JP5817804B2 - High strength steel sheet with small in-plane anisotropy of elongation and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel sheet having a small in-plane anisotropy of elongation, which is useful for uses such as automobiles and electric machines, and a method for producing the same.

In recent years, in order to suppress CO ₂ emissions from the viewpoint of global environmental conservation, there has been a demand for improved fuel efficiency of automobiles. In addition, in order to ensure the safety of passengers in the event of a collision, it is also required to improve safety centering on the collision characteristics of the automobile body. For this reason, the weight reduction and reinforcement of the automobile body are being actively promoted. In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is effective to increase the strength of the component materials and reduce the thickness in a range that does not cause a problem with rigidity. Is actively used in automotive parts. In the field of electrical machinery, there is a great need for increasing the strength of parts for the purpose of suppressing deformation during product transportation or accidental dropping. For example, there is a trend to use steel sheets having a yield strength (YP) of 300 MPa or more.

  On the other hand, since many automobile parts and electrical parts made of steel plates are formed by pressing, the steel plates need to have excellent press formability. However, high strength steel sheets are required to be improved because press formability and ductility are greatly deteriorated as compared with ordinary mild steel sheets.

  As the high strength steel plate, for example, when the yield strength (YP) is 440 MPa class, Ti and Nb are added to an extremely low carbon steel plate excellent in formability to add solid solution C and solid solution N in an amount of IF (Interstitial free). ), And a steel plate to which a solid solution strengthening element such as Si, Mn, or P is added.

  Further, when the yield strength (YP) is 500 MPa or more, a composite structure steel sheet is put into practical use, and there are a DP steel sheet having a two-phase structure of ferrite and martensite, and a TRIP steel sheet utilizing residual austenite. The former is characterized by low yield strength and high work hardening ability due to residual strain around the martensite. The latter is characterized by high uniform elongation due to plasticity-induced martensitic transformation.

  In general, the mechanical properties of a high-strength steel sheet are evaluated by tensile properties in a specific direction such as a direction perpendicular to rolling. However, analysis of actual press forming has revealed that part formability, for example, the height that can be formed by stretch forming or the Erichsen test, is greatly influenced by the in-plane anisotropy of elongation. Therefore, improvement in press formability can be expected by reducing the in-plane anisotropy of elongation.

  Regarding a steel sheet having a small in-plane anisotropy, for example, Patent Document 1 discloses a cold-rolled steel sheet having excellent bake hardenability and a small in-plane anisotropy and a method for manufacturing the same. In this technique, Δr is defined by the amount of C and the rolling reduction during cold rolling, and both in-plane anisotropy and dent resistance can be achieved. Moreover, it is necessary to start cooling within 2 seconds after hot rolling and to cool at a cooling rate of 70 ° C./s or more over a temperature range of 100 ° C. or more. However, the in-plane anisotropy here is Δr and does not necessarily match the in-plane anisotropy of elongation.

  As for the steel sheet relating to the in-plane anisotropy of elongation, for example, Patent Document 2 discloses a high-strength steel sheet having a small in-plane anisotropy of elongation and a method for manufacturing the same. Such a steel sheet is a composite structure steel including martensite having a ferrite phase in an area ratio of 85% or more and 99% or less, and an area ratio of 1% or more and 13% or less. Of α fibers (φ1 = 0 °, φ2 = 45 °, Φ = 0-55 °) represented by ODF (crystal orientation distribution function), the average crystal orientation density I in the range of Φ = 25-35 ° is 2 0.0 or more and 4.0 or less. However, the composite steel containing martensite has a problem in that the yield strength (YP) is low, so that the effect of suppressing deformation at the time of product transportation or accidental dropping becomes small. Even if martensite is included, if the tensile strength (TS) is increased by high alloying, the yield strength (YP) also increases. However, in this case, there is a problem that the manufacturing cost increases.

  Further, as a technique for reducing the anisotropy of a high-strength steel plate, for example, in Patent Document 3, after completion of hot rolling, cooling to 720 ° C. within 0.4 sec, preferably at a cooling rate of 400 ° C./second or more. By doing so, the in-plane anisotropy of the r value can be reduced. However, the in-plane anisotropy here is Δr and does not necessarily match the in-plane anisotropy of elongation. And when a hot-rolled steel sheet having a thickness of 2 mm or more is cooled at a cooling rate of 400 ° C./second or more, there is a problem that the temperature difference between the steel sheet surface layer and the inside is large, resulting in uneven structure and material variations. . Further, in order to cool a hot-rolled steel sheet having a thickness of 2 mm or more at a cooling rate of 400 ° C./second or more, a large facility is required, which causes an increase in cost.

