JP4887818B2 - Manufacturing method of continuous cast slab and manufacturing method of high-tensile hot-rolled steel sheet, high-tensile cold-rolled steel sheet, and high-tensile galvanized 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 steel plate used for inner plate parts such as structural members and reinforcing members of automobiles, and a method for producing a high-tensile steel plate using this slab. is there.

  From the viewpoint of weight reduction and collision safety of automobiles, the application of high-tensile steel plates to various structural members and reinforcing members of automobiles is being promoted. When this high-tensile steel sheet is applied, press formability and weldability become difficult, and therefore, development of materials for solving such practical problems has been advanced. 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. This is because high-strength steel contains many alloying components to increase the strength, so slab surface cracks are likely to occur in the continuous casting process. When the oxide scale generated in the stage is easily embedded in the vicinity of the surface layer of the steel sheet, and the hot-rolled steel sheet, cold-rolled steel sheet and hot-dip galvanized steel sheet are manufactured with the scale embedded in this way, the surface appearance This is because the quality deteriorates significantly. When the surface properties are lowered, application to a press product becomes difficult and the yield is lowered, and a mold can be damaged at the time of press molding. Therefore, good surface quality is desired.

  For this purpose, various manufacturing techniques have been disclosed so far for improving the surface quality in the casting stage of the slab. For example, in Patent Document 1, in B-added steel, the precipitation of BN on austenite grain boundaries is suppressed by controlling the casting speed within a predetermined range in accordance with the B content, N content, and Ti content in the steel. A casting technique for avoiding slab surface cracking due to austenite grain boundary embrittlement is disclosed. According to Patent Document 1, a steel plate (hot rolled steel plate, cold rolled steel plate, electroplated steel plate, galvanized steel plate) having a tensile strength of 297 to 565 MPa can be stabilized without generating surface defects due to slab cracking. Can be manufactured.

  In Patent Document 2, in continuous casting of steel containing Al and Nb with a curved or vertical bending type continuous casting machine, the surface temperature of the slab is adjusted to the austenite + ferrite two-phase region before bending the slab. After that, by reheating again to the austenite region and correcting it, the grain boundary precipitation of Al and Nb carbonitrides causing the austenite grain boundary embrittlement is reduced, and there is no surface cracking Al and Nb added steel A technique for casting is disclosed.

In Patent Document 3, the surface of the slab is imparted with processing strain at a temperature of 650 to 950 ° C., thereby coarsening carbonitrides such as AlN and NbC, and austenite grains by austenite → ferrite → austenite transformation. A casting technique for reducing the surface cracking caused by grain boundary brittleness is disclosed.
JP 2002-20836 A JP-A-11-33688 JP 60-56453 A

  However, the above prior art has the following problems.

  That is, the technique disclosed in Patent Document 1 intends to control the cooling rate of the slab by adjusting the casting speed. However, when the high-temperature slab surface is cooled to a predetermined temperature, the casting speed increases. Since it is necessary to increase the cooling strength, it is considered extremely difficult to stably carry out temperature control in which BN does not precipitate only by adjusting the casting speed without optimizing the cooling conditions. In addition, increasing or decreasing the casting speed during casting brings about a decrease in slab surface quality and productivity due to the promotion of the mold surface fluctuation in the mold, and is not a preferable casting form. Furthermore, a product having a tensile strength of 590 MPa or more has not been obtained with this technology.

  In the technique disclosed in Patent Document 2, when a vertical bending type continuous casting machine is used, the distance from the mold outlet to the upper straightening zone is short, and within this short distance, the slab surface is austenite + ferrite two-phase region. A slab quenching facility is required to reduce the temperature. In addition, it is necessary to reheat to the austenite region after cooling to the two-phase region before correction, and it is necessary to perform recuperation in a short time. It is difficult to recover the heat stably and it is necessary to introduce heating equipment. Furthermore, since this technique is not intended to increase the strength of final products such as cold-rolled steel sheets and galvanized steel sheets, it is difficult to obtain a steel sheet having a strength of 590 MPa or more by this technique.

  In the technique disclosed in Patent Document 3, it is necessary to introduce equipment that applies impact force such as shot blasting and air hammer to the slab, which not only increases the equipment cost but also within a few meters from the mold outlet. In order to cool to 650 to 950 ° C. and perform surface processing, it is necessary to reduce the casting speed to 0.8 m / min as shown in the examples, and the production efficiency is extremely low. Moreover, in this technique, since an impact force is applied to the slab surface, there is a possibility that slight wrinkles are generated on the slab surface. Even if such fine wrinkles are not markedly cracked during slab correction, the oxygen supply is reduced during the slab heating stage during hot rolling, so that the selective oxidation elements (Si, Mn, The production of granular oxides by Al or the like) causes a reduction in hot ductility, promotes hot cracking, and may cause scale defects due to the hot cracking. For this reason, it is thought that it is difficult to obtain a good surface quality with a hot rolled steel sheet, a cold rolled steel sheet, and a galvanized steel sheet as the final product.

  As described above, in the above-described conventional technology, the introduction of new incidental equipment to the continuous casting machine leads to an increase in cost, or the productivity of the slab cannot be increased and the productivity is reduced. The fact was that there was a problem. Moreover, the tensile strength of the product obtained was less than 590 MPa, and it was not a method for producing a steel plate of 590 MPa or more.

  The present invention has been made in view of the above circumstances, and the object of the present invention is to achieve 590 MPa with high productivity and without occurrence of surface cracks without installing a new incidental equipment in a continuous casting machine. A method for producing a continuous cast slab that is a material of a high-tensile steel sheet (hot rolled steel sheet, cold rolled steel sheet, galvanized steel sheet) having the above-described tensile strength, and at the same time, using this continuous cast slab, has excellent surface properties. It is to provide a method for producing a tensile hot-rolled steel sheet, a high-tensile cold-rolled steel sheet, and a high-tensile galvanized steel sheet.

