JP3990549B2 - High stretch flangeability steel sheet with excellent shape freezing property and method for producing the same - Google Patents

Description

【００６８】
【表２】

【００６９】
表２には、各鋼板の製造条件が本発明の範囲内にあるか否かを示している。「熱延条件１」は熱延がＡｒ₃変態温度以上で完了する場合にＡｒ₃変態温度以上（Ａｒ₃＋１００）℃以下での圧下率の合計が２５％以上で熱間圧延終了温度もその温度範囲にある場合について「○」、その温度域での圧下率の合計が２５％未満の場合に「×」とした。
【００７０】
「熱延条件２−１」は（Ａｒ₃＋５０）〜（Ａｒ₃＋１５０）℃の温度範囲における圧下率の合計が２５％以上の場合に「○」、圧下率の合計が２５％未満の場合に「×」「熱延条件２−２」は、引き続き、（Ａｒ₃−１００）〜（Ａｒ₃＋３０）℃の温度範囲で圧下率の合計が５〜３５％である場合について「○」、この条件を満たさない場合を「×」とした。
【００７１】
以上のいずれの場合にも、それぞれの温度範囲で、少なくとも１パス以上についての摩擦係数が０．２以下の場合には「潤滑」の欄に「○」、全パスにおける摩擦係数が０．２超の場合には「△」とした。熱延巻取は全て（１）式で求まるＴｏ温度以下とした。
このような熱延鋼板を１．４ｍｍ厚に冷延する場合、冷延圧下率が８０％以上の場合には「冷延圧下率」を「×」とし、「８０％未満」の場合に「○」とした。
【００７２】
また、焼鈍温度が６００℃以上（Ａｃ₃＋１００）℃以下の場合に「焼鈍温度」を「○」とし、それ以外の場合を「×」と記した。製造の条件として関係のない項目は「―」とした。熱延鋼板および冷延鋼板のいずれに対しても、スキンパス圧延を０．５〜１．５％の範囲で施した。
表３に、前記の方法によって製造された１．４ｍｍ厚の熱延鋼板と冷延鋼板の鉄炭化物粒界占有率Ｍ／Ｎ、鉄炭化物の最大粒子径ｄ、機械的特性値を、表４および表５（表４の続き）にＸ線ランダム強度比、寸法精度、スプリング・バック量、壁そり量および穴拡げ率を示した。なお、本発明の条件を満たしている鋼板の組織はいずれもフェライト又はベイナイトが主相であった。
【００７３】
【表３】

【００７４】
【表４】

【００７５】
【表５】

【００７６】
本発明の番号のものは、発明外の番号のものに比べて、スプリング・バック量、壁そり量が小さくなり、結果として、寸法精度が向上していることがわかる。本発明のものは、いずれのケースも伸びフランジ性も良好である。
一方、鉄炭化物の粒界占有率Ｍ／Ｎ、鉄炭化物の最大粒子径ｄが本発明の規定を満足していないＩおよびＪは、形状凍結性は良好なものの、伸びフランジ性が劣化している。鋼Ｈに関しては形状凍結性も伸びフランジ性も劣化している。
【００７７】
即ち、本発明で限定される成分、各結晶方位のＸ線ランダム強度比、ｒ値、組織を満たして、初めて良好な形状凍結性を有する高伸びフランジ性鋼板の製造が可能になるのである。
図４には、それぞれ引張強度で規格化した寸法精度と穴拡げ率の関係を示す。これからも、本発明の条件を満足したものは、寸法精度と伸びフランジ性のいずれにも優れていることが明らかである。
【００７８】
各結晶方位のＸ線ランダム強度比やｒ値が形状凍結性に重要であることの機構については、現在のところ、必ずしも明らかとはなっていない。おそらく、曲げ変形時にすべり変形の進行を容易にすることで、結果的に曲げ変形時のスプリング・バック量、壁そり量が小さくなり、その結果、寸法精度、すなわち形状凍結性が向上したものと理解される。
【００７９】
【発明の効果】
本発明により薄鋼板の集合組織とｒ値を制御することにより、その曲げ加工性は著しく向上し、また、組織と炭化物を制御することによって穴拡げ性と曲げ加工性を両立でき、スプリング・バック量、壁そり量が少なく、曲げ加工を主体とする形状凍結性と、穴拡げ性に優れた薄鋼板が提供できるようになった。特に、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。自動車の軽量化を推進するためには、高強度鋼板の使用は是非とも必要である。スプリング・バック量、壁そり量が少なく、形状凍結性と穴拡げ性に優れた高強度鋼板が適用できるようになると、自動車車体の軽量化をより一層推進することができる。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】ハット曲げ試験に用いた試験片の断面図である。
【図２】スプリングバック量とＢＨＦ（しわ押さえ力）の関係を示す図である。
【図３】壁そり量とＢＨＦ（しわ押さえ力）の関係を示す図である。
【図４】引張強度で規格化された寸法精度と穴拡げ率の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high stretch flangeability steel sheet excellent in shape freezing property and a method for producing the same, and is mainly used for automobile parts and the like. The steel sheet of the present invention contains both hot-rolled steel sheets and cold-rolled steel sheets.
