KR100325109B1 - Method for manufacturing bake hardenable low carbon cold rolled steel sheet with improved formability - Google Patents

Abstract

PURPOSE: Provided is a method to produce bake hardenable low carbon cold rolled steel sheet with a tensile strength of 30 to 35 kgf/mm¬2 and improved bake hardenability of 3.0 to 5.0 kgf/mm¬2, which is valid range for aging resistance at ambient temperatures while confirming excellent formability without modification of conventional continuous annealing process only by addition of Mn, S, N and a trace of Ti. CONSTITUTION: The method includes the steps of soaking a Ti added low carbon aluminum killed steel comprising C 0.03 to 0.05 wt.%, Mn 0.1 to 0.6 wt.%, P 0.008 to 0.09 wt.%, S 0.02 to 0.05 wt.%, Soluble-Al 0.02 to 0.08 wt.%, 0.006 wt.% or less of N, Ti 0.015 to 0.05 wt.% at above 1200°C wherein Ti is incorporated to satisfy the relationship of Ti<=(48/14)N+(48/32)S; finish hot rolling the soaked Ti added low carbon aluminum killed steel at 900 to 950°C; winding the hot rolled Ti added low carbon aluminum killed steel at 700 to 750°C; cold rolling the hot rolled low carbon aluminum killed steel at a reduction rate of 75 %; quenching the cold rolled Ti added low carbon aluminum killed steel at 800 to 860°C at a cooling rate of 30 to 50°C/sec; continuous annealing the cold rolled Ti added low carbon aluminum killed steel for overaging; and temper rolling it at a reduction rate of 1.0 to 2.0 %.

Description

Manufacturing method of small hardening type low carbon cold rolled steel sheet with excellent formability

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold rolled steel sheet used in interior and exterior sheet materials of automobiles, and more particularly, to a method for producing a small hardened type low carbon cold rolled steel sheet having excellent moldability.

In recent years, the demand for higher strength and formability of cold rolled steel sheets for automobiles has been increased for the purpose of improving fuel efficiency and reducing the weight of automobile bodies. Properties required for automotive cold rolled steel sheet include yield strength, tensile strength, good press formability, spot weldability and paint corrosion resistance.

In general, steel sheets generally exhibit characteristics in which moldability and strength are opposite to each other. Examples of the steel that can satisfy both of these characteristics in a cold rolled steel sheet include a composite structure steel and a hardened cold rolled steel sheet. In general, composite tissue steel that can be easily manufactured has excellent yield strength because it has a yield strength close to a soft steel sheet during press molding in steels having a tensile strength of 40 kgf / mm2 or less, and is excellent in ductility and applied during baking treatment after press molding. It is a steel whose yield strength increases with temperature, and it is attracting attention as a very ideal steel compared with the conventional cold rolled steel sheet which deteriorates formability as strength increases.

Baking hardening is a kind of strain aging that occurs when carbon or nitrogen, an invasive element dissolved in steel, adheres to the potential generated during deformation during high temperature treatment such as automobile baking. Is increased, but due to the excessive content of solid elements, deterioration of moldability due to aging at room temperature, it is very important to control the appropriate amount of solid elements.

In general, as a method for producing a cold-rolled steel sheet having a hardening hardening property, the P-added low-carbon Al-kilted steel is simply wound at a low temperature, that is, in an ordinary annealing method in which the hot-rolled winding temperature is annealed for a long time by using a cold winding having a temperature range of 400 to 500 ° C. Is mainly used to produce steel having a hardened amount of about 4 to 5 kgf / mm 2. This is because both of the moldability and the baking hardenability are more easily achieved by the annealing. In the case of P-added Al-kilted steel by the continuous annealing method, it is easy to secure baking hardenability because it uses a relatively fast cooling rate, but there is a problem of deterioration in formability due to excessive addition of solid elements by rapid heating and short time annealing. It is limited only to the outer shell of the car, which is not required. Thanks to the recent rapid development of steelmaking technology, it is possible to control the appropriate amount of solid solution in the steel, and it is very formable by using ultra-low carbon Al-kilted steel sheet containing strong carbonitride-forming elements such as Ti or Nb. Eggplants have been increasingly used for automotive exterior materials, in which cold rolled steel sheets are manufactured to require dent resistance.

