JP2009132988A - Steel sheet, hot dip galvanized steel sheet, hot dip galvannealed steel sheet and steel pipe having low yield ratio and high young's modulus, and method for producing them - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet, a hot dip galvanized steel sheet, a hot dip galvannealed steel sheet and a steel pipe each having a low yield ratio and a high Young's modulus in the rolling direction measured by a static stretching method, and to provide methods for producing them. <P>SOLUTION: A low yield ratio high Young's modulus steel sheet having a composition comprising, by mass, ≤0.01% N, 0.005 to 0.10% Nb and 0.002 to 0.15% Ti, in which Ti and N satisfy Ti-48/14×N≥0.0005, the total volume ratio of either or both ferrite and bainite is >50%, the volume ratio of martensite is 2 to 25%, and, at the position in which the distance to the sheet thickness direction from the surface of the steel sheet is 1/6 the sheet thickness, the total of the X-ray random intensity ratio in the ä100}<001> orientation and the X-ray random intensity ratio in the ä110}<001> orientation is ≤5, and the total of the maximum value of the X-ray random intensity ratios in the group of the ä110}<111> orientation to the ä110}<112> orientation and the X-ray random intensity ratio in the ä211}<111> orientation is ≥5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention has a high Young's modulus in the rolling direction, high tensile strength, low yield strength, low yield ratio high Young's modulus steel plate, hot dip galvanized steel plate, alloyed hot dip galvanized steel plate, and steel pipe, Moreover, it is related with those manufacturing methods.

  The correlation between the Young's modulus and the crystal orientation of iron is very strong. For example, the Young's modulus in the <111> direction ideally exceeds 280 GPa and the Young's modulus in the <110> direction is about 220 GPa. On the other hand, the Young's modulus in the <100> direction is about 130 GPa, and the Young's modulus changes depending on the crystal orientation. Further, when the crystal orientation of the steel material does not have an orientation in a specific orientation, that is, the Young's modulus of the steel sheet having a random texture is about 205 GPa.

  So far, a number of techniques have been proposed for steel sheets that control the texture and increase the Young's modulus in a direction perpendicular to the rolling direction (referred to as the width direction). As a technique for simultaneously increasing the Young's modulus in the rolling direction and the width direction of a steel sheet, a method of manufacturing a thick steel sheet that performs rolling in a direction perpendicular to the rolling direction in addition to rolling in a certain direction has been proposed (for example, Patent Document 1). ,reference). Such a method of changing the rolling direction in the middle can be performed relatively easily in the rolling process of the thick steel plate.

  However, even when a thick steel plate is manufactured, the rolling direction may have to be constant depending on the width and length of the steel plate. In particular, in the case of a thin steel plate, since it is often manufactured by a continuous hot rolling process in which a steel slab is continuously rolled into a steel strip, a technique for changing the rolling direction in the middle is not realistic.

  A reinforcing member or the like at the time of a car collision may be processed by roll forming. In this case, the rolling direction is the longitudinal direction of the member, and it is necessary to improve the Young's modulus in the rolling direction in order to increase the rigidity in the longitudinal direction of the member.

  In response to such demands, some of the present inventors have proposed a method of imparting shear strain to the surface layer portion of the steel sheet and increasing the Young's modulus in the rolling direction of the surface layer portion (for example, Patent Documents 2, 3, reference). The steel sheet obtained by the methods proposed in Patent Documents 2 and 3 has a texture that develops a texture that increases the Young's modulus in the rolling direction in the surface layer portion. Therefore, these steel sheets have a high Young's modulus at the surface layer portion, and a Young's modulus measured by the vibration method is as high as 230 GPa.

  The vibration method, which is one of the methods for measuring Young's modulus, is a measuring method in which bending deformation is applied to the steel sheet while changing the frequency to determine the frequency at which resonance occurs, and this is converted into Young's modulus. The Young's modulus measured by such a method is also called the dynamic Young's modulus, and is the Young's modulus obtained at the time of bending deformation, and the contribution of the surface layer portion having a large bending moment is large.

  However, when the automobile collides, for example, not only a bending moment but also a tensile stress and a compressive stress are applied to the impact absorbing member and the like. In addition, from the viewpoint of collision safety, automobile structural members are required to have high impact absorption energy capability when subjected to compression deformation. Therefore, in order to improve the impact absorption energy as a member, it is necessary to ensure rigidity with respect to tensile stress and compressive stress. For such a requirement, it is effective to increase the Young's modulus for the tensile stress and the compressive stress in the longitudinal direction of the member.

  Therefore, regarding the Young's modulus of the member on which tensile stress and compressive stress act, it is extremely important to increase the Young's modulus measured by the static tension method, not the vibration method, that is, the static Young's modulus. Static Young's modulus is the Young's modulus obtained from the slope in the elastic deformation region of the stress-strain curve obtained when the tensile test is performed, and is determined only by the ratio of the thickness of the high Young's modulus layer to the low layer. The Young's modulus as a whole.

  Therefore, in order to increase the static Young's modulus in the rolling direction, it is necessary to control the texture from the surface layer to the deep part in the thickness direction. In addition, it is more preferable to control the texture at the entire plate thickness from the surface layer to the plate thickness center portion. However, with the methods proposed in Patent Documents 2 and 3, it is difficult to introduce shear strain to the center of the plate thickness during rolling.

  Further, depending on the components and manufacturing conditions, there is a possibility that an orientation that lowers the Young's modulus in the rolling direction develops in the texture at the center of the plate thickness. Therefore, although the Young's modulus measured by the vibration method can be increased to 230 GPa or more, the Young's modulus measured by the static tension method is not necessarily high. That is, there was no steel sheet having a Young's modulus in the rolling direction of 220 GPa or more as measured by the static tension method.

JP-A-4-147717 JP 2005-273001 A International Publication No. 06/011503

  Since steel plates for automobiles are processed into complex shapes, workability such as press formability is required. In order to achieve both workability and strength, it is necessary to maintain the tensile strength and reduce the yield strength. The present invention has a high Young's modulus in the rolling direction measured by a static tensile method, a low yield ratio, a low Young's ratio high Young's modulus steel sheet, a plated steel sheet, a steel pipe made from these steel sheets, and The present invention provides a manufacturing method thereof.

  The crystal orientation is usually represented by {hkl} <uvw>. {Hkl} indicates the plate surface orientation, and <uvw> indicates the rolling direction. Therefore, in order to obtain a high Young's modulus in the rolling direction, it is necessary to control <uvw>, which is an orientation in the rolling direction, to be aligned in a direction having a high Young's modulus as much as possible.

  Based on this principle, the present inventors obtain a high Young's modulus steel sheet in which the Young's modulus in the rolling direction measured by the static tension method is 220 GPa or more and the microstructure of the steel sheet is controlled so as to have a low yield ratio. In order to do so, a study was conducted.

  As a result, in order to improve the static Young's modulus in the rolling direction, Nb is added and a predetermined amount of Ti and N is contained to suppress recrystallization in the austenite phase (hereinafter sometimes referred to as γ phase). In addition, it was newly found out that the effect is remarkable when B is added in combination.

  Moreover, in hot rolling, the shape ratio obtained from the rolling temperature and the thickness and diameter of the rolling roll on the entry and exit sides of the rolling roll is important, and by controlling these to an appropriate range, On the surface of the steel sheet, the thickness of the layer to which shear strain is applied increases, and the distance from the surface in the thickness direction is 1/6 of the thickness (referred to as a 1/6 thickness portion). It was newly found that the texture is also optimized.

  In addition, there is a correlation between the stacking fault energy that affects the deformation behavior of the γ-phase subjected to hot working and the texture after transformation, from the surface layer to 1/6 plate thickness part, and the central part in the plate thickness direction (Referred to as 1/2 plate thickness). Therefore, in order to obtain a texture in which the orientation in which the Young's modulus in the rolling direction is improved is developed in both the surface layer and the center of the plate thickness, Mn, Mo, W, Ni, Cu affecting the stacking fault energy of the γ phase. They also learned that it is important to optimize the Cr relationship.

  Furthermore, after hot rolling, cooling conditions until winding, cold rolling, cooling conditions after annealing, without lowering the Young's modulus in the rolling direction, the steel sheet structure, ferrite, bainite as the main phase, We succeeded in obtaining a steel sheet with low yield ratio and high Young's modulus with a microstructure having martensite as the second phase.

The gist of the present invention is as follows.
(1) By mass%, C: 0.005 to 0.20%, Mn: 0.10 to 3.00%, Nb: 0.005 to 0.10%, Ti: 0.002 to 0.15% Si: 2.50% or less, P: 0.15% or less, S: 0.015% or less, Al: 0.15% or less, N: 0.01% or less, and the following (formula 1), the balance is made of Fe and inevitable impurities, and one or both of ferrite and bainite with a total volume ratio exceeding 50%, martensite with a volume ratio of 2 to 25%, and the total volume ratio X-ray random intensity ratio in the {100} <001> orientation at a position where the distance in the thickness direction from the surface of the steel sheet is 1/6 of the thickness, consisting of one or both of pearlite and cementite of 15% or less And the {110} <001> orientation X-ray random intensity ratio is 5 or less, {11 } <111> to {110} <112> The low yield characterized by the sum of the X-ray random intensity ratio of the orientation group and the X-ray random intensity ratio of the {211} <111> orientation being 5 or more Specific high Young's modulus steel plate.

Ti-48 / 14 × N ≧ 0.0005 (Formula 1)
Here, Ti and N are content [mass%] of each element.

(2) The low yield ratio high Young's modulus steel sheet according to (1), wherein the following (Formula 2) is satisfied.
4 ≦ 3.2Mn + 9.6Mo + 4.7W + 6.2Ni + 18.6Cu + 0.7Cr ≦ 10
... (Formula 2)
Here, Mn, Mo, W, Ni, Cu, and Cr are the content [% by mass] of each element.

  (3) By mass%, Mo: 0.01 to 1.00%, Cr: 0.01 to 3.00%, W: 0.01 to 3.00%, Cu: 0.01 to 3.00% Ni: 0.01 to 3.00% of 1 type or 2 types or more, The low yield ratio high Young's modulus steel plate as described in said (1) or (2) characterized by the above-mentioned.

  (4) The low yield ratio high Young's modulus steel plate according to any one of the above (1) to (3), wherein B: 0.0005 to 0.01% by mass.

  (5) By mass%, one or more of Ca: 0.0005 to 0.10%, Rem: 0.0005 to 0.10%, V: 0.001 to 0.10% The low yield ratio high Young's modulus steel sheet according to any one of (1) to (4) above.