  The manufacturing method of the continuous casting slab based on 1st invention for solving the said subject is the mass%, C: 0.03-0.10%, Si: 1.0% or less, Mn: 0.5- 3.0%, P: 0.1% or less, S: 0.02% or less, Al: 0.2% or less, N: 0.006% or less, specified by Al content and N content (14/27) × (% Al /% N) is 50 or less, and the remaining steel is composed of Fe and inevitable impurities. The following formula (1) defined by the Al content and the N content: The secondary cooling zone is spray-cooled with a specific water amount satisfying the following formula (2) defined by the casting speed. However, in the formulas (1) and (2), Vc is the casting speed (m / min), Q is the specific water amount in the secondary cooling zone (liter / kg),% Al is the Al content of the molten steel, and% N is It is N content of molten steel.

  The manufacturing method of the continuous casting slab which concerns on 2nd invention is 1st invention. WHEREIN: The said molten steel is further mass%, Cr: 0.02-1.5%, V: 0.003-0.1 %, Mo: 0.05-0.5%, Nb: 0.01-0.08%, Ti: 0.01-0.08%, B: 0.0002-0.003% It is characterized by containing 1 type (s) or 2 or more types.

  The method for producing a high-tensile hot-rolled steel sheet according to the third invention is a method of hot rolling a continuous cast slab produced by the method for producing a continuous cast slab according to the first or second invention, and a temperature of 700 ° C. or lower. It has the hot rolling process wound up by.

  The method for producing a high-tensile galvanized steel sheet according to the fourth invention comprises hot rolling the continuous cast slab produced by the method for producing a continuous cast slab according to the first or second invention, and a temperature of 700 ° C. or lower. A hot-rolling step of winding the steel plate, a pickling step of pickling the hot-rolled steel plate obtained by the hot rolling step, and a zinc-based plating film on the surface of the pickled steel plate obtained by the pickling step It has the plating process to perform.

  The manufacturing method of the high-tensile cold-rolled steel sheet according to the fifth invention includes a hot rolling step of hot rolling the continuous cast slab manufactured by the continuous cast slab manufacturing method according to the first or second invention, According to the pickling process for pickling the hot rolled steel sheet obtained by the hot rolling process, the cold rolling process for cold rolling the pickled steel sheet obtained by the pickling process, and the cold rolling process. A continuous annealing step in which the obtained cold-rolled steel sheet is continuously annealed at a temperature equal to or higher than the recrystallization temperature.

  The manufacturing method of the high-tensile galvanized steel sheet according to the sixth invention includes a hot rolling step of hot rolling the continuous cast slab manufactured by the continuous cast slab manufacturing method according to the first or second invention, According to the pickling process for pickling the hot rolled steel sheet obtained by the hot rolling process, the cold rolling process for cold rolling the pickled steel sheet obtained by the pickling process, and the cold rolling process. A continuous annealing step in which the obtained cold-rolled steel plate is continuously annealed at a temperature equal to or higher than a recrystallization temperature, and a plating step in which a zinc-based plating film is formed on the surface of the steel plate obtained by the continuous annealing step. It is what.

  According to the present invention, by appropriately controlling the chemical composition of steel, and the casting speed and cooling conditions in the continuous casting process, the continuous cast slab, which is a material of the high-tensile steel plate, is produced with high productivity and surface cracks. It is possible to manufacture stably without this. As a result, it is possible to stably produce a high-tensile steel plate excellent in 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.

  The inventors of the present invention have made extensive studies on the cause of scale surface defects in low-carbon steel. As a result, due to grain boundary embrittlement due to fine precipitation on the austenite grain boundaries of AlN produced by the combination of Al and N in the steel during casting of the slab, the surface cracking sensitivity of the slab is increased. It was found that the scale easily bites into the surface layer portion of the steel sheet during hot rolling, and scale surface defects are likely to occur.

  In addition, since it is considered important to control the fine precipitation of AlN in the slab surface layer during continuous casting for reducing such surface defects, the Al content, the N content and the casting effect on the slab surface quality. The effect of conditions was investigated. As a result, the surface quality of the slab largely depends on the ratio of Al content and N content (mass% Al / mass% N) and is appropriate according to this ratio (mass% Al / mass% N). Thus, it has been found that the surface quality deteriorates when the casting speed range is outside the proper casting speed range, and that the secondary cooling water amount needs to be controlled to an appropriate value according to the proper casting speed.

  The contents will be described below. First, the influence of the ratio of Al content to N content in steel (mass% Al / mass% N) and the casting speed on the surface properties will be described.

  C: 0.075-0.090 mass%, Si: 0.2-0.3 mass%, Mn: 1.7-2.1 mass%, P: 0.02-0.03 mass%, S: 0.002 to 0.005 mass%, Cr: 0.01 to 0.3 mass%, V: 0.002 to 0.06 mass%, Al: 0.02 to 0.15 mass%, N: 0.00. A molten steel containing 0008 to 0.0055% by mass and the balance of Fe and inevitable impurities is melted, and this molten steel is cast in a vertical bending slab continuous casting machine at a casting speed of 1.0 to 4. Continuous casting was carried out under the condition of 5 m / min and the specific amount of cooling water in the secondary cooling zone at a constant condition of 2.0 liters / kg. Here, the specific water amount is the secondary cooling zone from directly under the mold to the end of the continuous casting machine, and the amount of cooling water used for cooling per kg of the slab that has passed through the secondary cooling zone (liters). (Hereinafter referred to as “L”)). The above-mentioned constant condition that the specific water amount is 2.0 L / kg means that the amount of cooling water in the secondary cooling zone is increased or decreased in proportion to the casting speed, and the specific water amount is maintained at a constant value.

