JPH01259123A - Manufacture of steel bar and wire rod by continuous hot rolling - Google Patents

Abstract

PURPOSE:To form a directly spheroidized structure by controlling the continuous hot rolling stage of a steel bar of carbon steel containing specific amounts of C and temp. treatment directly after the above stage, respectively, so as to obviate the necessity of subsequent spheroidizing annealing treatment. CONSTITUTION:In the course of the continuous hot rolling stage of a steel bar or wire rod of carbon steel or alloy steel containing 0.03-1.5% C, the surface layer part of the rolled stock is subjected to forcedly cooling down to the Ms point or below, by which a hardened structure of martensite, etc., is formed. Subsequently, cooling conditions after the above forcedly cooling until the rolled stock reaches the next rolling mill are controlled and the temp. in the central part of the rolled stock in a recuperating stage is regulated to a temp. in the region of Ar1 point-Ae1 point and, then, finish rolling is applied to the above rolled stock, by which prestructure easy of spheroidizing in all the cross sections of the rolled stock can be formed. Further, the rolled stock is heated up to a temp. in the region of Ac1 point-Ac3 point directly after the finish rolling, cooled down to a temp. in the region between the Ac1 point and 600 deg.C, and held at constant temp. in the above temp. region for 20min-5h, by which the directly spheroidized structure can be obtained. By this method, the necessity of spheroidizing annealing treatment can be obviated and, as a result, the process can be simplified and energy can be economized.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to the production of steel bars and wire rods by continuous hot rolling, and in particular, the present invention relates to the production of steel bars and wire rods by continuous hot rolling, and in particular, by using the continuous hot rolling process and temperature control immediately thereafter, it is possible to directly produce steel bars and wire rods by omitting the subsequent spheroidizing annealing process. This invention relates to a method for obtaining a spheroidized tissue. (Conventional technology) For high carbon steels such as cold forging steels, bearing steels, and tool steels, prior to cold working, it is necessary to improve workability by imparting ductility or reducing hardness. For this purpose, a spheroidizing annealing process is often performed. Conventionally, methods for this spheroidizing annealing include a long-time heating method in which heat is maintained at a temperature just below the -■A1 point for an appropriate period of time, and then cooled;
There are several methods, such as a slow cooling method in which the material is heated to a two-phase region between point A1 and point A3 and then gradually cooled to complete the transformation, and a repeated method in which the material is repeatedly heated and cooled to a temperature just above and below A1. be. This type of spheroidizing annealing treatment is normally carried out by hot rolling to form a steel bar, wire rod, etc., and then reheating it to a predetermined temperature in a heat treatment furnace on a separate line. generally requires an extremely long treatment time of about 10 to 15 hours, resulting in low productivity and high heat treatment costs.Also, the above heat treatment method cannot be said to be preferable from the viewpoint of energy saving and the like. Therefore, as a method to improve these, there is a method of adjusting the pre-structure (structure after rolling) to facilitate spheroidization, or a method of omitting the spheroidization annealing process itself which is carried out after hot rolling. is being attempted. The former method is, for example, a method in which after hot rolling, the product is rapidly cooled (water-cooled) in a product cooling zone to form a quenched structure like martensite or an intermediate structure. It is not easy to obtain a microstructure or an intermediate structure even if a large amount of cooling water and/or a long cooling time are used, and layered pearlite is usually formed in the center of the rolled material. Therefore, there is a problem in that the desired pre-structure can be adjusted only in the surface portion of the rolled material where the above-mentioned quenched structure or intermediate structure can be obtained relatively easily. In addition, as a method that can omit the latter spheroidizing annealing process itself, that is, a method of directly obtaining a spheroidized structure by controlling the temperature during and after hot rolling, for example, JP-A No. 5, No. ; l-136421, 59-1302