  (6) X-ray random intensity ratio (A) of {332} <113> orientation at the center in the thickness direction of the steel sheet is 15 or less, X-ray random intensity ratio (B) of {112} <110> orientation 5 or more and satisfies (A) / (B) ≦ 1.00. The low yield ratio high Young's modulus steel plate according to any one of (1) to (5) above.

  (7) The X-ray random intensity ratio (A) in the {332} <113> orientation at the center in the thickness direction of the steel sheet is 15 or less, the {001} <011> orientation X-ray random intensity ratio and {112 } The simple average value (C) with respect to the X-ray random intensity ratio in the <110> orientation is 5 or more and (A) / (C) ≦ 1.10 is satisfied. The low yield ratio high Young's modulus steel plate according to any one of 6).

  (8) The low yield ratio high Young's modulus steel sheet according to any one of (1) to (7) above, wherein the Young's modulus in the rolling direction measured by a static tension method is 220 GPa or more.

  (9) A low yield ratio high Young's modulus hot dip galvanized steel sheet, wherein the low yield ratio high Young's modulus steel sheet according to any one of (1) to (8) is subjected to hot dip galvanization.

  (10) Low yield ratio high Young's modulus alloying and melting, characterized in that the low yield ratio high Young's modulus steel sheet according to any of (1) to (8) above is subjected to alloying hot dip galvanization. Galvanized steel sheet.

  (11) The low yield ratio high Young's modulus steel sheet according to any one of (1) to (8) above, the low yield ratio type high Young's modulus hot dip galvanized steel sheet according to (9) above, or the above (10) A low yield ratio high Young's modulus steel pipe characterized in that the low yield ratio high Young's modulus galvannealed steel sheet is wound in any direction.

(12) The shape calculated | required by the following (Formula 3) on the steel piece which has a chemical component in any one of said (1)-(5) as 1100 degrees C or less and the reduction rate to the last pass | pass 40% or more. After rolling with a ratio X of 2.3 or more is set to 2 passes or more, after finishing hot rolling at a temperature of Ar ₃ transformation point [° C.] or more and 900 ° C. or less at the final pass temperature and performing air cooling within 30 s A method for producing a low yield ratio high Young's modulus steel sheet, wherein the steel sheet is cooled to 25 to 300 ° C. at a cooling rate of 5 to 150 ° C./s and wound.

Shape ratio X = l _d / h _m (Equation 3)
Here, l _d (contact arc length of rolling roll and steel plate): √ (L × (h _in −h _out ) / 2)
h _m : (h _in + h _out ) / 2
L: Diameter of the rolling roll
h _in : Thickness on the entry side of the rolling roll
h _out : Plate thickness on the exit side of the rolling roll

(13) The low yield ratio high Young's modulus steel plate according to (12) above, wherein hot rolling is performed so that the effective strain amount ε ^* calculated by the following (Equation 5) is 0.4 or more. Manufacturing method.

  Here, n is the number of finishing hot rolling rolling stands, εj is the strain applied at the jth stand, εn is the strain applied at the nth stand, and ti is between i and i + 1th stands. The traveling time [s] and τi are numerical values calculated by the following (formula 6) from the gas constant R (= 1.987) and the rolling temperature Ti [K] of the i-th stand.

  (14) The low yield ratio high Young's modulus according to (12) or (13) above, wherein at least one rolling roll having a roll diameter of 700 mm or less is used when hot rolling is performed. A method of manufacturing a steel sheet.

  (15) The production of the low yield ratio high Young's modulus steel sheet according to any one of (12) to (14) above, wherein the different peripheral speed ratio of at least one pass of hot rolling is 1% or more. Method.

  (16) A method of producing the low yield ratio high Young's modulus hot-dip galvanized steel sheet according to (9), wherein the low yield ratio is produced by the production method according to any one of (12) to (15). A method for producing a hot dip galvanized steel sheet having a low yield ratio and a high Young's modulus, characterized by subjecting a high Young's modulus steel sheet to hot dip galvanizing.

(17) A method for producing a low yield ratio high Young's modulus galvannealed steel sheet as described in (10) above, up to 450 to 600 ° C. after the hot dip galvanizing as described in (16) above. A method for producing a galvannealed steel sheet having a low yield ratio and a high Young's modulus, characterized by performing a heat treatment for 5 seconds or more in a temperature range of

  (18) A method for producing the steel pipe according to (11) above, wherein the steel plate has a low yield ratio and a high Young's modulus obtained by the production method according to any one of (12) to (15) above, (16 The low yield ratio high Young's modulus hot dip galvanized steel sheet obtained by the production method described in the above), or the low yield ratio high Young's modulus alloyed hot dip galvanized steel sheet obtained by the production method described in (17) above, A method for producing a steel pipe with a low yield ratio and a high Young's modulus, wherein the pipe is rolled in an arbitrary direction to form a steel pipe.

  ADVANTAGE OF THE INVENTION According to this invention, the low Young's modulus high Young's modulus steel plate with the high static Young's modulus of the rolling direction measured with the static tension method and favorable workability can be obtained.

  First, the formation of the texture that is important for improving the static Young's modulus in the rolling direction and the texture by hot rolling will be described.

  When the texture changes in the plate thickness direction of the steel sheet and the texture in the central portion in the surface layer and the plate thickness direction is different, the stiffness, that is, the Young's modulus, does not necessarily match between the tensile deformation and the bending deformation. This is because the rigidity of tensile deformation is a characteristic affected by the texture of the entire thickness of the steel sheet, and the rigidity of bending deformation is a characteristic influenced by the texture of the surface layer portion of the steel sheet.

  The present invention is a steel sheet in which the texture from the surface to the part where the distance in the sheet thickness direction is 1/6 of the sheet thickness is optimized to increase the Young's modulus in the rolling direction. Therefore, the texture that contributes to the Young's modulus in the rolling direction has developed to at least a 1/6 plate thickness portion that is deeper than the 1/8 plate thickness portion.

  By increasing the thickness of the region where the Young's modulus in the rolling direction is increased, not only bending deformation but also Young's modulus against tensile deformation and compression deformation can be increased. Further, in order to introduce shear strain not only to the surface layer but also to 1/6 plate thickness part, it is manufactured by increasing the shape ratio determined by the plate thickness of the steel plate before and after the hot rolling of one pass and the diameter of the rolling roll. Is.

  The steel sheet of the present invention accumulates orientations that increase the Young's modulus in the rolling direction at least from the surface layer to the 1 / 6th plate thickness portion, and suppresses the accumulation of orientations that reduce the Young's modulus. In addition, the static Young's modulus in the rolling direction up to 1/6 plate thickness is high, and the rigidity in tensile deformation is high. In the steel sheet of the present invention, the accumulation of the orientation that lowers the Young's modulus is also suppressed by accumulating the orientation that increases the Young's modulus in the rolling direction in the portion from the surface layer to the 1/6 thickness portion.

  Specifically, in the steel sheet of the present invention, the sum of the {100} <001> orientation X-ray random intensity ratio and the {110} <001> orientation X-ray random intensity ratio of the 1/6 thickness portion The sum of the maximum value of the X-ray random intensity ratio of the {110} <111> to {110} <112> orientation group and the X-ray random intensity ratio of the {112} <111> orientation is 5 or more. . The steel plate of the present invention is obtained by applying a shearing force from the surface layer of the steel plate to at least 1/6 thickness portion in the hot rolling.

  In order to apply the hot rolling shearing force to the 1/6 thickness portion of the steel sheet, the shape ratio X defined by the following formula is 2.3 or more in at least two passes out of the total number of hot rolling passes. The present inventors have found that it is necessary to satisfy the above. As shown in the following (formula 3), the shape ratio X is a ratio between the contact arc tension of the roll and the steel plate and the average plate thickness. It is a knowledge newly obtained by the present inventors that the shear force acts on a deeper portion in the plate thickness direction of the steel sheet as the value of the shape ratio X is larger.

Shape ratio X = l _d / h _m (Equation 3)
Here, l _d (contact arc length of rolling roll and steel plate): √ (L × (h _in −h _out ) / 2)
h _m : (h _in + h _out ) / 2
L: Diameter of the rolling roll
h _in : Thickness on the entry side of the rolling roll
h _out : Plate thickness on the exit side of the rolling roll

  When the number of passes in which the shape ratio X calculated by the above (Equation 3) is 2.3 or more is one pass, the shear strain is not introduced to the 1/6 plate thickness part. Therefore, the thickness of the layer in which shear strain is introduced (referred to as a shear layer) is insufficient, the texture in the vicinity of the 1/6 plate thickness portion is deteriorated, and the Young's modulus measured by the static tension method is low. descend.

  Therefore, it is necessary to set the number of passes having a shape ratio X of 2.3 or more to 2 or more. A larger number of passes is more preferable, and the shape ratio X of all passes may be 2.3 or more. In order to increase the thickness of the shear layer, it is preferable that the value of the shape ratio X is also large, 2.5 or more, more preferably 3.0 or more.

  In addition, when rolling with a shape ratio X of 2.3 or more is performed at a high temperature, the texture that increases the Young's modulus may be destroyed by subsequent recrystallization. Therefore, rolling that limits the number of passes for which the shape ratio X is 2.3 or more needs to be performed at 1100 ° C. or less. In addition, since the effect of the shape ratio becomes more significant as the rolling temperature is lower, it is preferable to perform rolling with the shape ratio X of 2.3 or more in a rolling stand close to the end.

  Furthermore, in order to optimize the texture of the total thickness from the surface to the center of the plate thickness, the stacking fault energy of the austenite phase (γ phase) generated by hot rolling by limiting the components is made an optimal range, It is preferable to perform rolling under conditions that allow deep shear deformation. Thereby, the azimuth | direction which reduces the Young's modulus developed in plate | board thickness center part can also be suppressed, and the static Young's modulus as the whole plate | board thickness can be improved.

  It has been known that the difference in stacking fault energy has a great influence on the working texture of the γ phase having a face-centered cubic structure. In addition, after being subjected to the processing of the γ phase during hot rolling, it is cooled and transformed into a ferrite phase (hereinafter sometimes referred to as α phase) and a bainite phase. It transforms to an orientation having a certain orientation relationship with the crystal orientation of the previous γ phase. This is a phenomenon called variant selection.

  The present inventors have found that the change in texture due to the type of strain introduced by hot rolling is affected by the stacking fault energy of the γ phase. That is, in the surface layer into which shear strain is introduced and the central layer into which compressive strain is introduced, the texture changes depending on the stacking fault energy of the γ phase.