JP 2004-197155 A JP 2009-132981 A JP 2011-144414 A

  The present invention advantageously solves the above problems, and has a high yield strength (YP) of 300 MPa or more, which is suitable for automobile parts and electrical parts, and reduces the in-plane anisotropy of elongation and press molding. An object of the present invention is to provide a high-strength steel sheet having excellent properties and a method for producing the same.

  In general, the rolled texture of a cold rolled steel sheet develops an α fiber whose <100> direction is parallel to the rolling direction and a γ fiber whose <111> direction is parallel to the normal direction. However, when recrystallization proceeds in the annealing process, the α fiber becomes weaker and the γ fiber becomes stronger. Since α-fiber reduces the elongation in the 45 ° direction with respect to the rolling direction, the cold-rolled steel sheet produced by a normal process has a low elongation in the 45 ° direction with respect to the rolling direction and a strong elongation anisotropy. Become.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that in order to increase the elongation in the 45 ° direction with respect to the rolling direction and reduce the anisotropy, the ¼ thickness position of the steel plate In the texture of the plate surface at Φ = 25 ° -35 ° among α fibers (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° -55 °) represented by ODF (crystal orientation distribution function) the average crystal orientation density I _alpha in the range is not less 2.0 to 4.0, gamma fiber (φ1 = 0 ° ~60 °, φ2 = 45 °, Φ = 55 °) average crystal orientation density I _gamma of It was found that it is important to set the ratio to 2.0 or more and 10 or less. Moreover, in order to obtain the said texture, it discovered that control of a component composition, especially content control of Nb, and control of manufacturing conditions were important.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.040 to 0.090%, Si: 0.20% or less, Mn: 0.50 to 0.99%, P: 0.050% or less, S: 0.03 % Or less, sol. Al: 0.01 to 0.09%, N: 0.005% or less, Nb: 0.015 to 0.040%, with the balance being Fe and unavoidable impurities, martensite phase and residual austenite phase In the texture of the plate surface at the ¼ plate thickness position of the steel plate, α fiber (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° to 55) represented by ODF (crystal orientation distribution function) ), The average crystal orientation density I _{α in} the range of Φ = 25 ° to 35 ° is 2.0 to 4.0, and γ fiber (φ1 = 0 ° to 60 °, φ2 = 45 °, Φ = 55 °) high-strength steel sheet having a small in-plane anisotropy of elongation, wherein the average crystal orientation density I _γ is 2.0 or more and 10 or less.
[2] The high-strength steel sheet having a small in-plane anisotropy of elongation according to [1], wherein ΔEl expressed by the following formula (1) is −2.0% to 2.0%.
ΔEl = (El ₀ −2El ₄₅ + El ₉₀ ) / 2 (1)
However, El ₀ , El ₄₅ and El ₉₀ are values of elongation at break measured in directions of 0 °, 45 ° and 90 ° with respect to the rolling direction of the steel sheet.
[3] The in-plane elongation according to [1] or [2], wherein the yield ratio YR (YR = YP / TS), which is the ratio between the yield strength YP and the tensile strength TS, is 0.79 or more. High strength steel sheet with low anisotropy.
[4] The high-strength steel sheet having a small in-plane anisotropy of elongation according to any one of [1] to [3], wherein the surface has a zinc-based plating film.
[5] A steel slab having the composition described in [1] is heated and held for 60 minutes or more in a temperature range of 1150 ° C. or higher, followed by hot rolling, followed by finish rolling temperature. Finish rolling is performed at a final pass rolling ratio of 15 to 25% at 820 to 920 ° C., water cooling is started within 2 seconds after finishing rolling, cooling is performed, and pickling and cold rolling are performed, followed by annealing. A method for producing a high-strength steel sheet having a small in-plane anisotropy of elongation, characterized in that:
[6] The in-plane anisotropy of elongation according to [5], wherein the finish rolling is performed after the hot rolling roughing rolling is followed by water cooling to a finish rolling entry temperature of 1050 ° C. or lower. Method for producing high-strength steel sheet with small size
[7] The method for producing a high-strength steel sheet according to [5] or [6], wherein the steel sheet after annealing is subjected to galvanizing treatment, and the high-strength steel sheet having a small in-plane anisotropy of elongation is characterized. Method.
In addition, the high intensity | strength in this invention means that yield strength YP is 300 Mpa or more.