  After the cast slab is cooled to room temperature, the surface of the slab is inspected by the penetrant flaw detection method. The slab surface was evaluated at 4 levels. When cracks exist on the surface of the slab, it was determined in this way because the scale biting into the surface layer of the steel sheet was promoted during hot rolling, and there was concern about surface defects in the steel sheet. “Good” means that no cracks are detected, “Permitted” means that very few cracks are detected, “Deterioration” means that minor cracks are detected, and “Bad” means many cracks. Is detected.

  The slab whose surface inspection was completed was heated and hot-rolled (heating temperature: 1250 ° C., finishing temperature: 860 ° C., winding temperature: 550 ° C.) while leaving the surface defects, and the heat obtained by hot rolling. After pickling the cold-rolled steel sheet, cold-rolling the pickled steel sheet, and continuously annealing the cold-rolled steel sheet obtained by cold rolling (annealing temperature: 830 ° C.), continuous hot-dip galvanizing (zinc plating: The number of surface defects per unit surface area and the tensile properties of the galvanized steel sheet having a thickness of 1.4 mm obtained by carrying out 460 ° C. and plating alloying treatment: 500 ° C. were investigated.

The surface defects of the galvanized steel sheet are determined by measuring the number of surface defects with a length of 2 mm or more found on the front and back surfaces of the galvanized steel sheet (number / m ² ) per unit area of the plate surface (hereinafter referred to as “surface The number of defects ”is calculated, and the number of surface defects is evaluated. When the number of surface defects is more than 0.3 / m ² , there is a concern about damage to the mold during press molding, and it is difficult to apply to automobile inner plate parts. The target was m ² or less. Moreover, the tensile characteristic of the galvanized steel sheet was measured by conducting a tensile test using a JIS No. 5 tensile test piece at a tensile speed of 10 mm / min. It was confirmed that all the steel plates had a tensile strength of 600 to 615 MPa and an elongation of 30.5 to 31.7%.

  FIG. 1 shows a three-way relationship between the surface properties of the investigated slab, the Al content and N content in the slab, and the casting speed in the continuous casting process. In FIG. 1, the influence on the surface properties of Al and N using the value of “(14/27) × (mass% Al / mass% N)”, which is the atomic equivalent ratio of Al and N contained in the slab. Is captured.

In FIG. 1, among the slab surface evaluations evaluated to four levels, “good” is represented by a circle, “acceptable” is represented by a Δ, “deterioration” is represented by ●, and “bad” is represented by an x. Yes. The inventors of the present invention found that the number of surface defects of a galvanized steel sheet obtained from a slab whose slab surface determination was determined to be “good” was 0.15 / m ² or less, and the slab surface determination was “acceptable”. The number of surface defects of the galvanized steel sheet obtained from the slab determined to be more than 0.15 / m ² and not more than 0.3 / m ² and the slab surface determination was determined as “deteriorated” The number of surface defects of the galvanized steel sheet obtained from the above is more than 0.3 / m ² and less than 0.5 / m ² , and the slab surface judgment is obtained from the slab judged as “bad”. It has been confirmed that the number of surface defects in the galvanized steel sheet exceeds 0.5 / m ² . That is, it has been confirmed that there is a strong correlation between the surface determination result of the slab and the number of surface defects of the steel plate.

  As shown in FIG. 1, in order to obtain a low surface defect rate in a galvanized steel sheet having good slab surface properties, a “(14/27) × (mass% Al / mass% N)” value, and It has been found that there is an appropriate range of casting speeds.

That is, in the range where the “(14/27) × (mass% Al / mass% N)” value exceeds 50, the slab surface properties can be determined only as “deterioration” or “defect” regardless of the casting speed. The number of surface defects in the galvanized steel sheet exceeded 0.3 / m ² , and the surface properties deteriorated as the casting speed increased. This is because the “(14/27) × (mass% Al / mass% N)” value is large, so that the amount of AlN finely precipitated at the austenite grain boundary during casting increases, and the grains accompanying the increase in the amount of precipitated AlN. This is thought to be because the slab surface is more susceptible to cracking due to interfacial embrittlement and the scale bites into the surface layer during hot rolling. The reason why the surface properties deteriorated as the casting speed increased was that the slab cooling rate became non-uniform due to the increase in casting speed, and the thermal strain on the slab surface due to non-uniform cooling and precipitation of AlN deposited near the surface layer. It is considered that the surface cracking sensitivity was promoted because the amount became non-uniform.

  Even in the range where the “(14/27) × (mass% Al / mass% N)” value was 50 or less and the casting speed was less than 1.5 m / min, the slab surface properties were only “deteriorated”. This is presumably because when the casting speed is low, the slab surface layer is deformed at a low strain rate, so that AlN is strain-induced and the crack sensitivity on the slab surface is increased.

Further, the “(14/27) × (mass% Al / mass% N)” value is 50 or less, and the casting speed is “4.0-0.68 × log [(14/27) × (mass% Al). / Mass% N)] ”Even in a range exceeding m / min, the slab surface property is“ deterioration ”or“ defect ”determination, and the number of surface defects in the galvanized steel sheet is more than 0.3 / m ^2. The surface quality was degraded. This is because the slab cooling rate becomes non-uniform as the casting speed increases, and the thermal strain on the surface of the slab during non-uniform cooling and the amount of AlN deposited on the surface layer become non-uniform, increasing the surface cracking susceptibility of the slab. It is thought that it was because. In addition, the boundary line of the above casting speed: “4.0-0.68 × log [(14/27) × (mass% Al / mass% N)]” is the difference between Δ mark and ● mark in FIG. It is an approximate line that approximates the boundary.

