[0001]The present invention relates to a hot-rolled steel sheet, in particular, suitable for underbody part of an automobile to be molded into various shapes by press working or the like, to a high-strength hot-rolled steel sheet excellent in hole expandability.
Background technique
[0002]
　Hot-rolled steel sheet which is relatively inexpensive to manufacture, are widely used for various industrial devices such as automobiles. In recent years, from the viewpoint of carbon dioxide emissions regulations due to global warming, fuel efficiency of automobiles it has been required, for the purpose of vehicle weight reduction of the collision safety, high-strength hot-rolled steel sheet in automobile parts applied is expanding of.
　Needless to say, in the steel sheet is subjected to automotive parts, strength as well as press formability and weldability, etc., various workability required during component molding must be satisfied. For example, for suspension parts, with respect to the press molding, a very high frequency of use of stretch flangeability and burring. Therefore, the high-strength hot-rolled steel sheet is subjected to the same component is required to have excellent hole expandability. In addition, the underbody part, it is part is often necessary to avoid plastic deformation even when heavily loaded in secure. Therefore, the steel sheet is subjected to suspension components, a high yield ratio is required.
[0003]
　Generally, in a high-strength hot-rolled steel sheet, in order to achieve both high yield ratio and excellent hole expandability, the steel structure ferrite, bainitic ferrite, or a tissue consisting of either single phase such as bainite, Mn, the solid solution strengthening of Si or the like, and / or Ti, Nb, by precipitation strengthening due carbides or Cu of V or the like, and is considered to be uniformly enhance tissue.
[0004]
　For example, Patent Document 1, substantially consisting of a ferrite single-phase structure was characterized by uniformly finely disperse Ti carbide containing Mo, technology disclosed relates to high-strength hot-rolled steel sheet excellent in hole expandability It is. However, in the technique of Patent Document 1, since the addition of very expensive alloy elements that Mo is essential, not suitable for mass production from an economic point of view.
[0005]
　Patent Document 2, a predetermined amount of Mn, the Ti-added steel containing Si, a cooling of between hot-rolled to the winding by appropriately controlling, fine precipitation of TiC on which tissue was ferrite and bainite by a technique for improving the elongation and stretch flangeability of the high-strength hot-rolled steel sheet is disclosed. However, Patent Document 2, no consideration with respect to which is one yield ratio characteristics required hot-rolled steel sheet is applied to the underbody parts. Also, compared to the precipitation hardening ferritic bainite show low yield ratio, in the technique of Patent Document 2, which allows to contain 50% of bainite, it is inferred to be high yield ratio can not be maintained. In addition, ferrite defined in the patent literature 2 is unclear definition, not a polygonal ferrite, it is considered to contain a so-called bainitic ferrite and quasi-polygonal ferrite. The reason is that in Patent Document 2, also, as the first cooling stop temperature, because the polygonal ferrite is also allowable temperature range of 720 ° C. or less that do not produce enough. Bainitic ferrite and quasi-polygonal ferrite is an organization that shows a low yield ratio than the polygonal ferrite.
[0006]
　Patent Document 3, to reduce the Mn content, by further controlling the ratio of C precipitated as cementite, Ti added high-strength hot-rolled steel sheet having improved toughness and hole expandability are disclosed. However, assuming the application to the underbody parts in hot rolled steel sheet of Patent Document 3, a high yield ratio of, for example, 75% or more in the above high strength steel 540MPa has not been obtained.
　Further, Patent Document 4, to reduce the Mn and Si content, and to suppress the coarsening of TiC by adding a certain amount of Ti and B, and technology related to a high-strength hot-rolled steel sheet excellent in hole expandability It has been disclosed. However, B is so there is the effect of suppressing the recrystallization of austenite, the Ti composite addition with the same effect, the rolling load of hot rolling is significantly increased, causing an increase in the load of the hot rolling mill. Therefore, the technique of Patent Document 4, can cause operational troubles. Also, B is the strength of the final product just content varies several ppm is greatly changed, the steel for the content of B is essential is not suitable for mass production.
[0007]
　Patent Document 5, cooling the steel containing a large amount of Si, Mn and Ti in an appropriate cooling conditions, tissues were the granular Rabe yn tick ferrite single-phase structure, a high yield ratio and excellent hole expandability high strength hot rolled steel sheet having is disclosed. However, in the technique of Patent Document 5 since it is necessary to incorporate a large amount of Si, Mn in order to obtain a granular Rabe yn tick ferrite structure, which leads to an increase in alloy cost.
CITATION
Patent Literature
[0008]
Patent Document 1: Japanese Patent 2002-322540 JP
Patent Document 2: Japanese Patent 2007-009322 JP
Patent Document 3: Japanese Patent Laid-Open 10-287949 discloses
Patent Document 4: Japanese Patent 2012-026032 JP
Patent Document 5: Japanese Patent 2004-307919 JP
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　The present invention was made in view of the situation as described above. An object of the present invention is to provide a high-strength hot-rolled steel sheet having a high yield ratio and excellent hole expandability. Refers to high strength and tensile strength in the present invention (TS) is not less than 540 MPa.
Means for Solving the Problems
[0010]
