Technical field
[0001]
　The present invention relates to a high hot-rolled steel sheet in workability, in particular, it relates to good hot-rolled steel sheet in stretch flangeability.
Background technique
[0002]
　Recently, application of the fuel efficiency of an automobile relative demand for weight reduction of various members for the purpose, and thinning with high strength of a steel sheet of an iron alloy or the like used for members, the various members of light metal such as Al alloy It has been promoted. However, when compared to heavy metals such as steel, although a light metal such as Al alloy is advantageous in that the specific strength high, there is a disadvantage that it is significantly more expensive. Therefore, the application of light metals such as Al alloys are limited to special applications. Therefore, in order to apply the weight of the various components in more inexpensive wide range, thinning by high strength of a steel sheet is required.
[0003]
　If high strength steel sheet, material properties such as generally moldability (processability) is deteriorated. Therefore, in the development of high-strength steel sheet is an important challenge is to achieve high strength without deteriorating the material properties. In particular, the inner plate member, structural members, the steel sheet used as an automotive member such as underbody members, depending on the application, stretch flange formability, burring formability, ductility, fatigue resistance, impact resistance and corrosion resistance and the like I asked, and these material properties and strength, it is important to achieve both.
[0004]
　For example, among automobile parts, steel sheets used for structural members and underbody members and the like accounts for approximately 20% of body weight, after performing the opening blanking and hole by shearing or stamping, stretch flanging or burring press molding, which was mainly composed of is applied. Therefore, these steel sheets, good stretch flangeability is required.
[0005]
　For the above problems, for example, Patent Document 1, ductile possible to limit the size of TiC, stretch flangeability, is disclosed and can provide a hot-rolled steel sheet excellent in material homogeneity.
　Further, Patent Document 2, kinds of oxides, inventions to provide a hot rolled steel sheet having excellent flange formability and fatigue characteristics elongation by defining the size and number density is disclosed.
　Further, Patent Document 3, by defining the hardness difference between the area ratio and the second phase of the ferrite phase, small variations in strength, and the invention is disclosed to provide a hot rolled steel sheet excellent in the ductility and hole expansion It is.
[0006]
　However, in the above Patent Document 1 a technique disclosed, it is necessary to secure a ferrite phase over 95% in the tissue of the steel sheet. Therefore, in order to ensure sufficient strength, even when the 590MPa class (TS more than 590MPa), it is necessary to include Ti 0.08% or more. However, in the steel having a ferrite phase soft 95% or more, to secure the strength of more than 590MPa by precipitation hardening of TiC, decrease in ductility is a problem.
　The addition of rare metals such as La and Ce are essential in the technique disclosed in Patent Document 2. Disclosed in Patent Document 3 technologies, it is necessary to limit the Si is an inexpensive strengthening element to 0.1% or less. Thus, the technique disclosed in Patent Documents 2 and 3 both have a problem that constraints alloying elements.
[0007]
　As described above, in recent years, the automobile member, there is an increasing demand for the application of high-strength steel sheet. When forming a high-strength steel plate and pressed with cold, cracks from the edge portion serving as flanging stretch during molding tends to occur. This is believed to be due to causes proceed hardening only the edge portion by distortion introduced into the end face punching during blanking. Conventionally, as a test method for evaluating stretch flangeability, hole expanding test has been used. However, the hole expansion test is to fracture with little distribution strain in the circumferential direction, in the processing of actual part, for the strain distribution is present, the influence of the fracture limit with a gradient of strain and stress around breaks there exist. Therefore, in the case of high strength steel sheet, even it showed sufficient stretch flangeability in hole expanding test, when performing cold press, there have been cases where cracks by strain distribution is generated.
[0008]
　The disclosed in Patent Documents 1-3 techniques, by defining only the tissue that is also observed with an optical microscope at any invention, to improve the hole expansion is disclosed. However, whether a sufficient stretch flangeability even when considering strain distribution can be secured is not known.
CITATION
Patent Document
[0009]
Patent Document 1: International Publication WO2013 / 161090 Patent Publication No.
Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-256115
Patent Document 3: Japanese Laid-Open Patent Publication No. 2011-140671
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　The present invention has been devised in view of the problems described above.
　The present invention aims at providing a can be applied to members that strict stretch flange formability yet high strength is required, a high-strength hot-rolled steel sheet excellent in stretch flange formability at low cost. In the present invention, the stretch flangeability, strain distribution is indicative of the stretch flangeability considering, tensile saddle stretch flange test method in the flange obtained as a result of the test limit forming height H (mm) Strength shows the values to be evaluated by the product of the (MPa), that the excellent stretch flangeability, the product of the limit forming height H (mm) and the tensile strength of the flange (MPa) is 19500mm · MPa or higher show. Further, the high strength, indicating that tensile strength of not less than 590 MPa.
Means for Solving the Problems
[0011]
　According to conventional wisdom, the improvement of stretch flangeability (hole expansion), as shown in Patent Documents 1-3, inclusions control, tissue homogenization, hardness difference between a single organization and / or organizational It was done by such a reduction. In other words, conventionally, by controlling the tissue being observed by an optical microscope, improvements such as stretch flangeability has been achieved.
[0012]
　However, we view the inability to improve the stretch flangeability when even strain distribution by controlling only the tissue that is observed with an optical microscope is present, the azimuth difference in the grains of the grain the focused, advanced intensive studies. As a result, the percentage of the total grain misorientation in the crystal grains is 5 ~ 14 ° crystal grain by controlling within a certain range, it found that it is possible to greatly improve the stretch flangeability.
