of ferrite, tempered martensite, tempered bainite, and martensite is 100%.

[0052] (The fraction of the area of residual austenite from 0% to 5%)

With respect to a phase or structure other than ferrite, tempered martensite, tempered bainite, and martensite, in the case where the area fraction of specifically mixed (residual) austenite is more than 5%, the flexibility is reduced. Therefore, residual austenite should not be present in the structure, or if it should be present, then in an amount (by area fraction) of 5% or less, and preferably 3% or less. The area fraction of residual austenite is most preferably 0%.

[0053] The fraction of the area of each phase and structure in the structure of the steel of the hot-formed component from the steel sheet is a quantity measured by a method exactly described in the Example below.

[0054] A steel sheet component in accordance with the present invention means a component made by hot forming from a steel sheet, including, for example, a steel sheet component formed by hot stamping. As a typical example, you can give a protective door trim, used as a component of the car body structure. Additionally, in relation to the car, you can specify, for example, the bumper stiffener. As a component of the machine structure, a hot-formed steel pipe for a building structure made of steel sheet as a starting material can be cited.

[0055] 3. Mechanical properties

Preferably, the hot-formed steel sheet component according to the present invention has a temporary resistance (TS) of 980 MPa or more, which is sufficient to help reduce the weight of the vehicle.

[0056] 4. Method of production

A preferred method for manufacturing a hot-formed component from a steel sheet in accordance with the present invention having the characteristics described above will be described below.

[0057] In order to obtain favorable ductility and flexibility of the hot-formed component from a steel sheet in accordance with the present invention while guaranteeing high tensile strength (for example, temporary resistance of 980 MPa or more), the structure of the steel after hot stamping (after quenching), as described above, it should be not a single-phase martensitic, but a two-phase structure, in which the fraction of the ferrite area is from 5% to 50%, the total fraction of the area of tempered martensite and tempered bainite is from 20% to 70%, the proportion loschadi martensite of 25% to 75%, the total area fraction of ferrite, tempered martensite, tempered bainite, and martensite of 90% or more, and the proportion of residual austenite area from 0% to 5%.

[0058] In order to obtain a steel structure for a hot-formed component from a steel sheet in accordance with the present invention, a steel sheet of the above chemical composition is preferably used as the steel sheet (hot-formed steel sheet), which is the starting material for hot forming. having a structure (two-phase structure) containing ferrite with an aspect ratio of 2.0 or less, and at least martensite or bainite, with a fraction of the ferrite area from 5% to 50%, the total fraction is flat spares of martensite and bainite from 45% to 90% and the total area of ferrite, martensite, and bainite 90% or more. Then, this steel sheet (steel sheet for hot molding) is heated to a temperature range of 720 ° C. or higher, but below Ac ₃ , after which hot stamping is carried out for a period of time from 3 s to 20 s, during which the steel sheet is subjected to air cooling from the end of heating to the start of hot stamping, and then cooled to a temperature not higher than the point Ms with an average cooling rate of 10 ° C / s to 500 ° C / s.

[0059] By hot stamping a steel sheet for hot forming, including a chemical composition and structure corresponding to the above described parameters, it is possible to obtain a hot-formed component from a steel sheet having the desired steel structure after hot stamping, high tensile strength (for example, a temporary resistance of 980 MPa or more), as well as excellent ductility and flexibility.

[0060] (Steel sheet structure for hot forming)

- Aspect ratio of ferrite 2.0 or less -

When the aspect ratio of ferrite is higher than 2.0, the aspect ratio of the ferrite component from the steel sheet after hot stamping can also exceed 2.0, and in addition, the fraction of the ferrite area in this component after hot stamping can drop below 5%, since the ferrite is excessively converted to austenite in heating process. When the aspect ratio of the ferrite component from the steel sheet exceeds 2.0, the anisotropy of ferrite (crystalline grain of ferrite) increases and becomes a source of stress concentration so that flexibility can be reduced. Therefore, the aspect ratio of ferrite should be 2.0 or less, and preferably 1.8 or less. Moreover, when the aspect ratio of ferrite approaches 1.0, the anisotropy of ferrite (crystalline grain of ferrite) is further reduced, therefore, the lower limit of the aspect ratio of ferrite is preferably 1.0. However, from the point of view of increasing the yield strength of the component from the steel sheet after hot stamping, the lower limit of the aspect ratio of ferrite is preferably 1.2.

[0061] The aspect ratio of ferrite is a quantity measured by a method exactly described in the Example below.

[0062]

- The fraction of the area of ferrite from 5% to 50% -

When the ferrite area fraction is less than 5%, the ferrite area fraction in the steel structure of the steel sheet component after hot stamping can also become less than 5%. Therefore, the area fraction of ferrite should be 5% or more, and preferably 15% or more. Similarly, when the ferrite area fraction is higher than 50%, the ferrite area fraction in the steel structure of the steel sheet component after hot stamping can also exceed 50%. Therefore, the area fraction of ferrite should be 50% or less, and preferably 45% or less.

