KR20120134384A - Cold-rolled steel sheet having excellent formability, surface properties for working and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet excellent in workability and discoloration resistance, and a method for manufacturing the same, in terms of weight% C: 0.001-0.004%, Sn: 0.05-0.35%, Mn: 0.07-0.55%, Al: 0.01-0.10% , B: 0.0005 ~ 0.0020, P: 0.003 ~ 0.020%, N: 0.001 ~ 0.004%, S: 0.015% or less, cold rolled steel sheet having excellent workability and discoloration resistance including residual Fe and other unavoidable impurities, and a method of manufacturing the same It is a technical point.
According to the present invention, it can be manufactured at a lower cost than conventional stainless steel sheet, has various processing characteristics such as extension flange and bending, and is excellent in room temperature workability, and suppresses surface oxidation of the plating layer even if it is maintained for a long time at high temperature after hot dip plating. The cold rolled steel sheet excellent in discoloration resistance can be manufactured.

Description

COLD-ROLLED STEEL SHEET HAVING EXCELLENT FORMABILITY, SURFACE PROPERTIES FOR WORKING AND MANUFACTURING METHOD THEREOF}

The present invention relates to a high heat-resistant cold-rolled steel sheet used in automobiles, home appliances and boilers and the like, and more particularly, to a cold rolled steel sheet produced by optimizing steel components and process conditions and having excellent discoloration resistance. It is about a method.

Cast iron is generally used for parts of products such as automobile exhaust systems, household flues, ovens and boilers, but aluminum is melted to secure such high temperature characteristics because it is continuously exposed to high temperatures of several hundred degrees (° C.) and requires high heat resistance. Plated steel sheet, stainless steel sheet, etc. were processed and used.

     Here, the high temperature property is one of the characteristics when the processed product is used in a high temperature environment, and includes sag resistance, high temperature strength, and discoloration resistance, and the product property may be degraded due to local temperature rise when using a product such as an automobile exhaust system. High temperature characteristics are required. The sag resistance is a phenomenon in which the steel sheet is repeatedly exposed to high temperature, causing a change in material and sagging. When such a phenomenon occurs, it is difficult to maintain the shape of the molded part. Since the product shape is deformed or destroyed due to deterioration, it is necessary to satisfy the yield strength of 55 MPa or more at a high temperature of about 700 ° C. in order to secure the shape freezing property. Discoloration resistance, which is a surface property for securing, is required.

     Conventionally, stainless steel sheet has been mainly used for heat resistance. However, stainless steel sheet is not only expensive to manufacture due to the addition of expensive alloying elements such as Cr and Ni, but also has a grain boundary of Cr and C when heated at high temperature. By combining, grain boundary corrosion occurs in the Cr depleted zone, which is caused by the precipitation of chromium carbide (chromium carbide) at the grain boundary, resulting in poor corrosion resistance.

     In addition, in order to secure oxidation resistance at high temperature, an aluminum hot dip galvanized steel sheet may be used, but when the aluminum hot dip galvanized steel sheet is heated to a high temperature of 400 ° C. or higher, an alloy layer at an interface due to the interdiffusion reaction between Fe and Al is grown. There is a problem in that the application can be applied because it loses the gloss of the surface in a short time and the color change is insufficient heat resistance.

     In addition, in addition to the high temperature characteristics described above, heat-resistant steel sheet is manufactured so that products such as automobile exhaust system, home communication, oven and boiler, etc. are accommodated in a limited space, and formed into a complicated shape by various methods to expand or bend after piping. In addition to the high temperature characteristics, a room temperature processability is also required because a process is required.

     In order to solve this problem, Japanese Patent Laid-Open No. Hei 8-319548 intends to manufacture a hot-dip aluminum plated steel sheet having excellent high temperature strength by forming a coating layer on the surface of the steel sheet, but improving the plating conditions rather than improving the characteristics of the steel sheet. Since the present invention is not only difficult to adjust when the plating layer is changed, there is a problem that it is difficult to obtain target heat resistance.

In addition, Japanese Laid-Open Patent Publication No. 9-176816 attempts to improve heat resistance and workability by heat-treating a steel sheet after adjusting aluminum content of Al and solid solution N in steel, but the steel sheet is used at a temperature of 550 ° C. or higher. It is difficult to be applied to a member of, and it is not easy to adjust the amount of the steel component added.

