KR20030049731A - Steel strip for the automotive reinforcement parts and method of manufacturing thereof - Google Patents

Abstract

PURPOSE: A cold rolled strip capable of obtaining high strength from the final product by heat treatment (or cooling) after processing as it being processed into a complicated shaped product using low strength and high forming property before heat treatment, and a manufacturing method of the cold rolled strip are provided. CONSTITUTION: The cold rolled strip with superior heat treatment characteristics for automobile reinforcements comprises 0.20 to 0.30 wt.% of carbon, 0.5 wt.% or less of silicon, 0.02 wt.% or less of sulfur, 0.005 wt.% or less of nitrogen, 0.02 to 0.06 wt.% of aluminum, 0.02 wt.% or less of phosphorus, 1.0 to 2.0 wt.% of manganese, 0.002 to 0.006 wt.% of boron, and a balance of Fe and other inevitable impurities, wherein atomic ratio of boron/nitrogen satisfies 1 or more. The manufacturing method of the cold rolled strip with superior heat treatment characteristics for automobile reinforcements comprises the processes of hot rolling the soaked aluminum killed steel in the finish rolling temperature range of 850 to 950 deg.C that is right above Ar3 transformation point by soaking at a temperature of 1,050 to 1,300 deg.C an aluminum killed steel comprising 0.20 to 0.30 wt.% of carbon, 0.5 wt.% or less of silicon, 0.02 wt.% or less of sulfur, 0.005 wt.% or less of nitrogen, 0.02 to 0.06 wt.% of aluminum, 0.02 wt.% or less of phosphorus, 1.0 to 2.0 wt.% of manganese, 0.002 to 0.006 wt.% of boron, and a balance of Fe and other inevitable impurities, wherein atomic ratio of boron/nitrogen satisfies 1 or more; coiling the hot rolled strip in the temperature range of 650 to 800 deg.C; and cold rolling the coiled strip to a cold reduction ratio of 30 to 80%.

Description

Cold rolled steel sheet for automobile reinforcement with excellent heat treatment characteristics and method for manufacturing thereof {Steel strip for the automotive reinforcement parts and method of manufacturing technique}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cold rolled steel sheet used for a reinforcement or structural member of an automobile body, and more particularly, a heat treatment after processing (or a high temperature mold) while processing into a product having a complex shape using low strength and high workability before heat treatment. It is to provide a cold rolled steel sheet and a method of manufacturing the same that can be obtained high strength in the final product by cooling after processing).

Conventional high strength steel for automobile reinforcement is a low carbon steel containing a large amount of alloying elements has been manufactured to a tensile strength of 100kgf / mm ² using a continuous annealing equipment. In this case, since many hard phases such as martensite or bainite are generated in the microstructure of the steel sheet, the workability is very low, so that it is difficult to apply to a complex part, and the steel sheet has a lot of constraints due to its high strength. In addition, the limit of tensile strength that can be produced in conventional continuous annealing equipment does not exceed 120kgf / mm ² . There is a method to increase the strength by quenching after the high temperature annealing by installing a water cooling equipment in the quenching zone of the continuous annealing equipment, but there is a disadvantage to remove the oxide layer formed on the surface after water cooling, and the limit of tensile strength does not exceed 140kgf / mm ² have. In addition, since it has a characteristic of delayed fracture that occurs after a certain time after processing, it has been known as a material that is not suitable as a reinforcing material of a vehicle that places importance on crash characteristics.

The present invention is to process a product of complex shape using low strength and high processability before heat treatment, and then heated to high temperature and then subjected to water cooling to obtain high strength in the product, or to process at high temperature by utilizing high processability in hot. At the same time to provide a cold rolled steel sheet and a method for producing the same that can be obtained by high cooling in the final product by cooling.

Cold rolled steel sheet of the present invention for achieving the above object,

By weight ratio of carbon 0.20 to 0.30%, silicon 0.5% or less, sulfur 0.02% or less, nitrogen 0.005% or less, aluminum 0.02 to 0.06%, phosphorus 0.02% or less, manganese 1.0 to 2.0%, boron 0.002 to 0.006%, boron / nitrogen The atomic ratio satisfies 1 or more and is composed of the remaining Fe and other unavoidable impurities.

In addition, the cold rolled steel sheet manufacturing method of the present invention, by homogenizing the aluminum-kilted steel composition as described above at 1050 ~ 1300 ^o C hot-rolled under the conditions of the finish rolling temperature of 850 ~ 950 ^o C that is directly above the Ar ₃ transformation point and 650 ~ 800 ^o winding in the temperature range of C, followed by cold rolling at a cold reduction rate of 30 to 80%.