[0002]
[Prior art]
In order to reduce carbon dioxide emissions from automobiles, the weight of automobile bodies is being reduced 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 sheet, because the shape after processing is high-strength, the spring-back phenomenon that makes it easy to return to the direction of the shape before processing away from the shape of the processing jig or bending-bending during forming Due to the elastic recovery from the return, a wall warp phenomenon occurs in which the plane of the side wall becomes a curved surface, resulting in a defective dimensional accuracy in which 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.
[0004]
In other words, for automobile bodies, it is necessary to use a high-strength steel plate of 490 MPa or higher to reduce the weight of the vehicle body, but spring back and wall warpage are unlikely to occur, and dimensional accuracy is good. The fact is that there is no high-strength steel sheet with good freezing properties. 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.
[0005]
Japanese Patent Application Laid-Open No. 10-72644 discloses a spring back amount (dimensional accuracy in the present invention) characterized in that the degree of accumulation of {200} texture in a plane parallel to the rolling surface is 1.5 or more. A small austenitic stainless cold rolled steel sheet is disclosed. However, there is no description about a technique for reducing the springback phenomenon and the wall warp phenomenon of the ferritic steel sheet.
[0006]
As a technique for reducing the springback amount of ferritic stainless steel, Japanese Patent Laid-Open No. 2001-32050 discloses a reflection X-ray intensity ratio of 2 in the {100} plane parallel to the plate surface in the texture at the center of the plate thickness. The invention as described above is disclosed. However, this publication does not describe any reduction of wall warp, and {100} <011> to {223} <110> orientation groups and {112} <110 which are important for wall warp reduction. There is no description about>.
[0007]
In addition, some of the present inventors disclosed a ferritic thin steel sheet in which the ratio of the {100} plane to the {111} plane is 1 or more for the purpose of improving the shape freezing property in WO 00/06971. In this publication, as in the present invention, the values of {100} <011> to {223} <110> orientation group and {112} <110> are not described.
[0008]
Further, some of the present inventors have disclosed in Japanese Patent Application Laid-Open No. 2001-64750 that the reflected X-ray intensity ratio of the {100} plane parallel to the plate surface is 3 or more as a technique for reducing the amount of springback. Although a cold-rolled steel sheet has been disclosed, this invention is characterized in that it defines a {100} plane reflection X-ray intensity ratio at the outermost surface of the sheet thickness, and { 100} <011> to {223} <110> The average X-ray intensity ratio of the orientation group differs from the X-ray measurement position. Also, the {112} <110> orientation is not described in the above publication.
[0009]
Japanese Patent Application Laid-Open No. 2000-297349 discloses a hot-rolled steel sheet having an r-value in-plane anisotropy Δr of 0.2 or less as a steel sheet having a good shape freezing property. However, this invention is characterized by improving the shape freezing property by lowering the yield ratio, and regarding texture control for the purpose of improving the shape freezing property based on the idea described in the present invention. It is not described in the above publication.
[0010]
Stretch flangeability, on the other hand, is an indispensable characteristic when processing steel sheets into automotive parts, etc., but in the above-mentioned patent publications, both stretch flangeability and shape freezing properties are provided. There is no description from the viewpoint of compatibility. By improving the shape freezing property of the high-stretch flangeable steel sheet, the range of application of the high-strength steel sheet to the automobile body becomes even wider.
[0011]
[Problems to be solved by the invention]
When a mild steel plate or a high strength steel plate is subjected to bending, a shape failure such as a large spring back or wall warp occurs depending on the strength of the steel plate, and the shape freezing property of the processed molded part is poor. Stretch flangeability is an indispensable characteristic when processing steel sheets. In order to apply high-strength steel sheets to automobile parts, etc., it must be excellent in both shape freezing properties and stretch flangeability. desired.
[0012]
The present invention fundamentally solves this problem and provides a high stretch flangeability steel sheet having excellent shape freezing property and a method for producing the same.
[0013]
[Means for Solving the Problems]
According to the conventional knowledge, it has been considered to be important for the time being to lower the deformation stress of the steel sheet as a measure for suppressing shape defects such as spring back and wall sled. And in order to make a deformation stress low, the steel plate with low tensile strength had 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 fundamentally solve the occurrence of spring back and wall warp by improving the bending workability, the present inventors newly pay attention to the influence on the bending workability of the texture of the steel sheet, The effects were investigated and studied in detail. And it discovered the steel plate excellent in bending workability.
That is, {100} <011> to {223} <110> orientation group, three crystal orientations of {554} <225>, {111} <112>, {111} <110>, and {112} By controlling the strength of each orientation of <110> or {100} <011>, 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 It is clarified that the bending workability is remarkably improved.
[0015]
Also, in order to achieve both high stretch flangeability and shape freezing properties, it is important that the ferrite phase or bainite phase be the maximum phase and that coarse cementite at the grain boundaries that hinder stretch flangeability be reduced as much as possible. Newly found.
The present invention is configured based on the above-described knowledge, and the main points thereof are as follows.