There is a method shown in No. 4 etc. However, with either method, it is impossible to control the temperature over the entire cross section of the rolled material during hot rolling, and it is impossible to obtain a spheroidized structure over the entire cross section of the rolled material in the hot rolled finish. The problem is that there is no. In order to solve such problems, the present applicant first carried out continuous hot rolling or by temperature control immediately thereafter,
The subsequent spheroidizing annealing process makes it easier to obtain a spheroidized structure,
Alternatively, a method of omitting the spheroidizing annealing treatment and directly obtaining a spheroidized structure was proposed (Japanese Patent Applications No. 61-294309, No. 61-294310, No. 62-221129). In other words, one is: (1) In the hot rolling process, especially in the intermediate cooling zone before finish rolling, the surface layer of the rolled material is strongly cooled to below the Ms point with a coolant such as water, and then transferred to the next rolling mill (finishing mill). By adjusting the reheating time after strong cooling, the center of the rolled material is lowered to a temperature below the Ar1 point during the reheating process and transformed to Barley 1. By completing the transformation of the untransformed structure in the cross section of the rolled material by the start of finish rolling, and dividing (or promoting the division) the transformed structure, especially the plate-like or rod-like carbides of the layered pearlite structure, by the finish rolling process. This is a method of realizing a pre-structure that facilitates the formation of a spheroidized structure in the subsequent heat treatment process (see FIG. 1). Alternatively, (1) In order to reduce operational problems such as rolling load and rolling adjustment during finish rolling, the temperature at the center of the rolled material at the end of the reheating described above is lowered to a temperature range of Ar1 to Ae1, Finish rolling is started from such a temperature range, and plate-like or rod-like carbides in the layered pearlite structure produced by the deformation-induced transformation caused by rolling are split (or splitting is promoted) during the finish rolling process. This is a method (see FIG. 2) of realizing a prestructure that facilitates the formation of a spheroidized structure in the heat treatment process. Furthermore, (1) In both of the above (2) and (2), it is a method of directly realizing a spheroidized structure by controlling the temperature (slow cooling or constant temperature maintenance) immediately after rolling. Among these temperature controls, the slow cooling method is
A rolled material having a pre-structure that is easily spheroidized in the entire cross section obtained in the hot rolling process is heated to a temperature range of Acm point to Ac3 point immediately after finish rolling, and then a cooling rate of 50 ° C / hr or less is applied. This is a method to obtain a spheroidized tissue by slow cooling at
0 in the temperature range below the Acm point and up to 600℃ immediately after finish rolling
This is a method of obtaining a spheroidized tissue by maintaining the temperature at a constant temperature of 5 to 5 hours. (Problems to be Solved by the Invention) Among the above methods proposed by the present applicant, a method in which temperature control is performed by constant temperature maintenance immediately after hot rolling, that is, immediately after finishing rolling, Ac, up to 600 °C at will. 0 in the temperature range.