Hereinafter, the present invention will be described in detail.

In the present invention, C, B, or Cr, Mo on cold-rolled steel sheet to secure high strength through heat treatment after processing (or cooling after high temperature mold processing) instead of processing into a complicated shape product using low strength and high workability before processing. It is characteristic to add alloying elements, such as N and Mn, suitably with a hardenability improvement element. That is, the addition of carbon in the production of the steel sheet serves to improve the hardenability of the steel when cooling occurs during processing at high temperatures to increase the strength of the final product. In addition, since the role of improving the strength of the martensite formed during the cooling process contributes further to the strength. Since boron is an element with a large hardenability, even if it is added in small amounts, big hardenability can be obtained. However, since boron nitride (BN) is formed in the nitrogen-rich steel, hardening by boron is greatly reduced. Therefore, it is important to properly control the amount of boron added through the B / N atomic ratio. Furthermore, molybdenum or chromium is an element with high hardenability, so it is added to improve the strength of the steel. The reason for component limitation of the cold rolled steel sheet of this invention is demonstrated more concretely.

[Cold rolled steel sheet]

, Carbon (C): 0.20 ~ 0.30%

If the amount of C is less than 0.20% by weight (hereinafter referred to as%), since the curing ability is low, sufficient strength cannot be secured even when heated to high temperature and water cooled. In addition, even when processing and cooling in a mold after heating to a high temperature, it is difficult to secure sufficient strength, it is not possible to manufacture a product having a high tensile strength even if the heat treatment method is applied. If the amount of carbon is too low, the tensile strength of the product is not high because the strength of the martensite formed after the heat treatment is not high enough. If the amount of carbon is more than 0.30%, the final product with high tensile strength can be obtained by improving the hardenability during heat treatment, but various difficulties occur in the manufacturing stage of the cold rolled steel sheet. In other words, not only makes it difficult to weld the steel sheet for continuous work in the pickling and rolling process, but also because the strength of the steel sheet is too high in the annealing process, thus degrading the sheet metal in the continuous annealing furnace to damage the furnace body. This can cause serious problems in the manufacturing process.

, Silicon (Si): 0.5% or less

Si slightly improves the hardenability of the steel sheet and increases the strength of the steel sheet produced by the solid solution strengthening effect, but the effect is not so large, it is preferable to limit the amount of silicon to 0.5% or less.

, Manganese (Mn): 1.0 ~ 2.0%

Since Mn has a great effect of delaying the transformation of austenite to ferrite and lowering the transformation temperature of austenite to ferrite, the amount of addition should be appropriately controlled. If the amount of Mn added is less than 1.0%, the temperature at which the microstructure of the steel sheet is completely austenized is increased when the processed product is heated or heated for high temperature processing. In other words, in order to obtain sufficient strength in the final product, the heating temperature of the steel sheet should be increased. If the heating temperature of the steel sheet is high, since the thickness of the oxide layer formed on the surface is increased, the cost for removing the oxide layer is increased, so that possible low temperature heating is advantageous. And if the microstructure is not fully austenitized, sufficient strength of the product cannot be obtained during water cooling or mold processing cooling after heating. However, when a large amount of manganese is added more than 2.0%, the manufacturing cost is greatly increased, making it difficult to weld for continuous operation during pickling and rolling of the hot rolled sheet.

, Boron (B): 0.002 ~ 0.006%

Since B is an element with a large hardenability, even when a trace amount is added, great strength can be obtained in the product of heat-treated steel. However, if nitrogen (N) is present in the steel, B / N atomic ratio should be added to 1 or more because it forms BN compound by combining with nitrogen in hot rolling step. Therefore, since there is generally 0.001 to 0.005% of solid solution nitrogen in steel, the lower limit of boron is 0.002%. Since the hardening effect does not increase significantly even if the addition amount of boron continues to increase, it is preferable to make the upper limit addition amount into 0.0060%.

And nitrogen (N): 0.005% or less

N is an element that is inevitably added to steel, but when the amount is too large, coarse BN precipitates are formed, which greatly reduces the hardenability of the steel. Therefore, it is preferable to make the minimum addition amount of nitrogen into 0.005% or less.

