JP4160840B2 - High formability and high strength hot-rolled steel sheet with excellent shape freezing property and its manufacturing method - Google Patents

Description

【０１３２】
【表２】

【０１３３】
【表３】

【０１３４】
【表４】

【０１３５】
表３および表４において、Ｎｏ．７はＴＦＳ−ＴＦＥが１８０℃と本発明の範囲外であり、同時に△εが本発明の条件を満たさないために、集合組織が本発明の範囲外となり結果として良好な形状凍結性が得られていない。
【０１３６】
Ｎｏ．８およびＮｏ．９は、ＣＴが発明範囲外であり良好な形状凍結性が得られていない。Ｎｏ．１０は△εが所定の範囲になく、結果として集合組織の発達が不充分となり、良好な形状凍結性が得られていない。
【０１３７】
Ｎｏ．２４は、Ｃ量が本発明の範囲外であり、その結果、残留γ量がゼロとなり良好な成形性と形状凍結性のバランスが得られていない。Ｎｏ．２５は、Ｃ量が本発明の上限より高く溶接性が低いばかりでなく、△εが低いことと合わせて、良好な形状凍結性が得られていない。
【０１３８】
Ｎｏ．２６は、ＡｌとＳｉの添加量が十分でないために、良好な成形性と形状凍結性のバランスが得られていない。Ｎｏ．２７は、Ｓｉが本発明の上限を超えているために、溶接性が悪いばかりでなく、十分な集合組織発達が得られず、良好な形状凍結性が得られていない。
【０１３９】
Ｎｏ．２８は、Ｐが高い為に溶接性が悪い。Ｎｏ．２９は、Ｂの添加量が本発明の上限以上であるため、加工性が悪く、良好な成形性と形状凍結性のバランスが得られていない。
【０１４０】
Ｎｏ．３０は、Ｍｎ、Ｎｉ、Ｃｕの添加量の総和が本発明の範囲外となっており、溶接性が悪いばかりでなく良好な成形性と形状凍結性のバランスが得られていない。Ｎｏ．３１は、焼入れ性に重要なＭｎ、Ｎｉ、Ｃｒ、Ｃｕ、Ｍｏ、Ｓｎの添加量の総和が本発明の下限値以下であるために焼入れ性が不足し、その結果、良好な成形性と形状凍結性のバランスが得られていない。
【０１４１】
Ｎｏ．３２は、Ｎｂ、Ｔｉ、Ｖの添加量の総和が本発明の上限値を超えているために、成形性が悪いばかりでなく局部伸びの異方性も大きくなり良好な形状凍結性が得られていない。
【０１４２】
その他に示した本発明範囲内の化学成分の鋼を本発明範囲内の熱延条件によって製造した場合には良好な延性、異方性とともに、良好な加工性と形状凍結性のバランスが得られることがわかる。
【０１４３】
【発明の効果】
本発明によって、スプリング・バック量が少なく、形状凍結性に優れると同時に異方性が少ない良好なプレス成形性を有する薄鋼板が提供できるようになり、従来は形状不良の問題から高強度鋼板の適用が難しかった部品にも、高強度鋼板が使用できるようになると同時に、効率的に自動車の安全性と車体の軽量化を両立することが可能となり、ＣＯ₂排出削減等の環境・社会からの要請に応える自動車製造に大きく貢献することができる。
【０１４４】
従って、本発明は、工業的に極めて高い価値のある発明である。
【図面の簡単な説明】
【図１】（１０００／ρ）×（１０００／ＴＳ）の値をＴＳ（引張り強度）＊Ｅｌ（破断伸び）に対してプロットした図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high workability and high strength hot-rolled steel sheet having excellent shape freezing properties and a method for producing the same, which can be used for automobile members and the like and can efficiently reduce the weight of automobile members.
[0002]
[Prior art]
In order to suppress the amount of carbon dioxide emission from automobiles, it has been promoted to reduce the weight of automobile bodies using high-strength steel sheets. In addition, in order to ensure the safety of passengers, high strength steel plates are often used in automobile bodies in addition to mild steel plates. Furthermore, in order to reduce the weight of automobile bodies in the future, new demands for increasing the strength level of use of high-strength steel sheets are increasing.
[0003]
However, when bending deformation is applied to a high-strength steel plate, the shape after processing tends to return away from the shape of the processing jig and return to the shape before processing because of its high strength. The phenomenon of returning to the original shape direction even after processing is called spring back.
[0004]
When this spring back occurs, the shape of the target processed part cannot be obtained. Therefore, the conventional automobile body has been mainly used only for high-strength steel sheets of 440 MPa or less.
[0005]
Although it is necessary for automobile bodies to use a high-strength steel sheet of 490 MPa or more to reduce the weight of the car body, there is no high-strength steel sheet with less spring back and good shape freezing. It is.
[0006]
Needless to say, increasing the shape freezing property after processing of a high-strength steel plate or mild steel plate of 440 MPa or less is also extremely important for improving the shape accuracy of products such as automobiles and home appliances.
[0007]
Patent Document 1 discloses an austenitic stainless cold-rolled steel sheet having a small springback amount, characterized in that the degree of {200} texture accumulation in a plane parallel to the rolling surface is 1.5 or more. However, there is no description about a technique for reducing the springback amount of the ferritic steel sheet.
[0008]
On the other hand, it is also required to ensure good press formability capable of being press-formed into a complex-shaped automobile part to which a high-strength steel plate is applied.
[0009]
As a method for improving the press formability of a high-strength steel sheet, for example, Patent Document 2 proposes a method in which a certain amount or more of austenite remains in the steel and a work-induced transformation from the retained austenite to martensite is utilized. However, in such a high-strength steel sheet having good workability, a method for reducing the anisotropy of formability and a method for improving the above-described shape freezeability are not clarified.
[0010]
Furthermore, as for a method for improving impact energy absorption capacity at the time of a car collision while having good workability, for example, Patent Document 3 proposes a method using residual austenite, but such good In a high-strength steel sheet having excellent workability and impact energy absorption capability, the method for reducing the anisotropy of formability and the method for improving the shape freezing property described above are not clarified.