[0013]
　The present invention is constructed based on the above findings and has as its gist is as follows.
[0014]
　(1) hot-rolled steel sheet according to one embodiment of the present invention, chemical components, by mass%, C: 0.020 ~ 0.070%, Si: 0.10 ~ 1.70%, Mn: 0.60 ~ 2.50%, Al: 0.01 ~ 1.00%, Ti: 0.015 ~ 0.170%, Nb: 0.005 ~ 0.050%, Cr: 0 ~ 1.0%, B: 0 ~ 0.10%, Mo: 0 ~ 1.0%, Cu: 0 ~ 2.0%, Ni: 0 ~ 2.0%, Mg: 0 ~ 0.05%, REM: 0 ~ 0.05 %, Ca: 0 ~ 0.05%, Zr: 0 contains ~ 0.05%, P: 0.05% or less, S: 0.010% or less, N: 0.0060% or less, limited to , the balance being Fe and impurities, tissue, an area ratio, comprises 5 to 60% ferrite 30 to 95% bainite, in the tissue, the boundary misorientation is 15 ° or more And field, the grain boundary surrounded by, and if the circle equivalent diameter of the region is 0.3μm or more was defined as the crystal grains, the proportion of the grain misorientation is 5 ~ 14 ° in the grains is, the area at a rate, which is 20 to 100%.
[0015]
　(2) hot-rolled steel sheet according to the above (1), the tensile strength, more than 590 MPa, the tensile strength and the product of the limit forming height of saddle stretch flange tests may be 19500mm · MPa or higher.
[0016]
　(3) hot-rolled steel sheet according to (1) or (2), the chemical composition, in mass%, Cr: 0.05 ~ 1.0%, B: 0.0005 ~ 0.10%, it may comprise one or more selected from.
[0017]
　(4) hot-rolled steel sheet according to any one of the above (1) to (3), the chemical composition, in mass%, Mo: 0.01 ~ 1.0%, Cu: 0.01 ~ 2.0%, Ni: it may include one or more selected from 0.01% to 2.0%.
[0018]
　(5) hot-rolled steel sheet according to any one of the above (1) to (4), the chemical composition, by mass%, Ca: 0.0001 ~ 0.05%, Mg: 0.0001 ~ 0.05%, Zr: 0.0001 ~ 0.05%, REM: may include one or more selected from 0.0001 to 0.05%.
Effect of the Invention
[0019]
　According to this aspect of the present invention, it is possible to provide that can be applied to members that strict stretch flange formability yet high strength is required, a high-strength hot-rolled steel sheet having excellent stretch flangeability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
Is a [1] Analysis results of EBSD in 1 / 4t part (1/4 of the position of the sheet thickness from the surface in the thickness direction) of the hot rolled steel sheet according to the present embodiment.
[2] used in the saddle-type stretch flange test method is a diagram showing a molded article in the shape of a saddle-shaped.
DESCRIPTION OF THE INVENTION
[0021]
　Hereinafter, the hot rolled steel sheet according to an embodiment of the present invention (hereinafter sometimes referred to as hot-rolled steel sheet according to the present embodiment) will be described in detail.
[0022]
　Hot rolled steel sheet according to the present embodiment, chemical composition, in mass%, C: 0.020 ~ 0.070% , Si: 0.10 ~ 1.70%, Mn: 0.60 ~ 2.50% , Al: 0.01 ~ 1.00%, Ti: 0.015 ~ 0.170%, Nb: contains from 0.005 to 0.050%, if necessary, Cr: 1.0% or less , B: 0.10% or less, Mo: 1.0% or less, Cu: 2.0% or less, Ni: 2.0% or less, Mg: 0.05% or less, REM: 0.05% or less, Ca 0.05% or less, Zr: contain one or more than 0.05% of, P: 0.05% or less, S: 0.010% or less, N: 0.006% or less, limited to, the balance being Fe and impurities.
　Also, tissue in the area ratio, comprises 5 to 60% ferrite 30 to 95% bainite, in the tissue and the boundary misorientation is 15 ° or more and the grain boundaries, surrounded by the grain boundary and if the circle equivalent diameter of the region is 0.3μm or more was defined as the crystal grains, the proportion of the grain misorientation in the grains is 5 ~ 14 ° is, an area ratio is 20-100% .
　First, a description will be given reasons for limiting the chemical components of the hot rolled steel sheet according to the present embodiment. % Of the content of each component is mass%.
[0023]
　C: 0.020 ~ 0.070%
　C is, Nb, and form precipitates in the steel sheet combines with Ti or the like, an element which contributes to improving the strength of the steel by precipitation strengthening. To obtain this effect, and 0.020% or lower limit of the C content. The lower limit of preferred C content is 0.025%, more preferable lower limit of C content is 0.030%. On the other hand, C content becomes to 0.070% greater, tend to azimuth dispersion in the bainite is increased, the misorientation in the grains is the proportion of crystal grains of 5 ~ 14 ° is reduced. Also, harmful cementite increases for stretch flange formability, stretch flangeability is degraded. Therefore, to 0.070% of the upper limit of the C content. The upper limit of the preferred C content is 0.065%, and more preferable upper limit of the C content is 0.060%.