  For example, when the stacking fault energy increases, the degree of accumulation in the {110} <111> orientation, which is the orientation that maximizes the Young's modulus in the rolling direction, increases in the surface layer portion of the steel sheet, and the Young's modulus in the rolling direction increases in the center of the plate thickness. The {332} <113> orientation that develops develops.

  On the other hand, when the stacking fault energy decreases, the {110} <111> orientation does not increase in the 1/6 plate thickness portion from the surface layer, and in particular, the orientation decreases the Young's modulus in the vicinity of the 1/6 plate thickness portion { 100} <001> and <110> <001> are easily developed.

  On the other hand, when the stacking fault energy decreases, the {225} <110> orientation, the {001} <011> orientation, and the {001} <011> orientation, which are relatively advantageous to the Young's modulus in the rolling direction, 112} <110> orientation develops.

Therefore, in order to improve the static Young's modulus, control to an appropriate stacking fault energy range in which the Young's modulus of both the plate thickness surface layer and the central portion becomes high, specifically, the following (Equation 2) is satisfied. It is preferable to do.
4 ≦ 3.2Mn + 9.6Mo + 4.7W + 6.2Ni + 18.6Cu + 0.7Cr ≦ 10
... (Formula 2)
Here, Mn, Mo, W, Ni, Cu, and Cr are the content [% by mass] of each element.

  The above (Formula 2) is the one that the present inventors have conducted a test and corrected it based on a formula that quantifies the influence of each element on the stacking fault energy of the austenitic stainless steel having a γ phase. It is.

  Specifically, 0.03% C-0.1% Si-0.5% Mn-0.01% P-0.0012% S-0.036% Al-0.010% Nb-0.015 % Ti-0.0012% B-0.0015% N as a basic component composition, and static Young in the rolling direction when the amount of Mn, Cr, W, Cu, and Ni are varied. The rate was investigated.

In hot rolling, rolling at a final pass temperature of not less than Ar ₃ transformation point and not more than 900 ° C., a reduction rate from 1100 ° C. to the final pass of not less than 40%, and a shape ratio of not less than 2.3 is not less than 2 passes. went. The Ar ₃ transformation temperature was calculated by the following (formula 4).

Ar ₃ = 901-325 × C + 33 × Si + 287 × P + 40 × Al
−92 × (Mn + Mo + Cu) −46 × (Cr + Ni) (Formula 4)
Here, C, Si, P, Al, Mn, Mo, Cu, Cr, and Ni are the contents [% by mass] of each element, and are 0 when the contents are about impurities. Moreover, in order to simulate winding at 700 degrees C or less after rolling, the heat processing hold | maintained at 650 degreeC for 2 hours was performed.

  JIS Z 2201 No. 13 test piece was taken from the steel sheet with the rolling direction as the longitudinal direction, and the static Young's modulus was measured by applying a tensile stress corresponding to 1/2 of the yield strength of each steel sheet. The measurement was performed 5 times, and among the Young's modulus calculated based on the slope of the stress-strain diagram, the average value of three measured values excluding the maximum value and the minimum value was defined as the Young's modulus by the static tension method.

  The results are shown in FIG. From this, when the value of this relational expression found by the present inventors is 4 or more and 10 or less, a high rolling direction rate static Young's modulus exceeding 220 GPa is obtained, whereas when it exceeds 4 or 10, It can be seen that the value drops significantly.

  Hereinafter, the X-ray random strength ratio and Young's modulus of the steel sheet of the present invention will be described.

  Sum of X-ray random intensity ratio of {100} <001> orientation and X-ray random intensity ratio of {110} <001> orientation at 1/6 plate thickness portion:

  The {100} <001> orientation and the {110} <001> orientation are orientations that significantly reduce the Young's modulus in the rolling direction. When the Young's modulus of a steel sheet is measured by the vibration method, the influence of the texture of the outermost layer is large, and the influence of the texture inside the thickness direction is small. However, when the Young's modulus of a steel sheet is measured by the static tension method, not only the surface layer but also the internal texture in the sheet thickness direction has an effect.

  In order to increase the Young's modulus measured by the static tension method, it is necessary to increase at least the Young's modulus from the surface layer to the 1/6 plate thickness portion. Therefore, in order to increase the Young's modulus in the rolling direction measured by the static tension method, the {100} <001> orientation X-ray random intensity ratio and the {110} <001> orientation at the 1/6 thickness portion The X-ray random intensity ratio must be 5 or less. In this respect, it is more preferably 3 or less.

  Note that the {100} <001> orientation and the {110} <001> orientation are likely to develop in the vicinity of the 1/6 thick portion when shear strain is applied only to the surface layer of the steel plate. On the other hand, when shear strain is introduced to the vicinity of the 1/6 plate thickness portion, the development of {100} <001> orientation and {110} <001> orientation at this portion is suppressed, and {110} described below <111> to {110} <112> orientation groups and {211} <111> orientations develop.

  Sum of maximum value of X-ray random intensity ratio of {110} <111> to {110} <112> azimuth group and {211} <111> azimuth X-ray random intensity ratio in 1/6 plate thickness portion:

  These orientations (groups) are crystal orientations effective for increasing the Young's modulus in the rolling direction, and develop due to shear strain introduced during hot rolling. The sum of the maximum value of the X-ray random intensity ratio of the {110} <111> to {110} <112> orientation group and the X-ray random intensity ratio of the {211} <111> orientation in the 1/6 plate thickness portion is 5 or more. This means that a texture that increases the Young's modulus in the rolling direction is developed from the surface of the steel plate to the 1/6 thick portion.

  Thereby, the static Young's modulus of the rolling direction measured by the static tension method becomes 220 GPa or more. Preferably, it is 10 or more, more preferably 12 or more.

  The {100} <001> orientation, {110} <001> orientation, {110} <111> to {110} <112> orientation group, and the {211} <111> orientation X-ray random intensity ratio is X Crystal orientation distribution function representing a three-dimensional texture calculated by a series expansion method based on a plurality of pole figures among {110}, {100}, {211}, {310} pole figures measured by line diffraction (Orientation Distribution Function, ODF).

  Note that the X-ray random intensity ratio means that the X-ray intensity of a standard sample that does not accumulate in a specific orientation and the test material is measured under the same conditions by the X-ray diffraction method or the like. It is a numerical value obtained by dividing the line intensity by the X-ray intensity of the standard sample.

FIG. 2 shows an ODF of a φ ₂ = 45 ° cross section where the crystal orientation of the present invention is displayed. FIG. 2 is a Bunge display showing a three-dimensional texture by a crystal orientation distribution function, where Euler angle φ ₂ is 45 °, and a specific crystal orientation (hkl) [uvw] is expressed by Euler of the crystal orientation distribution function. Angles φ ₁ and φ are shown.

As shown by the point on the axis of Φ = 90 ° in FIG. 2, the {110} <111> to {110} <112> orientation groups are strictly Φ = 90 ° and φ ₁ = 35.26. It refers to a range of ˜54.74 °.

However, since a measurement error due to specimen processing or sample setting may occur, the maximum value of the X-ray random intensity ratio in the {110} <111> to {110} <112> orientation groups is Φ = 85. The maximum X-ray random intensity ratio in the range of ~ 90 ° and φ ₁ = 35 to 55 °.

For the same reason, in the cross section of φ ₂ = 45 ° of the three-dimensional texture, the {211} <111> orientation is φ ₁ = 85 to 90 °, φ = 30 to 40 ° range, {100} <001> orientation is φ ₁ = 40 to 50 °, Φ = 0 to 5 ° range, {110} <001> orientation is φ ₁ = 85 to 90 °, The maximum value in the range of Φ = 85 to 90 ° is represented as the intensity ratio of each direction.

  Here, as for the crystal orientation, the orientation perpendicular to the plate surface is usually represented by [hkl] or {hkl}, and the orientation parallel to the rolling direction is represented by (uvw) or <uvw>. {Hkl} and <uvw> are generic names of equivalent planes, and [hkl] and (uvw) indicate individual crystal planes.

  That is, in the present invention, since a body-centered cubic (bcc structure) is targeted, for example, (111), (−111), (1-11), The (11-1), (-1-11), (-11-1), (1-1-1), and (-1-1-1) planes are equivalent and cannot be distinguished. In such a case, these orientations are collectively referred to as {111}.

The ODF is also used for displaying the orientation of a crystal structure with low symmetry, and is generally expressed as φ ₁ = 0 to 360 °, Φ = 0 to 180 °, and φ ₂ = 0 to 360 °. Individual orientations are displayed in [hkl] (uvw). However, in the present invention, b. c. c. Since that is targeted to the structure, for Φ and phi _2, it is expressed in a range of 0 to 90 °.

In addition, the range of φ ₁ varies depending on whether or not symmetry due to deformation is taken into account when performing calculations. In the present invention, φ ₁ = 0 to 90 ° in consideration of symmetry. A method of notation, that is, a method for expressing an average value in the same direction at φ ₁ = 0 to 360 ° on an ODF of 0 to 90 ° is selected. In this case, [hkl] (uvw) and {hkl} <uvw> are synonymous.

Therefore, for example, the X-ray random intensity ratio of (110) [1-11] of ODF in the φ ₂ = 45 ° cross section shown in FIG. 2 is the X-ray random intensity ratio of {110} <111> orientation. .

  The sample for X-ray diffraction is produced as follows. After the steel plate is polished to a predetermined position in the thickness direction by mechanical polishing or chemical polishing, and finished to a mirror surface by buffing, the distortion is removed by electrolytic polishing or chemical polishing, and at the same time, the 1/6 thickness portion is Adjust to the measurement surface. Since it is difficult to accurately set the measurement surface to 1/6 plate thickness, the sample should be placed so that the measurement surface is within 3% of the plate thickness with the target position at the center. What is necessary is just to produce.

  When measurement by X-ray diffraction is difficult, a statistically sufficient number of measurements may be performed by an EBSP (Electron Back Scattering Pattern) method or an ECP (Electron Channeling Pattern) method.

  {100} <001> orientation and {110} <001> orientation development is suppressed to a deeper position in the plate thickness direction, and {110} <111> to {110} <112> orientation group and {211} When the <111> orientation is developed, the Young's modulus is further improved. Therefore, by forming a texture similar to that of the surface layer to a position deeper than the １ ／ plate thickness portion, preferably ¼ plate thickness portion, more preferably １ ／ plate thickness portion, in the rolling direction. The static Young's modulus is significantly improved.

  However, as in the present invention, even if shear strain is introduced from the surface layer to a position deeper than usual, it is impossible to introduce shear strain into the center portion of the plate thickness. For this reason, the same texture as the surface layer cannot be developed in the ½ plate thickness portion, and a texture different from the surface layer develops in the plate thickness center layer.

