JPS6386815A - Production of steel having excellent cold workability - Google Patents

Abstract

PURPOSE:To shorten the total heat-treatment time by controlling cooling velocity at the way of hot rolling to adjust the structure and next finish-rolling at the specific working ratio and temp. CONSTITUTION:In the hot rolling for various kinds of steel, the steel is cooled at about 150 deg.C/sec cooling velocity at the way of rolling till martensite transformation is completes. Next, it is rapidly resin to its temp., and rolled at >=10% and <=70% the working ratio in the range of temp. <=Ac3 point and air-cooled to the room temp. In this way, the structure in the finish-rolled steel has finely dispersive carbides and also working strain remains in the rolled steel. Therefore, the cold-workability is improved and the time for subsequent spheroidizing heat-treatment is shortened.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to the production of steel materials such as structural carbon steel, low alloy steel, etc., such as wire rods, steel bars, and steel plates, by hot rolling. The present invention relates to a method of manufacturing steel materials suitable for manufacturing parts such as bolts, sockets, and screws.

(Conventional technology and problems to be solved) Prior to cold working, high carbon steels such as carbon steels for machine structures, low alloy steels, bearing steels, and tool steels are given ductility and deformation resistance. Spheroidizing annealing is generally performed to reduce hardness and improve workability. Conventionally, this spheroidizing annealing method has been used (
1) A long-time heating method in which the material is heated to a temperature just below the A1 point for a long time and then cooled down; (2) A slow cooling method in which the product is heated to a two-phase region between the A0 point and the A3 point for an appropriate time and then gradually cooled; (3) ) There are methods such as the repeated heating and cooling method, which repeatedly heats and cools to the temperature directly above and below point A4.

This spheroidizing annealing treatment is often performed on steel products such as wire rods, steel bars, and steel plates manufactured by hot rolling, by reheating them to a predetermined temperature in a heat treatment furnace on a separate line. Heat treatment typically requires an extremely long treatment time of more than ten hours, resulting in low productivity and high heat treatment costs.

Further, from the viewpoint of energy saving, it is desired to shorten the heat treatment time.

Therefore, the following methods have been tried as improvement measures for this purpose. In other words, the structure of rolled steel products currently produced industrially is normally the same.

It has a ferrite-pearlite structure, and in order to convert steel materials with such a structure into a spherical carbide structure suitable for cold working, the long heat treatment described above is required. In order to
As disclosed in No. 550℃~M after hot rolling,
As disclosed in Japanese Patent Publication No. 55-31165, there is a method in which the temperature range of the s point is cooled at a cooling rate of 100°C/sec or more and maintained in the above temperature range for 1 minute or more, or as disclosed in Japanese Patent Publication No. 55-31165. A method of rapid cooling to a temperature range of °C to Ms point has been proposed. However, the effect of significantly shortening the heat treatment time has not necessarily reached a satisfactory level.

The present invention has been made in view of these circumstances, and it significantly shortens the heat treatment time to obtain a spherical carbide structure having good workability (ductility, hardness) during the cold working described above. That is, it is an object of the present invention to provide a method for manufacturing hot rolled steel having a structure that can significantly shorten the conventional total heat treatment time by 40% or more.

(Means for Solving the Problems) In order to achieve the above object, the present invention attempts to obtain in advance a structure that is likely to become spheroidized in the subsequent heat treatment process by adjusting the structure of the hot-rolled finish rolled material. , Specifically, after controlling the cooling rate during hot rolling and adjusting one structure, finish rolling (warm working or two-phase region working) is performed at a specific rolling degree and working temperature. This was made possible by

That is, one aspect of the present invention is to cool various types of steel to a temperature at which martensitic transformation is completed during hot rolling, and then rapidly raise the temperature after the transformation is completed.

The gist of this invention is a method for producing steel materials with excellent cold workability, which is characterized by performing processing of 10% or more and 70% or less in a temperature range of Ac 3 or lower.

The present invention will be explained in detail below based on examples.