[0011]
[Patent Document 1]
JP-A-10-72644
[Patent Document 2]
Japanese Patent Laid-Open No. 6-145892
[Patent Document 3]
JP 11-080879 A
[0012]
[Problems to be solved by the invention]
Increasing the strength of steel plates applied to automotive parts subjected to bending increases the amount of spring back as the strength of the steel plates increases, resulting in shape defects and limiting the application of high strength steel plates. Currently. In addition, good press formability and high impact energy absorption capability are indispensable characteristics for high strength steel sheets to be applied to automobile parts and the like. The present invention drastically solves this problem and provides a high-strength hot-rolled steel sheet having a good shape freezing property and a good press formability, and a method for producing the same.
[0013]
[Means for Solving the Problems]
According to the conventional knowledge, as a measure for suppressing the spring back, it was considered to be important to lower the yield point of the steel plate for the time being. In order to lower the yield point, a steel plate having a low tensile strength has to be used. However, this alone is not the fundamental solution for improving the bending workability of the steel sheet and keeping the amount of spring back low.
[0014]
Therefore, in order to improve the bending workability and fundamentally solve the occurrence of springback, the present inventors have newly focused on the influence on the bending workability of the texture of the steel sheet, and its effects Were investigated and studied in detail. And the steel plate excellent in bending workability was found.
[0015]
That is, as a result of the research, X-ray randomness in the {100} <011> to {223} <110> orientation group and each orientation of {554} <225>, {111} <112> and {111} <110> It is clear that bending workability is dramatically improved by controlling the strength ratio, and further by setting at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction as low as possible. I made it.
[0016]
However, if at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is set to a low value, the press formability is expected to deteriorate, and it becomes difficult to achieve both shape freezing property and workability. . Therefore, as a result of earnest research, the inventors of the present invention simultaneously established the texture control and the austenite residue in the microstructure, and further controlled the properties of the retained austenite, thereby freezing the shape and the workability. It was clarified that the collision energy absorption ability can be enhanced simultaneously.
[0017]
In addition, not limiting the blank sampling direction for forming various parts contributes greatly to improving the yield of steel, but for this purpose, the anisotropy of ductility, especially the anisotropy of uniform elongation, should be reduced. Has an important meaning. The inventors of the present invention can reduce the anisotropy of uniform elongation while controlling the shape freezing property and workability by controlling the start temperature and end temperature of finish hot rolling of the steel sheet by experiment. I found it possible.
[0018]
The present invention is configured based on the above-mentioned knowledge, and the main points thereof are as follows.
[0019]
  (1)Containing C, Si, Al, Mn and P, in mass%,
  Group A
    C: 0.02 to 0.3%,
    Mn; 0.01 to 3%
    Ni: 3% or less,
    Cr: 3% or less,
    Cu: 2% or less,
    Mo; 2% or less,
    W: 2% or less,
    Sn: 0.3% or less,
  Group B
    Si; 0.003 to 3%,
    Al: 3% or less,
2% or 3 or more types including C and Mn of group A in total including 0.5% or more and 5% or less, and 2 types of group B including 0.5% or more and 4% or less in total In addition, P is contained 0.2% or less, the balance consists of Fe and inevitable impurities,The microstructure has ferrite or bainite as the volume fraction maximum phase,Including residual austenite of 1% to 25% in volume fraction,The average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at least 1/2 plate thickness is 2.5 or more, and {554} <225>. The average value of the X-ray random intensity ratio of the three crystal orientations of {111} <112> and {111} <110> is 3.5 or less, the local elongation anisotropy ΔLEl is 4% or less, and A highly workable and high strength hot-rolled steel sheet having excellent shape freezing property, characterized by having a uniform elongation anisotropy ΔuE1 or less.
    However, ΔuEl = {| uEl (L) −uEl (45 °) | + | uEl (C)
                    -UEl (45 °) |} / 2
          ΔLEl = {| LEl (L) −LEl (45 °) | + | LEl (C)
                    -LEl (45 °) |} / 2
The uniform elongation in the direction parallel to the rolling direction (L direction), vertical (C direction), and 45 ° is uEl (L), uEl (C), and uEl (45 °), respectively, and parallel to the rolling direction. The local elongations in the (L direction), vertical (C direction), and 45 ° directions are denoted by LEl (L), LEl (C), and LEl (45 °), respectively.
[0022]
  (2) Anisotropy ΔuE of the uniform elongationlIs 3% or more (1)RecordHigh workability and high strength hot-rolled steel sheet with excellent freezing shape.
[0024]
  (3Further, it is characterized in that it contains 0.01% or more and 3% or less of Co by mass% (1)Or (2)High workability and high strength hot-rolled steel sheet with excellent shape freezing property as described in 1.
[0025]
  (4And (1) to (%) containing, in mass%, one or more of Nb, Ti, and V in total of 0.001% to 0.8%.3The high workability high-strength hot-rolled steel sheet having excellent shape freezing property according to any one of the above.
[0027]
  (5And (1) to (1), further comprising 0.01% or less by mass of B.4The high workability high-strength hot-rolled steel sheet having excellent shape freezing property according to any one of the above.
[0028]
  (6) Furthermore, in mass%,
    Ca; 0.0005 to 0.005%,
    Rem; 0.001 to 0.02%,
    Ce; 0.0001 to 0.05%,
    La; 0.0001-0.05%,
    Mg; 0.0001 to 0.05%,
    Ta; 0.0001 to 0.05%,
(1) to (1) characterized by containing one or more of5The high workability high-strength hot-rolled steel sheet having excellent shape freezing property according to any one of the above.
[0029]
  (7) (1)-(6The high workability high-strength hot-rolled steel sheet with excellent shape-freezing property, which is obtained by plating the high-workability high-strength hot-rolled steel plate with excellent shape-freezing property described in any of the above.
[0030]
  (8) (1)-(7In producing a high workability high strength hot-rolled steel sheet having excellent shape freezing property according to any one of(1), (3)-(6)When the cast slab having the chemical component according to any one of the above is cast as it is or after being once cooled to 1000 ° C to 1300 ° C and hot rolled, Ar3 ° C to (Ar3 + 150) ° C The total rolling reduction in the temperature range is controlled to be 25% or more, and the finishing hot rolling start temperature TFS (° C.), the finishing hot rolling completion temperature TFE (° C.), and the calculated residual strain Δε at the completion of finishing hot rolling are The hot rolling is finished so that the following formulas (1) to (4) are all satisfied at the same time, and after the hot rolling is cooled, the critical temperature To (° C.) or less determined by the chemical components of the steel shown in the following formula (5) And the manufacturing method of the high workability high-strength hot-rolled steel plate excellent in the shape freezing property characterized by winding up at the temperature of 480 degrees C or less and 300 degrees C or more.
