JP2009019265A - High young's modulus steel sheet excellent in hole expansion property and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high Young's modulus steel sheet which has a high Young's modulus in the rolling direction measured by a static tension method and which is excellent in workability, especially in hole expanding property; and its production method. <P>SOLUTION: The high Young's modulus steel sheet with the excellent hole expanding property contains, by mass, 0.0100% or less of N, 0.005-0.100% of Nb, and 0.002-0.150% of Ti, N and Ti satisfying Ti-48/14×N≥0.0005, and has a microstructure wherein the sum of area ratio of one or both of polygonal ferrite and bainite is 98% or higher. At the position of which the distance from the surface of the steel sheet in the sheet thickness direction is 1/6 of the sheet thickness, the sum of an X-ray random strength ratio in the direction of ä100}<001> and an X-ray random strength ratio in the direction of ä110}<001> is 5 or less, and the sum of the maximum value of an X-ray random strength ratio of ä110}<111> to ä110}<112> orientation group and the maximum value of an X-ray random strength ratio of ä211}<111> orientation is 5 or higher. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high Young's modulus steel plate, hot-dip galvanized steel plate, alloyed hot-dip galvanized steel plate and steel pipe excellent in hole expansibility, and methods for producing them.

  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. In addition, when the crystal orientation of the steel material does not have an orientation in a specific orientation, that is, the Young's modulus of a steel plate with a random texture is about 205 GPa.

  So far, a number of techniques have been proposed for steel sheets that control the texture of the steel sheets 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). ). 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. Furthermore, the width of the thin steel plate manufactured by the continuous hot rolling process is about 2 m at the maximum. When considering application of high Young's modulus steel sheet to a reinforcing member in the event of a car collision, the forming method is not limited to pressing, but the member may be processed along the rolling direction by roll forming. Therefore, it is necessary to improve the Young's modulus in the rolling direction.

  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 to 2). 5). The steel sheet obtained by the methods proposed in Patent Documents 2 to 5 has a developed 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 of the surface layer portion, and a high value of Young's modulus measured by the vibration method is over 230 GPa.

  The vibration method, which is one of the Young's modulus measurement methods, is a measurement 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. The contribution of the surface layer portion having a large bending moment is large.

  However, for example, when a load is applied to a building material such as a long beam or column, or a long frame member such as a pillar or member that is a structural member of an automobile, the stress acting on these is tensile stress and compression. It is stress, not bending stress. In addition, from the viewpoint of collision safety, automobile structural members are required to have high impact absorption energy capability when subjected to compressive 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 a member on which such 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 performing a tensile test, and is determined only by the ratio of the thickness of the high Young's layer to the low layer. The Young's modulus as a whole.

  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, in the methods proposed in Patent Documents 2 to 5, it was difficult to introduce shear strain to the center of the plate thickness during rolling. In addition, depending on the components and manufacturing conditions, an orientation that reduces the Young's modulus in the rolling direction may develop 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.

  Furthermore, workability becomes important when considering application to automotive steel sheets. In this case, the maximum tensile strength is high and, in addition to high uniform elongation, it is desirable to have excellent stretch flange formability. However, in Patent Documents 2 to 5, the steel sheet for obtaining excellent stretch flange formability is disclosed. The microstructure and manufacturing cooling conditions are not disclosed.

JP-A-4-147717 JP 2005-273001 A International Publication No. 06/011503 JP 2007-46146 A JP 2007-146275 A

  Automotive steel sheets are sometimes processed into complex shapes and subjected to hole expansion. The present invention relates to a high Young's modulus steel plate having a long Young's modulus measured by a static tensile method in the longitudinal direction, such as an automobile member, of 220 GPa or more, and excellent in workability, particularly hole expansibility, and a method for producing the same. Is to provide.

  The crystal orientation is usually indicated by the indication {hkl} <uvw>, {hkl} is the plate orientation, and <uvw> is the rolling direction. Therefore, in order to obtain a high Young's modulus in the rolling direction, it is necessary to control so that <uvw>, which is the orientation in the rolling direction, is aligned in the direction with the highest Young's modulus as much as possible. Based on this principle, the inventors have studied to obtain a high Young's modulus steel sheet having a Young's modulus in the rolling direction measured by a static tension method of 220 GPa or more. As a result, in order to improve the static Young's modulus in the rolling direction, it is important to suppress recrystallization in the austenite phase (hereinafter referred to as γ phase) by adding Nb and containing a predetermined amount of Ti and N. In addition, the effect is remarkable when B is added in combination, and in hot rolling, the shape ratio obtained from the rolling temperature, the sheet thickness on the entry side and the exit side of the rolling roll, and the diameter of the rolling roll. By controlling these within an appropriate range, the thickness of the layer to which shear strain is applied is increased on the surface of the steel sheet, and the distance from the surface to the sheet thickness direction is 1/6 of the sheet thickness. It was newly found that the texture formed in the vicinity of (1/6 plate thickness) is also optimized.

  At the same time, by controlling the cooling conditions from hot rolling to winding, cold rolling, and cooling conditions after annealing, the main phase is bainite and tempered bainite without reducing the Young's modulus in the rolling direction. It has been found that a high Young's modulus steel sheet having an excellent hole expansibility can be obtained.

  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 the 1/6 plate thickness part and the center part in the plate thickness direction ( It is called 1/2 plate thickness part.) It affects the texture in the vicinity. 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 central portion of the plate thickness, Mn, Mo, W, Ni, which affect the stacking fault energy of the γ phase, We have also found that it is important to optimize the relationship between Cu and Cr.

This invention is made | formed based on such knowledge, The summary is as follows.
(1) By mass%, C: 0.005 to 0.100%, Si: 2.50% or less, Mn: 0.10 to 3.00%, P: 0.150% or less, S: 0.0150 %: Al: 0.150% or less, N: 0.0100% or less, Nb: 0.005 to 0.100%, Ti: 0.002 to 0.150%, and the following (formula 1): Satisfactory, the balance consists of Fe and inevitable impurities, has a microstructure in which the total area ratio of one or both of polygonal ferrite and bainite is 98% or more, and the distance in the thickness direction from the surface of the steel sheet is The sum of the {100} <001> orientation X-ray random intensity ratio and the {110} <001> orientation X-ray random intensity ratio at a position that is 1/6 of the plate thickness is 5 or less, and {110} <111> to {110} <112> Maximum X-ray random intensity ratio of orientation group And {211} <111> high Young's modulus steel sheet excellent in hole expandability characterized in that the sum of the azimuth X-ray random intensity ratio of 5 or more.
Ti-48 / 14 × N ≧ 0.0005 (Formula 1)
Here, Ti and N are content [mass%] of each element.

