JP4840269B2 - High-strength steel sheet and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength steel sheet having a high tensile strength of 950 MPa or more and having good shape freezing property, ductility and stretch flangeability, and a method for producing the same. Furthermore, in a preferred embodiment of the present invention, the present invention relates to a high-strength hot-rolled steel sheet having better surface properties and a method for producing the same.

With the recent trend toward lower fuel consumption in the automobile industry, steel sheets, which are the main structural material, have been required to be thinner and higher in strength from the viewpoint of reducing vehicle weight.
In order to reduce the weight, it is necessary to reduce the gauge of the old member. When a steel plate having a strength of 900 MPa or more is used with respect to a 590 MPa material that is currently relatively applied, the gauge down is facilitated, and the vehicle weight reduction effect becomes remarkable.

  On the other hand, relatively inexpensive hot-rolled steel sheets are applied to members such as automobile wheels and arms that are not directly visible by an outer plate body or a resin coating material. Usually, such members are molded mainly in various deformation modes. Therefore, in order to reduce the vehicle weight, the hot-rolled steel sheet is required to have various properties such as high strength and ductility and stretch flangeability (local ductility). In many cases, steel sheets having superior characteristics when applied to specific members have been proposed and used, but from the viewpoint of improving efficiency by integrating steel types, steel sheets having all these various characteristics at a high level have been proposed. desirable.

  By the way, forming with a press becomes difficult as the material becomes stronger, so low yielding of the steel sheet is desired in order to suppress the shape distortion caused by springback, etc., so-called dual phase in which martensite is dispersed in the ferrite structure Steel (hereinafter also referred to as “DP steel”) is used. However, DP steel is inferior in stretch flangeability due to the difference in hardness between the ferrite phase and martensite phase, and the interface between the two becomes the starting point of microcracks and voids during hole expansion processing. There were various issues in coexistence.

  As means for solving this, for example, Patent Document 1 discloses a low yield ratio / high hole expanding steel sheet having a three-phase structure of ferrite, bainite, and martensite. However, the strength level according to the disclosed technology does not reach 900 MPa.

  On the other hand, Patent Document 2 discloses a steel plate having a low yield ratio and a high hole expansion type with a ferrite / martensite structure in which ε-Cu is finely dispersed. However, hole expansion in a region having a strength of 950 MPa or more is disclosed. The rate is not sufficient, and there is a concern that the cost will increase with the addition of Cu.

  Further, Patent Document 3 proposes a low yield ratio high hole expanding steel sheet having a strength of 980 MPa or more, but there is a problem in chemical conversion treatment because Cr addition is essential to ensure hardenability and strength. .

Both are intricately meshed and cannot be completely removed by normal descaling, and there is a scale (island scale) that becomes Fe ₂ O ₃ and is unevenly distributed on the surface of the steel material during the rolling process. appear. This island scale causes unevenness in the cooling temperature of the steel sheet in the process of cooling the steel sheet after finish rolling, and hinders the uniformity of the mechanical properties in the coil. Moreover, since this island-shaped scale remains as a concave scale ridge (island-shaped scale ridge) after pickling, it tends to be a starting point of fatigue failure. Moreover, in the steel plate which does not perform pickling (a steel plate with a scale), an island-like scale is not removed but it remains on the steel plate surface as a red scale, and surface properties are impaired.

Patent Document 4 discloses a method for improving surface properties by uniformly generating scale on the entire surface of the steel sheet by adding a large amount of Si to the steel sheet, but excessive Si addition is applied to the structural material for automobiles. There is a problem that the corrosion resistance is deteriorated because chemical conversion processability is impaired. On the other hand, when the amount of Si added is low, it is difficult to uniformly generate the scale over the entire surface, so that the surface properties deteriorate, and the strength-workability balance decreases.
JP 9-41078 A JP-A-5-315991 JP 2000-282175 A Japanese Patent Laid-Open No. 3-79718

  An object of the present invention is to provide a high-strength steel sheet having a tensile strength of 950 MPa or more and having good shape freezing property, ductility and stretch flangeability. Furthermore, it is to provide an inexpensive high-strength hot-rolled steel sheet having substantially no island scale on the surface of the steel sheet, and a manufacturing method that can be carried out relatively easily using existing equipment.

  The present inventors first described the mechanical properties such as the shape freezing property evaluated by the yield ratio (hereinafter also referred to as “YR”) and the stretch flangeability confirmed by the hole expansion test. 30% to 80%, and the remaining structure contains no martensite or bainite containing martensite that has been reduced to 10% or less. It has been found that it is compatible with high ductility. Here, bainite contains bainitic ferrite.