In the case of a method in which the temperature is maintained for 5 to 5 hours, a spheroidized structure can be directly obtained by coagulating and growing carbides during the constant temperature maintenance. However, the temperature at the center of the rolled material before finish rolling is Ar,
In the case of Ae1 point (that is, when strain-induced transformation occurs during finish rolling), if the processing (finish rolling) start temperature is low, the layered pearlite produced is relatively fine, and therefore, this layered pearlite is formed during the finish rolling process. The carbides that are generated by splitting or promoting splitting from the plate-shaped or rod-shaped carbides in the structure are also relatively fine, so when these structures are kept at constant temperature using the temperature control method described above, the hardness after being kept at constant temperature is lower than that of the conventional method. There was a problem that the temperature remained at a higher level than that of offline spheroidized annealed materials. Among the methods previously proposed by the applicant, the present invention is a method in which the temperature is controlled by constant temperature maintenance immediately after hot rolling. This method was developed to solve the problems of the method, and by using a continuous hot rolling process and temperature control immediately after that, a directly spheroidized structure with the same level of hardness as the conventional offline spheroidized annealed material was created. The purpose is to provide a method for obtaining (Means for Solving the Problems) In order to achieve the above object, the present inventors have developed a method to solve the problem, especially when the temperature of the center of the rolled material before the start of finish rolling is between Ar□ point and Ae□ point (deformation-induced We are conducting intensive research on a method to obtain the desired spheroidal structure and the same level of hardness as conventional offline spheroidized annealed materials even when the processing temperature in finish rolling is low. layered. As a result, by regulating the heating conditions and constant temperature holding conditions immediately after finish rolling and before constant temperature holding = 7-, if the residual carbides are reagglomerated and grown as nuclei,
It has been found that the degree of refinement of the spheroidal structure is relaxed, and the hardness is comparable to that of conventional offline spheroidized annealed materials. That is, in the method for producing steel bars and wire rods by hot continuous rolling according to the present invention, during the hot continuous rolling process of carbon steel or alloy steel bars or wire rods containing 0.03 to 1.5% C,
By strongly cooling the surface layer of the rolled material to below the Ms point and adjusting the cooling conditions after the hard cooling until the rolled material reaches the next rolling mill, the central part of the rolled material is heated to Ar□ during the reheating process. point~
Lower the temperature to a temperature range between Ae After obtaining a pre-structure that is easy to spheroidize, and further heating to a temperature range of AcI point to Ac3 point immediately after finishing rolling, it is cooled to a temperature range of up to 600°C, and then heated to a temperature of 20°C in this temperature range.
It is characterized in that a spheroidized structure is directly obtained by maintaining the temperature at a constant temperature in the range of m''n to 5 hours. The present invention will be explained in more detail below. First, in the present invention, the hot-rolled material to be subjected to temperature control immediately after hot rolling is a rolled material having a pre-structure that is easily spheroidized and obtained by the method described in (1) or (2) below, which was previously proposed by the applicant. It is. ■During the hot continuous rolling process of carbon steel or alloy steel bars or wire rods containing 0.03 to 1.5% C, the surface layer of the rolled material is strongly cooled to below the Ms point, and the rolled material is then rolled. By adjusting the cooling conditions after strong cooling before reaching the mill, the center of the rolled material is lowered to a temperature range between Ar1 point and Ae□ point in the reheating process, and then finish rolling is performed to reduce the rolling A method to obtain a pre-structure that facilitates spheroidization within the entire cross section of the material (see Figure 2). ■During the hot continuous rolling process of carbon steel or alloy steel bars or wire rods containing 0.03 to 1.5% C, the surface layer of the rolled material is strongly cooled to below the Ms point, and the rolled material is rolled next time. By adjusting the cooling conditions after strong cooling before reaching the mill, the temperature of the center of the rolled material is lowered to below the Ar1 point in the reheating process, transforming it into barley 1, and then finishing rolling is performed. A method for obtaining a pre-structure that facilitates spheroidization within the entire cross section of a rolled material (see Figure 1). The present inventor conducted basic experiments to investigate the relationship between the hardness and the structure obtained by changing the constant temperature holding conditions on a rolled material having a pre-structure that easily becomes spheroidized after hot rolling. Figure 3 shows an example of the results of the basic experiment. In this experiment, 348C was used as the test steel, and the processing conditions (temperature pattern and processing rate) were such that during rolling from a 155mm billet to a 30mmφ steel bar, the surface of the rolled material was kept below the Ms point in the intermediate cooling zone before finish rolling. It was determined by calculating the temperature history of the recuperation process after strong cooling and the subsequent finish rolling process through temperature analysis of the rolling process using an empirical formula for the cooling zone cooling capacity, which was obtained in detail on the actual machine. The total processing rate is approximately 50%, and the processing start temperature (finish rolling start temperature) is 640"C (temperature between Ar1 and Ae□ points)
