KR20020055279A - Cold work embrittlement free high strength steel sheet - Google Patents

Abstract

PURPOSE: To improve conventional problems found in interstitial free steel such as embrittlement at low temperatures, a cold work embrittlement free high strength steel sheet having higher carbon and titanium content is provided. CONSTITUTION: The cold work embrittlement free high strength steel sheet comprises C 0.01 to 0.04 wt.%, P 0.06 to 0.08 wt.%, Mn 1.2 to 1.5 wt.%, Si 0.001 to 0.05 wt.% and a balance of Fe, and it further comprises Ti 0.03 wt.% or less. The present high strength steel sheet show superior embrittlement resistance at temperatures less than -40°C.

Description

Cold work embrittlement free high strength steel sheet

The present invention relates to a high tensile strength steel sheet, and more particularly, to a high tensile strength steel sheet which can be widely used as an automobile member or a reinforcing material due to its high strength and low temperature brittleness.

Conventionally, steel sheets that have been used as materials for automobile members or reinforcement parts generally have a carbon content of 0.005% or less, phosphorus (P) of 0.08 to 0.10%, manganese (Mn) of 1.05 to 1.25%, and silicon (Si). Interstitial free (IF) steel sheets containing 0.2 to 0.3% and 0.04 to 0.06% of titanium (Ti) have been mainly used. That is, most of the substituted solid solution to which P, Mn, and Si were added were Ti added while the content of C was extremely low in order to increase the strength.

Such IF steel sheets have been widely used in parts with complex shapes, which have no aging at room temperature and have good formability. However, since excessive P is added and segregates at grain boundaries, cracks may occur due to brittle grains during secondary press processing, or at low temperatures. There was a disadvantage that the parts were broken.

Accordingly, the present inventors have developed a high strength steel sheet having high strength and low temperature brittleness by increasing the content of carbon and titanium in order to improve the problems of the conventional IF steel sheet as described above.

The present invention is characterized by a high tensile strength steel sheet containing 0.01 to 0.04% of carbon, 0.06 to 0.08% of phosphorus, 1.2 to 1.5% of manganese, and 0.001 to 0.05% of silicon.

In addition, the high-strength steel sheet of the present invention may include 0.03% or less of titanium.

The present invention will be described in more detail as follows.

In the present invention, the carbon makes the intrusion-type solid solution to improve the strength of the steel sheet, and some of them move to the grain boundaries to increase the bonding strength between the grains to serve to prevent the occurrence of cracks during secondary processing. At this time, the carbon content should be 0.01 to 0.04%, but if the carbon content is 0.01% or less, the steel sheet is easily ruptured at low temperature collisions.

Phosphorus should be content of 0.06 to 0.08% as a substituted solid solution. If the phosphorus content is 0.06% or less, it is difficult to expect a sufficient strength increase effect, and if it is 0.08% or more, the desired grain boundary precipitation prevention effect cannot be obtained.

Manganese is also a substituted solid solution, and is added in an amount of 1.2% or more to improve strength, but is limited to 1.5% or less to prevent elongation reduction.

If silicon is contained in the steel sheet excessively, the adhesion of the plated steel sheet is impaired, so it is limited to 0.05% or less.

The high-strength steel sheet according to the present invention, although some room temperature aging is generated, there is no problem in the manufacturing of parts for the vehicle body inner plate. Since the automobile member or the reinforcing material has to undergo a process of bending deformation, a small amount of elongation molding, drawing, etc., the high tensile strength steel of the present invention is greatly improved and low temperature brittleness is expected to be widely used as a steel sheet for automobiles in the future.

Although this invention is demonstrated in detail based on an Example, this invention is not limited by an Example.

Examples 1-2 and Comparative Examples 1-2.

The steel sheet having the chemical composition of Table 1 is hot rolled to make a hot rolled steel sheet having a thickness of 2.8 mm. The product was cold rolled through a pickling process to a final thickness of 1.2 mm, followed by continuous annealing at 820 ° C., and cooling.

After the tensile test specimen of KS5 was made from the steel sheet thus manufactured, a tensile test was performed with a Zwick tensile tester. The test speed was 20mm / min and yield strength was calculated by 0.2% offset method. Tensile strength was determined by dividing the maximum load by the cross section of the specimen, and the elongation was calculated by increasing the length from breakage to 50mm. In addition, the soft brittle transition temperature of the steel sheet was made as a cylindrical cup having a drawing ratio of 1.92 for each steel sheet, the cup was exposed to each temperature and the weight is dropped to drop the cup.

Table 2 shows a comparison of the test results of the steel sheets of Examples and Comparative Examples.

As can be seen in Table 2, the high-strength steel sheet of the present invention is inexpensive and light in weight, and is particularly safe due to high impact absorption energy during low temperature collisions because it exhibits good brittleness even at temperatures below 40 ° C. It is expected to contribute greatly to the development of the vehicle.

Claims (1)

A high tensile strength steel sheet comprising from 0.01 to 0.04% of carbon, from 0.06 to 0.08% of phosphorus, from 1.2 to 1.5% of manganese and from 0.001 to 0.05% of silicon.