  By the way, as already stated, today, a high strength steel plate is required, and specifically, a high strength steel plate of 950 MPa or more is required. However, even in such a high-strength material, it has not been considered that the problem can be realized by setting the above-described ferrite + martensite + bainite structure ratio.

  Therefore, basically, focusing on precipitation strengthening by fine dispersion of metal carbide, which is a means for increasing the strength of the originally soft polygonal ferrite single phase structure while having the above-described structure ratio, the strength of 950 MPa or more Further study was made on the component system and manufacturing conditions that realize both the ductility and stretch flangeability while ensuring the stability.

  As a result, for steel materials containing V, Ti and Nb, 30-80% ferrite is formed during cooling after hot finish rolling and / or in the temperature range of 600 to 780 ° C., and further during subsequent cooling and / or When a structure composed of bainite containing martensite of 0% or more and 10% or less is formed in the winding process, V, Ti and Nb alone or a composite carbonized precipitate is finely dispersed inside and a strength of 950 MPa or more is obtained. It has been found that a steel sheet having an excellent balance between ductility and stretch flangeability can be obtained.

However, V is an element that affects the chemical conversion property of a steel sheet as in the case of Si, and from a practical standpoint, it has also been found that the total amount of Si and V must be limited to a certain value or less.
In addition, in this process, the formation of ferrite is promoted by positively containing V, and the target structure can be obtained relatively easily in the hot rolling line of the existing equipment, thereby stabilizing the characteristics and improving the production efficiency. It has been found possible.

  Furthermore, with regard to the surface properties of the steel sheet, it is possible to extract the descaling effect to the maximum extent relatively easily by performing an appropriate reheating treatment according to the contents of P, Si, and Al in the steel. As a result, it was found that it is possible to obtain a low yield ratio steel sheet having a strength of 950 MPa or more and having an excellent balance between ductility and stretch flangeability and having substantially no island scale on the steel sheet surface.

The present invention is as follows.
(1) By mass%, C: 0.08 to 0.30%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.0%, P: 0.10% or less, S: 0.007% or less, Al: more than 0.1% to 2.0% or less, N: 0.01% or less, further Ti: 0.01-0.5%, Nb: 0.01-0 0.1%, V: 0.05 to 0.5% is contained so as to satisfy the following formulas (1) to (3), the chemical composition is composed of the remaining Fe and impurities, and the area ratio is 30% or more. 80% or less of ferrite and 0% or more and 10% or less of martensite, the balance comprising a steel structure consisting of bainite, tensile strength (TS) of 950 MPa or more, yield ratio (YR) of 0.75 or less, Tensile strength (TS) x total elongation (El) is 15000 MPa% or more and tensile strength (TS) x hole expansion ratio (HER) is 500 A high-strength steel sheet having mechanical properties of 00 MPa ·% or more.

Here, the element symbol in each formula shows content (unit: mass%) of each element in steel.
(2) The high-strength steel plate according to the above (1), wherein the chemical composition is mass% and further contains Cr: 1.0% or less and / or Mo: 0.5% or less.

  (3) The high-strength steel sheet according to the above (1) or (2), wherein the chemical composition is mass% and further contains Mg: 0.02% or less and / or Ca: 0.02% or less.

  (4) A long hot strip in which the steel sheet has a coverage of island scale ridges or red scales of 3% or less in a region excluding a region of 100 mm from both ends in the width direction and a region of 30 m from each end in the longitudinal direction. The high-strength steel plate according to any one of (1) to (3) above, which is a rolled steel plate.

(5) A method for producing a high-strength steel sheet comprising the following steps (A) to (D):
(A) A step of subjecting the steel ingot or steel slab having the chemical composition according to any one of the above (1) to (3) to a temperature of 1200 ° C. or more and then subjecting it to rough hot rolling to obtain a rough bar;
(B) A step of performing descaling after setting the coarse bar to a temperature T _BRT (° C.) that satisfies the following formula (4);
(C) A step of subjecting the coarse bar subjected to descaling to finish hot rolling in a temperature range of Ar ₃ point temperature or more to obtain a hot rolled steel sheet; and (D) finishing hot rolling the hot rolled steel sheet. After cooling to a temperature range of 600 ° C. or higher and 780 ° C. or lower at an average cooling rate of 30 ° C./second or higher, and then retaining in the temperature range for 2 seconds or longer and 20 seconds or shorter, an average cooling rate of 30 ° C./second or higher Step of cooling to 350 ° C. or lower and winding up.