In order to solve the above problems, the present inventors have repeatedly conducted research and experiments and propose the present invention based on the results, and the present invention has high temperature characteristics and room temperature processability, such as automobile exhaust system, home communication, oven and boiler. At the same time, it is an object of the present invention to provide a cold rolled steel sheet having excellent workability and discoloration resistance which can be manufactured at low cost by optimizing steel components and processing conditions while reducing the addition of expensive alloying elements for use in required applications. .

In order to achieve the above object, the present invention provides a weight% of C: 0.001 to 0.004%, Sn: 0.05 to 0.35%, Mn: 0.07 to 0.55%, Al: 0.01 to 0.10%, B: 0.0005 to 0.0020, and P: 0.003 to Provided is a cold rolled steel sheet excellent in workability and discoloration resistance containing 0.020%, N: 0.001 to 0.004%, S: 0.015% or less, balance Fe and other unavoidable impurities.

     At this time, the atomic ratio of B and N, the value of B / N is also characterized by 0.5 to 1.5.

In addition, the cold rolled steel sheet is characterized in that the B-based nitride precipitates are formed.

Furthermore, the Sn-based oxide layer is formed on the surface of the cold rolled steel sheet. In addition, the Sn-based oxide layer is characterized by including a Sn ₂ O ₃ layer.

     In addition, the present invention is C: 0.001 ~ 0.004%, Sn: 0.05 ~ 0.35%, Mn: 0.07 ~ 0.55%, Al: 0.01 ~ 0.10%, B: 0.0005 ~ 0.0020, P: 0.003 ~ 0.020%, N : 0.001 ~ 0.004%, S: 0.015% or less, steel slab containing remainder Fe and other unavoidable impurities is heated and hot rolled, cold rolled after winding, and then cold rolled steel sheet is annealed at a temperature of 800 ° C. or higher. And it provides the manufacturing method of the cold rolled sheet steel excellent in workability and discoloration resistance characterized by cooling with the cooling rate 40 degreeC / sec or more with respect to the annealed steel plate.

     At this time, the steel slab is characterized in that the atomic ratio of the B and N, the value of B / N is 0.5 ~ 1.5.

     In addition, the hot rolling is also characterized by finish rolling at 900 ~ 940 ℃.

     In addition, the hot rolling is characterized in that it comprises the step of cooling the hot rolled hot rolled plate at a cooling rate of 20 ~ 80 ℃ / sec.

     The winding is also characterized by being performed at 560 to 680 ° C.

Furthermore, the annealing treatment is also characterized by being performed at 800 to 900 占 폚.

As described above, according to the present invention, it can be manufactured at a lower cost than the conventional stainless steel sheet, and satisfies various processing characteristics of extension flangeability, bending property, and deep drawing property, and thus has excellent room temperature workability, and solid solution element. By reducing the aging resistance to increase the yield point does not occur, not only excellent moldability, but also cold-rolled steel sheet excellent in discoloration resistance when maintained at high temperature for a long time.

Hereinafter, the cold rolled steel sheet of the present invention will be described in detail.

The inventors of the present invention at low cost satisfy various processing characteristics such as stretch-flangeability, bendability, deep drawing, and anti-aging properties, and discoloration resistance at high temperatures. Repeated research and experiment to secure the same time to complete the present invention. As a result, the present invention contains an extremely low carbon content in the steel component, improves the aging resistance by controlling the addition amount of B and the atomic ratio of B and N, while optimizing annealing and cooling conditions to secure strength-ductility By forming Sn-based oxide layer on the surface of steel sheet by adding Sn, it has excellent discoloration resistance at high temperature, aging resistance and workability at room temperature, and makes cold rolled steel sheet suitable for product parts such as automobile exhaust system, home communication, oven, boiler, etc. There is this.

     First, the reason for component limitation of this invention is demonstrated. (Hereinafter, the weight percentage is simply expressed as%)

Carbon (C) is an element added to improve the strength of the steel sheet, the tensile strength and yield strength increases as the amount of C is increased, but if excessively added, the workability is sharply lowered by aging, so the upper limit is preferably 0.004%. However, if the content is less than 0.001%, the grain size increases and the material not only softens rapidly, but also abnormal growth occurs at high temperature, and thus it is difficult to secure the properties, so the content of C is limited to 0.001 to 0.004%.