[0024]
　Si: 0.10 ~
　1.70% Si is an element that contributes to improving the strength of the steel. Moreover, Si is an element having a role as a deoxidizer for molten steel. To obtain these effects, 0.10% of the lower limit of the Si content. The lower limit of the preferred Si content is 0.30%, and more preferable lower limit of Si content is 0.50%, the lower limit of more preferable Si content is 0.70%. On the other hand, when the Si content exceeds 1.70% deteriorates elongation flange formability, surface flaws or generated. In addition, too high a transformation point, need to occur higher the rolling temperature. In this case, recrystallization during hot rolling is promoted considerably, misorientation in the grains is the proportion of 5 ~ 14 ° of the crystal grains is reduced. Therefore, to 1.70% of the upper limit of the Si content. The upper limit of the preferred Si content is 1.50%, and more preferable upper limit of the Si content is 1.30%.
[0025]
　Mn: 0.60 ~
　2.50% Mn, by improving the hardenability of solid solution by strengthening or steel, an element which contributes to the improvement of the strength of the steel. To obtain this effect, 0.60% of the lower limit of the Mn content. The lower limit of the preferred Mn content is 0.70%, and more preferable lower limit of Mn content is 0.80%. On the other hand, when the Mn content exceeds 2.50%, hardenability becomes excessive, that the degree of orientation deviation in bainite is increased, the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains reduced, stretch flangeability is degraded. Therefore, 2.50% of the upper limit of the Mn content. The upper limit of the preferred Mn content is 2.30%, and more preferable upper limit of the Mn content is 2.10%.
[0026]
　Al: 0.010 ~
　1.00% Al is an element effective as a deoxidizer for molten steel. To obtain this effect, and 0.010% the lower limit of the Al content. The lower limit of the preferred Al content is 0.020%, more preferable lower limit of Al content is 0.030%. On the other hand, when the Al content exceeds 1.00%, such as weldability and toughness are deteriorated. Therefore, 1.00% of the upper limit of the Al content. The upper limit of the preferred Al content is 0.90%, and more preferable upper limit of the Al content is 0.80%.
[0027]
　Ti: 0.015 ~
　0.170% Ti is finely precipitated in the steel as carbide, is an element that improves the strength of steel by precipitation strengthening. Further, Ti is a carbide (TiC) to secure the C by forming a element for suppressing generation of harmful cementite for stretch flangeability. To obtain these effects, 0.015% of the lower limit of the Ti content. The lower limit of the preferred Ti content is 0.020%, more preferable lower limit of Ti content is 0.025%. On the other hand, Ti content exceeds 0.170%, the ductility is degraded. Therefore, to 0.170% the upper limit of the Ti content. The upper limit of the preferred Ti content is 0.150%, and more preferable upper limit of the Ti content is 0.130%.
[0028]
　Nb: 0.005 ~
　0.050% Nb is finely precipitated in the steel as carbide, it is an element that improves the strength of steel by precipitation strengthening. Further, Nb is a carbide (NbC) and fixing C by forming a element for suppressing generation of harmful cementite for stretch flangeability. To obtain these effects, 0.005% of the lower limit of the Nb content. The lower limit of the preferred Nb content is 0.010%, more preferable lower limit of Nb content is 0.015%. On the other hand, when the Nb content exceeds 0.050% ductility is degraded. Further, since recrystallization during hot rolling is significantly inhibited, too large misorientation in the grains, resulting misorientation in the grains is the proportion of crystal grains of 5 ~ 14 ° is reduced. Therefore, 0.050% of the upper limit of Nb content. The upper limit of the preferred Nb content is 0.040%, and more preferable upper limit of Nb content is 0.035%.
[0029]
　P: 0.05% or less
　P is an impurity. P is the toughness, ductility, so deteriorating the welding property, P content is preferably as low. However, since the deterioration of the flange of the indicator extends when P content exceeds 0.05% is significant, P content should be limited to 0.05% or less. More preferably, P content is 0.03% or less, more preferably 0.02% or less. You need not limit the specifically defined for P, excessive reduction because undesirable in view of manufacturing cost may be 0.005% the lower limit of the P content.
[0030]
　S: 0.010% or less
　S is not only causes cracking during hot rolling, which is an element to form a A-based inclusions which deteriorate the stretch flangeability. Therefore, S content is preferably as low. However, since a significant flange deterioration elongation when the S content exceeds 0.010%, may be the upper limit of the S content to 0.010%. Preferably, S content is 0.005% or less, more preferably 0.003% or less. The lower limit of S is not particularly defined, excessive reduction because undesirable from the viewpoint of production cost, the lower limit of the S content may be 0.001%.
[0031]
　N: 0.0060% or less
　N is preferentially than C, to form a with Ti and Nb precipitate, which is an element to reduce the effective Ti and Nb for fixing the C. Therefore, N content is preferably as low as. However, if the N content exceeds 0.0060%, the remarkable deterioration of the stretch flange formability may be limiting the N content to 0.0060% or less. Preferably, N content is 0.0050%. The lower limit of the N content is not particularly defined, excessive reduction because undesirable in view of manufacturing cost, the N content may be 0.0010% or more.
[0032]
　Basic or more chemical elements is the basic component contained in the hot-rolled steel sheet according to the present embodiment comprises these elements, the chemical composition the balance being iron and impurities, hot rolled steel sheet according to the embodiment it is a composition. However, (in place of some of the remainder Fe) The addition to the basic components, the hot rolled steel sheet according to the present embodiment, furthermore, one or more optionally be selected from the following chemical elements (selection elements) it may contain. Since the following elements are not always necessary to contain, the lower limit of the content thereof is 0%. Inevitably (e.g., the amount an amount less than the lower limit of the selected element) These selection elements in the steel be mixed, does not impair the effects of the present embodiment.