  According to the present invention, a high-strength steel sheet having a small in-plane anisotropy of elongation and excellent press formability can be obtained. Moreover, since the yield strength (YP) is high, deformation at the time of product transportation or accidental dropping is suppressed. The high-strength steel sheet of the present invention can be applied to automotive parts and electrical parts, and is extremely useful.

It is a graph which shows the relationship between the average crystal orientation density Iα in Φ = 25 ° to 35 ° of the _α fiber, the average crystal orientation density I _γ in Φ = 55 ° of the γ fiber, and ΔEl.

  Hereinafter, the present invention will be specifically described.

  First, the reason for the component composition will be described. The unit of the content of each element is mass% unless otherwise specified.

C: 0.040 to 0.090%
C is an element necessary for refining the crystal and increasing the strength. Moreover, it has the effect of forming a precipitate with Nb, which will be described later, and increasing the yield strength (YP). If the amount of C is less than 0.040%, the effect of increasing the strength due to fine graining is low. On the other hand, when the amount of C exceeds 0.090%, it becomes easy to form the second phase, and the elongation decreases. Therefore, the C content is in the range of 0.040 to 0.090%. Preferably it is 0.040 to 0.060% of range.

Si: 0.20% or less Si has the effect of improving the surface quality by delaying the scale formation in hot rolling in a small amount. In addition, there is an effect of increasing the work hardening ability of the ferrite phase. From such a viewpoint, it is preferable to contain about 0.01% or more. However, when the Si content exceeds 0.20%, the appearance quality deteriorates. Therefore, the Si content is 0.20% or less. Preferably it is 0.10% or less.

Mn: 0.50 to 0.99%
Mn is an element useful for increasing the strength of a steel sheet through solid solution strengthening and crystal grain refining effects. If the amount of Mn is less than 0.50%, the effect of solid solution strengthening and refining is low, so 0.50% or more is required. On the other hand, when the amount of Mn exceeds 0.99%, it becomes easy to form a martensite phase and the yield strength (YP) decreases. Therefore, the Mn content is in the range of 0.50 to 0.99%. Preferably it is 0.61 to 0.79% of range.

P: 0.050% or less When the amount of P exceeds 0.050%, surface defects due to deterioration of weldability or segregation occur. Therefore, the P content is 0.050% or less. Preferably, it is 0.040% or less.

S: 0.03% or less S has an effect of improving the peelability of the primary scale of the steel sheet and improving the appearance quality. However, when the amount of S increases, the amount of MnS precipitated in the steel increases. For this reason, ductility, such as elongation of a steel plate and stretch flangeability, is reduced, and press formability is reduced. Moreover, the hot ductility at the time of hot-rolling a slab is reduced, and surface defects are likely to occur. From such a viewpoint, the S amount is set to 0.03% or less. Preferably it is 0.01% or less, More preferably, it is 0.005% or less, More preferably, it is 0.002% or less.

sol. Al: 0.01 to 0.09%
sol. In addition to being useful as a deoxidizing element for steel, Al has the effect of fixing solid solution N present as an impurity and improving formability. For this reason, sol. The amount of Al is 0.01% or more. On the other hand, sol. If the amount of Al exceeds 0.09%, the cost increases, and further surface defects are induced. Therefore, sol. The Al content is in the range of 0.01 to 0.09%. Preferably it is 0.02 to 0.07%.

N: 0.005% or less If the amount of N is too large, the moldability is deteriorated and a large amount of Al is required to fix the solid solution N. For this reason, it is preferable to reduce as much as possible. From such a viewpoint, the N amount is set to 0.005% or less.

Nb: 0.015-0.040%
Nb is an element necessary for refining the crystal and increasing the strength. Moreover, the above-mentioned C and precipitates are formed, and in particular, it has the effect of increasing the yield strength (YP). Furthermore, since Nb precipitates are finely precipitated in the finish rolling step of the hot rolling process to partially suppress recrystallization of the steel, and Nb has the effect of increasing the α fiber after cold rolling and annealing. It is the most important element of the invention. In order to acquire such an effect, it is necessary to contain Nb amount 0.015% or more. On the other hand, if it exceeds 0.040%, recrystallization in the finish rolling step of the hot rolling process is completely suppressed, and the α fiber after cold rolling and annealing is excessively increased to reduce elongation anisotropy. The hot rolling load becomes high. Therefore, the Nb content is in the range of 0.015 to 0.040% or less. Preferably, it is 0.030% or less.