Japanese Patent Publication No. 63004899 discloses a Ti and Ti, Nb composite-added ultra low carbon cold rolled steel sheet having a C: 0.005 to 0.015% and an S + N content of 0.005%. In Japanese Patent Publication No. 57089437, C: 0.005% or less Disclosed is a method for producing a cold rolled steel sheet having a bake hardening amount of about 4 kgf / mm 2 or more using Ti, Nb composite additive steel. This method is to control the amount of Ti, Nb added or the cooling rate during annealing to appropriately control the solid solution carbon in the steel to prevent deterioration of the material and to impart proper baking hardening.

On the other hand, as a method for securing the aging resistance and baking hardening resistance using low carbon steel in the continuous annealing method, Japanese Patent Publication No. 5055573, C: 0.01 to 0.06%, S: 0.002 to 0.025%, Mn: 0.05 to 0.04% Cold-rolled steel sheet heated at low temperature to 1000 ~ 1130 ℃ at 50 ℃ to 200 ℃ / sec rapid cooling, maintained at 200 ~ 310 ℃ for several seconds, and then reheated and hardened with low age It introduces the manufacturing method of cold rolled steel sheet.

However, in the case of Ti or Ti, Nb composite addition ultra low carbon steel, strict control such as Ti, nitrogen, sulfur, etc. is required to add a vacuum degassing process and to secure an adequate baking hardening step in the steelmaking step to manufacture ultra low carbon steel. This raises the problem of cost increase. In addition, in the case of Nb-added ultra-low carbon steel, problems such as cost increase due to the addition of a vacuum degassing process and the addition of special elements, and deterioration of workability due to high temperature annealing are caused.

In addition, the method disclosed in Japanese Patent Application Laid-Open No. 5055573 uses a special annealing method such as reheating and quenching and aging at the time of continuous annealing as well as strict control of the components, so to produce cold rolled steel sheet having aging resistance and hardening hardening. It is inevitable that the front side of the facility will be retrofitted, and due to the low temperature slab reheating temperature, material degradation due to the mixed structure during hot rolling is expected.

The present invention was made in order to solve the problems of the prior art described above, to minimize the increase in manufacturing cost by vacuum degassing to produce a hardened type cold-rolled steel sheet by low carbon steel, carbon, Mn simply without modification of continuous annealing equipment , S, N and a small amount of strong carbonitride-forming element, the addition of Ti, which is excellent in formability by the usual continuous annealing method, and the appropriate hardening hardening property of 3.0 to 5.0 kgf / mm 2, which can suppress the aging resistance at room temperature It is an object of the present invention to provide a method for producing an Al-killed low carbon hardened hardened cold rolled steel sheet having a tensile strength of 30 to 35 kgf / mm 2.

In order to achieve the above object, the method for producing a low carbon calcined hardened cold rolled steel sheet excellent in formability of the present invention is, by weight, C: 0.03 to 0.05%, Mn: 0.1 to 0.6%, P: 0.008 to 0.09%, and S: 0.02 to 0.05%, Soluble Al: 0.02 to 0.08%, N: 0.006% or less, Ti: 0.015 to 0.05%, while satisfying the relationship of Ti≤ (48/14) N + (48/32) S Low-carbon Ai-killed steel with Ti added to be homogenized heat treatment at 1200 ℃ or higher and finished hot rolling at temperature range of 900 ~ 950 ℃, cold rolling at about 75% and 800 ~ 860 ℃ after winding in temperature range of 700 ~ 750 ℃ In the temperature range of, a rapid annealing is performed at a cooling rate of 30 ° C. to 50 ° C./sec, followed by ramping and aging, followed by temper rolling of 1.0% to 2.0%.