And aluminum (Al): 0.02 ~ 0.06%

Al is added for the deoxidation of steel. When the amount of aluminum is less than 0.02%, oxygen is present in the steel. When the oxide-forming elements such as manganese, silicon, boron, and chromium are added during steelmaking, manganese oxide, silicon oxide, boron oxide, Since chromium oxide is formed, it is difficult to control components of manganese, silicon, boron, and chromium. And when the amount of aluminum is more than 0.06%, the amount of aluminum is added more than necessary to increase the manufacturing cost and generate a large amount of surface defects of the steel sheet.

, Sulfur (S) and the (P): not more than 0.02% each

S and P are inevitably contained in the production of steel, so the addition range is preferably 0.02% or less.

At least one selected from molybdenum (Mo) and chromium (Cr): 0.1 ~ 1.5%

Mo and Cr not only increases the hardenability but also increases the toughness of the heat-treated steel sheet, so when added to a steel sheet characterized by high impact energy absorption, the effect is very large. Therefore, the brittleness of the water-cooled product by the production of a large amount of martensite can be greatly improved. In addition, since the hardenability is improved, the strength reduction of a portion not directly contacting the mold during high temperature mold processing can be prevented. If the addition amount of molybdenum or chromium is not more than 0.1%, sufficient hardenability cannot be obtained, and even if the amount is continuously increased, the hardenability does not increase significantly and the manufacturing cost required for steel sheet production is greatly increased, so it is preferable to limit it to 1.5% or less. .

In the case of processing the cold rolled steel sheet formed as described above, and water-cooling after heating, it can be made into a product having a tensile strength of 170kgf / mm ² or more, and in the case of hot mold processing and cooling after heating the cold rolled steel sheet to a high temperature It can be made into products with a tensile strength of 150kgf / mm ² or more.

The prepared product, when applied to high-strength steel, because the intensity is very high, it can be applied to automobile parts, have not applied a complex shape because of low workability, despite 150kgf / mm ² or more ultra-high strength steel for automotive reinforcement of high tensile steel as well It is also applicable to structural parts such as members and pillars.

[Manufacturing method of cold rolled steel sheet]

· Hot rolling

The steel melted in the above composition is subjected to homogenization treatment at about 1050 to 1300 ^o C similarly to normal conditions, and the finished homogenized specimen is finished hot under normal conditions in the range of 850 to 950 ^o C, which is directly above the Ar ₃ temperature. Rolling is performed, and hot rolling is carried out at 650 to 800 ^° C. If the hot-rolled coiling temperature is too low, bainite or martensite is formed in the hot-rolled steel sheet, which increases the strength of the hot-rolled steel sheet, making cold rolling difficult. Therefore, the hot rolled coil temperature was limited to over 650 ^° C.

- cold rolling

In order to match the shape and thickness of a steel plate, the hot rolled sheet obtained above is cold-rolled. Automotive parts are made of cold rolled sheet or annealed steel sheet of uniform thickness because they require high thickness uniformity. Since the strength of the final product is determined according to the heat treatment conditions after processing or in the high temperature processing cooling process, the effect of the reduction ratio on the material is not significant. However, if the reduction ratio increases, the productivity is lowered, so it is preferable to limit the reduction ratio upper limit to 80%. If the cold reduction rate is less than 30%, since the grain size of the heat-treated product is coarse, it has a weak brittleness to impact. And by cold rolling to produce a cold rolled steel sheet with a high precision of the thickness, if the cold reduction rate is too low, the thickness of the hot rolled steel sheet is lowered, so the load during hot rolling increases. Therefore, the lower limit of the reduction ratio is limited to 30%.

When the cold rolled plate obtained above is made into a product by high temperature mold processing, the cold rolled plate may be supplied to the demander by cold rolled plate.

· Annealing Process

If the annealing temperature of the cold rolled sheet is too low, it is difficult to secure sufficient workability, so it is preferable to limit the annealing temperature to 750 ^° C. or more.

The present invention will be described in more detail with reference to the following examples.

EXAMPLE

Table 1 shows the chemical composition of the inventive steel and the comparative steel, and the hot-roll was performed after maintaining the ingot of the molten steel for 1 hour in a 1250 ^o C heating furnace. At this time, the hot rolling finish temperature was 900 ^° C, the coiling temperature was 680 ^° C. Pickling was performed using a hot rolled steel sheet, and cold rolling was performed with a cold reduction ratio of 50%. Cold-rolled specimens were subjected to continuous annealing with an annealing temperature of 800 ^o C and an overaging temperature of 400 ^o C. After the hot working heat treatment, the cold-rolled specimens were heated to investigate the material properties, and transferred to a mold to process the heated sheet, and the steel sheet was heated. The specimens were quenched by a mold at the same time as they were processed at high temperatures. At this time, the heating temperature is 900 ^° C., the holding time was 5 minutes. And after the end of the processing was cut to the portion that can be cut tensile specimens were prepared tensile specimens. In order to investigate the properties of the material after heat treatment of the annealed steel sheet, tensile specimens were processed and loaded into a 900 ^° C. holding furnace, and homogenized for 5 minutes, and water cooled. Tensile tests were carried out using a universal tensile tester.