[0016]
      (1) In mass%,
            C:0.034%Or more, 0.15% or less,
          Si: 0.001% or more, 3.5% or less,
          Mn: 0.05% or more, 3.0% or less,
            P: 0.2% or less,
            S: 0.03% or less,
          Al: 0.01% or more, 3.0% or less,
            N: 0.01% or less,
          O: 0.01% or less,
Furthermore,
      Ti: 0.01% or more, 2.0% or less,
      Nb: 0.01% or more and 2.0% or less
And the balance is composed of iron and inevitable impurities, ferrite or bainite is the largest phase in area ratio, the occupation ratio of iron carbide at grain boundaries is 0.1 or less, and this iron The maximum particle diameter of carbide is 1 μm or less, and 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 is 3.0 or more. And, among these orientation groups, the X-ray random intensity ratio of {112} <110> orientation is maximum and satisfies 4.0 or more, and {554} <225>, {111} <112> and The average value of the X-ray random intensity ratio of the three crystal orientations of {111} <110> is 3.5 or more, and in addition, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is Shape characterized by being 0.7 or less High elongation flange of steel sheet excellent in fixability.
[0019]
(2) The high elongation flangeable steel sheet having excellent shape freezing property according to (1), further containing B: 0.01% or less in mass%.
(3) Furthermore, in mass%, Cr: 3% or moreDownA high-stretch flangeable steel sheet having excellent shape freezing properties as described in (1) or (2).
[0020]
(4) Furthermore, in mass%,
Ca: 0.0005% or more, 0.005% or moreDownThe high stretch flangeability steel sheet excellent in shape freezing property according to any one of (1) to (3), characterized in that it is contained.
[0021]
    (5) (1) to (4A high-stretch flangeable steel plate excellent in shape freezing property, wherein the steel plate according to any one of 1) is plated.
    (6) (1)~(5The method for producing a steel sheet according to any one of (1),1) ~ (4) When hot-rolling a steel slab composed of the component according to any one of items 1) to 1150-1350 ° C., Ar_ThreeTransformation temperature ~ (Ar_ThreeHot rolling is performed so that the total reduction ratio in the temperature range of +100) ° C. is 25% or more, and Ar_ThreeHot rolling is completed at the transformation or higher, and the cooling is performed at an average cooling rate of 10 ° C./s or higher from the hot rolling completion temperature to the critical temperature To (° C.) determined by the chemical composition (mass%) of the steel shown in the formula (1). A method for producing a high-stretch flangeable steel sheet having excellent shape freezing property, characterized by winding at 450 to 750 ° C.
[0022]
      To = −650.4 × C% + B (1)
here,
      B = −50.6 × Mneq + 894.3
      Mneq = Mn% +0.13 x Si% +0.55 x Cr%-0.50 × Al%
(7) Ar₃Transformation temperature ~ (Ar₃In the temperature range of +100) ° C., at least one pass or more is rolled so that the coefficient of friction is 0.2 or less. The production of a high stretch flangeability steel sheet having excellent shape freezing property according to (6) Method.
[0024]
    (8) (6)Or (7)The steel sheet described above is pickled, cold-rolled to less than 80%, heated to a temperature range of 600 ° C. to 800 ° C. and cooled, and is a high stretch flangeability steel plate having excellent shape freezing property Manufacturing method.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
The contents of the present invention will be described in detail below.
Average values of X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness, {554} <225>, {111} <112> and {111 } The average value of the X-ray random intensity ratio of the three crystal orientations of <110> and the X-ray random intensity ratio of the {112} <110> or {100} <011> orientation:
These values are particularly important characteristic values 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 3. Must be greater than or equal to zero. If this is less than 3.0, the shape freezing property is poor.
[0026]
The main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} < 110> and {223} <110>. The X-ray random intensity ratio in each of these directions is obtained by calculating a three-dimensional texture calculated by the vector method based on the {110} pole figure, or a plurality of pole figures of {110}, {100}, {211}, {310} pole figures. What is necessary is just to obtain | require from the three-dimensional texture calculated | required by the series expansion method using the pole figure (preferably 3 or more).
[0027]
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] strengths may be used as they are. .
[0028]
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.
[0029]
Among these orientation groups, {100} <011> and {112} <110> orientations are extremely effective orientations for reducing wall warpage. Therefore, among these orientation groups, {100} < It is preferable that the X-ray random intensity ratio in the 011> or {112} <110> orientation is maximum and 4.0 or more because the shape freezing property is further improved.
Furthermore, the average value of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness is 3.5 or less. It must be. 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. The X-ray random intensity ratio of {554} <225>, {111} <112>, and {111} <110> may be obtained from the three-dimensional texture calculated according to the above method.
[0030]
More preferably, the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group is 4.0 or more, and the {100} <011> or {112} <110> orientation X The linear random intensity ratio is 5.0 or more, and the arithmetic average value of the X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> is less than 2.5.
[0031]
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. .
The sample to be subjected to X-ray diffraction is thinned to a predetermined plate thickness by mechanical polishing or the like, and then the distortion is removed by chemical polishing or electrolytic polishing, and at the same time, the 1/2 plate thickness becomes the measurement surface. To make. When there is a segregation zone or a defect in the thickness center layer of the steel plate, which causes inconvenience in measurement, the above-described surface is set 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 prepared and measured according to the method.
[0032]
As a matter of course, the above-described limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness ½ but also as much as possible, so that the shape freezing property is further improved. 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>.
R value (rL) in the rolling direction and r value (rC) in the direction perpendicular to the rolling direction:
The r value is 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. 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.
[0033]
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%. However, when the uniform elongation is less than 15%, the tensile strain may be evaluated as close to 15% as possible within the range of uniform elongation.
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.
[0034]
By the way, it is generally known that there is a correlation between the texture and the r value. However, 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.