On the other hand, the “(14/27) × (mass% Al / mass% N)” value is 50 or less and the casting speed is 1.5 m / min or more, “4.0-0.68 × log [ In the range of (14/27) × (mass% Al / mass% N)] m / min or less, the slab surface judgment is only “good” and “acceptable”, the slab surface properties are improved, and the galvanized steel sheet The number of surface defects was 0.3 / m ² or less, and it was found that the surface quality was excellent. When the casting speed is 2.0 m / min or more and “3.5-0.68 × log [(14/27) × (mass% Al / mass% N)]” m / min or less, the slab All the surface determinations were “good”, and it was found that the number of surface defects of the galvanized steel sheet was reduced to 0.15 / m ² or less, and better surface quality was obtained. This is considered to be because the precipitation of fine AlN to the austenite grain boundaries is small in this range, and the slab surface cracking susceptibility is reduced due to the reduction of grain boundary embrittlement and the reduction of thermal strain on the slab surface. . In addition, the boundary line of the above casting speed: “3.5-0.68 × log [(14/27) × (mass% Al / mass% N)]” It is an approximate line that approximates the boundary.

  Thus, in order to obtain a good surface quality, the “(14/27) × (mass% Al / mass% N)” value of the steel composition is set to 50 or less, and the casting speed is set to 1.5 m / min or more. , “4.0-0.68 × log [(14/27) × (mass% Al / mass% N)]” m / min or less, preferably, the casting speed is 2.0 m / min. As mentioned above, it turned out that it is preferable to set it as "3.5-0.68xlog [(14/27) x (mass% Al / mass% N)]" m / min or less.

  Next, the influence of the cooling water amount and casting speed on the surface properties in the secondary cooling zone will be described. The amount of cooling water in the secondary cooling zone was evaluated by the specific water amount.

  C: 0.060-0.075 mass%, Si: 0.02-0.05 mass%, Mn: 1.9-2.0 mass%, P: 0.01-0.02 mass%, S: A component containing 0.003 to 0.005% by mass, Al: 0.05% by mass, N: 0.005% by mass, Mo: 0.1 to 0.15% by mass, the balance being Fe and inevitable impurities The molten steel was melted in a vertical bend type slab continuous casting machine, the casting speed was in the range of 2.2 to 3.0 m / min, and the specific water amount in the secondary cooling zone was 0.5 to 7 Continuous casting was carried out at a range of 5 L / kg.

  After the cast slab is cooled to room temperature, the surface of the slab is inspected by a penetrant flaw detection method, and, as described above, “good”, “acceptable”, “deteriorated” depending on the occurrence of cracks (number and length) The slab surface was evaluated to 4 levels of “bad”. The evaluation criteria are the same as above.

  Heat obtained by hot rolling by heating the slab after the surface inspection while leaving the surface flaws and hot rolling (heating temperature: 1250 ° C., finishing temperature: 850 ° C., winding temperature: 550 ° C.) After pickling the cold-rolled steel sheet, cold-rolling the pickled steel sheet, and continuously annealing the cold-rolled steel sheet obtained by cold rolling (annealing temperature: 840 ° C.), continuous hot-dip galvanizing (zinc plating: 460 ° C., plating alloying treatment: 550 ° C.), and the obtained galvanized steel sheet having a thickness of 1.4 mm was examined for the number of surface defects and tensile properties. The method for investigating the number of surface defects and tensile properties is the same as described above. It was confirmed that all the steel plates had a tensile strength of 600 to 620 MPa and an elongation of 30.7 to 31.5%.

  FIG. 2 shows a three-way relationship between the surface properties of the investigated slab, the casting speed, and the specific water amount. 2 are the same as those in FIG. That is, “good” in the slab surface determination is indicated by a circle, “acceptable” is indicated by a Δ, “deterioration” is indicated by ●, and “bad” is indicated by ×. As shown in FIG. 2, it was found that it was necessary to control the specific water amount within an appropriate range in accordance with the casting speed in order to obtain a good slab surface property and to obtain a low surface defect rate in the galvanized steel sheet. .

In other words, when the specific water amount is less than 1.0 L / kg, the slab surface property is only “degraded” regardless of the casting speed, and the number of surface defects of the galvanized steel sheet is 0.3 / m ² . It was over. This is presumably because the surface austenite grains become coarse due to insufficient cooling of the slab, the grain boundaries become brittle due to the coarsening, and the surface cracking susceptibility of the slab increases.

In addition, when the casting speed is Vc (m / min), the slab surface property is only determined as “deterioration” or “defect” even when the specific water amount exceeds “2.5 + Vc / 1.5” L / kg. The number of surface defects of the galvanized steel sheet also exceeded 0.3 / m ² . This is because the cooling rate of the slab surface tends to be non-uniform as the specific water amount increases, and the thermal strain on the slab surface and the amount of AlN finely precipitated at the austenite grain boundaries in the surface layer become non-uniform, increasing the surface cracking susceptibility. It is thought that it was because. The boundary line of the specific water amount: “2.5 + Vc / 1.5” is an approximate straight line that approximates the boundary between the Δ mark and the ● mark in FIG.

On the other hand, when the specific water amount is 1.0 L / kg or more and “2.5 + Vc / 1.5” L / kg or less, the slab surface judgment is “good” and “acceptable”, and galvanization It was found that the number of surface defects in the steel sheet was reduced to 0.3 / m ² or less. This is presumed to be due to the reduction in slab surface cracking susceptibility due to uniform thermal strain on the slab surface and suppression of non-uniform precipitation of AlN. Further, when the specific water amount is 1.5 L / kg or more and “2.0 + Vc / 1.5” L / kg or less, the slab surface judgment is all “good”, and the number of surface defects of the galvanized steel sheet is 0.8. It was found that the surface quality was reduced to 15 / m ² or less, and better surface quality was obtained. The boundary line of the specific water amount: “2.0 + Vc / 1.5” is an approximate line that approximates the boundary between the circles and the triangles in FIG.