　Ti is relatively inexpensive, and express significant precipitation strengthening at a content of trace. The present inventors have found that in order to achieve good hole expandability, and the assumption that the tissue of the hot-rolled steel sheet and polygonal ferrite mainly. Then, the present inventors have found that in polygonal ferrite principal structure having excellent hole expandability, for improving the strength, was studied to utilize precipitation strengthening of Ti. Further, the Ti-containing high-strength hot-rolled steel sheet Ti precipitates precipitated on the polygonal ferrite mainly tissues was conducted extensive studies on techniques for improving hole expandability. As a result, the following findings were obtained.
[0011]
　The present inventors have polygonal ferrite was measured individual ferrite grains micro hardness of the steel having a main body of tissue. As a result, it was found that significantly different for each particle hardness thereof is measured. Furthermore, by reducing the variation in the hardness of individual ferrite grains were found to be able to significantly improve the hole expandability.
[0012]
　Further, the present inventors, the inside grains of polygonal ferrite of hole expandability of subordinate samples were observed using a transmission electron microscope. As a result, it has been deposited Ti-based carbides anisometric shape elongated along a particular orientation of the ferrite is a number, which is found to have an adverse effect on the properties hole expansion. Conventionally, almost no reports of the shape of Ti carbides affect the properties hole expansion affects mechanisms hole expandability shape of Ti-containing carbide is not clear. However, compared to the Ti-containing carbide of equiaxed shape, Ti-based carbides anisometric shape is highly consistent with the ferrite matrix, it is estimated that a large matching strain is accumulated in the periphery thereof since this alignment strain that promotes the propagation of cracks during machining hole expansion, it is estimated that the hole expandability is deteriorated.
[0013]
　The present invention was made based on the above findings. It shows the summary below.
(1) hot-rolled steel sheet according to one embodiment of the present invention, the chemical composition, in mass%, C: 0.010% ~ 0.200 %, Si: 0.001% ~ 2.50%, Mn: 0 .001% ~ 1.50%, P: 0.050% or less, S: 0.010% or less, N: 0.0070% or less, Al: 0.001% ~ 0.50% , Ti: 0.050 % ~ 0.30%, V: 0 % ~ 0.50%, Nb: 0% ~ 0.090%, Cr: 0% ~ 0.50%, Ni: 0% ~ 0.50%, Cu: 0 ~ 0.50%, Mo: 0% ~ 0.50%, B: 0% ~ 0.0050%, Ca: 0% ~ 0.01%, Mg: 0% ~ 0.01%, Bi: 0% contains ~ 0.01%, balance being Fe and impurities, tissue, an area ratio, 80% or more polygonal ferrite, 5% martensite in total and austenite If, containing 5% or less of pearlite and cementite in total, the balance is not less than one selected from bainitic ferrite and bainite, any present from the central plane in the thickness direction within the range of ± 100 [mu] m when the 50 micro hardness standard deviation of the polygonal ferrite was σHV, Ti-containing carbide 5 × 10 to the ShigumaHV is 30 or less the polygonal intragranular ferrite 7 pieces / mm 2 exist or , 50% or more of the Ti-containing carbide, the aspect ratio is the ratio of the length of the long side to the length of the shorter side is less than 3, the tensile strength is not less than 540 MPa.
[0014]
(2) hot-rolled steel sheet according to the above (1), the chemical composition, in mass%, V: 0.010% ~ 0.50%, Nb: 0.001% ~ 0.090%, Cr: 0.001% ~ 0.50%, Ni: 0.001% ~ 0.50%, Cu: 0.001% ~ 0.50%, Mo: 0.001% ~ 0.50%, B: 0. it may contain one or more kinds selected from 0001% to 0.0050%.
[0015]
(3) hot-rolled steel sheet according to (1) or (2), the chemical composition, by mass%, Ca: 0.0001% ~ 0.01%, Mg: 0.0001% ~ 0.01 %, Bi: it may contain one or more kinds selected from 0.0001% to 0.01%.
[0016]
(4) hot-rolled steel sheet according to any one of the above (1) to (3) may have a galvanized layer on the surface.
Effect of the invention
[0017]
　According to this aspect of the present invention, it is possible to manufacture an inexpensive high-strength hot-rolled steel sheet having a high yield ratio and excellent hole expandability. Further, the steel sheet according to the above aspect of the invention, parts for automobiles, in particular stretch flangeability which is frequently used in such underbody part, has excellent hole expandability. Therefore, in particular, weight reduction of the vehicle body in the automotive field and collision contributes to safety.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
FIG. 1 is a schematic diagram showing an example of a processing pattern of the hot rolling.
FIG. 2 is a schematic diagram showing an example of a heat treatment pattern in the alloyed hot-dip galvanizing line used in Example 2.
Is a diagram illustrating an example of a micro-hardness distribution in FIG. 3A] polygonal ferrite measured in Example 1.
[FIG 3B] is a diagram showing an example of a micro-hardness distribution of the polygonal ferrite was measured in Example 1.
DESCRIPTION OF THE INVENTION
[0019]
　High strength hot rolled steel sheet according to an embodiment of the present invention below (hereinafter sometimes referred to as hot-rolled steel sheet according to the present embodiment) will be described in detail.
　Hot rolled steel sheet according to the present
embodiment, (a) the chemical composition, in mass%, C: 0.010% ~ 0.200 %, Si: 0.001% ~ 2.50%, Mn: 0.001 % ~ 1.50%, P: 0.050 % or less, S: 0.010% or less, N: 0.0070% or less, Al: 0.001% ~ 0.50% , Ti: 0.050% ~ containing 0.30%, further, V optionally: 0.50% or less, Nb: 0.090% or less, Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0. 50% or less, Mo: 0.50% or less, B: 0.0050% or less, Ca: 0.01% or less, Mg: 0.01% or less, Bi: 1 or more selected from 0.01% or less containing the balance is Fe and impurities,
(b) tissue, an area ratio, 80% or more polygonal ferrite, 5% or less in total And martensite and austenite, containing 5% or less of pearlite and cementite in total, the balance is not less one or more selected from bainitic ferrite and bainite,
100 [mu] m ± from the center plane (c) In the thickness direction 50 when the said polygonal σHV the standard deviation of the ferrite micro hardness any present in the range of, the σHV is 30 or less
(d) Ti-containing carbide in grains of the polygonal ferrite 5 × 10 7 cells / mm 2 occur more than 50% or more of the Ti-containing carbide, the aspect ratio is the ratio of the length of the long side to the length of the shorter side is less than 3,
the tensile (e) the intensity is greater than or equal to 540MPa.
[0020]