Organization:
From the viewpoint of hole expansibility and shape freezing property, the structure has a maximum phase of ferrite or bainite phase. However, when the textures of ferrite and bainite are compared, the texture of {100} <011> to {223} <110> orientation, which is advantageous for shape freezing, easily develops in the bainite portion. The reason for this is not clear, but it is considered that the bainite structure easily inherits the austenite texture that is superior in shape freezing property formed during hot rolling.
[0035]
Therefore, it is more desirable that the bainite space factor is large. From this viewpoint, it is desirable that the area ratio of bainite is more than 35%.
The area ratio of ferrite or bainite is determined from an average value obtained by observing the central part of the plate thickness with an optical microscope at 100 to 500 times at least 5 views. Further, as-processed ferrite remarkably impairs formability, so it is not included in the area ratio described here.
[0036]
Further, when the occupation ratio of iron carbide at the grain boundary exceeds 0.1 or the maximum particle diameter of the iron carbide exceeds 1 μm, these iron carbides are connected at the grain boundary, and the stretch flangeability is remarkably deteriorated. Therefore, it is necessary that the occupation ratio of iron carbide at the grain boundary is 0.1 or less and the maximum particle diameter of the iron carbide is 1 μm or less.
Since it is desirable that the iron carbide occupancy and the maximum particle size be smaller, the lower limit is not particularly defined. The grain boundary occupation ratio (-) by iron carbide is a ratio d / L of the total length L of grain boundaries in a certain region in the cross-sectional sample of iron material and the sum d of the lengths of grain boundaries occupied by iron carbide. Given in. In the measurement, L and d may be directly obtained by image processing in an optical microscope observation photograph having a magnification of 200 times or more.
[0037]
As a simpler method, the number N of the intersections of the n straight lines drawn on the photograph and the grain boundary, and the number M when iron carbide exists at the position of the intersections among the N intersections are as follows. It may be obtained by M / N. Sufficient accuracy can be ensured by setting the number N of straight lines to be 3 or more. In addition, sufficient accuracy can be ensured by selecting the magnification of the photograph so that the number of intersections of the one straight line and the grain boundary is 10 or more.
[0038]
  Next, the limiting conditions of the component range will be described. The reason why the lower limit of C is set to 0.001% is that the lower limit value obtained from practical steel is used. If it exceeds 0.15%, the stretch flangeability deteriorates, so the upper limit is set to 0.15%.In addition, the lower limit of C was set to 0.034% based on the C amount of steel type B in Table 1 of the Examples.
Si is an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 3.5%, workability deteriorates or surface flaws occur, so 3.5% is made the upper limit. On the other hand, since it is difficult to make Si less than 0.001% in practical steel, 0.001% is made the lower limit.
[0039]
Mn is also an effective element for increasing the mechanical strength of the steel sheet. However, if it exceeds 3.0%, the workability deteriorates, so 3.0% is made the upper limit. On the other hand, if Mn is less than 0.05% in practical steel, the cost is high and there is no merit in material, so 0.05% is made the lower limit. In addition to Mn, when an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added, it is desirable to add an amount of Mn that satisfies Mn / S ≧ 20 by mass%.
[0040]
P and S are 0.2% or less and 0.03% or less, respectively. This is to prevent workability deterioration and cracking during hot rolling or cold rolling.
Al is added in an amount of 0.01% or more for deoxidation. However, if the amount is too large, the workability deteriorates or the surface properties become poor, so the upper limit is made 3.0%.
N and O are impurities, and are 0.01% or less and 0.01% or less, respectively, so as not to deteriorate the workability.
[0041]
Ti and Nb are important elements in the present invention. These elements are finely precipitated as carbides and nitrides, and are effective in increasing strength and reduce iron carbides, so that stretch flangeability is also improved. Further, when these elements exist in a solid solution state in the austenite region, there is an effect of sharpening the texture that contributes to the improvement of the shape freezing property, particularly the {112} <110> orientation. Therefore, 0.01% or more is added depending on the desired strength. However, even if added excessively, there is no remarkable effect, but rather the workability and surface properties are deteriorated, so 2.0% was made the upper limit respectively.
[0042]
  CrHas the effect of increasing the mechanical strength and improving the material., 0. It is desirable to add 001% or more. However, excessive addition adversely degrades workability, so the upper limit3%And
  Ca isControlling the form of sulfide improves stretch flangeability, so if necessary, 0. 0005% or moreSuperordinateIt is desirable to add. However, adding too much will not have a significant effect and will increase the cost.0. 005%WhenSet.
[0043]
In the present invention, although not particularly limited, 0.01% or less of Mg may be added for the purpose of deoxidation or the control of the form of sulfide.
plating:
The type of plating is 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.
[0044]
Next, a manufacturing method will be described.
The production method preceding hot rolling is not particularly limited. That is, various secondary smelting may be performed following smelting by a blast furnace, converter, electric furnace or the like, and then cast by a method such as thin slab casting in addition to normal continuous casting and casting by an ingot method. In the case of continuous casting, after cooling to low temperature once, it may be heated again and then hot rolled, or the cast slab may be continuously hot rolled. Scrap may be used as a raw material.
[0045]
The steel sheet having excellent shape freezing property of the present invention is cast after steel of the above components, and after cooling after hot rolling, after hot rolling, heat treatment after hot rolling, cooling after hot rolling, annealing after cold rolling, or It can also be obtained by subjecting the hot-rolled steel sheet or cold-rolled steel sheet to plating or heat treatment in a hot dipping line, and further subjecting these steel sheets to surface treatment.