  From the above results, in producing a high strength steel sheet having a tensile strength of 590 MPa or more by rolling a continuously cast slab, in order to obtain a good surface quality, it is determined from the Al content and N content of the steel composition. The (14/27) × (mass% Al / mass% N) ”value is 50 or less, and the casting speed (Vc) in the continuous casting process is“ (14/27) × (mass% Al / According to the value of “mass% N)”, it is 1.5 m / min or more and “4.0−0.68 × log [(14/27) × (mass% Al / mass% N)]” m / min or less. Furthermore, the specific water amount in the secondary cooling zone in the continuous casting process needs to be 1.0 L / kg or more and “2.5 + Vc / 1.5” L / kg or less depending on the casting speed (Vc). I found out that

  Based on the above knowledge, the present inventors have invented a technique for stably producing a high-tensile steel sheet having a good surface quality in a high-tensile steel sheet applied to an automotive inner plate part or the like. Below, the reason for component addition of the present invention, the reason for component limitation, and the reason for limitation of production conditions will be described.

(1) Chemical component range C: 0.03 to 0.10% by mass
C is effective for strengthening steel, but if the addition amount is less than 0.03% by mass, a tensile strength of 590 MPa or more cannot be obtained stably. If the C content exceeds 0.10% 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 are remarkably caused by the concentration of thermal stress due to solidification shrinkage on the thin portion of the 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 0.03-0.10 mass%. In addition, the preferable range of C amount is 0.04-0.085 mass%, More preferably, it is 0.05-0.075 mass%.

Si: 1.0% by mass or less Si is an element effective for strengthening a steel sheet, and can be appropriately added. When adding Si for strengthening steel, it is preferable to add 0.03% by mass or more. A more preferable amount of Si is 0.08% by mass or more. However, if the addition amount of Si exceeds 1.0 mass%, the amount of Si oxide generated on the slab surface during casting increases, and surface cracks occur during slab bending correction. In addition, the occurrence of red scale due to the formation of firelite becomes significant during hot rolling, and the surface properties of the steel sheet deteriorate. For this reason, the amount of Si shall be 1.0 mass% or less. Further, when hot dip galvanizing treatment is performed on a steel sheet, there is a concern about non-plating or a decrease in plating adhesion, and therefore the Si content is preferably 0.7% by mass or less. A more preferable amount of Si is 0.5% by mass or less.

Mn: 0.5 to 3.0% by mass
Mn is an element effective for strengthening the steel sheet, but if the addition amount is less than 0.5% by mass, a tensile strength of 590 MPa or more cannot be stably obtained. On the other hand, when the amount of Mn exceeds 3.0 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 0.5-3.0 mass%. In addition, the preferable range of the amount of Mn is 1.0-2.3 mass%.

P: 0.1% by mass or less P is an element effective for strengthening the steel sheet, and is also an element preferable for scale peelability during hot rolling, and therefore can be added as appropriate. When adding P, it is preferable that the amount of P shall be 0.01 mass% or more. However, when the amount of P exceeds 0.1% by mass, surface cracks occur in the slab due to grain boundary embrittlement accompanying P segregation to the austenite grain boundaries during casting. For this reason, the amount of P shall be 0.1 mass% or less.

S: 0.02 mass% or less When the amount of S exceeds 0.02 mass%, hot brittleness is caused and the generation of scale surface defects is promoted. For this reason, S amount shall be 0.02 mass% or less. Further, from the viewpoint of scale peelability, S is preferably 0.001% by mass or more.

Al: 0.2% by mass or less Nitride formed by combining Al and N precipitates finely on the austenite grain boundary during casting and causes embrittlement at the grain boundary. Causes surface cracking. Due to the surface crack of the slab, the oxide scale formed during casting and hot rolling is easily embedded in the surface layer portion during hot rolling. If a cold-rolled steel sheet and a galvanized steel sheet are manufactured in a state where the scale is embedded, the portion becomes a scale surface defect and cannot be applied to press forming. In order to suppress such deterioration of the surface properties, the Al content is 0.2% by mass or less.

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.

“(14/27) × (mass% Al / mass% N)” value: 50 or less “(14/27) × (mass% Al / mass% N)” value defined by Al content and N content Is an important parameter for controlling the form of nitride affecting the surface properties, and as shown in FIG. 1 described above, the value of “(14/27) × (mass% Al / mass% N)” is 50 In the case of exceeding, the amount of precipitation of AlN on the austenite grain boundary increases during casting, and the surface quality of the steel sheet is remarkably deteriorated due to slab surface cracking caused by grain boundary embrittlement. For this reason, the “(14/27) × (mass% Al / mass% N)” value is 50 or less.

  In the present invention, in addition to the above alloy elements, one or more selected from the group of Cr, V, Mo, Nb, Ti, and B can be contained according to the strength characteristics of the high-tensile steel plate. . The reasons for limiting the components will be described below.

Cr: 0.02-1.5 mass%
Cr is effective for strengthening the steel sheet and may be added as necessary. When the addition amount is less than 0.02% by mass, the strengthening ability is small. Moreover, since the production | generation of Cr oxide will be accelerated | stimulated on the steel plate surface at the time of continuous annealing when the addition amount exceeds 1.5 mass%, the chemical conversion property of a steel plate will fall remarkably. For this reason, when adding Cr, it is 0.02-1.5 mass%. From the viewpoint of strengthening the steel, the lower limit of the Cr content is desirably 0.05% by mass, and from the viewpoint of chemical conversion properties, the upper limit of the Cr content is preferably 1.2% by mass.

V: 0.003-0.1% by mass
The fine carbide formed by combining V and C is effective for strengthening the steel sheet, and V may be added as necessary. If the amount of V is less than 0.003 mass%, the effect is small. On the other hand, if the amount of V exceeds 0.1% 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 adding V, it is set as 0.003-0.1 mass%. From the viewpoint of strengthening the steel, the lower limit of the V amount is desirably 0.01% by mass, and from the viewpoint of surface properties, the upper limit of the V amount is preferably 0.07% by mass.