　will be described first reason for limiting the chemical composition of the hot rolled steel sheet according to the present embodiment. Hereinafter, defining a chemical composition "%" are all "mass%".
[0021]
　[C: 0.010% ~ 0.200%]
　C is an essential element for increasing the strength by precipitation strengthening or solid solution strengthening of the steel sheet. To obtain this effect, the C content is 0.010% or more. Preferably, 0.020% or more, more preferably 0.040% or more. On the other hand, tends cementite is formed with the generation of the polygonal ferrite and the C content is excessive is suppressed. Further, there is a tendency that the hardness difference between the crystal grains of polygonal ferrite increases. As a result, the hole expandability is deteriorated. Moreover, it weldability significantly degraded. Therefore, the C content is 0.200% or less. Preferably 0.130% or less, more preferably not more than 0.110%.
[0022]
　[Si: 0.001% ~
　2.50%] Si is a solid solution strengthening element, which is an element effective for increasing the strength of the steel sheet. To obtain this effect, the Si content is 0.001% or more. Preferably, 0.01% or more, more preferably 0.04% or more. On the other hand, when the Si content is excessive, the island-shaped scale is generated, the surface quality deteriorates. Therefore, the Si content is set to less 2.50%. Preferably 1.30% or less, more preferably 0.80%.
[0023]
[Mn: 0.001 Pasento ～ 1.50
　Pasento] Mn is an effective element to the improvement of the strength of the steel plate. Further, by fixing the S in the steel as MnS, it is an element inhibiting hot embrittlement due to the solid solution S. To obtain these effects, the Mn content is 0.001% or more. Preferably, 0.10% or more, more preferably 0.45% or more. On the other hand, the ferrite transformation from the austenite Mn content is excessive it is difficult to obtain a 80 area% or more polygonal ferrite is delayed, hole expandability is deteriorated. Therefore, the Mn content is set to 1.50% or less. Preferably 1.00% or less, more preferably 0.80%.
[0024]
[P: 0.050% or less]
　P is an element contained as an impurity deteriorates the weldability and toughness of the steel sheet. Therefore, P content is preferably small. However, since the above effect becomes remarkable when the P content exceeds 0.050% as a range deterioration is not noticeable in weldability and toughness, the P content to 0.050% or less. Preferably 0.020% or below, more preferably 0.010% or less.
[0025]
[S: 0.010% or less]
　S is an element contained as an impurity, and forms MnS in steel, deteriorating the hole expandability of the steel sheet. Therefore, S content is preferably small. However, since the above-described effects when the S content exceeds 0.010% is remarkable, as the range hole expandability deterioration is not noticeable, the S content to 0.010% or less. Preferably 0.0050% or less, more preferably 0.0020%.
[0026]
[N: 0.0070% or less]
　N is an element contained as an impurity, significantly degrade the hole expandability of the steel sheet to form a coarse nitrides in the steel. Therefore, N content is preferably small. However, since the above-described effects when the N content exceeds 0.0070 percent becomes remarkable, as the range hole expandability deterioration is not noticeable, the N content to less 0.0070%. Preferably is 0.0050% or less.
[0027]
[Al: 0.001% ~
　0.50%] Al is an element effective for deoxidation of steel. To obtain this effect, the Al content is 0.001% or more. On the other hand, the Al content exceeds 0.50%, the cost is increased, not only the effect is saturated. Therefore, the Al content to 0.50% or less. Preferably 0.20% or less, more preferably 0.10% or less.
[0028]
[Ti: 0.050% ~
　0.30%] Ti is an element which forms a carbide in the steel, uniformly precipitation strengthening of ferrite. Further, to reduce the amount of solute C by being precipitated as TiC, is also an element having an effect of inhibiting the precipitation of cementite degrading hole expandability. Therefore, it is particularly important element in the hot rolled steel sheet according to the present embodiment. Since the Ti content, the effect is not sufficient at less than 0.050%, the Ti content is 0.050% or more. Preferably 0.100% or more, more preferably, not less than 0.130%. On the other hand, if the Ti content exceeds 0.30%, leading to unnecessary costs increase with toughness is remarkably deteriorated. Therefore, the Ti content is set to 0.30% or less. Preferably 0.25% or less, more preferably 0.20% or less.
[0029]
　Hot rolled steel sheet according to the present embodiment contains the above chemical components, and the balance basically in that it consists of Fe and impurities. However, further, in order to enhance the strength and hole expandability, instead of a part of Fe, to the extent that the following are selected V, Nb, Cr, Ni, Cu, Mo, B, Ca, Mg, Bi, it may contain one or more that. However, since these elements do not necessarily have to be contained, the lower limit is 0%. Here, the impurities, in producing the steel industrially, refers to the components to be mixed ore raw material scraps and other factors.
[0030]
[V: 0.010% ~ 0.50%]
　V is an element which forms carbide in similarly in steel and Ti. Also, V is large solubility product in austenite than Ti, the high strength of the steel sheet is an effective element. Therefore, although it is expensive compared to Ti, it may be contained as needed. Since V content above effect is not sufficiently obtained is less than 0.010%, the case of obtaining the above effect, the V content is 0.010% or more. Preferably 0.070% or more, and more preferably not less than 0.140%. On the other hand, since the cost is increased when the V content is excessive, even if the inclusion of V, the V content is set to 0.50% or less.
[0031]
[Nb: 0.001% ~
　0.090%] Nb is a carbide is formed in the steel similarly to Ti, is an element effective in increasing the strength of the steel sheet. Therefore, although it is expensive compared to Ti, it may be contained as needed. The Nb content is less than 0.001%, the above effect is not sufficiently obtained, the case of obtaining the above effects, the Nb content is 0.001% or more. On the other hand, the plastic anisotropy of the steel sheet and the Nb content is excessive increases, hole expandability is deteriorated. Therefore, even if the inclusion of Nb, the Nb content is less 0.090%.
[0032]
[Cr: 0.001% ~ 0.50%] [Ni: 0.001% ~ 0.50%] [Cu: 0.001% ~ 0.50%] [Mo: 0.001% ~ 0.50 %] [B: 　0.0001% ~ 0.0050%] Cr, Ni, Cu, Mo, Bはいずれも鋼板の高強度化に有効な元素である.そのため必要に応じて単独で,または2種以上を複合で含有させてもよい.上記効果を得るには,それぞれ, Cr: 0.001%以上, Ni: 0.001%以上, Cu: 0.001%以上, Mo: 0.001%以上B: 0.0001%以上とする必要がある.一方,これらの元素は, Mnと同様に熱間圧延後のフェライト変態を遅延させる.そのため,含有量が過剰であると熱延鋼板の組織において面積率で80%以上のポリゴナルフェライトを得ることが困難と り,熱延鋼板の穴拡げ性が劣化する.従って,各元素を含有させる場合でも,その含有量をそれぞれ, Cr: 0.50%以下, Ni: 0.50%以下, Cu: 0.50以下%, Mo: 0.50%以下, B: 0.0050%以下とする.好ましくは, Cr: 0.20%以下, Ni: 0.20%以下, Cu: 0.20%以下, Mo:以下0.09%, B: 0.0040%以下である.