[0063]

- The total area fraction of martensite and bainite is from 45% to 90% -

When the total area ratio of martensite and bainite is less than 45%, the total area ratio of tempered martensite and tempered bainite in the steel structure of a component from a steel sheet after hot stamping can become less than 20%. Additionally, the fraction of martensite area in the steel structure of this component after hot stamping can become less than 25%. Therefore, the total area fraction of martensite and bainite should be 45% or more, and preferably 50% or more. Similarly, when 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 component after hot stamping can also exceed 70%. Additionally, the fraction of martensite in the steel structure of this component after hot stamping can exceed 75%. Therefore, the total area fraction of martensite and bainite should be 90% or less, and preferably 80% or less.

[0064]

- The total area of ferrite, martensite, and bainite is 90% or more -

When the total area fraction of ferrite, martensite, and bainite is less than 90%, the impurity of a phase or structure other than ferrite, tempered martensite, tempered bainite, and martensite in the steel structure of a steel sheet component after hot stamping can exceed 10%. Especially, the fraction of residual austenite may exceed 5%. Therefore, the total area ratio of ferrite, martensite, and bainite should be 90% or more, and preferably 93% or more. The upper limit of the total fraction of ferrite, martensite, and bainite is 100%.

[0065] The fraction of the area of each phase and structure in the structure of the hot-formed steel sheet is a quantity measured by a method exactly described in the Example below.

[0066] (Production of hot-formed steel sheet)

The hot forming steel sheet may be any — hot rolled steel sheet, cold rolled steel sheet, and coated steel sheet. Examples of coated steel sheet include aluminized steel sheet, and galvanized steel sheet.

[0067] A hot-rolled steel sheet having the structure described above can be produced at the hot rolling stage, for which the carbon, silicon and manganese contents are set in the chemical composition ranges of the steel, finish rolling is completed at a temperature of 850 ° C to 930 ° C, and the finished product in the production line for 3 s or more in the range from 740 ° C to 660 ° C, and then the steel is wound onto a roll in the temperature range of 450 ° C or less. Additionally, a cold-rolled steel sheet having the structure described above can be produced by cold rolling, after which the product is heated in a production line from 780 ° C to 900 ° C, and then, at the annealing stage, the steel is cooled at an average speed of 10 ° C / s or more . After the production of the hot-rolled steel sheet or cold-rolled steel sheet, it is possible to produce a coated steel sheet by performing the well-known surface cladding treatment on the surface of this hot-rolled or cold-rolled steel sheet.

[0068] (Heating of the steel sheet for hot forming: heating to a temperature range of 720 ° C. or higher, but below the Ac ₃ point)

The hot forming steel sheet is heated to a temperature of 720 ° C. or higher, but below the Ac ₃ point. In this case, the Ac ₃ point (° C) is the temperature determined by the following empirical equation (i), which is the Ac ₃ point (° C) of a single-phase austenitic structure.

Ac ₃ = 910-203x (C ^0.5 ) -15.2xNi + 44.7xSi + 104xV + 31.5xMo-30xMn-11xCr - 20xCu + 700xP + 400 x sol.Al + 50xTi (i)

In this case, the element symbols in equation (i) represent the contents of the corresponding elements (wt.%) In the chemical composition of the steel sheet. In this case, equation (i) is derived by inserting the content value of the element, the content of which in the steel sheet is not equal to 0 (0 wt.%).

[0069] When the heating temperature is less than 720 ° C, austenization becomes insufficient and martensite is absent in the hot stamped steel sheet so that it is difficult to guarantee high tensile strength (for example, temporary resistance of 980 MPa or more) after hot stamping (after quenching). Therefore, the heating temperature should be 720 ° C or higher, and preferably 750 ° C or higher. At the same time, when the heating temperature is lower than the Ac ₃ point, even in the case of subsequent air cooling of the steel sheet, the fraction of martensite area in the steel structure after hot stamping (after quenching) exceeds 75%, and a decrease in ductility becomes noticeable. Therefore, the heating temperature should not be higher than the Ac ₃ point, and preferably not higher than Ac ₃ - 30 ° C.

[0070] Although there are no particular restrictions on the heating rate up to 720 ° C and the exposure time in this temperature range, it is preferable to maintain these parameters in the following ranges.

[0071] The average heating rate up to 720 ° C is preferably from 0.2 ° C / s to 100 ° C / s. When the average heating rate is 0.2 ° C / s or more, high productivity can be guaranteed. Additionally, if the average heating rate is 100 ° C / s or less, it is possible to easily control the heating temperature even in the case of heating in a conventional furnace.