    TFE ≧ Ar3 (1)
    TFS ≦ 1100 ° C (2)
    20 ° C. ≦ TFS−TFE ≦ 150 ° C. (3)
    Δε ≧ (TFS−TFE) / 375 (4)
    To = −650.4 × {C% / (1.82 × C% −0.001)} + B (5)
  Here, B is obtained from the chemical composition of steel expressed in mass%.
    B = −50.6 × Mneq + 894.3
    Mneq = Mn% + 0.24 x Ni% + 0.13 x Si% + 0.38 x Mo% + 0.55 x Cr% + 0.16 x Cu%
          −0.50 × Al% −0.45 × Co% + 0.90 × V%
    However,
    Ar3 = 901-325 × C% + 33 × Si% + 287 × P% + 40 × Al%
          −92 × (Mn% + Mo% + Cu%) − 46 × (Cr% + Ni%)
    [Delta] [epsilon] is equivalent strain [epsilon] i (i is 1 to n) given at each stand of n-stage finish rolling for rolling, time ti (seconds) between each stand (i = 1 to n-1), and from the last stand Time tn (seconds) until the start of cooling, rolling temperature Ti (K) (i = 1 to n) at each stand, and constant R = 1.987.
          Δε = Δε1 + Δε2 ++. + Δεn
    Where Δεi = εi × exp {-(ti * / τn)^(2/3)}
          τi = 8.46 × 10^-9Xexp {43800 / R / Ti}
          ti * = τn × {ti / τi + t (i + 1) / τ (i + 1) +.
                  + Tn / τn}
  (9) It is characterized by controlling the friction coefficient to be 0.2 or less in at least one pass or more of hot rolling in the temperature range of Ar3 to (Ar3 + 150) ° C. (8The manufacturing method of the high workability high strength hot-rolled steel plate excellent in the shape freezing property of description.
[0031]
  (10) (8)Or(9)A method for producing a high-workability, high-strength hot-rolled steel sheet excellent in shape freezing, characterized by subjecting a hot-rolled steel sheet produced by the described production method to skin pass rolling of 0.1% to 5%.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
The contents of the present invention will be described in detail below.
[0033]
The average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness, and {554} <225>, {111} <112> and Average value of X-ray random intensity ratio of three crystal orientations of {111} <110>:
This is a particularly important characteristic value in the present invention. The average value of {100} <011> to {223} <110> orientation groups when the X-ray diffraction of the plate surface at the plate thickness center position and the intensity ratio of each orientation with respect to the random sample is obtained is 2. Must be 5 or more. If this is less than 2.5, the shape freezing property is poor.
[0034]
The main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} < 110> and {223} <110>.
[0035]
The X-ray random intensity ratio in each of these directions is a three-dimensional texture calculated by the vector method based on the {110} pole figure, or a plurality of poles among {110}, {100}, {211}, {310} pole figures. What is necessary is just to obtain | require from the three-dimensional texture calculated by the series expansion method using the figure (preferably three or more).
[0036]
For example, the X-ray random intensity ratio of each crystal orientation in the latter method includes (001) [1-10], (116) [1-10], (114 in the φ2 = 45 ° cross section of the three-dimensional texture. ) [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are.
[0037]
The average value of {100} <011> to {223} <110> orientation group is an arithmetic average of each of the above-mentioned orientations. When the strengths of all the above directions cannot be obtained, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> Alternatively, an arithmetic average of each direction may be substituted.
[0038]
Furthermore, the average value of the X-ray random intensity ratio of the three crystal orientations {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness is 3.5 or less. Must-have. If this exceeds 3.5, it will be difficult to obtain good shape freezing properties even if the strength of the {100} <011> to {223} <110> orientation groups is appropriate.
[0039]
The X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> may be obtained from the three-dimensional texture calculated according to the above method. More preferably, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> orientation groups is 3.0 or more, more preferably 4.0 or more, {554} <225>, { The arithmetic average value of the X-ray random intensity ratios of 111} <112> and {111} <110> is less than 2.5.
[0040]
The reason why the X-ray intensity of the crystal orientation described above is important for the shape freezing property during bending is not necessarily clear, but it is presumed to be related to the sliding behavior of the crystal during bending deformation. .
[0041]
Samples to be subjected to X-ray diffraction are prepared so that the steel plate is reduced to a predetermined thickness by mechanical polishing, etc., and then distortion is removed by chemical polishing, electrolytic polishing, etc., and at the same time, the 1/2 surface thickness becomes the measurement surface To do. When there is a segregation zone or a defect in the thickness center layer of the steel plate, causing inconvenience in measurement, the above method is used so that an appropriate surface becomes the measurement surface in the range of 3/8 to 5/8 of the plate thickness. The sample may be adjusted according to the above and measured.
[0042]
As a matter of course, the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness ½ but also as much as possible (especially the outermost layer to ¼ of the plate thickness), thereby further freezing the shape. Good. The crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw>.
[0043]
R value (rL) in the rolling direction and r value (rC) in the direction perpendicular to the rolling direction:
Important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that even if the X-ray intensities of the various crystal orientations described above are appropriate, good shape freezing properties cannot always be obtained.
[0044]
At the same time as the above X-ray intensity, it is essential that at least one of rL and rC is 0.7 or less. More preferably, it is 0.55 or less.
[0045]
The lower limit of rL and rC is not particularly defined, and the effect of the present invention can be obtained. The r value is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually 15%, but when the uniform elongation is less than 15%, the strain may be evaluated as close to 15% as possible within the range of uniform elongation.
[0046]
The direction in which the bending process is performed is not particularly limited because it varies depending on the processed part. However, it is preferable that the bending process is mainly performed in a direction that is perpendicular or nearly perpendicular to the direction in which the r value is small.
[0047]
Incidentally, it is generally known that there is a correlation between the texture and the r value, but in the present invention, the above-described limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are not synonymous with each other. If both limitations are satisfied at the same time, good shape freezing property cannot be obtained.
[0048]
Microstructure:
Compared with ferrite and other low-temperature products (bainite, martensite, acicular ferrite, Widmanstetten ferrite, etc.), the latter has a stronger texture development to ensure high shape freezing For this, it is preferable to adjust the volume fraction of ferrite so as not to exceed 80%. Here, bainite may or may not contain iron carbide particles in the microstructure.
