KR940007366B1 - Method of manufacturing cold rolling steel sheet - Google Patents

Abstract

The high strength cold roll steel plate having a good press workability is mfd. by hot rolling a steel composed of 0.005-0.01 wt.% C, 0.1-0.2 wt.% Mn, at most 0.015 wt.% S, at most 0.004 wt.% N, 0.02-0.07 wt.% Al and 3.2 wt.% C + 1.5 wt.% S + 3.43 wt.% N <= Ti <= 3.8 wt.% C + 1.5 wt.% S + 3.43 wt.% N at 1100-1250 deg.C slab heating temp. and 890-930 deg.C hot finishing temp., winding it, acid washing the rolled steel plate in the chloric acid soln., cold rolling it, and continuously annealing it at 800-830 deg.C.

Description

Manufacturing method of high strength cold rolled steel sheet with excellent press workability

1 is a graph showing the tensile strength and the r value of the steel and the comparative steel according to the present invention.

The present invention relates to a method for producing a high strength steel sheet used in automobiles, and more particularly, to a method for manufacturing a high strength cold rolled steel sheet having excellent porosity.

Cold rolled steel with a tensile strength of 35-40 kg / mm ^{2 produced} at the same time using the conventional continuous annealing method and excellent press workability is P and Ti (Shinil Iron, product name SAFC) or Mn and Nb () in ultra low carbon Al-killed steel. There were solid-solution-reinforced cold-rolled steel sheets manufactured by adding Kawasaki, product name CHRX), Si and Ti (Japanese steel pipe, product name NKCA), and Mn and Ti (Japanese specialty 60-54373). At this time, the addition ratio of Ti * / C is usually 1.0 or more.

In general, however, in the manufacture of high strength cold rolled steel sheets for high-strength-reinforced deep processing, the manufacturing cost increases according to the addition of solid-solution elements such as P, Mn, and Si. There is a problem that this increases and the secondary workability is lowered.

Accordingly, the present invention is to solve the conventional problems as described above, by adding a small amount of Ti to the ultra-low carbon Al-killed steel containing a C content of 0.005-0.01% by weight of tensile strength 35-40kg / mm ² , r value To produce a high strength cold rolled steel sheet excellent in press workability of 1.7 or more, the purpose is.

The object of the present invention as described above is achieved by producing a cold rolled steel sheet by adding only a small amount of Ti to the Al-killed steel having a C content of 0.005-0.01% without adding a solid solution element such as P, Mn, and Si.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, by weight%, C: 0.005-0.01%, Mn: 0.1-0.2%, N: 0.004% or less, Al: 0.02-0.07%, S: 0.015% or less of Ti 3.2C% in Al-killed steel + 1.5S% + 3.43N% Ti Melting the steel added on the basis of 3.8C% + 1.5S% + 3.43N% (Ti * / C = 0.8-0.95, Ti *: effective Ti amount), the slab heating temperature is set to 1100-1250 ℃, and hot finishing Hot rolling was carried out at a rolling temperature of 890-930 ° C., and then hot rolled coils wound at 650-700 ° C. and air-cooled to room temperature were pickled in hydrochloric acid solution and cold-rolled to a reduction ratio of 40-85%. The present invention relates to a method for producing a high strength cold rolled steel sheet having excellent press workability by continuously annealing a coil with a tensile strength of 35-40 kg / mm ² and an r value of 1.7 or more.

The reason for limiting each of the component range, slab heating temperature at the time of hot rolling, hot finishing temperature, winding temperature after hot rolling, and the continuous annealing cycle in the present invention will be described in detail.

Limiting the carbon content in steel to 0.005-0.01% requires a lot of time in the decarburization process in the range of 0.005% or less, and it is difficult to obtain the target strength due to the small increase in strength due to the employment element. This is because it is lowered.

Mn in steel is required at least 0.1% to prevent red brittleness during hot rolling.However, if Mn content in steel exceeds 0.2%, Mn interacts with carbon in the steel to inhibit the development of annealing aggregates, which is advantageous for press workability. It is preferable to limit the content to 0.1-0.2%.

The content of P in the steel is preferably limited to 0.015% or less, which does not cause brittle fracture at the time of manufacturing and grain brittleness at the time of secondary processing, and S is to prevent cracking at both ends of the hot rolled sheet due to red brittleness during hot rolling. It is limited to 0.015% or less, which is about 1/15 of Mn content in consideration of the atomic equivalence ratio within the range in which MnS can be combined with Mn.