　Here, the impurity, when the industrial production of alloys, ores, from a raw material of scraps or by various factors of the manufacturing process, and a component to be mixed into the steel, according to the present embodiment heat It means what is allowed in a range that does not adversely affect the properties of the rolled steel sheet.
[0033]
　Cr: 0 ~
　1.0% Cr is an element which contributes to the improvement of the strength of the steel. To obtain this effect, it is preferable to contain Cr of 0.05% or more. On the other hand, when the Cr content exceeds 1.0%, its effect is lowered economical efficiency saturated. Therefore, even if the inclusion of Cr, it is preferable that the 1.0% upper limit of the Cr content.
[0034]
　B: 0 ~ 0.10% B
　increases the hardenability, it is an element that increases the structural fraction of the low-temperature transformation product phase is a hard phase. To obtain this effect, the content of B is preferably set to 0.0005% or more. On the other hand, the effect when the B content is 0.10% or more, along with saturated, is uneconomical. Therefore, even if the inclusion of B, the upper limit of the B content is preferably made 0.10%.
[0035]
　Mo: 0.01 ~
　1.0% Mo is an element having an effect of improving the strength by forming a carbide improves the hardenability. To obtain these effects, the Mo content is preferably set to 0.01% or more. On the other hand, when the Mo content is 1.0 percent, there is a possibility that the ductility and weldability is decreased. Therefore, even if the inclusion of Mo, the upper limit of the Mo content is preferably set to 1.0%.
[0036]
　Cu: 0.01 ~
　2.0% Cu, together with increasing the strength of the steel sheet is an element for improving the corrosion resistance and scale peelability. To obtain these effects, the Cu content is preferably set to 0.01% or more. More preferably, it is 0.04% or more. On the other hand, when the Cu content of 2.0 percent, is a concern that the surface flaws generated. Therefore, even if the inclusion of Cr, it is desirable to 2.0% the upper limit of the Cr content, it is more desirable to 1.0%.
[0037]
　Ni:
　0.01% of Ni with increasing the strength of the steel sheet is an element for improving the toughness. To obtain these effects, the Ni content is preferably set to 0.01% or more. On the other hand, when the Ni content of 2.0 percent, the ductility is reduced. Therefore, even if the inclusion of Ni, it is desirable that the 2.0% upper limit of the Ni content.
[0038]

　Ca:　
　0.0001 ~ 0.05% Mg: 0.0001
　~ 0.05% Zr: 0.0001 ~ 0.05% REM: 0.0001
　~ 0.05% Ca, Mg, Zr and REM are both is an element that improves the toughness by controlling the shape of sulfides and oxides. Therefore, for this purpose, it is desirable to include one or more of these elements each 0.0001% or more. More preferably 0.0005%. However, stretch flangeability and the contents of these elements is excessive is deteriorated. Therefore, even if the inclusion of these elements, it is preferable that the upper limit of the content of 0.05%, respectively.
[0039]
　Next, a description will be given tissue (metal structure) of the hot rolled steel sheet according to the present embodiment.
　Hot rolled steel sheet according to the present embodiment, in tissues was observed with an optical microscope, the area ratio, including 5 to 60% ferrite 30 to 95% bainite. Such tissue by a, thereby improving good balance and strength and workability. When the ferrite fraction (area ratio) is less than 5%, ductility is deteriorated, generally ensuring properties sought in automotive parts and the like becomes difficult. On the other hand, when the ferrite fraction is 60 percent, or degraded elongation flange formability, it is difficult to obtain the desired steel sheet strength. For this reason, ferrite fraction is 5 to 60%.
　Further, when the bainite fraction is less than 30%, stretch flangeability is degraded. On the other hand, bainite fraction, if it is 95%, the ductility is deteriorated. Therefore, the bainite fraction is 30 to 95%.
　Ferrite and the remainder other than the bainite structure need not be particularly limited, for example, martensite residual austenite may like perlite. However, since the stretch flangeability and the remainder of the structure fraction is large is degraded is concerned, the percentage of the remainder is preferably a total of 10% or less in area ratio. In other words, it is preferable ferrite and bainite is a total of 90% or more by area ratio. More preferably, a total of 100% in the ferrite and bainite and an area ratio.
[0040]
　Tissue fraction (area ratio) can be obtained by the following method. First, a sample collected from the hot-rolled steel sheet is etched with nital. To structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m in 1/4 depth position of the plate thickness using an optical microscope after etching, by performing image analysis, the area ratio of ferrite and pearlite, and bainite and martensite to get the total area ratio of the site. Then, using the Repera corroded samples, with respect to structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m at the position of 1/4 depth of thickness with an optical microscope, by performing image analysis, residual austenite and martensite to calculate the total area ratio of the site.
　Further, using the cutting plane from the rolled surface direction normal to 1/4 depth of thickness sample by X-ray diffraction measurement determining the volume fraction of retained austenite. The volume fraction of retained austenite are the equivalent to the area ratio, which is the area ratio of residual austenite.
　By this method, it is possible to obtain ferrite, bainite, martensite, residual austenite, the respective area ratios perlite.
[0041]
　Hot rolled steel sheet according to the present embodiment, after controlling the tissue being observed with an optical microscope within the above range, further, the crystal orientation analysis in many EBSD method used (electron beam backscatter diffraction pattern analysis) using resulting Te, it is necessary to control the rate of grain misorientation is 5 ~ 14 ° in the grains. Specifically, the boundary misorientation is 15 ° or more and the grain boundaries, when you define a region surrounded by the grain boundaries and crystal grains, of all the grains, misorientation in the grains is 5 ~ the proportion of the crystal grains is 14 °, the area ratio, it is necessary to 20% or more.