[0049]
If the phase with the largest volume fraction is other than ferrite or bainite, the strength of the steel will be increased more than necessary to deteriorate its workability, or unnecessary carbide precipitation will not ensure the required amount of retained austenite. Since the workability of the steel sheet is significantly deteriorated, the phase with the largest volume fraction is limited to ferrite or bainite.
[0050]
Further, martensite not only increases strength but also improves fatigue strength and impact energy absorption capacity. Therefore, if necessary, it is preferable to contain 1% or more by volume fraction, but its volume ratio is 25%. If the ratio exceeds 50%, not only the strength increases more than necessary, but also the workability and toughness deteriorate significantly, so the content is preferably made 25% or less.
[0051]
In an actual automobile part, not only the shape freezing property caused by bending as described above becomes a problem in one part, but also other parts of the same part have good overhangability and drawing workability. There are many cases where high press workability is required.
[0052]
Therefore, it is necessary to improve the press workability of the steel sheet itself, in addition to the improvement of the shape freezing property during the bending process in which the texture is controlled.
[0053]
While satisfying that at least one of rL and rC, which is a feature of the steel of the present invention, is 0.7 or less, the present inventors have incorporated a steel sheet as a method for enhancing drawability as well as stretchability. We have found that it is most desirable to retain austenite.
[0054]
At this time, when the volume fraction of retained austenite was less than 1%, the effect was small, so 1% was set as the lower limit of the retained austenite volume fraction. Preferably, the lower limit is 2%. The larger the amount of retained austenite, the better the formability. However, when the retained austenite with a volume fraction of more than 25% is included, the working stability of austenite decreases, and conversely, the workability of the steel material decreases. Therefore, it is preferable to set 25% as the upper limit of the retained austenite volume fraction.
[0055]
When the average particle size of retained austenite is larger than the particle size of ferrite or bainite, which is the phase with the largest volume fraction, the stability of retained austenite itself decreases, and the formability and impact energy absorption capacity also decrease. The retained austenite grains are preferably made as fine as possible.
[0056]
Therefore, the ratio of the maximum grain size of retained austenite to the grain size of ferrite or bainite, which is the phase with the largest volume fraction, is desirably 0.6 or less.
[0057]
Although the lower limit of this ratio is not particularly defined, the effect of the present invention can be obtained. However, extremely reducing the retained austenite grains to reduce the effect of retained austenite by stabilizing the austenite more than necessary. The ratio of the maximum grain size of retained austenite to the grain size of ferrite or bainite, which is the phase with the largest volume fraction, is preferably 0.05 or more.
[0058]
The amount of residual austenite is, for example, that of the (200) plane, (211) plane of ferrite, and the (200) plane, (220) plane, (311) plane of austenite by X-ray analysis using Mo Kα rays. It can be calculated by the method shown in Journal of The Iron and Steel Institute, 206 (1968) p60 using the integrated reflection intensity.
[0059]
Further, ferrite or bainite, which is the phase having the maximum volume fraction, can be measured by image processing or a point counting method based on a nital corrosion photograph.
[0060]
Ductile anisotropy:
In the case of press forming a steel plate, the uniform elongation of the steel plate, that is, the n value is important. Especially in the case of high-strength steel sheets that are mainly stretch-formed, if this uniform elongation (n value) has anisotropy, it is necessary to carefully select the blank cutting direction depending on the part. Degradation and steel sheet yield decrease. In some cases, it may not be possible to mold into a desired shape.
[0061]
When anisotropy parameters are defined by the following equation using elongation in the direction parallel to the rolling direction (L direction), vertical (C direction) and 45 ° direction (uniform elongation uEl, local elongation LEl):
ΔuEl = {| uEl (L) −uEl (45 °) | + | uEl (C)
-UEl (45 °) |} / 2
ΔLEl = {| LEl (L) −LEl (45 °) | + | LEl (C)
-LEl (45 °) |} / 2
It becomes.
[0062]
In steels having a tensile strength of about 400 MPa or more (the maximum strength obtained by a tensile test), if this local elongation anisotropy ΔLEl exceeds 4%, the stretch flangeability that plays an important role in local deformation is Since the good shape freezing property cannot be obtained at the same time as deterioration, this was made the upper limit of ΔLEl.
[0063]
In addition, when ΔLEl is larger than the uniform elongation anisotropy ΔuEl, not only the ductility of the steel sheet is lowered but also good shape freezing property cannot be obtained. did.
[0064]
The influence of the chemical component of the present invention will be described below.
[0065]
C:
C is the most important element in the present invention because C is the cheapest element that contributes to the stabilization of austenite necessary for stabilizing and retaining austenite at room temperature.
[0066]
The average C content of the steel material not only affects the retained austenite volume fraction that can be secured at room temperature, but also improves the stability of residual austenite to processing by concentrating in the untransformed austenite during manufacturing heat treatment. Can be made.
[0067]
However, when the amount added is less than 0.02% by mass, the volume fraction of the residual austenite finally obtained cannot be ensured to be 1% or more, so 0.02% by mass was made the lower limit.
[0068]
On the other hand, the retained austenite volume fraction that can be secured increases as the average C content of the steel material increases, and the stability of retained austenite can be secured while securing the retained austenite volume fraction.
[0069]
However, when the amount of C added to the steel material becomes excessive, the strength of the steel material is increased more than necessary, and not only the formability such as press working is inhibited, but also the dynamic stress increase inhibition compared to the static strength increase. At the same time, the use of steel as a part is limited by reducing weldability. Therefore, the upper limit of C mass% of the steel material is set to 0.3 mass%.
[0070]
Mn, Ni, Cr, Cu, Mo, W, Sn:
Mn, Ni, Cr, Cu, Mo, W, and Sn are all important elements for controlling the formation of a structure due to transformation from austenite to ferrite. In particular, when the addition amount of C is limited from the viewpoint of weldability, austenite can be effectively left by adding an appropriate amount of such an element.
[0071]
These elements, although not as much as Al and Si, have an effect of suppressing the formation of cementite, and also serve to assist the concentration of C into austenite. Furthermore, these elements have the function of increasing the dynamic deformation resistance at high speed by strengthening the solution of ferrite and bainite as a matrix together with Al and Si.
[0072]
However, when the total of the addition of one or more of these elements including C is less than 0.5% by mass, the necessary retained austenite cannot be secured, and the strength of the steel material is reduced. Since effective vehicle weight reduction cannot be achieved, the lower limit of the total of one or more of these elements including C is set to 0.5 mass%.