This is the case where the temperature was raised to 740°C just above the Ac point after finishing the processing, and then cooled at a rate of 1200C/hr to the holding temperature of 680°C. In addition, strictly speaking, Ar1 point is the steel type,
Although it varies depending on the cooling rate etc., here Ar1 = 550
It was considered as ℃. Moreover, the Ae1 point is 2720°C. As is clear from FIG. 3, when the holding time exceeds approximately 20 minutes, the hardness decreases significantly. This is because some of the carbides solid-solved in the austenite precipitate, aggregate, and grow with the remaining carbides as nuclei during the constant temperature holding process, resulting in a spheroidized structure. On the other hand, if the holding time is less than approximately 20 minutes, carbide precipitation, aggregation, and growth will not occur sufficiently, and the structure will not reach a spheroidal structure but will remain in a granular structure, resulting in a relatively high level of hardness. It has become. Therefore, a good spheroidized structure cannot be obtained by maintaining the temperature for such a short time. The above results show that the processing start temperature is 640℃ (Ar1 point ~ A
(e□ temperature), but if the processing start temperature is lower in the temperature range from Ar1 point to Aei point (e.g. 600
℃), it was confirmed that the desired hardness and structure could be obtained by holding the temperature at a constant temperature of about 20 minutes or more. In this respect, the conventional temperature control method does not raise the temperature to the temperature range of Acm point to Ac3 point after finishing processing (finish rolling).
In the method of holding the temperature just below the A01 point, the temperature at the center of the rolled material at the start of processing (finish rolling) is between the Ar1 point and the Ae1 point.
In the case where deformation-induced transformation occurs during finish rolling), when the deformation start temperature is low (eg, 600° C.), the hardness after constant temperature maintenance is at a higher level than that of conventional offline spheroidized annealed materials. Therefore, the present invention was completed based on the results of the above-mentioned various basic experiments and further detailed study of temperature control conditions through constant temperature maintenance. Next, the reason for limiting the conditions will be explained along with an example of a continuous hot rolling line in the present invention. Figure 4 shows a continuous hot rolling line, with 20
is a heating furnace that heats billets, etc., and in the latter stage is an Nα1
Rough rolling mill row 21 consisting of rolling mills of ~Nα8, Nα9~N
Intermediate rolling mill row 22 consisting of α12 rolling mills and No.
A finishing rolling mill row 23 consisting of rolling mills No. 13 to No. 16 is arranged in series. 24 is an intermediate cooling zone provided between the intermediate rolling mill row 22 and the finishing rolling mill row 23;
Reference numeral 5 denotes a rear cooling zone provided between the finishing rolling mill row and the cooling bed 26, and the flow rate of cooling water supplied to the rolled material in these cooling zones 24 and 25 is controlled by a control device 29 including a process control computer. It is now possible to do so. 30, 31, and 32 are thermometers that measure the surface temperature of the rolled material, and 27 is a heat treatment furnace. In the above rolling line, billets and the like extracted from the heating furnace 20 are sequentially rolled in a rough rolling mill row 21 and an intermediate rolling mill row 22, and the rolled material is passed through an intermediate cooling zone 24 to
A hardened structure such as martensite is used below the s point.1.
In the reheating process of the surface layer between the intermediate cooling zone 24 and the finishing rolling mill row 23, the temperature at the center of the rolled material is in the temperature range from Ar1 point to 80 points or below Ar1 point. Finish rolling is started by controlling the cooling conditions (cooling water flow rate and reheating time). If finish rolling is started in such a temperature range, a pre-structure that is easily spheroidized within the entire cross section of the rolled material can be obtained. That is,
When the temperature at the center of the rolled material is in the temperature range from Ar1 point to Ae1 point, deformation-induced transformation occurs due to rolling and a layered pearlite structure is generated (when the temperature at the center of the rolled material is below Ar1 point, barley 1 - At the same time, the plate-shaped or rod-shaped carbides in this structure are divided (or their division is promoted), so that a pre-structure that can easily be spheroidized is obtained. The reason why the finish rolling start temperature is set in the temperature range from Ar1 point to Ae□ point is that finish rolling can be performed from a higher temperature range, which improves operability by reducing the rolling load during finish rolling and the difficulty of rolling adjustment. This is because it improves and has a great practical effect. In addition, Ms.