　The reason for 20% or more at 5 ~ 14 ° crystal grain area ratio the ratio of the is less than 20%, because the desired steel sheet strength and stretch flangeability can not be obtained. Grain proportion of 5 ~ 14 °, since the higher may be, the upper limit is 100%.
　Since such intragranular crystal grains having an orientation difference is effective for obtaining a steel sheet having excellent balance between strength and workability, and by controlling the ratio, while maintaining the desired steel sheet strength, stretch flange it is possible to improve the resistance.
[0042]
　Here, the crystal orientation difference in the grains are considered to be correlated with the dislocation density contained in the crystal grains. Generally an increase in the dislocation density of intragranular reduces the workability while providing improved strength. However, the crystal grains misorientation in the grains is controlled to be 5 ~ 14 ° it is possible to improve the strength without reducing the workability. Therefore, in hot rolled steel sheet according to the present embodiment, the misorientation in the grains to control the crystal grain ratio of 5 ~ 14 ° to 20% or more. Misorientation in the grains is less than 5 ° crystal grain is excellent in processability is difficult high strength, orientation difference within the grains 14 ° more than the crystal grains, since deformability in the crystal grain are different , it does not contribute to the improvement of the stretch flange formability.
[0043]
　The proportion of crystal grain misorientation is 5 ~ 14 ° in the grains can be measured by the following method.
　First, the rolling direction vertical section of the 1/4 depth position of the sheet thickness t from the surface of the steel sheet (1 / 4t part), 200 [mu] m in the rolling direction, the area of 100μm in the rolling surface normal direction in measured intervals 0.2μm EBSD obtain crystal orientation information by analyzing. Here EBSD analysis, using a thermal field emission scanning electron microscope (JEOL Ltd. JSM-7001F) and EBSD detector (TSL manufactured HIKARI detector) device constituted by, carried in the analysis speed of 200 to 300 points / sec to. Next, the crystal orientation information obtained by defining a 0.3μm or more regions misorientation 15 ° or more and a circle equivalent diameter and grain, calculates the mean misorientation in grain grain, grain determining the proportion of crystal grain misorientation is 5 ~ 14 ° of the inner. Average misorientation of crystal grains and the grains defined above can be calculated using a software "OIM Analysis (registered trademark)" supplied with the EBSD analysis device.
　The definitive present invention "intragranular orientation difference" represents the azimuthal distribution of the crystal grains "Grain Orientation Spread (GOS)" miss in its value plastic deformation of the stainless steel by "EBSD method and X-ray diffraction method analysis of orientation ", Hidehiko Kimura et al., Japan Society of mechanical Engineers (a ed.), Vol. 71, 712 Nos., 2005, p. 1722-1728, as described in, is determined as an average value of misorientation between the crystal orientation and all the measurement points as a reference in the same crystal grains. In this embodiment, crystal orientation as the reference is an orientation obtained by averaging all the measured points in the same grain, GOS value software that came with the EBSD analyzer "OIM Analysis (TM) Version 7.0 it can be calculated by using the .1 ".
[0044]
　1, the hot rolled steel sheet according to the present embodiment, in the 1 / 4t part is EBSD analysis results of 100 [mu] m × 100 [mu] m area of ​​the rolling direction vertical section. In Figure 1, the azimuth difference is surrounded by grain boundaries is 15 ° or more, the region misorientation is 5 ~ 14 ° in the grains are displayed in gray.
[0045]
　In the present embodiment, stretch flangeability was used saddle molded article is evaluated in a saddle-type stretch flange test method. Specifically, the molded article of the saddle shape simulating the stretch flange shape composed of a straight portion and an arc portion as shown in FIG. 2 by pressing, the stretch flangeability using limiting forming height at that time evaluate. In the saddle-type stretch flange testing of the present embodiment, 50 ~ 60 mm radius of curvature R of the corner, with the opening angle theta 120 ° and the saddle-shaped molded article, when the 11% clearance when punching the corner portion measuring the limits forming height H (mm). Here, the ratio of the thickness of the clearance between the clearance of the punching die and a punch test specimens. Since the clearance is actually determined by a combination of punching tool and the plate thickness, and 11%, which means that within the ranges of 10.5 to 11.5%. Determination limit forming height observes the presence or absence of cracks with more than one-third of the length of the plate thickness by visual observation after molding, and the forming height limit cracks is not present.
[0046]
　And hole expanding test as is used as a test method corresponding to stretch flangeability conventional, since the circumferential strain is to fracture with little distribution, the time of actual stretch flanging strain near the tear portion and stress gradient is different. The hole expanding test, such as an evaluation at the time of breakage of the through thickness occurs, not in evaluation reflecting the original stretch flangeability. On the other hand, in the saddle-type stretch flange tests used in this embodiment, since the stretch flangeability considering strain distribution can be evaluated, it can be evaluated that reflect the original stretch flangeability.
[0047]
　In the hot rolled steel sheet according to the present embodiment, the area ratio of the tissue to be observed with an optical microscope organizations such as ferrite or bainite, is directly related to the crystal grain ratio of misorientation is 5 ~ 14 ° in the grains not. In other words, for example, even if the hot-rolled steel sheet having the same area ratio of ferrite and bainite area ratio, not necessarily the ratio of crystal grain misorientation is 5 ~ 14 ° in the grains are the same. Therefore, the ferrite area ratio, by merely controlling the bainite area ratio, it is impossible to obtain a characteristic corresponding to a hot rolled steel sheet according to the present embodiment.