[0072] The duration of heating in the temperature range of 720 ° C. or higher and below the Ac ₃ point is preferably from 2 minutes to 10 minutes Moreover, the heating time is the period of time from the moment when the temperature of the steel sheet reaches 720 ° C until the end of heating. In particular, in the case of using furnace heating, the moment of termination of heating means the moment when the steel sheet is removed from the heating furnace, and in the case of applying ohmic heating or induction heating, this is the moment the power is turned off. When the heating time is 2 minutes or more, a more stable strength can be formed after hot stamping (after quenching). If the exposure time is 10 minutes or less, then the structure of the component from the steel sheet may be more micrograin so that the toughness of this component can be increased.

[0073] (The time period from the end of heating to the start of hot stamping, during which the steel sheet is subjected to air cooling: from 3 s to 20 s)

Typically, a hot forming steel sheet after being heated in a heating furnace is transported to a hot forming press. In this case, for example, during extraction from the heating furnace or during transport to the press or when loaded into the press, the steel sheet may be partially subjected to air cooling. Since ferrite is formed anew or grows during such cooling, the duration of this cooling affects the temporary resistance. Therefore, to ensure consistently high strength after hot stamping (after hardening), the duration of air cooling should be preferably short. Especially when the time period from the end of heating to the start of hot stamping, during which the steel sheet is air-cooled, exceeds 20 s, the temporary resistance of the component from the steel sheet after hot stamping (after hardening) is reduced, or even if high temporary resistance is guaranteed (for example, temporary resistance 980 MPa or more), the carbon concentration in austenite becomes noticeable, and the transformed martensite region acquires a tendency to crack so that flexibility decreases. Therefore, the period from the end of heating to the start of hot stamping, during which the steel sheet is air-cooled, should be 20 s or less, and preferably 16 s or less. Meanwhile, austenite formed during heating falls out in the form of a needle. Since part of the precipitated austenite is converted into ferrite during cooling, and the shape of this austenite gradually changes from needle to spherical, when hot stamping (quenching) is carried out for a period of less than 3 s from the end of heating to the start of hot stamping and during this period steel the sheet is subjected to air cooling, causing martensitic transformation, the transformation region of the needle martensite forms a source of stress concentration, as a result of which not only decreases flexibility, but appears endentsiya the formation of residual austenite. Therefore, the period from the end of heating to the start of hot stamping, during which the steel sheet is air-cooled, should be 3 s or more, and preferably 7 s or more, and even more preferably 10 s or more. Moreover, the period of time during which air cooling is permissible can be controlled by changing the length of the transportation period from extraction from the heating furnace to the die, during which the steel is usually subjected to air cooling.

[0074] (Average cooling rate to the temperature of the point Ms or lower: from 10 ° C / s to 500 ° C / s)

When the hot forming steel sheet is hot stamped and cooled to a temperature range of Ms or lower (Ms is the martensitic transformation onset temperature) with an average cooling rate of 10 ° C / s to 500 ° C / s, the diffusion transformation is suppressed. If the average cooling rate is less than 10 ° C / s, then bainitic transformation proceeds excessively. Alternatively, pearlite transformation occurs so that the necessary fraction of the martensite area, which is the hardening phase, cannot be guaranteed and it becomes difficult to guarantee high tensile strength (for example, temporary resistance of 980 MPa or more) after hot stamping (after quenching). Austenite is likely to stabilize so that flexibility is reduced. Therefore, the average cooling rate in the indicated temperature range should be 10 ° C / s or more, and preferably 30 ° C / s or more. However, when the average cooling rate exceeds 500 ° C / s, it is very difficult to maintain the exposure of the component from the steel sheet and the strength becomes unstable. Therefore, this average cooling rate should be 500 ° C / s or less, and preferably 200 ° C / s or less.

The average cooling rate is the value obtained from dividing the difference between the temperature for hot stamping (° C) and the point Ms (° C) by the period of time required for cooling from this temperature to the point Ms.

[0075] In this case, during cooling after reaching 400 ° C, a very strong heat generation occurs due to phase transformation, therefore, an adequate cooling rate cannot be guaranteed using the same cooling method as used in a temperature range of not lower than 400 ° C. Therefore, from 400 ° C to the point Ms it is necessary to apply stronger cooling than cooling to 400 ° C, and it is preferable to perform it as described below. In the process of hot stamping, cooling is usually achieved through the use of a steel stamp at normal temperature or several tens of degrees Celsius. Therefore, to change the cooling rate, the dimensions of the stamp can be changed so as to change the heat capacity. Additionally, the cooling rate can also be changed by replacing the stamp material with another metal (e.g., copper). In the case when the stamp size cannot be changed, a water-cooled stamp can be used to change the cooling rate by controlling the flow of cooling water. In addition, using a stamp in which several grooves are pre-cut, it is possible to change the cooling rate by passing water through the grooves during the stamping process, or by increasing the pressure between the interruption of the stamping and the water supply. And finally, the cooling rate can also be adjusted by changing the clearance of the stamp so as to change the contact area with sheet steel. As means for changing the cooling rate before and after 400 ° C., for example, the following means are applicable.

