JP5929556B2 - Method for producing continuous cast slab and method for producing high-strength cold-rolled steel sheet - Google Patents

Description

  The present invention relates to a method for producing a continuous cast slab that is a material of a high-strength cold-rolled steel plate used for inner plate parts such as structural members and reinforcing members of automobiles, and a method for producing a high-strength cold-rolled steel plate using the slab. Is. In particular, the present invention relates to a method for producing a high-strength cold-rolled steel sheet having a tensile strength of 1180 MPa or more and a breaking elongation of 9.0% or more.

  From the viewpoint of weight reduction and collision safety of automobiles, application of high-strength steel sheets is being promoted for various structural members and reinforcing members of automobiles. When this high-strength steel sheet is applied, press forming and welding become difficult, and therefore, material development for solving such practical problems is underway. Further, in the manufacturing process of steel sheets, there are problems such as reduction of fluctuation range of product material and improvement of surface quality, and improvement of manufacturing technology is being promoted. Above all, the surface quality is particularly improved. Since a high strength steel sheet contains many alloy components for increasing the strength, slab surface cracks are likely to occur in the continuous casting process. Occurrence of surface cracks makes it easy for the oxide scale generated in the casting stage and the hot rolling stage to be embedded in the vicinity of the surface layer of the steel sheet. When a cold-rolled steel sheet is manufactured with an oxide scale embedded, the surface appearance is significantly deteriorated. If there is a problem with the surface properties, not only will it be difficult to apply to press products, the yield will decrease, but it will also cause damage to the mold during press molding, and corrosion resistance due to deterioration of the conversion coating and paint adhesion. Therefore, good surface quality is desired.

Various slab manufacturing techniques have been disclosed so far for improving surface properties of cold-rolled steel sheets due to surface cracks of slabs and surface cracks of slabs. For example, in Patent Document 1, when continuous casting is performed, the temperature of the slab corner portion is measured with a temperature detector installed after the exit side of the secondary cooling zone, and a portion lower than 675 ° C. is detected. The amount of cooling water in the secondary cooling zone is corrected during casting so that the surface temperature of the corner portion becomes 675 ° C. or higher, and the portion where the surface temperature of the slab corner portion is less than 675 ° C. is adjusted to 675 ° C. or higher using a heating device. A casting method for heating to become is disclosed. Patent Document 2 predicts or measures the surface temperature distribution of the slab long side surface and the slab short side surface when bending strain is applied to the slab in the continuous casting machine, and the surface temperature is determined to be an embrittlement region from the slab surface temperature distribution. Casting that continues to cool until the slab part where the surface temperature is in the brittle temperature range is transformed into either a ferrite single-phase structure or a pearlite structure in the secondary cooling zone immediately below the mold. A method is disclosed. Patent Document 3 includes mass%, C: 0.03 to 0.10%, Si: 1.0% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.02% or less, Al: 0.2% or less, N: 0.006% or less, (14/27) × (% Al /% N) is 50 or less, the balance is Fe and The molten steel composed of inevitable impurities is expressed by the formula 1.5 ≦ Vc ≦ 4.0−0.68 × log [(14/27) × (% Al /% N)] defined by the Al content and the N content. The specific water amount that satisfies the formula 1.0 ≦ Q ≦ 2.5 + Vc / 1.5 defined by the casting speed (Q: liter / kg) A casting method in which the secondary cooling zone is spray-cooled with a slab manufactured by the above manufacturing method, and a tensile strength of 592 to 820 MPa. Method for producing a high-strength steel sheet is disclosed. Furthermore, Patent Document 4, the mold is vibrated in a sine waveform or biased sinusoidal waveform Upon continuous casting of steel, and NSL negative strip length of the mold oscillation, the negative strip time t _n, a positive strip time t _p Then, the characteristic value Z _defined by the characteristic value Z = NSL ^A × (t _p / 2 + t _n ) ^0.5 by NSL, t _n and t _p is empirically not problematic in terms of the surface quality of the slab. A casting method in which the vibration conditions of the mold are set so as to be equal to or less than the value is disclosed.