   Here, the element symbol in a formula shows content (unit: mass%) of each element in steel.

  According to the present invention, a high-strength steel sheet having excellent shape freezing property during press molding, excellent ductility and stretch flangeability even at a high strength of 950 MPa or more can be obtained by an inexpensive means. Contributes to the drastic reduction of

  The reason why the chemical composition is limited as described above in the present invention will be described. In this specification, all chemical compositions are indicated by “mass%”. Moreover, the element symbol in each formula which prescribes | regulates a chemical composition shows content (mass%) of each element.

C: 0.08 to 0.30%
If the C content is less than 0.08%, a tensile strength of 950 MPa or more may not be obtained, and if it exceeds 0.30%, weldability is impaired and mechanical properties such as stretch flangeability are caused by the occurrence of pearlite. It may be damaged. Therefore, the C content is set to 0.08 to 0.30%. Preferably it is 0.09 to 0.20%.

V: 0.05-0.5%
V is one of the important elements in the present invention. By containing V, ferrite transformation is promoted and the ductility of the steel sheet is improved, and the steel sheet is strengthened by precipitation of fine carbides combined with C. If the V content is less than 0.05%, ferrite of 30 area% or more may not be obtained in the steel structure, or tensile strength of 950 MPa or more may not be obtained. On the other hand, even if the content exceeds 0.5%, the above effects are saturated and the manufacturing cost is increased. Therefore, the V content is 0.05 to 0.5%. Preferably it is 0.1 to 0.3%. V, as will be described later, affects the chemical conversion treatment similarly to Si, so that the total amount with V is limited to 1.5% or less.

Ti: 0.01 to 0.5%
Ti is one of the important elements in the present invention. By containing Ti, N in the steel is fixed and stretch flangeability is improved. Therefore, the Ti content is 0.01% or more. Ti further has the effect of increasing the strength of the steel sheet by the precipitation of fine carbides combined with C. From such a viewpoint, the Ti content is preferably 0.05% or more. More preferably, it is 0.08% or more. On the other hand, if the Ti content is excessive, coarse carbonitrides may be formed in austenite and the mechanical properties of the steel sheet may be deteriorated. Therefore, the Ti content is 0.5% or less. Preferably it is 0.3% or less.

Nb: 0.01 to 0.1%
Nb is one of the important elements in the present invention. By containing Nb, carbonitrides are formed in the steel, the austenite grains are refined to increase the nucleation sites of ferrite and suppress the final coarsening of the steel structure, and the combined addition of V and Ti Thus, it has the effect of increasing the strength of the steel sheet by forming fine precipitates. Therefore, the Nb content is 0.01% or more. On the other hand, if the Nb content is excessive, ferrite transformation may be delayed and 30% by area or more of ferrite may not be obtained in the steel structure. Therefore, the Nb content is 0.1% or less. Preferably it is 0.05% or less.

  The above C, V, Ti, and Nb are contained so as to satisfy the following formula (1) within the range determined for each element, and an excellent shape is obtained after securing the tensile strength of 950 MPa or more as intended by the present invention. Freezing property, stretch flangeability, and ductility can be effectively expressed.

 That is, if the value of the middle side of the formula (2) is less than 0.9, the increase in YR, which seems to be due to the decrease in the hardness of the second phase, becomes remarkable, and the ductility deteriorates. The reason for this is not necessarily clear, but if the value falls below the lower limit of equation (2), C is trapped by Ti, V, and Nb during transformation, and C enrichment to untransformed austenite is suppressed. It is considered that the hardness of the second phase is not sufficiently high. On the other hand, when the value of the middle side exceeds 2.0, the hole expandability is lowered due to the formation of coarse cementite. In order to balance YR, ductility, and hole expansion ratio more preferentially, it is preferable that the left side of equation (1) is 1.0 and the right side of equation (1) is 1.5.

Mn: 0.2 to 2.0%
Mn is an element effective for securing the strength, and the content is made 0.2% or more in order to obtain the desired strength. Mn further lowers the transformation temperature from austenite to ferrite, so that the finishing temperature can be lowered and the ferrite grain size can be expected to be reduced. Therefore, Mn is preferably contained from this viewpoint. On the other hand, if the Mn content is excessive, ferrite transformation after finish rolling is delayed, and the production stability is impaired. For this reason, Mn content shall be 2.0% or less. From the viewpoint of the balance of strength, YR, ductility, and hole expansion rate, it is preferably 0.8 to 1.5%.