[0076]

(1) Immediately after reaching 400 ° C, the component is transferred to a stamp having a different heat capacity, or to a stamp with a room temperature, changing the cooling rate.

(2) In the case of using a water-cooled stamp, immediately after reaching 400 ° C, the flow rate of the stamp is changed, changing the cooling rate.

(3) Immediately after reaching 400 ° C, water is passed between the die and the component, and the water flow rate is changed, changing the cooling rate.

[0077] In this application, there are no particular restrictions on the type of molding by hot stamping in accordance with the present invention. Examples of molding include bending, drawing, tightening, dispensing, flanging. A suitable type of stamping is selected according to the type of the proposed hot formed component from the steel sheet. Typical examples of such a component include, as described above, a door trim and a bumper stiffener, which are car reinforcing components.

[0078] The hot-formed steel sheet component in accordance with the present invention is characterized in that it has excellent ductility and flexibility. As the ductility corresponding to the practical use, the total elongation in the tensile test is preferably 12% or more, and even more preferably 14% or more. As flexibility, a limiting bending radius of 5t or less is preferred when tested for V-bend with an apex angle of 90 °.

[0079] The hot-formed component from the steel sheet after hot stamping can be shot blasted to remove scale. This treatment has the effect of creating compressive stresses on the surface and therefore gives an advantage in suppressing delayed fracture, as well as in increasing fatigue resistance.

[0080] Although hot forming is described in the above description by the example of hot forming, which is a specific embodiment, the present invention is similarly applicable to hot forming using means for cooling a steel sheet at the same time as or immediately after molding, for example, to roll forming.

Examples

[0081] Next, Examples in accordance with the present invention will be described, provided that the invention is in no way limited to these Examples.

[0082] Steel sheets having the chemical composition shown in table 1 were used as test materials. Each of these sheets was made by heating a slab cast in the laboratory to 1250 ° C for 30 min, which was then (except for test materials No. 6 and No. 22) hot rolled at a temperature of rolling end of 880 ° C to 910 ° C. after which this material was kept for 5 s in the range from 720 ° C to 680 ° C, obtaining a hot-rolled steel sheet with a thickness of 2.6 mm. After hot rolling, this sheet was cooled by irrigation with water to 420 ° C or lower, and then slowly cooled at a rate of 20 ° C / h to room temperature, simulating the stage of winding into a coil of hot rolled sheet steel at a temperature of 420 ° C or lower.

[0083]

Table 1 Steel Chemical composition, wt.% (The rest is iron and impurities) A _C3 (° C) Ms (° C) C Si Mn P S sol.Al * N Ti Nb V Cr Mo Cu Ni B Ca Mg REM ** Zr Bi A 0.167 1.20 1.51 0.014 0.0013 0,035 0.0042 0.150 858 429 B 0,200 0.25 1.42 0.014 0.0015 0,033 0.0046 0,021 0.0012 812 419 C 0.139 1.46 2.03 0.012 0,0009 0,029 0.0043 0,023 859 428 D 0.171 1.17 1.88 0.014 0.0013 0,037 0.0046 0.017 0,0009 847 418 E 0.202 1.23 1,64 0.013 0.0011 0,033 0.0040 0,033 0.0035 848 411 F 0.195 1.28 1.45 0.011 0.0010 0,040 0.0039 0.0010 858 421 G 0.152 1.18 1,59 0.011 0.0011 0,027 0.0042 0.015 0,0009 855 436 H 0.362 1.89 1.24 0.014 0.0012 0,041 0.0039 861 348 I 0.163 1,52 1.78 0.009 0.0012 0,033 0.0041 0.0020 862 424 J 0.155 1.21 1,63 0.011 0.0014 0,038 0.0042 0,025 858 434 K 0.158 1.22 1.66 0.013 0,0009 0,029 0.0046 855 431 L 0.134 1,02 0.81 0.011 0.0015 0,029 0.0043 876 471 M 0.183 1.35 1.72 0.009 0.0014 0,033 0.0042 0,100 0,100 0.0010 848 415 N 0.202 1.23 1,64 0.013 0.0011 0,033 0.0040 847 411 O 0.154 1.24 1.51 0.010 0.0012 0,041 0.0044 0,032 0.0010 865 438 P 0,084 1.12 2.44 0.013 0.0011 0,036 0.0048 852 441 Q 0.163 1.19 1,62 0.012 0.0011 0,036 0.0040 0,032 859 431 R 0.162 1.13 1.47 0.012 0,0008 0,039 0.0039 0,100 0.0010 862 435 * Soluble aluminum ** Rare earth element

[0084] The hot rolled steel sheet thus obtained had a complex structure of ferrite and martensite, or ferrite and bainite.

[0085] In this case, the hot rolling mode of the tested materials No. 6 and No. 22 differs from the mode indicated above. Imitating the stage of winding into a coil of hot rolled steel at room temperature, material No. 6 was kept in the range from 740 ° C to 660 ° C for 2 s, and then it was cooled by irrigation with water to room temperature. Imitating the stage of winding into a coil of hot rolled steel at 670 ° C, material No. 22 was cooled by irrigation with water to 670 ° C, after which it was slowly cooled at a rate of 20 ° C / h to room temperature.