JP 2008-183608 A JP 2010-253481 A JP 2007-216247 A JP 2010-120044 A

However, the above prior art has the following problems.
In the technique disclosed in Patent Document 1, the slab of a steel slab having a component composition in which C equivalent (= (C (%) + Si (%) / 24 + Mn (%) / 7.4) × 100) is 39 or less. Although corner cracking has been improved, slab corner cracking of a steel slab having a C equivalent outside the above range has not been studied.
In the technique disclosed in Patent Document 2, when applied to a steel slab having a high Mn content, the ferrite transformation and the pearlite transformation are slower compared to the steel in Patent Document 2, so that the steel slab must be removed before bending straightening. It is extremely difficult to control the ferrite single phase structure or pearlite structure.
In the technique disclosed in Patent Document 3, since the management range of the slab corner temperature is not specifically defined, it cannot be said that the surface properties of the steel slab are sufficiently improved. Furthermore, in Patent Document 3, a high-strength cold-rolled steel sheet that satisfies both a tensile strength required for a high-strength cold-rolled steel sheet of 1180 MPa or higher and a ductility of 9.0% or higher in breaking elongation has not been obtained.
The technique disclosed in Patent Document 4 is characterized by improving the surface properties of the continuous casting slab by reducing the oscillation depth of the steel slab by controlling the vibration method of the continuous casting mold. The surface property of the steel slab having the component composition having the amount of Mn is not improved. Furthermore, in patent document 4, content of the component composition which contributes to the tensile strength of final products, such as C, Si, and Mn, is small, and the tensile strength required as a high-strength cold-rolled steel sheet is 1180 MPa or more and the elongation at break is 9. It is difficult to stably produce a high-strength cold-rolled steel sheet that satisfies both the ductility of 0% or more.

  As described above, in the above-described conventional technology, a continuous cast slab excellent in surface properties within a range of component compositions having a high amount of C, Si and Mn, a continuous cast slab for high-strength cold-rolled steel sheets, and a tensile strength of 1180 MPa. A high-strength cold-rolled steel sheet with excellent surface properties that satisfies both the above strength and elongation at break of 9.0% or more has not been obtained.

  The present invention has been made in view of the above, and even a steel having a high composition of C, Si, and Mn, which contributes to both high strength and high ductility, has high strength cooling without causing surface cracks. It aims at providing the method of manufacturing the continuous cast slab which is the raw material of a rolled steel plate, and the method of manufacturing the high strength cold-rolled steel plate excellent in surface property using this continuous cast slab.

The inventors of the present invention have made extensive studies on the cause of surface cracking of a continuously cast slab (hereinafter sometimes simply referred to as slab). As a result, the steel composition was optimized, the mold frequency was 80 to 200 cycles / minute, the casting speed was 1.30 to 1.70 m / minute, and the slab corner temperature on the bending band exit side was 720. It was found that surface cracking of the continuously cast slab can be suppressed by casting into the slab under the conditions of ˜780 ° C. and the slab corner temperature on the straightening banding side of 720 to 780 ° C. In addition, by appropriately controlling other conditions and using it as a continuous cast slab for high-strength cold-rolled steel sheets applied to automotive interior plate parts, etc., stable production of high-strength cold-rolled steel sheets with good surface properties is possible. Invented the technology to do.
The gist of the present invention is as follows.
[1] By mass%, C: 0.10 to 0.20%, Si: 0.6 to 2.0%, Mn: 1.8 to 3.0%, P: 0.02% or less, S: A molten steel containing 0.003% or less, Al: 0.01 to 0.10%, N: 0.006% or less, with the balance being Fe and unavoidable impurities, with a mold frequency of 80 to 200 cycles / minute, Continuous casting slab characterized by casting into a slab at a casting speed of 1.30 to 1.70 m / min, a slab corner temperature of 720 to 780 ° C. on the bending strip side and a slab corner temperature of 720 to 780 ° C. on the straight strip side Manufacturing method.
[2] The method for producing a continuous cast slab according to [1], further comprising 0.01 to 0.10% by mass of any one or more of Nb, Ti, and V.
[3] The method for producing a continuous cast slab according to [1] or [2], further comprising 0.01 to 0.30% by mass of any one or more of Mo or Cr. .
[4] The continuous cast slab according to any one of [1] to [3], further comprising 0.01 to 0.30% by mass of any one or more of Cu or Ni. Manufacturing method.
[5] The method for producing a continuous cast slab according to any one of [1] to [4], further comprising 0.0003 to 0.0020% B in mass%.
[6] The method for producing a continuous cast slab according to any one of [1] to [5], wherein the slab is a continuous cast slab for cold rolled steel sheets.
[7] The slab manufactured by the method according to [6] is hot-rolled at a heating temperature of 1150 to 1300 ° C, a finish rolling temperature of 820 to 920 ° C, and a winding temperature of 520 to 600 ° C, and then a cold rolling rate of 30 Cold rolling at ˜65%, followed by soaking for 1 to 10 minutes at an annealing temperature of 750 to 860 ° C., followed by primary cooling to 600 to 700 ° C. at an average cooling rate of 10 ° C./s or less, and an average cooling rate of 500 ° C. / A method for producing a high-strength cold-rolled steel sheet, characterized in that after secondary cooling to 50 ° C. or less at s or more, reheating to 100 to 450 ° C. and soaking for 1 to 15 minutes.
The high-strength cold-rolled steel sheet in the present invention refers to a cold-rolled steel sheet having a tensile strength of 1180 MPa or more and a breaking elongation of 9.0% or more.
Hereinafter, the mass% may be simply referred to as%.