     Manganese (Mn) is a solid solution strengthening element that increases the strength of steel and improves hot workability, but is an element that inhibits ductility and workability by MnS formation. Therefore, when Mn is added excessively, ductility falls and it becomes a factor of economic deterioration and center segregation by addition of a large amount of alloying elements, so an upper limit is 0.55%. However, if less than 0.07%, the workability is improved, but it is difficult to secure the target strength, the Mn content is limited to 0.07 ~ 0.55%.

     Aluminum (Al) is an element added for deoxidation of molten steel and is preferably contained in an amount of 0.01% or more because it is combined with solid solution elements in steel to improve aging characteristics. However, when excessively added in excess of 0.10%, the amount of inclusions in the steel increases to cause surface defects and workability is lowered, so the Al content is limited to 0.01 to 0.10%.

     Phosphorus (P) is an element that improves the strength and corrosion resistance of steel, and it is preferable to contain 0.003% or more in order to secure these properties. However, if the content exceeds 0.020%, it causes central segregation during casting and lowers workability. The content is limited to 0.003 to 0.020%.

     Nitrogen (N) exists in solid solution inside the steel and is effective for strengthening the material. If it contains less than 0.001%, sufficient rigidity cannot be obtained, and precipitate formation sites are reduced, and if the content exceeds 0.004%, Since it causes aging and hardening occurs to deteriorate moldability, the content of N is limited to 0.001% to 0.004%.

     Sulfur (S) combines with Mn in the steel to form non-metallic inclusions that act as corrosion initiation points and causes red shortness, so it is desirable to reduce the content as much as possible, so the content of S is limited to 0.015% or less. . However, in order to ensure the said effect reliably, it is preferable to manage at 0.010% or less.

     Boron (B) is combined with a solid solution element in the steel to form a B-based nitride precipitate to improve the aging resistance and workability, and by forming the precipitate inhibits grain growth of the steel even at high temperature conditions, thereby miniaturizing ferrite particles. It is preferable to contain at least 0.0005%, but if the content exceeds 0.0020%, the material is cured and brings about reduced annealing operability, so the content of B is limited to 0.0005 to 0.0020%.

Tin (Sn) is an element that exists in a solid state inside the steel to improve high temperature characteristics and corrosion resistance, and is heated and oxidized by heat treatment to form a Sn-based oxide layer such as Sn ₂ O ₃ on the surface of the steel sheet to form an alloying layer on the surface of the steel sheet. It is preferable to add 0.05% or more in order to obtain this effect, but to increase the manufacturing cost rather than contributing to the improvement of corrosion resistance or discoloration resistance. And since it acts as a factor of cold rolling workability deterioration, the content of Sn is limited to 0.05 ~ 0.35%.

     In order to secure the high temperature and room temperature characteristics targeted by the present invention, it is necessary to consider the effects of precipitate formation and solid solution elements in the steel. For this purpose, it is also important to maintain the atomic ratio of B and N and the B / N ratio in a certain range to ensure room temperature aging resistance, processability and high temperature discoloration resistance at the same time.

     In order to optimize the aging resistance at room temperature, processability, and the effect of inhibiting grain growth at high temperature, it is necessary to keep the value of B / N atomic ratio B / N at 0.5 to 1.5. If the B / N value is less than 0.5, there is a problem that the room temperature aging resistance is deteriorated due to excessive solid solution elements in steel, and the effect of suppressing grains at low temperature is insufficient, and when the B / N value exceeds 1.5, ductility is lowered and workability is decreased. It is preferable to limit the value of B / N to 0.5-1.5.

     The cold rolled steel sheet of the present invention comprises the above components and consists of the balance Fe and other unavoidable impurities. And if necessary, an alloying element may be further added to improve the characteristics of the present cold rolled steel sheet, an alloying element that is not disclosed in the embodiment of the present invention is added and is not interpreted to be excluded from the scope of the present invention.

     Hereinafter, the manufacturing method of the cold rolled steel sheet excellent in workability and discoloration resistance of this invention is demonstrated in detail.

     After reheating the steel slab having the above composition, it is hot rolled, cold rolled after winding, and cold-rolled steel sheet having excellent workability and discoloration resistance by cooling at a cooling rate of 40 ° C./sec or more on a steel sheet subjected to annealing at a temperature of 800 ° C. or higher. Can be prepared.