Si: 0.05 to 1.5%
Si is known as a useful solid solution strengthening element that has a relatively small ductility deterioration due to an increase in strength, and further has an action of promoting ferrite transformation and suppressing the coarsening of cementite in steel. In order to acquire the effect by such an effect | action, Si content shall be 0.05% or more. From the viewpoint of strength / ductility balance, the Si content is preferably 0.2% or more. On the other hand, when the Si content is excessive, chemical conversion treatment with phosphoric acid to zinc phosphate applied to the product surface is impaired. For this reason, Si content shall be 1.5% or less. From the viewpoint of balance between strength and ductility, it is more preferably 0.1 to 1.0%.

  Further, since Si affects the chemical conversion treatment similarly to V, the total content of Si + V satisfies the following formula (2) from the viewpoint of chemical conversion treatment.

 When the value of the left side of the formula (2) is 1.0 or less, the chemical conversion property is remarkably improved. Therefore, the right side of the formula (2) is preferably set to 1.0.

Al: more than 0.10% and not more than 2.0% Al is one of the important elements in the present invention. By containing Al, ferrite transformation is promoted, and it becomes easy to obtain a sufficient ferrite phase even at a low temperature at which deterioration of stretch flangeability due to coarsening of precipitates hardly occurs. For this reason, Al content is made more than 0.10%. Preferably it is 0.12% or more. On the other hand, if the Al content is excessive, the transformation temperature from austenite to ferrite is remarkably increased, and the finishing temperature is increased to deteriorate the production stability. Therefore, the Al content is set to 2.0% or less. Preferably it is 0.5% or less.

P: 0.10% or less P is generally contained as an impurity, but may contribute to improving the strength. However, if the P content is excessive, P segregates at the grain boundaries to cause embrittlement, so the P content is 0.10% or less.

Here, Si contributes to the formation of FeO—Fe ₂ SiO ₄ and induces island scales during heating of the steel ingot or steel slab to be subjected to hot rolling. On the other hand, Al and P have the effect of reducing the melting point of Fe ₂ SiO ₄ and improving the scale peelability to facilitate descaling, and are therefore effective in suppressing the formation of island scales on the steel sheet surface. is there. Therefore, considering the descaling temperature during actual operation, the contents of Si, Al and P are within the ranges described above.

It is contained so as to satisfy the relational expression. More preferably

It is.
By satisfying the above relational expression, it is possible to obtain a steel sheet having substantially no island scale. Here, “substantially no island scale” means that the coverage of the island scale is 3% or less in the portion excluding the area of 100 mm from both ends in the width direction and 30 m from each end in the longitudinal direction. I point to that. The reason why the respective end portions in the width direction and the longitudinal direction are excluded in this way is that such regions are usually cut and removed in the final product before shipment. In other words, the island-shaped scale coverage as described above means an average value on the surface of the steel plate as a product, so in practice it is not necessary to measure in all the surface areas of the product, so-called average It is sufficient if the above-mentioned scale coverage as a value can be estimated.

Cr: 1.0% or less Cr may be contained because it has an effect of suppressing the formation of pearlite which improves hardenability and impairs stretch flangeability. In order to surely obtain the effect by the above action, the Cr content is preferably 0.05% or more. On the other hand, if the Cr content exceeds 1.0%, ferrite transformation is suppressed and production stability is impaired, so the Cr content is set to 1.0% or less. Further, since Cr has an effect because it lowers the chemical conversion treatment with phosphoric acid to zinc phosphate, the Cr content is preferably 0.8% or less.

Mo: 0.5% or less Mo may be contained because it has an effect of suppressing the formation of pearlite which enhances hardenability and impairs stretch flangeability and an effect of suppressing coarsening of metal carbonitride due to composite addition. In order to surely obtain the effect by the above action, the Mo content is preferably 0.05% or more. On the other hand, when the Mo content exceeds 0.5%, chemical conversion treatment with phosphoric acid to zinc phosphate is impaired, so the Mo content is set to 0.5% or less. Preferably it is 0.3% or less.

Mg: 0.02% or less Mg forms MgO and has the effect of improving the hole expanding property to suppress the formation of coarse TiN, which causes the mechanical properties to be impaired and to finely disperse, and the same effect Since the addition efficiency is higher than Ca having Ca, it may be contained. In order to surely obtain the effect by the above action, the Mg content is preferably 0.001% or more. On the other hand, if the Mg content exceeds 0.02%, the effect is saturated, so the Mg content is set to 0.02% or less. A more preferable Mg content is 0.0010 to 0.0050%.

