High strength steel sheet and method for manufacturing the same

Abstract

The invention provides a high strength steel sheet which exhibits excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case where the steel sheet has a high Si content, and a method for manufacturing such steel sheets. The method includes continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at ≦0.01%, the balance being represented by Fe and inevitable impurities, while controlling the dew-point temperature of the atmosphere to become not less than −10° C. when the heating furnace inside temperature is in the range of not less than A° C. and not more than B° C. during the course of heating (A: 600≦A≦780, B: 800≦B≦900).

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]This application is the U.S. National Phase application of PCT International Application No. PCT / JP2010 / 067612, filed Sep. 30, 2010, the disclosure of which is incorporated herein by reference in its entirety for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to a high strength steel sheet having excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case where the steel sheet has a high Si content, as well as to a method for manufacturing such steel sheets.BACKGROUND OF THE INVENTION[0003]From the viewpoint of the improvements in automobile fuel efficiency and crash safety of the automobiles, there have recently been increasing demands for car body materials to be increased in strength for thickness reduction in order to reduce the weight and increase the strength of car bodies themselves. For this purpose, the use of high strength steel sheets in automobiles has been...

AI Technical Summary

Benefits of technology

[0058]After the steel sheet is cooled from the temperature range of not less than 750° C., hardening and tempering may be performed as required. Although the conditions for these treatments are not particularly limited, it is desirable that tempering be performed at a temperature of 150 to 400° C. The reasons are because elongation tends to be deteriorated if the temperature is less than 150° C. as well as because hardness tends to be decreased if the temperature is in excess of 400° C.

[0059]According to the present invention, good chemical convertibility can be ensured even without performing electrolytic pickling. However, it is preferable that electrolytic pickling be performed in order to remove trace amounts of oxides that have been inevitably generated by surface segregation during annealing and thereby to ensure better chemical convertibility.

[0060]The electrolytic pickling conditions are not particularly limited. However, in order to efficiently remove the inevitably formed surface segregation of silicon and manganese oxides formed during the annealing, alternating electrolysis at a current density of not less than 1 A / dm2 is desirable. The reasons why alternating electrolysis is selected are because the pickling effects are low if the steel sheet is fixed to a cathode as well as because if the steel sheet is fixed to an anode, iron that is dissolved during electrolysis is accumulated in the pickling solution and the Fe concentration in the pickling solution is increased with the result that the attachment of iron to the surface of the steel sheet causes problems such as dry contamination.

[0061]The pickling solution used in the electrolytic pickling is not particularly limited. However, nitric acid or hydrofluoric acid is not preferable because they are highly corrosive to a facility and require careful handling. Hydrochloric acid is not preferable because chlorine gas can be generated from the cathode. In view of corrosiveness and environment, the use of sulfuric acid is preferable. The sulfuric acid concentration is preferably not less than 5 mass % and not more than 20 mass %. If the sulfuric acid concentration is less than 5 mass %, the conductivity is so lowered that the bath voltage is raised during electrolysis possibly to increase the power load. On the other hand, any sulfuric acid concentration exceeding 20 mass % leads to a cost problem because a large loss is caused due to drag-out.

[0062]The temperature of the electrolytic solution is preferably not less than 40° C. and not more than 70° C. Because the bath temperature is raised by the generation of heat by continuous electrolysis, the pickling effect may be lowered if the temperature is less than 40° C. Further, maintaining the temperature below 40° C. is sometimes difficult. Furthermore, a temperature exceeding 70° C. is not preferable in view of the durability of the lining of the electrolytic cell.

[0063]The high strength steel sheets of the present invention are obtained in the above manner.

Problems solved by technology

Consequently, as mentioned above, various problems have been encountered such as difficulties in controlling the dew-point temperature or the oxidation, the occurrence of uneven chemical convertibility and a decrease in corrosion resistance after electrodeposition coating.