  Therefore, in order to further improve the static Young's modulus, in addition to the texture from the surface layer to the 1/6 plate thickness part, the texture of the 1/2 plate thickness part is also advantageous for the Young's modulus in the rolling direction. It is preferable to improve the orientation.

  {332} <113> orientation X-ray random intensity ratio (A), {112} <110> orientation X-ray random intensity ratio (B), and (A) / (B) :

  The {332} <113> orientation is a typical crystal orientation that develops in the center portion of the plate thickness and is an orientation that lowers the Young's modulus in the rolling direction, whereas the {112} <110> orientation is a Young in the rolling direction. This is a relatively advantageous orientation for rate.

  Therefore, in order to improve the static Young's modulus in the rolling direction of the plate thickness center portion, the X-ray random intensity ratio (A) of the {332} <113> orientation at the plate thickness center portion is 15 or less, and It is preferable that the X-ray random intensity ratio (B) in the {112} <110> orientation satisfies 5 or more.

  In addition, the orientation (A) for reducing the Young's modulus in the rolling direction should be equal to or less than the orientation (B) for improving the Young's modulus in the rolling direction, specifically, (A) / (B) is set to 1 It is preferable to make it 0.000 or less. From this viewpoint, it is more preferable that (A) / (B) is 0.75 or less, and further preferably 0.60 or less. By satisfying the above conditions, the difference between the dynamic Young's modulus and the static Young's modulus can be made within 10 GPa.

  Average value (C) and (A) / (C) of the X-ray random intensity ratio of {001} <011> orientation and {112} <110> orientation at the center of the plate thickness:

  In order to set the static Young's modulus in the rolling direction to 220 GPa or more, it is desirable to control the rolling texture developed at the center portion of the plate thickness, and to set the Young's modulus in this rolling direction to a value exceeding 215 GPa.

  The {001} <011> orientation and the {112} <110> orientation are representative orientations in which the <110> direction is aligned with the rolling direction called α fiber. This orientation is a relatively advantageous orientation with respect to the Young's modulus in the rolling direction. In order to improve the static Young's modulus in the rolling direction at the center of the plate thickness, {001} <011 at the center of the plate thickness. It is preferable that the average value (C) of the X-ray random intensity ratio between the> orientation and the {112} <110> orientation satisfies 5 or more.

  In addition, the orientation (A) for reducing the Young's modulus in the rolling direction is made equal to or less than the orientation (C) for improving the Young's modulus in the rolling direction, specifically, (A) / (C) is set to 1. It is preferable to make it 10 or less.

  The sample for X-ray diffraction in the 1/2 plate thickness part is also polished to remove the distortion in the same manner as the sample for the 1/6 plate thickness part, and the measurement surface is within 3% of the 1/2 plate thickness part. What is necessary is just to adjust and produce so that it may become. When abnormalities such as segregation are observed at the center of the plate thickness, the sample may be prepared by avoiding the segregated portion within the range of 7/16 to 9/16 of the plate thickness.

However, as with the 1/6 plate thickness portion, measurement errors due to test piece processing, sample setting, and the like may occur. Therefore, in the cross section of φ ₂ = 45 ° of the three-dimensional texture shown in FIG. 2, the {001} <011> orientation and the {112} <110> orientation are φ ₁ = 0 to 5 ° and Φ = The range of 0-5 ° and φ ₁ = 0-5 °, the range of Φ = 25-35 °, the {332} <113> orientation is the range of φ ₁ = 85-90 °, Φ = 60-70 °. The maximum value of each will be represented as the intensity ratio in that direction.

In addition, the {112} <110> orientation is in a range of φ ₁ = 0 to 5 ° and Φ = 30 to 40 °. Therefore, for example, when φ ₁ = 0 to 5 ° and the maximum value in the range of Φ = 30 to 35 ° is larger than Φ = 25 to 30 ° and Φ = 35 to 40 °, {225} The <110> orientation X-ray random intensity ratio and the {112} <110> orientation X-ray random intensity ratio are evaluated as the same numerical value.

  The Young's modulus is measured by the static tension method by applying a tensile stress corresponding to ½ of the yield strength of the steel sheet using a tensile test piece according to JIS Z 2201. That is, the Young's modulus is calculated based on the slope of the obtained stress-strain diagram by applying a tensile stress corresponding to ½ of the yield strength. In order to eliminate variations in measurement, the same test piece was used, and five measurements were performed. Among the obtained results, the value calculated as the average value of the three measured values excluding the maximum and minimum values was determined as Young. Rate.

  The sum of the X-ray random intensity ratio of {100} <001> orientation and the X-ray random intensity ratio of {110} <001> orientation of the 1/6 plate thickness part is 5 or less, and {110} <111 > ~ {110} <112> The steel sheet of the present invention in which the sum of the maximum value of the X-ray random intensity ratio of the orientation group and the X-ray random intensity ratio of the {112} <111> orientation is 5 or more is hot-rolled. In this case, it is obtained by applying a shearing force from the surface layer of the steel plate to at least 1/6 thickness portion.

  In order to apply the hot rolling shearing force to the 1/6 thickness portion of the steel sheet, the shape ratio X defined by the following formula is 2.3 or more in at least two passes out of the total number of hot rolling passes. Need to be satisfied. As shown in the following (formula 3), the shape ratio X is a ratio between the contact arc tension of the roll and the steel plate and the average plate thickness.

  It is a knowledge newly obtained by the present inventors that the shear force acts on a deeper portion in the plate thickness direction of the steel sheet as the value of the shape ratio X is larger.

Shape ratio X = l _d / h _m (Equation 3)
Here, l _d (contact arc length of rolling roll and steel plate): √ (L × (h _in −h _out ) / 2)
h _m : (h _in + h _out ) / 2
L: Diameter of the rolling roll
h _in : Thickness on the entry side of the rolling roll
h _out : Plate thickness on the exit side of the rolling roll

  When the number of passes in which the shape ratio X calculated by the above (Equation 3) is 2.3 or more is one pass, the shear strain is not introduced to the 1/6 plate thickness part. Therefore, the thickness of the layer in which shear strain is introduced (referred to as a shear layer) is insufficient, the texture in the vicinity of the 1/6 plate thickness portion is deteriorated, and the Young's modulus measured by the static tension method is low. descend. Therefore, it is necessary to set the number of passes having a shape ratio X of 2.3 or more to 2 or more.

  A larger number of passes is more preferable, and the shape ratio X of all passes may be 2.3 or more. In order to increase the thickness of the shear layer, it is preferable that the value of the shape ratio X is also large, 2.5 or more, and more preferably 3.0 or more.

  In addition, when rolling with a shape ratio X of 2.3 or more is performed at a high temperature, the texture that increases the Young's modulus may be destroyed by subsequent recrystallization. Therefore, rolling that limits the number of passes for which the shape ratio X is 2.3 or more needs to be performed at 1100 ° C. or less. Since the effect of the shape ratio becomes more remarkable as the rolling temperature is lower, it is preferable to perform rolling in which the shape ratio X is 2.3 or more in a rolling stand close to the end.

When performing hot rolling, in order to effectively introduce shear strain from the surface layer of the steel plate to at least 1/6 thickness portion, the effective strain amount ε ^* calculated by the following (Equation 5) is 0. More preferably, the number is 4 or more.

  Here, n is the number of finishing hot rolling rolling stands, εj is the strain applied at the jth stand, εn is the strain applied at the nth stand, and ti is between i and i + 1th stands. The traveling time [s] and τi are numerical values calculated by the following (formula 6) from the gas constant R (= 1.987) and the rolling temperature Ti [K] of the i-th stand.

The effective strain ε ^* is an index of the cumulative strain introduced into the surface layer of the steel sheet taking into account the recovery of dislocations during hot rolling. The strain introduced into the layer can be secured. As the effective strain ε ^* is higher, the thickness of the shear layer is increased and a texture preferable for improving the Young's modulus is developed. Therefore, 0.5 or more is preferable, and 0.6 or more is more preferable.

When the effective strain ε ^* is 0.4 or more, it is preferable that the friction coefficient between the rolling roll and the steel plate is more than 0.2 in order to effectively introduce strain into the shear layer. The friction coefficient can be adjusted by controlling the rolling load, rolling speed, type and amount of lubricant.

  Next, the microstructure of the low yield ratio high Young's modulus steel sheet of the present invention will be described.

  The structure of the steel sheet of the present invention is a composite structure of one or both of relatively soft ferrite and bainite and martensite which is a hard phase. When the total volume ratio of one or both of ferrite and bainite is more than 50% and the volume ratio of martensite (hereinafter also referred to as martensite fraction) is 2 to 25%, the yield strength relative to the tensile strength can be reduced. The ratio, i.e. the yield ratio, decreases.

  When the volume ratio of one or both of ferrite and bainite is 50% or less and the volume ratio of martensite exceeds 25%, the yield strength increases and the yield ratio increases. On the other hand, when the volume ratio of martensite is less than 2%, the strength decreases. In order to increase the strength, the martensite fraction is preferably 5% or more. On the other hand, in order to reduce the yield ratio, it is preferable to make the martensite fraction 20% or less.

  Structures other than ferrite, bainite, and martensite, that is, the balance is cementite and pearlite, and the balance structure needs to be 15% or less. This is because when the volume ratio of pearlite and cementite increases, the volume ratio of martensite decreases, the strength decreases, and the yield ratio increases at the same time.

  Note that the area ratio of ferrite, bainite, martensite, cementite, and pearlite measured by microstructural observation with an optical microscope and image analysis of the structure photograph is equivalent to the volume ratio. In addition, fine cementite that is not observed with an optical microscope has little effect on the properties of the steel sheet. Therefore, in the present invention, when cementite is not observed with an optical microscope, the volume ratio is regarded as 0%.

  In the case of a hot-rolled steel sheet, the microstructure can be controlled by cooling after hot rolling. When the transformation structure is controlled during cooling after hot rolling, that is, when the structure of the hot rolled sheet is made into a composite structure containing martensite, the cooling conditions after finish rolling are important. Control cooling is performed by limiting the air cooling time.

  Although it is preferable to perform control cooling immediately after finish rolling, air cooling may occur until the start of control cooling due to restrictions on equipment. When the air cooling time after finish rolling exceeds 30 s, martensite is less than 5%, so the air cooling time is set within 30 s.

  The cooling rate of the controlled cooling after hot rolling is 5 ° C./s or more. This is because when the cooling rate after hot rolling is less than 5 ° C./s, the volume ratio of martensite decreases to less than 2% and the volume ratio of pearlite and cementite increases. On the other hand, the upper limit of the cooling rate is not particularly limited, but it is difficult to make it 150 ° C./s or more in production. Note that controlled cooling with a cooling rate of 5 to 150 ° C./s can be performed by water cooling or mist cooling.