[0033]
[Ca:
0.0001% ~
0.01%] [Mg: 0.0001% ~ 0.01%] [Bi: 0.0001% ~
　0.01%] Ca and Mg are finely dispersed in the steel inclusions an element contributing to reduction, Bi is an element to reduce micro-segregation of substitutional alloying elements Mn, Si and the like in the steel. Each element is also so contribute to the hole expandability improvement of the steel sheet, alone, or two or more kinds may be contained in the composite as required. To obtain the above effect, it is necessary to contain 0.0001% or more respectively. On the other hand, since the ductility and the content of these elements is excessive to deteriorate, even if to be contained, the content of each element is 0.01%, respectively less.
[0034]

　Next, a description will be given reasons for limiting the structure of the hot rolled steel sheet according to the present embodiment.
[0035]
[Polygonal ferrite area ratio of 80% or more]
　polygonal ferrite is an effective organization in improving the hole expandability. The area ratio of polygonal ferrite to the hole expandability secure 80% or more. Preferably 90% or more, more preferably 95% or more. Area percentage of polygonal ferrite may be 100%, that is, hot rolled steel sheet according to the present embodiment may be a polygonal ferrite single phase.
[0036]
Total area fraction of martensite and austenite: 5% or less]
　martensite and more than 5%, the hole expandability area ratio in total austenite is remarkably deteriorated. Therefore the total area fraction of martensite and austenite is 5% or less. Preferably 2% or less. The% area ratio of the total is 0 (i.e., martensite and austenite is not contained) may be used. Further, the austenite here, a so-called retained austenite.
[0037]
Total area ratio of pearlite and cementite: 5% or less]
　area ratio of pearlite and cementite exceeds 5%, the hole expandability in total is remarkably deteriorated. Therefore, at most 5% area ratio of pearlite and cementite in total. Preferably 3% or less, more preferably 1% or less. The% area ratio of the total is 0 (i.e., pearlite and cementite is not contained) may be used.
[0038]
[Rest of Organization
　of the remainder other than the above tissue is at least one selected from the bainitic ferrite and bainite. However, when the total area ratio of the above tissues is 100%, bainitic ferrite and bainite is not included.
[0039]
　The above tissue samples were cut from the hot rolled steel sheet on which to appear tissue by etching, it can be identified from its structural photograph.
　Polygonal ferrite produced by diffusion mechanism does not have an internal structure in the grains, and the grain boundary is a straight line or arc. On the other hand, bainitic ferrite or bainite has an internal structure, and a grain boundary shape Ashukyura have distinctly different tissues and polygonal ferrite. Therefore, the polygonal ferrite, and bainite or bainitic ferrite, a structure photograph obtained by using an optical microscope after etching by nital, can be determined by the presence or absence of grain boundary shape and internal structure. The internal structure is not clearly appear, and the grain boundary shape Ashukyura - if shape structure (quasi polygonal ferrite) is present counts as bainitic ferrite.
　Further, it is possible to identify clearly the tissue since the cementite and pearlite are black etched.
　Further, using the Repera corroded samples, with respect to structural photograph obtained by an optical microscope, by performing image analysis, it is possible to calculate the total area ratio of retained austenite and martensite.
　In this embodiment, the tissue of the steel sheet shows a representative structure of the steel sheet is observed at the position of the sheet thickness 1/4 depth.
[0040]
[Optional 50 of polygonal ferrite present from the center plane in the range of ± 100 [mu] m in the thickness direction, the standard deviation of micro-hardness σHV: 30 below]
　As described above, the individual ferrite grains hardness of by reducing the variation, it is possible to remarkably improve the hole expandability of the hot-rolled steel sheet. Specifically, in the thickness direction, the center plane of any 50 of polygonal ferrite present in the range of ± 100 [mu] m from (including the thickness center portion of the steel sheet, the surface intersecting the thickness direction and vertical) Hardness It is measured (micro hardness), that when the micro-hardness standard deviation of the ShigumaHV, excellent hole expandability by 30 or less ShigumaHV is obtained. Therefore, to 30 or less ShigumaHV. Since the standard deviation is smaller the better, the lower limit of σHV is 0.
　The specific measuring method of σHV described below. The hardness measurement sample, the rolling direction cross-section of the steel sheet was mirror-polished, further subjected to chemical polishing with a colloidal silica to remove the surface layer of the working layer, and revealing the grain boundaries and then nital corrosion use things. Micro hardness, microhardness measuring device (trade name: FISCHERSCOPEHM2000 XYp) using a 50 polygonal ferrite randomly selected existing from the central plane in the thickness direction within the range of ± 100 [mu] m (each crystal grain) for the Vickers indenter quadrangular pyramid apex angle 136 °, the indentation is measured by pushing in the grains so as not to overlap the ferrite grain boundaries. The indentation load to 20N. 50 pieces of data obtained, the standard deviation σHV micro hardness.
[0041]
[Polygon Ti-containing carbide present in the grains of ferrite: 5 × 10 7 cells / mm 2 or more]
[more than 50% of the Ti-containing carbide present in polygonal ferrite grains is, the aspect ratio of the long side / short side in less than 3]
　the hot rolled steel sheet according to the present embodiment, in grains of polygonal ferrite, 5 × 10 7 cells / mm 2 contains more Ti-containing carbide. 5 × 10 7 cells / mm 2 or less, the precipitation strengthening is insufficient insufficient strength. On the other hand, it is not necessary to define the upper limit, usually, if the components within the above range, 1 × 10 11 pieces / mm 2 does not become more than.
　Further, this polygonal among Ti-containing carbide present in the grains of ferrite, 50% or more in number ratio, expressed as the ratio (long side / short side of the length of the long side to the length of the short side Aspect ratio) that is to less than 3 carbide, superior hole expandability can be obtained. Preferably, it is 2/3 or more of the Ti-containing carbide aspect ratio of the long side / short side of the Ti-containing carbide having smaller than 3 are present in grains of polygonal ferrite. Ratio of Ti containing carbide having an aspect ratio of less than 3 may be 100%.
　Ratio of Ti containing carbide having an aspect ratio of less than 3, the parallel incident direction of the electron beam of ferrite matrix and <001>, a transmission electron microscope: at least 100 pieces of Ti-containing carbide by (magnification 200,000 ×) when observed above, with respect to the total number of Ti-containing carbide which is observed, the aspect ratio of the long side / short side is obtained by obtaining the less than 3 carbides.
　In the present embodiment, the Ti-containing carbide is a carbide containing Ti, V, may further contain one or more Nb. That is, has a crystal structure of the Ti-containing carbide (NaCl structure), and several positions of Ti is but also a state where replaced by V or Nb.
[0042]
[Galvanized layer]
　hot rolled steel sheet according to the present embodiment, the on the surface may have a known galvanized layer. Galvanized layer may be alloyed galvanized layer is alloyed. If a hot-dip galvanizing layer, the generation of rust can be suppressed, thereby improving the corrosion resistance of the hot-rolled steel sheet.
[0043]