     After reheating the steel slab formed like the above-described component system at a normal temperature, the finish rolling temperature during hot rolling is 900 to 940 ° C. If the finish rolling temperature is less than 900 ° C, the hot rolling is completed in a relatively low temperature region. When the final formed grains are mixed and workability and rolling property are lowered, and when the finish rolling temperature exceeds 940 ° C, uniform hot rolling is not performed throughout the thickness, resulting in insufficient grain refinement, which is caused by grain coarsening. Since impact toughness falls, finish rolling temperature is restrict | limited to 900-940 degreeC.

     After the hot finish rolling, the hot rolled steel sheet is cooled at a run-out table at a cooling rate of 20 to 80 ° C./sec. If the cooling rate is less than 20 ° C./sec, the grain growth is promoted. This is because coarse crystal grains are formed, which is a factor of deterioration of strength and workability, and when the cooling rate exceeds 80 ° C / sec, it acts as a cause of variation of materials due to non-uniform width cooling.

     After cooling in the runout table, the hot rolled steel sheet is wound at a temperature of 560 ~ 680 ℃. When the coiling temperature is less than 560 ℃, the material of the hot rolled material is somewhat hardened, so that the load in the cold rolling process, which is the next process, becomes large, making it difficult to secure rolling property, and the nonuniformity of the width direction increases, resulting in a difference in the production behavior of low-temperature precipitates. Due to this material variation is caused workability is lowered, there is a problem that the high temperature characteristics are deteriorated. In addition, when the coiling temperature exceeds 680 ℃, there is a problem that the structure of the final product is produced coarse, deterioration of workability and corrosion resistance, the coiling temperature is limited to 560 ~ 680 ℃.

     After the wound steel sheet is subjected to pickling treatment and cold rolling to a target thickness, the steel sheet undergoes a continuous annealing process at a temperature of 800 ° C. or higher for recrystallization and microstructure control. The annealing temperature of 800 ° C. or more corresponds to a temperature for sufficiently securing the transformation driving force of the second phase tissue. If the annealing temperature is less than 800 ° C., the final microstructure of the target steel sheet cannot be obtained, and there is a problem that it is difficult to secure excellent workability and high temperature characteristics. However, in the case of high temperature annealing, the surface defect of the steel sheet may increase, so the annealing temperature is preferably managed at 900 ° C. or less.

     The annealed steel sheet can secure the target tissue characteristics through the cooling process, for this purpose to maintain a cooling rate of 40 ℃ / sec or more. If the cooling rate is less than 40 ° C / sec, it is difficult to obtain the target hard structure due to slow cooling.

     Hereinafter, the present invention will be described in more detail with reference to Examples.

     Invented steels 1 and 2 and Comparative steels 1 to 5 prepared by melting the composition as shown in Table 1 below under the process conditions as shown in Table 2 to produce cold rolled steel invention materials 1 to 5 and Comparative materials 1 to 10 After that, the characteristics of the cold rolled steel sheet material produced at room temperature and high temperature are shown in Table 3. Table 1 shows the components of the invention steel and comparative steel, Table 2 shows the conditions of the manufacturing process for each steel sheet.

Among the characteristics shown in Table 3, the bending resistance test was divided according to the degree of occurrence of surface bending after steel sheet processing, and the bending index representing this was divided into five stages. As much as possible, the 3 to 5 steps which generate | occur | produced the bending phenomenon were judged as defect. Table 3 shows the characteristic evaluation for each steel sheet.

In addition, the high temperature discoloration resistance test was judged to be poor when the surface glossiness was lowered by 30% or more after maintaining the steel sheet at a temperature of 500 ° C for 48 hours.

In addition, the workability test was judged to be defective when a work crack occurred during normal temperature processing.

     As shown in Table 3, Inventive Materials 1 to 5 in which the steel component and the process conditions satisfy the scope of the present invention had good bending resistance and excellent aging resistance, and produced Sn-based nitride precipitates and Sn on the surface of the steel sheet. As the system oxide layer was formed, the high temperature discoloration resistance was good, and processing defects did not occur during normal temperature processing, and thus the normal temperature processability was also excellent.

     On the other hand, although the steel components of Table 1 and the like are the invention steels satisfying the scope of the present invention, Comparative Materials 1 to 5, in which some of the process conditions of Table 2 are out of the scope of the present invention, were difficult to secure high temperature properties in most cases. And poor aging resistance in many cases.