[0046]
In any case, the heating temperature for hot rolling is 1150 to 1350 ° C. When the heating temperature is less than 1150 ° C., the carbides of Ti and Nb are not re-dissolved, so that the effect of sharpening the texture is reduced and the hole expandability is deteriorated as coarse carbides after hot rolling. Further, even if the heating temperature exceeds 1350 ° C., the effect is saturated and the cost and equipment are disadvantageous, so the upper limit is set to 1350 ° C.
[0047]
In order to achieve each crystal orientation of the predetermined X-ray intensity level described in (1) above, Ar_ThreeHot rolling is performed above the transformation temperature. In the second half of hot rolling, Ar_ThreeAbove transformation temperature (Ar_ThreeIf a total of 25% or more is not rolled at +100) ° C. or lower, the rolled austenite texture does not develop sufficiently. Therefore, even if such cooling is applied, the finally obtained hot rolled steel sheet Each crystal orientation of the predetermined X-ray intensity level described in the invention of (1) cannot be obtained on the plate surface. Therefore, Ar_ThreeAbove transformation temperature (Ar_ThreeThe lower limit of the total rolling reduction at +100) ° C. or lower was set to 25%.
[0048]
Ar_ThreeAbove transformation temperature (Ar_ThreeThe higher the total rolling reduction at +100) ° C. or less, the sharper the texture formation is expected. Therefore, it is preferable to set it to 35% or more, but if the total rolling reduction exceeds 97.5%, Since it is necessary to increase the rigidity excessively and cause economic disadvantages, it is desirably 97.5% or less.
Hot rolling finish temperature is Ar_ThreeIf the temperature is lower than the transformation temperature, the phenomenon that the {112} <110> orientation is particularly developed in the {100} <011> to {223} <110> orientation groups does not develop, and (Ar_ThreeWhen the transformation temperature exceeds +100) ° C., the shape freezing property deteriorates because the entire texture is randomized. Therefore Ar_ThreeTransformation temperature ~ (Ar_ThreeThe transformation temperature is defined as +100) ° C. From this viewpoint, the upper limit of the hot rolling end temperature is (Ar_ThreeThe transformation temperature +50) ° C. is desirable.
[0049]
Ar_ThreeAbove transformation temperature (Ar_ThreeWhen the friction coefficient between the hot rolling roll and the steel plate at the time of hot rolling at +100) ° C. or less exceeds 0.2, the crystal orientation mainly consisting of {110} plane is present on the plate surface in the vicinity of the steel plate surface. As it develops and the shape freezing property deteriorates, when aiming for a better shape freezing property, Ar_ThreeAbove transformation temperature (Ar_ThreeIt is desirable that the friction coefficient between the hot rolling roll and the steel plate is 0.2 or less for at least one pass during hot rolling at +100) ° C. or less.
[0050]
The lower the friction coefficient, the lower the limit is preferably not set. However, when better shape freezing properties are required, Ar_ThreeAbove transformation temperature (Ar_ThreeThe friction coefficient is desirably 0.15 or less for all passes of hot rolling at +100) ° C. or less. The method for measuring the friction coefficient is not particularly defined, but it is desirable to obtain it from the advanced rate and the rolling load, as is generally well known.
[0051]
In order to transfer the austenite texture formed in this way to the final hot-rolled steel sheet, the average cooling rate of 10 ° C./s until the To temperature shown in the following formula (1) is reached after the end of hot rolling. It is necessary to cool by the above.
Therefore, To determined by the steel component (mass%) was taken as the upper limit of the coiling temperature. This To temperature is thermodynamically defined as the temperature at which the same component of austenite and austenite ferrite have the same free energy, and considering the influence of components other than C, the chemistry of the steel sheet using equation (1) It can be simply calculated by the component (mass%). The influence of the other components defined in the present invention on To (° C.) is not so great and thus ignored here.
[0052]
      To = −650.4 × C% + B (1)
here,
      B = −50.6 × Mneq + 894.3
      Mneq = Mn% +0.13 x Si% +0.55 x Cr%-0.50 × Al%
  When the average cooling rate increases, the driving force for precipitation of TiC or NbC during winding increases, so the average cooling rate is preferably 30 ° C./s or more, more preferably 50 ° C./s or more. On the other hand, since it is practically difficult to set the average cooling rate to more than 200 ° C./s, it is desirable that the average cooling rate be 200 ° C./s or less.
[0053]
Winding after cooling is performed in a temperature range of 450 to 750 ° C. When the coiling temperature is less than 450 ° C., fine precipitation of TiC or NbC is reduced, and iron carbide that deteriorates hole expansibility increases. On the other hand, when the temperature exceeds 750 ° C., TiC or NbC becomes coarse at the grain boundaries, and the hole expandability is deteriorated. From the above viewpoint, it is desirably wound up in a temperature range of 500 to 700 ° C.
