JPS5934778B2 - Nb-added cold-rolled steel sheet with excellent hardening properties during paint baking treatment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cold-rolled steel sheet that has the property of being hardened by heat treatment during paint baking.

Cold-rolled steel sheets have a wide range of uses, including automobiles, electrical appliances, furniture, and office supplies, and have achieved significant growth in both quantity and quality.

During this period, cold-rolled steel sheets were given properties such as non-aging properties, deep drawability, and shape fixability in response to customer demands, and new products were developed one after another.

In recent years, cold-rolled steel sheets have been made to have higher tensile strength for the purpose of reducing the weight of automobile bodies, and in order to solve the difficulty of press work that occurs with this, we have developed a new steel sheet that has a low yield strength during press forming and can be easily press-formed. 150-18 after applying the product paint
The baking of paint at a temperature of 0°C increases the strength of the steel sheet, and cold-rolled steel sheets are now required to have the ability to make products with high strength.

This performance is A, A (Accelerated A
ging) and B, H (Bake Hardeni)
ng) Evaluated by gender.

The former is obtained by processing a cold-rolled steel sheet product that has been temper-rolled after annealing to a strain of about 3 to 8 degrees, and then heat-treating the product at 170°C for about 30 minutes, which corresponds to the processing, for paint baking. The difference between the yield stress after processing and heat treatment and the product yield stress is defined as the A, A characteristic value, and the larger this difference is, the better the A, A properties are.

In addition, for the latter, cold-rolled steel sheet products are heat-treated as they are at 170°C for about 30 minutes, and the difference between the yield stress after heat treatment and the product yield stress is defined as the B and H characteristic values, and the larger this difference is, the better the B and H properties are. It is said that

The present inventors provide a cold-rolled steel sheet with excellent B and H properties or A and A properties, the gist of which is C:
0.02-0.12%, Mn 20.5% or less, Si
: 0.05% or less, Al: 0.01 to 0.10%, N20.0100 or less, Nb: 0.010 to 0.030%
The remainder consists of iron and unavoidable impurities, which are cold-rolled steel sheets with excellent hardening properties during paint baking treatment.

The reason why the present invention is limited to the above-mentioned steel components will be explained in detail below.

C is a necessary component to impart good H properties or A, A properties to steel, but this effect cannot be obtained if it is less than 0.02%, and if the amount of C increases, it increases the strength of the steel plate matrix. Therefore, the upper limit is set to 0.12%, preferably 0.12%.
08%.

As the content of Mn and Si increases and the strength of the steel sheet matrix increases, the B, H, and A, A properties decrease, so the upper limit for Mn and Si is limited to 0.5% and 0.05%, respectively. do.

AA is added to deoxidize steel and prevent coarsening of crystal grains. If it is less than 0.01%, the degree of deoxidation will vary from lot to lot, causing surface defects on the product, so the lower limit should be set. 0.
01%.

Furthermore, addition of more than 0.10% will only unnecessarily increase manufacturing costs, so the upper limit is set at 0.10%.

If Nb is less than 0.010%, the amount of fine NbC precipitated, which is the cause of good B, H properties or A, A properties in the steel of the present invention, will be insufficient and sufficient properties cannot be maintained, so the lower limit is set to 0.
．． 010%.

Further, if excessive Nb exceeding 0.030% is contained, the strength of the steel sheet becomes too high and the B, H properties and A, A properties decrease, so the upper limit is set at 0.030%.

In addition, P, S, etc. contained as unavoidable impurities deteriorate the properties of steel, so they must be reduced as much as possible.

The cold-rolled steel sheet of the present invention having the above-mentioned composition can be produced by melting in a melting furnace such as a converter or an electric furnace, and if necessary, further performing vacuum degassing treatment. After being made into a steel billet, it is manufactured through the usual manufacturing process of hot rolling, cold rolling, and continuous annealing.

The cold-rolled steel sheet produced in this way has B, H and A properties.
Because of its extremely good A property, it is used as a cold-rolled steel sheet that can be easily pressed and has the characteristics of increasing the strength of secondary products.

Next, examples of the present invention will be described.

After hot-rolling and pickling a steel billet produced in a converter and continuous casting, it is cold-rolled at a reduction rate of 65% to produce a cold-rolled plate with a thickness of O, 5TL7n, and this cold-rolled plate is A cold-rolled steel sheet was produced by continuous annealing at 700° C. for 5 minutes and then temper rolling at a rolling reduction of 1.5%.

The cold rolled steel sheet composition and its characteristic values are shown in the table.

Note that steel types F, G, H, and I are materials for comparison that deviate from the composition of the present invention.

As is clear from the above results, the cold-rolled steel sheet of the present invention has better hardenability during paint baking treatment than other general cold-rolled steel sheets, including steel sheets outside the scope of the invention. The effect is extremely strong.

Claims (1)

[Claims] IC: 0.0:II'-0.12%, Mn 20.5
% or less, Si: 0.05% or less, Al20, 01-0.
10%, N: 0.0100% or less, Nb: 0.010~
A Nb-added cold-rolled steel sheet containing 0.030% and the remainder consisting of iron and unavoidable impurities, which has excellent hardenability during paint baking treatment.