Ca: 0.02% or less Ca forms CaO, and suppresses the formation of coarse TiN that causes mechanical properties to be impaired, so that fine dispersion is achieved. . In order to surely obtain the effect by the above action, the Ca content is preferably 0.001% or more. On the other hand, if the Ca content exceeds 0.02%, the effect is saturated, so the upper limit of the Ca content is set to 0.02% or less.

Examples of impurities in steel include S, N, and Sn. For example, the content of S and N is preferably regulated as follows.
S: 0.007% or less The content is preferably 0.007% or less in order to combine with Mn or the like to form coarse sulfide inclusions and significantly impair the workability. More preferably, it is 0.003% or less.

N: 0.01% or less An impurity element that impairs workability such as stretch flangeability when contained excessively, and its content is preferably 0.01% or less. More preferably, it is 0.006% or less.

(Organization)
In the steel sheet according to the present invention, ferrite is composed of bainite containing martensite in an area ratio of 30% or more and 80% or less of the entire structure and the balance is 0% or more and 10% or less.

When ferrite is less than 30% of the entire structure, ductility is remarkably impaired, and when it exceeds 80%, YR is increased and shape freezeability is impaired. In order to balance the strength, YR, ductility, and stretch flangeability at a higher level, it is preferable to make ferrite 50 to 70% of the entire structure.
The reason why the structure of the portion excluding ferrite is bainite containing martensite of 0% or more and 10% or less is that when a structure other than these, for example, a pearlite structure is included, stretch flangeability is significantly impaired. . On the other hand, if the martensite exceeds 10%, the stretch flangeability deteriorates significantly. Therefore, martensite may not be contained.

(Production conditions)
First, a steel ingot or steel slab having the above chemical composition is brought to a temperature of 1200 ° C. or higher and then subjected to rough hot rolling. This is because a coarse carbonitride that inhibits stretch flangeability is completely dissolved to avoid deterioration of mechanical properties such as strength reduction and stretch flangeability. In addition, when the steel ingot obtained by continuous casting and the steel slab after the partial rolling are subjected to mechanical compression processing in the width direction or the plate thickness direction, at least the center of the steel ingot or the steel slab in the plate thickness direction If the temperature over 1/4 is 1200 ° C. or higher, heating is not necessary.

  The upper limit of the temperature of the steel ingot or steel slab when subjected to rough hot rolling is not particularly limited, but the temperature is 1400 ° C. or less in consideration of durability of the heat resistant wall in the furnace and a decrease in yield due to scale loss. It is preferable that

  Next, depending on the content of Si, Al, P in the steel, the coarse bar obtained by rough hot rolling

Descaling is performed after setting the temperature to be equal to or higher than T _BRT (° C.). A FeO—Fe ₂ SiO ₄ eutectic compound layer is formed during the high-temperature heating of the steel containing Si, and the subsequent cooling causes island scales. However, the rough bar has a temperature that satisfies the above T _BRT (° C.). By doing so, FeO and Fe ₂ SiO ₄ are in a semi-molten state, and this peeling and removal (descaling) becomes easy.

There is no particular limitation on the means for setting the coarse bar to the above T _BRT (° C.), but industrially, for example, an induction installed between a roll stand group for rough hot rolling and a roll stand group for finishing hot rolling Use of a heating device is suitable. When the temperature of the rough bar after rough hot rolling can be set to T _BRT (° C.) by increasing the temperature of the steel ingot or steel slab, heating may not be performed. There are no particular limitations on the means / equipment for descaling, but in practice, injection of high-pressure water by spraying is appropriate.

The finish hot rolling is performed in the austenite temperature range, and the finish hot rolling finish temperature may be not less than the Ar ₃ point temperature. The finish hot rolling finish temperature is more preferably as close as possible to the Ar ₃ point temperature. This is because the effect of accelerating ferrite transformation due to processing strain introduced during rolling is enhanced. The Ar ₃ point of the steel defined in the present invention is approximately 750 ° C. to 980 ° C.

  After finishing hot rolling, after cooling to 600-780 ° C. at an average cooling rate of 30 ° C./second or more, and then retaining at that temperature for 5 seconds to 20 seconds, the average of 30 ° C./second or more Cool down to 350 ° C. or lower at a cooling rate and wind up. In the following description, the cooling and cooling rate at this time are also referred to as first, second cooling, and first and second cooling rates, respectively, for convenience.

