JPH0125811B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing steel bars and wire rods, and in particular to a method of manufacturing steel bars and wire rods, and in particular, uses processing heat during hot rolling to control the temperature of steel materials.
A This relates to a method for manufacturing steel bars and wire rods that allows a spheroidized carbide structure to be obtained during rolling by moving the material up and down at _one point. Prior Art In order to impart deformability to steel materials for cold forging and lower deformation resistance, and to improve wear resistance in bearing steels, carbides in the steel are generally spheroidized. In order to spheroidize the carbides in these steels, conventionally, after hot rolling, the steel was allowed to cool and formed into a straight bar or wire coil, which was then reheated in a heat treatment furnace and subjected to spheroidizing annealing. Conventionally, the following methods have been adopted as such spheroidization processing. (1) After heating to a temperature of Ac ₁ or more and Ac ₃ or Acm or less, it is slowly cooled until the transformation is completed, or it is cooled to an appropriate temperature just below Ac ₁ and kept at this temperature for a certain period of time to ensure that the transformation occurs. After completion, air cool. (2) Heat for a long time to a temperature just below Ac ₁ and then cool. (3) Heating and cooling is repeated above and below the Ac ₁ transformation point. (4) After heating to a temperature just above Ac ₃ or Acm, and cooling at an appropriate cooling rate that can prevent the formation of network carbides and coarse pearlite, the carbides are spheroidized by the method of (2) or (3). In other words, in as-rolled steel bars or wire rods, carbon steel with low hardenability and large diameter materials with slow cooling rate after rolling have a pearlite structure or ferrite/pearlite structure, and alloy steels with high hardenability and cooling after rolling have a pearlite structure. Small-diameter materials with a fast bainite structure were annealed to spheroidize over a long period of time using one of the above methods. In this case, the annealing time is 10 to 20 hours for carbon steel that is easy to spheroidize (e.g. S45C), and for alloy steel that is difficult to spheroidize (e.g. SCM435) or bearing steel.
The process took more than 20 hours, which was a manufacturing bottleneck and was also a problem from an energy-saving perspective. Furthermore, due to long-term heat treatment, the steel surface becomes oxidized.
The problem of decarburization also arose, and therefore it was 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 has been proposed in which the spheroidizing annealing treatment time can be shortened by this method. However, although this method shortened the spheroidizing annealing time, it added a cold working step, so it was less effective in terms of shortening the processing time throughout the entire process. In view of these problems in the prior art, the applicant has developed a method to raise the temperature of the steel again by utilizing the processing heat generated during hot working, and to make the steel into a spherical shape as rolled by adjusting cooling or maintaining the temperature after rolling. In January 21, 1982, a method of obtaining a spheroidized structure, or a method of obtaining a spheroidized structure as rolled by repeating hot working and cooling and raising and lowering the steel temperature at _one point Ar using the generated processing heat, etc. Patent application dated 1984-8584
No. 58-8585 and Japanese Patent Application No. 58-8586. In particular, the latter method, that is, the method of spheroidizing by repeatedly hot working and cooling and raising and lowering Ar _{at one} point, is disclosed in Japanese Patent Application No. 58-8586, the gist of which is as follows: % or less of C is heated to Ac ₁ point or more and then deformed. During rolling, the steel is cooled to a temperature range of Ar ₁ point or less or Ar ₁ -200℃ or more, and then subsequently rolled. 15
The objective is to obtain a spheroidized structure by repeating at least twice a controlled rolling pattern in which plastic deformation of % or more is applied and the resulting deformation heat reaches a temperature range of Ac ₁ point or more and Ac ₃ points or less. OBJECTS OF THE INVENTION An object of the present invention is to provide a novel method for producing steel bars and wire rods, which solves the problems of the prior art described above and can significantly shorten the processing time for spheroidizing carbide annealing. More specifically, an object of the present invention is to provide a method for manufacturing steel bars and wire rods that can obtain a spheroidized structure as they are rolled, with improved rolling efficiency. Structure of the Invention In order to achieve the above-mentioned object, the manufacturing method according to the present invention uses the processing heat generated by hot rolling to raise the steel temperature, and by repeating hot working and cooling, This method is characterized by the combination of raising and lowering the steel temperature with _one Ar point as the boundary, and the effect of deformation and fracture of carbide due to deformation processing. Therefore, according to the present invention, in hot working where steel containing 2% or less of C is heated to Ac ₁ point or more and then deformed, Ae is ₁ point or less during rolling, and
Cool to a temperature range exceeding Ar ₁ point, then apply plastic deformation of 15% or more by finish rolling,
By promoting pearlite or bainite transformation, this structure is generated, and at the same time, at least a controlled rolling pattern is created that allows the temperature range of Ac ₁ point or more and Ac ₃ point or Accm point or less to be reached again by deformation heat. A method for manufacturing a steel bar and wire rod having a spheroidized structure characterized by repeating the process twice is provided. That is, the difference between the present invention and the invention related to Japanese Patent Application No. 58-8586 is that the cooling temperature during rolling is Ae ₁