Hereinafter, the present invention will be described in detail.

First, the alloy component and the manufacturing method in this invention are demonstrated.

Carbon (C) is an element exhibiting a solid solution hardening and a hardening hardening effect. The smaller the content, the more favorable the moldability and the aging resistance. When the carbon content is 0.03% or less, the production cost is expected to increase due to the addition of a vacuum degassing operation in the steelmaking step in order to lower the carbon content, which is against the object of the present invention to minimize the cost increase. In addition, when the carbon content is 0.05% or more, the amount of carbon is very high compared to the amount of Ti added, so that the amount of solid solution carbon in the steel becomes excessive and room temperature aging resistance is not secured.

Manganese (Mn) is an element for minimizing particles without damaging ductility and precipitating sulfur in the steel to MnS to prevent hot brittleness due to the formation of FeS. In particular, in low carbon steels, in addition to improving moldability and preventing room temperature aging resistance Precipitation from the hot rolling step is a very important element for forming MnS precipitates in steel-based MnS and Ti-based precipitates which act as a nucleation site for carbides. Therefore, when the manganese content is 0.1% or less, the strength decreases and control of the appropriate MnS precipitate size and the amount of precipitates become small. When 0.6% or more is added, the strength increases rapidly due to solid solution strengthening. In order to secure the appropriate size and distribution of MnS precipitates to secure proper baking hardenability, the content of sulfur is increased to cause deterioration of the material, so the content is limited to 0.1 to 0.6%.

Phosphorus (P) is a substitution-type alloy element having the highest solid solution strengthening effect in the element or steel that does not significantly affect the aging resistance, and serves to improve in-plane anisotropy and enhance strength. However, when the phosphorus content is 0.008% or less, the above-mentioned effect cannot be obtained. When the phosphorus content is added at 0.09% or more, the addition amount is 0.008 to 0.009% because the drop in ductility is inevitable such as sudden increase in strength and embrittlement of the material by segregation at grain boundaries. Limited to

Sulfur (S) precipitates as TiS sulfide, which acts as another carbide nucleation site by the addition of MnS or Ti with manganese at high temperatures. However, when the sulfur content is less than 0.02%, it is difficult to secure the proper size and distribution of the MnS precipitates. When the sulfur content is more than 0.05%, the amount of manganese added is relatively high in order to obtain the appropriate hardening hardening property by controlling the appropriate carbide nucleation site size and distribution. Since the decrease in ductility due to the increase in strength is inevitable, the addition amount is preferably limited to 0.02 to 0.05%.

Aluminum (Al) is an element added for deoxidation and denitrification of steel, and when added in an amount of 0.02% or less, it is disadvantageous to mechanical properties such as increase of oxidation inclusions in steel and deterioration of workability. In addition, when excessively added to more than 0.08% will cause an increase in the curing and manufacturing cost of the material is preferably limited to the content of 0.02 to 0.08%.

Nitrogen (N) is a very important element in the present invention to secure proper baking hardenability and formability by promoting TiN precipitation in MnS, TiN and TiS precipitates serving as intracarbide carbide nucleation sites within the carbon range of the present invention. As a large amount of addition is required. However, if the content is more than 0.0060%, the amount of Ti added for a large amount of TiN precipitation is excessively increased, so that the production cost is increased and the material is expected to deteriorate. Therefore, it is necessary to limit the content to 0.0060% or less.