[0073]
On the other hand, Mn: over 3% by mass, Ni: over 3% by mass, Cr: over 3% by mass, Cu: over 2% by mass, Mo: over 2% by mass, W: over 2% by mass, or Sn; If it exceeds 3% by mass, or if the total of one or more of these elements exceeds 5% by mass, it will cause hardening of the parent phase ferrite or bainite, resulting in a decrease in workability and toughness of the steel material. In order to increase the steel material cost and to raise the steel material cost, an upper limit is set for the content of each element and the upper limit of the total amount is set to 5 mass%.
[0074]
However, if the Mn addition amount is less than 0.01% by mass, an economic disadvantage is caused, so this is set as the lower limit of the Mn addition amount.
[0075]
Al, Si:
Al and Si are both ferrite stabilizing elements and have the function of improving the workability of the steel material by increasing the ferrite volume fraction. In addition, since it suppresses the formation of cementite for both Al and Si, it is possible to effectively concentrate C in austenite, so it is inevitable to leave austenite with an appropriate volume fraction at room temperature. Additive element.
[0076]
Examples of the additive element having such a function include P, Cu, Cr, Mo and the like in addition to Al and Si. A similar effect can be expected by appropriately adding such an element.
[0077]
However, when Si is less than 0.003% by mass, or when the total of one or both of Al and Si is less than 0.5% by mass, the effect of suppressing the formation of cementite is not sufficient, and is most effective for stabilizing austenite. Most of the effective added C is wasted in the form of carbides, and the volume ratio of retained austenite necessary for the present invention cannot be ensured, or the production conditions necessary for securing retained austenite are in the mass production process. Not suitable for conditions.
[0078]
Accordingly, the Si content is set to 0.003 mass% or more, and the lower limit of the total amount of one or two of Al and Si is set to 0.5 mass%.
[0079]
Moreover, when Al or Si exceeds 3% by mass, or when the total of one or both of Al and Si exceeds 5% by mass, the parent phase ferrite or bainite is hardened and embrittled, Lowering the workability of steel, lowering the toughness, and further increasing the cost of steel, and because the surface treatment characteristics such as chemical conversion treatment are remarkably deteriorated, the upper limit of each of Al and Si is 3% by mass, Moreover, the upper limit of the total amount of both was 5 mass%.
[0080]
Co:
Co is an element that is effective for controlling the microstructure through phase transformation, and at the same time, is an element that increases impact energy absorption capacity by stabilizing retained austenite.
[0081]
However, when the addition amount is less than 0.01% by mass, the effect is small, so this is set as the lower limit. Further, excessive addition causes a drop in ductility as the cost increases, so 3% in mass% was made the upper limit.
[0082]
Nb, Ti, V:
Nb, Ti, and V can increase the strength of steel by forming carbide, nitride, or carbonitride, but the total of one or more of them exceeds 0.8% by mass. In some cases, a large amount of carbide, nitride or carbonitride precipitates in the ferrite or bainite grains as the parent phase or in grain boundaries, and becomes a source of movable dislocations during high-speed deformation to obtain high dynamic deformation resistance. I can't do that.
[0083]
Moreover, since the formation of carbides inhibits the concentration of C in the retained austenite, which is most important for the present invention, and wastes C, the upper limit was set to 0.8% by mass.
[0084]
However, in order to increase the strength by adding these elements, it is necessary to add 0.001% by mass or more in total of one or more of Nb, Ti and V.
[0085]
P:
P is effective in increasing the strength of steel materials and securing retained austenite as described above, but when added in an amount exceeding 0.2% by mass, ferrite or bainite, which is the phase with the largest volume fraction, is used. The deformation resistance is increased more than necessary, and the increase in deformation resistance during high-speed deformation is hindered.
[0086]
Furthermore, the upper limit is set to 0.2% by mass because it causes deterioration of the crack resistance, fatigue characteristics, and toughness. However, in order to acquire the effect of addition of P, it is preferable to contain 0.005 mass% or more.
[0087]
B:
B is effective for strengthening the grain boundaries and increasing the strength of the steel material. However, when the amount added exceeds 0.01% by mass, not only the effect is saturated, but the steel plate strength is increased more than necessary, The upper limit was set to 0.01% by mass because it hinders the increase in deformation resistance during high-speed deformation and also reduces the workability to parts. However, in order to obtain the effect of addition of B, the content is preferably 0.0002% by mass or more.
[0088]
Ca, rare earth element (Rem):
It is an element effective for inclusion control, and the addition of an appropriate amount improves the hot workability, but excessive addition conversely promotes hot embrittlement. Therefore, if necessary, each of Ca: 0.0005 to 0.00. 005% by mass, Rem: 0.001 to 0.02% by mass.
[0089]
Here, Rem is an abbreviation for Rera Earth Metal and represents a lanthanoid element starting from La. Industrially, it is often added in the form of misch metal, and in this case, the inclusion of La and Ce is the main component.
[0090]
Rem refers to elements of Y, Sc and lanthanoid series, and is industrially a mixture thereof. In particular, among the lanthanoid-based elements, Ce contains 0.0001% by mass or more and 0.05% by mass or less, and La contains one or two of 0.0001% by mass or more and 0.05% by mass or less. It is preferable for obtaining the effect.
[0091]
Further, addition of one or two Mg of 0.0001% by mass to 0.05% by mass and Ta of 0.0001% by mass to 0.05% by mass also exhibits an equivalent effect.
[0092]
Here, in all cases, the lower limit value indicates the minimum addition amount at which the inclusion control effect is manifested, and when the maximum value is exceeded, conversely, the inclusions grow too much, thereby limiting the ultimate deformability such as hole expandability. Reduce. Addition as misch metal is advantageous in terms of cost.
[0093]
The amount of S is not particularly limited, but in order to improve ultimate deformability such as hole expansibility, S is 0.01% by mass or less, desirably 0.002% by mass or less, and more severe limit deformability is required. In such a case, the content is preferably 0.001% by mass or less.
[0094]
Similarly, for the purpose of improving ultimate deformability and toughness, N is desirably 0.01% by mass or less, and desirably 0.005% by mass or less.
[0095]
In addition, O forms an oxide and deteriorates the workability of steel as inclusions, particularly the ultimate deformability represented by hole expandability, the fatigue strength and toughness of steel, so 0.01% by mass or less. It is desirable to control.
[0096]
The production method of the present invention will be described below.
