KR101263790B1 - High heat-resistance cold-rolled steel sheet having excellent formability, heat resistance for working and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a high heat-resistant cold-rolled steel sheet and a method for manufacturing the same excellent in workability and heat resistance, by weight% C: 0.002 ~ 0.005%, Nb: 0.02 ~ 0.06%, Co: 0.02 ~ 0.20%, Mn: 0.10 ~ 0.35 High-temperature cold rolled steel for processing and excellent in workability and heat resistance, including%, Al: 0.02 to 0.08%, P: 0.003 to 0.020%, N: 0.002 to 0.006%, S: 0.015% or less, balance Fe and other unavoidable impurities The manufacturing method is a technical subject matter. According to the present invention, it can be manufactured at low cost compared to the existing stainless steel sheet, has various processing characteristics of elongation flangeability, bending property and deep drawing (excellent room temperature workability), and age-resistant by solid element precipitation The increase in yield point does not occur, not only excellent moldability, but also excellent in high temperature strength, it is possible to manufacture a high-temperature cold-rolled steel sheet for processing by extending the life of the facility by securing the shape freezing properties of high-temperature applications.

Cold Rolled Steel Sheet, Needle Ferrite, Heat Resistance, Processability, Aging Resistance

Description

High heat resistant cold rolled steel sheet for processing and excellent manufacturing method and its manufacturing method {HIGH HEAT-RESISTANCE COLD-ROLLED STEEL SHEET HAVING EXCELLENT FORMABILITY, HEAT RESISTANCE FOR WORKING AND MANUFACTURING METHOD THEREOF}

The present invention relates to a high heat resistant cold rolled steel sheet and a method for manufacturing the same for use in automobiles, home appliances and boilers, and more particularly, high heat resistant cold rolled steel sheet for processing and excellent heat resistance produced by optimizing steel components and process conditions; It relates to a manufacturing method.

In the past, cast iron was generally used in parts of products such as automobile exhaust systems, home communication systems, ovens, boilers, etc., but high heat resistance is required because it is continuously exposed to a high temperature environment of several hundred degrees (° C.) or higher. In order to secure an aluminum hot-dip galvanized steel sheet, stainless steel sheet, etc. were processed and used.

The high temperature property is one of the characteristics when the processed product is used in a high temperature environment. The high temperature property, that is, sag resistance and high temperature strength, is required because the product property may be deteriorated by a local temperature rise when using a product such as an automobile exhaust system. . The sag resistance is a sagging phenomenon in which the steel sheet is repeatedly exposed to high temperature to cause a change in the material. When such a phenomenon occurs, it is difficult to maintain the shape of the molded part, and when the thermal stress is concentrated to a specific place, the high temperature strength decreases. Since the product shape is deformed or destroyed, it is necessary to satisfy the yield strength of 55MPa or more at a high temperature of about 700 ° C to secure the shape freezing property.

Stainless steel sheet has been mainly used for such heat-resistance, but stainless steel sheet is not only expensive to manufacture due to the addition of expensive alloy elements such as Cr and Ni, but also has a high Cr and There is a problem in that corrosion resistance (corrosion resistance) is lowered because grain boundary corrosion occurs in the Cr depleted zone region formed by the precipitation of chromium carbide at the grain boundary by bonding.

Also, in order to secure oxidation resistance at high temperature, an aluminum hot dip galvanized steel sheet may be used. When the aluminum hot dip galvanized sheet is heated to a high temperature of 400 ° C. or higher, an alloy layer of an interface is grown due to the interdiffusion reaction between Fe and Al. 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, products such as automobile exhaust system, home communication, oven, boiler, etc. are manufactured to be accommodated in a limited space, and formed into a complex shape by various methods, so that the processing process to expand or bend after piping is required, in addition to the above high temperature characteristics Processability is required.

Thus, 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 to be added, which may cause workability deterioration and processing defects due to aging.

In addition, Japanese Patent Application Laid-open No. Hei 8-319548 attempts 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 suggests a method of improving the plating conditions rather than improving the properties of the steel sheet. Not only is the adjustment difficult when the ingredients are changed, but also there is a problem that it is difficult to obtain target heat resistance.

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, the present invention provides a high-temperature cold-rolled steel sheet and a method for manufacturing the same 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 demanding applications. have.