In the conventional hot rolling method, a layered pearlite structure is formed in the rolled material after finish rolling due to transformation from the normal austenite state, which is difficult to form into spheres in the subsequent heat treatment process. In order to apply the above-mentioned heat treatment method to such a rolled material and obtain a spheroidized structure from this stable layered pearlite structure, one method is to perform the above-mentioned two-phase region heating.
In this case, it is necessary to slow the cooling process, and in any heat treatment method, the heat treatment time must be significantly increased.

Therefore, in the present invention, a pre-structure that facilitates the formation of a spheroidized structure in the subsequent heat treatment process is realized in the rolled finish. That is, it is necessary to prevent the formation of plate-like carbides found in the above-mentioned layered pearlite structure, and for this purpose, usually.

The pearlite transformation from austenite that occurs after finish rolling is prevented by rapidly cooling the steel material during the hot rolling process, thereby preventing the formation or precipitation of carbides, creating a structure in which C is supersaturated as a solid solution in the steel, and the subsequent elevating process is prevented. Warm and/or
After the formation and termination of fragmented carbides during processing to form a tempered martensitic structure, appropriate processing is further applied to fragment and refine the precipitated carbides.

In order to prevent pearlite transformation, rapid cooling is performed at a cooling rate of 150° C./sec or more during hot J-rolling. As a result, a martensitic structure is obtained, so the temperature is rapidly raised (e.g., 100~B), and after the temperature rise, machining with a working degree of 10~70% is performed.
By finishing the process in a temperature range of 3 points or less, the precipitated carbide is divided and refined.

A working degree of at least 10% or more is required to divide and refine the carbide, but a working degree of 70% or less is sufficient. Further, since such processing is carried out at low temperatures such as Ac3 points or lower, processing strain remains in the rolled material, and the presence of this strain energy has the effect of promoting spheroidization of carbides during subsequent heat treatment.

Note that the temperature at which the above-mentioned processing is performed is below the Ac3 point, but two modes are possible for this. Firstly, A c
This is a case where processing (warm processing) is performed at a low temperature of 1 point or less, preferably below the recrystallization temperature, and the second is 1 point or more of Ac,
This is a case where processing is performed in a two-finger temperature range of 3 Ac points or less.

Especially when processing in the latter two-finger temperature range, Acm
By rapidly increasing the temperature above the point, the martensitic structure that has completed its transformation undergoes a partial reverse transformation, thereby causing a small amount of γ (austenite) to appear, and by applying processing deformation to this austenite, it is possible to The pearlite transformation structure that occurs during cooling can be refined.

According to the above-mentioned hot working process, the processing time to obtain a steel material with a spheroidized structure after heat treatment for spheroidization of Arashi 3 or higher according to JIS G3539 and a hardness of Hv≦180 is longer than that of the conventional heat working process. The same method as before can be applied to the 6-spheroidizing annealing method, which can reduce the time to more than 1/3 of that for inter-rolled materials, but the processing time is particularly shortened when slow cooling is used. The effect of change is large.

It should be noted that the steel to which the method of the present invention is applied is not particularly limited and is not limited to carbon steel and chromium steel, and similar effects can be obtained on boron steel and molybdenum-containing rust.1 For example, 510C-355C (C: 0.1
0-0.58%), 5CR420-3CR440,SC
Examples include M420-5CM440.

(Example) Next, examples of the present invention will be shown.

Example 1 Carbon steel 845C for mechanical structures having the chemical composition (wt%) shown in Table 1 was heated to 950°C, and then continuously hot rolled to give 4
．． After rolling to 5-13mat, cooling rate>150℃/
It was cooled to 200°C using ssc. After that, continue to 650
The temperature was raised to ℃ at a heating rate of 200'C/win, and immediately after the temperature was raised, rolling was performed to 4oH1t.

Air cooled to room temperature.

As shown in Fig. 1, plate-shaped carbides, which are found in the layered pearlite structure that is most difficult to form into spheroids, are not formed.
Austenite becomes martensite by rapid cooling before finish rolling, and then becomes tempered martensite by increasing temperature.