The hardened structure obtained by hard cooling to below the point becomes a pre-structure in which it is easier to form spheroidization due to the increase in strain energy due to finish rolling, and the area other than the center and surface layer of the rolled material has an intermediate structure. Since it has a mixed structure including these, it also becomes a pre-structure that can be easily spheroidized by finish rolling. After this, that is, immediately after finishing rolling, A
After heating to a temperature range of cm point to Ac3 point, Ac1 point to Ac6 point
The temperature is maintained at a constant temperature in the range of 00° C., and during this constant temperature maintenance, some of the carbides solid-solved in the austenite are precipitated, aggregated, and grown using the remaining carbides as nuclei, thereby obtaining a spheroidized structure. The reason why the lower limit of constant temperature holding temperature is set at 600℃ is because the aggregation of carbides is delayed when constant temperature is kept below this temperature range, so the time required to grow these into a spheroidal structure is extremely long. This is because it becomes impractical. The reason why the upper limit is set to Ac1 point is that when this temperature is exceeded, carbides begin to form a solid solution in austenite, and layered pearlite is generated in the subsequent cooling process, making it impossible to obtain a good spheroidized structure. The reason why the lower limit of constant temperature holding time is set to 20 m1n is that if the time is shorter than this, the carbide will re-agglomerate using the remaining carbide as a nucleus.
In addition to insufficient growth, processing (
This is because the desired hardness and spheroidized structure cannot be obtained even when the starting temperature (for finish rolling) is lower (eg, 600° C.). The reason why the upper limit is set at 5 hr is that even if the holding time is longer than this, it will not have much effect on the increase in the spheroidizing effect, and since it is an online process, if the holding time is longer, a large-scale heat treatment will be required, which will improve productivity. The reason for this is that the temperature decreases, making it impractical. The cooling rate after the end of constant temperature maintenance does not need to be specially adjusted as in slow cooling, and air cooling may be used.

[Left below]

(Example) Next, an example of the present invention will be shown. Example 1 Steel type 548C (Ae
After heating a 155mm-mouth billet (1z720°C) to 9°0°C, it is continuously hot rolled, and in the intermediate cooling zone 24 before the start of finish rolling, the surface layer of the rolled material (45mmφ) is heated to below the Ms point (=350°C). After water cooling, during the reheating process of the surface layer of the rolled material for about 12 seconds up to the Nα13 rolling mill of the finishing mill row 23, the temperature at the center of the rolled material ranged from point Ar□ to point A.
Forced water cooling was performed for 4 to 5 see at a cooling water flow rate of 100 m3/hr to bring the temperature into the e1 point temperature range (cooling rate at the outermost surface: approximately 300°C/5eC). At this time, the center temperature of the rolled material at the start of finish rolling is approximately 650°C, the surface temperature is approximately 520°C, and the average temperature within one cross section is approximately 560°C. Subsequently, finish rolling was performed to produce a 30 mmφ steel bar. In this case, the untransformed structure (suitably cooled austenite structure) represented in the center of the rolled material is transformed during the finish rolling process,
The plate-like or rod-like carbides of the produced layered pearlite are divided by finish rolling and exhibit a granular structure, and the quenched structure in the surface layer of the rolled material easily forms a granular structure due to the increase in strain energy caused by rolling. It has become an organization. Immediately after this, the rolled material (30mmφ steel bar) was transferred to the heat treatment furnace 27.
The temperature range from this temperature to 680°C is approximately 150°C.