Tensile Strength (TS): 540 MPa or more]
[tensile strength (TS) and a ratio of 0.2% yield strength (YS) (yield ratio): 75% or more]
Tensile strength (TS) hole expansion ratio defined by JFST1001 (λ) product of (TS · λ): 50000MPa · % or more]
　to meet the stringent performance required for the recent high-strength hot-rolled steel sheet for automobiles, the mechanical properties as the tensile strength TS is 540MPa or more and a tensile strength TS and 0.2% yield strength YS ratio (YR (yield ratio)) of 75% or more, and the tensile strength TS and is the hole expansion rate λ defined in JFST1001 it is preferred product (TS · λ) is 50000 mPa ·% or more. The hot rolled steel sheet according to the present embodiment, by controlling the chemical composition and structure, high tensile strength as described above, yield ratio, and tensile strength - by including all hole expandability balance (TS · λ) the target.
　The tensile strength is preferably 590MPa or more. The tensile strength, at a 1180MPa than, the fatigue properties of the welded portion is deteriorated, the following are preferred 1180MPa.
[0044]
　It will now be described preferred method for obtaining a hot rolled steel sheet according to the present embodiment. Hot rolled steel sheet according to the present embodiment, according to the manufacturing method comprising the following of (A) ~ (D) step, preferably it is possible to manufacture stably.
(A) heating a slab obtained from the molten steel having the chemical composition of the aforementioned range of about 1200 ° C.,
the (B) cumulative reduction rate in the heated slab 1050 ° C. or higher 1150 ° C. or less is 50% or more volumes of rough rolling is
performed, (C) a steel sheet after the rough rolling, 20% to 80% cumulative rolling reduction at 1050 ° C. or less, reduction rate of the final pass is 15% to 35%, and the final pass temperature ( finishing temperature) performs finish rolling so that the 930 ° C. or
higher, the temperature of the (D) Thereafter, the hot rolled steel sheet, i) as primary cooling, the finish rolling final pass temperature ~ MT (720 ℃ ≦ MT ≦ 830 ℃) the area, cooling under the condition average cooling rate is 30 ° C. / s or higher, then, ii) a secondary cooling, the temperature range of MT ~ Tx (720 ℃ ≦ Tx
　In the heating step, heating the slab having the chemical composition as described above to about 1200 ° C.. Precipitation density of Ti-containing carbide in polygonal ferrite grains, Ti, Nb, affects the solid solution state of the carbide-forming elements of the V or the like, from the viewpoint of suppressing the formation of coarse carbides, in order to obtain the desired performance the heating temperature is preferably a temperature range of 1250 ° C. from 1150 ° C..
[0046]