[0054]
  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 is 3.0 or more, and among these orientation groups, { 100} <011> orientation has a maximum X-ray random intensity ratio of 4.0 or more, and three crystal orientations {554} <225>, {111} <112> and {111} <110> The average value of the X-ray random intensity ratio is 3.5 or less,To achieve each crystal orientation at the X-ray intensity level (Ar_Three+50) ° C. or higher (Ar_ThreeIt is necessary to perform rolling of a total of 25% or more at +150) ° C. or less. If this condition is not satisfied, the processing of austenite is insufficient and the texture is not sufficiently developed. (Ar_Three+50) to (Ar_ThreeAs the total rolling reduction at +150) ° C. is higher, sharper texture formation is expected. Therefore, it is preferable to set the rolling reduction to 35% or more. However, if the total rolling reduction exceeds 97.5%, Since it is necessary to increase the rigidity excessively and cause economic disadvantages, it is desirably 97.5% or less.
[0055]
In order to significantly increase the texture accumulation in the {100} <011> orientation, (Ar_Three−100) to (Ar_ThreeIt is very important to apply a reduction of 5 to 35% at +30) ° C. This is because, when austenite sufficiently processed in a high temperature region is at least partially recrystallized, a further appropriate amount of reduction is applied, and immediately after that, ferrite transformation is performed, thereby developing the {100} <011> orientation. Because it is extremely important.
[0056]
Therefore, (Ar_ThreeEven when the rolling is performed at a temperature lower than −100) ° C., {100} <011> does not develop because the region where the ferrite transformation has already been completed is too large.
(Ar_ThreeWhen the reduction is applied above +30) ° C., the strain introduced before the ferrite transformation is recovered, and {100} <011> does not develop.
When the rolling reduction is less than 5%, the entire texture including {100} <011> to {223} <110> is randomized, and when it exceeds 35%, accumulation in the {100} <011> orientation is performed. (Ar)_Three−100) to (Ar_ThreeThe rolling reduction in the temperature range of +30) ° C. is 5 to 35%. From the above viewpoint, the rolling reduction is desirably 10 to 25%.
[0057]
Hot rolling (Ar_Three−100) to (Ar_ThreeEnd in the temperature range of +30) ° C. Hot rolling end temperature is (Ar_ThreeWhen it is less than −100), the workability is remarkably deteriorated, and (Ar_ThreeWhen the temperature exceeds +30) ° C., the shape freezing property deteriorates due to insufficient accumulation of texture.
Here, when the friction coefficient between the hot rolling roll and the steel plate during hot rolling exceeds 0.2, the crystal orientation mainly consisting of {110} plane develops on the plate surface near the steel plate surface. When the shape freezing property is deteriorated and the better shape freezing property is directed, (Ar_Three−100) to (Ar_ThreeIn the temperature range of +150) ° C., it is preferable to perform rolling with a coefficient of friction between the roll and the steel sheet being 0.2 or less for at least one pass.
[0058]
The lower the coefficient of friction, the better. Especially when severe shape freezing properties are required, (Ar_Three−100) to (Ar_ThreeThe friction coefficient is desirably 0.15 or less for all passes of hot rolling at +150) ° C.
Winding is performed in a temperature range of 450 to 750 ° C. When the coiling temperature is less than 450 ° C., fine precipitation of TiC or NbC is reduced, and iron carbide that deteriorates hole expansibility increases. On the other hand, when the temperature exceeds 750 ° C., TiC or NbC becomes coarse at the grain boundaries, and the hole expandability is deteriorated. From the above viewpoint, it is desirably wound up in a temperature range of 500 to 700 ° C.
[0059]
In hot rolling, sheet bars may be joined after rough rolling, and finish rolling may be performed continuously. At that time, the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function, if necessary, and rewound again before joining. The hot-rolled steel sheet may be subjected to skin pass rolling as necessary. Needless to say, the skin pass rolling has the effect of preventing stretcher strain generated during processing and shape correction.
[0060]
When the hot-rolled steel sheet thus obtained is cold-rolled and annealed to obtain a final thin steel sheet, when the total rolling reduction of the cold-rolling is 80% or more, a general cold-rolled steel sheet is used. The {111} plane and {554} plane components of the X-ray diffraction integral plane intensity ratio of the crystal plane parallel to the plate surface, which is a cold-rolled and recrystallized texture, are increased. Since the crystal orientation of the invention is not satisfied, the upper limit of the cold rolling reduction is set to less than 80%. In order to increase the shape freezing property, it is desirable to limit the cold rolling reduction to 70% or less.
[0061]
The lower limit of the cold rolling rate is not particularly defined, and the effects of the present invention can be obtained. However, in order to control the strength of the crystal orientation within an appropriate range, it is preferably 3% or more.
When annealing a cold-rolled steel sheet cold-worked in such a range, if the annealing temperature is less than 600 ° C, the processed structure remains and the formability deteriorates significantly, so the lower limit of the annealing temperature is 600 ° C. And On the other hand, when the annealing temperature exceeds 800 ° C., TiC and NbC are coarsened, the hole expandability is deteriorated and the shape freezing property is also lowered. Accordingly, the annealing temperature is set to 800 ° C. or less. The cold-rolled steel sheet may be subjected to skin pass rolling as necessary.
[0062]
Note that the steel sheet according to the present invention can be applied not only to bending, but also to composite forming mainly composed of bending, such as bending, overhanging, drawing, and the like.
[0063]
【Example】
(Example)
The technical contents of the present invention will be described with reference to examples of the present invention.