[0086] A portion of the hot-rolled steel sheets obtained as described above was descaled by pickling and then cold rolled to a thickness of 1.6 mm, after which it was heated to a temperature of 780 ° C to 900 ° C and annealed by cooling with medium 30 ° C / s. However, test material No. 27 was heated to 920 ° C, and annealed, cooling at an average speed of 30 ° C / s.

[0087] The fractions of the area of ferrite, martensite, and bainite in the steel sheet intended for hot stamping were measured by the EBSP method (electron backscattering pattern). In particular, thin cross-section disks located along the rolling direction and perpendicular to this direction were cut from a steel sheet intended for hot stamping. Then, thin disks of the cross section were polished and etched with nital. Further, using a scanning electron microscope (SEM) equipped with an EBSP detector (trade name QUANTA 200, manufactured by FEI), an IQ image (image quality map: 2000x magnification) of EBSP was obtained for each thin cross-sectional disk using the EBSP method. Then, the corresponding area fractions of ferrite, martensite, and bainite were determined as the average value of the area fractions measured on the corresponding IQ images of EBSP samples cut along the rolling direction and samples cut perpendicular to the rolling direction. At the same time, the following conditions were set for EBSP analysis: accelerating voltage of 25 kV, working distance of 15 mm, scanning pitch of 0.2 μm.

[0088] Further, an aspect ratio of ferrite in a hot stamping steel sheet was measured as follows. In particular, thin cross-section disks located along the rolling direction and perpendicular to this direction were cut from a steel sheet intended for hot stamping. Then, thin disks of the cross section were polished and etched with nital. Further, using a scanning electron microscope (SEM) equipped with an EBSP detector (trade name QUANTA 200, manufactured by FEI), an IQ image (image quality map: 2000x magnification) of EBSP was obtained for each thin cross-sectional disk using the EBSP method. After that, the aspect ratio of ferrite was determined as the average value of aspect ratios of 50 crystalline ferrite grains measured with each EBSP IQ image for samples along the rolling direction and samples perpendicular to the rolling direction. At the same time, the following conditions were set for EBSP analysis: accelerating voltage of 25 kV, working distance of 15 mm, scanning pitch of 0.2 μm.

[0089] Structures of steel sheets for hot stamping are presented in table 2.

[0090]

table 2 No. of test material Steel Type of steel sheet Aspect ratio of ferrite The fraction of the area of ferrite (%) The proportion of martensite area (%) The proportion of bainite area (%) *one * 2 one A GKSL 1.4 36 64 0 64 one hundred 2 A GKSL 1.3 36 64 0 64 one hundred 3 A SLP 1.3 28 5 65 70 98 four B GKSL 1,5 35 0 60 60 95 5 C GKSL 1.4 29th 0 66 66 95 6 C GKSL 1,6 32 0 64 64 96 7 D GKSL 1.3 0 one hundred 0 one hundred one hundred 8 E GKSL 1.4 0 one hundred 0 one hundred one hundred 9 F GKSL 1,2 32 68 0 68 one hundred 10 F GKSL 1,2 32 68 0 68 one hundred eleven G GKSL 1.4 22 78 0 78 one hundred 12 H GKSL 1.3 0 5 78 83 83 13 I GKSL 1,6 37 0 56 56 93 fourteen I GKSL 2.1 40 60 0 60 one hundred fifteen I Xxxl 1.3 44 56 0 56 one hundred 16 I Xxxl 1.4 44 56 0 56 one hundred 17 J GKSL 1.3 35 0 60 60 95 eighteen J Xxxl 1,6 43 57 0 57 one hundred 19 K GKSL 1.4 34 66 0 66 one hundred twenty K GKSL 1.3 34 66 0 66 one hundred 21 K Xxxl 1,5 42 fifty 5 55 97 22 K SLP 1.7 36 6 52 58 94 23 L GKSL 1.4 54 37 3 40 94 24 M GKSL 1.3 33 67 0 67 one hundred 25 M GKSL 1.3 33 67 0 67 one hundred 26 N GKSL 1,2 45 0 32 32 77 27 N GKSL 1.4 41 59 0 59 one hundred 28 N GKSL 1,6 41 59 0 59 one hundred 29th O GKSL 1.4 39 0 55 55 94 thirty P GKSL 1,5 57 43 0 43 one hundred 31 Q GKSL 1.3 37 63 0 63 one hundred 32 Q Xxxl 1.9 21 79 0 79 one hundred 33 R GKSL 1.4 28 0 68 68 96 34 R Xxxl 1.4 9 91 0 91 one hundred Notes: 1 *: total area fraction of martensite and bainite (%);
2 *: total area fraction of ferrite, martensite, and bainite (%);
GKLS - hot rolled steel sheet; HKLS - cold rolled
steel sheet; SLP - coated steel sheet