　The heated slab, a hot-rolled steel sheet through a hot rolling step consisting of rough rolling step and the finish rolling step. In preparing the hot rolled steel sheet according to the present embodiment, in each step of the rough rolling and finish rolling, it is preferable to control the temperature, the reduction ratio and the like.
　In the rough rolling step of hot rolling, it is preferable that the cumulative rolling reduction at 1050 ° C. ~ 1150 ° C. and 50% or more. Cumulative rolling reduction at 1050 ° C. ~ 1150 ° C. become a tissue heterogeneity below 50%, the σHV becomes hole expandability and is greatly reduced. The cumulative reduction ratio in the present invention, prior to the first pass inlet plate thickness with respect to the, before the first pass in the cumulative reduction ratio (rolling with respect to the reference inlet thickness and outlet thickness Metropolitan after the final pass in the rolling it is the percentage of difference). Further, the cumulative rolling reduction, rough rolling, is calculated separately respectively finish rolling. That is, the cumulative rolling reduction in rough rolling is the percentage of the difference between the exit thickness Metropolitan after the final pass in the first inlet plate thickness of the pass before the rough rolling in rough rolling, the cumulative reduction rate at finish rolling, the percentage of the difference between the exit thickness Metropolitan after the final pass in the first pass before the inlet plate thickness and finish rolling in the finish rolling.
[0047]

　In the finish rolling step of hot rolling, the cumulative reduction rate at 1050 ° C. or less preferably 20% to 80%. Cumulative rolling reduction at 1050 ° C. or less anisotropic organization of the finally obtained hot-rolled steel sheet exceeds 80% obvious. In this case, there is a case where the hole expandability σHV increases is reduced. This is presumed to be due to the deviation of the crystal orientation of the ferrite grains is conducive hardness difference. On the other hand, when the cumulative rolling reduction at 1050 ° C. or less below 20% by austenite grain size is insufficient strain accumulation is to austenite with coarse, ferrite transformation after the finish rolling is suppressed, and finally the standard deviation of micro-hardness of the resulting polygonal ferrite fraction and polygonal ferrite is disengaged from the desired range increases the likelihood that hole expandability is deteriorated.
[0048]
[Reduction ratio in the final pass: 15% to 35%
　reduction ratio of the final pass is preferably set to 15% to 35%. Manifested anisotropy tissue and reduction ratio of the final pass exceeds 35%, as a result, the σHv becomes hole expandability and is greatly reduced. Therefore the reduction ratio of the final pass is 35% or less. More preferably, 25% or less. On the other hand, when the reduction ratio of the final pass is less than 15% insufficient strain accumulation is to austenite is suppressed ferrite transformation after the finish rolling, the micro hardness of the finally obtained polygonal ferrite fraction and polygonal ferrite out of the range the standard deviation is desired is the possibility of hole expandability is deteriorated is high.
[0049]
[Finishing temperature: 930 ° C. or higher]
　finishing temperature (temperature of the steel sheet after the final pass of the finish rolling) is preferably set to 930 ° C. or higher. Easy anisotropy of the tissue is manifested in the finally obtained hot-rolled steel sheet when the finishing temperature is below 930 ° C., resulting σHv hole expandability and increases more likely to decrease. On the other hand, as the finishing temperature becomes higher, the austenite grain size is ferrite transformation after the finish rolling by insufficient accumulation of strain to austenite suppressed with coarse, polygonal ferrite fraction finally obtained and polygons the standard deviation of micro-hardness of the ferrite is enlarged, hole expandability is more likely to deteriorate. Therefore, it is preferable that the upper limit of the finishing temperature of about 1000 ° C..
[0050]