First, the results of studies using steels A to K having the composition shown in Table 1 will be described. After casting, these steels are either reheated as they are or once cooled to room temperature, heated to 1250 ° C., and then hot-rolled to a final thickness of 1.4 mm, 3 mm, or 8.0 mm. A hot-rolled steel sheet was obtained. The 3.0 mm and 8.0 mm thick hot-rolled steel sheets were cold rolled to a thickness of 1.4 mm, and then annealed in a continuous annealing process.
[0064]
Test pieces having a width of 50 mm and a length of 270 mm were prepared from these 1.4 mm thick steel plates, and a hat bending test was performed using a die having a punch width of 78 mm, a punch shoulder R5, and a die shoulder R5.
The test piece subjected to the bending test was measured for the shape of the central part of the plate width with a three-dimensional shape measuring device, and as shown in FIG. 1, the tangent of point (1) and point (2) and point (3) The value obtained by subtracting 90 ° from the angle of the intersection of the tangent of point (4) is the average value on the left and right, and the value obtained by averaging the reciprocal of the curvature between points (3) and (5) on the left and right The shape freezing property was evaluated using the value obtained by subtracting the punch width from the wall warp amount and the length between the left and right points (5) as the dimensional accuracy. The bending was performed so that a polygonal line was inserted perpendicular to the direction of low r value.
[0065]
By the way, as shown in FIG. 2 and FIG. 3, the amount of spring back and the amount of wall warp also change depending on BHF (wrinkle pressing force). The effect of the present invention does not change even if the evaluation is performed with any BHF, but when pressing an actual part with an actual machine, a very high BHF cannot be applied. The hat bending test was performed.
The X-ray measurement was performed by adjusting a sample parallel to the plate surface at a position of 7/16 thickness of the plate thickness as a representative value of the steel plate.
[0066]
In the hole expansion test, a punched hole with a diameter of 10 mm is processed in the center of a test piece with a side of 100 mm, the initial hole is expanded with a conical punch with an apex angle of 60 °, and the hole diameter when the crack penetrates the steel plate. Evaluation was made with a hole expansion ratio λ (the following formula) for an initial hole diameter of 10 mm of d.
λ = {(d−10) / 10} × 100 (%)
The grain boundary occupancy of iron carbide draws four straight lines on a 200 × optical microscope observation photograph, and the number of intersections between the straight lines and the grain boundaries is N, and iron carbide is located at the position of the N intersections. It calculated | required from M / N using the number M when it exists.
[0067]
[Table 1]

[0068]
[Table 2]

[0069]
Table 2 shows whether the manufacturing conditions of each steel sheet are within the scope of the present invention. “Hot-rolling condition 1” means that hot-rolling is Ar_ThreeAr when completed above the transformation temperature_ThreeAbove transformation temperature (Ar_ThreeWhen the sum of the rolling reductions at +100) ° C. or lower is 25% or more and the hot rolling end temperature is also in that temperature range, “◯”, and when the total rolling reductions in that temperature range is less than 25%, “× "
[0070]
“Hot rolling condition 2-1” is (Ar_Three+50) to (Ar_ThreeWhen the sum of the rolling reductions in the temperature range of +150) ° C. is 25% or more, “O”, and when the sum of the rolling reductions is less than 25%, “x” and “hot rolling condition 2-2” continue to be (Ar_Three−100) to (Ar_ThreeIn the temperature range of +30) ° C., the case where the total rolling reduction was 5 to 35% was indicated as “◯”, and the case where this condition was not satisfied was indicated as “X”.
[0071]
In any of the above cases, in each temperature range, when the friction coefficient for at least one pass is 0.2 or less, “O” in the “Lubrication” column, and the friction coefficient in all passes is 0.2. In the case of exceeding, “△” was given. All the hot rolling operations were performed at a temperature equal to or lower than the To temperature determined by equation (1).
When such a hot-rolled steel sheet is cold-rolled to a thickness of 1.4 mm, when the cold-rolling reduction ratio is 80% or more, the “cold-rolling reduction ratio” is “x”, and when it is “less than 80%” ○ ”.
[0072]
  Also, the annealing temperature is 600 ° C. or higher (Ac_ThreeIn the case of +100) ° C. or lower, the “annealing temperature” was “◯”, and the other cases were marked “X”. Items that have no relation to manufacturing conditions were marked with “-”. Skin pass rolling was performed in a range of 0.5 to 1.5% for both hot-rolled steel sheets and cold-rolled steel sheets.
  Table 3 shows the iron carbide grain boundary occupancy M / N, the maximum particle diameter d of iron carbide, and the mechanical characteristic values of 1.4 mm thick hot-rolled steel sheet and cold-rolled steel sheet manufactured by the above method. And Table 5 (continuation of Table 4) shows the X-ray random intensity ratio, dimensional accuracy, spring back amount, wall warp amount and hole expansion rate.. NaThe structure of the steel sheet satisfying the conditions of the present invention was mainly composed of ferrite or bainite.
[0073]
[Table 3]

[0074]
[Table 4]

[0075]
[Table 5]

[0076]
  The present inventionTurn ofNo.Turn ofIt can be seen that the amount of spring back and the amount of wall warp are smaller than those of No. 1, and as a result, the dimensional accuracy is improved. The present invention has good stretch flangeability in any case.
  On the other hand, I and J in which the grain boundary occupancy M / N of iron carbide and the maximum particle diameter d of iron carbide do not satisfy the provisions of the present invention are good in shape freezing property, but stretch flangeability deteriorates. Yes. Regarding steel H, both the shape freezing property and stretch flangeability are deteriorated.