  According to the present invention, by appropriately controlling the component composition of the steel, the frequency of the mold in the continuous casting process, the casting speed, the slab corner temperature on the bending banding side, and the slab corner temperature on the correction banding side, A continuous cast slab can be stably produced with high productivity and without generating surface cracks. As a result, it is possible to stably produce a high-strength steel sheet having excellent surface quality required for materials such as automobile interior plate parts, and the utility value of the present invention in the automobile and steel industry is extremely large.

  In the present invention, molten steel having a specific composition is obtained by correcting the mold frequency of 80 to 200 cycles / minute, casting speed of 1.30 to 1.70 m / minute, slab corner temperature of 720 to 780 ° C. on the bending band side, and straightening. The slab is cast at a slab corner temperature of 720 to 780 ° C. on the banding side. Hereinafter, the reasons for limiting the composition of the present invention and the production conditions of the continuously cast slab will be described.

(1) Component composition C: 0.10 to 0.20 mass%
C is effective for strengthening steel. When the amount of C is less than 0.10% by mass, it is difficult to achieve both stable tensile strength of 1180 MPa or more and elongation at break of 9.0% or more. On the other hand, if it exceeds 0.20% by mass, the thickness of the solidified shell of the molten steel tends to be non-uniform during continuous casting, and the slab surface cracks remarkably due to the concentration of thermal stress accompanying solidification shrinkage on the thinned portion of the solidified shell. Become. When such a slab surface crack occurs, the oxide scale generated in the casting stage and the hot rolling stage is likely to bite into the surface layer portion by hot rolling, resulting in a surface defect of the final product. For this reason, C amount shall be 0.10-0.20 mass%. Moreover, Preferably it is 0.12-0.16 mass%.

Si: 0.6-2.0 mass%
Si is effective for strengthening steel. In a high-strength steel sheet having a tensile strength of 1180 MPa or more, a tensile strength of 1180 MPa or more cannot be stably obtained in a high-strength cold-rolled steel sheet if the Si amount is less than 0.6 mass%. Moreover, when it exceeds 2.0 mass%, the production amount of Si oxide will increase at the time of forging, and a surface crack will generate | occur | produce at the time of bending correction of a slab. In addition, the occurrence of red scale due to the formation of firelite becomes noticeable during hot rolling, and a Si oxide layer is formed on the surface layer during continuous annealing. As a result, the surface properties of the final product deteriorate. For this reason, Si amount shall be 0.6-2.0 mass%. Moreover, Preferably it is 0.6-1.5 mass%.

Mn: 1.8 to 3.0% by mass
Mn is an element effective for strengthening steel sheets. When the amount of Mn is less than 1.8% by mass, a tensile strength of 1180 MPa or more cannot be stably obtained in a high-strength cold-rolled steel sheet. On the other hand, if it exceeds 3.0% by mass, the slab cracking sensitivity increases due to segregation during casting. Moreover, the peelability of the oxide scale formed on the surface of the steel sheet during hot rolling is remarkably reduced, and the occurrence rate of scale surface defects is increased. Further, the decrease in elongation becomes significant due to segregation of Mn during casting. For this reason, the amount of Mn shall be 1.8-3.0 mass%. Moreover, Preferably it is 2.0-2.5 mass%.