  The coiling temperature after hot rolling is 25 to 300 ° C. When the coiling temperature exceeds 300 ° C., 2% or more of martensite cannot be obtained, and the volume ratio of pearlite and cementite increases. The lower limit of the coiling temperature is 25 ° C. because it is difficult to make it lower than 25 ° C. due to manufacturing restrictions.

  Hereinafter, the reason for limiting the steel composition in the present invention will be further described. In addition,% means the mass%.

  Nb is an important element in the present invention, and remarkably suppresses recrystallization when the γ phase is processed in hot rolling, and remarkably promotes the formation of a processed texture in the γ phase. From this viewpoint, Nb needs to be added in an amount of 0.005% or more. Moreover, addition of 0.010% or more is preferable, and addition of 0.015% or more is more preferable.

  However, if the amount of Nb added exceeds 0.10%, the Young's modulus in the rolling direction decreases, so the upper limit is made 0.10%. The reason why the Young's modulus in the rolling direction decreases due to the addition of Nb is not clear, but it is presumed that Nb affects the stacking fault energy of the γ phase. From this viewpoint, the amount of Nb added is preferably 0.08% or less, and more preferably 0.06% or less.

  Ti is also an important element in the present invention. Ti forms nitrides in the high temperature region of the γ phase, and suppresses recrystallization when the γ phase is processed in hot rolling. Further, when B is added, the formation of Ti nitride suppresses the precipitation of BN, so that solid solution B can be ensured. This promotes the development of a texture preferable for improving the Young's modulus. In order to obtain this effect, 0.002% or more of Ti needs to be added.

  On the other hand, if Ti is added in an amount exceeding 0.15%, the workability deteriorates remarkably, so the upper limit is made 0.15%. From this viewpoint, 0.10% or less is preferable. More preferably, it is 0.06% or less.

  N is an element inevitably contained in the steel, and the lower limit is not particularly set. However, in order to make N less than 0.0005%, the cost becomes high, and the effect of improving the formability due to the reduction of N is not remarkable, so the lower limit is preferably made 0.0005%. Further, N forms a nitride with Ti and suppresses the recrystallization of the γ phase, so it may be added positively, but it reduces the effect of suppressing the recrystallization of B, so it is 0.01% or less. suppress. In this respect, the content is preferably 0.005% or less, and more preferably 0.002% or less.

Furthermore, Ti and N must satisfy the following (formula 1).
Ti-48 / 14 × N ≧ 0.0005 (Formula 1)

  As a result, the effect of suppressing recrystallization of the γ phase due to TiN precipitation is exhibited, and in the case of adding B, formation of BN can be suppressed, and the development of a texture preferable for improving the Young's modulus is promoted. .

C is an element that increases the strength and needs to be added in an amount of 0.005% or more. From the viewpoint of Young's modulus, the lower limit of the C content is preferably 0.010%. This is because when the C content decreases to less than 0.010%, the Ar ₃ transformation temperature increases, hot rolling at low temperatures becomes difficult, and the Young's modulus may decrease. Furthermore, 0.020% or more is preferable in order to suppress deterioration of the fatigue characteristics of the weld.

  On the other hand, if the C content exceeds 0.20%, the moldability deteriorates, so the upper limit is made 0.20%. If the C content exceeds 0.10%, weldability may be impaired, so the C content is preferably 0.10% or less. Moreover, since the Young's modulus of a rolling direction may fall when C amount exceeds 0.06%, 0.06% or less is still more preferable.

  Si is a deoxidizing element, and the lower limit is not particularly specified, but in order to make it less than 0.001%, the production cost becomes high. Si is an element that increases the strength by solid solution strengthening. Therefore, it may be added positively according to the target strength level, but if the added amount exceeds 2.50%, the press formability deteriorates, so 2.50% is made the upper limit.

  Moreover, since chemical conversion processability will fall when there is much Si amount, 1.20% or less of Si amount is preferable. Furthermore, when hot dip galvanizing is performed, problems such as a decrease in plating adhesion and a decrease in productivity due to a delay in the alloying reaction may occur, so the Si amount is preferably 1.00% or less.

  From the viewpoint of Young's modulus, it is more preferable that the Si content be 0.60% or less. Furthermore, Preferably it is 0.30% or less.

Mn is an important element in the present invention. Mn is an element that lowers the Ar ₃ transformation point at a temperature that transforms from the γ phase to the ferrite phase when heated to a high temperature during hot rolling. By adding Mn, the γ phase becomes stable to a low temperature, and the temperature of finish rolling can be lowered. In order to obtain this effect, it is necessary to add 0.10% or more of Mn.

  Further, as will be described later, Mn has a correlation with the stacking fault energy in the γ phase, affects the formation of the work texture in the γ phase, and the variant selection at the time of transformation. The crystal orientation that increases the Young's modulus is developed, and conversely, the formation of the orientation that lowers the Young's modulus is suppressed. From this viewpoint, it is preferable to add 1.00% or more of Mn. More preferably, the addition is 1.20% or more, and the addition of 1.5% or more is most preferable.

  On the other hand, if the amount of Mn added exceeds 3.00%, the political enemy Young's modulus in the rolling direction decreases. In addition, since the strength becomes too high and the ductility is lowered, the upper limit of the amount of Mn is set to 3.00%. Moreover, when the amount of Mn exceeds 2.00%, the adhesiveness of galvanization may be inhibited and it is preferable to set it as 2.00% or less also from a viewpoint of the Young's modulus of a rolling direction.

  P is an impurity, but may be positively added if the strength needs to be increased. P also has the effect of making the hot-rolled structure fine and improving workability. However, if the addition amount exceeds 0.15%, the fatigue strength after spot welding deteriorates, the yield strength increases, and a surface shape defect is caused during pressing. Furthermore, during continuous hot dip galvanization, the alloying reaction is extremely slow and productivity is reduced. Also, the secondary workability is deteriorated. Therefore, the upper limit of the P content is 0.15%.

  S is an impurity. If it exceeds 0.015%, it causes hot cracking or deteriorates workability, so 0.015% is made the upper limit.

  Al is a deoxidation preparation agent, and the lower limit is not particularly limited, but is preferably 0.01% or more from the viewpoint of deoxidation. On the other hand, Al significantly increases the transformation point, so if added over 0.15%, γ region rolling at low temperatures becomes difficult, so the upper limit is made 0.15%.

In order to increase the static Young's modulus of both the plate thickness surface layer and the central portion, it is preferable to satisfy the following (Formula 2).
4 ≦ 3.2Mn + 9.6Mo + 4.7W + 6.2Ni + 18.6Cu + 0.7Cr ≦ 10
... (Formula 2)
Here, Mn, Mo, W, Ni, Cu, and Cr are the content [% by mass] of each element.

  In addition, when the addition amount of Mo, W, Ni, Cu, and Cr is less than a preferable lower limit value, the relational expression of the above (Formula 2) is calculated as 0. If the above (Formula 2) is satisfied, the orientation that increases the Young's modulus in the rolling direction is accumulated in the vicinity of the shear layer of the surface layer of the steel sheet and the center of the thickness, and the accumulation of the orientation that reduces the Young's modulus in the rolling direction is suppressed. The

  In addition, since the Young's modulus in the rolling direction is increased together with the numerical value of the relational expression (Formula 2), Mn and necessary to be preferably 4.5 or more, more preferably 5.5 or more. Accordingly, one or two of Mo, W, Ni, Cu, and Cr are added.

  However, (Formula 2) is not satisfied, and if the value of the relational expression exceeds 10, the mechanical properties deteriorate, the texture at the center of the plate thickness deteriorates, and the static Young's modulus in the rolling direction decreases. Therefore, the value of the relational expression is preferably 10 or less. From this viewpoint, it is more preferably 8 or less.

  Mo, Cr, W, Cu, and Ni are elements that affect the stacking fault energy of the γ phase during hot rolling, and one or two or more of them are added in an amount of 0.01% or more, respectively. Is preferred.

  In addition, if one or more of Mo, Cr, W, Cu, and Ni and Mn are added in combination, it affects the formation of the processed texture, and in the 1/6 plate thickness portion from the surface layer, in the rolling direction. Develop {110} <111> and {211} <111>, which are crystal orientations that increase the Young's modulus, and suppress formation of {100} <001> and {110} <001>, which are orientations that lower the Young's modulus To express the effect.

  Moreover, it is preferable to add one or more of Mo, Cr, W, Cu, and Ni in combination with Mn so as to satisfy the above (2). This suppresses the accumulation of {332} <113> orientation, which lowers the Young's modulus in the rolling direction, and increases the Young's modulus in the rolling direction at the center portion of the plate thickness, and {001} < The accumulation of the 011> orientation and the {112} <110> orientation can be enhanced.

  In particular, since Mo and Cu have a high coefficient of the above (Formula 2) and exhibit the effect of increasing the Young's modulus even when added in a small amount, it is more preferable to add one or both of Mo and Cu.

  On the other hand, the addition of Mo significantly increases the strength and may reduce the workability, so the addition amount is preferably 1.00% or less. From the viewpoint of cost, addition of 0.50% or less of Mo is preferable.

  The upper limit of one or more of Cr, W, Cu, and Ni is preferably 3.00% from the viewpoint of workability. Furthermore, from the viewpoint of economy, the upper limit of one or more of Cr, W, Cu, and Ni is 2.00% for Cr, 0.10% for W, 0.15% for Cu, For Ni, the content is preferably 1.00%.

  B is an element that significantly suppresses recrystallization and enhances hardenability in a solid solution state when added in combination with Nb, and affects the variant selectivity of crystal orientation at the time of transformation from austenite to ferrite. it is conceivable that.

  Therefore, the {110} <111> to {110} <112> orientation groups that increase the Young's modulus are promoted and the {100} <001> orientation and {110} < It is considered that the development of 001> orientation is suppressed. From this viewpoint, it is preferable to add 0.0005% or more.

  On the other hand, even if B is added in excess of 0.01%, no further effect is obtained, so the upper limit is made 0.01%. If B is added in excess of 0.005%, the workability may deteriorate, so 0.005% or less is preferable. More preferably, it is 0.003% or less.

  Ca, Rem, and V have an effect of increasing mechanical strength or improving the material, and therefore it is preferable to contain one or two or more as necessary. If the addition amount of Ca and Rem is less than 0.0005% and the addition amount of V is less than 0.001%, sufficient effects may not be obtained.