After cooling at a cooling rate of /hr, at this temperature about 40 m1
n and then air-cooled. The obtained tissues are shown in FIGS. 5 and 6. A spheroidized tissue of approximately Nα2 is obtained in the surface layer, and a structure of Nα2 is obtained in the center.
A spherical structure of approximately α3 was obtained. In this way, by appropriate temperature control after hot rolling, a directly spheroidized structure with a structure Nα3 or more can be obtained over the entire cross section of the rolled material, and
The hardness was also Hv180 or less in the entire cross section, and the same level of value as the conventional offline spheroidized annealed material was obtained by online processing. Example 2 As in Example 1, steel type S Cr420 (Ae,
After heating a 155 mm billet at 725° C.) to 930° C., it was continuously hot rolled, and the temperature of the surface layer of the rolled material (45 mmφ) was reduced to Ms points 1 and 5 in the intermediate cooling zone 24 before finish rolling.
380°C) or less, and after water cooling, the temperature at the center of the rolled material falls within the temperature range of the Ar□ point to the Ae□ point during the approximately 10 sec reheating process of the surface layer of the rolled material until it reaches the finish rolling mill Example 23. , 5,5 s at a cooling water flow rate of 90 m3/hr
Forced water cooling during ee (cooling rate of outermost surface: approx. 250°C)
ec). At this time, the center temperature of the rolled material at the start of finish rolling is about 660°C, the surface temperature is about 510°C, and the cross-sectional average temperature is about 560°C. Subsequently, it was finished into a 30mmφ steel bar. In this case as well, the untransformed structure (suitably cooled austenite structure) represented in the center of the rolled material is transformed during the second rolling process, and the plate-like or rod-like carbides of the formed layered pearlite structure take on a granular structure during finish rolling. In addition, the hardened structure in the surface layer of the rolled material has a structure in which it is easy to form a spheroidized structure due to an increase in strain energy caused by rolling. Immediately after this, the rolled material (30mmφ steel bar) was transferred to the heat treatment furnace 27.
After heating to 770°C above Ac1 point (725°C),
The temperature range up to 680° C. was slowly cooled at a cooling rate of approximately 150° C./hr, and approximately 40 ml was maintained at this temperature, followed by air cooling. The obtained tissues are shown in FIGS. 7 and 8. in this way,
Even in low-alloy steel, by proper temperature control after hot rolling, a spheroidized structure approximately equivalent to structure No. 2 can be obtained in the entire cross section, and the hardness is approximately Hv140 in the entire cross section, which is superior to conventional offline spheroidized annealed materials. The same level of value was obtained by online processing. Comparative Example 1 In the hot continuous rolling process of steel type 548G into a 30 mmφ steel bar with a 155 mm diameter similar to that in Example 1, the temperature range up to 680 °C was heated to 740°C immediately after finish rolling, and the temperature range up to 680°C was
Cooled at a cooling rate of 50℃/hr, and at this temperature 15ml
The temperature was maintained at constant temperature, and as in Example 1, the mixture was air-cooled to room temperature. In this case, the structure contained granular carbide and layered barley 1-, and a good spheroidizing machine could not be obtained. Comparative Example 2 155 mm of steel type 5Cr420 similar to Example 2
In the continuous hot rolling process of the billet into a 30 mmφ steel bar, it was heated to 770 ° C. immediately after finish rolling, cooled at a cooling rate of 150 ° C. / hr in the temperature range up to 680 ° C., and kept constant at this temperature for 15 m1n. As in Example 2, it was air cooled to room temperature. In this case, the structure contained granular carbide and layered pearlite, and a good spheroidized structure could not be obtained. Example 3 In the same manner as in Example 1, a 155 mm billet of steel type 548C was heated to 900°C, then continuously hot rolled, and rolled material (45
After water cooling, the temperature at the center of the rolled material increases during the approximately 12 sec reheating process of the surface layer of the rolled material until it reaches the Nα13 rolling mill of the finishing mill row 23. 4 to 5 sec at a cooling water flow rate of 20011Ia/hr to enter the temperature range from Ar□ point to Ae1
(Cooling rate of the outermost part: approx. 4.0°C)
/5ee). At this time, the center temperature of the rolled material at the start of finish rolling is about 600°C, the surface temperature is about 480°C, and the cross-sectional circle average temperature is about 510°C. Subsequently, finish rolling was performed to obtain a 30 mmφ steel bar. Immediately after this, the rolled material (30mmφ steel bar) was transferred to the heat treatment furnace 27.