P: 0.02% by mass or less P is an element effective for strengthening a steel sheet, and is an element that acts on scale peelability during hot rolling. When the P content exceeds 0.02 mass%, surface cracks occur in the slab due to grain boundary embrittlement accompanying P segregation to the austenite grain boundaries during casting. Furthermore, the spot weldability required for the steel sheet for automobiles is also deteriorated. For this reason, the amount of P shall be 0.02 mass% or less. Moreover, 0.005 mass% or more is preferable from a viewpoint of scale peelability, and 0.015 mass% or less is preferable from a viewpoint of slab surface crack and spot weldability.

S: 0.003 mass% or less When the amount of S exceeds 0.003 mass%, hot brittleness is caused and the generation of scale surface defects is promoted. For this reason, S amount shall be 0.003 mass% or less. Further, from the viewpoint of peelability of the scale, S is preferably 0.001% by mass or more, and more preferably 0.001 to 0.002% by mass.

Al: 0.01-0.10 mass%
Al forms oxides to reduce the amount of Si-based oxides produced, and is liberated into the slag during casting, thus improving the surface properties. If the amount of Al is less than 0.01% by mass, a significant effect cannot be obtained. When the Al content exceeds 0.10% by mass, the nitride formed by the combination of Al and N precipitates finely on the austenite grain boundaries during casting and causes grain boundary embrittlement. Causes surface cracks at the corners of the slab. Oxide scale formed during casting and hot rolling is easily embedded in the surface layer during hot rolling due to surface cracks in the slab. When a cold-rolled steel sheet is manufactured with the scale embedded, the portion becomes a scale surface defect and cannot be applied to press forming. For this reason, Al amount shall be 0.01-0.10 mass%. Moreover, Preferably it is 0.01-0.05 mass%.

N: 0.006% by mass or less Nitride formed by combining N and Al adversely affects the surface properties as described above. When the amount of N exceeds 0.006% by mass, the surface property is greatly reduced by the Al nitride, and the elongation is significantly reduced due to the increase in the solid solution N. For this reason, N amount shall be 0.006 mass% or less.

  In the present invention, when further improving the characteristics, any one or more of Nb, Ti, and V may be contained. Each reason for limitation will be described.

Nb: 0.01 to 0.10% by mass
Since the carbide formed by combining Nb and C contributes to precipitation strengthening of the steel sheet, Nb may be contained as necessary. If the amount of Nb is less than 0.01% by mass, the effect is small. Moreover, when it exceeds 0.10 mass%, the elongation of a steel plate will fall remarkably by the production | generation of an excess carbide. For this reason, when it contains Nb, it is preferable to set it as 0.01-0.10 mass%. Moreover, it is more preferable to set it as 0.01-0.06 mass%.

Ti: 0.01-0.10 mass%
Since the carbide formed by combining Ti and C contributes to precipitation strengthening of the steel sheet, Ti may be contained as necessary. Further, Ti precipitates as carbonitrides and sulfides at a high temperature when the slab is cooled, and prevents surface precipitation of the slab by suppressing precipitation of AlN and grain boundary precipitation of Nb and V carbides that occur at a relatively low temperature. . If the amount of Ti is less than 0.01% by mass, the effect is small. Moreover, when it exceeds 0.10 mass%, the production | generation amount of a carbide | carbonized_material will increase and the fall of the elongation of a steel plate will become remarkable. For this reason, when it contains Ti, it is preferable to set it as 0.01-0.10 mass%. Moreover, it is more preferable to set it as 0.01-0.07 mass%.

V: 0.01-0.10 mass%
The fine carbide formed by combining V and C is effective for strengthening the precipitation of the steel sheet, and may contain V as necessary. When the amount of V is less than 0.01% by mass, the effect is small. On the other hand, if it exceeds 0.10% by mass, the formation of fine carbides at the austenite grain boundaries is promoted during slab casting, and slab surface cracks due to austenite grain boundary embrittlement become prominent. For this reason, when it contains V, it is preferable to set it as 0.01-0.10 mass%. Moreover, it is more preferable to set it as 0.01-0.06 mass%.

  In this invention, when improving a characteristic further, you may contain any 1 or more types of Mo and Cr. Each reason for limitation will be described.

Mo: 0.01-0.30 mass%
Mo is effective for strengthening the quenching of the steel sheet, and may be contained as necessary. When the amount of Mo is less than 0.01% by mass, the strengthening ability is small. Moreover, when it exceeds 0.30 mass%, formation of Mo oxide will be accelerated | stimulated on the steel plate surface at the time of continuous annealing, and the chemical conversion treatment property of a steel plate will fall remarkably. For this reason, when it contains Mo, it is preferable to set it as 0.01-0.30 mass%.