  On the other hand, if Ca and Rem are added so that the addition amount exceeds 0.10% and the addition amount of V exceeds 0.10%, ductility may be impaired. Therefore, Ca, Rem, and V are preferably added in the range of 0.0005 to 0.10%, 0.0005 to 0.10%, and 0.001 to 0.10%, respectively.

  Next, the reasons for limiting the manufacturing conditions other than the above-described shape ratio, effective strain, and cooling rate will be described.

  Steel is melted and cast by a conventional method to obtain a steel piece to be subjected to hot rolling. Although this steel slab may be a forged or rolled steel ingot, it is preferable to manufacture the steel slab by continuous casting from the viewpoint of productivity. Moreover, you may manufacture with a thin slab caster.

Moreover, normally, a steel slab is cooled after casting, and is heated again in order to perform hot rolling. In this case, it is preferable that the heating temperature of the steel slab when hot rolling is 1100 ° C. or higher. This is because when the heating temperature of the steel slab is less than 1100 ° C., it is difficult to set the finishing temperature of hot rolling to the Ar ₃ transformation point or higher. In order to heat the steel piece efficiently and uniformly, the heating temperature is preferably 1150 ° C. or higher.

  The upper limit of the heating temperature is not particularly specified, but when heated to over 1300 ° C., the crystal grain size of the steel sheet becomes coarse and the workability may be impaired. Further, a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting the molten steel may be employed.

  In the production of the steel sheet of the present invention, the conditions for hot rolling at 1100 ° C. or lower are important, and the definition of the shape ratio is as described above. In addition, the diameter of a rolling roll is measured at room temperature, and it is not necessary to consider the flatness during hot rolling.

  The sheet thicknesses on the entry side and exit side of each rolling roll may be measured on the spot using radiation or the like, or may be determined by calculation in consideration of deformation resistance and the like from the rolling load. Further, the hot rolling at a temperature exceeding 1100 ° C. is not particularly defined and may be appropriately performed. That is, the rough rolling of the steel slab is not particularly limited, and may be performed by a conventional method.

  In hot rolling, the rolling reduction to 1100 ° C. or less and the final pass is 40% or more. This is because even after hot rolling above 1100 ° C., the processed structure recrystallizes, and the X-ray random strength of the {110} <111> to {110} <112> orientation groups in the 1/6 plate thickness part This is because the effect of increasing the ratio cannot be obtained.

  1100 ° C. or less, the reduction ratio until the final pass is a numerical value expressed as a percentage obtained by dividing the difference between the plate thickness of the steel plate at 1100 ° C. and the plate thickness of the steel plate after the final pass by the plate thickness of the steel plate at 1100 ° C. It is. The reason why the rolling reduction is 40% or more is that if it is less than 40%, the texture that increases the Young's modulus in the rolling direction is not sufficiently developed in the 1/6 plate thickness portion.

  Moreover, it is preferable that the rolling reduction be 40% or more in order to increase the texture that increases the Young's modulus in the rolling direction at the ½ plate thickness portion. In order to increase the Young's modulus in the rolling direction at the 1/6 plate thickness portion and the 1/2 plate thickness portion, it is preferable that the rolling reduction rate to the final pass is 50% or more.

  The upper limit of the rolling reduction until the final pass is not particularly set, but setting the rolling reduction to 95% or lower at 1100 ° C. or less not only increases the load on the rolling mill but also changes the texture. Since the Young's modulus starts to decrease, the rolling reduction is preferably 95% or less. In this respect, 90% or less is more preferable.

The temperature of the final pass of hot rolling is not less than the Ar ₃ transformation point. This is because {110} <001> texture that is unfavorable for the Young's modulus in the rolling direction and the width direction develops in the 1/6 thickness portion when rolling is performed below the Ar ₃ transformation point.

  In addition, when the temperature of the final pass of hot rolling exceeds 900 ° C., it is difficult to develop a texture preferable for improving the Young's modulus in the rolling direction, and {110} <111> to 1/6 plate thickness portion. The X-ray random intensity ratio of the {110} <112> orientation group decreases.

In order to improve the Young's modulus in the rolling direction, it is preferable to lower the rolling temperature in the final pass. In particular, in order to increase the Young's modulus in the rolling direction of the ½ plate thickness part, the rolling is performed at a lower temperature. It is preferable to increase the rate. The rolling temperature in the final pass is preferably 850 ° C. or lower, more preferably 800 ° C. or lower, provided that it is not lower than the Ar ₃ transformation point.

  Thereby, the X-ray random intensity ratio (A) of {332} <113> orientation at the center of the plate thickness is 15 or less, the X-ray random intensity ratio (B) of {112} <110> orientation is 5 or more, and , (A) / (B) can be 1.00 or less.

  When the value of (Expression 2) is high, if the rolling reduction is increased, the {332} <113> orientation that reduces the Young's modulus in the rolling direction tends to be easily developed in the 1/2 sheet thickness portion. However, the development of {225} <110> orientation, {001} <110> orientation, and {112} <110> orientation that enhance the Young's modulus in the rolling direction is promoted.

  Accordingly, the average value (C) of the X-ray random intensity ratio of the {001} <011> orientation and the {112} <110> orientation is set to 5 or more, and the X-ray random intensity ratio (A) of the {332} <113> orientation (A ) Ratio (A) / (C) can be 1.10 or less.

  When carrying out hot rolling, forming the texture in the vicinity of the surface layer is promoted by carrying out at least one pass of different circumferential speed rolling with a rolling roll having a different circumferential speed ratio of 1% or more. The Young's modulus improves more than the present invention when not implemented. From this point of view, it is desirable that the different peripheral speed rate is 1% or more, preferably the different peripheral speed ratio is 5% or more, and more preferably, the different peripheral speed ratio is 10% or more.

  Although the upper limit of the different peripheral speed ratio and the number of different peripheral speed rolling passes is not particularly specified, it goes without saying that the larger or larger one is advantageous in obtaining a large Young's modulus improvement effect for the above reasons. . However, a different peripheral speed ratio of 50% or more is currently difficult, and the finishing hot rolling pass is usually up to about 8 passes.

  Here, the different peripheral speed ratio in the present invention represents a percentage obtained by dividing the peripheral speed difference between the upper and lower rolling rolls by the peripheral speed of the low peripheral speed roll. Further, the different peripheral speed rolling of the present invention has no difference in Young's modulus improvement effect regardless of the upper and lower roll peripheral speeds.

  In addition, when one or more work rolls having a roll diameter of 700 mm or less are used in a rolling mill used for finishing hot rolling, texture formation in the vicinity of the surface layer is promoted, so that Young's modulus is higher than that of the present invention when not used. improves. Therefore, it is desirable to use a work roll having a roll diameter of 700 mm or less. From this viewpoint, the work roll diameter is 700 mm or less, desirably 600 mm or less, and more desirably 500 mm or less.

  The lower limit of the work roll diameter is not particularly defined, but if it is 300 mm or less, the sheet passing control becomes difficult. Although the upper limit of the number of passes using the small-diameter roll is not particularly defined, as described above, the finish hot rolling pass is usually up to about 8 passes.

The hot-rolled steel sheet may be annealed with a maximum heating temperature in the range of 500 ° C. or more and 0.5 × (Ac ₁ + Ac ₃ ) [° C.] or less. This further improves the Young's modulus in the rolling direction. The reason for this is not clear, but it is presumed that the dislocations introduced by the transformation after hot rolling are due to rearrangement by heat treatment.

When the maximum heating temperature is less than 500 ° C., the above effect is not remarkable. On the other hand, when it exceeds 0.5 × (Ac ₁ + Ac ₃ ) [° C.], transformation to the austenite phase at the annealing temperature is promoted, and the texture May be weakened and Young's modulus may decrease.

  In order to improve the Young's modulus, the maximum heating temperature for hot-rolled sheet annealing is more preferably 650 to 800 ° C. Although it may be cooled immediately after reaching the maximum heating temperature, in order to make the temperature of the steel plate uniform, it is preferable to hold for 120 s or more, and holding for over 1800 s impairs productivity.

  In order to achieve both the homogeneity of the material of the steel plate and the productivity, it is more preferable that the holding time is 300 s or more and 600 s or less. Further, the hot-rolled steel sheet may be subjected to a skin pass having a reduction rate of 10% or less by pickling, in-line or off-line as necessary.

  The heating rate for annealing the hot-rolled steel sheet is not particularly limited, but is preferably 3 ° C./s or more. When the heating rate is less than 3 ° C./s, recrystallization proceeds during heating, and the extreme density of the texture that is advantageous for improving the Young's modulus may decrease. On the other hand, even if the heating rate is higher than 70 ° C./s, the material characteristics are not changed.

  The hot-rolled steel sheet may be hot dip galvanized or alloyed hot dip galvanized. The composition of the galvanizing is not particularly limited, and besides zinc, Fe, Al, Mn, Cr, Mg, Pb, Sn, Ni, etc. may be added as necessary.

  The alloying treatment is performed within a range of 450 to 600 ° C. after hot dip galvanizing. If the temperature is lower than 450 ° C., alloying does not proceed sufficiently, and if it exceeds 600 ° C., alloying proceeds excessively and the plating layer becomes brittle. Trigger. The alloying time is 5 s or longer. If it is less than 5 s, alloying does not proceed sufficiently. The upper limit is not particularly defined, but is preferably about 10 s in consideration of plating adhesion.

  After annealing and hot dip galvanization, pickling may be performed as necessary, and then a skin pass with a rolling reduction of 10% or less may be performed inline or offline.

  The hot-rolled steel sheet may be subjected to Al-based plating or various electroplating. Furthermore, the hot-rolled steel sheet, the cold-rolled steel sheet, and the various plated steel sheets can be subjected to surface treatments such as an organic film, an inorganic film, and various paints according to the purpose.

  The low yield ratio high Young's modulus steel plate, hot dip galvanized steel plate, and alloyed hot dip galvanized steel plate of the present invention are wound on a steel pipe so that the angle between the rolling direction and the longitudinal direction of the steel pipe is within 0 to 30 °. Then, a high Young's modulus steel pipe with a high Young's modulus in the longitudinal direction of the steel pipe can be manufactured.

  When the wire is wound in parallel with the rolling direction, the Young's modulus is the highest, so this angle is preferably as small as possible. From this viewpoint, it is preferable to wind at an angle of 15 ° or less.

  As long as the relationship between the rolling direction and the longitudinal direction of the steel pipe is satisfied, the pipe forming method may be any method such as UO pipe, electric resistance welding, spiral or the like. Of course, it is not necessary to limit the direction with a high Young's modulus parallel to the longitudinal direction of the steel pipe, and there is no problem even if a steel pipe with a high Young's modulus is produced in any direction depending on the application.