[0091] The obtained steel sheets were heated in a gas furnace at an air / fuel ratio of 0.85 and under the conditions shown in Table 3. Then, the heated steel sheets were removed from the heating furnace and after a period of air cooling to hot stamping (time period from steel extraction from furnace before placing this steel in the stamp, namely, the period of time during which the steel sheet is subjected to air cooling between the end of heating and the start of hot stamping), set as shown in table 3, was subjected to hot stamping, using a steel plate in the form of a plate. Further, after hot stamping, the steel sheets were cooled at an average speed indicated in Table 3 to a temperature of 150 ° C, which was not higher than the point Ms, holding the sheets in contact with the stamp, and then they were removed from the stamp and left to cool, obtaining As a result, various test steel sheets (below test steel sheet) are referred to as “hot stamped steel sheet”.

[0092] Cooling was performed 1) after cooling the periphery of the mold with cooling water, 2) after cooling in the stamp, which was at normal temperature, or 3) after cooling in the heated stamp by cooling the periphery of the stamp with cooling water. The average cooling rate to 150 ° C was determined by reading the readings of a thermocouple attached to the edge of a steel sheet intended for hot stamping. Moreover, the duration of heating means the period of time from the moment when the temperature of the steel sheet after placing it in the furnace reached 720 ° C, until the sheet is removed from the furnace. At the same time, in Examples 6, 18 and 25, various test steel sheets were prepared with gas cooling at a predetermined speed after air cooling for a predetermined period of time, in order to simulate the hot stamping mode at which the cooling rate is controlled using a stamp with grooves.

[0093] The area fractions of ferrite, tempered martensite, tempered bainite, and martensite of hot-stamped steel sheet were measured similarly to the corresponding areas of ferrite, martensite, and bainite of hot-formed steel sheet by EBSP. The results are shown in Table 4. The aspect ratio of ferrite in the hot stamped steel sheet was measured similarly to the aspect ratio of ferrite in the hot stamped steel sheet.

[0094] The mechanical properties of the hot stamped steel sheet were determined by the following methods. The results are also shown in table 4.

[0095] A JIS prototype No. 5 for tensile testing was taken from each steel sheet in a direction normal to the rolling direction, and tensile tests were carried out, measuring TS (tensile strength) and EL (total elongation).

[0096] Additionally, a rectangular sample was cut from each steel sheet so that the fold line was normal to the rolling direction, and then one surface of the sample was machined to produce a bend test sample 1 mm thick, 30 mm wide, and 60 length mm The test specimen was subjected to a V-bend test with an apex angle of 90 ° and mandrel radii of 5 mm, 4 mm, 3 mm to assess flexibility. In this case, the treated surface of the sample was the inner surface of the bend. After testing, this surface was visually inspected and the results were evaluated according to the following evaluation criteria.

- Criteria for assessing flexibility -

A: After testing for V-bend by means of a mandrel with a radius of 4 mm, no cracks were noticed.

B: After testing for V-shaped bend by means of a mandrel with a radius of 4 mm, a microcrack or neck formation is noticed.

C: After testing for V-shaped bend, a crack is seen by means of a mandrel with a radius of 4 mm.

D: After testing for V-shaped bend by means of a mandrel with a radius of 5 mm, a crack is noticed.

[0097] The steel sheets manufactured in this Example were not hot stamped in the stamp, however, their thermal history was similar to the thermal history of the hot stamped component from the steel sheet. Therefore, the mechanical properties of the steel sheet were almost the same as the hot stamped component of the steel sheet, which has the same thermal history.

[0098] In Tables 1-4, underlined values mean that the content, condition, or mechanical property expressed by that value is outside the scope of the present invention.

[0099]

Table 3 No. of test material The heating rate from room temperature to 720 ° C (° C / s) Heating temperature
(° C) Duration of heating (min) Duration of air cooling (s) Average cooling rate to Ms or lower (° C / s) one 12 800 5 10 70 2 12 900 four fifteen 70 3 12 800 5 10 70 four 12 775 5 10 70 5 12 800 5 10 70 6 12 800 5 10 fifteen 7 12 800 5 10 70 8 12 790 5 10 70 9 12 800 5 10 70 10 12 800 5 10 400 eleven 12 800 5 10 70 12 12 790 5 10 70 13 12 800 5 10 70 fourteen 12 840 5 10 70 fifteen 12 800 5 10 70 16 12 820 5 25 70 17 12 800 5 10 70 eighteen 12 800 5 10 8 19 12 800 5 10 70 twenty 12 680 5 10 70 21 12 800 5 10 70 22 12 800 5 10 70 23 12 800 5 10 70 24 12 800 5 10 70 25 12 800 5 10 5 26 12 800 5 10 70 27 12 775 5 5 70 28 12 775 5 17 70 29th 12 800 5 10 70 thirty 12 800 5 10 70 31 12 800 5 10 70 32 12 770 8 one 70 33 12 800 5 10 70 34 12 740 5 10 70