Cr: 0.01-0.30 mass%
Cr is effective for strengthening the quenching of the steel sheet, and may be contained as necessary. When the Cr amount is less than 0.01% by mass, the strengthening ability is small. Moreover, when it exceeds 0.30 mass%, since the production | generation of Cr oxide will be accelerated | stimulated on the steel plate surface at the time of continuous annealing, the chemical conversion property of a steel plate will fall remarkably. For this reason, when it contains Cr, it is preferable to set it as 0.01-0.30 mass%.

  In this invention, when improving a characteristic further, you may contain any 1 or more types of Cu and Ni. Each reason for limitation will be described.

Cu: 0.01-0.30 mass%
Cu suppresses the ferrite transformation from austenite during continuous annealing cooling and promotes the formation of martensite. Therefore, Cu is effective for strengthening the steel sheet, and may be contained if necessary. When the amount of Cu is less than 0.01% by mass, this effect is small. Moreover, when it exceeds 0.30 mass%, a ferrite transformation will be suppressed too much and ductility will fall. For this reason, when it contains Cu, it is preferable to set it as 0.01-0.30 mass%.

Ni: 0.01-0.30 mass%
Ni suppresses the ferrite transformation from austenite during the cooling of continuous annealing and promotes the formation of martensite. Therefore, Ni is effective for strengthening the steel sheet, and may be contained as necessary. If the amount of Ni is less than 0.01% by mass, this effect is small. Moreover, when it exceeds 0.30 mass%, a ferrite transformation will be suppressed too much and ductility will fall. For this reason, when it contains Ni, it is preferable to set it as 0.01-0.30 mass%.

  In the present invention, when further improving the characteristics, B may be contained. The reason for the limitation will be described.

B: 0.0003 to 0.0020 mass%
B segregates at the austenite grain boundary during heating in continuous annealing, suppresses ferrite transformation from austenite during cooling, and promotes the formation of martensite. Therefore, B is effective in strengthening the steel sheet, and is contained as necessary. Also good. If the amount of B is less than 0.0003 mass%, this effect is small. Moreover, when it exceeds 0.0020 mass%, this effect will be saturated. For this reason, when it contains B, it is preferable to set it as 0.0003-0.0020 mass%.

  The balance other than the above is Fe and inevitable impurities.

(2) Manufacturing Conditions for Continuous Casting Slab In the present invention, the continuous casting slab may be manufactured using a continuous casting machine having a mold vibration mechanism, a bending band, and a correction band. Below, the reason for limitation of manufacturing conditions is demonstrated.

Mold vibration frequency: 80 to 200 cycles / minute When the mold vibration frequency is less than 80 cycles / minute, the slab surface is uneven due to the oscillation mark, and slab corner cracks occur from that. On the other hand, if it exceeds 200 cycles / minute, the friction between the mold and the slab increases and surface cracks occur. For this reason, the frequency of the mold is set to 80 to 200 cycles / minute.

Casting speed: 1.30 to 1.70 m / min When the casting speed is less than 1.30 m / min, productivity is significantly impaired. Moreover, when the steel slab passes through the bending band or the straightening band when it exceeds 1.70 m / min, the strain rate of the tensile stress applied to the bottom surface of the steel slab in the bending band and the top surface of the steel slab in the straightening band is excessively high. Therefore, a bending crack occurs. For this reason, the casting speed is set to 1.30 to 1.70 m / min.

Slab corner temperature on the bending band exit side: 720-780 ° C
When the slab corner temperature on the bending band exit side is less than 720 ° C., the deformation resistance of bending deformation is large. Moreover, when it exceeds 780 degreeC, the ductility of a slab corner will fall remarkably and a slab corner crack will generate | occur | produce. For this reason, the slab corner temperature on the bending band exit side is set to 720 to 780 ° C. Further, from the viewpoint of further stabilizing the ductility of the slab corner, it is preferably 760 ° C. or lower.

Slab corner temperature on the straightening side: 720-780 ° C
When the slab corner temperature on the straightening side is less than 720 ° C., the deformation resistance of straightening deformation is large. Moreover, when it exceeds 780 degreeC, the ductility of a slab corner will fall remarkably and a slab corner crack will generate | occur | produce. For this reason, the slab corner temperature on the correction banding side is set to 720 to 780 ° C. Further, from the viewpoint of further stabilizing the ductility of the slab corner, it is preferably 760 ° C. or lower.