  If the first cooling rate is lower than 30 ° C./second, the strength becomes insufficient due to the excessive formation of the ferrite phase, or the shape freezeability is impaired due to the high YR, and further, stretch flangeability due to pearlite precipitation during cooling. Or drop. For this reason, a 1st cooling rate shall be 30 degrees C / sec or more.

  When the cooling stop temperature of the first cooling is higher than 780 ° C., the metal carbide precipitated in the ferrite is rapidly coarsened, and the balance between strength and stretch flangeability is deteriorated. On the other hand, if the cooling stop temperature of the first cooling is less than 600 ° C., the balance between strength, ductility and stretch flangeability deteriorates due to insufficient formation of ferrite. For this reason, the cooling stop temperature of 1st cooling shall be 600-780 degreeC. It is more preferable to set it as 630-730 degreeC from a viewpoint of a balance of intensity | strength, YR, ductility, and stretch flangeability.

  If the residence time at the cooling stop temperature of the first cooling is less than 2 seconds, a sufficient amount of ferrite cannot be obtained, and ductility and stretch flangeability deteriorate. On the other hand, if the residence time at the cooling stop temperature of the first cooling exceeds 20 seconds, the strength is reduced due to excessive ferrite phase growth and coarse growth of precipitates. For this reason, the residence time at the cooling stop temperature of the first cooling is 2 seconds or more and 20 seconds or less. Preferably, it is 5 seconds or more and 15 seconds or less.

  When the second cooling rate is lower than 30 ° C./second, the ratio of the second phase is decreased due to the ferrite transformation during cooling, the YR is increased and the shape freezing property is impaired, or the stretch flangeability is deteriorated by the formation of pearlite. Damaged. For this reason, the second speed is set to 30 ° C./second or more. Preferably, it is 50 ° C./second or more.

  In the subsequent winding process, if the temperature at which the winding is performed exceeds 350 ° C., the YR increases and the shape freezing property is impaired. Therefore, the coiling temperature is set to 350 ° C. or lower. If the coiling temperature is further reduced, martensite can be easily formed and YR can be further lowered. Therefore, the coiling temperature is preferably 300 ° C. or lower.

  Thus, according to the present invention, the tensile strength (TS) is 950 MPa or more, the yield ratio (YR) is 0.75 or less, the tensile strength (TS) × total elongation (El) is 15000 MPa% or more, and the tensile strength (TS) × hole. A high-strength steel sheet having mechanical properties with a spreading ratio (HER) of 50000 MPa ·% or more is obtained.

  Of steels having the chemical composition shown in Table 1, steel types containing components within the scope of the invention and N and Q steels outside the scope of the present invention were each made into slabs by continuous casting, and then a steel slab was collected from the vicinity of the slab center. Then, hot forging was performed to a thickness of 30 mm. The obtained steel slab was reheated, and the steel slab was hot-rolled with a small test tandem mill to obtain a hot-rolled steel plate having a thickness of 2.6 mm. During the hot rolling, various investigations were made on the heating temperature, finishing temperature, first cooling rate, first cooling stop temperature, residence time at that temperature, second cooling rate, and coiling temperature of the steel slab.

  For cooling, a water spray device is used, and for the winding process, a winding simulation is performed using a slow cooling furnace that simulates the heat history after winding the hot-rolled coil found in an actual production line. Obtained.

About the mechanical property of the obtained steel plate, the following methods were investigated about tensile strength and stretch flangeability.
Tensile strength is measured with a JIS No. 5 test piece. For stretch flangeability, a 100 mm square test piece is sampled, a punched hole with a clearance of 15% and a diameter of 10 mm is formed in the center, and a conical punch with a tip angle of 60 ° is used. The hole was expanded and evaluated by the limiting hole expansion rate calculated from the hole diameter when the crack generated at the edge of the hole penetrated in the plate thickness direction. Further, the section in the rolling direction was treated with a repeller corrosion, and the structure was observed with a microscope.

 Table 2 shows the details of the steel types and hot rolling conditions in the implementation, and Table 3 shows the properties of the steel sheets obtained thereby. A steel sheet to which the steel grade and hot rolling conditions within the ranges specified in the present invention are applied satisfies TS ≧ 950 MPa, YR ≦ 0.75, and satisfies the ranges of strength × ductility and strength × hole expansion ratio values defined by the present invention. In addition, a steel sheet having a high balance of high strength, low yield ratio, high ductility, high hole expansion ratio, that is, excellent stretch flangeability is obtained.