In the present invention, C: 0.002 to 0.005%, Nb: 0.02 to 0.06%, Co: 0.02 to 0.20%, Mn: 0.10 to 0.35%, Al: 0.02 to 0.08%, P: 0.003 to 0.020%, and N: Provides a high heat resistant cold rolled steel sheet for processing having excellent workability and heat resistance including 0.002% to 0.006%, S: 0.015% or less, balance Fe and other unavoidable impurities.

At this time, there is also a feature that the value of the effective addition ratio (Nb × Co) / C of C to Nb and Co is 0.6 to 3.0.

In addition, the cold rolled steel sheet is characterized in that (Nb, Co) C-based carbonized composite precipitates are formed.

In addition, the cold rolled steel sheet is characterized in that it is made of an equiaxed ferrite and acicular ferrite structure.

Here, the volume fraction of the acicular ferrite structure is also characterized by 5 to 15%.

In addition, the present invention is C: 0.002 ~ 0.005%, Nb: 0.02 ~ 0.06%, Co: 0.02 ~ 0.20%, Mn: 0.10 ~ 0.35%, Al: 0.02 ~ 0.08%, P: 0.003 ~ 0.020%, Steel slab containing N: 0.002 ~ 0.006%, S: 0.015% or less, balance 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 above 800 ℃ It provides a process for producing a high heat-resistant cold rolled steel sheet for processing and excellent heat resistance, characterized in that the treated and annealed steel sheet is cooled at a cooling rate of 30 ℃ / sec or more.

In this case, the steel slab is characterized in that the value of the effective addition ratio (Nb × Co) / C of C to Nb and Co is 0.6 to 3.0.

In addition, the hot rolling is also characterized by finish rolling at 900 ~ 950 ℃.

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 is possible to manufacture at low cost compared to the existing stainless steel sheet, and has various processing characteristics of elongation flangeability, bending property, and deep drawing property, and thus excellent room temperature processability, Due to the increase in aging resistance due to elemental precipitation, there is no yield point elongation, which is not only excellent in formability but also excellent in high temperature strength, and can be manufactured for high temperature cold-rolled steel sheet for processing which has extended equipment life by securing shape freezing property of high temperature applied products. have.

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

The present inventors have satisfied various processing characteristics such as stretch-flangeability, bendability, deep drawing, etc., anti-aging properties, and corrosion resistance at low cost and 55 MPa at 700 ° C. The present invention was completed by repeating the research and experiment to secure the above yield strength simultaneously. As a result, the present invention contains a very low carbon content in the steel components, and control the addition amount and the addition ratio of Nb, Co to form a fine (Nb, Co) C-based carbonized composite precipitate, and optimize the annealing and cooling conditions By securing the volume fraction of ferrite structure, it is excellent in heat resistance at high temperature, corrosion resistance and aging resistance at room temperature, and workability, so that it is manufactured to manufacture high heat resistant cold rolled steel sheet suitable for product parts such as automobile exhaust system, home communication, oven and boiler. have.

First, the reasons for limiting components of the present invention will be described.

Carbon (C) is an element added to improve the strength of the steel sheet and is mainly consumed by the reaction with Nb for forming Nb carbide-based precipitates in the present invention. As the amount of C added increases, the tensile and yield strength increases, but when added excessively, the workability decreases, so the upper limit is preferably 0.005%. However, if the content is less than 0.002%, a sufficient (Nb, Co) C-based composite precipitate cannot be obtained, and the grain size increases and a sudden change in the material occurs. Therefore, the C content is limited to 0.002 to 0.005%.

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 preferably 0.35%. However, if less than 0.10% workability is improved, but the target strength is difficult to secure, the Mn content is limited to 0.10 to 0.35%.

Aluminum (Al) is an element added for deoxidation of molten steel and is preferably contained in an amount of 0.02% or more because it is combined with solid solution elements in steel to improve aging characteristics. However, excessive addition of more than 0.08% increases the amount of inclusions in the steel, causing surface defects and lowering the workability, Al content is limited to 0.02 ~ 0.08%.

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 reinforcing the material. If it contains less than 0.002%, sufficient rigidity cannot be obtained, and the precipitate formation site is reduced. If the content exceeds 0.006%, Since it causes aging and hardening occurs and deteriorates moldability, N content is limited to 0.002% to 0.006%.

