JP2866107B2 - Method for producing high-carbon hot-rolled steel sheet in which carbides are uniformly dispersed - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a high-carbon hot-rolled steel sheet in which carbides are uniformly dispersed and workability is excellent.

(Prior art) Generally, carbon (hereinafter simply referred to as C) is 0.2% or more and 0.5% or more.
% Carbon steel (high carbon steel) has high hardness, high strength, high toughness, and excellent abrasion resistance. Utilizing these characteristics, it can be used in a wide range of fields such as cutting tools, gears, springs, and structural parts. It is used for These various parts are often manufactured by stamping steel sheets. The workability as seen in this stamping strongly depends on the dispersion state and degree of spheroidization of carbide, and the more uniformly dispersed, the more spherical The higher the degree of conversion, the better the workability.

As a method of spheroidizing carbide of high carbon steel, for example,
As disclosed in Japanese Unexamined Patent Publication No. 59-205417, a pearlite transformation is terminated on a cooling floor after hot rolling, and subsequently rolling is performed at a rolling reduction of 10% or more to deform and break carbides, and subsequently rolled steel sheet Is heated to a temperature of ₁ point or less of Ac and held for a certain period of time. Incidentally, the dispersion state and degree of spheroidization of the carbide after the spheroidizing heat treatment depend on the spheroidizing treatment conditions and the structure before the spheroidizing heat treatment. For example, a spheroidized carbide structure can be obtained by heating the spheroidizing heat treatment at a temperature as high as possible near Ae ₁ for a long time. Further, by performing cold rolling before the spheroidizing treatment to cut the layered carbide structure and then performing the spheroidizing heat treatment, a more uniformly dispersed spheroidized carbide structure can be obtained. At present, for applications requiring severe cold workability, a steel sheet is manufactured by a process of performing spheroidizing heat treatment after cold rolling as in the latter case.

(Problems to be Solved by the Invention) As described above, in order to obtain cold workability in a high C steel, it is necessary to form a uniformly dispersed spheroidized carbide structure. Can not avoid. Another object of the present invention is to produce a steel sheet having an unprecedented uniformly dispersed spheroidized carbide structure simply by performing spheroidizing heat treatment without performing cold rolling after hot rolling.

(Means and Actions for Solving the Problems) The gist of the present invention is to produce a high carbon hot rolled steel sheet by hot rolling high carbon steel containing 0.2% or more and 0.5% or less of carbon and performing spheroidizing annealing. In doing so, after hot rolling, cool at an average cooling rate of 30 ° C / s or more until the temperature reaches 670 ° C or less, and then
A method for producing a high-carbon hot-rolled steel sheet having a uniformly dispersed spherical carbide structure, wherein winding is performed at 0 or more to reduce the ferrite structure to 20% or less.

 Hereinafter, the present invention will be described in detail.

The reason why the amount of carbon is adjusted to 0.2% is that if the amount of carbon is less than this, the amount of carbide generated after cooling is so small that an appropriate spherical carbide distribution cannot be obtained even if the present invention is applied.

On the other hand, the upper limit of the carbon content is set to 0.5% because it is difficult to spheroidize the hot rolled sheet as it is at a carbon content higher than that, and it is necessary to perform cold rolling and spheroidizing annealing after cooling under normal cooling conditions. Because you do.

The present inventors conducted a basic heating and cooling experiment to hot-roll a steel material containing 0.2 to 0.5% of carbon and then cooled it to 670 ° C or less at a cooling rate of 30 ° C / s or more and to a temperature of 670 ° C or less. It has been found that the ferrite structure can be reduced to 20% or less. FIG. 1 shows, as a typical example, the relationship between the quenching stop temperature in the case of cooling from the austenite region at 100 ° C./s after hot rolling and the ferrite structure ratio in the case of slow cooling thereafter. From this figure, it can be seen that in the case of the quenching stop temperature exceeding 670 ° C., the ferrite microstructure ratio of the steel sheet after cooling becomes a value almost expected from the equilibrium diagram according to the carbon content. 30 ℃
Even at a cooling rate lower than / s, the ferrite microstructure ratio of the steel sheet becomes a value almost expected from the equilibrium diagram according to the carbon content. For example, a steel containing 0.3% of carbon has a ferrite structure ratio of 50 to 60%, and the others have a pearlite structure.
When a steel sheet having a high ferrite structure ratio is subjected to spheroidizing annealing, carbides in pearlite are spheroidized, but since a large amount of ferrite structure is present, a uniformly dispersed structure of carbides cannot be obtained. Therefore, here, it is limited that cooling is performed at an average cooling rate of 30 ° C./s or more until the temperature becomes 670 ° C. or less after hot rolling.

The reason why the winding temperature is set to 500 ° C. or higher is that when winding is performed at a temperature lower than 500 ° C., the risk of cracking due to martensite formation increases.

The steel sheet manufactured by applying the method of the present invention has a low ferrite structure ratio, and a structure in which carbides are uniformly dispersed can be obtained only by performing spheroidizing annealing thereafter. Further, even if spheroidizing annealing is performed after cold rolling, a carbide uniformly dispersed structure can be obtained in the same manner.

(Examples) Next, examples of the present invention will be described together with comparative examples.

Table 1 shows the chemical components of the test materials, hot rolling conditions, ferrite structure ratio after hot rolling, spheroidizing annealing conditions, and the degree of carbide dispersion. Steels 1, 2 and 3 having the chemical compositions shown in this table were melted and rolled to a thickness of 3.2 mm by hot rolling, cooled to 600 ° C at a low cooling rate and a high cooling rate, respectively, and wound. Then 700
The spheroidizing annealing was performed at 10 ° C. for 10 hours. As shown in Table 1, 67
When the cooling rate to 0 ° C was 30 ° C / s or more, the ferrite structure ratio of the hot-rolled sheet was 20% or less, and the carbide dispersion after annealing was uniform. As described above, a steel material in which carbides are uniformly dispersed is excellent in punchability, and by applying the method of the present invention, a high-carbon hot rolled steel sheet having excellent workability represented by punchability is manufactured. We can see that we can do it.

(Effect of the Invention) As described above, by applying the method of the present invention, a high-carbon hot-rolled steel sheet in which carbides are uniformly dispersed only by spheroidizing annealing without cold rolling after hot rolling is manufactured. The industrial effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the ferrite structure ratio after cooling and the quenching stop temperature for explaining the present invention.

Claims (1)

(57) [Claims] 1. A high-carbon steel containing 0.2% or more and 0.5% or less of carbon is hot-rolled and spheroidized to produce a high-carbon hot-rolled steel sheet. Cooling at an average cooling rate of 30 ° C / s or more until it reaches a minimum, and then winding at 500 or more to produce a high-carbon hot-rolled steel sheet with uniformly dispersed carbide characterized by reducing the ferrite structure to 20% or less Method.