  In the present invention, in addition to the above components, the following elements may be contained. However, the following elements are elements that have particularly high hardenability and facilitate the formation of a martensite phase. Therefore, the following range is preferable.

Cr: 0.05% or less Cr, like Mn, is an element that easily forms a martensite phase, and yield strength (YP) decreases when the martensite phase is generated. For this reason, the Cr content is 0.05% or less. Preferably it is 0.02% or less, More preferably, it is 0.01% or less. Since excessive reduction causes an increase in cost, the lower limit is preferably set to 0.001%.

Mo: 0.05% or less Mo, like Mn, is an element that easily forms a martensite phase. When a martensite phase is generated, yield strength (YP) decreases. For this reason, the Mo amount is 0.05% or less. Preferably it is 0.02% or less, More preferably, it is 0.01% or less. Since excessive reduction causes an increase in cost, the lower limit is preferably set to 0.001%.

  In the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

  Next, the reasons for limiting the steel structure and texture in the steel sheet of the present invention will be described.

Steel structure: Does not include martensite phase and retained austenite phase When martensite phase and retained austenite phase are generated, the yield strength (YP) decreases, and the effect of suppressing deformation during product transportation or accidental dropping is reduced. Therefore, it is necessary to contain no martensite phase and residual austenite phase. The microstructure of the steel sheet of the present invention is a ferrite + pearlite structure or a structure made of cementite or the like, and does not include a martensite phase and a retained austenite phase. The martensite phase and the retained austenite phase are based on the entire structure. The volume fraction is 1% or less. Moreover, in order not to contain a martensite phase and a retained austenite phase, it can control by the manufacturing conditions mentioned later.

Texture: α fiber (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° -55 °) represented by ODF (crystal orientation distribution function) in the texture of the plate surface at the 1/4 plate thickness position of the steel plate ), The average crystal orientation density I _{α in} the range of Φ = 25 ° to 35 ° is 2.0 or more and 4.0 or less, and γ fiber (φ1 = 0 ° to 60 °, φ2 = 45 °, Φ = 55 °) average crystal orientation density I _gamma conventional 2.0 but no more than 10, the analysis of the texture pole figure by an X-ray diffraction (XRD) have been used. The pole figure represents a statistical crystal orientation distribution regarding a large number of crystal grains, and is therefore a method suitable for determining the preferred orientation. However, the texture of polycrystalline materials often exhibits a number of preferred orientations as well as a single preferred orientation. For example, in a fiber texture such as an α fiber or γ fiber that is an orientation group rotated around a crystal axis, it is difficult to accurately evaluate the existence ratio of individual orientations from the pole figure. Therefore, a three-dimensional crystal orientation distribution function is created based on the pole figure information, and the existence ratio of each orientation is evaluated. In the evaluation of the three-dimensional crystal orientation distribution function, it is obtained by the series expansion method from the incomplete pole figures (200), (211), and (110) obtained by the reflection method. As a result, in the steel structure that does not contain the martensite phase or the residual austenite phase as described above, Φ = 25 ° to 35 ° of α fiber (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° to 55 °). The average crystal orientation density I _{α in} the range of ° is 2.0 or more and 4.0 or less, and the average crystal orientation of the γ fiber (φ1 = 0 ° -60 °, φ2 = 45 °, Φ = 55 °) It was investigated that the in-plane anisotropy of elongation becomes small when the density _Iγ is 2.0 or more and 10 or less. The reason why the in-plane anisotropy of elongation becomes small when the texture is in the above range is not necessarily clear, but the γ fiber that improves the elongation in the rolling direction and the direction of 90 ° with respect to the rolling direction. Of the α fiber (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° -55 °) that improves the abundance ratio and the elongation in the direction of 45 ° with respect to the rolling direction, Φ = 25 ° -35 ° The balance of the existence ratio is considered good.