  Next, the manufacturing method of the high intensity | strength cold-rolled steel plate of this invention is demonstrated. The high-strength cold-rolled steel sheet of the present invention is obtained by hot rolling a continuous cast slab obtained by the above production conditions at a heating temperature of 1150 to 1300 ° C, a finish rolling temperature of 820 to 920 ° C, and a winding temperature of 520 to 600 ° C. , After cold rolling at a cold rolling rate of 30 to 65% and then soaking at an annealing temperature of 750 to 860 ° C. for 1 to 10 minutes, primary cooling to 600 to 700 ° C. at an average cooling rate of 10 ° C./s or less, After secondary cooling to 50 ° C. or less at an average cooling rate of 500 ° C./s or more, the sample is reheated to 100 to 450 ° C. and kept soaked for 1 to 15 minutes. Hereinafter, the reason for limitation will be described.

Heating temperature of continuous casting slab: 1150-1300 ° C
The continuously cast slab may be reheated as a hot piece, or may be reheated after being cooled to room temperature. The surface property can be improved by peeling the primary scale of the surface layer generated during slab heating by descaling. When the slab heating temperature is less than 1150 ° C., a sufficient amount of scale peeling cannot be obtained, so that the desired surface texture effect cannot be obtained. On the other hand, when the temperature exceeds 1300 ° C., the fuel consumption increases or the yield decreases due to excessive scale peeling, and the productivity is remarkably lowered. For this reason, the heating temperature of a continuous casting slab shall be 1150-1300 degreeC. Moreover, 1200-1280 degreeC is preferable from a viewpoint of surface property improvement and manufacturability.

Finishing rolling temperature: 820-920 ° C
If the finish rolling temperature is less than 820 ° C., the structure of the hot-rolled sheet becomes non-uniform and the ductility of the cold-rolled steel sheet after annealing is significantly impaired. On the other hand, when the temperature exceeds 920 ° C., the scale formed immediately before the finish rolling is rolled without being removed, so that a scale defect is generated and the surface properties are deteriorated. For this reason, finish rolling temperature shall be 820-920 degreeC. Moreover, in order to highly stabilize the ductility of the cold-rolled steel sheet after annealing, Ar ₃ points or more are preferable.

Winding temperature: 520-600 ° C
When the coiling temperature is less than 520 ° C., the cold rolling resistance increases due to the hardening of the hot-rolled sheet structure, so the productivity is lowered. In addition, when the temperature exceeds 600 ° C., grain boundary oxidation occurs mainly in the steel sheet surface layer inside the steel strip after winding, and the grain boundary oxidation part peels off in the subsequent process, or the peeled material adheres to the roll. As a result, the surface properties are deteriorated. For this reason, the coiling temperature is set to 600 ° C. or less. Moreover, 540-580 degreeC is preferable from a viewpoint of reduction of cold rolling resistance, and suppression of grain boundary oxidation.

Cold rolling rate: 30-65%
If the cold rolling rate is less than 30%, the ferrite phase is not sufficiently recrystallized during annealing, and the local ductility is lowered, so that the desired ductility cannot be obtained. On the other hand, if it exceeds 65%, the cold rolling resistance increases, so the productivity is lowered. For this reason, a cold rolling rate shall be 30 to 65%. Further, 40% or more is preferable from the viewpoint of promoting recrystallization during annealing.

Annealing temperature and soaking conditions: If the soaking annealing temperature is less than 750 ° C. for 1 to 10 minutes at an annealing temperature of 750 to 860 ° C., the austenite phase at the time of soaking is small, and the amount of martensite necessary for securing the strength is obtained. Therefore, a tensile strength of 1180 MPa or more cannot be obtained. Further, when the temperature exceeds 860 ° C., a Si-based oxide is formed on the surface layer of the steel sheet during soaking, so that when surface strain is applied, the surface layer peels off from the oxide as a starting point and the surface properties deteriorate. For this reason, annealing temperature shall be 750-860 degreeC. Moreover, 780-830 degreeC is preferable from a viewpoint of high strengthening and surface property improvement.
If the soaking time is less than 1 minute, the austenite phase at the time of soaking is small and the amount of martensite necessary for securing the strength cannot be obtained, so that a tensile strength of 1180 MPa or more cannot be obtained. If it exceeds 10 minutes, Si-based oxides increase on the surface of the steel plate during soaking, and the surface properties deteriorate. Therefore, the soaking time is 1 to 10 minutes. Moreover, 2 minutes or more are preferable from a viewpoint of intensity | strength and ductility.