Mo: 0.05-0.5 mass%
Mo is effective for strengthening the steel sheet and may be added as necessary. When the addition amount is less than 0.05% by mass, the strengthening ability is small. Moreover, when Mo amount exceeds 0.5 mass%, formation of Mo oxide will be accelerated | stimulated on the steel plate surface at the time of continuous annealing, and the chemical conversion property of a steel plate will fall remarkably. For this reason, when adding Mo, it is 0.05-0.5 mass%. Moreover, it is preferable that the upper limit of the amount of Mo shall be 0.3 mass% from a viewpoint of chemical conversion property.

Nb: 0.01-0.08 mass%
Since the carbide formed by combining Nb and C contributes to strengthening of the steel sheet, Nb may be added as necessary. When the amount of Nb added is less than 0.01% by mass, the effect is small. On the other hand, when the Nb content exceeds 0.08 mass%, the elongation of the steel sheet is remarkably reduced due to the generation of excess carbides. For this reason, when adding Nb, it is set as the range of 0.01-0.08 mass%. From the viewpoint of strength and elongation, the preferable range of the amount of Nb is 0.02 to 0.06% by mass.

Ti: 0.01-0.08 mass%
Since the carbide formed by combining Ti and C is effective for strengthening the steel sheet, Ti may be added as necessary. If the amount of Ti is less than 0.01% by mass, the effect is small. Moreover, when the amount of Ti exceeds 0.08 mass%, the amount of carbides generated increases, and the reduction in the elongation of the steel sheet becomes significant. For this reason, when adding Ti, it is 0.01-0.08 mass%. From the viewpoint of strength and elongation, the Ti content is preferably in the range of 0.02 to 0.06% by mass.

B: 0.0002 to 0.003 mass%
B segregates at the austenite grain boundaries during heating in continuous annealing, suppresses ferrite transformation from austenite during cooling, and promotes the formation of martensite. Therefore, B is effective for strengthening steel sheets, and is added as necessary. Also good. When the amount of B is less than 0.0002% by mass, this effect is small. Moreover, when the amount of B exceeds 0.003 mass%, this effect is not only saturated, but also when applied to hot dip galvanizing, the alloying rate of the plating is remarkably reduced, resulting in poor plating adhesion. For this reason, when adding B, it is 0.0002 to 0.003 mass%. Moreover, the preferable range of B amount is 0.0005 to 0.002 mass%.

  About chemical components other than said steel component, the effect of this invention will not be impaired unless it adds excessively. For example, if Ni and Cu are 0.2 mass% or less, the target characteristics of the present invention are not adversely affected. That is, other alloy elements may be contained as long as they do not adversely affect the target characteristics of the present invention.

(2) Manufacturing method of continuous casting slab and manufacturing method of steel plate Molten steel having the chemical components described in the above (1) is melted, and this molten steel is continuously cast. In order to reduce the occurrence of scale surface defects due to increased slab surface cracking susceptibility during continuous casting, as shown in FIG. 1 described above, “(14/27) × (mass% Al / mass of molten steel) % N) ”value, the casting speed is 1.5 m / min or more, and“ 4.0-0.68 × log [(14/27) × (mass% Al / mass% N)] m / min ”. Preferably, the casting speed is 2.0 m / min or more, and “3.5-0.68 × log [(14/27) × (mass% Al / mass% N)] m / min”. Further, the specific water amount of the cooling water in the secondary cooling zone is 1.0 L / kg or more and “2.5 + Vc / 1.5” L / kg or less depending on the casting speed (Vc). Range. Preferably, the specific water amount is 1.5 L / kg or more and “2.0 + Vc / 1.5” L / kg or less.

The continuously cast slab may be hot-rolled as a hot piece, or may be hot-rolled after being heated. When the surface properties are improved by increasing the amount of scale peeling by promoting the generation of the surface primary scale, it is preferable to heat the slab at 1100 ° C. or higher. After rough rolling, finish rolling and winding into a coil. At the time of finish rolling, when the rolling is finished at a temperature lower than the Ar ₃ point, the shape of the steel sheet is deteriorated. Therefore, the finishing temperature is preferably an Ar ₃ point or higher.

  When a hot-rolled steel sheet is used as a final product, when the coiling temperature exceeds 700 ° C., it becomes difficult to stably obtain a strength of 590 MPa or more, and the structure tends to coarsen in the surface layer portion of the steel sheet. Decrease increases. In order to prevent this, the coiling temperature is set to 700 ° C. or lower. Further, from the viewpoint of the shape of the steel sheet during coil winding, the winding temperature is preferably room temperature or higher. A more preferable winding temperature is 100 to 650 ° C. When the pickling step is performed, the winding temperature is preferably 700 ° C. or lower from the viewpoint of the peelability of the scale. When cold rolling is subsequently performed, the winding temperature is 400 ° C. or higher in order to reduce rolling load. Is desirable. A more preferable winding temperature in this case is 450 to 650 ° C.

  When a cold rolled steel sheet is used, if the cold rolling rate exceeds 90%, an increase in the rolling load becomes remarkable. On the other hand, if the rolling rate is less than 30%, it is not preferable for promoting recrystallization of ferrite during annealing. For this reason, the cold rolling rate is preferably 30 to 90%. More preferably, cold rolling is performed at a rolling rate of 40 to 80%.

  After cold rolling, continuous annealing is performed at a temperature equal to or higher than the recrystallization temperature of ferrite for the purpose of imparting good ductility to the steel sheet. In this case, if annealing is performed at a temperature exceeding 900 ° C., there is a concern that the toughness of the steel sheet is reduced due to the coarsening of ferrite. For this reason, it is preferable that an annealing temperature shall be 900 degrees C or less, More preferably, an annealing temperature shall be 880 degrees C or less.

  The hot-rolled steel sheet and cold-rolled steel sheet obtained in the present invention can be treated with a zinc-based plating film. In this case, either continuous hot dip galvanizing or electrogalvanizing may be used, and the type of plating may be pure galvanizing, alloying galvanizing, zinc + nickel alloy plating, or zinc + aluminum alloy plating. In the case of producing a hot-dip galvanized steel sheet based on a hot-rolled steel sheet, the hot-rolled steel sheet after pickling is subjected to continuous annealing at a temperature of 600 to 900 ° C., more preferably 700 to 880 ° C., and then predetermined plating is performed. What is necessary is just to process. When producing a galvanized steel sheet based on a cold-rolled steel sheet, a predetermined plating treatment may be applied to the annealed cold-rolled steel sheet.