[0048]
　Hot rolled steel sheet according to the present embodiment can be obtained by a production method comprising hot rolling and cooling steps such as follows.
[0049]

　In the hot rolling process, by heating the slab having the above-described chemical composition to obtain a hot-rolled steel sheet subjected to hot rolling. Slab heating temperature is preferably set to SRTmin ° C. or higher 1260 ° C. or less represented by the following formula (a).
　SRTmin = 7000 / {2.75-log ([Ti] × [C])} - 273 ··· (a)
　where, [Ti] in the formula (a), [C] is the mass% Ti, shows a content of C.
　Hot rolled steel sheet according to the present embodiment is contained Ti, the slab heating temperature is less than SRTmin ° C., Ti does not sufficiently solution. When Ti during slab heating is not solution, by fine precipitation of Ti as a carbide (TiC), it is difficult to improve the strength of steel by precipitation strengthening. Also, the carbide (TiC) to secure the C by forming, it is difficult to suppress the generation of harmful cementite to burring properties. In this case, undesirable because crystal orientation difference in the grains is also reduced the proportion of crystal grains of 5 ~ 14 °.
　On the other hand, if the heating temperature in the slab heating step is at 1260 ° C. greater than the yield by the scale off is reduced. Therefore, the heating temperature is preferably set to 1260 ° C. or less than SRTmin ° C..
[0050]
　If the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains more than 20%, in the hot rolling to be performed on heated slabs, in finish rolling of the subsequent three-stage (final 3 passes) cumulative strain on that 0.5 to 0.6, is effective to perform the cooling to be described later. This grain misorientation in the grains is 5 ~ 14 ° is relatively low because it generates by transformation para equilibrium, thereafter while limited to a certain range the dislocation density of austenite before transformation by limiting the range of the cooling rate is because it is possible to control the generation of crystal grains misorientation in the grains is 5 ~ 14 °.
　That is, by controlling the cumulative strain and subsequent cooling in the subsequent three stages of the finishing rolling, it is possible to control the nucleation frequency and subsequent growth rate of the crystal grains misorientation is 5 ~ 14 ° in the grains, the volume fraction resulting can also be controlled. More specifically, the dislocation density of austenite introduced by the finish rolling is involved primarily in the nucleation frequency, the cooling rate after rolling is mainly involved in the growth rate.
　The cumulative strain of the subsequent three stages of the finishing rolling is less than 0.5, the dislocation density of austenite to be introduced is not sufficient and the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains is less than 20% undesirable since. Further, when the accumulation of subsequent three stages of the finishing rolling strain is 0.6 greater than recrystallization austenite during hot rolling occurs, the accumulation dislocation density during the transformation is reduced. This is not preferable because the ratio of crystal grain misorientation is 5 ~ 14 ° in the grains becomes less than 20%.
　Finish rolling subsequent three stages in the present embodiment cumulative strain (Ipushironeff.) Can be determined by the following equation (1).
　εeff. = Σεi (t, T ) ··· (1)
where,
　.epsilon.i (t, T) = Ipushiron'ai0 / exp {(t / .tau.R) 2/3 },
　.tau.R = .tau.0 · exp (Q / RT),
　= 8.46 × .tau.0 10 -6
　,
　Q = 183200J, R = 8.314 J / K · mol are,,
　Ipushiron'ai0 represents the strain logarithm of time pressure, t is the accumulated time immediately before cooling in the path shows, T is showing the rolling temperature in the path.
[0051]
　Rolling end temperature is preferably set to Ar @ 3 ° C. or higher. When the rolling finishing temperature lower than Ar @ 3 ° C., rising to excessive dislocation density of austenite before transformation, misorientation in the grains is grain is 5 ~ 14 ° and it is difficult to 20% or more.
　Further, the hot rolling, including a rough rolling and finish rolling, finish rolling be carried out using a tandem mill to obtain a predetermined thickness by continuously rolling the linearly arranged to one direction a plurality of rolling mill It is preferred. Also, when performing finish rolling by using a tandem rolling mill, by performing cooling between the rolling mill and the rolling mill (between stand cooling), and scope steel temperature below ~ Ar3 + 150 ℃ Ar3 ℃ higher in the finish rolling it is preferably controlled to become. When the maximum temperature of the steel sheet during finish rolling exceeds Ar @ 3 + 0.99 ° C., toughness is feared that degradation to the particle size is too large.
　By performing the hot rolling conditions as above, to limit the dislocation density range before austenite transformation, and facilitates orientation differences in the grains to obtain a crystal grain which is 5 ~ 14 ° in the desired proportions Become.
[0052]
　Ar3 is based on the chemical components of the steel sheet, in consideration of the influence of the transformation point by pressure, is calculated by the following equation (2).
Ar3 = 970-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] -92 × ([Mn] + [Mo] + [Cu]) - 46 × ([Cr] + [Ni ]) ... (2)
　where, [C], [Si] , [P], [Al], [Mn], [Mo], [Cu], [Cr], [Ni] , respectively, shows C, Si, P, Al, Mn, Mo, Cu, Cr, the content by mass percent and Ni. For elements that are not contained, calculated as 0%.
[0053]

　against hot-rolled steel sheet after hot rolling, to cool. Against hot-rolled steel sheet hot rolling is completed in the cooling step, at 10 ° C. / s or more cooling rate, then cooled to a temperature range of 650 - 750 ° C. (first cooling), in this temperature range, 1 - and held for 10 seconds, then, to a temperature range of 450 ~ 650 ° C. and cooled at 30 ° C. / s or more cooling rate to (second cooling).