  On the other hand, the steels using the steel types outside the scope of the present invention shown in Test Nos. 10 and 17 show a significant decrease in the value of strength × hole expansion rate, which seems to be due to the formation of coarse inclusions and carbonitrides. Further, in the sample No. 11 in which the heating temperature of the steel slab is outside the range of the present invention, carbides are coarsely precipitated and the amount of ferrite is excessive, which does not satisfy the strength specification defined in the present invention. Further, in trial numbers 12 and 13 in which hot rolling conditions outside the scope of the present invention were applied for the cooling stop temperature and the residence time at that temperature, the structure defined by the present invention could not be obtained, and the mechanical properties defined in the present invention were Not achieved.

  Further, for the trial numbers 14 and 15 in which the cooling rate was outside the range of the present invention, as in the case of the trial number 14, when the first cooling rate was outside the range of the present invention, the coarsening of precipitates and excessive ferrite phase were observed. When the specified cooling strength is not achieved and the value of strength x elongation is decreased, and the value of strength x hole expansion rate is decreased, and the second cooling rate is outside the range of the present invention as shown in test number 15. The increase in YR, which is expected because the second phase hardness is not sufficiently obtained, is observed, and the characteristics defined in the present invention cannot be obtained. Further, in the test No. 16 in which the steel containing the element for improving the hardenability was retained for a long time at a relatively low temperature outside the scope of the present invention and then quenched to room temperature, the martensite was excessively more than the provisions of the present invention. As a result, the value of strength × hole expansion rate decreases.

  Of the steels having the chemical composition shown in Table 1, steel types A, F, I within the component range of the present invention and steel types L, M, K, O, P outside the range of the present invention were each made into slabs by continuous casting. Later, steel sheets were produced on an actual production line. After slab heating at a temperature higher than 1200 ° C., the slab extracted from the heating furnace was roughly rolled by a tandem mill to obtain a coarse bar. 1. The rough bar is reheated to a predetermined temperature (rough bar temperature) by a heating device installed between the rough rolling mill group and the finish rolling mill group, and then finish hot rolling is performed. A hot rolled steel sheet having a thickness of 6 mm and a width of 1000 mm was obtained. The steel sheet after finish hot rolling is water-cooled at a cooling rate within the range of the present invention, set to a predetermined temperature (cooling stop temperature), immediately retained at this temperature for a predetermined time, and then again within the range of the present invention. The steel sheet coil was obtained by winding with a down coiler at a cooling rate at a temperature within the range of the present invention.

  About the mechanical property of the obtained steel plate, the following methods were investigated about tensile strength and stretch flangeability. Tensile strength is obtained from JIS No. 5 test piece, and stretch flangeability is obtained by taking a 100 mm square test piece, drilling a punched hole with a clearance of 15% and a diameter of 10 mm at the center, and using a conical punch with a tip angle of 60 °. The hole was expanded and evaluated by the limiting hole expansion rate calculated from the hole diameter when the crack generated at the edge of the hole penetrated in the plate thickness direction. Further, the section in the rolling direction was treated with a repeller corrosion, and the structure was observed with a microscope.

  Furthermore, the island scale scales and the red scale coverage were determined by the red scale coverage. Since the island scale ridge is a depressed indentation ridge that is formed by removing the red scale after the steel plate pickling, the position and area ratio of the red scale of the steel plate before pickling and the island scale basin after pickling are almost the same, Therefore, if the red scale area ratio before pickling is investigated, the area ratio of the island scale ridge after pickling can be obtained. Therefore, the red scale coverage of the surface before pickling of the steel sheet manufactured by the actual hot rolling line. The coverage of island scale ridges and red scales was obtained.

  In the obtained hot-rolled steel sheet before pickling, the surface of the steel sheet consisting of a portion excluding the region of 100 mm from both ends in the rolling width direction and the region excluding 30 m from the start end and the end end in the longitudinal direction is coated with red scale. When the rate was 3% or less, it was judged as good (surface property ○ in Table 4), and when it exceeded 3%, it was judged as poor (surface property x in Table 4). The survey was conducted on the rolling front and back surfaces, and the surface properties were judged on the surface with the higher red scale coverage.