  Next, an example explains the present invention.

  Steel having the composition shown in Table 1 was melted to produce a steel slab, and the steel slab was heated, followed by rough rolling and finish rolling under the conditions shown in Table 2. The blank in Table 1 means that no element was intentionally added. The finish rolling stand consists of 6 stages, and the roll diameter is 650 to 830 mm. Moreover, the finishing board thickness after the last pass was 1.6-10 mm.

In Tables 2 and 3, SRT [° C.] is the heating temperature of the steel slab, FT [° C.] is the temperature after the final pass of the rolling, that is, the finishing side, and t _AC1 [s] is It is the air cooling time from the finish rolling of hot rolling until the start of cooling, and CR ₁ [° C./s] is the average cooling rate during cooling after air cooling.

CT [° C.] is the coiling temperature, V _α1 [%] is the ferrite volume fraction of the hot-rolled steel sheet, VB1 [%] is the bainite volume ratio of the hot-rolled steel sheet, and V _M1 [ %] Is the martensite volume ratio of the hot-rolled steel sheet, and V _other1 [%] is the sum of the volume ratios of cementite and pearlite of the hot-rolled steel sheet.

  The rolling reduction is a value obtained by dividing the difference between the plate thickness at 1100 ° C. and the finished plate thickness by the plate thickness at 1100 ° C., and is expressed as a percentage.

The area ratio of ferrite, bainite, martensite, cementite, and pearlite was determined by microstructure observation and image analysis using an optical microscope, and this was defined as the volume ratio. Ar ₃ , Ac ₁ , and Ac ₃ were determined by measuring the thermal expansion change of the test piece during heating / cooling at 10 [° C./s] using a thermal dilatometer. In Tables 1 to 9, the underline means outside the scope of the present invention or outside the preferred range.

  Further, in the pass / fail column for the shape ratio, “◯” is shown when at least two or more of the shape ratios of each pass are over 2.3, and “x” is shown when they are not over.

Moreover, Formula 1 of Table 1 is the value of the left side of the following (Formula 1) calculated by content [mass%] of Ti and N.
Ti-48 / 14 × N> 0.0005 (Formula 1)

Moreover, Formula 2 of Table 1 is the value of the left side of the following (Formula 2) calculated by content [mass%] of each element of Mn, Mo, W, Ni, Cu, and Cr.
3.2Mn + 9.6Mo + 4.7W + 6.2Ni + 18.6Cu + 0.7Cr ≧ 4
... (Formula 2)
When the content of Mn, Mo, W, Ni, Cu, and Cr is about an impurity, for example, when Mo, W, Ni, Cu, and Cr in Table 1 are blank, 0 is set as (Formula 2) Calculate the left side.

  A tensile test piece based on JIS Z 2201 was collected from the obtained steel plate, a tensile test was performed based on JIS Z 2241, and the tensile strength was measured. Young's modulus was measured by both the static tension method and the vibration method.

  Measurement of Young's modulus by the static tension method was performed by applying a tensile stress corresponding to ½ of the yield strength of the steel sheet using a tensile test piece according to JIS Z 2201. The measurement was performed five times, and among the Young's modulus calculated based on the slope of the stress-strain diagram, the average value of the three measured values excluding the maximum and minimum values was taken as the Young's modulus by the static tension method. The Young's modulus is shown in Table 3.

  The vibration method was performed by a transverse resonance method at room temperature in accordance with JIS Z 2280. That is, the vibration was applied without fixing the sample, the frequency of the oscillator was gradually changed, the primary resonance frequency was measured, and the dynamic Young's modulus was calculated from the frequency.

  Also, {100} <001> and {110} <001> orientations, {110} <111> to {110} <112> orientation groups, and {211} <111> in the 1/6 thickness portion of the steel plate. The azimuth X-ray random intensity ratio was measured as follows.

  First, the steel plate was mechanically polished and buffed, and then electropolished to remove strain, and a sample was prepared so that the 1/6 plate thickness portion became the measurement surface. X-ray diffraction was performed using this sample. Note that the X-ray diffraction of a standard sample having no accumulation in a specific orientation was also performed under the same conditions.

  Next, ODF was obtained by the series expansion method based on {110}, {100}, {211}, {310} pole figures obtained by X-ray diffraction. From this ODF, the X-ray random intensity ratios of {100} <001> and {110} <001> orientations and {110} <111> to {110} <112> orientation groups were obtained.

  The X-ray random intensity ratio of the {332} <113> orientation and {112} <110> orientation of the 1/2 plate thickness part of the steel plate is the same as that of the 1/6 plate thickness part sample. X-ray diffraction was performed using a sample prepared so that the two plate thickness portions became the measurement surface, and obtained from ODF.

  Moreover, among these steel plates, when hot dip galvanizing was performed after the hot rolling was completed, “melting” and when alloying hot dip galvanizing at 520 ° C. for 15 seconds was performed, it was described as “alloy”.

  The results are shown in Table 3. In the Young's modulus column, RD means rolling direction, and TD means transverse direction which is a direction perpendicular to the rolling direction.

  As is apparent from Table 3, when the steel having the chemical composition of the present invention is hot-rolled under appropriate conditions, the Young's modulus by the static tension method exceeds 220 GPa in both the rolling direction and the direction perpendicular to the rolling. It is. In particular, when the texture condition of the thickness center layer is simultaneously satisfied, it can be seen that the Young's modulus by the static tension method is high and the difference from the vibration method is small.

On the other hand, production No. Nos. 21 to 26 are steel Nos. Whose chemical components are outside the scope of the present invention. It is a comparative example using K-P. Production No. 21 is an example in which the amount of C added is small and the strength is lowered. Production No. Nos. 22 and 23 are examples of excessive addition of Si and Al. Since the Ar ₃ point was raised, the hot rolling finishing temperature became lower than the Ar ₃ point, and the texture of γ could be developed during rolling. This is an example in which the Young's modulus has decreased.

  Production No. 24 is an example in which the amount of Mn added is small and the Young's modulus is lowered. Production No. 25 and 26 are examples in which the Nb addition amount and the Ti addition amount are small, and the Young's modulus in the rolling direction is lowered.

  In addition, production No. As shown in FIG. 12, if there are few paths having a shape ratio of 2.3 or more, even if a high Young's modulus is obtained by the vibration method, it cannot exceed 220 GPa by the static tension method. Production No. Nos. 10 and 18 have a high hot rolling finishing temperature FT [° C.]. 16 is an example in which the rolling reduction of the hot rolling is low and the Young's modulus is reduced.

  Furthermore, production No. No. 8 has a high CT temperature. No. 14 is an example in which a sufficient martensite fraction could not be obtained because the cooling rate was slow. Production No. No. 20 is an example in which the air cooling time after hot rolling was long, ferrite transformation and pearlite transformation proceeded, and a sufficient martensite fraction was not obtained.

  Using steels A and E shown in Table 1, hot rolling was performed under the conditions shown in Table 4. Production No. shown in Table 4 28, 31, and 32 perform different peripheral speed rolling with different peripheral speed ratios in the final three stages of the finish rolling stand consisting of all six stages, that is, four passes, five passes, and six passes. This is an example. Table 4 also shows the effective strain amount when this hot rolling is performed. All the hot rolling conditions not displayed in Table 4 are the same as those in Example 1.

  In the same manner as in Example 1, measurement of tensile properties, texture of 1/6 plate thickness part and 1/2 plate thickness part, and measurement of Young's modulus were performed. The results are shown in Table 5.

  As is clear from this, when hot rolling the steel having the chemical component of the present invention under appropriate conditions, when 1 pass or more of different peripheral speed rolling is added for one pass or more, formation of a texture in the vicinity of the surface layer is promoted, The Young's modulus improves, and the Young's modulus improves as the effective strain increases.

  Using the steels A and E shown in Table 1, hot rolling was performed under the conditions shown in Table 6. Production No. shown in Table 6 33 to 35 and 37 to 39 are the final three stages of the finish rolling stand consisting of all six stages, that is, four passes, five passes, and six passes, and rolling is performed using a roll having a diameter of 700 mm or less. This is an example. Table 6 also shows the effective strain amount when this hot rolling is performed. All the hot rolling conditions not displayed in Table 6 are the same as those in Example 1.

  In the same manner as in Example 1, measurement of tensile properties, texture of 1/6 plate thickness part and 1/2 plate thickness part, and measurement of Young's modulus were performed. The results are shown in Table 7.

  As is apparent from this, when a steel having the chemical component of the present invention is hot-rolled, if a small-diameter roll is used, the shear strain amount of the surface layer increases, and the Young's modulus can be further increased. Furthermore, the Young's modulus is improved as the rolling conditions have a higher effective strain.

Using the steels A and E shown in Table 1, hot rolling was performed under the conditions shown in Table 8. Production No. shown in Table 8 Nos. 48 to 52 are examples in which rolling was performed so that the effective strain amount ε ^* was 0.4 or more in a finish rolling stand having 6 stages. All the hot rolling conditions not displayed in Table 8 are the same as in Example 1.

  In the same manner as in Example 1, measurement of tensile properties, texture of 1/6 plate thickness part and 1/2 plate thickness part, and measurement of Young's modulus were performed. The results are shown in Table 9.

  As is clear from this, when the steel having the chemical component of the present invention is hot-rolled and rolled so that the effective strain amount is 0.4 or more, the shear strain amount of the surface layer increases and the Young's modulus is further increased. Is possible.

  In the same manner as in Example 1, hot rolling was performed under the conditions shown in Table 11 using steel pieces having the compositions shown in Table 10. All the hot rolling conditions not displayed in Table 11 are the same as in Example 1. In addition, as in Example 1, a tensile test is performed to observe the metal structure and measure the area ratio, measure the texture of the 1/6 plate thickness part and 1/2 plate thickness part, and measure the Young's modulus. went. Furthermore, in the 1/2 plate thickness part, the X-ray random intensity ratio of {001} <011> orientation was also measured. The results are shown in Table 12.

  As is clear from this, when the steel having the chemical composition of the present invention is hot-rolled under appropriate conditions, the Young's modulus by the static tension method exceeds 220 GPa in both the rolling direction and the direction perpendicular to the rolling direction. . In particular, when the texture condition of the thickness center layer is satisfied at the same time, the Young's modulus by the static tension method is high and the difference from the vibration method is small. In addition, among the examples of the present invention, a production No. which does not satisfy (Formula 2). 62 and 63 have a slightly low static Young's modulus.