This is because it has been processed. Therefore, as for the structure of the rolled material, fine precipitation of deformed ferrite and granular carbide is observed during processing, resulting in a structure that can promote the formation of a spheroidized structure. Incidentally, FIG. 1 shows the case of a rolled material which was processed by 50% at 650° C. after martensitic transformation.

After reheating this steel material to 740℃ using a heat treatment furnace,
The structure after heat treatment is shown in Fig. 3 and Fig. 4.The structure after heat treatment is shown in Fig. 3 and Fig. 4. , FIG. 3 shows the case where 10% processing was performed at 650° C. after martensitic transformation, and FIG. 4 shows the case where 50% processing was similarly performed. Further, the obtained spherical synthetic fibers 11aNQ and hardness values are shown in Table 2.

Furthermore, the relationships between the obtained spheroidized structure Nα, hardness, and degree of rolling are shown in FIGS. 5 and 6.

For comparison, the same material was heated to 950°C, then finished to a thickness of 4 mm at 800°C by continuous hot rolling, and then air cooled to room temperature. After reheating this steel material to 740°C using a heat treatment furnace, soaking it for 3 hours and cooling it to 680°C.
Figure 7 shows the structure after heat treatment, which was slowly cooled at 5''C/hr and then air cooled (see Figure 2(b)).

As is clear from the comparison between Figures 3 to 6 and Figure 7 as well as from Table 2, the material processed at low temperatures by the method of the present invention takes significantly longer time to spheroidize than conventionally rolled material. Despite being shortened.

It has the same degree of spheroidization and hardness as long-term processed materials.

Example 2 Case-hardened tlIISCR having the chemical components shown in Table 1
420 by continuous hot rolling after heating to 950°C.
After rolling to 5~13m+ut, cooling rate>150℃/
The mixture was cooled to 200° C. with a see. After that, continue to 650
℃ at a heating rate of 200° C./+++ in. Immediately after raising the temperature, rolling was performed to 4 mmt, and air cooling to room temperature.

As shown in Figure 8, the plate-like carbides found in the layered pearlite structure, which is the most difficult to form into spheroids, are not formed. This is because it became a website and was processed. Therefore, as for the structure of the rolled material, fine precipitation of deformed ferrite and granular carbide is observed during processing, resulting in a structure that can promote the formation of a spheroidized structure. Incidentally, FIG. 8 shows the case of a rolled material which was subjected to 50% processing at 650° C. after martensitic transformation.

After reheating this steel material to 770℃ using a heat treatment furnace,
The specimen was kept soaked for 0 ml, slowly cooled to 680°C at a cooling rate of 25°C/hr, and then cooled in air (see Figure 9(a)). The structure after heat treatment is shown in Figures 10 and 11. , 1st
Figure 0 shows the case where 10% processing was performed at 650''C after martensitic transformation, and Figure 11 shows the case where 50% processing was similarly performed.

Also, the obtained spherical synthetic fiber! The values of aNc and hardness are shown in Table 3, and the relationship between the obtained spheroidized structure Nα, hardness, and rolling degree is shown in FIGS. 12 and 13.

For comparison, the same material was heated to 950°C and then finished to 4 mat at 920°C by continuous hot rolling.

Air cooled to room temperature. This steel material is heated to 770% using a heat treatment furnace.
After reheating to ℃, it was kept in a homogenizer for 3 hours, slowly cooled to 680"C at a cooling rate of 12.5℃/hr, and then air-cooled (9th
(See figure (b)).

As is clear from Figures 10 to 13 and Table 3, although the material processed at low temperatures by the method of the present invention has a significantly shorter time required for spheroidization than ordinary rolled material, The degree of spheroidization is equivalent to that of long-time treated materials.
It has a hardness level.

011 Carbon steel for mechanical structures 34 having the chemical composition shown in Table 1
After heating 8C to 950℃, it is continuously hot rolled to 4.5
After rolling to ~13++vt, cooling rate>150℃/s
It was cooled down to 200"C by EC. Then, it was continued to 735
℃ at a heating rate of 200"C/ll1n, and immediately after the temperature was raised, it was rolled to a thickness of 4 mm and air cooled to room temperature.