[0100]

Table 4 Test Material Number Steel structure and area share (%) Steel structure Mechanical properties Fer rit Released Marten Sieve Released bainite Martin sit Tempered austenite * 3 *four Aspect ratio of ferrite TS
(MPa) Ei
(%) Flexibility one 25 39 0 36 0 39 one hundred 1.4 1174 fifteen A I / E 2 eighteen 0 0 82 0 0 one hundred 1.3 1275 10 D C / e 3 fifteen 2 fifty 33 0 52 one hundred 1.3 1136 fourteen A I / E four fifteen 0 47 37 0 47 99 1.4 1102 eleven A C / e 5 22 0 46 32 0 46 one hundred 1.4 1089 17 A I / E 6 23 0 40 33 four 40 96 1,5 1027 16 B I / E 7 0 58 0 42 0 58 one hundred 1.3 1245 9 D C / e 8 0 62 0 38 0 62 one hundred 1.4 1305 eleven D C / e 9 23 44 0 33 0 44 one hundred 1,2 1194 16 A I / E 10 22 45 0 33 0 45 one hundred 1,2 1243 fourteen B I / E eleven fifteen 49 0 36 0 49 one hundred 1.3 1142 17 A I / E 12 0 0 55 45 0 55 one hundred 1,2 1486 7 D C / e 13 26 0 38 36 0 38 one hundred 1,5 1186 fifteen A I / E fourteen four 28 0 68 0 28 one hundred 2.1 1198 8 D C / e fifteen 32 33 0 35 0 33 one hundred 1.3 1203 fourteen A I / E 16 43 21 0 23 2 21 87 1.3 968 fourteen A C / e 17 24 0 40 36 0 40 one hundred 1.3 1256 fourteen A I / E eighteen 31 31 0 31 7 37 93 1,5 1043 eighteen D C / e 19 22 43 0 35 0 43 one hundred 1.4 1178 16 A I / E twenty 34 66 0 0 0 66 one hundred 1.3 848 16 A C / e 21 33 28 3 36 0 31 one hundred 1.4 1146 fifteen A I / E 22 28 0 39 33 0 39 one hundred 1,5 1137 fourteen A I / E 23 52 eighteen 0 23 0 eighteen 93 1.4 925 fifteen D C / e 24 26 39 0 35 0 39 one hundred 1,2 1203 fourteen A I / E 25 thirty 37 0 four 3 37 71 1.3 878 24 A C / e 26 38 0 eighteen 44 0 eighteen one hundred 1,2 1278 13 D C / e 27 36 thirty 0 34 0 thirty one hundred 1.3 1175 13 C I / E 28 38 31 0 31 0 31 one hundred 1.3 1136 17 C I / E 29th 25 0 43 32 0 43 one hundred 1.3 1246 fifteen A I / E thirty 35 29th 0 36 0 29th one hundred 1.4 946 16 A C / e 31 24 42 0 34 0 42 one hundred 1.3 1143 fifteen A I / E 32 17 51 0 26 6 51 one hundred 1.8 1126 fourteen D C / e 33 eighteen 0 45 37 0 45 one hundred 1.3 1189 fourteen A I / E 34 7 85 0 8 0 85 one hundred 1.3 949 10 A C / e * 3 - the total fraction of the area of tempered martensite and tempered bainite (%); * 4 - the total fraction of the area of ferrite, tempered martensite, tempered bainite and martensite; I / E is an example of the present invention; C / E - a comparative example

 [0101] Test materials No. 1, 3, 5, 6, 9, 10, 11, 13, 15, 17, 19, 21, 22, 24, 27, 28, 29, 31, and 33 as examples of the present the invention in table 4 are components from a steel sheet according to the present invention, namely, hot stamped components from a steel sheet that satisfy all the requirements in accordance with the present invention. Any of these hot stamped components in the hot formed state has a temporary resistance of up to 980 MPa or more, excellent ductility, and also flexibility.

[0102] However, with respect to the test material No. 2, since the heating temperature of the steel sheet exceeded the upper limit of the range specified in accordance with the present invention, the required structure could not be obtained, and the ductility and flexibility were low.

[0103] Regarding test material No. 4, since the silicon content was below the lower limit of the range specified in accordance with the present invention, the ductility was low.

[0104] Regarding test material No. 7, since the steel sheet for hot stamping and the hot stamped component of the steel sheet did not have the structure specified in accordance with the present invention, the ductility and flexibility were low.

[0105] Regarding the test material No. 8, since the required structure of the steel sheet for hot stamping and the hot stamped component from the steel sheet was not obtained, the ductility and flexibility were low

[0106] Regarding test material No. 12, since the carbon content exceeded the upper limit of the range specified in accordance with the present invention, and the hot stamping steel sheet and the hot stamped steel sheet component did not have the structure specified in accordance with the present invention, then ductility and flexibility were low.