　After finish rolling described above, cooling the hot-rolled steel sheet is subjected.
　Finish rolling in the final pass temperature region of the temperature ~ 720 ° C., i) by the growth of Ti-containing carbide precipitated in the ferrite (coarsening), changes in the Ti-containing carbide density in the grains of polygonal ferrite, and ii) polygonal change in the aspect ratio of the long side / short side of the Ti-containing carbide present in the ferrite grains increases. Therefore, the average cooling rate in a temperature range of finish rolling final pass temperature ~ 720 ° C. be 30 ° C. / s is effective in obtaining the desired performance.
　Furthermore, after the above cooling, in a temperature range of 830 ° C. ~ 720 ° C., the desired time determined in accordance with content Mn content, cooling the hot rolled steel sheet at a low average cooling rate, ferrite transformation and Ti-containing carbide It promotes the deposition is finally obtained micro hardness standard deviation of the polygonal ferrite fraction and polygonal ferrite effective in that the desired range.
　Then, after further cooling, take up the hot-rolled steel sheet. At that time, or less than the cooling rate of 30 ° C. / s, when the coiling temperature is 650 ° C. greater than after cooling during or winding, Ti-containing carbide in the hot-rolled steel sheet is excessively coarsened, desired intensity there is a case that it is difficult to ensure. On the other hand, when the coiling temperature is less than 450 ° C., since the coiling temperature control accuracy decreases undesirably. Therefore, the coiling temperature in the range of 450 ~ 650 ° C., and it is effective to cool to the coiling temperature at a predetermined average cooling rate higher.
　That is, in the cooling step after the finish rolling, the hot-rolled steel sheet after finish rolling, i) as primary cooling, the temperature region of the finish rolling final pass temperature ~ MT (720 ℃ ≦ MT ≦ 830 ℃), average cooling rate cooled in comprising such conditions as 30 ° C. / s or higher, then, ii) a secondary cooling, the temperature range of MT ~ Tx (720 ℃ ≦ Tx
　After the winding process, it may comprise a hot-dip galvanizing step of performing galvanizing treatment to the hot-rolled steel sheet. The galvanizing treatment the plated layer is formed on the surface of the steel sheet by applying, it is possible to improve the corrosion resistance of the steel sheet. Further, after the galvanizing treatment, by performing alloying treatment, it may form a galvannealed layer on the surface of the steel sheet. Also this time, in order to suppress the reduction in strength of the steel sheet, the maximum heating temperature during the annealing before galvanizing immersion is preferably set to 800 ° C. or less. May According to the conventional method for other hot-dip galvanizing conditions.

　The hot rolled steel sheet according to the present embodiment in a usual manner, after the hot rolling step may be carried out pickling. Further, after pickling before or pickling, may be subjected to skin pass rolling for the flat straightening and scale peeling accelerator. Elongation when subjected to skin pass rolling is not particularly defined, preferably less than 0.1% or more to 3.0%.
Example
[0052]
　The embodiments of the present invention shall now be described.
[Example 1]
　was melted in a laboratory steel having a chemical composition shown in Table 1 was cast slab, heating the pattern shown in FIG. 1, the hot rolling was carried out cooling, the winding. In this case, the conditions in each step were as shown in Table 2. In Table 2, SRT, R1, R2, R3, FT, MT, t, CT shows the following respectively.
　SRT: Slab heating temperature
　R1: 1050 ° C. or higher 1150 cumulative rolling reduction at ° C. or less
　R2: 1050 cumulative rolling reduction at ° C. or less
　R3: finishing pass rolling reduction in the
　FT: finishing rolling temperature
　MT: the primary cooling stop temperature
　t: secondary cooling time
　CT: coiling temperature
　in this way and pickled hot-rolled steel sheet thus obtained, was subjected to galvanizing of conditions that are shown with the plating in the column labeled processed in Table 3 It was taken JIS5 No. tensile specimen from the direction perpendicular to the rolling direction of the hot-rolled steel sheet. Using this test matter, subjected to a tensile test, yield strength (YS), tensile strength (TS), yield ratio (YR), to measure the total elongation (EL).
　Also, make a hole expanding test according to the "JFS T 1001 hole expansion test method" of the Japan Iron and Steel Federation standard, to measure the hole expansion ratio (λ).
　Further, a sample containing a rolling direction cross-section of the hot-rolled steel sheet was taken, after the corrosion of the surface corresponding to the rolling direction cross-section of the sample by nital solution, 1/4 depth of thickness with an optical microscope or a scanning electron microscope photographed structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m at the position were tissue identification. From the obtained tissue photograph was calculated area ratio of each tissue by point counting method. Polygonal ferrite, and bainite, bainitic ferrite is determined by the presence or absence of grain boundary shape and internal structure, the is black etched tissues were identified as cementite and pearlite. Further, using the Repera corroded samples, with respect to structural photograph obtained by an optical microscope, by performing image analysis to calculate the total area ratio of retained austenite and martensite.
　Further, the film sample was taken from each hot-rolled steel sheet, a transmission electron microscope: Ti precipitated in the ferrite grains, V, a carbide containing one or more Nb observed with (magnification 200,000 ×) to obtain the percentage number density and aspect ratio of 3 or less.
　Also, the 80 area% or more of polygonal ferrite was obtained steel, and the standard deviation of micro-hardness was measured by the method described above. Figure 3A of Run No. 14, the measurement results of the micro hardness of Run No. 15 in FIG. 3B, respectively as an example.
[0053]
　The results obtained are shown in Tables 3 and 4. In Table 3 and 4, Vα, VPθ, VMA, B, BF, σHV represents less. Blank tissues demonstrates that was not observed.
　V.alpha: the area ratio of the ferrite
　VPshita: pearlite and cementite of the total area ratio
　VMA: total area fraction of martensite and austenite
　B, BF: bainite and bainitic ferrite
　ShigumaHV: standard deviation of the ferrite micro hardness
　Run No. 1-3, 5 ~ 6,11,17 ~ 19,22,25 ~ 34 are desired mechanical characteristics can be obtained for a range of chemical composition and structure defines all the invention. On the other hand, Run No. 4,10,12 ~ 16 and 20 ~ 21,24,36 are beyond the limit that defines the σHV by the present invention, as a result, not obtained the desired mechanical properties. Run No. 7,8,18,36 area ratio of polygonal ferrite is below the lower limit prescribed in the present invention so that the desired mechanical properties are not obtained. Run No. 9 is the sum of the area ratio of the martensite and austenite is above the upper limit defined in the present invention so that the desired mechanical properties are not obtained. Run No. 36 and 38 the sum of the area ratio of pearlite and cementite is above the upper limit defined in the present invention so that the desired mechanical properties are not obtained.
　Also, in Run No. 7,8,12,23,24,35,38, carbides number density is low, also, the Run No. 7,8,12,23,24,36 In an aspect ratio of 3 or less Ti-containing carbide ratio has become many, not the desired mechanical characteristics.
　Run No. 37 has low toughness, because the broken during specimen processing, can not be tested.
[0054]
[Table 1]