Ti is another high temperature Ti-based precipitate other than MnS precipitate that acts as a nucleation nucleation site in the mouth in order to prevent moldability and aging deterioration due to excessively dissolved solid carbon in low carbon steels and to obtain an appropriate high carbon content to obtain proper hardening hardenability. It is an element which precipitates TiN and TiS. However, when Ti is added at 0.015% or less, the absolute amount of Ti-based precipitates, which are carbide nucleation sites, is expected to deteriorate formability and aging resistance due to the increase of solid solution carbon in the steel. Due to the increase of manufacturing cost and the increase of Ti-based precipitates, TiC precipitates are produced. Therefore, precipitation hardening due to TiC precipitation occurs and moldability is deteriorated. Therefore, it is necessary to manage the range as narrowly as possible. There is. Therefore, in order to obtain stable bake hardenability due to the action of carbide nucleation sites due to the formation of Ti-based precipitates, the Ti content should be limited to the range of Ti ≤ (48/14) N + (48/32) S while being 0.015 to 0.05%. There is a need.

Slab slab (Slab) is melted in the converter after the composition in the composition is heated at more than 1200 ℃ can be sufficiently homogenized austenite structure before hot rolling to finish the hot rolling in the temperature range of 900 ~ 950 ℃ immediately above the Ar3 temperature .

If the slab cracking temperature is less than 1200 ℃ MnS that can act as a carbide nucleation site because the steel structure does not become uniform austenite grains, and the mixture is generated, especially coarse precipitates occur in the steel, Difficult to control the appropriate size of Ti-based precipitates, it is difficult to secure appropriate baking hardenability.

When the hot rolling finish temperature is 900 ° C. or less, the top, tail and edges of the hot rolled coil become single phase regions, thereby increasing in-plane anisotropy and degrading formability. In addition, when the temperature is higher than 950 ° C, remarkable coarse grains are generated and defects such as orange peel are likely to occur on the surface after processing.

High temperature winding of 700-750 degreeC is preferable in order to prevent the deterioration of the workability by the solid solution carbon which remain | survives in a hot rolled sheet after the said hot rolling process. However, when the coiling temperature exceeds 750 ℃ abnormal grain growth occurs and a good material can not be obtained, and if the temperature is less than 700 ℃, yield strength increase and moldability deterioration due to fine grain of the hot-rolled structure is caused.

After hot rolling, the steel is pickled by a conventional method, followed by cold rolling to a thickness of 0.8 mm at a cold rolling rate of 75%, and a recrystallization is completed after raising the temperature at a normal temperature raising rate of 8 to 10 ° C / sec. Slow cooling at 3-6 ° C / sec after crack temperature heat treatment in the temperature range of 800-860 ° C where growth can occur, and rapid cooling at 30-50 ° C / sec after cooling Heat treatment which is air-cooled after inclination and aging treatment at the temperature of 400 degreeC for 3 to 5 minutes is performed. In particular, when the annealing crack temperature is higher than 860 ℃, it is difficult to control the tension during continuous annealing due to high temperature annealing and deterioration of workability such as reducing the life of the burner is expected to limit the temperature range to 800 ~ 860 ℃. .

By using the low carbon Al-kilted steel prepared as described above, temper rolling of 1.0-2.0%, which is somewhat higher than the usual temper rolling ratio, is performed as a means for securing appropriate bake hardening resistance and aging resistance. When the temper rolling ratio is 1.0% or less, aging occurs during long-term preservation at room temperature, yielding strength is increased and yield point elongation, which is fatal to press processing, is very disadvantageous in terms of aging resistance. However, when the temper rolling ratio is 2.0% or more, work hardening occurs due to excessive temper rolling to increase the strength and deteriorate the ductility.

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

Table 1 below shows the chemical composition and manufacturing conditions of the invention steel and the comparative steel with strict control of carbon, manganese, sulfur and Ti. Steels 1 to 5 are invention steels and steels 6 to 10 are comparative steels.

Table 1

After hot-rolling the steels of Table 1 above and performing cold rolling at 75%, which is the normal condition, and after cooling at a temperature rising rate of 8 ° C / sec and 800 ° C annealing at a cooling rate of 5 ° C / sec, about 40 After rapid quenching at a cooling rate of ℃ / sec, the mechanical properties were measured after the continuous continuous annealing and air-tight rolling of about 1.5%, followed by gradient and aging treatment for 3 minutes from 400 ℃ to 350 ℃. Is shown in Table 2.

TABLE 2

Carbon: 0.022-0.044%, Manganese: 0.11-0.17%, Phosphorus: 0.013-0.014%, Sulfur: 0.022-0.026%, Nitrogen: 0.0037-0.0059%, Ti: 0.015-0.047% Inventive steel Nos. 1 to 5, in which the amount of Ti was controlled to satisfy the formula of N + (48/32) S, the baking hardening amount (BH) was 4.0 to 4.7 kgf / mm 2, the elongation was 40 to 42.2%, Tensile strength of 33.4 ~ 34.9 kgf / mm 2 and the aging index of 3.0 kgf / mm 2 or less, it can be seen that according to the present invention can be produced a small hardening type low carbon cold rolled steel sheet excellent in moldability and room temperature aging resistance.

Steel 6 satisfies the range of components of the present invention, but the sulfur content is very low as 0.007%, so the precipitation hardening amount of MnS and TiS, which acts as a carbide nucleation site in the steel, cannot be sufficiently precipitated. The aging index is 3.5 ㎏f / mm2, which is very high as mm2, and the aging resistance is deteriorated at room temperature. Also, even though the amount of Ti is within the range of the present invention, the amount of Ti added due to the low sulfur content is Ti Since it was higher than ≤ (48/14) N + (48/32) S, the material was degraded due to precipitation hardening caused by the formation of TiC precipitates in the steel.

Steel 7 also satisfies the component regulation range of the present invention, but the amount of Ti is very high, which causes material degradation due to excessive Ti-based precipitates, and also increases the manufacturing cost due to the addition of a large amount of Ti.

Steel No. 8 has a sulfur content of 0.012%, which is somewhat lower than the scope of the present invention, resulting in a decrease in the amount of MnS and TiS precipitates acting as a carbide nucleation site and a very high carbon content of 0.062%. In addition, it remains in the steel, and as the material deteriorates, the bake hardening amount is 5.9 kgf / mm 2, which is very high.

Although steel 9 satisfies the component range of the present invention well, the addition of Ti did not have an effect as a carbide nucleation site due to Ti-based precipitates, and thus the hardening hardenability was very high as 6.9 kgf / mm 2. The room temperature aging resistance was deteriorated as 4.2 kgf / mm <2>.

In steel 10, all the components satisfy the component regulation range of the present invention, but in the manufacturing conditions, the slabs reheating temperature is 1030 ° C lower than the conditions specified when manufacturing the present invention steel, so the precipitates acting as carbide nucleation sites are reheated at low temperature. Because of the coarse growth of more than the proper size and the decrease of precipitate distribution density, the material was softened by the low slab reheating temperature, but the baking hardening amount was very high as 5.8kgf / mm2.

Therefore, according to the present invention as described above, the low-carbon bake hardened cold-rolled steel sheet having a bake hardening amount of 3.0 to 5.0 kgf / mm 2, an elongation of 40% or more, and a tensile strength of 30 to 35 kgf / mm 2 formability is improved. Is obtained.

Claims (1)

By weight%, C: 0.03-0.05%, Mn: 0.1-0.6%, P: 0.008-0.09%, S: 0.02-0.05%, Soluble Al: 0.02-0.08%, N: 0.006% or less, Ti : Temperature of 900-950 ° C after homogenization heat treatment of low-carbon Ai-killed steel containing Ti to satisfy the relation of Ti≤ (48/14) N + (48/32) S containing 0.015-0.05% at 1200 ° C or higher Finish hot rolling in the range, cold rolling of about 75% after winding in the temperature range of 700 to 750 ° C, and continuous cooling and aging after rapid cooling at a cooling rate of 30 to 50 ° C / sec, which is a normal condition in the temperature range of 800 to 860 ° C. A method for producing a low carbon calcined hardened cold rolled steel sheet having improved formability, characterized by performing annealing and tempering rolling at 1.0 to 2.0%.