  Further, even if the hot-rolled steel sheet, cold-rolled steel sheet, zinc-plated hot-rolled steel sheet, and zinc-plated cold-rolled steel sheet thus obtained are subjected to surface treatment such as chemical conversion treatment or organic coating treatment The intended characteristics of the invention are not impaired.

  As described above, according to the present invention, by appropriately controlling the chemical composition of steel and the casting speed and cooling conditions in the continuous casting process, the continuous cast slab, which is the material of the high-tensile steel plate, can be produced with high productivity. It becomes possible to produce stably without generating surface cracks, and as a result, it is possible to stably produce high-tensile steel sheets with excellent surface quality required for materials such as automotive interior plate parts. .

  Molten steel of the components shown in Table 1 (No. 1-3, 6-14: Steel of the present invention, No. 4, 5, 15-21: Comparative steel) is melted, and this molten steel is continuously bent into a slab of vertical bending type. The slab was cast by spray cooling using a machine under the conditions of a casting speed of 2.5 m / min and a specific amount of secondary cooling zone cooling water of 2.5 L / kg. The cast slab was cooled to room temperature, and then the surface property of the slab was inspected and evaluated by the penetration flaw detection method. The casting conditions in which the casting speed is 2.5 m / min and the specific water amount in the secondary cooling zone is 2.5 L / kg are within the scope of the present invention, as is apparent from FIGS. 1 and 2 described above. This is the casting condition.

  The slab surface was inspected by the penetrant flaw detection method, and the slab surface was evaluated according to four levels of “good”, “acceptable”, “deteriorated”, and “bad” according to the occurrence state (number and length) of cracks. Here, “good” means that no cracks are detected, “allowable” means that very few cracks are detected, “deterioration” means that minor cracks are detected, and “bad” means many A crack was detected. Table 2 shows the slab surface properties examined for each heat cast.

  As shown in Table 2, in the heat whose steel composition is within the range of the present invention, the determination of the slab surface was all “good”, but in the heat whose steel composition was outside the range of the present invention, the casting conditions were the same. Even if it was, the judgment of the slab surface was only “deterioration” and “defect”.

  Without carrying out surface care of the slab, these slabs are conveyed to a hot rolling step, hot-rolled at a heating temperature of 1250 ° C., a finishing temperature of 860 ° C., and a winding temperature of 550 ° C. A hot-rolled steel sheet was produced.

  Heat Nos. 1-5 investigated the properties (surface properties, tensile properties) of the hot-rolled steel sheet, and Heat Nos. 6-21 were cold-rolled steel sheets, hot-dip galvanized steel sheets, hot-rolled bases. Hot-dip galvanized steel sheets were manufactured, and the characteristics (surface properties, tensile characteristics) of each steel sheet were evaluated.

  Cold-rolled steel sheets and cold-rolled base hot-dip galvanized steel sheets are pickled hot-rolled steel sheets, cold-rolled to a thickness of 1.4 mm (rolling rate 50%), and then subjected to continuous annealing or continuous hot-dip galvanizing. Manufactured. The hot-rolled base hot-dip galvanized steel sheet was produced by subjecting the hot-rolled steel sheet after pickling to continuous hot-dip galvanizing. In continuous annealing or continuous hot dip galvanizing, the annealing temperature is 820 ° C., and in continuous hot dip galvanizing, zinc is plated at 460 ° C. and heated to 550 ° C. in the cooling stage after annealing. After the crystallization treatment, it was cooled to room temperature. The steel sheet thus obtained was subjected to temper rolling of 0.5%, and then the characteristics were investigated.

The surface properties of the steel sheet are obtained by measuring the number of surface defects with a length of 2 mm or more found on the front and back surfaces and calculating the number of the surface area per unit area (pieces / m ² ). When the number of surface defects was greater than 0.3 / m ^2, it was determined that application to press molding was difficult. In addition, the tensile properties were obtained by conducting a tensile test using a JIS No. 5 tensile test piece at a tensile speed of 10 mm / min. Table 3 shows the survey results of the heat numbers used, the classification, the tensile test values, and the number of surface defects of each steel plate. Table 3 also shows the slab surface determination results.

As shown in Table 3, in steel plates No. 1 to 3 and 6 to 32 whose steel composition is within the range of the present invention, good surface properties are obtained in the slab stage, and the hot rolled steel plate (the final product) Hot), cold-rolled steel sheet (Cold), cold-rolled base hot-dip galvanized steel sheet (CG (Cold)), and hot-rolled base hot-dip galvanized steel sheet (CG (Hot)) have 0.10-0. 13 / m ² , 0.008 to 0.012 / m ² , 0.006 to 0.013 / m ² , and 0.007 to 0.012 / m ² , all having good surface quality Had. Steel plates No. 1 to 3, Steel plates No. 6 to 29, and Steel plates No. 30 to 32 have a tensile strength of 615 to 620 MPa, an elongation of 27.5 to 28.3%, and a tensile strength of 592 to 620 MPa, respectively. It had an elongation of 26.9-31.8%, a tensile strength of 785-820 MPa, and an elongation of 21.5-22.5%.