[0055]
[Table 2]

[0056]
[table 3]

[0057]
[Table 4]

[0058]
[Example 2]
　A ~ C and G of the next steel having a chemical composition shown in Table 1, the 5 steels H, was subjected to hot rolling and cooling is shown in FIG. Then subjected to descaling treatment, without performing the cold rolling, using a continuous heat treatment simulator, it was subjected to a heat treatment simulating the galvannealed line pattern shown in FIG. In this case, the conditions in each step were as shown in Table 5. In Table 5, RA, LTH, DIP, GA represents the following.
　RA: maximum heating temperature
　LTH: low holding temperature
　DIP: Zn bath temperature
　GA: alloyed temperature
　from the hot-rolled steel sheet obtained in this manner was collected JIS5 No. tensile specimen from the direction perpendicular to the rolling direction. Using this test matter, subjected to a tensile test, yield strength (YS), tensile strength (TS), yield ratio (YR), to measure the total elongation (EL). Also, make a hole expanding test according to the "JFS T 1001 hole expansion test method" of the Japan Iron and Steel Federation standard, to measure the hole expansion ratio (λ).
　Further, a sample containing a rolling direction cross-section of the steel sheet was taken, to calculate the area ratio of each tissue in the same manner as in Example 1.
　Further, the film sample was taken from each hot-rolled steel sheet, a transmission electron microscope: Ti precipitated in the ferrite grains, V, a carbide containing one or more Nb observed with (magnification 200,000 ×) to obtain the percentage number density and aspect ratio of 3 or less. About 80 area% or more of polygonal ferrite was obtained steel, and the standard deviation of micro-hardness was measured by the method described above.
[0059]
　The results obtained are shown in Table 6. Run No. 39 to 42 and 44 to 47 is within a range of chemical composition and structure defines all the invention, the desired mechanical characteristics are obtained. On the other hand, Run No. 43 σHV has exceeded the upper limit prescribed in the present invention so that the desired mechanical properties are not obtained. Run No. 48 is polygonal ferrite area ratio is below the lower limit prescribed in the present invention so that the desired mechanical properties are not obtained.
[0060]
[table 5]

[0061]
[Table 6]

Industrial Applicability
[0062]
　According to the present invention, it is possible to manufacture an inexpensive high-strength hot-rolled steel sheet having a high yield ratio and excellent hole expandability. Further, the steel sheet according to the present invention, parts for automobiles, in particular stretch flangeability which is frequently used in such underbody part, has excellent hole expandability. Therefore, industrially, in particular, it contributes to weight reduction and collision safety of the vehicle body in the automotive field.

claims

[Claim 1]Chemical composition, inmass%,
C:
0.010% ~ 0.200%, Si: 0.001% ~ 2.50%, Mn:
0.001% ~ 1.50%, P: 0.050%
hereinafter, S: 0.010% or
less, N: 0.0070% or
less,
Al: 0.001% ~ 0.50%, Ti:
0.050% ~ 0.30%, V: 0% ~ 0.50
%,
Nb:
0% ~ 0.090%, Cr: 0%
~ 0.50%, Ni: 0% ~ 0.50%, Cu:
0% ~ 0.50%, Mo: 0% ~ 0.50
%,
B: 0% ~
0.0050%, Ca: 0% ~ 0.01%, Mg:
0% ~ 0.01%, Bi: 0% ~ 0.01%
containing the balance Fe and impurities , and the
　tissue, an area ratio, 80% or more polygonal ferrite, martensite and austenite 5% in total, less than 5% Pa in total Contains a write and cementite, the balance is not less than one selected from bainitic ferrite and bainite,
　any present from the central plane in the thickness direction within the range of ± 100 [mu] m of 50 of the polygonal ferrite when the standard deviation of micro-hardness and ShigumaHV, the ShigumaHV is not less than 30,
　the Ti-containing carbide of 5 × 10 in polygonal intragranular ferrite 7 pieces / mm 2 occur more than, of the Ti-containing carbide 50% or more, the aspect ratio is the ratio of the length of the long side to the length of the shorter side is less than 3,
　the tensile strength is not less than 540MPa
high-strength hot-rolled steel sheet, characterized in that.
[Claim 2]
　The chemical composition, in mass%, V: 0.010% ~ 0.50%, Nb: 0.001% ~ 0.090%, Cr: 0.001% ~ 0.50%, Ni: 0.001 % ~ 0.50%, Cu: 0.001% ~ 0.50%, Mo: 0.001% ~ 0.50%, B: containing one or more kinds selected from 0.0001% - 0.0050% high strength hot rolled steel sheet according to claim 1, characterized in that.
[Claim 3]
　The chemical composition, by mass%, Ca: 0.0001% ~ 0.01%, Mg: 0.0001% ~ 0.01%, Bi: 0.0001% ~ one or more selected from 0.01% high strength hot rolled steel sheet according to claim 1 or 2, characterized in that it contains.
[Claim 4]
　High strength hot rolled steel sheet according to any one of claims 1 to 3, characterized in that it has a hot-dip galvanizing layer on the surface.