[0097]
Slab reheating temperature:
The steel adjusted to a predetermined component is hot-rolled after being reheated directly after casting or once cooled to the Ar3 transformation temperature or lower. When the reheating temperature at this time is less than 1000 ° C., it is difficult to ensure a predetermined finish hot rolling completion temperature, so this was made the lower limit.
[0098]
Further, when the reheating temperature exceeds 1300 ° C., the yield is deteriorated due to scale generation during heating, and at the same time, the manufacturing cost is increased, so this is set as the upper limit of the reheating temperature. Even if the steel slab after heating is locally or entirely heated during hot rolling, the properties of the present invention are not affected at all.
[0099]
Hot rolling conditions:
It is controlled to a predetermined microstructure and texture by hot rolling and subsequent cooling. The texture of the steel sheet finally obtained varies greatly depending on the temperature range of hot rolling. When the finish hot rolling completion temperature TFE was less than Ar3 ° C., this was regarded as the lower limit of the finish hot rolling completion temperature TFE (formula (1)) in order to significantly deteriorate the moldability.
[0100]
The TFE is generally measured behind the stand where the hot rolling final rolling is performed, but if necessary, a temperature obtained by calculation may be used.
[0101]
The upper limit of the hot rolling completion temperature is not particularly limited, but when it exceeds 1050 ° C., the surface quality is lowered by the oxide layer formed on the surface of the steel sheet, and therefore it is desirable that the upper limit is lower than this.
[0102]
If more strict surface quality is required, it is desirable to set TFE to 980 ° C. or lower.
[0103]
Further, even when the finish hot rolling start temperature TFS is higher than 1100 ° C., the steel sheet surface quality is remarkably lowered, and this is set as the upper limit of TFS (Equation (2)).
[0104]
In addition, when the difference between TFS and TFE exceeds 150 ° C., the texture is not sufficiently developed, and good shape freezing property and low anisotropy are not compatible, so this is the upper limit of (TFS−TFE). Value. Moreover, since it is difficult in operation to make this difference less than 20 ° C., this was set as the lower limit (formula (3)).
[0105]
In hot rolling, the rolling reduction in the temperature range of Ar3 ° C. to (Ar 3 +150) ° C. has a great influence on the formation of the texture of the final steel sheet, and the rolling reduction in this temperature range is less than 25%. Since the texture is not sufficiently developed and the finally obtained steel sheet does not exhibit a good shape freezing property, this rolling reduction is defined as the lower limit of the rolling reduction in the temperature range of Ar3 ° C. to (Ar 3 +150) ° C. .
[0106]
The higher the rolling reduction, the more the desired texture develops, so 50% or more is preferable, and 75% or more is more preferable.
[0107]
However,
Ar3 = 901-325 × C% + 33 × Si% + 287 × P% + 40 × Al%
−92 × (Mn% + Mo% + Cu%) − 46 × (Cr% + Ni%)
And
[0108]
Furthermore, the amount of strain effective for texture formation in hot rolling also changes depending on the temperature of hot rolling, and the calculated residual strain Δε calculated by the following equation according to the strain, temperature, and time between passes in hot rolling. Is less than (TFS−TFE) / 375, the texture is insufficiently developed, and this is set as the lower limit of Δε (Equation (4)).
[0109]
In addition, the relationship of Formula (4) was newly found by the present inventors from the results of investigating the characteristics of steel sheets with various hot rolling conditions.
[0110]
[Delta] [epsilon] is equivalent strain [epsilon] i (i is 1 to n) given at each stand of n-stage finish rolling for rolling, time ti (seconds) between each stand (i = 1 to n-1), and from the last stand Time tn (seconds) until the start of cooling, rolling temperature Ti (K) (i = 1 to n) at each stand, and constant R = 1.987.
Δε = Δε1 + Δε2 ++. + Δεn
Where Δεi = εi × exp {-(ti * / τn)^(2/3)}
τi = 8.46 × 10^-9Xexp {43800 / R / Ti}
ti * = τn × {ti / τi + t (i + 1) / τ (i + 1) +.
+ Tn / τn}
[0111]
Although hot rolling in the temperature range of Ar3 ° C to (Ar3 + 150) ° C is performed under normal conditions, the shape freezing property of the final steel sheet is high, but at least one pass of hot rolling performed in this temperature range When the friction coefficient is controlled to be 0.2 or less, the shape freezing property of the final steel plate is further increased.
[0112]
In addition, it is preferable that processing for removing the scale, high-pressure water injection, fine particle injection, or the like is performed prior to finish hot rolling because it has an effect of improving the surface quality of the final steel plate.
[0113]
In the cooling after hot rolling, it is most important to control the coiling temperature, but the average cooling rate is preferably 15 ° C./second or more. It is desirable that cooling be started immediately after hot rolling. Further, providing air cooling in the middle of cooling does not deteriorate the properties of the final steel plate.
[0114]
In order to transfer the austenite texture thus formed to the final hot-rolled steel sheet, it is necessary to wind it at a temperature equal to or lower than the To temperature shown in the equation (5). Therefore, To determined by the steel components was taken as the upper limit of the coiling temperature.
[0115]
This To temperature is thermodynamically defined as the temperature at which austenite and ferrite of the same component as austenite have the same free energy, and considering the influence of components other than C, it can be simplified using equation (5). Can be calculated.
[0116]
Since the influence of the other components specified in the present invention on the To temperature is not so great, it was ignored here.
[0117]
When the cooling is completed above the temperature To determined by the chemical composition of the steel material and the winding process is performed as it is, even if the above hot rolling conditions are satisfied, the desired steel sheet is finally obtained. The texture does not develop sufficiently and the shape freezing property of the steel sheet does not increase.
[0118]
To = −650.4 × {C% / (1.82 × C% −0.001)} + B (5)
Here, B is obtained from the chemical composition of steel expressed in mass%.
B = −50.6 × Mneq + 894.3
Mneq = Mn% + 0.24 × Ni% + 0.13 × Si% + 0.38 × Mo% + 0.55 × Cr% + 0.16 × Cu%
−0.50 × Al% −0.45 × Co% + 0.90 × V%
In addition, when the coiling temperature is higher than 480 ° C., a sufficient amount of austenite does not remain in the steel sheet, and this is set as the upper limit value of the coiling temperature. On the other hand, when the coiling temperature is less than 300 ° C., the retained austenite in the steel sheet becomes unstable, and this greatly reduces the workability of the steel sheet.