(2) The high Young's modulus steel sheet excellent in hole expansibility 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, and the total of these contents is 5.00% or less, described in (1) or (2) High Young's modulus steel plate with excellent hole expandability.
(4) The high Young's modulus steel plate excellent in hole expansibility according to any one of (1) to (3), wherein B: 0.0005 to 0.0100% in mass%.
(5) Containing one or more of Ca: 0.0005 to 0.1000%, Rem: 0.0005 to 0.1000%, and V: 0.001 to 0.100% in mass%. A high Young's modulus steel sheet excellent in hole expansibility according to any one of (1) to (4).

(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 {225} <110> orientation 5 or more and satisfies (A) / (B) ≦ 1.00. The high Young's modulus steel plate excellent in hole expansibility according to any one of (1) to (5).
(7) The 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, and the X-ray random intensity ratio of {001} <110> orientation and {112 } The simple average value (C) with the X-ray random intensity ratio in the <110> orientation is 5 or more and (A) / (C) ≦ 1.10 is satisfied (1) to (6) A high Young's modulus steel plate excellent in hole expansibility described in any of the above.
(8) The Young's modulus in the rolling direction measured by a static tension method is 220 GPa or more, and the high Young's modulus steel plate having excellent hole expansibility according to any one of (1) to (7).

(9) A high Young's modulus hot-dip galvanized steel sheet excellent in hole expansibility, wherein the high Young's modulus steel sheet according to any one of (1) to (8) is hot-dip galvanized.
(10) High Young's modulus alloyed hot dip zinc excellent in hole expansibility, characterized in that alloyed hot dip galvanizing is applied to the high Young's modulus steel plate according to any one of (1) to (8) Plated steel sheet.
(11) The high Young's modulus steel sheet, the high Young's modulus hot-dip galvanized steel sheet or the high Young's modulus alloyed hot-dip galvanized steel sheet according to any one of (1) to (10) is wound in any direction. High Young's modulus steel pipe with excellent hole expandability.

(12) The shape ratio obtained by the following (formula 3), with the steel slab having the chemical component according to any one of (1) to (5) being 1100 ° C. or less and a rolling reduction rate to the final pass of 40% or more. Rolling with X being 2.3 or more is set to 2 passes or more, hot rolling is performed so that the temperature of the final pass is Ar ₃ transformation point [° C.] or more and 900 ° C. or less, and after the hot rolling is finished, 5 to 150 ° C. The manufacturing method of the high Young's modulus steel plate excellent in the hole expansibility characterized by cooling to above 300 degreeC-650 degreeC with the cooling rate of / s, and winding up.
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

ここで、ｎは仕上げ熱延の圧延スタンド数、ε_jはｊ番目のスタンドで加えられたひずみ、ε_nはｎ番目のスタンドで加えられたひずみ、ｔ_iはｉ〜ｉ＋１番目のスタンド間の走行時間［ｓ］、τ_iは気体常数Ｒ（＝１．９８７）とｉ番目のスタンドの圧延温度Ｔｉ［Ｋ］によって下記（式６）で計算できる。

(13) The hot rolling is performed according to claim 12, wherein the hot rolling is performed so that an effective strain amount ε ^* calculated by the following (formula 5) is 0.4 or more. Manufacturing method of Young's modulus steel plate.

Here, n is the number of finishing hot rolling rolling stands, ε _j is the strain applied at the j-th stand, ε _n is the strain applied at the n-th stand, and ti is between _{i to i} + 1th stands. The traveling time [s] and τ _i can be 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) When carrying out the hot rolling, at least one rolling roll having a roll diameter of 700 mm or less is used. The high Young excellent in hole expanding property according to (12) or (13) Of steel sheet.
(15) The high Young's modulus excellent in hole expansibility according to any one of (12) to (14), wherein a different peripheral speed ratio of at least one pass or more in the hot rolling is 1% or more. A method of manufacturing a steel sheet.
(16) After winding up, the highest heating temperature is further annealed in a temperature range of Ac ₁ [° C.] or less, and the high hole expansibility according to any one of (12) to (15) Manufacturing method of Young's modulus steel plate.

(17) Highly excellent hole-expanding property, characterized by subjecting a high Young's modulus steel plate having excellent hole-expandability manufactured by the manufacturing method according to any one of (12) to (16) to hot-dip galvanization. A method for producing a Young's modulus hot-dip galvanized steel sheet.
(18) The method for producing a high Young's modulus hot-dip galvanized steel sheet having excellent hole expansibility according to (17), wherein the hot-dip galvanizing is performed in a continuous line following hot rolling.
(19) A high Young's modulus alloy excellent in hole expansibility, characterized by performing a heat treatment for 5 seconds or more in a temperature range from 450 to 600 ° C. after performing the hot dip galvanizing described in (17) or (18) A method for producing a hot-dip galvanized steel sheet.
(20) A high Young's modulus steel sheet, a high Young's modulus hot-dip galvanized steel sheet, or a high Young's modulus alloyed hot-dip galvanized steel sheet obtained by the production method according to any one of (12) to (19) is wound in any direction. A method for producing a high Young's modulus steel pipe excellent in hole expansibility, characterized by being made into a steel pipe.

  According to the present invention, it is possible to obtain a high Young's modulus steel sheet having an improved static Young's modulus in the rolling direction measured by a static tension method and good workability, particularly good hole expansibility.

First, formation of a texture during hot rolling, which is important for improving Young's modulus, 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, Young's modulus, does not necessarily match between the tensile deformation and the bending deformation. This is due to the fact that the rigidity of tensile deformation is affected by the texture of the entire surface of the steel sheet, and the rigidity of bending deformation is affected by the texture of the surface layer 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 thickness portion is high, and the rigidity in tensile deformation is high. Moreover, the accumulation | aggregation of the direction which reduces a Young's modulus is also suppressed by accumulating the direction which raises the Young's modulus of a rolling direction in the site | part from a surface layer to 1/6 board thickness part.

  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 {211} <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 entire thickness from the surface to the center of the plate thickness, the stacking fault energy of the austenite phase generated by hot rolling heating is limited to an optimum range, and the shear deformation is deep. It is preferable to perform rolling under the conditions to enter. 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 so far that the difference in stacking fault energy greatly affects the working texture of the γ phase having a face-centered cubic structure. In addition, after undergoing γ phase processing during hot rolling, when it is cooled and transformed into a ferrite phase (referred to as α phase), the α phase has a certain orientation relationship with the crystal orientation of the γ phase before transformation. The ferrite transforms in the orientation it has. 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 accumulation degree 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. } <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 Investigation of the static Young's modulus in the rolling direction when the basic component composition is% Ti-0.0012% B-0.0015% N and the amount of Mn, Cr, W, Cu, and Ni are varied. did.

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. In the case where the value of this relational expression found by the present inventors from FIG. 1 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, the value is It turns out that it falls remarkably.

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 tensile method, it is necessary to increase the Young's modulus at least 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 tensile method, the {100} <001> orientation X-ray random intensity ratio and the {110} <001> orientation in the 1/6 plate thickness part The sum with 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 group and {211} <111> orientation develop.

Sum of maximum X-ray random intensity ratio of {110} <111> to {110} <112> orientation group and {211} <111> orientation X-ray random intensity ratio at 1/6 plate thickness portion:
These 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 tension method becomes 220 GPa or more. Preferably it is 10 or more, more preferably 12 or more.

X-ray diffraction intensity ratio of {100} <001> orientation, {110} <001> orientation, {110} <111> to {110} <112> orientation group and {211} <111> orientation measured by {110}, {100}, {211}, {310} was calculated by a series expansion method based on a plurality of pole figures out of the pole figures, crystal orientation distribution function representing a three-dimensional texture (O rientation D istribution F unction, may be obtained from that.) 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. Euler angle φ ₂ is 45 °, and a specific crystal orientation (hkl) [uvw] is expressed as Euler of the crystal orientation distribution function. Angles φ1 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 of {110} <111> to {110} <112> orientation groups is The maximum X-ray random intensity ratio in the range of Φ = 85 to 90 ° and φ ₁ = 35 to 55 ° indicated by the hatched portion.

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 about the position indicated by the point in FIG. 40 ° range, {100} <001> orientation is φ ₁ = 40-50 °, Φ = 0˜5 ° range, {110} <001> orientation is φ ₁ = 85-90 °, Φ = 85 The maximum value in the range of 90 ° is represented as the intensity ratio in each direction.

  Here, the orientation of the crystal is usually expressed as [hkl] or {hkl} in the direction perpendicular to the plate surface, and (uvw) or <uvw> in the direction parallel to the rolling direction. {Hkl} and <uvw> are generic terms for equivalent planes, and [hkl] and (uvw) indicate individual crystal planes. That is, in the present invention, a body-centered cubic structure (referred to as a body-centered cubic, bcc structure) is targeted, and for example, (111), (−111), (1-11), (11 -1), (-1-11), (-11-1), (1-1-1), and (-1-1-1) planes are equivalent and indistinguishable. In such a case, these orientations are collectively referred to as {111}.

The ODF is also used to display the orientation of a crystal structure with low symmetry, and is generally expressed as φ ₁ = 0 to 360 °, Φ = 0 to 180 °, φ ₂ = 0 to 360 °, Individual orientations are displayed in [hkl] (uvw). However, in the present invention, b. c. c. Since the structure is targeted, Φ and φ ₂ are expressed in the range of 0 to 90 °. Further, the range of φ ₁ changes depending on whether or not symmetry due to deformation is taken into account when performing calculation, but in the present invention, φ ₁ = 0 to 90 ° in consideration of symmetry. That is, a method of selecting 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. 1 is the X-ray random intensity ratio of {110} <111> orientation.

The sample for X-ray diffraction is manufactured as follows. The steel plate is polished to a specified position in the plate thickness direction by mechanical polishing or chemical polishing, and finished to a mirror surface by buffing, and then the distortion is removed by electrolytic polishing or chemical polishing. Adjust so that Since it is difficult to accurately set the measurement surface to 1/6 plate thickness portion, the sample should be prepared so that the measurement surface is within a range of 3% of the plate thickness with the target position as the center. That's fine. Further, when it is difficult to measure due to X-ray diffraction, even if the statistical measure sufficient number of the EBSP (E lectron B ack S cattering P attern) method or ECP (E lectron C hanneling P attern ) Method good.

  {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} If the <111> orientation is developed, the Young's modulus is further improved. Therefore, by forming a texture similar to that of the surface layer up to a position deeper than the 板 plate thickness portion, preferably ¼ plate thickness portion, more preferably １ ／ plate thickness portion, The rate 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 1/2 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 sheet thickness part, the texture of the 1/2 sheet 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) and {225} <110> orientation X-ray random intensity ratio (B) and (A) / (B) at the center of the plate thickness:
The {332} <113> orientation is a typical crystal orientation that develops in the center of the plate thickness, and is an orientation that lowers the Young's modulus in the rolling direction, whereas the {225} <110> orientation is the Young's modulus in the rolling direction. Is a relatively advantageous orientation. Therefore, in order to improve the static Young's modulus in the rolling direction at the thickness center portion, the X-ray random intensity ratio (A) of {332} <113> orientation at the thickness center portion is 15 or less, and {225 } It is preferable that the X-ray random intensity ratio (B) in the <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 00 or less. From this viewpoint, (A) / (B) is more preferably 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.

Simple 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. If an abnormality such as segregation is observed at the center of the plate thickness, the sample may be prepared within the range of 7/16 to 9/16 of the plate thickness while avoiding the segregated portion.

However, as with the 1/6 plate thickness portion, measurement errors may occur due to test piece processing, sample setting, and the like. Therefore, in the cross section of φ ₂ = 45 ° of the three-dimensional texture shown in FIG. 2, the {001} <110> orientation and the {225} <110> orientation are φ ₁ = 0 to 5 ° and Φ = 0-5 ° range, φ ₁ = 0-5 °, Φ = 25-35 ° range, {332} <113> orientation is φ ₁ = 85-90 °, Φ = 60-70 ° The maximum value of each will be represented as the intensity ratio of that direction. The {112} <110> orientation is in the range of φ1 = 0 to 5 ° and Φ = 30 to 40 °. Therefore, for example, when φ1 = 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} <110> azimuth X-ray random intensity ratio and {112} <110> azimuth 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 variation in measurement, the same test piece was used for five measurements, and the value calculated as the average of the three measured values excluding the maximum and minimum values was obtained. And

Next, the microstructure of the steel sheet for making the hole expandability excellent will be described.
In the microstructure of the steel sheet of the present invention, the total area ratio of one or both of polygonal ferrite and bainite is 98% or more. Thereby, the outstanding hole expansibility is expressed. Polygonal ferrite and bainite may be mixed. The bainite as defined herein includes a needle-like ferrite structure (which may be called acicular ferrite or bainitic ferrite) that is similar in form to bainite but contains little or no cementite. On the other hand, a structure composed of grains having a shape close to a polygon is called a polygonal ferrite structure (edited by the Japan Iron and Steel Institute Basic Research Group, Bainite Research Group, “Steel Bainite Photobook 1 ", Japan Steel Association, published in June 1992).

  The balance of one or both of polygonal ferrite and bainite is one or more of pearlite, martensite, and cementite, and may include all of them. The decrease in hole expansibility is caused by the fact that voids are likely to occur at the interface between hard pearlite, martensite, and cementite, and softer polygonal ferrite and bainite. Therefore, the total volume ratio of one or both of polygonal ferrite and bainite is set to 98% or more. Moreover, since the hole expansion property is improved as the hard phase is smaller, it is more preferable that the total volume ratio of one or both of polygonal ferrite and bainite is 99.8% or more.