Sulfur (S) combines with Mn in the steel to form non-metallic inclusions that serve as the starting point of corrosion 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 above effect, it is preferable to manage at 0.010% or less.

Niobium (Nb) is an effective element for increasing the strength of steel sheet and refining grains. In the present invention, niobium (Nb) is combined with C dissolved in steel to form a (Nb, Co) C-based carbonized composite precipitate to improve aging and formability. It is preferable to contain 0.02% or more because the strength is increased by the formation of (Nb, Co) C-based carbonized composite precipitates and the ferrite grains are suppressed at high temperature to refine the ferrite particles, but the content is 0.06%. If exceeded, the material is hardened, and the workability of the continuous annealing treatment is lowered and the surface properties of the steel sheet are deteriorated, so the Nb content is limited to 0.02 to 0.06%.

Cobalt (Co) is an element that promotes the formation of precipitates in the steel to increase strength and improve corrosion resistance. The cobalt (Co) is preferably contained in an amount of 0.02% or more in order to obtain such an effect. The Co content is limited to 0.02 to 0.20% because it acts as a factor in manufacturing cost by adding a large amount of expensive alloying elements rather than the effect of promoting precipitation.

In the case of Nb and Co alone, it is important to manage them alone.However, the effective weight addition ratio (Nb × Co) / C of C to Nb and Co is maintained in a certain range to ensure room temperature aging resistance, processability and high temperature strength simultaneously. It is also important.

That is, when the value of (Nb × Co) / C is 0.6 to 3.0, solid solution element C is fixed in steel by formation of (Nb, Co) C-based carbonized composite precipitates, thereby ensuring room temperature aging resistance and workability, and annealing. And by controlling the cooling conditions appropriately, through the formation of fine (Nb, Co) C-based carbide composite precipitates to suppress the grain growth at a high temperature to control the ferrite microstructure can ensure excellent high temperature properties.

However, when the value of (Nb × Co) / C is less than 0.6, there is a problem of deterioration of room temperature aging resistance and processability due to excessive elemental solids in the steel, and the amount of (Nb, Co) C-based carbonized composite precipitates is excellent. High temperature strength cannot be secured. In addition, when the value exceeds 3.0, there is a problem that the material hardens, the temperature of the recrystallization is raised rapidly, the surface properties deteriorate and the workability of the post-process is lowered. Therefore, the value of (Nb × Co) / C Is limited to 0.6 to 3.0.

The cold rolled steel sheet of the present invention comprises the above components and consists of the remainder Fe and other unavoidable impurities. And alloy elements may be further added to improve the properties of the present cold-rolled steel, if necessary, it is not interpreted to be excluded from the scope of the present invention because the alloy element is not added in the embodiment of the present invention.

On the other hand, the cold-rolled steel sheet of the present invention is characterized by consisting of an equiaxed ferrite (polygonal ferrite) and acicular ferrite (acicular ferrite) tissue, the needle-like ferrite tissue is a kind of bainite grown by nucleation in the mouth of austenite before transformation As it grows by nucleation from fine non-metallic inclusions dispersed in steel, ferrite laths are relatively excellent in ductility and toughness due to the disordered structure in which ferrite laths form a high angle boundary with each other. .

Therefore, the present invention by controlling the conditions of the annealing and cooling process appropriately secures the volume fraction of the needle-like ferrite structure 5-15% to form a high-potential-density type microstructure to suppress abnormal growth of grains at high temperature to increase the high temperature strength By securing it, heat resistance can be improved. If the volume fraction of the acicular ferrite structure is less than 5%, it is difficult to secure the high temperature strength, so that target heat resistance is difficult to be obtained. If the volume fraction of the acicular ferrite structure is more than 15%, the workability is deteriorated by hardening of the material. There is a problem, and the volume fraction of the needle ferrite is limited to 5 to 15%.

Hereinafter, the manufacturing method of the cold rolled steel sheet excellent in heat resistance and workability 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 cooled at 30 ° C./sec or more on a steel sheet annealed at a temperature of 800 ° C. or higher, which is excellent in workability and heat resistance. Steel sheet can be produced.