As shown in FIG. 14, plate-like carbides, which are found in the layered pearlite structure that is most difficult to form into spherules, were not generated. This is because austenite becomes martensite due to rapid cooling before finish rolling, and then becomes tempered martensite when heated.

This is because it is processed. Therefore, in the structure of the rolled material, fine precipitation of deformed ferrite and granular carbide is observed during processing, resulting in a structure that can promote the formation of a spheroidized structure. Incidentally, FIG. 14 shows the case of a rolled material that was 50% processed at 735° C. after martensitic transformation.

After reheating this steel material to 740"C using a heat treatment furnace,
The material was soaked for 0 ml, then slowly cooled to 680"C at a cooling rate of 25°C/hr, and then air cooled (see Figure 2 (a)).
. The structure after heat treatment is shown in FIGS. 15 and 16.
FIG. 15 shows the case where 10% processing was performed at 735° C. after martensitic transformation, and FIG. 16 shows the case where 50% processing was similarly performed.

Table 4 shows the obtained spheroidized structure Nα and hardness values. Furthermore, the relationship between the obtained spheroidized structure Na, hardness, and rolling degree is shown in FIGS. 17 and 18.

For comparison, the same material was heated to 950'C, finished to 4 mmt by continuous hot rolling at 800C, and then air cooled to room temperature. After reheating this steel material to 740°C using a heat treatment furnace, it was kept soaked for 3 hours, and the cooling rate was 12°C until it reached 680'C.
．． It was slowly cooled at a rate of 5° C./hr and then air-cooled (see FIG. 2(b)). The structure after heat treatment is shown in FIG.

As is clear from the comparison between Figures 15 to 18 and Figure 19 as well as from Table 4, the material processed at low temperatures by the method of the present invention takes significantly longer time to spheroidize than the conventionally rolled material. Despite being shortened, it has the same degree of spheroidization and hardness as the long-term treated material.

Carbon for mechanical structures having the chemical components shown in Table 1
After heating s48G to 950℃, continuous hot rolling
After rolling to 1.5-13mmt, cooling rate>150℃
/sec to 200°C. After that, 76
The temperature was raised to 0°C at a heating rate of 200''C/win, and immediately after the temperature was raised, it was rolled to 411IIIt, and then air cooled to room temperature.

As shown in FIG. 20, plate-like carbides, which are found in the layered pearlite structure that is most difficult to form into spherules, were not generated. This is because austenite becomes martensite due to rapid cooling before finish rolling, and then becomes tempered martensite when heated.

This is because it is processed. Therefore, as for the structure of the rolled material, fine precipitation of deformed ferrite and granular carbide is observed during processing, resulting in a structure that can promote the formation of a spheroidized structure. Note that FIG. 20 shows the case of a rolled material that was 50% processed at 760'C after martensitic transformation.

After reheating this steel material to 740℃ using a heat treatment furnace,
The structure after heat treatment is shown in Figs. 21 and 22.The microstructure after heat treatment is shown in Fig. 21 and Fig. 22. , 21st
The figure shows the case of a rolled material that has been processed by 10% at 760° C. after martensitic transformation, and FIG. 22 shows the case of a rolled material that has been similarly processed by 50%. Furthermore, Table 5 shows the obtained spheroidized structure +tα and hardness values.

For comparison, the same material was heated to 950°C, finished to a thickness of 4 mm at 800°C by continuous hot rolling, and then air cooled to room temperature. After reheating this steel material to 740°C using a heat treatment furnace, it was soaked for 3 hours and cooled at a cooling rate of 12.0°C to 680'C.
It was slowly cooled at a rate of 5° C./hr and then air-cooled (see FIG. 2(b)). The structure after heat treatment is shown in FIG.

As is clear from the comparison between Figures 21 and 22 and Figure 19, and from Table 5, the material processed at low temperatures by the method of the present invention takes significantly longer time to spheroidize than the normally rolled material. Despite being shortened, it has a spheroidized structure equivalent to that of long-term treated materials.

It has a hardness level.