Next, the inventors hot-rolled a steel slab having the composition of the present invention into a hot-rolled steel sheet, pickled, cold-rolled into a cold-rolled steel sheet, and then annealed. Cold-rolled annealed steel sheet was used, and temper rolling was performed. A crystal orientation distribution function (hereinafter referred to as ODF: Orientation Distribution Function) obtained from the X-ray diffraction result of the plate surface at the ¼ plate thickness position of each cold-rolled annealed steel plate thus obtained is created, and from this ODF, Focusing on the texture called α-fiber, which is thought to affect the workability such as elongation, we investigated the relationship between this α-fiber and moldability. As a result, the average crystal orientation density I _α and the average crystal orientation density I _{γ of the} γ fiber in the range of Φ = 25 ° to 35 ° among Φ = 0 ° to 55 °, which is the orientation group of the α fiber. It was found that there was a strong correlation with ΔEl. In the present invention, when the value of ΔEl is −2.0% to 2.0%, it is determined that the in-plane anisotropy of elongation is small and the press formability is good. ΔEl is obtained by the following equation (1).
ΔEl = (El ₀ −2El ₄₅ + El ₉₀ ) / 2 (1)
However, El ₀ , El ₄₅ and El ₉₀ are JIS No. 5 test pieces in the 0 ° direction (L direction), 45 ° direction (D direction) and 90 ° direction (C direction) from the cold-rolled annealed steel sheet to the rolling direction. Is a value of elongation at break measured by performing a tensile test at a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241.

FIG. 1 shows the relationship between the average crystal orientation densities I _α and I _γ thus obtained and the absolute value of ΔE1 (hereinafter sometimes simply referred to as | ΔE1 |). From FIG. 1, when the average crystal orientation density I _α is 2.0 or more and 4.0 or less and the average crystal orientation density I _γ is 2.0 or more and 10 or less, the value of | ΔEl | is 2.0%. The following good results were obtained. That is, the average crystal orientation density I _{α in} the range of Φ = 25 ° to 35 ° in the α fiber (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° to 55 °) is 2.0 or more and 4.0. A high-strength steel sheet having a texture where the average crystal orientation density I _{γ of} the γ fiber (φ1 = 0 ° to 60 °, φ2 = 45 °, Φ = 55 °) is 2.0 or more and 10 or less, It was found that the in-plane anisotropy of elongation was small. Therefore, in the steel sheet of the present invention, ΔE1 represented by the following formula (1) is preferably −2.0% or more and 2.0% or less.
ΔEl = (El ₀ −2El ₄₅ + El ₉₀ ) / 2 (1)
However, El ₀ , El ₄₅ and El ₉₀ are values of elongation at break measured in directions of 0 °, 45 ° and 90 ° with respect to the rolling direction of the steel sheet.

  In the present invention, the following limitations can be applied to the yield strength and yield ratio.

Yield strength YP is 300 MPa or more, and yield ratio YR (YR = YP / TS), which is the ratio of yield strength YP and tensile strength TS, is 0.79 or more. Is prevented from being deformed. In order to obtain this effect, the yield strength YP is preferably 300 MPa or more. On the other hand, if it is too high, the spring back becomes large and it is difficult to maintain the component shape. The yield ratio YR is preferably 0.79 or more. If the tensile strength is higher than the yield strength, the press load becomes higher than necessary, and a large press must be introduced. For this reason, it is preferable that the tensile strength TS is 560 MPa or less.

  Next, the manufacturing method of this invention is demonstrated.

  First, the steel slab to be used is preferably manufactured by a continuous casting method in order to prevent macro segregation of components. In addition, you may manufacture by an ingot-making method or a thin slab casting method. In addition to the conventional method in which the slab is manufactured and then cooled to room temperature and then reheated, it is charged directly into the heating furnace without being cooled and placed in a heating furnace and hot rolled, or a little heat retention After carrying out, an energy saving process such as direct rolling which is immediately hot rolled can be applied without any problem.

  Next, conditions for the hot rolling process will be described.

Slab heating temperature: Hold for 60 minutes or more in a temperature range of 1150 ° C or higher In slab heating, Nb precipitates are completely dissolved and finely precipitated in the finish rolling step of the hot rolling process to recrystallize the steel. In order to partially suppress the above and increase the α fiber after cold rolling and annealing, it is preferable that the heating temperature is high and the holding time is long. From such a viewpoint, in this invention, it hold | maintains for 60 minutes or more in the temperature range of slab heating temperature 1150 degreeC or more. On the other hand, when the slab heating temperature is too high or the holding time is too long, the scale loss accompanying the increase in the oxidized weight increases, so the heating temperature is preferably 1300 ° C. or less, and the holding time is 500 minutes or less. It is preferable.

  The steel slab heated under the above conditions is subjected to hot rolling consisting of rough rolling and finish rolling. Here, the steel slab is made into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be performed according to a conventional method. In addition, it is effective to use a so-called sheet bar heater or edge heater that heats the sheet bar for the purpose of preventing troubles during hot rolling or improving temperature unevenness in the width direction.

