JPH0347922A - Production of hot rolled high carbon steel plate containing uniformly dispersed carbide - Google Patents

Abstract

PURPOSE:To easily produce a hot rolled high carbon steel plate having a structure of uniformly dispersed spheroidal carbide and excellent in workability by hot-rolling a high carbon steel slab and subjecting the resulting plate stock to cooling and coiling under respectively specified conditions. CONSTITUTION:A slab of a high carbon steel containing, by weight, 0.2-0.5% C is hot-rolled, and the resulting hot rolled steel plate is cooled rapidly down to <=670 deg.C at >=30 deg.C/sec cooling rate to regulate the amount of ferrite in a structure to <=20%, and further, the above hot rolled steel plate is coiled at >=500 deg.C to spheroidize iron carbide in a structure and form a structure in which the above iron carbide is uniformly dispersed in the form of fine spheroidal carbide, by which the hot rolled high carbon steel plate having superior workability, such as blanking characteristic, and excellent in strength, toughness, and wear resistance can be easily produced while obviating the necessity of cold rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing a high carbon hot rolled steel sheet in which carbides are uniformly dispersed and excellent in workability.

(Prior art) Generally, carbon (hereinafter simply referred to as C) is 0.2% or more and 0.
．． Carbon steel containing 5% or less (high carbon steel) has high hardness,
It has great strength, toughness, and excellent wear resistance, and these characteristics are used to make cutlery and gears.

It is used in a wide range of fields such as springs and structural parts.

These various parts are often manufactured by punching steel plates.The workability seen in this punching strongly depends on the dispersion state of carbide and the degree of spheroidization, and the more uniformly dispersed the carbide is, the more spherical it becomes. The greater the degree of oxidation, the better the processability.

As a method for spheroidizing carbides in high carbon steel, for example, JP-A No. 5
As disclosed in Publication No. 9-205417, pearlite transformation is completed on a cooling bed after hot rolling, followed by rolling at a reduction rate of 10% or more to deform and fracture carbides, and then the steel plate is wound. There is a method in which Ac is heated to the temperature below the fish gear and held for a certain period of time.

By the way, the dispersion state of carbides and the degree of spheroidization after the spheroidization heat treatment depend on the spheroidization treatment conditions and the structure before the spheroidization heat treatment. For example, a spheroidized carbide structure can be obtained by performing spheroidization heat treatment at Ae and heating at the highest possible temperature in the vicinity for a long time. In addition, by performing cold rolling before the spheroidization treatment to divide the layered carbide structure, and then performing the spheroidization heat treatment,
A more uniformly dispersed spheroidal carbide structure can be obtained.

Currently, for applications requiring severe cold workability, steel sheets are manufactured using the latter process, which involves performing a spheroidizing heat treatment after cold rolling.

(Problems to be Solved by the Invention) As mentioned above, in order to obtain cold workability in high C steel, it is necessary to have a uniformly dispersed spheroidized carbide structure, so cold rolling and subsequent spheroidization are required. Heat treatment cannot be avoided. The purpose of the present invention is to produce a steel sheet having an unprecedented uniformly dispersed spheroidal carbide structure by simply subjecting the steel plate to a spheroidizing heat treatment after hot rolling without performing cold rolling.

(Means and effects for solving the problems) The gist of the present invention is to hot roll high carbon steel containing 0.2% or more and 0.5% or less of carbon, and then annealing it to form a spheroid. In manufacturing rolled steel sheets, after hot rolling, the temperature is cooled at an average cooling rate of 30°C/s or more until the temperature reaches 870°C or less, and then 500°C/s or more is cooled.
The present invention provides a method for producing a high carbon hot rolled steel sheet having a uniformly dispersed spherical carbide structure, characterized in that winding is performed at a temperature of 0.degree. C. or higher.

The present invention will be described in detail below.

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

On the other hand, the upper limit of the carbon content was set at 0.5% because if the carbon content is higher than this, it is difficult to form the hot-rolled sheet into spheroids. Because you need it.

The present inventors conducted basic heating and cooling experiments to obtain steel materials containing 0.2 to 0.5% carbon at 30°C after hot rolling.
It has been found that the ferrite structure of a steel sheet can be reduced to 20% or less by cooling to 670° C. or less at a cooling rate of /s or more. FIG. 1 shows, as a representative example, the relationship between the rapid cooling stop temperature when cooling from the austenite region at 100° C./s after hot rolling and the ferrite structure ratio when slowly cooling. From this figure, it can be seen that when the rapid cooling stop temperature exceeds 670°C, the ferrite structure ratio of the steel plate after cooling becomes approximately the value expected from the equilibrium phase diagram, depending on the carbon content. Furthermore, even at a cooling rate slower than 30° C./s, the ferrite structure ratio of the steel sheet takes approximately the value expected from the equilibrium phase diagram depending on the carbon content. For example, 0.3% carbon
In the steel containing the ferrite structure, the percentage of ferrite structure is 50 to 60%, and the rest has a pearlite structure.

When a steel sheet having a high ferrite structure is annealed to form a spheroid, the carbides in the pearlite become spheroidized, but a uniformly dispersed carbide structure cannot be obtained because of the presence of a large amount of ferrite structure. Therefore, here, it is limited to cooling at an average cooling rate of 30°C/s or more until the temperature reaches 670°C or less after hot rolling.

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

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 simply by subsequent spheroidizing annealing. Further, even if spheroidizing annealing is performed after cold rolling, a uniformly dispersed carbide structure can be similarly obtained.

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

Table 1 shows the chemical composition, hot rolling conditions, ferrite structure ratio after hot rolling, spheroidizing annealing conditions, and degree of carbide dispersion of the sample materials. Steel with chemical composition as shown in this table 1. 2. No. 3 was melted and rolled to a thickness of 8.2 m+a by hot rolling, cooled to 600° C. at a low cooling rate and a high cooling rate, respectively, and wound up. Thereafter, spheroidizing annealing was performed at 700°C for 10 hours.

As shown in Table 1, the cooling rate up to 670℃ is 30℃/
For those with s or more, the ferrite structure ratio of the hot rolled sheet is 20%.
The dispersion of carbides after annealing is also uniform. As mentioned above, steel materials in which carbides are uniformly dispersed have excellent punchability, and by applying the method of the present invention, high-carbon hot-rolled steel sheets with excellent workability represented by punchability can be manufactured. I know what I can do.

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

[Brief explanation of drawings]

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

Claims (1)

[Claims] When hot-rolling high-carbon steel containing 0.2% or more and 0.5% or less of carbon and annealing it to form a high-carbon steel sheet, the temperature becomes 670°C or less after hot rolling. Cool at an average cooling rate of 30℃/s or higher until 500℃
A method for producing a high carbon hot rolled steel sheet in which carbides are uniformly dispersed, the method comprising winding as described above.