On the other hand, in steel plates No. 4, 5, 33 to 53 whose steel composition is outside the range of the present invention, all have a tensile strength of 590 MPa or more, but the slab surface properties deteriorate and a good surface in the final product. Quality was not obtained. That is, the steel plates No. 4 and 5, steel plates No. 33 to 41, and steel plates No. 48 to 50 are 0.34 to 0.51 pieces / m ² , 0.35 to 0.60 pieces / m ² , 0, respectively. .52 to 0.54 / m ² and the number of surface defects is large. Further, the number of surface defects of the steel plates No. 42 to 44 is as large as 0.62 to 0.65 / m ^2, and the non-plating occurs in the hot dip galvanized steel plates No. 43 and 44. Furthermore, the steel sheets No. 45 to 47 and 51 to 53 had deteriorated surface properties, and the elongation of the steel sheets was also reduced.

  The steel type No. 12 (invention steel) shown in Table 1 described above was melted, and this molten steel was fixed at a constant water volume of 2.5 L / kg in the secondary cooling zone using a vertical bending slab continuous casting machine. The casting speed was changed to a range of 1.0 to 4.5 m / min as a value, and the specific water amount of the secondary cooling zone was 0.5 to 5.5 L with the casting speed being a constant value of 2.5 m / min. Casting changed to / kg was carried out for a total of 19 heats. The cast slab was cooled to room temperature, and then the surface property was determined by the same method as in Example 1.

  Next, hot rolling, pickling and cold rolling were performed under the same production conditions as in Example 1, followed by continuous annealing or continuous hot dip galvanizing treatment at an annealing temperature of 830 ° C. (galvanizing at 460 ° C. and 550 ° C. And temper rolling with an elongation of 0.5%. The surface properties and tensile properties of the cold-rolled steel sheet and hot-dip galvanized steel sheet thus obtained were examined by the same method as in Example 1. Table 4 shows the examination results of the casting conditions, slab surface determination results, classification, tensile test values, and number of surface defects of each steel plate.

In the steel plates No. 56 to 58 and 64 to 68 in which the casting speed and the specific water amount are within the range of the present invention, no cracks were observed on the slab surface, and the number of surface defects of the steel plates was 0.10 to 0.14 / m respectively. ² and 0.09 to 0.15 / m ^2, and the surface quality of the product was extremely good. In the steel plates No. 55 and 59 and the steel plates No. 63 and 69 in which the slab surface judgment was “acceptable”, although some cracks occurred in the slab, the number of surface defects of the steel plates was 0.20 to 0.22 respectively. Pieces / m ² , 0.17 to 0.24 pieces / m ² , and the target value of 0.3 pieces / m ² or less was achieved.

On the other hand, the steel plate No. 54, 60, 61 whose casting speed is outside the scope of the present invention and the steel plate No. 62, 70-72 whose specific water content is outside the scope of the present invention have poor slab surface properties. surface defects number of steel sheets, each 0.32 to 0.53 pieces / m ^2, more and 0.35 to 0.43 pieces / m ^2, the surface quality was not satisfactory.

It is a figure which shows the influence of ratio and casting speed of Al content and N content in a slab which have on slab surface property. It is a figure which shows the influence of the casting speed and specific water quantity which has on slab surface property.

Claims (6)

In mass%, C: 0.03-0.10%, Si: 1.0% or less, Mn: 0.5-3.0%, P: 0.1% or less, S: 0.02% or less, Al: 0.2% or less, N: 0.006% or less, (14/27) × (% Al /% N) defined by Al content and N content is 50 or less, and the balance Is cast at a casting speed that satisfies the following formula (1) defined by the Al content and N content, and the following (2) defined by the casting speed: The method for producing a continuously cast slab, characterized in that the secondary cooling zone is spray-cooled with a specific water amount satisfying the above formula.
1.5 ≦ Vc ≦ 4.0-0.68 × log [(14/27) × (% Al /% N)]… (1)
1.0 ≦ Q ≦ 2.5 + Vc / 1.5… (2)
However, in the formulas (1) and (2), Vc is the casting speed (m / min), Q is the specific water amount in the secondary cooling zone (liter / kg),% Al is the Al content of the molten steel, and% N is It is N content of molten steel.   The molten steel is further in mass%, Cr: 0.02-1.5%, V: 0.003-0.1%, Mo: 0.05-0.5%, Nb: 0.01-0. It contains 1 type (s) or 2 or more types chosen from the group of 0.08%, Ti: 0.01-0.08%, B: 0.0002-0.003%, It is characterized by the above-mentioned. Manufacturing method for continuous casting slabs.   A continuous cast slab produced by the method for producing a continuously cast slab according to claim 1 or 2 is hot-rolled and wound at a temperature of 700 ° C or lower, and a high-temperature rolling process is provided. A method for producing a tension hot-rolled steel sheet.   A continuous cast slab produced by the method for producing a continuous cast slab according to claim 1 or 2 is hot-rolled and wound at a temperature of 700 ° C. or less, and obtained by the hot-rolling step. A high-tensile galvanized steel sheet comprising a pickling process for pickling the hot-rolled steel sheet obtained, and a plating process for forming a zinc-based plating film on the surface of the pickled steel sheet obtained by the pickling process Manufacturing method.   A hot rolling step of hot rolling a continuous cast slab manufactured by the method for manufacturing a continuous cast slab according to claim 1 or claim 2, and pickling the hot rolled steel sheet obtained by the hot rolling step The pickling step, the cold rolling step for cold rolling the pickled steel plate obtained by the pickling step, and the cold rolled steel plate obtained by the cold rolling step at a temperature above the recrystallization temperature. A method for producing a high-tensile cold-rolled steel sheet, comprising: a continuous annealing step for annealing.   A hot rolling step of hot rolling a continuous cast slab manufactured by the method for manufacturing a continuous cast slab according to claim 1 or claim 2, and pickling the hot rolled steel sheet obtained by the hot rolling step The pickling step, the cold rolling step for cold rolling the pickled steel plate obtained by the pickling step, and the cold rolled steel plate obtained by the cold rolling step at a temperature above the recrystallization temperature. A method for producing a high-tensile galvanized steel sheet, comprising: a continuous annealing process for annealing; and a plating process for forming a zinc-based plating film on the surface of the steel sheet obtained by the continuous annealing process.

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