　When the first is a cooling rate of less than 10 ° C. / s of cooling it is not preferred because the crystal orientation difference in the grains is the proportion of crystal grains of 5 ~ 14 ° is reduced. Further, when the cooling stop temperature of the first cooling is lower than 650 ° C., it becomes difficult to obtain more than 5% ferrite area ratio, the crystal orientation difference in the grains is 5 ~ 14 ° grains ratio undesirably lowered.
　Further, when the cooling stop temperature of the first cooling is at 750 ° C. greater, it becomes difficult to an area ratio obtained more than 30% of bainite, misorientation of the grains is 5 ~ 14 ° grains ratio undesirably lowered. Further, when the holding time at 650 ~ 750 ° C. greater than 10 seconds, with harmful cementite is likely to generate in burring workability, it is difficult to an area ratio obtained more than 30% of bainite, further intragranular crystals undesirably misorientation proportion of 5 ~ 14 ° of the crystal grains is reduced. When the holding time at 650 ~ 750 ° C. is less than 1 second, decreases as it becomes difficult to obtain a ferrite area ratio of 5% or more, the crystal orientation difference in the grains is the proportion of crystal grains of 5 ~ 14 ° which is not preferable.
　Moreover, the cooling rate of the second cooling is less than 30 ° C. / s, with harmful cementite is likely to generate the burring properties, crystal orientation difference in the grains is the proportion of crystal grains of 5 ~ 14 ° drop which is not preferable. Or a second cooled below stop temperature is 450 ° C. cooling, when it comes to 650 ° C. greater, it is difficult misorientation in the grains to obtain a crystal grain which is 5 ~ 14 ° in the desired proportions.
　First cooling, the upper limit of the cooling rate in the second cooling, in particular but not necessarily limited to, the installed capacity of the cooling equipment may be less considered to 200 ° C. / s.
[0054]
　According to the manufacturing method described above, an area ratio, with including 5 to 60% ferrite, 30 to 95 percent of bainite, surrounded by grain boundary misorientation is 15 ° or more and a circle equivalent diameter of 0 If .3μm the at whose area less was defined as the crystal grains can be the ratio of the grain misorientation is 5 ~ 14 ° in the grains is in the area ratio, obtaining a tissue is 20-100%.
　In the above-described manufacturing method, after introducing the processed dislocations austenite by controlling the hot rolling conditions, it is important to leave moderately the introduced processed dislocations by controlling the cooling conditions. That is, even when controlling the hot rolling conditions and cooling conditions alone can not be obtained hot rolled steel sheet according to the present embodiment, it is important to control the hot rolling conditions and cooling conditions at the same time. The conditions other than the above, for example, wound in a known manner after the second cooling, may be a known method, it is not necessary to particularly limit.
Example
[0055]
　Hereinafter, examples of the hot-rolled steel sheet of the present invention, the present invention will be described more specifically, the present invention is not intended to be well limited to the following examples, prior to, be adapted to the purports described hereinbefore and hereinafter it is also possible to practice after appropriate modifications or variations within a range, they both are included in the technical scope of the present invention.
[0056]
　In the present embodiment, first, Steels having the composition shown in Table 1 to produce a billet, and heating the billet, after rough rolling in hot, subsequently, following It was finished rolling under the conditions shown in Table 2. Thickness after the finish rolling was 2.2 ~ 3.4mm. Described in Table 2, Ar @ 3 (° C.) was determined using the following equation (2) from components shown in Table 1.
Ar3 = 970-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] -92 × ([Mn] + [Mo] + [Cu]) - 46 × ([Cr] + [Ni ]) (2)
in addition, the cumulative distortion of finishing three stages was determined from the following equation (1).
　εeff. = Σεi (t, T ) ··· (1)
where,
　.epsilon.i (t, T) = Ipushiron'ai0 / exp {(t / .tau.R) 2/3 },
　.tau.R = .tau.0 · exp (Q / RT),
　= 8.46 × .tau.0 10 -6
　,
　Q = 183200J, R = 8.314 J / K · mol are,,
　Ipushiron'ai0 represents the strain logarithm of time pressure, t is the accumulated time immediately before cooling in the path shows, T is showing the rolling temperature in the path.
Table 1 blank means that analytical values were below the detection limit.
[0057]
[Table 1]

[0058]
[Table 2]

[0059]
　The obtained hot-rolled steel sheet, structural fraction of each tissue (area ratio), and misorientation of intragranular was determined the percentage of the crystal grains is 5 ~ 14 °. Tissue fraction (area ratio) was determined by the following method. First, the sample taken from the hot-rolled steel sheet was etched with nital. To structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m in 1/4 depth position of the plate thickness using an optical microscope after etching, by performing image analysis, the area ratio of ferrite and pearlite, and bainite and martensite to obtain the total area ratio of the site. Then, using the Repera corroded samples, with respect to structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m at the position of 1/4 depth of thickness with an optical microscope, by performing image analysis, residual austenite and martensite to calculate the total area ratio of the site.
　Further, using the cutting plane from the rolled surface direction normal to 1/4 depth of thickness sample, it was determined volume fraction of residual austenite by X-ray diffraction measurement. The volume fraction of retained austenite are the equivalent to the area ratio, which was used as a area ratio of residual austenite.