[0107] Regarding the test material No. 14, since the required structure of the steel sheet for hot stamping and the hot stamped component from the steel sheet was not obtained, the ductility and flexibility were low.

[0108] Regarding the materials tested No. 16, 20, and 25, since the duration of the air cooling, the heating temperature, and the average cooling rate are out of the respective ranges defined in accordance with the present invention, the desired structure of the hot stamped component from the steel sheet was not obtained, but The required temporary resistance was not obtained.

[0109] Regarding test material No. 18, since the average cooling rate was out of the range defined in accordance with the present invention, the required structure of the hot stamped component from the steel sheet was not obtained and the flexibility was low.

[0110] Regarding test material No. 23, since the manganese content was below the lower limit of the range limited in accordance with the present invention, and the hot stamping steel sheet and the hot stamped steel sheet component did not have the structure defined in accordance with the present invention , then the required temporary resistance was not obtained, and the flexibility was low.

[0111] Regarding test material No. 26, since the steel sheet for hot stamping and the hot stamped component of the steel sheet did not have the structure defined in accordance with the present invention, the flexibility was low.

[0112] Regarding test material No. 30, since the carbon content was below the lower limit of the range defined in accordance with the present invention, the required temporary resistance was not achieved.

[0113] Regarding test material No. 32, since the duration of the air cooling exceeded the range specified in accordance with the present invention, the required structure of the hot stamped component from the steel sheet was not obtained, and the flexibility was low.

[0114] Finally, with respect to test material No. 34, since the steel sheet for hot stamping and the hot stamped component of the steel sheet did not have the structure defined in accordance with the present invention, the tensile strength and ductility were low.

[0115] All descriptions of Japanese Patent Application No. 2013-247814 are incorporated herein by reference.

All literature, patent applications and technical standards mentioned in this document are also included in this document to the same extent as is provided in particular and separately with respect to a separate literature, patent application and technical standard in the sense that the same most should be included by reference.

Claims (9)

1. A hot stamped steel component comprising: chemical composition containing, wt.%: from 0.100 to 0.340 carbon, from 0.50 to 2.00 silicon, from 1.00 to 3.00 manganese, 0.050 or less phosphorus, 0.0100 or less sulfur, from 0.001 to 1,000 soluble aluminum, 0,0100 or less nitrogen, the rest is iron and impurities; and the structure of the steel containing ferrite, at least one of the phases is tempered martensite or tempered bainite, and martensite, whereby the fraction of the ferrite area is 5 to 50%, the total fraction of the tempered martensite and tempered bainite is 20 to 70%, the proportion of martensite from 25 to 75%, the total fraction of the area of ferrite, tempered martensite, tempered bainite and martensite is 90% or more, and the fraction of the area of residual austenite is from 0 to 5%. 2. The steel component according to claim 1, in which the chemical composition further comprises at least one element selected from the group consisting of, wt.%: 0.200 or less titanium, 0.200 or less niobium, 0.200 or less vanadium, 1,000 or less chromium, 1,000 or less molybdenum, 1,000 or less copper, 1,000 or less nickel, 0.0025 or less boron, 0.0100 or less calcium, 0.0100 or less magnesium, 0.0100 or less rare earth, 0.0100 or less zirconium and 0.0100 or less bismuth. 3. Steel sheet for hot stamping, containing: chemical composition containing, wt.%: from 0.100 to 0.340 carbon, from 0.50 to 2.00 silicon, from 1.00 to 3.00 manganese, 0.050 or less phosphorus, 0.0100 or less sulfur, from 0.001 to 1,000 soluble aluminum, 0,0100 or less nitrogen, the rest is iron and impurities; and a steel structure containing ferrite with an aspect ratio of 2.0 or less, and at least one of the phases is martensite or bainite, in which the proportion of the ferrite area is from 5 to 50%, the total fraction of the area of martensite and bainite is from 45 to 90%, and the total area fraction of ferrite, martensite and bainite is 90% or more. 4. The steel sheet for hot stamping according to claim 3, in which the chemical composition further comprises at least one element selected from the group consisting of, wt.%: 0.200 or less titanium, 0.200 or less niobium, 0.200 or less vanadium, 1,000 or less chromium, 1,000 or less molybdenum, 1,000 or less copper, 1,000 or less nickel, 0,0025 or less boron, 0,0100 or less calcium, 0,0100 or less magnesium, 0,0100 or less rare earth, 0 0100 or less zirconium and 0.0100 or less bismuth. 5. A method of manufacturing a hot stamped steel component, comprising heating a steel sheet for hot stamping according to claim 3 or 4 to a temperature in the range of 720 ° C or higher and below the Ac ₃ point, performing hot stamping in a period of time from 3 to 20 s, during which the steel sheet is subjected to air cooling from the end of heating to the beginning of hot stamping, and cooling to a temperature not higher than the point Ms at an average speed of 10 to 500 ° C / s.

Images