Primary cooling temperature and average cooling rate at the time of primary cooling: The steel sheet after annealing at the average cooling rate of 10 ° C./s or less up to 600 to 700 ° C. under the above-mentioned conditions is to achieve both high strength and high ductility. For the purpose of illustration, it is rapidly cooled after cooling to the primary cooling temperature. When the primary cooling temperature is less than 600 ° C., a tensile strength of 1180 MPa or more cannot be obtained. On the other hand, if the temperature exceeds 700 ° C., the ductility decreases as the strength increases, so that the elongation at break of 9.0% or more cannot be obtained. For this reason, primary cooling temperature shall be 600-700 degreeC. Moreover, 620-680 degreeC is preferable from a viewpoint of the balance improvement of intensity | strength and ductility.
If the average cooling rate at the time of primary cooling exceeds 10 ° C./s, the ductility decreases with increasing strength, and therefore, elongation at break of 9.0% or more cannot be obtained. For this reason, the average cooling rate at the time of primary cooling shall be 10 degrees C / s or less. Moreover, 3-8 degrees C / s is preferable from a viewpoint of the balance improvement of intensity | strength and ductility.

Secondary cooling condition: average cooling rate of 500 ° C./s or more to 50 ° C. or less If the secondary cooling average cooling rate is less than 500 ° C./s, a ferrite phase is excessively generated during cooling, so that a tensile strength of 1180 MPa or more is obtained. I can't get it. For this reason, an average cooling rate shall be 500 degrees C / s or more. Moreover, 1000 degrees C / s or more is preferable from a viewpoint of ensuring intensity | strength.
When not rapidly cooled to 50 ° C. or lower, a ferrite phase is excessively generated during cooling, and a tensile strength of 1180 MPa or more cannot be obtained. For this reason, secondary cooling is performed to 50 degrees C or less. Further, from the viewpoint of securing strength, 35 ° C. or lower is preferable. Although cooling is preferably water cooling, it is also possible to perform cooling by combining gas cooling, mist cooling, roll cooling, and the like.

Reheating conditions: If the reheating temperature for reheating to 100 to 450 ° C. and keeping soaked for 1 to 15 minutes is lower than 100 ° C., the martensite phase is not tempered sufficiently and the balance between strength and ductility is lowered. In addition, since the hard phase increases, the hardness difference from the ferrite phase also increases, and stretch flangeability and bendability deteriorate. If it exceeds 450 ° C., tempering proceeds excessively rapidly, and the martensite phase decomposes into a ferrite phase and carbide and softens, so that a tensile strength of 1180 MPa or more cannot be obtained. For this reason, reheating temperature shall be 100-450 degreeC. Moreover, Preferably it is set as 200-400 degreeC.
If the soaking time is less than 1 minute, the tempering of the martensite phase is insufficient, the strength is excessively increased, and the balance between strength and ductility is reduced. Moreover, when it exceeds 15 minutes, not only the effect tends to be saturated, but also tempering proceeds excessively, and a tensile strength of 1180 MPa or more cannot be obtained. For this reason, the soaking time is set to 1 to 15 minutes. Moreover, 3-12 minutes is preferable from a viewpoint of the balance improvement of intensity | strength and ductility. In addition, cooling until it returns to room temperature after reheating can be performed by air cooling, furnace cooling, gas cooling, mist cooling, water cooling, etc.

  As described above, according to the present invention, surface cracks can be generated even in a steel having a component composition that contributes to high strength by appropriately controlling the component composition of steel, continuous casting conditions, annealing conditions, and the like. It is possible to produce a continuous cast slab that is a raw material of a high-strength cold-rolled steel sheet without being generated, and to produce a high-strength cold-rolled steel sheet having excellent surface properties using this continuous cast slab. As a result, it is possible to stably manufacture a high-strength steel plate having excellent surface quality required for materials such as automobile interior plate parts.

  Steels A to R having the component compositions shown in Table 1 were melted in a converter and continuously cast under the conditions shown in Table 2 to cast a continuous casting slab having a thickness of 260 mm.