　This method gave ferrite, bainite, martensite, residual austenite, the respective area ratios perlite.
　The ratio of the crystal grains misorientation is 5 ~ 14 ° in the grains was measured by the following method. First, the rolling direction vertical section of the 1/4 depth position of the sheet thickness t from the surface of the steel sheet (1 / 4t part), 200 [mu] m in the rolling direction, the area of 100μm in the rolling surface normal direction in measured intervals 0.2μm to obtain the crystal orientation information by EBSD analysis. Here EBSD analysis, using a thermal field emission scanning electron microscope (JEOL Ltd. JSM-7001F) and EBSD detector (TSL manufactured HIKARI detector) device constituted by, carried in the analysis speed of 200 to 300 points / sec did. Next, the crystal orientation information obtained by defining a 0.3μm or more regions misorientation 15 ° or more and a circle equivalent diameter and grain, calculates the mean misorientation in grain grain, grain misorientation inner was determined the ratio of 5 ~ 14 ° a crystal grain. Average misorientation of crystal grains and the grains defined above was calculated using the software provided with EBSD analyzer "OIM Analysis (registered trademark)".
[0060]
　Next, the tensile test, determined and a yield strength tensile strength, the saddle stretch flange tested to determine the limits forming height. Also, evaluated as an index of stretch flangeability the product of tensile strength and (MPa) limit molding height as (mm), if the product is more than 19500mm · MPa, is determined that excellent stretch flangeability.
　Tensile tests were taken from the direction perpendicular to the rolling direction JIS5 No. Tensile test pieces, using the test pieces were tested in accordance with JISZ2241.
　Also, the saddle-type stretch flange test uses R60mm curvature radius of the corner, and the opening angle theta 120 ° the saddle-type molded article was performed with 11% clearance when punching the corner portion. Further, the limit forming height observes the presence or absence of cracks with more than one-third of the length of the plate thickness by visual observation after molding, and the forming height limit cracks is not present.
　The results are shown in Table 3.
[0061]
[table 3]

[0062]
　As is apparent from the results shown in Table 3, when hot rolling at preferred conditions chemical components defined in the present invention (Test No.1 ~ 17), the strength of not less than 590 MPa, and the stretch flangeability index there was obtained a high-strength hot-rolled steel sheet is 19500mm · MPa or higher.
　On the other hand, production No. 18-24, the steel No. chemical composition is outside the range of the present invention It is a comparative example using a ~ g. In addition, No. 25-37 As a result of out of the range manufacturing conditions is desired, the scope of any or both the present invention, the proportion of crystal grain misorientation tissue and intragranular is observed with an optical microscope is 5 ~ 14 ° which is a comparative example that did not meet. In these examples, the stretch flangeability has not satisfied the target value. In addition, tensile strength in some cases was also lower.
Industrial Applicability
[0063]
　According to the present invention, it is possible to provide that can be applied to members that strict stretch flange formability yet high strength is required, a high-strength hot-rolled steel sheet having excellent stretch flangeability. These steel sheets, in order to contribute to improved fuel efficiency of an automobile, has high industrial applicability.

The scope of the claims
[Claim 1]
　Chemical composition, in
　mass%,
　C:
　0.020 ~ 0.070%, Si: 0.10 ~ 1.70%,
　Mn: 0.60 ~ 2.50%, Al: 0.01 ~ 1.00
　%,
　Ti:
　0.015 ~ 0.170%, Nb: 0.005 ~
　0.050%, Cr: 0 ~ 1.0%, B:
　0 ~ 0.10%, Mo: 0 ~ 1.0% , 　Cu:
　　0 ~ 　2.0%, Ni: 0 ~ 2.0%, Mg: 0 ~ 0.05%, 　REM: 0 ~ 0.05%, Ca: 0 ~ 　0.05%, Zr: 0 ~ 0.05% containing,　P: 0.05% or 　less, S: 0.010% or 　less, N: 0.0060% or less, limited to, 　the balance being Fe and impurities; 　tissue, an area ratio comprises 5 to 60% ferrite 30 to 95% bainite, 　in the tissue and the boundary misorientation is 15 ° or more and the grain boundary, by the grain boundary Rarely, and if the circle equivalent diameter of the region is 0.3μm or more was defined as the crystal grains, the proportion of the grain misorientation is 5 ~ 14 ° in the grains is an area ratio, with 20-100% there; hot-rolled steel sheet, characterized in that.

[Claim 2]
　Tensile strength, more than 590 MPa, the tensile strength and hot rolled steel sheet according to claim 1, the product of the marginal forming height of saddle stretch flange test is equal to or is 19500mm · MPa or higher.
[Claim 3]
　The chemical composition, in
　mass%,
　Cr: 0.05 ~ 1.0%, B: 0.0005 ~ 0.10%,
characterized in that it comprises at least one selected from claims 1 or 2 hot-rolled steel sheet according to.
[Claim 4]
　The chemical composition, in
　mass%,
　Mo: 0.01 ~ 1.0%,
　Cu: 0.01 ~ 2.0%, Ni: 0.01% ~ 2.0%
of one or more selected from hot-rolled steel sheet according to any one of claims 1 to 3, characterized in that it comprises.
[Claim 5]
　The chemical composition, by
　mass%,
　Ca:
　0.0001 ~ 0.05%, Mg: 0.0001 ~ 0.05%,
　Zr: 0.0001 ~ 0.05%, REM: 0.0001 ~ 0. 0.05%
hot-rolled steel sheet according to any one of claims 1 to 4, characterized in that it comprises one or more selected from.