Further, a 50 mm × 70 mm size sample was taken from the pickled steel sheet, and subjected to chemical conversion treatment with VALBOND WL35 chemical conversion solution manufactured by Nippon Parkerizing Co., Ltd. The amount of chemical crystals deposited on the surface of the sample steel sheet immersed in the chemical solution for 120 seconds at room temperature was investigated, and the coating form of the chemical crystals was observed with a scanning electron microscope. The amount of deposited crystals was 3.0 g / m ² or more. When the surface has no gap in the chemical conversion crystal coating (Chemical conversion treatment ○ in Table 4), the amount of adhesion is less than 3.0 g / m ² or there is a portion that is not covered with the chemical conversion crystal on the sample steel plate surface The chemical conversion processability was evaluated as bad (chemical conversion processability x in Table 4).

  According to Table 4, samples 18 to 21 produced within the scope of the present invention satisfy TS ≧ 950 MPa, YR ≦ 0.75, and satisfy the range of strength × ductility and strength × hole expansion ratio values defined by the present invention, Steels that balance high strength, low yield, and high hole expansion ratio have been obtained.

  However, the trial numbers 22 and 23 manufactured with the steel types outside the scope of the present invention do not satisfy the specified strength or have the specified structure despite the hot rolling conditions within the scope of the present invention. The balance between strength, ductility and hole expandability is low. Furthermore, sample 24 produced under hot rolling conditions outside the scope of the present invention cannot obtain the structure defined by the book, and the balance between strength, ductility and hole expansion ratio is poor, as well as the contents of V and Si. Since the steel type K outside the scope of the present invention is used, the chemical conversion property of this steel sheet is unsatisfactory.

On the other hand, the test numbers 25 to 27 carried out at a temperature equal to or lower than the T _BRT that sets the rough bar heating temperature within the range of the present invention remain on the steel plate surface with a red scale exceeding 3%, and the surface properties are inferior. Nos. 18 to 21 produced within the range of No. 1 have substantially no red scale on the steel sheet surface, no problem with chemical conversion treatment, have good surface properties, and realize the mechanical properties defined in the present invention. .

Claims (5)

In mass%, C: 0.08 to 0.30%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.0%, P: 0.10% or less, S: 0.007 %: Al: Over 0.10% and 2.0% or less, N: 0.01% or less, Ti: 0.01-0.5%, Nb: 0.01-0.1%, V: 0.05 to 0.5% is contained so as to satisfy the following formulas (1) to (3), the chemical composition is composed of the remaining Fe and impurities, and the area ratio is 30% to 80%. It has a steel structure containing ferrite and 0% to 10% martensite with the balance being bainite, the tensile strength (TS) is 950 MPa or more, the yield ratio (YR) is 0.75 or less, and the tensile strength (TS ) X total elongation (El) is 15000 MPa% or more and tensile strength (TS) x hole expansion rate (HER) is 50,000 MP High-strength steel sheet, characterized in that it comprises the mechanical properties is -% or more.

Here, the element symbol in each formula shows content (unit: mass%) of each element in steel. The high-strength steel sheet according to claim 1, wherein the chemical composition further includes, by mass%, Cr: 1.0% or less and / or Mo: 0.5% or less. The high-strength steel sheet according to claim 1 or 2, wherein the chemical composition further includes, by mass%, Mg: 0.02% or less and / or Ca: 0.02% or less. The steel sheet is a long hot-rolled steel sheet in which the coverage of island scale ridges or red scales is 3% or less in a region excluding a region of 100 mm from both ends in the width direction and a region of 30 m from both ends in the longitudinal direction. The high-strength steel sheet according to any one of claims 1 to 3, wherein the high-strength steel sheet is provided. A method for producing a high-strength steel sheet comprising the following steps (A) to (D):
(A) A step of subjecting the steel ingot or steel slab having the chemical composition according to any one of claims 1 to 3 to a temperature of 1200 ° C. or higher and then subjecting the steel ingot or steel slab to a rough bar;
(B) A step of performing descaling after setting the coarse bar to a temperature T _BRT (° C.) that satisfies the following formula (4);
(C) A step of subjecting the coarse bar subjected to descaling to finish hot rolling in a temperature range of Ar ₃ point temperature or more to obtain a hot rolled steel sheet; and (D) finishing hot rolling the hot rolled steel sheet. After cooling to a temperature range of 600 ° C. or higher and 780 ° C. or lower at an average cooling rate of 30 ° C./second or higher, and then retaining in the temperature range for 2 seconds or longer and 20 seconds or shorter, an average cooling rate of 30 ° C./second or higher Step of cooling to 350 ° C. or lower and winding up.

Here, the element symbol in a formula shows content (unit: mass%) of each element in steel.