After reheating the steel slab formed as the above-described component system at a normal temperature, the finish rolling temperature during hot rolling is 900 to 950 ° C., and when the finish rolling temperature is less than 900 ° C., the hot rolling is completed in a relatively low temperature region. Due to the occurrence of hybridization of the finally formed crystal grains, the workability and rolling properties are lowered, and if the finish rolling temperature exceeds 950 ° C, uniform hot rolling is not performed throughout the thickness, resulting in insufficient grain refinement, resulting in grain coarsening. Since impact toughness falls, finish rolling temperature is restrict | limited to 900-950 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 grains are formed, which may be a cause of deterioration in strength and workability, and when the cooling rate exceeds 80 ° C / sec, it acts as a cause of variation in materials due to widthwise cooling unevenness.

After cooling in the runout table, the hot rolled steel sheet is wound at a temperature of 560 ~ 680 ℃. If the coiling temperature is less than 560 ℃, the material of the hot rolled material is somewhat hardened to increase the load in the cold rolling process, which is the next process, and thus it is difficult to secure rolling property, and the nonuniformity of the widthwise temperature 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 needle-like ferrite structure. If the annealing temperature is less than 800 ° C., there is a problem that it is difficult to obtain a volume fraction of the acicular ferrite structure present in the microstructure of the target steel sheet, and thus it is difficult to secure excellent high temperature characteristics. However, since the surface defects of the steel sheet may increase during high temperature annealing, the annealing temperature is preferably managed at 900 ° C. or lower.

The annealed steel sheet can secure a volume fraction of the target acicular ferrite structure through a cooling process, for which the cooling rate is maintained at 30 ℃ / sec or more. This is because when the cooling rate is less than 30 ° C./sec, it is difficult to obtain a volume fraction of the target acicular ferrite structure due to slow cooling.

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

[Example]

Invented steels 1 and 2 and Comparative steels 1 to 5 prepared by melting the composition shown in Table 1 below under the process conditions as shown in Table 2 to produce cold rolled steel invention materials 1 to 4 and Comparative materials 1 to 10 After that, the volume fraction of the acicular ferrite structure and the characteristics at room temperature and high temperature were evaluated for each of the prepared cold rolled steel sheets.

 Yield elongation phenomenon of the characteristics described in Table 3 is expressed as occurrence when the elongation is generated by measuring the yield point elongation, and not occurred when the elongation is not generated, and the bending resistance test is performed after the steel sheet processing. Dividing the bending index that expresses it by the degree of bending, divided into five stages, and judges 1 to 2 stages that are relatively insignificant, and 3 to 5 stages where the bending occurs to the extent that visual observation is possible. It was.

In addition, the sag resistance test was determined to be defective when the sagging of the steel sheet after measuring the sagging of the steel sheet after heating the material of 250mm in length, 30mm in width for 100 hours at 700 ℃ using a heat treatment equipment, The high temperature yield strength test was determined to be poor when the yield strength was less than 55 MPa at 700 ° C., and to be good if the yield strength was higher than that. The workability test was judged to be bad when a crack occurred in normal temperature processing.

Table 1 Components of Inventive and Comparative Steels

[Table 2] Condition of Manufacturing Process by Steel Plate

[Table 3] Evaluation of properties by steel plate

As shown in Table 3, Inventive Materials 1 to 4 in which the steel component and the like satisfy the scope of the present invention, the yield point stretching phenomenon did not occur and the bending resistance was good, and the aging resistance was excellent, and (Nb, Co) C The yield strength at high temperatures and the sag resistance after heat treatment (sag resistance) were good due to the generation of the system carbonized composite precipitate and the optimization of the volume fraction of the needle-like ferrite structure.

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 deviate from the scope of the present invention, are mostly excellent in low volume fraction of acicular ferrite structure. It did not have high temperature characteristics, and workability and aging resistance were also poor 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, Nb, Co, etc. of Table 1 are out of the scope of the present invention have a low volume fraction of acicular ferrite structure, and room temperature. And high temperature characteristics were often poor.

Among them, the comparative material 6 had no yield point stretching phenomenon and good bending resistance, so that aging resistance and workability were good, but sag resistance and high temperature strength corresponding to high temperature characteristics were poor. This is because the formation and promotion effects of Nb, Co) C-based carbonized composite precipitates are reduced. In addition, Comparative Materials 7 to 10 had a high amount of solid solution carbon in steel, which was poor in workability and aging resistance, and had a low amount of Nb, Co or its addition ratio. The volume fraction was also low, resulting in poor high temperature properties, making it difficult to satisfy both workability and high temperature properties at the same time.