[0077]
That is, the high-strength flangeable steel sheet having good shape freezing property can be produced only after satisfying the components limited by the present invention, the X-ray random intensity ratio of each crystal orientation, the r value, and the structure.
FIG. 4 shows the relationship between the dimensional accuracy normalized by the tensile strength and the hole expansion rate. It is clear from this that those satisfying the conditions of the present invention are excellent in both dimensional accuracy and stretch flangeability.
[0078]
At present, the mechanism that the X-ray random intensity ratio and the r value of each crystal orientation are important for the shape freezing property is not necessarily clear at present. Probably, by facilitating the progress of slip deformation at the time of bending deformation, the amount of spring back and wall warpage at the time of bending deformation is reduced, resulting in improved dimensional accuracy, that is, shape freezing. Understood.
[0079]
【The invention's effect】
By controlling the texture and r value of the thin steel sheet according to the present invention, the bending workability is remarkably improved, and by controlling the structure and carbide, both the hole expandability and bending workability can be achieved. It has become possible to provide a thin steel plate with a small amount of wall warpage and a shape freezing property mainly composed of bending and a hole expandability. In particular, a high-strength steel sheet can be used even for parts that have conventionally been difficult to apply a high-strength steel sheet due to the problem of shape defects. In order to promote the weight reduction of automobiles, the use of high-strength steel sheets is absolutely necessary. If a high-strength steel sheet having a small amount of spring back and wall warpage and having excellent shape freezing properties and hole expandability can be applied, the weight reduction of the automobile body can be further promoted. Therefore, the present invention is industrially extremely valuable.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a test piece used in a hat bending test.
FIG. 2 is a diagram showing a relationship between a springback amount and BHF (wrinkle pressing force).
FIG. 3 is a diagram showing the relationship between the amount of wall warpage and BHF (wrinkle holding force).
FIG. 4 is a diagram showing the relationship between dimensional accuracy normalized by tensile strength and hole expansion rate.

Claims (8)

% By mass
C: 0.034% or more, 0.15% or less,
Si: 0.001% or more, 3.5% or less,
Mn: 0.05% or more, 3.0% or less,
P: 0.2% or less,
S: 0.03% or less,
Al: 0.01% or more, 3.0% or less,
N: 0.01% or less,
O: 0.01% or less,
In addition,
Ti: 0.01% or more, 2.0% or less,
Nb: 0.01% or more and 2.0% or less of one or two kinds, the balance is made of iron and unavoidable impurities, ferrite or bainite is the maximum phase in area ratio, and iron carbide at the grain boundary The occupation ratio is 0.1 or less, the maximum particle diameter of this iron carbide is 1 μm or less, and the {100} <011> to {223} <110> orientation group X of the plate surface at 1/2 plate thickness The average value of the line random intensity ratio is 3.0 or more, and among these azimuth groups, the X-ray random intensity ratio of the {112} <110> azimuth satisfies the maximum value of 4.0 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, in addition, the r value in the rolling direction and At least one of r-values perpendicular to the rolling direction High stretch-flange formability steel sheet excellent in shape fixability characterized in that is 0.7 or less. Furthermore, in mass%,
B: 0.01% or less is contained, The high elongation flangeability steel plate excellent in the shape freezing property of Claim 1 characterized by the above-mentioned. Furthermore, in mass% ,
C r: 3% or less claim 1 or 2 high stretch-flange formability steel sheet excellent in shape fixability according to characterized in that it contains under. Furthermore, in mass%,
Ca: 0.0005% or more, high stretch flangeability steel sheet excellent in shape fixability according to any one of claims 1 to 3, characterized in that it contains hereinafter 0.005%.   A steel sheet according to any one of claims 1 to 4, which is plated with the steel sheet according to any one of claims 1 to 4, and a high stretch flangeability steel sheet excellent in shape freezing property. It is a method of manufacturing the steel plate of any one of Claims 1-5, Comprising: In hot-rolling the steel slab which consists of a component of any one of Claims 1-4, 1150-1350 Heated to 0 ° C., hot-rolled so that the total rolling reduction in the temperature range of Ar ₃ transformation temperature to (Ar ₃ +100) ° C. is 25% or more, and the hot rolling is finished at the Ar ₃ transformation temperature or higher, From the hot rolling end temperature to the critical temperature To (° C.) determined by the chemical composition (mass%) of the steel shown in the formula (1), cooling at an average cooling rate of 10 ° C./s or more and winding at 450 to 750 ° C. A method for producing a high stretch flangeability steel sheet having excellent shape freezing characteristics.
To = −650.4 × C% + B (1)
here,
B = −50.6 × Mneq + 894.3
Mneq = Mn % + 0.13 × Si % + 0.55 × Cr % − 0.50 × Al % In the temperature range of the Ar ₃ transformation temperature to (Ar ₃ +100) ° C., at least one pass is rolled so that the friction coefficient is 0.2 or less. A method for producing an excellent high stretch flangeability steel sheet.   The steel sheet according to claim 6 or 7 is pickled and subjected to cold rolling of less than 80%, and then heated to a temperature range of 600 to 800 ° C and cooled, and has excellent shape freezing property. Manufacturing method of high stretch flangeability steel sheet.

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