heated to 740 °C above Ac1 point (z720 °C),
The temperature range from this temperature to 680℃ is approximately 150℃/
After cooling at a cooling rate of hr, at this temperature about 50ml1n
It was held and then air cooled. The obtained structure is shown in FIGS. 9 and 10. A spheroidized tissue with a texture of about Nα2 was obtained in the surface layer, and a spheroidized tissue with a texture of about Nα3 was obtained in the center. In this way, by appropriate temperature control after hot rolling, the temperature at the center of the rolled material before the start of finish rolling is approximately 600°C and Ar□
Even in the case of a low temperature range from point to Ae
The same level of value was obtained online as that of the conventional offline spheroidized annealed material. Example 4 As in Example 2, steel type S Cr420 (Ac□
After heating a 155 mm billet at 725° C.) to 930° C., it is continuously hot rolled, and the temperature of the surface layer of the rolled material (4.5 mmφ) is reduced to the point Ms (
z380℃) or less, and after water cooling, the rolling material center temperature is within the temperature range of Ar□ point to Ae1 point during the approximately 10 sec reheating process of the surface layer of the rolled material until it reaches the finish rolling mill row 23. /hr cooling water flow rate 5,5
forced water cooling between
°C/5ec). At this time, the center temperature of the rolled material at the start of finish rolling is about 610°C, the surface temperature is about 470°C, and the cross-sectional circle average temperature is about 520°C. Subsequently, it was finished into a 30mmφ steel bar. Immediately after this, the rolled material (30mmφ steel bar) was transferred to the heat treatment furnace 27.
After heating to 770°C, which is above the Ac1 point (z725°C), it was slowly cooled in the temperature range up to 680°C at a cooling rate of about 150°C/hr, and then air-cooled. The obtained structure is shown in FIGS. 11 and 12. A spheroidized structure of about Nn 2 was obtained in both the surface layer and the center. In this way, even with low-alloy steel, by proper temperature control after hot rolling, even when the center temperature of the rolled material is about 610°C before the start of finish rolling, which is a low temperature reduction in the range of Ar1 point to Ae□ point, A spheroidized structure with a structure of about Nα2 was obtained in the entire cross section of the rolled material, and the hardness was about Hv140 in the entire cross section, which was the same value as that of the conventional offline spheroidized annealed material. In the hot continuous rolling process of a 155 mm diameter billet of steel type 548G into a 30 mm diameter steel bar as in the case of Example 3, immediately after finishing rolling, the temperature at Ac1 point was 680 °C below 720 °C.
After maintaining the temperature at a constant temperature of about 15 ml, it was air-cooled to room temperature in the same manner as in Example 3. In this case, the hardness remained at about Hv195, which is higher than the hardness level of the conventional off-line spheroidized annealed material (Hv180 or less), and the structure had a five-grain structure. 155 m of steel type S Cr420 similar to that of Example 4
In the continuous hot rolling process of m-neck billets into 30mmφ steel bars, immediately after finishing rolling, the temperature is reduced to 680°C below the Ac1 point for about 5 seconds.