The N is a major element that causes the aging of steel sheet to cause material degradation at room temperature due to the large elongation deterioration effect is limited to less than 0.004% of the relatively small effect, Al is 0.02% by combining with N in steel to form AIN If less than N, it is difficult to sufficiently precipitate with AIN, and if there is not enough deoxidation and 0.07% or more, there is a disadvantage that surface defects occur in the product, so it is limited to 0.02-0.07%.

Ti is an element that combines with C, S and N, and precipitates with TiC, TiN, etc. in the hot rolling process to reduce the solid solution C to increase the r value, but in the present invention, the processability of TiC ₂ S ₄ and the like is minimized. Ti was 3.2C% + 1.5S% + 3.43N% for the purpose of particle size reduction by precipitates and strength increase by residual solid solution C. Ti 3.8 C% + 1.5S% + 3.43N% (Ti * / C = 0.8-0.95, Ti *: amount of effective Ti) is added as a reference.

The slab heating temperature during hot rolling is appropriately re-dissolved TiC, AIN and MnS that have already been deposited in the steel, and then re-precipitated to an appropriate size during hot rolling to develop {111} texture structure effective for the workability of cold rolled steel sheet during annealing. Hold at 1150-1250 ° C. for 1 hour. If the hot finish temperature is below 890 ℃, the mixed structure and coarse grains occur in the hot-rolled sheet, which inhibits the development of {111} texture upon annealing, and the r value decreases again when it exceeds 930 ℃, so the hot finish temperature is 890-930 It is set at ° C.

Winding temperature is limited to 630-670 ° C in consideration of scale peelability and mechanical properties.

The cold rolling rate is limited to 40%, which is a critical workability for recrystallization, to 85%, which is a typical cold rolling range. The annealing method applies a continuous annealing cycle, and the annealing temperature is 800-830, which is a temperature at which recrystallization is sufficiently completed. Limit to ℃.

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

EXAMPLE

By using the Al-killed steel having the chemical composition of Table 1 hot-rolled in the slab heating temperature range of 1150-1250 ℃, hot finish rolling temperature 890-930 ℃, winding temperature 630-670 ℃, and then hot rolled steel sheet Pickled in hydrochloric acid solution, cold rolled to 70% reduction ratio, heated to 800-820 ℃ at 6 ℃ / sec using normal heat treatment cycle of continuous annealing, cracked at this temperature for 40 seconds, and then heated to 5 ℃ / The steel sheet which was slowly cooled to sec and then quenched to 450 ° C. was subjected to overaging at 450 ° C., and then cooled to room temperature at 400 ° C. for 3 minutes, and then subjected to temper rolling.

The mechanical properties of the preparation were then tested and the results are shown in FIG. According to FIG. 1, by the method of the present invention, only Ti is added on the basis of Ti * / C = 0.8-0.95 without adding solid solution elements such as Si, Mn, and P, and the tensile strength is 35 kg / It can be seen that a cold rolled steel sheet having a thickness of ² mm or more can be manufactured.

TABLE 1

As described above, the present invention reduces the decarburization time and P, because only Ti is added to the steel containing 0.005-0.01% carbon without P, compared to P-added ultra low carbon steel to which Ti-P and Nb-P are added. It is possible to reduce the cost of adding high-strength strengthening element, and there is no effect of brittleness and secondary processing cracks during manufacturing by P. Moreover, since it has a component system similar to that of Ti-added ultra low carbon steel for deep processing, the effect of productivity improvement by simplifying steel grade is expected.

As described above, according to the present invention, the tensile strength is 35-40 kg / mm ² by the continuous annealing method using a steel in which a small amount of Ti is added to the Al-killed steel having a C content of 0.005-0.01%, and at the same time, it has excellent press workability. High strength cold rolled steel sheet for automobiles can be manufactured.

Claims (1)

3.2C% + 1.5S in Al-killed steel containing C: 0.005-0.01%, Mn: 0.1-0.2%, S: 0.015% or less, N: 0.004% or less, Al: 0.02-0.07% by weight. % + 3.43N% Ti Steel with added Ti in the range of 3.8C% + 1.5S% + 3.43N% (Ti * / C = 0.8-0.95, Ti *: effective Ti content) is slab heating temperature 1100-1250 ℃ and hot finishing rolling temperature 890 After hot rolling under the condition of -930 ° C, after winding at 650-700 ° C for pickling, and cold rolling at a reduction ratio of 40-85%, the continuous annealing is carried out at a temperature range of 800-830 ° C. Method for producing high strength cold rolled steel sheet having excellent press workability.