point or less and exceeds Ar ₁ point. In the case of the present invention, the cooling temperature is set in a relatively high range as described above, and the steel structure at the start of finish rolling or the start of the next rolling is made of a single austenite phase or a mixed phase of austenite and ferrite or cementite with low deformation resistance. It is characterized by its organized structure. Natural cooling or forced cooling (air cooling, water cooling, etc.) is used to cool the steel in the method of the present invention. In this way, a method for producing a steel bar or wire rod with high rolling efficiency and having a spheroidized structure as rolled is provided. Below, the constituent elements of the present invention will be explained one by one. (1) Reason for setting the C content to 2% or less If the C content exceeds 2%, the austenite phase region in the phase diagram becomes extremely narrow, and the amount of pro-eutectoid cementite precipitated on the austenite grain boundaries increases. The amount of C was set to 2% or less because it deteriorates workability and tends to cause cracks during hot rolling. Furthermore, in order to provide the desired strength and ductility to the steel to which the method of the present invention is applied, in addition to Si and Mn, Cr and Mo are added.
It may contain alloying elements such as. Furthermore, in addition to including Sol.Al as a deoxidizing agent, impurities such as PS are limited to a predetermined range depending on the desired characteristics of the product and the manufacturing method, but these are not characteristics of the present invention and will not be described in further detail. (2) Reason for heating to Ac ₁ point or more In order to roughly roll a large bloom or billet, heating before rolling is necessary because at temperatures lower than Ac ₁ point, the deformation resistance increases rapidly and rolling becomes virtually impossible. The temperature should preferably be ₁ point or higher on Ac. (3) Reason for cooling to a temperature range where Ae is below ₁ point and Ar is above ₁ point before finish rolling To prevent spheroidization of carbides, processing is applied in advance to deform and fracture the carbides, followed by spheroidization annealing. It is a well-known fact that it is extremely effective when administered.
The present invention also utilizes this deformation fracture of carbide. That is, conventionally, a method has been adopted in which deformation and fracture of the carbide is performed by performing cold working after rolling and cooling, but the present invention attempts to perform this by finish rolling. Therefore, carbides must already precipitate before finish rolling is completed. However, if the pearlite transformation or bainite transformation in which carbides are precipitated is completed before the start of finish rolling, the deformation resistance during finish rolling becomes extremely large, resulting in an excessive load on the rolling mill. Therefore, in the present invention, it is specified that before the start of finish rolling, there is a single austenite phase or a mixed phase of austenite and ferrite or cementite. However, the austenite in this case is supercooled austenite, and carbides are precipitated by work-induced transformation during finish rolling. Therefore, since the processing is performed while the carbide is precipitated, deformation fracture of the carbide is sufficiently performed. For this purpose, it is necessary to cool the steel to a supercooled austenite temperature range before finish rolling, that is, a temperature range where Ae is below ₁ point and Ar is above ₁ point, so the above limitations were made. (4) Reason for applying plastic deformation of 15% or more by rolling As stated in the previous section, it is necessary to cause deformation fracture in the carbide by rolling, and as will be explained later, the deformation heat during rolling causes In order to increase the temperature,
It is necessary to reach a temperature of at least Ac ₁ point, but for this purpose it is necessary to give at least 15% plastic deformation, so the above limitations were made. However, rolling in this case is not limited to one pass, and may be performed in two or more consecutive passes to give a cumulative deformation of 15% or more. (5) Ac ₁ point or more, Ac ₃ points or more due to deformation heat
Reason for repeating the controlled rolling pattern at least twice to reach a temperature range below the Accm point As already mentioned, the conventional spheroidizing annealing method (repetitive method) in which heating and cooling are repeated above and below _one point A
is being carried out. This principle is to heat the carbide precipitated below A1 _point to above A1 point to dissolve (segment) some of _the carbide, and then to coagulate the separated carbide while cooling to _A1 point or below. By repeating this process, it is possible to obtain a completely spherical structure. In the method of the present invention, as described above, the _A1 point is raised or lowered by natural cooling or forced cooling and temperature increase due to deformation heat generated by rolling of 15% or more. Therefore, the temperature after heating due to deformation heat is Ac ₁
You need to reach a point or higher. However, if the Ac ₃ point or the Accm point is exceeded, the carbide becomes a complete solid solution, so if the Ac point is ₁ or more and the Ac ₃ point or
Limited to below the Accm point. Furthermore, this controlled rolling pattern was repeated at least twice because its effect would not be apparent unless it was repeated two or more times. Examples Hereinafter, the method of the present invention will be explained using examples. A steel having the components shown in Table 1 was manufactured by a conventional melting method, and the steel bars were made into steel bars having a diameter of 15.4 mm to 164.0 mm, which were soaked for 4 hours and then rolled to a size of 11.0 mm. For rolling, #1 to #3 rolling, #4 to #6 rolling, and #7 to #9 rolling were each carried out continuously.
Controlled cooling was performed between #3 and #4 and between #6 and #7 using forced air cooling. Table 2 shows the heating temperature of each steel, #1~#3 rolling, #4~#6 rolling, #7~
Rolling start temperature, rolling end temperature in #9 rolling,
The reduction rate and equilibrium transformation temperature are listed. In addition, those rolled under exactly the same conditions are #1,
#1, #4, #7 were cooled with water immediately before and immediately after rolling, and their structures were observed.
Just before rolling, austenite, #1, #4, #7
It has been confirmed that bainite or pearlite is generated immediately after rolling. From the above facts, it is clear that this example was rolled within the manufacturing range of the present invention. Next, 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.