  In order to finely precipitate Nb precipitates in the finish rolling step, it is preferable to perform finish rolling at a low temperature, and the finish rolling entry temperature is preferably 1050 ° C. or lower. In the present invention, since the slab is heated at a high temperature of 1150 ° C. or higher, the sheet bar is preferably water-cooled before finish rolling in order to cool to 1050 ° C. on the finish rolling entry side. On the other hand, excessively low temperature increases the load during hot rolling, and is preferably 930 ° C. or higher.

Finish rolling temperature: 820-920 ° C
Next, the sheet bar is finish-rolled to obtain a hot-rolled steel sheet. At this time, the finishing temperature, that is, the finish rolling outlet temperature (FT) is 820 to 920 ° C. This is for obtaining a texture preferable for in-plane anisotropy of elongation after cold rolling and recrystallization annealing. When the FT is less than 820 ° C., not only the load during hot rolling becomes high, but in some component systems, rolling occurs in the ferrite region, and the texture changes greatly. On the other hand, when FT exceeds 920 ° C., not only the structure becomes coarse, but austenite cannot be rolled in a partially recrystallized state, and the in-plane anisotropy of elongation increases after cold rolling annealing. For this reason, finishing temperature shall be 820-920 degreeC, More preferably, it shall be 820-890 degreeC.

Finish rolling final pass rolling ratio: 15-25%
Texture formation by rolling in the austenite region during finish rolling enhances the α fiber after cold rolling and annealing. This effect is most strongly affected by the final rolling final pass. When the final pass rolling ratio of the finish rolling is less than 15%, the texture formation by rolling in the austenite region is insufficient, and the α fiber after cold rolling and annealing does not become strong. On the other hand, if it exceeds 25%, the load during rolling becomes high, so it is 25% or less.

Time until start of water cooling after finish rolling: within 2 seconds Since it is necessary to transform austenite in a partially recrystallized state after finish rolling, holding in the austenite region is not preferable. Therefore, water cooling is started within 2 seconds after finish rolling. More preferably, it is within 0.5 seconds.

  The cooling rate from finish rolling to coil winding is not particularly specified, but is preferably 20 ° C./second or more in order to suppress recrystallization in the austenite region during cooling. Moreover, since excessive cooling tends to cause temperature unevenness in the plate thickness direction and in-plane direction, it is preferably 200 ° C./second or less. More preferably, it is 99 degrees C / second or less, More preferably, it is 40 degrees C / second or less.

  The coil winding temperature (CT) is not particularly specified, but is preferably 400 ° C. or higher and 720 ° C. or lower. In particular, when CT exceeds the upper limit, the crystal grains become coarse and the strength is reduced.

  Then, the high strength steel plate of this invention can be obtained by performing pickling, cold rolling, and annealing.

  Pickling is not particularly specified and is performed by a conventional method. In order to suppress scale defects, shot blasting or leveling may be performed before pickling.

  Cold rolling is not particularly specified, but the rolling rate is preferably 30 to 80%. If the rolling rate is less than 30%, recrystallization at the time of annealing becomes unstable and the elongation is lowered. Moreover, when it exceeds 80%, the load at the time of rolling will become high.

Although annealing is not particularly specified, it is preferable that the annealing temperature is 700 to 900 ° C. in continuous annealing. When the annealing temperature is less than 700 ° C., the crystal is not sufficiently recrystallized and the elongation is lowered. Moreover, when it exceeds 900 degreeC, the austenite fraction at the time of annealing will become high, the balance of (alpha) fiber and (gamma) fiber will collapse, and the in-plane anisotropy of elongation will increase. In the case of batch annealing, it is preferable to set it as 600-800 degreeC. If it is less than 600 ° C., it will not be sufficiently recrystallized, leading to a decrease in elongation. Moreover, when it exceeds 800 degreeC, sticking will arise and a steel plate shape will deteriorate.
When the cooling after annealing is fast, a martensite phase is likely to be generated, and therefore it is desirable to cool at an average cooling rate of 50 ° C./sec or less.

  As mentioned above, although the basic process of the manufacturing method of this invention was demonstrated, you may add the following process.

  A plating layer may be formed on the surface of the steel sheet by adding a surface treatment such as electroplating or hot dipping after the cold-rolled steel sheet annealing step. The plating layer is not limited to pure zinc plating or zinc-based alloy plating, but may be various plating layers conventionally applied to the surface of a steel sheet, such as Al plating or Al-based alloy plating. Moreover, you may apply | coat a chemical conversion treatment film for the purpose of improving corrosion resistance or fingerprint resistance after plating.