On the other hand, Figure 1 is a photograph comparing the microstructure of the invention material 3 and the comparative material 10, in the invention material 3 of Figure 1 (a) the ferrite of the shape close to the polygon is an equiaxed ferrite structure, a long stretched ferrite Denotes acicular ferrite structure. Although the volume fraction of the needle-like ferrite structure of the inventive material 3 was 7.9%, both the evaluation of the characteristics of the room temperature and the high temperature were satisfactory, but the comparative material 10 of FIG. It can be confirmed in Table 3 that the aging resistance, processability and heat resistance are poor.

As a result, it is possible to form fine (Nb, Co) C-based carbonized composite precipitates by optimizing the compositional conditions of the steel and the conditions of the manufacturing process (especially annealing and cooling conditions), and to control the volume fraction of the acicular ferrite structure. The high temperature resistant cold rolled steel sheet for processing at the same time satisfying the high temperature characteristics and workability could be manufactured.

The above embodiment is only one example, and the present invention is not limited thereto. Anything having substantially the same constitution as the technical idea described in the claims of the present invention and achieving the same operational effect is included in the technical scope of the present invention.

1 is a photograph comparing the microstructure of Inventive Material 3 and Comparative Material 10.

Claims (11)

By weight% C: 0.002 ~ 0.005%, Nb: 0.02 ~ 0.06%, Co: 0.02 ~ 0.20%, Mn: 0.10 ~ 0.35%, Al: 0.02 ~ 0.08%, P: 0.003 ~ 0.020%, N: 0.002 ~ 0.006 %, S: 0.015% or less, including balance Fe and other unavoidable impurities, A high heat-resistant cold rolled steel sheet for processing having excellent workability and heat resistance, wherein the effective addition ratio (Nb × Co) / C of C to Nb and Co is 0.6 to 3.0. delete The method of claim 1, The cold rolled steel sheet is a high heat-resistant cold-rolled steel sheet for processing and excellent heat resistance, characterized in that the (Nb, Co) C-based carbide composite precipitate is formed. By weight% C: 0.002 ~ 0.005%, Nb: 0.02 ~ 0.06%, Co: 0.02 ~ 0.20%, Mn: 0.10 ~ 0.35%, Al: 0.02 ~ 0.08%, P: 0.003 ~ 0.020%, N: 0.002 ~ 0.006 A high heat resistant cold rolled steel sheet having excellent workability and heat resistance, comprising:%, S: 0.015% or less, balance Fe, and other unavoidable impurities. 5. The method of claim 4, A high heat resistant cold rolled steel sheet for processing and excellent heat resistance, characterized in that the volume fraction of the acicular ferrite structure is 5 ~ 15%. By weight% C: 0.002 ~ 0.005%, Nb: 0.02 ~ 0.06%, Co: 0.02 ~ 0.20%, Mn: 0.10 ~ 0.35%, Al: 0.02 ~ 0.08%, P: 0.003 ~ 0.020%, N: 0.002 ~ 0.006 Steel slab containing%, S: 0.015% or less, balance Fe and other unavoidable impurities is heated and hot rolled, cold rolled after winding, and then the cold rolled steel sheet is annealed at a temperature of 800 ° C. or higher, and annealed. Cooled at a cooling rate of 30 ℃ / sec or more for the finished steel sheet, The steel slab is a method for producing a high heat resistant cold rolled steel sheet having excellent workability and heat resistance, characterized in that the value of the effective addition ratio (Nb × Co) / C of C to Nb and Co is 0.6 to 3.0. delete The method of claim 6, The hot rolling is a method of manufacturing a high heat-resistant cold rolled steel sheet for processing and excellent heat resistance, characterized in that the finish rolling at 900 ~ 950 ℃. The method of claim 6, The hot rolling is a method of manufacturing a high heat resistance cold rolled steel sheet having excellent workability and heat resistance, characterized in that it comprises the step of cooling the hot rolled hot rolled sheet at a cooling rate of 20 ~ 80 ℃ / sec. The method of claim 6, Said winding is performed at 560-680 degreeC, The manufacturing method of the high heat resistant cold rolled steel sheet for processing excellent in workability and heat resistance characterized by the above-mentioned. The method of claim 6, The annealing treatment is carried out at 800 ~ 900 ℃ a manufacturing method of high heat resistance cold rolled steel sheet for processing excellent in workability and heat resistance.

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