JPS59136423A - Preparation of rod steel and wire material having spheroidal structure - Google Patents

Abstract

PURPOSE:To shorten the treating time required in the spheroidizing annealing of carbide in preparing rod steel and a wire material to a large extent, by up and down shifting a steel temp. on the basis of an Ac1 temp. by the processing heat generated during hot processing. CONSTITUTION:Steel capable of containing Si, Mn or the like and containing 2% or less of C is heated to an Ac1 temp. or more and thereafter subjected to hot processing for applying deformation. In this case, the heated steel is cooled to a temp. region ranging from an Ar1 temp. to Ar1 -200 deg.C or more on the way of rolling by forcible cooling (water cooling or air cooling). Succeedingly, plastic deformation of 15% or more is applied to the rolled steel and the temp. thereof is up and down shifted on the basis of the Ac1 temp. by the temp. rising due to deformation heat generated by said plastic deformation. By repeating this control pattern two times or more, the coagulation of carbide is attained and rod steel and a wire material each having a perfect spheroidal structure are obtained. In addition, the aforementioned rolling of 15% or more may be performed in two continuous passes without limiting the same to one pass.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing steel bars and wire rods, and in particular, a method for producing steel bars and wire rods that can obtain a spheroidized carbide structure as rolled by utilizing processing heat during hot rolling. It is related to.

In order to impart deformability to steel materials for cold forging and reduce deformation resistance, and to improve wear resistance in steel for bearings, it is common to spheroidize the carbides in the steel.

In order to spheroidize the carbides in these steels, conventionally, after hot rolling, the steel was allowed to cool, formed into straight rods or wire rod coils, and then reheated in a heat treatment furnace and subjected to spheroidizing annealing.

In other words, in the as-rolled state, carbon steel with low hardenability and large diameter materials with a slow cooling rate after rolling have a pearlite structure or ferrite-pearlite structure, and alloy steel with high hardenability and a large diameter material with a slow cooling rate after rolling have a pearlite structure. In the case of fast m-warp materials, the bainitic structure was subjected to spheroidizing annealing over a long period of time.

In this case, the annealing time should be set for carbon steel that easily becomes spheroidized (for example, 3
45C), it takes 10 to 20 hours1, while alloy steels that are difficult to spheroidize (e.g. SCM 435) and bearing steels have a lifespan of more than 20 hours, which is a bottleneck in manufacturing as well as from an energy saving perspective. That was a problem.

Furthermore, problems of oxidation and decarburization of the steel surface occurred due to the long heat treatment. Therefore, it has been desired to shorten the spheroidizing annealing time.

Therefore, before performing spheroidizing annealing, the steel is subjected to cold working (for example, cold wire drawing) to cause deformation failure of the carbides in the steel, thereby promoting fragmentation and agglomeration of the carbides in the subsequent spheroidizing annealing. A method was proposed in which the spheroidizing annealing treatment time was shortened by this. However, although this method shortened the spheroidizing annealing time, it added a cold working step, so it was not very effective in shortening the processing time throughout the entire process.

In addition, as well-known methods for spheroidizing annealing are A, a method of intentionally heating and then slow cooling (slow cooling method), and 4.
There is a method of holding the temperature directly below point 7 (constant temperature holding method), and a method of repeating heating and cooling above and below point A (repetitive method).

An object of the present invention is to provide a method for manufacturing steel bars and wire rods that can significantly shorten the processing time for spheroidizing carbide annealing.

In order to achieve the above-mentioned object, the manufacturing method according to the present invention is characterized in that a spheroidized structure is obtained in the as-rolled state by raising and lowering the steel temperature from point A using processing heat generated during hot working. It has the following.

According to the present invention, in hot working where steel containing 2% or less of C is heated above the bladder I point and then deformed, it is cooled to a temperature range of below the Ar7 point and above 200"C during rolling, and then A controlled rolling process is repeated at least twice to obtain a spheroidized structure, in which a plastic deformation of 15% or more is applied by rolling, and the resulting deformation heat is used to reach a temperature range from the ACl point to the AcB point. A method of manufacturing steel bars and wire rods is provided.

The requirements of the present invention will be explained in detail below.

First, regarding the amount of C, when the amount of Ci exceeds 2%, the austenite phase region in the phase diagram becomes very narrow, and
Since the amount of pro-eutectoid cementite precipitated on austenite grain boundaries increases, hot workability deteriorates and cracks are more likely to occur during hot rolling, the Ci content is set to 2% or less.

Furthermore, the steel to which the method of the present invention is applied can contain alloying elements such as Cr, Mo, etc. in addition to Si and Mn in order to provide desired strength and ductility. Furthermore, in addition to containing 5olAl as a deoxidizing agent, impurities such as P and S are limited to a predetermined range depending on the desired characteristics of the product and the manufacturing method, but since these are not characteristics of the present invention, they will not be described in further detail.

In order to roughly roll a large bloom or billet, the heating temperature before rolling should be AC7, since the deformation resistance increases rapidly at a temperature lower than the HeC/point, making rolling virtually impossible.
Points or higher are desirable.

It is a well-known fact that it is extremely effective to process the carbide in advance to cause deformation and fracture, and then perform spheroidization annealing to prevent the deformation and fracture of the carbide. It was used.

That is, conventionally, a method has been adopted in which deformation fracture of carbides is performed by cold working after rolling and cooling, but the present invention attempts to perform this during hot rolling. Therefore, carbides must have already precipitated during rolling, and for this purpose it is necessary to cool the steel to a total of two points or less.