[0119]
Skin pass rolling:
Applying skin pass rolling to the steel of the present invention manufactured by the above method before shipment makes the shape of the steel plate good. At this time, if the skin pass reduction ratio is less than 0.1%, this effect is small, so this was set as the lower limit of the skin pass reduction ratio.
[0120]
In addition, in order to perform skin pass rolling exceeding 5%, it is necessary to modify a normal skin pass rolling mill, resulting in economic demerits and significant deterioration of workability. This is the upper limit of the skin pass rolling reduction. .
[0121]
In order for the workability of the obtained steel sheet to be good, the product of the breaking strength (TS / MPa) and the total elongation (El /%) obtained by a normal JIS No. 5 tensile test (TS × El / MPa ·%) Is desirably 18000 or more.
[0122]
Further, when the demand for press formability is strong, this value is preferably 19000 or more. When particularly severe press workability is required, it is effective to control this value to 20000 or more. .
[0123]
In addition, in order to show good impact energy absorption ability after being formed into a member by press molding / bending molding or hydraulic molding, the ratio of the residual austenite volume ratio before and after applying a pre-strain of 10% to the equivalent strain Is preferably 0.35 or more, and the work hardening index of 5 to 10% after adding 10% pre-strain to an equivalent strain satisfies 0.130 or more.
[0124]
Plating:
The type and method of plating are not particularly limited, and the effects of the present invention can be obtained by any of electroplating, hot dipping, vapor deposition plating and the like.
[0125]
The steel sheet of the present invention can be applied not only to bending, but also to composite molding mainly consisting of bending, such as bending, overhanging, drawing, etc., and a method of forming a tube or the like by applying pressure from the inside after forming and joining.
[0126]
【Example】
(Example 1): 25 types of steel materials shown in Table 1 and Table 2 (continuation of Table 1) were heated from 1100 ° C to 1280 ° C, and hot-rolled under the hot rolling conditions shown in Table 3, and 2.8 mm. A thick hot-rolled steel sheet was used.
[0127]
The shape freezing property was evaluated by using a strip-shaped sample of 270 mm long × 50 mm wide × plate thickness, and after forming into a hat shape with various wrinkle holding thicknesses with a punch width of 80 mm, a punch shoulder R5 mm, and a die shoulder R5 mm. The amount of warpage of the wall was measured as the curvature ρ (mm), and the reciprocal was 1000 / ρ. The smaller the 1000 / ρ, the better the shape freezing property.
[0128]
In general, it is known that the shape freezeability deteriorates when the strength of a steel plate increases. From the results of actual part molding by the present inventors, 1000 / ρ at a wrinkle holding pressure of 90 kN measured by the above method is (1000 / ρ) × with respect to the tensile strength TS and breaking elongation El of the steel sheet. When (1000 / TS) ≦ (7/15000) × TS × E1 is satisfied, the balance between shape freezing property and press formability is remarkably good, and this is evaluated as the shape freezing property. (See FIG. 1).
[0129]
Here, when the wrinkle pressure is increased, 1000 / ρ tends to decrease. However, no matter what wrinkle holding pressure is selected, the order of superiority of the shape freezing property of the steel sheet does not change. Therefore, the evaluation at the wrinkle holding pressure of 90 kN well represents the shape freezing property of the steel sheet.
[0130]
The r value, ductility and its anisotropy were measured using a JIS No. 5 tensile test piece. For spot weldability, the optimum spot welding conditions are investigated by changing the current density for each steel, welding is performed under conditions where the nugget diameter is 6.5 mm under conditions where no dust is generated, and shear and peel tensile tests are performed. went. At this time, when the peel tensile strength was 0.9 times or less that of mild steel, the weldability was poor (marked with x in the spot weldability column in Table 4 (continuation of Table 3)).
[0131]
[Table 1]

[0132]
[Table 2]

[0133]
[Table 3]

[0134]
[Table 4]

[0135]
In Table 3 and Table 4, no. No. 7 is TFS-TFE of 180 ° C., which is outside the scope of the present invention. At the same time, Δε does not satisfy the conditions of the present invention, so that the texture is outside the scope of the present invention and as a result good shape freezing property is obtained. Not.
[0136]
No. 8 and no. In No. 9, CT is outside the scope of the invention, and good shape freezing property is not obtained. No. No. 10 has Δε not in the predetermined range, and as a result, the development of the texture is insufficient, and good shape freezing property is not obtained.
[0137]
No. In No. 24, the amount of C is outside the range of the present invention. As a result, the amount of residual γ is zero, and a good balance between moldability and shape freezing property is not obtained. No. No. 25 not only has a C content higher than the upper limit of the present invention but low weldability, but also has a good shape freezing property in combination with a low Δε.
[0138]
No. In No. 26, since the addition amounts of Al and Si are not sufficient, a good balance between good moldability and shape freezing property is not obtained. No. In No. 27, since Si exceeds the upper limit of the present invention, not only the weldability is bad, but also sufficient texture development cannot be obtained, and good shape freezing property is not obtained.
[0139]
No. No. 28 has poor weldability due to high P. No. In No. 29, since the amount of addition of B is not less than the upper limit of the present invention, workability is poor, and a good balance between good moldability and shape freezing property is not obtained.
[0140]
No. In No. 30, the total amount of Mn, Ni and Cu added is outside the range of the present invention, and not only the weldability is bad, but also a good balance between good formability and shape freezing property is not obtained. No. No. 31, because the total amount of Mn, Ni, Cr, Cu, Mo, Sn, which is important for hardenability, is below the lower limit of the present invention, hardenability is insufficient, and as a result, good formability and shape The freezing balance is not obtained.
[0141]
No. In No. 32, the total amount of Nb, Ti, and V exceeds the upper limit of the present invention, so that not only the moldability is poor, but also the anisotropy of local elongation is increased and a good shape freezing property is obtained. Not.
[0142]
When steels with chemical components within the scope of the present invention shown in the above are manufactured under hot rolling conditions within the scope of the present invention, a good balance between good workability and shape freezing properties is obtained along with good ductility and anisotropy. I understand that.
[0143]
【The invention's effect】
According to the present invention, it is possible to provide a thin steel sheet having a good press formability with a small amount of spring back and excellent shape freezing properties and at the same time low anisotropy. High-strength steel sheets can be used for difficult-to-apply parts, and at the same time, it is possible to efficiently balance the safety of automobiles and the weight of the vehicle body.₂It can greatly contribute to automobile manufacturing that responds to environmental and social demands such as emission reduction.