Shogo Yudan Case hardened steel 5CR420 with the chemical composition shown in Table 1
After heating to 950℃, continuous hot rolling gives 4.5 to 1
After rolling to 3 mmt, it was cooled to 220°C at a cooling rate>150°C/sec. after that.

Subsequently, the temperature was raised to 740°C at a heating rate of 100°C/+nin, and immediately after the temperature was raised, it was rolled to a thickness of 4 mm and air cooled to room temperature.

As shown in FIG. 23, the plate-like carbide of the layered pearlite structure, which is the most difficult to form into spheroids, is fragmented and deformed by processing, resulting in a structure that can promote the formation of a spheroidized structure. Note that FIG. 23 shows the case of a rolled material that was processed by 33% at 740° C. after martensitic transformation.

After reheating this steel material to 770℃ using a heat treatment furnace,
The structure after heat treatment is shown in FIGS. 24 and 25. , 24th
The figure shows the case of a rolled material which was processed by 10% at 740° C. after martensitic transformation, and FIG. 25 shows the case where the material was similarly processed by 50%. Also, the obtained spherical credit union lQi
The values of Nc and hardness are shown in Table 6, and the relationships between the obtained spheroidized structure No., hardness, and rolling degree are shown in Table 26.
27.

For comparison, the same material was heated to 950°C, then finished to 4 mat at 920°C by continuous hot rolling, and then air cooled to room temperature. This steel material was reheated to 770℃ using a heat treatment furnace, and after soaking for 3 hours, the cooling rate was 12.5℃ to 680℃.
It was slowly cooled at °C/hr and then air-cooled (see FIG. 9(b)).

As is clear from Figures 24 to 27 and Table 6, although the material processed at low temperatures by the method of the present invention has a significantly shorter time required for spheroidization than ordinary rolled material, The degree of spheroidization is equivalent to that of long-time treated materials.
It has a hardness level.

Example 6 Case hardened 5CR420 having the chemical components shown in Table 1
After heating to 950'C, it is continuously hot rolled to 4.5 ~
After rolling to 13mm+t, cooling rate>150℃/se
The mixture was cooled to 200°C at c. After that, the temperature was continued to be raised to 820°C at a heating rate of 100°C/win, and immediately after the temperature was raised, the temperature was increased to 820°C.
Rolling was performed on a mat and air-cooled to room temperature.

As shown in FIG. 28, the plate-like carbide of the layered pearlite structure, which is the most difficult to form into spheroids, is divided and deformed by processing, resulting in a structure that can promote the formation of a spheroidized structure.

After reheating this steel material to 770℃ using a heat treatment furnace,
The microstructure after heat treatment is shown in FIGS. 29 and 30. , second
Figure 9 shows the case of a rolled material that has been subjected to 10% processing at 820° C. after martensitic transformation, and Fig. 30 shows the case of a rolled material that has been similarly processed to 50%. Table 7 shows the obtained spheroidized structure Nα and hardness values. Furthermore, the relationships between the obtained spherical strands gi Ha, hardness, and rolling degree are also shown in FIGS. 26 and 27.

As is clear from Figures 26 to 30 and Table 7, although the material processed at low temperatures by the method of the present invention has a significantly shorter time required for spheroidizing treatment than conventionally rolled material. The degree of spheroidization is equivalent to that of long-time treated materials.
It has a hardness level.

(Blank below) (Effects of the Invention) As detailed above, according to the present invention, after rapidly cooling from the austenitic state during hot rolling and completing martensitic transformation, warm rolling or two-phase region Because rolling is performed, the structure of the finish-rolled material is a structure in which granular carbide is finely dispersed, and there is residual processing strain in the rolled material, whereas conventionally rolled material exhibits a layered carbide structure. In comparison, the subsequent spheroidizing heat treatment time can be significantly shortened.