  As shown in Table 2, all the inventive examples in which the steel component composition and casting conditions are within the scope of the present invention have good slab surface properties. On the other hand, the comparative example in which either the component composition of steel or the casting conditions is out of the scope of the present invention has poor slab surface properties.

Next, this continuous cast slab was subjected to hot rolling, cold rolling and continuous annealing under the conditions shown in Table 3, and then subjected to temper rolling at a pickling treatment and an elongation rate of 0.15 to 0.20% to obtain a steel material 1 A cold-rolled steel sheet shown in No. 31 was produced. Then, the surface properties of the continuously cast slab, the surface properties of the cold-rolled steel sheet, and the tensile properties were evaluated by the following methods.
Surface property of slab The surface property of slab was evaluated by three levels of “◯”, “Δ”, and “×” according to the state of occurrence (number and length) of cracks by inspecting the slab surface by a penetrant flaw detection method. Here, “◯” means that no crack is detected, “Δ” means that a crack is detected, but 5 mm or less of cracks are 0.5 pieces / m ² or less, and “×” means long Even if the crack exceeds 5 mm or the length is 5 mm or less, it is assumed that a crack exceeding 0.5 pieces / m ² is detected, and “◯” and “Δ” are determined to have good slab surface properties. .
Surface properties of the cold-rolled steel sheet The surface of the cold-rolled steel sheet is visually inspected, and “○”, “△”, “×” according to the occurrence state (number and length) of surface defects found on the front and back surfaces of the cold-rolled steel plate. ”Was evaluated at three levels. Here, “◯” indicates that no defect is observed, “Δ” indicates that a defect is recognized, but defects having a length of 30 mm or less are 0.5 pieces / m ² or less, and “×” is a length It was determined that defects longer than 30 mm were observed, or defects having a length of 30 mm or less exceeded 0.5 / m ² , and “◯” and “Δ” were determined to have good product surface properties.
Tensile properties Tensile test based on JIS Z 2241 (1998) is performed using No. 5 tensile test piece of JIS Z 2201 (1998) whose longitudinal direction is 90 ° to the rolling direction of the product steel plate after annealing. The tensile strength and elongation at break were measured.

  As shown in Table 3, all of the inventive examples in which the steel component composition and production conditions are within the scope of the present invention have desired properties in terms of tensile strength, elongation at break and product surface properties. On the other hand, in the comparative examples in which either the steel component composition or the production conditions are out of the scope of the present invention, any of tensile strength, breaking elongation, and product surface properties is inferior.

Claims (7)

  In mass%, C: 0.10 to 0.20%, Si: 0.6 to 2.0%, Mn: 1.8 to 3.0%, P: 0.02% or less, S: 0.003 %, Al: 0.01 to 0.10%, N: 0.006% or less, with the balance being Fe and inevitable impurities, casting frequency of 80 to 200 cycles / min, casting speed 1 Casting into a slab at a slab corner temperature of 720 to 780 ° C. on the bending strip outlet side and a slab corner temperature of 720 to 780 ° C. on the straight strip strip side, .30 to 1.70 m / min .   2. The method for producing a continuous cast slab according to claim 1, further comprising 0.01 to 0.10% of any one or more of Nb, Ti, and V by mass%.   The method for producing a continuous cast slab according to claim 1 or 2, further comprising 0.01 to 0.30% of at least one of Mo and Cr.   The method for producing a continuously cast slab according to any one of claims 1 to 3, further comprising 0.01 to 0.30% of at least one of Cu and Ni in terms of mass%.   The method for producing a continuous cast slab according to any one of claims 1 to 4, further comprising 0.0003 to 0.0020% of B in mass%.   The said slab is a continuous casting slab for cold-rolled steel sheets, The manufacturing method of the continuous casting slab in any one of Claims 1-5 characterized by the above-mentioned.   The slab manufactured by the method according to claim 6 is hot-rolled at a heating temperature of 1150 to 1300 ° C, a finish rolling temperature of 820 to 920 ° C, and a winding temperature of 520 to 600 ° C, and then a cold rolling rate of 30 to 65%. And then soaked at an annealing temperature of 750 to 860 ° C. for 1 to 10 minutes, followed by primary cooling to 600 to 700 ° C. at an average cooling rate of 10 ° C./s or less, and an average cooling rate of 500 ° C./s or more. A method for producing a high-strength cold-rolled steel sheet, comprising: secondary cooling to 50 ° C. or lower, reheating to 100 to 450 ° C., and soaking for 1 to 15 minutes.