JPS6115132B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a continuous heat treatment method for cold rolled steel sheets. According to the method according to the invention, it is possible to provide a cold-rolled steel sheet with good elastic limit and elongation properties, resulting in excellent drawability and homogeneous properties over the entire width of the steel sheet. can. Such qualities are particularly required in the case of steel sheets with high elastic limits used in the automobile industry. When it is desired to obtain a cold-rolled steel sheet with good ductility, drawability, and elongation, the coiled steel sheet is generally placed in a bell-shaped annealing furnace and annealed for recrystallization. However, such processing is time consuming and therefore inefficient, resulting in high costs, and furthermore, the results of the processing are highly variable. To overcome these drawbacks, it has been proposed to replace such annealing treatment with continuous heat treatment.
Various modification methods have been proposed. According to some of them, sufficiently uniform properties and good ductility can be obtained over the entire width of the steel plate, while at the same time the processing time can be kept to a reasonable length. The basic principle of these methods is to achieve the following in a short period of time (several minutes), taking into account the throughput and the length of the processing line. (b) At least partial recrystallization of the hardened structure of the cold-rolled steel sheet (b) Formation of a structure that is advantageous for drawing (c) Expansion of crystal grains to a desired size (d) Reduction of interstitial carbon To be precipitated in the form of carbides. Certain methods involve, inter alia, heat treating the cold rolled steel sheet by heating it to its recrystallization temperature and cooling it by immersing it in an aqueous bath. In one particular embodiment of this method, the heating step is advantageously carried out at a rate faster than 4° C./sec. In another embodiment of this method, the rate of cooling by immersing the steel plate in an aqueous bath is increased, e.g.
The temperature should be quite fast, such as ℃/second to 250℃/second. The present invention is directed to an improvement in a treatment process consisting of cooling a steel plate in an aqueous bath. The purpose of this improvement is to ensure that the steel sheet emerges from the cooling bath in a non-oxidized state, regardless of the temperature of the cooling bath. The continuous heat treatment method for rolled steel sheets that is the object of this invention is characterized in that the aqueous bath in which the steel sheets to be quenched are immersed contains formic acid. It has actually been observed that when such a substance is added to an aqueous bath, the steel sheet coming out of the bath is not oxidized. Furthermore, it has been found that the corrosive action can be easily stopped upon exiting the bath, and that the reaction products can also be easily removed. This is not possible when cleaning is carried out with inorganic acids. Furthermore, it has also been found to be advantageous to add reducing substances, such as hydrazine, to the aqueous bath, which reduce the amount of dissolved oxygen in the water. Thus, such treatment has the effect of limiting the formation of oxides and thus reducing the amount of formic acid required to carry out the method. It is likewise within the scope of this invention to add surfactants to the aqueous bath to facilitate drying and removal of deposits from the steel sheet, for example by blowing air. In one advantageous embodiment of the invention, the temperature of the aqueous bath can be above 75° C. to facilitate the action of the organic substances. According to the present invention, when a rolled steel sheet is continuously treated using an aqueous solution containing formic acid, the degree of oxidation is significantly reduced even when the treated steel sheet comes into contact with air. Regarding the precise mechanism of these effects,
Although it has not yet been completely elucidated, formic acid combines with the iron content in the surface layer of the steel plate, causing It is believed that by forming a monomolecular layer or a layer of a compound close to it, a layer containing a type of aldehyde group is created, which has a strong function of resisting oxidation. This effect is not observed with other organic acids. The table below shows the conditions under which this invention may be practiced, but these are merely examples. A 1 mm thick steel plate was heated to 850 °C in a protective atmosphere and then immersed in various aqueous baths at a temperature of 100 °C. The residence time in the bath was 30 seconds, during which time the temperature of the steel plate was lowered from 850°C to 100°C. In the table below,
Method (1) is the standard method of treatment with an aqueous bath containing no organic acid, and methods (2), (3), (4) and (5) are
This corresponds to the case where the aqueous bath contains formic acid and its content is gradually increased. 【table】

Claims (1)

[Claims] 1. A continuous heat treatment method for rolled steel plates, which comprises immersing the rolled steel plates in an aqueous bath to rapidly cool them, and using an aqueous bath containing formic acid.