Table 2 shows the change in mechanical properties according to the heat treatment conditions or manufacturing conditions of the inventive steel and comparative steel.

As shown in Tables 1 and 2, Sample Nos. 4 to 8, which are steels of the present invention, have a tensile strength of 150kgf / mm ² or more in the final product when the cold rolled sheet is heated and processed after annealing material. In the case of cold heat treatment, the tensile strength is 170kgf / mm ² or more. The steel sheet is processed at a high temperature so that even products with complex shapes can be processed. And the elongation in the final product is not critical because no further processing is required. It can be used as structural member and reinforcing material of automobile because it can process parts with complicated shape and secure high strength.

Sample No. 1-3 steel does not have high tensile strength in the product because the added amount of carbon does not give sufficiently strong martensite structure after heat treatment and mold processing.

Steel No. 9 has a small amount of manganese and carbon in the steel, so sufficient austenite cannot be obtained when heated, and sufficient transformation strength is obtained in the final product because austenite-to-ferrite phase transformation may occur even during cooling of mold processing. Can not.

Steel Nos. 10 and 11 do not have sufficient strength in the final product after mold processing at high temperature because molybdenum, boron, and carbon are not added sufficiently.

As described above, since the cold rolled sheet of the present invention has excellent hardenability by heat treatment, when the cold rolled steel sheet is subjected to high temperature processing, it is possible to obtain a strength of 150kgf / mm ² or more in the final product by cooling by a mold. In addition, when cold-rolled annealing steel sheet is processed and heated to high temperature and then water cooled, tensile strength of 170kgf / mm ² or more can be obtained from the product, so when applied to structural members or reinforcement materials of automobiles requiring excellent tensile strength to density, There is an advantage that can greatly reduce the weight.

Claims (7)

By weight ratio of carbon 0.20 to 0.30%, silicon 0.5% or less, sulfur 0.02% or less, nitrogen 0.005% or less, aluminum 0.02 to 0.06%, phosphorus 0.02% or less, manganese 1.0 to 2.0%, boron 0.002 to 0.006%, boron / nitrogen Cold rolled steel sheet for automobile reinforcement with an atomic ratio of 1 or more and excellent heat treatment characteristics composed of the remaining Fe and other unavoidable impurities. [Claim 2] The cold rolled steel sheet for automobile reinforcement of claim 1, wherein at least one member selected from the group consisting of molybdenum or chromium is added to the aluminum-kilted steel. The cold rolled steel sheet according to claim 1 or 2, wherein the cold rolled steel sheet is used as a product having a high temperature mold and a tensile strength of 150 kgf / mm ² or more. The cold rolled steel sheet according to claim 1 or 2, wherein the cold rolled steel sheet is continuously annealed and heat treated after processing to be used as a product requiring a tensile strength of 170 kgf / mm ² or more. By weight ratio of carbon 0.20 to 0.30%, silicon 0.5% or less, sulfur 0.02% or less, nitrogen 0.005% or less, aluminum 0.02 to 0.06%, phosphorus 0.02% or less, manganese 1.0 to 2.0%, boron 0.002 to 0.006%, boron / nitrogen Aluminum-kilted steel, which has an atomic ratio of 1 or more and is composed of the remaining Fe and other unavoidable impurities, is homogenized at 1050 to 1300 ^o C, hot rolled to a finish rolling temperature of 850 to 950 ^o C, which is directly above the Ar ₃ transformation point, and 650 to 800 ^o A method for producing cold rolled steel sheet for automobile reinforcement having excellent heat treatment characteristics, including winding in a temperature range of C, followed by cold rolling at a cold reduction rate of 30 to 80%. [Claim 6] The cold rolled steel sheet for automobile reinforcing materials according to claim 5, wherein at least one selected from the group consisting of molybdenum or chromium is added to the aluminum-kilted steel. The method of claim 5, wherein the cold rolled sheet comprises continuous annealing at a temperature of 750 ^° C. or higher.