     In addition, although the process conditions of Table 2 satisfy the scope of the present invention, Comparative Materials 6 to 10 in which the steel components C, B, Sn, or the atomic ratio of B and N of Table 1, etc., are outside the scope of the present invention, are room temperature and high temperature. Often the characteristics were poor. Among them, Comparative Material 7 had good bending resistance, so that aging resistance and workability were good, but the discoloration resistance corresponding to high temperature characteristics was poor. This is due to high B / N value, resulting in deterioration of surface properties. This is because no Sn-based oxide layer is formed on the surface. In addition, the comparative materials 8 to 10 had a high amount of solid carbon in steel, and not only poor workability and aging resistance, but also poor high temperature characteristics due to inadequate B / N atomic ratios, which made it difficult to simultaneously satisfy normal temperature workability and high temperature characteristics.

As a result, fine B-based carbide precipitates are formed by optimizing the steel component conditions and manufacturing process conditions (especially annealing and cooling conditions), and Sn is added to form Sn-based oxide layers such as Sn ₂ O ₃ on the surface of the steel sheet, thereby lowering the cost. As a result, it was possible to produce a cold rolled steel sheet satisfying workability and discoloration resistance at the same time.

     The above embodiment is only one example, and the present invention is not limited thereto. Any thing that has substantially the same structure and the same effect as the technical idea described in the claim of the present invention is included in the technical scope of this invention.

Claims (11)

By weight% C: 0.001 ~ 0.004%, Sn: 0.05 ~ 0.35%, Mn: 0.07 ~ 0.55%, Al: 0.01 ~ 0.10%, B: 0.0005 ~ 0.0020, P: 0.003 ~ 0.020%, N: 0.001 ~ 0.004% , S: 0.015% or less, cold rolled steel sheet having excellent workability and discoloration resistance, including residual Fe and other unavoidable impurities. The method of claim 1,
Cold rolled steel sheet excellent in workability and discoloration resistance, characterized in that the value of the atomic ratio B / N of the B and N is 0.5 ~ 1.5. The method of claim 1,
The cold rolled steel sheet is a cold rolled steel sheet excellent in workability and discoloration resistance, characterized in that the B-based nitride precipitates are formed. 4. The method according to any one of claims 1 to 3,
The cold rolled steel sheet excellent in workability and discoloration resistance, characterized in that a Sn-based oxide layer is formed on the surface of the cold rolled steel sheet. 5. The method of claim 4,
The Sn-based oxide layer is a cold rolled steel sheet excellent in workability and discoloration resistance, characterized in that it comprises a Sn ₂ O ₃ layer. By weight% C: 0.001 ~ 0.004%, Sn: 0.05 ~ 0.35%, Mn: 0.07 ~ 0.55%, Al: 0.01 ~ 0.10%, B: 0.0005 ~ 0.0020, P: 0.003 ~ 0.020%, N: 0.001 ~ 0.004% , S: 0.015% or less, steel slab containing remainder Fe and other unavoidable impurities is heated and hot rolled, cold rolled after winding, and then cold-rolled steel sheet at an temperature of 800 ℃ or more, annealing Process for producing a cold rolled steel sheet excellent in workability and discoloration resistance characterized by cooling at a cooling rate of 40 ℃ / sec or more with respect to the steel sheet. The method according to claim 6,
The steel slab is a method of producing a cold rolled steel sheet having excellent workability and discoloration resistance, characterized in that the value of B / N atomic ratio B / N is 0.5 ~ 1.5. 8. The method according to claim 6 or 7,
The hot rolling is a method for producing a cold rolled steel sheet having excellent workability and discoloration resistance, characterized in that the finish rolling at 900 ~ 940 ℃. 8. The method according to claim 6 or 7,
The hot rolling is a method of manufacturing a cold rolled steel sheet excellent in workability and discoloration resistance comprising the step of cooling the hot rolled hot rolled sheet at a cooling rate of 20 ~ 80 ℃ / sec. 8. The method according to claim 6 or 7,
The winding is carried out at 560 ~ 680 ℃ manufacturing method of cold rolled steel sheet excellent in workability and discoloration resistance. 8. The method according to claim 6 or 7,
The annealing treatment is carried out at 800 ~ 900 ℃ the manufacturing method of the cold rolled steel sheet excellent in workability and discoloration resistance.