After maintaining the temperature at a constant temperature of 0 ml, it was air-cooled to room temperature in the same manner as in Example 4. In this case, the hardness is about Hv155, which is the level of hardness of the conventional off-line spheroidizing annealed material (Hv1
40 or less), and the structure exhibited a five-grain structure. The chemical composition of the test steel used on each side is shown in Table 1. Table 1 Chemical composition (wt%) (Continued) Although each side shows the case of a steel bar, the present invention can be implemented in exactly the same way for a wire rod that is wound into a coil after finishing rolling. be. Moreover, it is not limited to the steel types shown in Table 1, but can be similarly applied to other carbon steels and alloy steels containing 0.03 to 1.5% C. (Effects of the Invention) As detailed above, according to the present invention, a spheroidized structure can be obtained directly by appropriate temperature control online after finish rolling, which conventionally required a long time off-line. Since spheroidizing annealing can be omitted, this has an extremely large effect on process simplification and energy savings. Furthermore, according to the present temperature control method, particularly when the finish rolling start temperature is in the range of the Ar Since it is possible to directly obtain a spheroidized structure even if the processing temperature is low in the range from point z to point Ae1), it is extremely advantageous in terms of flexibility in actual operation.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing CC7 curves schematically showing the temperature patterns of the surface layer and center of the rolled material, and Figure 3 shows the hardness of the rolled material obtained by temperature control (constant temperature maintenance) immediately after hot rolling. FIG. 4 is a diagram showing an example of a continuous hot rolling line for realizing a direct spheroidized structure obtained by temperature control immediately after hot rolling according to the present invention; FIG. FIG. 12 is a micrograph showing the metallographic structure of the rolled material obtained in the example, and FIGS. 5 and 6 are for Example 1, and FIGS. 7 and 8 are for Example 2. , No. 9
Figures 10 and 10 are for Example 3, Figures 11 and 12
The figures each show the case of Example 4. 1 to 16: Rolling mill, 20: Heating furnace, 21: Rough rolling mill row, 22: Intermediate rolling mill row, 23: Finishing rolling mill row, 2
4...Intermediate cooling zone, 25...Late stage cooling zone, 26...Cooling bed, 27...Heat treatment furnace, 29...Control device, 30-
32...Thermometer. I/Kidney 21" Figure 9 Figure 11 Figure 7 and'y-7-convex-2-one 5-cho-l-1 Figure 10 Figure 12

Claims (2)

[Claims] (1) During the hot continuous rolling process of carbon steel or alloy steel bars or wire rods containing 0.03 to 1.5% C, the surface layer of the rolled material is strongly cooled to below the Ms point, and the rolled material is By adjusting the cooling conditions after strong cooling until reaching the next rolling mill,
During the reheating process, the center of the rolled material is heated from point Ar_1 to Ae_1.
After lowering the temperature to a temperature range between points, then performing finish rolling to obtain a pre-structure that facilitates spheroidization within the entire cross section of the rolled material, and further heating to a temperature range of Ac_1 point to Ac_3 point immediately after finish rolling, A process for producing steel bars and wire rods by continuous hot rolling, characterized by directly obtaining a spheroidized structure by cooling to a temperature range of 600°C below the Ac_1 point and maintaining the temperature in this temperature range for 20 min to 5 hr. Production method. (2) During the hot continuous rolling process of carbon steel or alloy steel bars or wire rods containing 0.03 to 1.5% C, the surface layer of the rolled material is strongly cooled to below the Ms point. By adjusting the reheating time after strong cooling until reaching the next rolling mill,
In the reheating process, the center of the rolled material is lowered to a temperature of Ar_1 point or less to undergo pearlite transformation, and then finish rolling is performed to obtain a prestructure that is easy to spheroidize within the entire cross section of the rolled material, and immediately after finish rolling, After heating to a temperature range of Ac_1 point to Ac_3 point, cooling to a temperature range of below Ac_1 point to 600°C, and maintaining the temperature in the range of 20 min to 5 hr, directly obtaining a spheroidized structure. A method for manufacturing steel bars and wire rods by continuous hot rolling, characterized by: