KR100544639B1 - A method for manufacturing high strength steel having superior aging index - Google Patents

Abstract

Provided is a method for producing a high strength steel sheet excellent in aging resistance.

In the production method of the present invention, in weight%, C: 0.07% or less, Mn: 0.2 to 0.5%, P: 0.051 to 0.1%, S: 0.015% or less, N: 0.002 to 0.01% or less, SA | Providing an aluminum killed steel comprising 0.1% residual Fe and unavoidable impurities; Hot-finishing the steel at 850-900 ° C. immediately above the Ar ₃ transformation point after homogenizing; Winding the hot rolled steel sheet at a temperature of 600 ° C. or lower and pickling the conventional method, followed by cold rolling at a cold reduction rate of 60 to 75%; Heating the cold rolled steel sheet at a rate of 10 ° C./hr or more and then performing annealing for 8 hours or more at 650˜700 ° C .; And temper rolling the annealed steel sheet at a cooling rate of 30 ° C./hr or less to 1.0-2.0% range.

Annealing, Aging Resistance, Aluminum Kilted Steel, Temper Rolling

Description

A method for manufacturing high strength steel having superior aging index}

1 is a graph showing a method for measuring age-resistance index (AI: Aging Indix)

The present invention relates to a method of manufacturing a high strength steel sheet having a tensile strength of 35Kgf / mm ² or more having excellent aging resistance that can be used for internal parts of domestic and overseas automobile bodies, and more specifically, the aging phenomenon and workability which are problems of low carbon BASE high tensile strength steel It relates to a manufacturing method of 35Kgf / mm class ² high tensile strength steel sheet using a box annealing (BAF) method to improve the.

The development of high strength steel sheet has been changed in close relation with various laws and regulations surrounding the automotive industry such as automobile safety regulation, fuel economy regulation, exhaust gas regulation law, etc. As the concern emerged, R & D accelerated, and many kinds of high strength steel sheet have been developed. In general, and calling 35Kgf / mm ² grade than the steel sheet based on the tensile strength of high strength steel, workability is required car within, high strength steel for the shell is of a low to medium carbon BASE P added Al-Killed steel extremely low BASE P, Mn added to deep drawing is High tensile steel is mainly used with tensile strength of 35Kg ~ 45Kg.

High-strength steel of medium-low carbon base has high strength due to excellent solidification effect of C and P. However, it has a high possibility of aging due to low-carbon low base. Or there is a problem that the workability is lowered, and thus it is mainly used for automotive interior.

In order to compensate for this, recently, Mn, P solid solution strengthening elements are added to ultra-low base to produce high tensile strength steel with both strength and machinability.In addition, carbonitride fish is removed to remove solid solution C, N in steel, which is the main cause of aging. Although it has inferiority by adding elemental Ti and Nb, it is inevitable to extend steelmaking operation time and reduce error rate due to high-clean steel work to produce ultra low carbon steel, and to add special elements such as Ti and Nb, strengthen Mn, P solid solution There is a problem such as an increase in manufacturing cost due to the addition of a large amount of elements.

Therefore, the present invention is to solve the problems of the prior art described above, age-resistant measurement using a box annealing (BAF) method to improve the aging phenomenon and lack of workability, which is a problem of low-carbon base high tensile strength steel currently used for automotive parts this index value iNDEX is AGING provide an 3Kgf / mm ² or less and 35Kgf / mm ² class high strength steel cold rolled plate a process for producing an elongation greater than 38% and for that purpose.

The present invention for achieving the above object,

By weight%, C: 0.07% or less, Mn: 0.2 ~ 0.5%, P: 0.051 ~ 0.1%, S: 0.015% or less, N: 0.002 ~ 0.01% or less, SA ㅣ: 0.05 ~ 0.1%, residual Fe and inevitable Providing an aluminum killed steel including impurities; Hot-finishing the steel at 850-900 ° C. immediately above the Ar ₃ transformation point after homogenizing; Winding the hot rolled steel sheet at a temperature of 600 ° C. or lower and pickling the conventional method, followed by cold rolling at a cold reduction rate of 60 to 75%; Heating the cold rolled steel sheet at a rate of 10 ° C./hr or more and then performing annealing for 8 hours or more at 650˜700 ° C .; And annealing the annealed steel sheet at a cooling rate of 30 ° C./hr or less, followed by temper rolling in a range of 1.0% to 2.0%.

Hereinafter, the steel composition component of the present invention and the reason for limitation thereof.

The present invention relates to a method for manufacturing a low carbon BASE cold rolled high tensile strength steel that improves the aging problem of low carbon high tensile strength steel and has a workability of the ultra low tensile steel level, the composition of steel and the reasons for its limitation are as follows.

Carbon [C] is an element that inhibits workability, and when its content exceeds 0.07%, a large amount of solid carbon remaining in solid solution in hot rolled steel sheet remains, and such solid carbon is excessively precipitated as carbide during cold rolling annealing. This will delay grain growth. As a result, the strength increases but the elongation decreases, and there may be a large amount of dissolved carbon even after cold rolling annealing, which may cause aging.

Therefore, in the present invention, the upper limit of carbon is limited to 0.07% in order to not only secure proper strength and elongation but also to ensure aging resistance.

Manganese [Mn] must contain at least 10 times of the [S] content to prevent red brittleness due to sulfur [S], and must contain at least 0.2% to increase the strength by solid solution strengthening. However, if the content exceeds 0.5%, the machinability decreases due to excessive solid solution effect and delayed grain growth, and therefore, the content is limited to 0.2-0.5%.

Phosphorus [P] is excellent in the employment strengthening effect even with a small amount of addition, but when exceeding 0.1%, the elongation is lowered due to excessive employment effect and the secondary processing brittleness due to grain boundary segregation is shown. It was limited to 0.051 ~ 0.1%.

Nitrogen [N] combines with [AL] in the steel in low carbon steels to precipitate AlN. However, AlN precipitation plays a very important role in the recrystallization of Al-Killed steel, so it is necessary to manage Al and N in an appropriate amount.

If the content of N is too small, the AlN precipitate decreases, resulting in insufficient AlN precipitates, resulting in poor workability, and in the case of too much N, it may remain in solid N form, thereby promoting aging. Therefore, in the present invention, the N content is limited to 0.002 to 0.01%.

Aluminum [Al] is added to the steel for deoxidation. Also, in Al-Killed steel, AlN precipitates can be formed to prevent ineffective solidity by preventing N from remaining in solid solution.In particular, in annealing materials, annealing recovery or recrystallization is performed. Precipitates as fine AlN and develops {111} orientation to improve deep drawing property.

In order to remove the solid solution residue of N and to form the processing structure with excellent AlN precipitate formation, more than 0.05% should be added. If the amount of aluminum is added more than necessary, manufacturing cost increases and a large amount of surface defect of steel sheet is generated. ] Is limited to 0.1%.

Hereinafter, the high tensile steel sheet manufacturing process of the present invention will be described.

In the present invention, after providing the aluminum-kilted steel composition as described above it is subjected to a homogenization treatment at about 1050 ~ 1200 ℃ normal working conditions.

And the homogenized steel is finished hot rolled at 850 ~ 900 ℃, which is directly above Ar ₃ transformation point. The crystal structure of the material may be degraded. If it exceeds 900 ° C, there is a problem of poor surface quality and hot rolling workability due to overgrowth of the hot rolled scale.

Next, the hot rolled steel sheet is wound at a temperature of 600 ° C. or lower and pickled by a conventional method, followed by cold rolling at a cold reduction rate of 60 to 75%. In the present invention, the coiling temperature is limited to 600 ° C. or less in order to maintain Al, N in solid solution in a hot rolled state in order to precipitate AlN in a subsequent annealing operation, thereby obtaining excellent deep drawing property. .

In the present invention, the cold reduction rate is limited to 60 to 75%, but if less than 60%, the workability of the product decreases due to the drop of Lankford value, which is a deep drawing performance evaluation item, and if it exceeds 75%, the rolling mill is overloaded. This is because workability is poor.

The cold rolled steel sheet is heated and then subjected to annealing for a predetermined time or more.

In the present invention, in order to make an excellent texture of the workability by AlN precipitates when recrystallized by the annealing (BAF), the cold rolled steel sheet is heated at a rate of 10 ℃ / hr or more and then annealed for 8 hours at a temperature range of 650 ~ 700 ℃ Process. If the annealing temperature is less than 650 ℃, the workability may decrease due to the lack of temperature in the mid part of the coil. If the annealing temperature exceeds 700 ℃, the facility load may be caused by exceeding the upper limit of the annealing temperature of BAF annealing furnace.

The annealed steel sheet is cooled and then subjected to temper rolling, where the temper elongation is limited to 1.0 to 2.5%. This is because at least 1.0% of temper rolling is required to secure aging, and if the stretching is excessive, material degradation may occur due to work hardening.

Meanwhile, in the present invention, it is preferable to cool the annealed steel sheet at a cooling rate of 30 ° C./hr or less. If this rate is exceeded, Fe ₃ C precipitates are not generated, which may cause aging due to the remaining solid solution C. Because.

Hereinafter, the present invention will be described in detail through examples.

(Example)

As shown in Table 1, Al-Killed steels having different compositions were prepared. Specifically, the comparative steel (1) is a low-carbon steel base high tensile strength steel for continuous annealing, Comparative steel (2) is a very low-base high-tensile strength steel for continuous annealing was added to the special element Ti to remove the residual C, N. On the other hand, the present invention steel is added to the S4Al and N components compared to the comparative steel in order to form a solid structure of the processability by the addition of solid solution strengthening elements Mn, P to 0.04% low carbon steel and the formation of precipitates of AlN.

The steel sheet prepared as described above was manufactured in a steel sheet by varying the manufacturing conditions of Table 2 and specifically, in the present invention, the hot finishing temperature was set at 860 ° C., which is directly above the Ar ₃ temperature, at 570 ° C. to secure the solid state of Al and N. It wound up and manufactured the hot rolled material. It was also pickled to a normal level and cold rolled at a 68% cold reduction rate, and, unlike the comparative materials, it was heat-treated in a continuous annealing (BAF) process rather than continuous annealing. On the other hand, the heating rate was at least 10 ℃ / hr, the cooling rate was controlled to less than 30 ℃ / hr.

And the mechanical properties of the steel sheets manufactured under these conditions were measured and the results are shown in Table 3. In Table 3, AI (aging resistance index) represents the measured value through the experiment under the same conditions as in FIG.

division Chemical composition (wt.%) C Mn P S SOL-AL N Ti Invention steel 0.041 0.28 0.07 0.008 0.066 0.0057 0 Comparative Steel 1 0.018 0.20 0.72 0.009 0.034 0.0031 0 Comparative Steel 2 0.004 0.61 0.042 0.007 0.037 0.0023 0.058

division Hot Rolling Condition Cold rolling reduction Annealing Condition SPM Elongation Remarks Filament rolling temperature Winding temperature Annealing Temperature Annealing time Invention 860 ℃ 570 ℃ 68% 680 ℃ 10Hr 1.40% Annealing Comparative Material 1 916 ℃ 700 ℃ 69% 826 ℃ 38 seconds 1.36% Continuous annealing Comparative Material 2 926 ℃ 620 ℃ 75% 819 ℃ 33 seconds 0.855 Continuous annealing

division Material performance Remarks YP (Kgf / mm) YP-EL TS (Kgf / mm) EL (%) AI (Kgf / mm) Invention 22.6 0 37.2 41 1.5 Low Carbon 35Kg Comparative Material 1 26.5 0 37.2 42 5.8 Mid-low 35Kg Comparative Material 2 21.1 0 36.2 42 1.3 Ultra low 35Kg

As shown in Table 3, in the case of the optimally controlled inventions such as hot rolling conditions and ordinary annealing conditions, the tensile strength was 35 Kgf / mm ² or more and the elongation was very good at 41%. It can be seen that the aging index of high tensile strength steel is 1.5 Kgf / mm ^2, which is less than 3 Kgf / mm ² .

As described above, according to the present invention, box annealing (BAF) method for use by my aging measured index of AGING INDEX value 3Kgf / mm ₂ or less and an elongation of 38% or more 35Kgf / mm ² grade cold rolled high tensile steel plate to be produced Therefore, it can be used not only as an inner plate of an automobile but also as a base steel sheet such as an outer plate material which requires workability and dent resistance of a roof, a door and a hood.

Claims (2)

By weight%, C: 0.07% or less, Mn: 0.2 ~ 0.5%, P: 0.051 ~ 0.1%, S: 0.015% or less, N: 0.002 ~ 0.01% or less, SA ㅣ: 0.05 ~ 0.1%, residual Fe and inevitable Providing an aluminum killed steel including impurities; Hot-finishing the steel at 850-900 ° C. immediately above the Ar ₃ transformation point after homogenizing; Winding the hot rolled steel sheet at a temperature of 600 ° C. or less and pickling in a usual manner, followed by cold rolling at a cold reduction rate of 60 to 75%; Heating the cold rolled steel sheet at a rate of 10 ° C./hr or more and then performing annealing for 8 hours or more at 650˜700 ° C .; And After cooling the annealing the steel sheet at a cooling rate of 30 ℃ / hr or less and temper rolling in the range of 1.0 ~ 2.0%; delete

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