  Further, temper rolling or leveler processing may be applied to the cold-rolled annealed plate or plated steel plate produced as described above for the purpose of adjusting the shape correction, surface roughness, and the like. The total elongation of temper rolling or leveler processing is preferably in the range of 0.2 to 15%. If it is less than 0.2%, the intended purpose of shape correction and roughness adjustment cannot be achieved. More preferably, it is 1.3% or more. On the other hand, if it exceeds 15%, it tends to cause a significant decrease in ductility, which is not preferable.

  Hereinafter, the present invention will be described in more detail by way of examples.

  Molten steel having various compositions shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. These steel slabs were hot rolled under the conditions shown in Table 2 to obtain 3.2 mm thick hot rolled steel sheets. These hot-rolled steel sheets were pickled and then made into 1.6 mm thick as roll material by cold rolling at a reduction rate of 50%. Next, these as roll materials were subjected to continuous annealing at 820 ° C. in a continuous annealing line and cooled at an average cooling rate of 15 ° C./sec. Furthermore, the obtained cold-rolled annealed steel sheet was subjected to temper rolling with an elongation of 1.3%. Some as roll materials were subjected to hot dip galvanization after annealing at 810 ° C. in a continuous hot dip galvanizing line and subjected to temper rolling with an elongation of 1.3%. The average cooling rate at this time was 10 ° C./sec.

  The thus obtained cold-rolled annealed sheet and hot-dip galvanized steel sheet were examined for tensile properties, steel structure and texture.

(1) Tensile properties JIS No. 5 tensile test specimens were sampled from 0 ° (L direction), 45 ° (D direction) and 90 ° (C direction) directions with respect to the rolling direction of each cold-rolled annealed sheet obtained, A tensile test was performed at a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241 to determine yield strength (YP), tensile strength (TS), and elongation (El). Here, the representative values of yield strength (YP), tensile strength (TS), elongation (El), and yield ratio (YP / TS) were values of test pieces taken from the 0 ° direction. A yield strength of 300 MPa or more is accepted.
Further, ΔEl was used as an index of in-plane anisotropy of elongation. This ΔEl indicates the in-plane anisotropy of elongation, and was calculated from the following equation (1).
ΔEl = (El ₀ −2El ₄₅ + El ₉₀ ) / 2 (1)
Here, El ₀ , El ₄₅ , and El ₉₀ indicate the elongation of the test specimens taken from 0 ° (L direction), 45 ° (D direction), and 90 ° (C direction) directions.
If ΔEl is −2.0% to 2.0%, it can be said that the in-plane anisotropy of elongation is excellent.

(2) Steel structure, texture (a) Volume fraction of phase The volume fraction of each phase is determined by measuring the area ratio of each phase by the point count method (according to ASTM E562-83 (1988)). The rate was defined as the volume fraction. The area ratio of each phase was obtained by collecting a test piece from each of the obtained cold-rolled annealed plates, and corroding the vertical section (L section) parallel to the rolling direction with nital after polishing, and using a scanning electron microscope (SEM). In addition, the type of phase was identified by observation at 4000 times, and the area ratio (martensite fraction) of the martensite phase was determined. In the structure photograph, particles with white contrast were martensite. Further, the abundance ratio of residual austenite phase (residual γ fraction) is obtained by performing X-ray diffraction of the plate surface at the 1/4 thickness, and measuring the integrated intensity of α phase (211) and γ phase (220), Obtained by standardization.
(B) Three-dimensional crystal orientation distribution function (200), (211), (110) obtained by the reflection method by performing X-ray diffraction of the plate surface at the 1/4 thickness of each cold-rolled annealed plate. ) To obtain a three-dimensional crystal orientation distribution function by the series expansion method, and from α fiber (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° -55 °), Φ = 25 °- 35 average crystal orientation density I in the range of ° _alpha, and, gamma fiber (φ1 = 0 ° ~60 °, φ2 = 45 °, Φ = 55 °) average seek crystal orientation density I _gamma, and was evaluated. A steel sheet having I _α of 2.0 or more and 4.0 or less and I _γ of 2.0 or more and 10 or less has small in-plane anisotropy of elongation.

  The results are shown in Table 3.