【table】

[Table] The spheroidization rate shown in Table 2 was determined by the following method. That is, the tissue is photographed using a scanning electron microscope,
Measure the long axis and short axis of the white carbide, and calculate the long axis/
The value of the short axis was determined. Measurements were performed randomly on 100 or more carbides, a histogram of the length/breadth ratio was obtained, and the ratio of the number of carbides with a value of 3.0 or less to the total number of carbides was expressed as a percentage, which was defined as the spheroidization rate. Each steel in Table 1 is normally rolled (heated at 1050℃ - 950℃
At the start of rolling - end of rolling at 1040°C - 60% reduction - natural cooling), almost all the carbides in the structure of ABE steel are lamellar, and the spheroidization rate is less than 2% (C , D steel has a bainite structure, so the spheroidization rate cannot be measured). On the other hand, the spheroidization rate of the rolled materials according to the present invention shown in Table 2 is all 70% or more, and reaches 85% 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 to 800℃), steel B (heated to 900℃), steel C (heated to 750℃), steel D
For steel (heated to 1000℃), #1 to #3 natural cooling after rolling, #1 to #6 natural cooling after rolling, #1 to #9
After rolling, the product was allowed to cool naturally (the above rolling conditions were
Figure 1 shows the spheroidization rates of the samples (as shown in the table) and those of the conventionally rolled samples. In the figure, the white circles are steel A heated to 800℃, the white triangles are steel B heated to 900℃, and the black circles are steel C heated to 750℃.
The black triangles indicate the spheroidization rate after rolling D steel heated to 1000°C under the respective conditions. According to the results shown in Figure 1, #1 to #6 and #1 to #9 that were naturally cooled after rolling were all 60%
Although the above-mentioned spheroidization is shown, those #1 to #3 which were naturally cooled after rolling showed a very low spheroidization rate of 20% or less. That is, it can be seen that the effect of the controlled rolling pattern of the present invention is not exhibited unless it is repeated two or more times. Effects of the Invention As explained above, in the method for producing steel bars and wire rods according to the present invention, the temperature of the steel is raised and lowered from point _A1 using the processing heat during hot rolling, thereby maintaining the rolling state. A spheroidized carbide structure can be obtained, and 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 drawings]

Figure 1 shows steel A (heated at 800°C), steel B (heated at 900°C), steel C (heated at 750°C), and steel D (heated at 1000°C) after normal rolling and natural release after rolling #1 to #3. cold,
It is a figure which showed the spheroidization rate of #1-#6 natural cooling after rolling, and #1-#9 natural cooling after rolling, respectively.

Claims (1)

[Claims] 1. In hot working where steel containing 2% or less of C is heated to Ac ₁ point or more and then deformed, it is cooled during rolling to a temperature range that is Ae ₁ point or less and Ar exceeds ₁ point, and then continued. 15 by finish rolling
By applying plastic deformation of % or more and thereby promoting pearlite or bainite transformation, these structures are generated, and at the same time, the temperature range is again reached at Ac ₁ point or more, Ac ₃ point, or Accm point or less by heat of deformation. A method for producing steel bars and wire rods having a spheroidized structure, the method comprising repeating a controlled rolling pattern at least twice.