  On the other hand, production No. 64 and 65 are examples in which the Ti addition amount is small and the Young's modulus in the rolling direction is lowered. In addition, production No. No. 46 has a high hot rolling finishing temperature FT [° C.]. 57 is an example in which the rolling reduction is low and the Young's modulus in the rolling direction is reduced. Production No. No. 60 has a small shape ratio, and if there are few paths having a shape ratio of 2.3 or more, even if a high Young's modulus is obtained by the vibration method, the Young's modulus is lowered by the static tension method. Production No. 48 is an example in which the air cooling time after hot rolling was long, the ferrite transformation and the pearlite transformation proceeded, and a sufficient martensite fraction was not obtained.

  Using the steel R and X shown in Table 10, hot rolling was performed under the conditions shown in Table 13. Production No. shown in Table 13 67 and 69 are examples in which different peripheral speed rolling was performed by changing the different peripheral speed ratios in the final three stages of the finish rolling stand including all six stages, that is, four passes, five passes, and six passes. . Table 13 also shows the effective strain amount when this hot rolling is performed. All the hot rolling conditions not displayed in Table 13 are the same as in Example 1.

  In the same manner as in Example 5, measurement of tensile properties, texture of 1/6 plate thickness part and 1/2 plate thickness part, and measurement of Young's modulus were performed. The results are shown in Table 14. As is clear from this, when hot rolling the steel having the chemical component of the present invention under appropriate conditions, when 1 pass or more of different peripheral speed rolling is added for one pass or more, formation of a texture in the vicinity of the surface layer is promoted, The Young's modulus improves, and the Young's modulus improves as the effective strain increases.

  Using the steel R and X shown in Table 10, hot rolling was performed under the conditions shown in Table 15. Production No. shown in Table 15 Nos. 71, 72, 74 and 75 were rolled using a roll having a diameter of 700 mm or less in the final three stages of a finish rolling stand consisting of all six stages, that is, four passes, five passes and six passes. It is an example. Table 15 shows the effective strain amount when the hot rolling is performed. All the hot rolling conditions not displayed in Table 15 are the same as in Example 1.

  In the same manner as in Example 5, measurement of tensile properties, texture of 1/6 plate thickness part and 1/2 plate thickness part, and measurement of Young's modulus were performed. The results are shown in Table 16. As is apparent from this, when a steel having the chemical component of the present invention is hot-rolled, if a small-diameter roll is used, the shear strain amount of the surface layer increases, and the Young's modulus can be further increased. Furthermore, the Young's modulus is improved as the rolling conditions have a higher effective strain.

Using the steel R and X shown in Table 10, hot rolling was performed under the conditions shown in Table 17. Production No. shown in Table 17 Nos. 76 to 79 are examples in which the rolling was performed so that the effective strain amount ε ^* was 0.4 or more in a finish rolling stand having 6 stages. All the hot rolling conditions not displayed in Table 17 are the same as in Example 1.

  In the same manner as in Example 5, measurement of tensile properties, texture of 1/6 plate thickness part and 1/2 plate thickness part, and measurement of Young's modulus were performed. The results are shown in Table 18. As is clear from this, when the steel having the chemical component of the present invention is hot-rolled and rolled so that the effective strain amount is 0.4 or more, the shear strain amount of the surface layer increases and the Young's modulus is further increased. Is possible.

  As described above, according to the present invention, it is possible to obtain a low Young's modulus high Young's modulus steel sheet having a high static Young's modulus in the rolling direction measured by a static tension method and good workability. Therefore, the present invention has a remarkable industrial contribution.

It is a figure which shows the relationship between (Formula 2) of this invention, and the static Young's modulus of a rolling direction. It is a figure which shows ODF and main azimuth | direction in a (phi) ₂ = 45 degree cross section.

Claims (18)

% By mass
C: 0.005-0.20%
Mn: 0.10 to 3.00%,
Nb: 0.005 to 0.10%,
Ti: 0.002 to 0.15%
Containing
Si: 2.50% or less,
P: 0.15% or less,
S: 0.015% or less,
Al: 0.15% or less,
N: Restricted to 0.01% or less, satisfies the following (formula 1), the balance is composed of Fe and inevitable impurities, and one or both of ferrite and bainite having a total volume ratio exceeding 50%, and the volume ratio Is composed of one or both of martensite of 2 to 25%, pearlite and cementite with a total volume ratio of 15% or less, and the distance from the surface of the steel sheet in the thickness direction is 1/6 of the thickness. , {100} <001> orientation X-ray random intensity ratio and {110} <001> orientation X-ray random intensity ratio is 5 or less, {110} <111> to {110} <112> A low yield ratio high Young's modulus steel plate, wherein the sum of the maximum value of the X-ray random intensity ratio of the orientation group and the X-ray random intensity ratio of the {211} <111> orientation is 5 or more.
Ti-48 / 14 × N ≧ 0.0005 (Formula 1)
Here, Ti and N are content [mass%] of each element. The low yield ratio high Young's modulus steel plate according to claim 1, wherein the following (Formula 2) is satisfied.
4 ≦ 3.2Mn + 9.6Mo + 4.7W + 6.2Ni + 18.6Cu + 0.7Cr ≦ 10
... (Formula 2)
Here, Mn, Mo, W, Ni, Cu, and Cr are the content [% by mass] of each element. % By mass
Mo: 0.01 to 1.00%,
Cr: 0.01 to 3.00%,
W: 0.01 to 3.00%
Cu: 0.01 to 3.00%,
Ni: 0.01 to 3.00%
The low yield ratio high Young's modulus steel plate according to claim 1 or 2, characterized by containing one or more of the following. % By mass
B: 0.0005 to 0.01%
The low yield ratio high Young's modulus steel plate according to any one of claims 1 to 3, wherein the steel plate contains % By mass
Ca: 0.0005 to 0.10%,
Rem: 0.0005 to 0.10%,
V: 0.001 to 0.10%
The low yield ratio high Young's modulus steel sheet according to any one of claims 1 to 4, wherein the steel sheet contains one or more of the following. The X-ray random intensity ratio (A) of {332} <113> orientation at the center of the steel sheet thickness direction is 15 or less, and the X-ray random intensity ratio (B) of {112} <110> orientation is 5 or more. ,And,
The low yield ratio high Young's modulus steel sheet according to claim 1, wherein (A) / (B) ≦ 1.00 is satisfied.   The X-ray random intensity ratio (A) in the {332} <113> orientation at the center in the thickness direction of the steel sheet is 15 or less, the X-ray random intensity ratio in the {001} <011> orientation and the {112} <110 The simple average value (C) with respect to the X-ray random intensity ratio in the azimuth is 5 or more and satisfies (A) / (C) ≦ 1.10. The low yield ratio high Young's modulus steel sheet as described in the section.   The low yield ratio high Young's modulus steel sheet according to any one of claims 1 to 7, wherein the Young's modulus in the rolling direction measured by a static tension method is 220 GPa or more.   A low yield ratio high Young's modulus hot dip galvanized steel sheet, wherein the low yield ratio high Young's modulus steel sheet according to any one of claims 1 to 8 is subjected to hot dip galvanization.   A low yield ratio high Young's modulus alloyed hot dip galvanized steel sheet, wherein the low yield ratio high Young's modulus steel sheet according to any one of claims 1 to 8 is subjected to galvannealing.   The low yield ratio high Young's modulus steel sheet according to any one of claims 1 to 8, the low yield ratio type high Young's modulus hot dip galvanized steel sheet according to claim 9, or the low yield ratio according to claim 10. A low yield ratio high Young's modulus steel pipe characterized in that a high Young's modulus galvannealed steel sheet is wound in any direction. The shape ratio X calculated | required by the following (Formula 3) is made into the steel slab which has a chemical component of any one of Claims 1-5 to 1100 degrees C or less, the rolling reduction rate to the last pass is 40% or more, and is 2 The rolling that is 3 or more is set to 2 passes or more, the final pass temperature is Ar ₃ transformation point [° C.] or more and 900 ° C. or less, the hot rolling is finished, and air cooling is performed within 30 s. A method for producing a low yield ratio high Young's modulus steel sheet, wherein the steel sheet is cooled to 25 to 300 ° C at a cooling rate of ° C / s and wound.
Shape ratio X = l _d / h _m (Equation 3)
Here, l _d (contact arc length of rolling roll and steel plate): √ (L × (h _in −h _out ) / 2)
h _m : (h _in + h _out ) / 2
L: Diameter of the rolling roll
h _in : Thickness on the entry side of the rolling roll
h _out : Plate thickness on the exit side of the rolling roll The method of manufacturing a low yield ratio high Young's modulus steel sheet according to claim 12, wherein hot rolling is performed so that an effective strain amount ε ^* calculated by the following (formula 5) is 0.4 or more.

Here, n is the number of finishing hot rolling rolling stands, εj is the strain applied at the jth stand, εn is the strain applied at the nth stand, and ti is between i and i + 1th stands. The traveling time [s] and τi are numerical values calculated by the following (formula 6) from the gas constant R (= 1.987) and the rolling temperature Ti [K] of the i-th stand.

  14. The method for producing a low yield ratio high Young's modulus steel sheet according to claim 12 or 13, wherein when performing hot rolling, at least one rolling roll having a roll diameter of 700 mm or less is used.   The method for producing a low yield ratio high Young's modulus steel sheet according to any one of claims 12 to 14, wherein a different peripheral speed ratio of at least one pass of hot rolling is 1% or more.   A method for producing a low yield ratio high Young's modulus hot-dip galvanized steel sheet according to claim 9, wherein the low yield ratio high Young's modulus steel sheet produced by the production method according to any one of claims 12 to 15 is used. A method for producing a hot dip galvanized steel sheet having a low yield ratio and a high Young's modulus, characterized by applying hot dip galvanizing.   A method for producing a low yield ratio high Young's modulus galvannealed steel sheet according to claim 10, wherein the hot dip galvanized steel sheet according to claim 16 is applied for 5 seconds in a temperature range from 450 to 600 ° C. A method for producing a galvanized steel sheet having a low yield ratio and a high Young's modulus alloy, characterized by performing the above heat treatment.   The method of manufacturing a steel pipe according to claim 11, wherein the steel plate has a low yield ratio high Young's modulus obtained by the manufacturing method according to any one of claims 12 to 15, and the manufacturing method according to claim 16. A low yield ratio high Young's modulus hot dip galvanized steel sheet obtained by the above, or a low yield ratio high Young's modulus alloyed hot dip galvanized steel sheet obtained by the production method according to claim 17, and rolled in an arbitrary direction. A method for producing a steel pipe having a low yield ratio and a high Young's modulus.

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