As is apparent from Table 3, steel types D, E, F 1 , N 2 , O, and R , which are steel plates of the present invention, are high strength and high yield ratio steel plates with YP ≧ 300 MPa and YR ≧ 0.79. And it has the structure | tissue which does not contain a martensite phase and a retained austenite phase but consists of ferrite + pearlite + cementite. Since I _α satisfies 2.0 or more and 4.0 or less and I _γ satisfies 2.0 or more and 10 or less, ΔEl is −2.0% to 2.0%, and the in-plane anisotropy of elongation is small. I understand. Further, among the steel plates D and E of the present invention, when the steel bars D and E are compared, the steel bar E with the sheet bar cooled with water and having a finish rolling entry temperature of 1050 ° C. or lower and a finishing temperature of 890 ° C. or lower is more It can be seen that the in-plane anisotropy of elongation is small. In addition, among the steel sheets of the present invention, when comparing the steel types O and R, the steel type R has lower strength and lower ductility, although they are the same component. This is presumably because the hot rolling cooling rate is high and the structure is non-uniform.

  On the other hand, steel types A and J, which are steel plates that deviate from the component range of the present invention, have a low strength with YP of less than 300 MPa. Moreover, the steel types G, K, L, and M, which are steel plates that deviate from the component range of the present invention, lose the texture balance and increase anisotropy. In particular, the steel type G, which is a steel sheet including a martensite phase and a retained austenite phase, not only has a large anisotropy but also has a low YR.

  Moreover, even within the component range of the present invention, the steel grades B, C, P, and Q are out of the texture balance because the manufacturing conditions such as the slab heating temperature and the cooling start time do not satisfy the scope of the present invention. As a result, anisotropy increases.

Claims (7)

In mass%, C: 0.040 to 0.060 %, Si: 0.20% or less, Mn: 0.50 to 0.99%, P: 0.050% or less, S: 0.03% or less, sol. Al: 0.01 to 0.09%, N: 0.005% or less, Nb: 0.015 to 0.040%, with the balance being Fe and unavoidable impurities, martensite phase and residual austenite phase In the texture of the plate surface at the ¼ plate thickness position of the steel plate, α fiber (φ1 = 0 °, φ2 = 45 °, Φ = 0 ° to 55) represented by ODF (crystal orientation distribution function) ), The average crystal orientation density I _{α in} the range of Φ = 25 ° to 35 ° is 2.0 to 4.0, and γ fiber (φ1 = 0 ° to 60 °, φ2 = 45 °, Φ = 55 °) high-strength steel sheet having a small in-plane anisotropy of elongation, wherein the average crystal orientation density I _γ is 2.0 or more and 10 or less. The high-strength steel sheet having a small in-plane anisotropy of elongation according to claim 1, wherein ΔEl expressed by the following formula (1) is −2.0% to 2.0%.
ΔEl = (El ₀ −2El ₄₅ + El ₉₀ ) / 2 (1)
However, El ₀ , El ₄₅ and El ₉₀ are values of elongation at break measured in directions of 0 °, 45 ° and 90 ° with respect to the rolling direction of the steel sheet.   The in-plane anisotropy of elongation according to claim 1 or 2, wherein a yield ratio YR (YR = YP / TS), which is a ratio between the yield strength YP and the tensile strength TS, is 0.79 or more. High strength steel plate.   The high-strength steel sheet having a small in-plane anisotropy of elongation according to any one of claims 1 to 3, wherein the surface has a zinc-based plating film. A method for producing a high-strength steel sheet having a small in-plane anisotropy of elongation according to any one of claims 1 to 4,
After heating the steel slab and holding it for 60 minutes or more in the temperature range of slab heating temperature of 1150 ° C or higher, rough rolling of hot rolling is performed, and then the final pass rolling ratio of the final rolling at the final rolling temperature of 820 to 920 ° C is set. In-plane anisotropic of elongation characterized by performing finish rolling at 15 to 25%, starting with water cooling within 2 seconds after finish rolling, cooling, and then pickling and cold rolling, followed by annealing Of producing high-strength steel sheets with low properties.   6. The in-plane anisotropy of elongation according to claim 5, wherein after the rough rolling of the hot rolling, the finish rolling is performed after the finish rolling entrance temperature is set to 1050 ° C. or less by water cooling. Manufacturing method of high strength steel sheet.   The method for producing a high-strength steel sheet according to claim 5 or 6, wherein the steel sheet after annealing is subjected to galvanizing treatment, wherein the in-plane anisotropy of elongation is small.

Images