However, if the temperature is lower than Ar 7200°C, the deformation resistance becomes very large and subsequent rolling becomes substantially difficult. Therefore, it was decided to cool down to a temperature range of 1 point or less in total and above Art 200'C. The cooling method is forced cooling (
Water cooling, wind cooling) are preferred, but natural cooling may also be used if possible.

As mentioned above, it is necessary to cause deformation fracture in the carbide by rolling, and as will be described later, it is necessary to raise the temperature by the heat of deformation during rolling to reach at least a temperature of -C/point. for at least 15
It is necessary to give plastic deformation of 15% by rolling.
% or more of plastic deformation was applied. However, in this case, rolling is not limited to one bath, but 15% for two or more continuous baths.
It is also possible to give a deformation of .

As already mentioned, the spheroidizing annealing method (repetitive method) in which heating and cooling are repeated above and below the A1 point has traditionally been used.
This principle is to intentionally heat the carbide precipitated at 47 points or below to partially dissolve (divide) the carbide, then cool it to below the A1 point to coagulate the separated carbide. By repeating this process, the aim is to obtain a completely spheroidized tissue.

In the manufacturing method according to the present invention, as described in the previous section, the temperature is increased by natural cooling or forced cooling and the heat of deformation generated by rolling by 15% or more to raise or lower the point.

Therefore, the temperature after heating due to deformation heat needs to reach the Ac1 point or higher. However, if the A'c B point is exceeded, the carbide will completely dissolve into solid solution, so the temperature range is limited to 7 Ac points or more and 3 Ac points or less. Furthermore, this controlled rolling pattern is
It is preferable to repeat the process at least twice, as the effect will not be noticeable unless the process is repeated more than once.

The method for producing steel bars and wire rods according to the present invention will be described below with reference to Examples.

Steel having the components shown in Table 1 was produced by a conventional melting method based on actual practice. Steel bars having a diameter of 15.4 mm to 164.0 mm were prepared in advance, and after soaking for 4 hours, the steel bars were rolled to a diameter of 11.0 mm. Rolling is #1 to #3 rolling, #4 to #6 rolling, and #7 rolling.
~ #9 rolling is continuously rolled, #3, #4, #
Controlled cooling was performed between #6 and #7 by forced air cooling. Second
The table shows the heating temperature of each stroke, #1 to #3 rolling, #4 to #6
The rolling start temperature, rolling end temperature, rolling reduction ratio, and equilibrium transformation temperature in rolling, #7 to #9 rolling were described.

After finish rolling under each condition, natural cooling and controlled cooling at 20° C./min using a heat insulating cover were performed, and the spheroidization rates obtained from these structures are also shown in Table 2. The spheroidization rate shown in Table 2 was determined by the following method. That is, the structure was photographed using a scanning electron microscope, the long axis and short axis of the white carbide were measured, and the value of the long axis/breadth axis was determined. Measurement is 1
This was carried out randomly for 00 or more carbides, a histogram of major axis/minor axis was obtained, and the ratio of the number of carbides whose value was 3.0 or less to the total number of carbides was expressed as %, and was defined as the spheroidization rate.

When each stroke in Table 1 is normally rolled (heating at 1050°C - start of rolling at 950°C - end of rolling at 1040°C - 60% reduction - natural cooling), carbides in the structure of steels A, B and E are almost All of them have a lamellar shape, and the spheroidization rate is 2% or less in all steels (C and D steels have a bainitic structure, so the spheroidization rate cannot be measured). On the other hand, the spheroidization rates of the rolled materials according to the present invention shown in Table 2 are all 75% or more, and reach 95% or more when conditioned cooling is performed after rolling.

Therefore, it is quite possible to omit the spheroidizing annealing.

Next, among the steels shown in Table 2, steel A (heated at 800°C), steel D
Steel (heated to 900℃), C steel (heated to 750℃), D steel (1
000℃ heating), #1 to #3 natural cooling after rolling,
Figure 1 shows the spheroidization rates of #1 to #6 rolled and naturally cooled, #1 to #9 rolled and then naturally cooled (the above rolling conditions are shown in Table 2), and normally rolled. show.

When sitting on this, #1 to #6 and #1 to #9 that were naturally cooled after rolling all showed a spheroidization rate of 65% or more, but #
1 to #3 that were allowed to cool naturally after rolling had a very low spheroidization rate of 25% or less. That is, it can be seen that the effect of the controlled rolling (i-turn) of the present invention is not exhibited much unless it is repeated two or more times.

As explained above, in the method for manufacturing steel bars and wire rods according to the present invention, the temperature of the steel is raised and lowered at a target point using the processing heat during hot rolling, thereby forming a spheroidized structure of carbides during rolling. Therefore, the processing time for subsequent spheroidizing annealing can be significantly shortened, and depending on the application, spheroidizing annealing can be completely omitted.

[Brief explanation of the drawing]

Figure 1 shows steel A (heated to 800°C) and steel B (heated to 900°C).
-=1 for steel C (heated to 700°C) and steel D (heated to 1000°C). Normal rolling, ■, #1 to #3 natural cooling after rolling, ■, #1 to #6 natural cooling after rolling, ■. It is a figure showing the spheroidization rate of #1 to #9 which were naturally cooled after rolling. Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Arai Yazuon Figure 1

Claims (1)

[Claims] In hot working where steel containing 2% or less of C is heated above the C7 point and then deformed, it is cooled to a temperature range of -200°C or higher above the Ar2 point in the middle of rolling, and then subsequently rolled. A steel bar characterized by obtaining a spheroidized structure by repeating a controlled rolling pattern of applying plastic deformation of 15% or more and causing the resulting deformation heat to reach a temperature range of Ac/point to Ac3 point at least twice. Method of manufacturing wire rods.