[0144]
Therefore, the present invention is industrially extremely valuable.
[Brief description of the drawings]
FIG. 1 is a diagram in which the value of (1000 / ρ) × (1000 / TS) is plotted against TS (tensile strength) * El (breaking elongation).

Claims (10)

Containing C, Si, Al, Mn and P, in mass%,
Group A
C: 0.02 to 0.3%,
Mn; 0.01 to 3%
Ni: 3% or less,
Cr: 3% or less,
Cu: 2% or less,
Mo; 2% or less,
W: 2% or less,
Sn: 0.3% or less,
Group B
Si; 0.003 to 3%,
Al: 3% or less,
2% or 3 or more types including C and Mn of group A in total including 0.5% or more and 5% or less, and 2 types of group B including 0.5% or more and 4% or less in total Furthermore, P is 0.2% or less, the balance is Fe and inevitable impurities, the microstructure is ferrite or bainite and the volume fraction is the maximum phase, and the volume fraction of retained austenite is 1% or more and 25% or less. comprises an average value of at least 1/2 {100} <011> - plate surface in the sheet thickness {223} <110> orientation component group X-ray random intensity ratio of 2.5 or more, and {554} <225>, {111} <112> and {111} <110>, the average value of the X-ray random intensity ratio of the three crystal orientations is 3.5 or less, and the local elongation anisotropy ΔLEl is 4% or less. And anisotropy of uniform elongation ΔuE Characterized in that it is less, high workability high-strength hot-rolled steel sheet excellent in shape fixability.
However, ΔuEl = {| uEl (L) −uEl (45 °) | + | uEl (C)
-UEl (45 °) |} / 2
ΔLEl = {| LEl (L) −LEl (45 °) | + | LEl (C)
-LEl (45 °) |} / 2
The uniform elongation in the direction parallel to the rolling direction (L direction), vertical (C direction), and 45 ° is uEl (L), uEl (C), and uEl (45 °), respectively, and parallel to the rolling direction. The local elongations in the (L direction), vertical (C direction), and 45 ° directions are denoted by LEl (L), LEl (C), and LEl (45 °), respectively. High workability high-strength hot-rolled steel sheet excellent in shape fixability according to claim 1 Symbol placement, wherein the anisotropy △ UEL is 3% or more of the uniform elongation. The high workability high-strength hot-rolled steel sheet having excellent shape freezing properties according to claim 1 or 2 , further comprising Co in an amount of 0.01% to 3% by mass. Furthermore, by mass%, Nb, Ti, according to any one of claims 1 to 3, characterized in that it comprises 0.8% or less than 0.001% in total of one or more of V High workability and high strength hot-rolled steel sheet with excellent shape freezing properties. Furthermore, the high workability high-strength hot-rolled steel sheet excellent in shape freezing property according to any one of claims 1 to 4 , further comprising 0.01% or less by mass of B. Furthermore, in mass%,
Ca; 0.0005 to 0.005%,
Rem; 0.001 to 0.02%,
Ce; 0.0001 to 0.05%,
La; 0.0001-0.05%,
Mg; 0.0001 to 0.05%,
Ta; 0.0001 to 0.05%,
The high workability high-strength hot-rolled steel sheet excellent in shape freezing property according to any one of claims 1 to 5 , characterized in that it contains one or more of the following. The high workability and high strength hot rolling excellent in shape freezing property, wherein the high workability and high strength hot rolled steel plate excellent in shape freezing property according to any one of claims 1 to 6 is plated. steel sheet. In producing a high workability high-strength hot-rolled steel sheet excellent in shape fixability according to any one of claims 1 to 7 the chemical composition according to any one of claims 1,3～6 When the cast slab is cast or is once cooled and then heated again in the range of 1000 ° C. to 1300 ° C. and hot-rolled, the total rolling reduction in the temperature range of Ar 3 ° C. to (Ar 3 +150) ° C. is 25 %, The finishing hot rolling start temperature TFS (° C.), the finishing hot rolling completion temperature TFE (° C.), and the calculated residual strain Δε at the time of finishing hot rolling completion are the following formulas (1) to (4) The hot rolling is finished so as to satisfy all of the above simultaneously, and after the hot rolling, the steel is cooled and cooled to a critical temperature To (° C) determined by the chemical composition of the steel shown in the following formula (5) or lower and 480 ° C or lower and 300 ° C or higher. Excellent shape freezing property, characterized by winding at temperature Method for producing a high workability high strength hot rolled steel sheet.
TFE ≧ Ar3 (1)
TFS ≦ 1100 ° C (2)
20 ° C. ≦ TFS−TFE ≦ 150 ° C. (3)
Δε ≧ (TFS−TFE) / 375 (4)
To = −650.4 × {C% / (1.82 × C% −0.001)} + B (5)
Here, B is obtained from the chemical composition of steel expressed in mass%.
B = −50.6 × Mneq + 894.3
Mneq = Mn% + 0.24 x Ni% + 0.13 x Si% + 0.38 x Mo% + 0.55 x Cr% + 0.16 x Cu%
−0.50 × Al% −0.45 × Co% + 0.90 × V%
However,
Ar3 = 901-325 × C% + 33 × Si% + 287 × P% + 40 × Al%
−92 × (Mn% + Mo% + Cu%) − 46 × (Cr% + Ni%)
[Delta] [epsilon] is equivalent strain [epsilon] i (i is 1 to n) given at each stand of n-stage finish rolling for rolling, time ti (seconds) between each stand (i = 1 to n-1), and from the last stand Time tn (seconds) until the start of cooling, rolling temperature Ti (K) (i = 1 to n) at each stand, and constant R = 1.987.
Δε = Δε1 + Δε2 ++. + Δεn
Where Δεi = εi × exp {-(ti * / τn) ^(2/3) }
τi = 8.46 × 10 ⁻⁹ × exp {43800 / R / Ti}
ti * = τn × {ti / τi + t (i + 1) / τ (i + 1) +.
+ Tn / τn} 9. The high shape-freezing property according to claim 8, wherein the friction coefficient is controlled to be 0.2 or less in at least one pass of hot rolling in a temperature range of Ar3 to (Ar3 + 150) ° C. Process for producing workable high-strength hot-rolled steel sheet. A hot-rolled steel sheet produced by the production method according to claim 8 or claim 9 is subjected to skin pass rolling of 0.1% or more and 5% or less. A method for producing a hot-rolled steel sheet.

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