[Brief explanation of the drawing]

Figure 1 shows the 650°C after martensitic transformation in Example 1.
This is a scanning electron micrograph showing the structure of a rolled material processed by 50% at ℃. Figure 2 is a diagram showing the spheroidizing heat treatment conditions for 545C and 548C materials, where (a) is for the inventive material, (b) is for the conventional material, and Figures 3 and 4 are for the example. 3 is a microscopic photo showing the structure of the rolled material processed at 650°C after martensitic transformation in 1 and subjected to spheroidization heat treatment.
Figure 4 shows the case of 10% processing, Figure 4 shows the case of 50% processing, and Figures 5 and 6 are diagrams showing the relationship between spheroidization Nα, hardness, and rolling rate of the spheroidization heat-treated material in Example 1. FIG. 7 is a micrograph showing the structure of a spheroidized heat-treated material made from a conventionally rolled material. Figure 8 shows the 650°C after martensitic transformation in Example 2.
FIG. 9 is a scanning electron micrograph showing the structure of a rolled material processed by 50% at ℃, and FIG. 9 is a diagram showing the spheroidization heat treatment conditions of 5CR420 material.
b) is the case of the conventional material, and Figures 10 and 11 are micrographs showing the structure when the rolled material was processed at 650°C after martensitic transformation in Example 2 and subjected to spheroidization heat treatment. , 1st
Figure 0 is for 10% processing. Figure 11 shows the case of 50% machining. 12 and 13 are diagrams showing the relationship between spheroidization, hardness, and rolling rate of the spheroidized heat-treated material in Example 2, and FIG.
15 and 16 are scanning electron micrographs showing the structure of a rolled material processed at 50% at 5°C, and FIGS. 15 and 16 are of the rolled material processed at 735°C after martensitic transformation in Example 3. These are micrographs showing the structure when subjected to spheroidization heat treatment. Figure 15 is for 10% processing, Figure 16 is for 50% processing, and Figures 17 and 18 are for the spherical structure in Example 3. FIG. 19 is a diagram showing the relationship between the spheroidization number and hardness of the heat-treated material and the rolling rate; FIG. 19 is a micrograph showing the structure of the spheroidization heat-treated material obtained from a normally rolled material; FIG. After martensitic transformation at 76
21 and 22 are scanning electron micrographs showing the structure of a rolled material processed at 50% at 0°C, and FIGS. 21 and 22 are of the rolled material processed at 760°C after martensitic transformation in Example 4. These are micrographs showing the structure when subjected to spheroidization heat treatment. Fig. 21 shows the case of 10% processing, Fig. 22 shows the case of 50% processing, and Fig. 23 shows the structure of the microstructure after martensitic transformation in Example 5.
24 and 25 are scanning electron micrographs showing the structure of a rolled material processed at 33% at 0°C, and FIGS. 24 and 25 are of the rolled material processed at 740°C after martensitic transformation in Example 5. FIG. 24 is a micrograph showing the structure when subjected to spheroidization heat treatment, and Fig. 24 shows the case of 10% processing. FIG. 25 shows the case of 50% processing, and FIGS. 26 and 27 are diagrams showing the relationship between the spheroidization Nα and hardness of the spheroidization heat-treated materials in Example 5 and Example 6, and the rolling ratio. Figure 28 shows 82 after martensitic transformation in Example 6.
This is a scanning electron micrograph showing the structure of a rolled material processed by 50% at 0°C. FIGS. 29 and 30 are micrographs of the structure of the rolled material processed at 820°C after martensitic transformation in Example 6, and subjected to spheroidization heat treatment. FIG. 30 shows the case of 50% processing. Patent applicant Hisashi Nakamura, Patent attorney representing Kobe Steel, Ltd. Figure 3 Figure 4 Figure 5 1,000 (Changing Figure 13 n Figure 14 Figure 15 Figure 116 Figure 17 Figure 18 Figure 19 Figure 26 Figure λ Figure 27

Claims (3)

[Claims] (1) For various steels, in the middle of hot rolling, after cooling to the temperature at which martensitic transformation is completed and the transformation is completed, the temperature is rapidly raised to 10% or more in the temperature range of Ac_3 points or less. A method for manufacturing a steel material with excellent cold workability, characterized by performing processing of 70% or less. (2) The method according to claim 1, wherein the temperature range is below Ac_1 point. (3) The method according to claim 1, wherein the temperature range is a two-phase temperature range of not less than Ac_1 point and not more than Ac_3 point.