  Further, even if the total area ratio of one or both of polygonal ferrite and bainite is 98% or more, if the area ratio of cementite is high among the remainder, polygonal ferrite and bainite than other hard structures. Voids are likely to occur at the interface. Therefore, the area ratio of cementite is preferably 0%.

  In order to obtain such a structure, it is necessary to perform transformation structure control during cooling after hot rolling. The coiling temperature after hot rolling is extremely important, and it is necessary to set it to more than 300 ° C. to 650 ° C. The upper limit of the coiling temperature is set to 650 ° C., because if the coil is wound at a temperature exceeding 650 ° C., a pearlite structure is generated without generating a bainite structure, and the hole expandability is deteriorated. Moreover, the reason why the lower limit value is set to be higher than 300 ° C. is that martensite is generated at 300 ° C. or lower and the hole expanding property is deteriorated. In order to suppress the formation of martensite, the lower limit is preferably set to 350 ° C. or higher.

  Further, the cooling rate after hot rolling needs to be 5 ° C./s or more in order to suppress the formation of pearlite. Furthermore, in order to shorten the cooling time until winding and improve productivity, it is preferable to set the cooling rate after hot rolling finish to 10 ° C./s or more. 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.

  Further, the reason for limiting the steel composition in the present invention will be described. In addition,% of element content 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 0.100%, the Young's modulus in the rolling direction decreases, so the upper limit is made 0.100%. Although the reason why the Young's modulus in the rolling direction decreases due to the addition of Nb is not clear, it is presumed that Nb affects the stacking fault energy of the γ phase. From this viewpoint, the amount of Nb added is preferably 0.080% or less, and more preferably 0.060% 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 secured. 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 0.150%, the workability deteriorates remarkably, so this value is made the upper limit. From this viewpoint, the content is preferably 0.100% or less. More preferably, it is 0.060% or less.

  N is an impurity, and the lower limit is not particularly set. However, if it is less than 0.0005%, the cost becomes high, and so much effect cannot be obtained, so 0.0005% or more is preferable. Further, N may form Ti and nitride to suppress recrystallization of the γ phase, so it may be positively added. However, N reduces the recrystallization suppressing effect of B to 0.0100% or less. suppress. From this viewpoint, it is preferably 0.0050% or less, and more preferably 0.0020% or less.

Furthermore, Ti and N must satisfy the following (formula 1).
Ti-48 / 14 × N ≧ 0.0005 (Formula 1)
Here, Ti and N are content [mass%] of these elements.
As a result, the effect of suppressing the recrystallization of the γ phase due to TiN precipitation is exhibited, and in the case of adding B, the 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. Further, from the viewpoint of Young's modulus, the lower limit of the C content is preferably set to 0.010% or more. This is because when the C content is reduced to less than 0.010%, the Ar ₃ transformation temperature rises, it becomes difficult to perform hot rolling at a low temperature, and the Young's modulus may be reduced. Furthermore, in order to suppress the deterioration of the fatigue characteristics of the welded portion, it is preferably 0.020% or more.

  On the other hand, if the C content exceeds 0.100%, the weldability may be impaired, or the workability may be extremely deteriorated due to an increase in the hard structure, so the upper limit is made 0.100%. Further, if the C content exceeds 0.080%, the moldability deteriorates, so the C content is preferably 0.080% or less. Further, if the C content exceeds 0.060%, the Young's modulus in the rolling direction may be decreased, so that it is more preferably 0.060% or less.

  Si is a deoxidizing element, and the lower limit is not specified, but if it is 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 positively added 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, it is preferable to set it as 1.20% or less. 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. Therefore, the Si content is preferably 1.00% or less. . From the viewpoint of Young's modulus, the Si content is more preferably 0.60% or less, and still more preferably 0.30% or less.

Mn is an important element in the present invention. Mn is an element that lowers the Ar ₃ transformation point, which is a temperature at which the γ phase is transformed into the ferrite phase when heated to a high temperature during hot rolling, and by adding Mn, the γ phase becomes stable to a low temperature, 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. 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, and the Young's modulus in the rolling direction after the transformation. There is an effect of suppressing the formation of an orientation that develops a higher crystal orientation and conversely lowers the Young's modulus. From this viewpoint, it is preferable to add 1.00% or more of Mn. More preferably, 1.5% or more is added. On the other hand, even if the amount of Mn added exceeds 3.00%, not only a significant Young's modulus improvement effect is not obtained, but also the strength becomes too high and the ductility decreases, so the upper limit is made 3.00%. Moreover, since the adhesiveness of galvanization may be inhibited when the amount of Mn exceeds 2.00%, it is preferable to set it as 2.00% or less.

  P is an impurity, but may be positively added when 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 0.150%, the fatigue strength after spot welding deteriorates, the yield strength increases, and a surface shape defect is caused during pressing. Furthermore, the alloying reaction becomes extremely slow during continuous hot dip galvanizing, and productivity is lowered. Also, the secondary workability is deteriorated. Therefore, the upper limit is made 0.150%.

  S is an impurity, and if it exceeds 0.0150%, it causes hot cracking or deteriorates workability, so 0.0150% is made the upper limit.

  Al is a deoxidizing element, and the lower limit is not particularly limited, but is preferably 0.010% or more from the viewpoint of deoxidation. On the other hand, Al is an element that remarkably raises the transformation point. If over 0.150% is added, γ-region rolling at low temperatures becomes difficult, so the upper limit is made 0.150%. In addition, γ-region rolling is hot rolling performed at a temperature at which the metal structure is an austenite single phase.

  Furthermore, Mo, Cr, W, Cu, Ni, B, Ca, Rem, and V may be selectively added. In addition, inclusion in a smaller amount than the preferable range described below does not have an adverse effect, and is acceptable as an impurity.

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. When the above (Formula 2) is satisfied, orientations that increase the Young's modulus in the rolling direction are accumulated near the shear layer of the surface layer of the steel sheet and the center of the plate thickness, and accumulation of orientations that reduce the Young's modulus in the rolling direction is suppressed. .

  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, if necessary, preferably 4.5 or more, more preferably 5.5 or more. 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 it is preferable to add one or two or more of each 0.01% or more. . In addition, when one or more of Mo, Cr, W, Cu, and Ni and Mn are added in combination, the formation of the processed texture is affected, and the Young's modulus in the rolling direction from the surface layer to the 1/6 thickness portion. Is the effect of suppressing the formation of {100} <001> and {110} <001> which are orientations that lower the Young's modulus by developing {110} <111> and {211} <111> Is expressed.

  Further, it is preferable to add one or more of Mo, Cr, W, Cu, and Ni together with Mn so as to satisfy the above (Formula 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, 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. Therefore, it is more preferable to add one or both of Mo and Cu.

  On the other hand, the addition of Mo increases strength and may impair workability, so the upper limit of the amount of Mo added is preferably 1.00%. From the viewpoint of cost, addition of 0.50% or less of Mo is preferable. Further, the upper limit of one or more of Cr, W, Cu, and Ni is preferably set to 3.00% from the viewpoint of workability. In addition, the more preferable upper limit of W, Cu, and Ni is 1.40%, 0.35%, and 1.00% in mass%, respectively.

  B is an element that significantly suppresses recrystallization by adding Nb together and enhances hardenability in the solid solution state, and is thought to affect the variant selectivity of crystal orientation during the transformation from austenite to ferrite. It is done. 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 0.0100%, no further effect is obtained, so the upper limit is made 0.0100%. Further, if B is added in excess of 0.005%, the workability may deteriorate, so 0.0050% or less is preferable. More preferably, it is 0.0030% or less.

  Since Ca, Rem, and V have the effect of increasing mechanical strength or improving the material, it is preferable to contain one or more as required. 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, when Ca and Rem are added so that the addition amount exceeds 0.1000% and the addition amount of V exceeds 0.100%, ductility may be impaired. Therefore, Ca, Rem and V are preferably added in the range of 0.0005 to 0.1000%, 0.0005 to 0.1000% and 0.001 to 0.100%, respectively.

  Next, the reasons for limiting the manufacturing conditions other than the above-described hot rolling shape ratio, the cooling rate after hot rolling, and the coiling temperature 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.

Usually, the steel slab is cooled again after casting, and then heated again for 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 slab efficiently and uniformly, the heating temperature is preferably 1150 ° C. or higher. The upper limit of the heating temperature is not specified, but when heated to over 1300 ° C., the crystal grain size of the steel sheet becomes coarse and the workability may be impaired. Also, 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. The diameter of the rolling roll was measured at room temperature. Therefore, it is not necessary to consider the flatness during hot rolling. The entry side and exit side plate thicknesses of each rolling roll may be measured on the spot using radiation or the like, or may be calculated from the rolling load in consideration of deformation resistance and the like. 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 lower and the final pass is 40% or higher. This is because even after hot rolling above 1100 ° C., the processed structure recrystallizes, and the {110} <111> to {110} <112> orientation group X-ray random intensity ratio in the 1/6 plate thickness part This is because the effect of increasing the thickness 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. From this viewpoint, 50% or more is preferable. Although there is no particular upper limit, increasing the rolling reduction to 1100 ° C. or less and the final pass to more than 95% not only increases the load on the rolling mill, but also changes the texture and starts to lower the Young's modulus. % Or less is preferable. From this viewpoint, 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 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. Further, if 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 {{ 110} <112> The X-ray random intensity ratio of the 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, preferably 850 ° C. or lower, more preferably 800 ° C. or lower, provided that the rolling temperature is not lower than the Ar ₃ transformation point.

The Ar ₃ transformation temperature may be obtained by measuring a change in thermal expansion during cooling, or may be 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 content [% by mass] of each element, and 0 when the content is about an impurity.

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) should be 0.4 or more. Is more preferable.

Here, n is the number of finishing hot rolling rolling stands, ε _j is the strain applied at the j-th stand, ε _n is the strain applied at the n-th stand, and ti is between _{i to i} + 1th stands. The traveling time [s] and τ _i can be calculated by the following (formula 6) by the gas constant R (= 1.987) and the rolling temperature T _i [K] of the i-th stand.

  Effective strain ε * is a cumulative strain index that takes into account the recovery of dislocations during hot rolling. If this is 0.4 or more, the strain introduced into the shear layer is more effectively secured. it can. 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, the friction coefficient between the rolling roll and the steel plate is preferably 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.

  When carrying out hot rolling, forming the texture in the vicinity of the surface layer is promoted if at least one pass of different circumferential speed rolling with a different circumferential speed ratio of the rolling roll of 1% or more is performed. If not, the Young's modulus is improved more than the present invention. 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 defined, it goes without saying that a larger Young's modulus can be obtained with a larger value 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 is a value obtained by dividing the peripheral speed difference between the upper and lower rolling rolls by the peripheral speed of the low peripheral speed roll in percentage. 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 near the surface layer is promoted, and thus the Young's modulus is improved over the present invention when not used. 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 in order to control the material, but the maximum heating temperature for annealing is preferably set to Ac ₁ [° C.] or less. This is because when a hot-rolled steel sheet is heated to a temperature higher than Ac ₁ [° C.], a part or all of the structure is austenitized and the texture obtained by hot rolling is destroyed, and the Young's modulus in the rolling direction may be reduced. Because. Although it may be cooled immediately after reaching the maximum heating temperature, it is preferable to hold it for 30 seconds or more in order to make the temperature of the steel plate uniform. In order to achieve both the homogeneity and the productivity of the steel plate material, the holding time is more preferably 300 s or more and 600 s or less, but the heating temperature may be held for 600 seconds or more. There are no restrictions on the heating rate and cooling rate during annealing.

  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 hot-rolled steel sheet may be hot dip galvanized or alloyed hot dip galvanized. When the steel sheet is annealed, it may be subjected to hot dip galvanization as it is in a continuous hot dip galvanizing line after cooling. 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 heat treatment is performed within a range of 450 to 600 ° C. after hot dip galvanization. If it is less than 450 ° C, alloying does not proceed sufficiently, and if it exceeds 600 ° C, alloying proceeds excessively and the plated layer becomes brittle, which causes problems such as peeling of the plating due to processing such as pressing. . The alloying time is 5 s or longer. If it is less than 5 s, alloying does not proceed sufficiently. Although the upper limit is not particularly defined, it is preferably about 10 s in consideration of plating adhesion.

  The hot-rolled steel sheet may be subjected to Al-based plating or various electroplating. Further, the hot-rolled steel sheet and various plated steel sheets can be subjected to surface treatment such as organic coating, inorganic coating, and various paints according to the purpose.

  When the high Young's modulus steel sheet, hot dip galvanized steel sheet, and galvannealed steel sheet of the present invention are rolled into a steel pipe so that the angle between the rolling direction and the longitudinal direction of the steel pipe is within 0 to 30 °, A high Young's modulus steel pipe having a high Young's modulus in the longitudinal direction can be produced. It is preferable that this angle be as small as possible since the Young's modulus is the highest when it is wound parallel to the rolling direction. From this viewpoint, it is more 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, the present invention will be described with reference to examples.

Steel pieces having the composition shown in Table 1 were melted to produce steel pieces. Formula 1 of Table 1 is the value of the left side of the following (Formula 1) calculated by the content [mass%] of Ti and N, and Formula 2 is Mn, Mo, W, Ni, Cu, Cr for each. It is the value of the left side of the following (Formula 2) calculated by the element content [% by mass].
Ti-48 / 14 × N ≧ 0.0005 (Formula 1)
3.2Mn + 9.6Mo + 4.7W + 6.2Ni + 18.6Cu + 0.7Cr ≧ 4
... (Formula 2)
When the content of Mn, Mo, W, Ni, Cu, Cr is about an impurity, for example, when Mo, W, Ni, Cu, Cr in Table 1 are blank, the left side of the above (Formula 2) is set to 0 Was calculated.

  The steel slab was heated, followed by hot rough rolling and finish rolling under the conditions shown in Tables 2 and 3. The stand for finish rolling consists of all six stages, and the roll diameter is 650 to 830 mm. The finished plate thickness after the final pass was set to 1.6 mm to 10 mm. Furthermore, 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 rolling, that is, the temperature on the finishing side, and CR [° C./s] is the average cooling rate during cooling. And CT [° C.] is the coiling temperature.

  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. 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, 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, “alloy” was indicated. For Ar ₃ and Ac ₁ , a sample was taken from a steel plate, and a thermal dilatometer was used to measure the change in thermal expansion of the test piece when heated and cooled at a heating rate and cooling rate of 10 ° C./s. I asked for it.

The structure of the obtained steel sheet was observed with an optical microscope, and the ferrite area ratio and bainite area ratio were determined by image analysis. Further, the remaining structure was confirmed by an optical microscope. V _α [%] is the area ratio of polygonal ferrite, and V _B [%] is the area ratio of bainite. The value obtained by dividing the sum of V _α [%] and V _B [%] from 100 is the area ratio of the balance. In the balance column, M is martensite, P is pearlite, and C is cementite.

  Further, the {100} <001> and {110} <001> orientations, {110} <111> to {110} <112> orientation groups, and {211} <111> orientations of the 1/6 thickness portion of the steel plate The X-ray random intensity ratio was measured as follows. First, after mechanically polishing and buffing the steel plate, X-ray diffraction was performed using a sample that was further electropolished to remove strain and adjusted so that the 1/6 thickness portion became the measurement surface. Note that X-ray diffraction of a standard sample having no accumulation in a specific orientation was 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 {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 the sample adjusted so that the thick part became the measurement surface, and obtained from ODF.

  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. The hole expansion test was evaluated according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. Young's modulus was measured by both static tension method and 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 5 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 rates are shown in Tables 4 and 5.

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

The results are shown in Tables 4 and 5. Incidentally, the Young's modulus of the column of RD is the rolling direction (R ollinng D irection), TD means transverse direction is the direction of the rolling direction and at right angles to (T ransverse D irection) respectively. Moreover, in Tables 1-11, an underline means that it is outside this invention range or a preferable range.

  As is apparent from Tables 4 and 5, when the steel having the chemical composition of the present invention is hot-rolled under appropriate conditions, the Young's modulus by static tension method exceeds 220 GPa in both the rolling direction and the direction perpendicular to the rolling direction. And a high strength-hole expansion value balance could be satisfied. In particular, 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 when the texture condition of the thickness center layer is simultaneously satisfied.

  On the other hand, hot rolling No. Nos. 34 to 41 are steel Nos. Whose chemical components are outside the scope of the present invention. This is a comparative example using Y to AE. Hot rolling No. 34 and 35 are steel Nos. With a large amount of C. In this example, Y is used, and the area ratios of pearlite and cementite are increased, so that the workability is deteriorated. Hot rolling No. No. 36 is a steel no. This is an example using Z, and the workability is deteriorated. Hot rolling No. No. 37 is a steel no. This is an example using AA, and the hole expandability is deteriorated due to segregation.

  Hot rolling No. No. 38 is a steel No. that does not satisfy (Equation 1). In this example, AB is used, the precipitation of TiN becomes insufficient, the texture advantageous for improving the Young's modulus does not develop, and the Young's modulus in the rolling direction is lowered. In addition, hot rolling No. No. 39 is a steel No. containing no Nb. This is an example using AC, and hot rolling No. No. 40 is a steel no. This is an example using AD, and hot rolling No. No. 41 is a steel No. containing no Nb and Ti. This is an example using AE. These are examples in which the Young's modulus in the rolling direction was lowered because a sufficient recrystallization inhibitory effect was not obtained and a texture preferable for improving the Young's modulus was not developed.

  In addition, hot rolling No. 3, 9, 13, 16, 18, 20, and 28 are steel B, F, I, J, K, L, and S whose components are within the scope of the present invention, and the production conditions are outside the scope of the present invention. This is a comparative example. Hot rolling No. No. 3 has a high coiling temperature, so No. 28 is an example in which pearlite increased due to the low cooling rate, and the value of TS × λ, which is an index of the balance between strength and hole expansion rate, decreased. Hot rolling No. No. 13 is an example in which martensite increases and TS × λ decreases because the winding temperature is low.

Furthermore, hot rolling No. 9 is an example in which FT [° C.] is set lower than Ar ₃ , and the Young's modulus in the rolling direction is lowered because the degree of integration in the {110} <001> orientation is increased. Hot rolling No. 20 has a high FT [° C.], and in the 1/6 plate thickness part, the X-ray random intensity ratio of {110} <111> to {110} <112> orientation group, which is preferable for improving the Young's modulus in the rolling direction, and {211 } Since the sum of the <111> orientations is reduced and the texture does not develop in all of the thickness direction, the Young's modulus in the width direction is also reduced. Hot rolling No. No. 16 is an example in which the Young's modulus decreased because the rolling reduction of hot rolling was small, sufficient shear strain was not introduced during rolling, and the texture did not develop. In addition, hot rolling No. As shown in FIG. 18, when the number of paths having a shape ratio of 2.3 or more is small, the Young's modulus measured by the static tension method is lowered even if a high Young's modulus is obtained by the vibration method.

  Using the steels A and E shown in Table 1, hot rolling was performed under the conditions shown in Table 6. Hot rolling No. shown in Table 6 43 and 46 to 48 are examples in which different peripheral speed rolling was performed by changing the different peripheral speed ratios in the final three stages of the finishing rolling stand including all six stages, that is, four passes, five passes, and six passes. . All the hot rolling conditions not displayed in Table 6 are the same as in Example 1. In addition, hot rolling No. 45, 46 and 48 are examples in which the effective strain ε * is 0.4 or more.

  In the same manner as in Example 1, tensile properties, hole expansion test, measurement of the 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 clear from this, when hot rolling the steel having the chemical component of the present invention under appropriate conditions, adding 1% or more of different peripheral speed rolling promotes the formation of a texture in the vicinity of the surface layer, Young's modulus is improved.

  In addition, hot rolling No. 45 is ordinary rolling, but the Young's modulus is improved by setting the effective strain to 0.4 or more. Hot rolling No. No. 48 performs different peripheral speed rolling, and further reaches an extremely high Young's modulus of about 240 GPa by setting the effective strain to 0.4 or more.

  Using steel J shown in Table 1, hot rolling was performed under the conditions shown in Table 8. Hot rolling No. shown in Table 9 Reference numerals 51 to 53 are examples in which rolling is performed using a roll having a diameter of 700 mm or less in the final three stages of the finish rolling stand including all six stages, that is, four passes, five passes, and six passes. The hot rolling conditions not displayed in Table 8 are all the same as in Example 1. In addition, hot rolling No. 48 and 51 are examples in which the effective strain ε * is 0.4 or more.

  In the same manner as in Example 1, tensile properties, hole expansion test, measurement of the 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 apparent from this, when a steel having the chemical component of the present invention is hot-rolled, a small-diameter roll is used, so that the shear strain amount of the surface layer increases and the Young's modulus can be further increased.

  In addition, hot rolling No. Although 50 is rolling using a medium diameter roll, the Young's modulus is improved by setting the effective strain to 0.4 or more. Hot rolling No. No. 53 performs rolling with a small-diameter roll, and further increases the effective strain to 0.4 or more, so that the Young's modulus in the rolling direction reaches a very high value of about 240 GPa.

  Steel No. Hot rolling No. which is an invention example of K. No. 17 hot-rolled steel sheet, hot-rolled No. 17 having an effective strain of 0.4 or more. No. 43 hot rolled steel sheet, hot rolled No. No. 45 hot rolled steel sheet and hot rolled No. 1 rolled with a small diameter roll. 50, hot-rolled sheet annealing was performed using a continuous annealing furnace or a batch annealing furnace under the conditions shown in Table 10. In the same manner as in Example 1, the microstructure, tensile properties, hole expansion test, measurement of the texture of 1/6 plate thickness part and 1/2 plate thickness part, measurement of Young's modulus, and hole expansion test were performed. The results are shown in Table 11.

  Annealing No. As shown in 1 to 7, by further annealing the hot-rolled steel sheet produced by the hot rolling conditions of the present invention, the workability, particularly the hole expanding property is further increased while maintaining a high Young's modulus. It becomes possible. On the other hand, annealing No. No. 8 is a steel no. Hot rolling No. which is a comparative example of K. This is an example using 18 hot-rolled steel sheets. Annealing No. In No. 8, since the hot rolling conditions of the hot rolled sheet are not appropriate, the Young's modulus in the rolling direction is less than 220 GPa even if annealing is performed under appropriate conditions.

  In addition, annealing No. As shown in 9 to 11, even with different peripheral speed rolling, rolling with a small diameter roll, and hot-rolled steel sheet with high effective strain, annealing is performed under appropriate conditions, so that the hole expansion property is maintained while maintaining a high Young's modulus. Can be increased.

Steel having a composition shown in Table 12 (the balance is Fe and inevitable impurities) is manufactured to produce a steel slab, the steel slab is heated, followed by hot rough rolling, under the conditions shown in Table 13 Finish rolling was performed. The finish rolling stand consists of 6 stages, and the roll diameter is 700 to 830 mm. The finished plate thickness after the final pass was set to 1.6 mm to 10 mm. “-” In the column of Formula 1 means that the comparative example does not contain Ti. “Melting” in Table 13 is a steel sheet that has been hot-dip galvanized after hot rolling is completed, and “alloy” is a steel sheet that has been subjected to alloying treatment at 520 ° C. for 15 seconds after hot-dip galvanizing. . Ar ₃ and Ac ₁ were measured using a thermal dilatometer in the same manner as in Example 1.

  In the same manner as in Example 1, the tensile strength, the hole expansion test, and the Young's modulus of the obtained steel plate were measured, and the texture of the 1/6 plate thickness part of the steel plate and the 1/2 plate thickness part of the steel plate was measured. . At the 1/2 plate thickness part, the X-ray random intensity ratio between the {001} <011> orientation and the {112} <110> orientation was also measured, and an average value was obtained.

  The results are shown in Table 14. As is apparent from Table 14, 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 should be more than 220 GPa in both the rolling direction and the direction perpendicular to the rolling direction. I was able to. In particular, 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 when the texture condition of the thickness center layer is simultaneously satisfied.

  On the other hand, hot rolling No. Nos. 75 to 78 are steel Nos. Whose chemical components are outside the scope of the present invention. It is a comparative example using AT-AV. Hot rolling No. No. 75 is an example using steel AT with a small amount of Ti. 76 is an example using steel AU with a small amount of Nb and has a low Young's modulus. Hot rolling No. No. 77 is an example using steel AV with a large amount of C. Since the pearlite fraction is high, the hole expandability is lowered.

  On the other hand, hot rolling No. 55 and 57 are comparative examples in which the hot rolling conditions are outside the scope of the present invention. Hot rolling No. In No. 55, since one pass satisfies the shape ratio X of 2.3 or more, shear strain is not sufficiently introduced into the steel sheet, and the Young's modulus is low. Hot rolling No. No. 57 is 1100 ° C. or lower, and the rolling reduction to the final pass is less than 40%.

  On the other hand, hot rolling No. 59, 68 and 74 are comparative examples in which the hot rolling finishing temperature (FT) or the winding temperature (CT) is outside the scope of the present invention. Hot rolling No. No. 59 has a high FT, insufficient crystal orientation to increase the Young's modulus, and has a low Young's modulus. Hot rolling No. No. 68 has a high CT and a high pearlite structure fraction. Since No. 74 has low CT and a high martensite structure fraction, the hole expandability is lowered in all cases.

  Using the steel AI and AL shown in Table 12, hot rolling was performed under the conditions shown in Table 15. Hot rolling No. shown in Table 15 79 and 82 to 84 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. . All the hot rolling conditions not displayed in Table 15 are the same as in Example 5. In addition, hot rolling No. 81, 82 and 84 are examples in which the effective strain ε * is 0.4 or more.

  In the same manner as in Example 1, tensile properties, hole expansion test, measurement of the 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 clear from this, when hot rolling the steel having the chemical component of the present invention under appropriate conditions, adding 1% or more of different peripheral speed rolling promotes the formation of a texture in the vicinity of the surface layer, Young's modulus is improved.

  In addition, hot rolling No. 81 is an example in which the effective strain is 0.4 or more, and the Young's modulus is improved. Hot rolling No. No. 84 is an example in which different peripheral speed rolling is performed and the effective strain is set to 0.4 or more, which reaches a very high Young's modulus of about 240 GPa.

  Using steel AF shown in Table 12, hot rolling was performed under the conditions shown in Table 17. Hot rolling No. shown in Table 17 Nos. 87 to 89 are examples in which rolling is performed using a roll having a diameter of 700 mm or less in the final three stages of the finish rolling stand including all six stages, that is, four passes, five passes, and six passes. All the hot rolling conditions not displayed in Table 17 are the same as in Example 5. In addition, hot rolling No. 86 and 89 are examples in which the effective strain ε * is 0.4 or more.

  In the same manner as in Example 1, tensile properties, hole expansion test, measurement of the 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 apparent from this, when a steel having the chemical component of the present invention is hot-rolled, a small-diameter roll is used, so that the shear strain amount of the surface layer increases and the Young's modulus can be further increased.

  In addition, hot rolling No. 86 is an example in which the Young's modulus is improved by setting the effective strain to 0.4 or more. Hot rolling No. 89 is an example in which rolling is performed with a small-diameter roll and the effective strain is 0.4 or more, and the Young's modulus in the rolling direction reaches a very high value of about 240 GPa.

It is a figure which shows the relationship between (Formula 2) and Young's modulus. It is a figure which shows ODF and main orientation in (phi) 2 = 45 degree cross section.

Claims (20)

% By mass
C: 0.005 to 0.100%,
Si: 2.50% or less,
Mn: 0.10 to 3.00%,
P: 0.150% or less,
S: 0.0150% or less,
Al: 0.150% or less,
N: 0.0100% or less,
Nb: 0.005 to 0.100%,
Ti: 0.002 to 0.150%
And the following (formula 1) is satisfied, the balance is Fe and inevitable impurities, and the total area ratio of one or both of polygonal ferrite and bainite is 98% or more, and a steel plate Between the X-ray random intensity ratio in the {100} <001> orientation and the X-ray random intensity ratio in the {110} <001> orientation at a position where the distance in the thickness direction from the surface is 1/6 of the plate thickness The sum is 5 or less, and the sum of the maximum 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 is 5 or more A high Young's modulus steel plate with excellent hole-expandability, characterized by
Ti-48 / 14 × N ≧ 0.0005 (Formula 1)
Here, Ti and N are content [mass%] of each element. The high Young's modulus steel plate excellent in hole expansibility 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 high Young's modulus steel plate excellent in hole expansibility according to claim 1 or 2, wherein the total content thereof is 5.00% or less. % By mass
B: 0.0005 to 0.0100%
The high Young's modulus steel plate excellent in hole expansibility according to any one of claims 1 to 3. % By mass
Ca: 0.0005 to 0.1000%,
Rem: 0.0005 to 0.1000%,
V: 0.001 to 0.100%
The high Young's modulus steel plate excellent in hole expansibility according to any one of claims 1 to 4, characterized by containing one or more of the following.   The X-ray random intensity ratio (A) in the {332} <113> orientation at the center of the steel sheet thickness direction is 15 or less, and the X-ray random intensity ratio (B) in the {225} <110> orientation is 5 or more. And the high Young's modulus steel plate excellent in hole expansibility according to any one of claims 1 to 5, 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 {001} <110> orientation X-ray random intensity ratio and the {112} <110 The simple average value (C) with respect to the X-ray random intensity ratio in the azimuth satisfies 5 or more and (A) / (C) ≦ 1.10 is satisfied. A high Young's modulus steel plate with excellent hole-expandability described in 1.   The Young's modulus in the rolling direction measured by a static tensile method is 220 GPa or more, and the high Young's modulus steel plate excellent in hole expansibility according to any one of claims 1 to 7.   A high Young's modulus hot-dip galvanized steel sheet excellent in hole expansibility, wherein the high Young's modulus steel sheet according to any one of claims 1 to 8 is hot-dip galvanized.   A high Young's modulus galvannealed steel sheet excellent in hole expansibility, wherein the high Young's modulus steel sheet according to any one of claims 1 to 8 is alloyed galvanized.   A hole characterized in that the high Young's modulus steel sheet, the high Young's modulus hot-dip galvanized steel sheet or the high Young's modulus alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 10 is wound in an arbitrary direction. High Young's modulus steel pipe with excellent spreadability. 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, and the rolling reduction rate to the last pass is 40% or more. .3 or more rolling is performed at 2 passes or more, and the final pass temperature is set to Ar ₃ transformation point [° C.] or more and 900 ° C. or less, and after the hot rolling is finished, 5 to 150 ° C./s. A method for producing a high Young's modulus steel sheet having excellent hole expansibility, wherein the steel sheet is cooled to a temperature exceeding 300 ° C. to 650 ° C. at a cooling rate.
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 high Young's modulus steel plate with excellent hole expansibility according to claim 12, wherein the hot rolling is performed so that an effective strain amount ε ^* calculated by the following (formula 5) is 0.4 or more. Manufacturing method.

Here, n is the number of finishing hot rolling rolling stands, ε _j is the strain applied at the j-th stand, ε _n is the strain applied at the n-th stand, and ti is between _{i to i} + 1th stands. The traveling time [s] and τ _i can be 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 method for producing a high Young's modulus steel plate with excellent hole expanding property according to claim 12 or 13, wherein at least one rolling roll having a roll diameter of 700 mm or less is used when performing the hot rolling. .   The production of a high Young's modulus steel plate excellent in hole expansibility according to any one of claims 12 to 14, wherein a different peripheral speed ratio of at least one pass or more in the hot rolling is 1% or more. Method. The steel sheet with high Young's modulus excellent in hole expansibility according to any one of claims 12 to 15, wherein the steel sheet is further annealed at a maximum heating temperature of Ac ₁ [° C] or less after winding. Manufacturing method.   A high Young's modulus fusion excellent in hole expansibility, characterized by subjecting the high Young's modulus steel plate excellent in hole expansibility manufactured by the manufacturing method according to any one of claims 12 to 16 to hot dip galvanization. Manufacturing method of galvanized steel sheet.   The method for producing a high Young's modulus hot-dip galvanized steel sheet having excellent hole expanding property according to claim 17, wherein the hot-dip galvanizing is performed in a continuous line following hot rolling.   19. A high Young's modulus galvannealed steel sheet excellent in hole expansibility, which is subjected to a heat treatment for 5 seconds or more in a temperature range of 450 to 600 ° C. after the hot dip galvanizing according to claim 17 or 18. Manufacturing method.   A high Young's modulus steel plate, a high Young's modulus hot-dip galvanized steel plate or a high Young's modulus alloyed hot-dip galvanized steel plate obtained by the production method according to any one of claims 12 to 19 is wound in an arbitrary direction into a steel pipe. A method for producing a high Young's modulus steel pipe excellent in hole expansibility.

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