JPS63149316A - Production of bar steel and wire rod by hot continuous rolling mill - Google Patents

Abstract

PURPOSE:To easily produce a bar steel and wire rod having prestructure, which easily attains spheroidizing structure, by hot-rolling a carbon steel and adjusting recuperating heat time till coming to the next rolling mill after forced-cooling the surface part to the Ms point or lower. CONSTITUTION:After heating the carbon steel billet containing 0.03-1.5% C in a heating furnace 20, it is rough-rolled by a rougher rolling mill line 21 and an intermediate rolling mill line 22. Next, the rolled material is forcedly cooled in the intermediate cooling zone 24 at the Ms point or lower on the surface part, to form the quenching structure, such as martensite. Next, by adjusting the recuperating heat time till the rolled material attains the finish rolling mill line 23, the temp. of the central part of rolled material is lowered to the Ar1 point or lower, to a pearlite transformation. Then, plate-like or bar-like carbide is cut into pieces by the finish rolling and the prestructure, which easily forms the spheroidizing structure, is obtd. Next, the rolled material is carried to a cooling bed 26 through the latter part cooling zone 25.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to the manufacture of steel bars and wire rods by hot continuous rolling, and in particular, the spheroidized structure is formed in the subsequent spheroidizing annealing by the continuous hot rolling process or by controlling the temperature immediately thereafter. The present invention relates to a method that can easily obtain a spheroidized FFI fabric or directly obtain a spheroidized FFI fabric by omitting a spheroidizing annealing process. (Conventional technology) For high carbon steels such as cold forging steel, bearing steel, and tool steel, ductility is imparted to them prior to cold working. With the aim of reducing hardness and improving its workability,
Generally, spheroidizing annealing treatment is often performed. Conventionally, methods for this spheroidizing annealing include: (1) a long-time heating method in which heating is maintained at a temperature just below the A1 point for an appropriate period of time, and then cooling; (2) heating to a two-phase region between the A0 point and the A1 point; A slow cooling method in which the transformation is then completed by slow cooling; ■ A repeated method in which the A
There are other methods. 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, the above heat treatment method requires an extremely long treatment time of about 10 to 15 hours, resulting in low productivity and high heat treatment cost.Also, the above heat treatment method cannot be said to be preferable from the viewpoint of energy saving. Therefore, as a remedy for this, there is a method of adjusting the anterior tissue as a method to facilitate spheroidization, or furthermore,
Attempts have been made to omit the spheroidizing annealing treatment performed after hot rolling. The former method is, for example, a method in which a quenched structure such as martensite or an intermediate structure is formed by quenching after hot rolling. It is not easy to obtain a deep structure 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 hardened 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 temperature control during and after hot rolling, there are, for example, JP-A-59-136421; There is a method shown in No. 59-13024. However, with either method, it is impossible to achieve temperature control 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. The present invention has been made in order to solve the problems of the above-mentioned conventional technology, and the present invention has been made by continuously hot rolling to create a pre-structure that makes it easy to obtain a spheroidized structure over the entire cross section of the rolled material in the subsequent spheroidizing annealing treatment. How to make it happen. Furthermore, it is an object of the present invention to provide a method for directly obtaining a spheroidized structure over the entire cross section of a rolled material by controlling the temperature immediately after rolling. (Means for Solving the Problems) In order to achieve the above object, the present inventor first developed a method for rolling the finished product of continuous hot rolling, and then applying a subsequent spheroidizing annealing treatment to the center of the rolled material within the entire cross section. Experimental research was carried out to find a rolling method to obtain a pre-spheroidized structure that would facilitate the formation of a spheroidized structure. As a result, the rolled material after finish rolling generally has a layered pearlite structure in the center or other regions within the cross section due to transformation from the austenite state that is difficult to spheroidize in the subsequent heat treatment process. In order to achieve the desired prestructure in the rolling finish, it is necessary to break up or accelerate the breakup of the plate-like or rod-like carbides in the layered pearlite structure by rolling during the continuous hot rolling process. focused on the fact that the pearlite transformation from the austenite structure that normally occurs after finish rolling should be completed during the continuous hot rolling process, especially before finish rolling, and after further research on specific measures for this purpose, they found that When the transformation of the untransformed austenite structure in the cross section of the rolled material is completed by adjusting the strong cooling and recuperation time after strong cooling during the process, the plate-like or rod-like carbides are separated at least by finish rolling. It has been found that a pre-structure is obtained in which the spheroidized structure is facilitated in the subsequent spheroidizing annealing treatment. Furthermore, according to the research of the present inventor, this finishing material. Of course, the subsequent spheroidizing annealing process easily turns into a spheroidal structure in a short time, but since it has the above-mentioned previous structure, it can be heated and slowly cooled immediately after rolling without using the conventional offline spheroidizing process. If proper temperature control is carried out, some of the carbides in the structure adjusted during the rolling process will dissolve into the austenite, and during the subsequent slow cooling process, the solute carbon will precipitate with the remaining carbides as nuclei, causing growth. It was discovered that a spheroidized tissue could be obtained directly by doing this. 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 reheating time after the hard cooling until the rolled material reaches the next rolling mill, the central part of the rolled material is heated to Ar1 in the reheating process. It is characterized by lowering the temperature to below the point point to cause pearlite transformation, and if necessary, immediately after finish rolling, heating it to a temperature range of Ac1 point to Ac3 point, and then slowly cooling it at a cooling rate of 50 ° C / hr or less. It is something to do. The present invention will be explained in detail below based on examples. Figure 1 shows an example of a hot continuous rolling line for achieving a rolled finish with a structure that facilitates the formation of a spheroidized structure. In the furnace, in the latter stage there is a rough rolling mill row 21 consisting of rolling mills Nα1 to NQ8, an intermediate rolling mill row 22 consisting of rolling mills &9 to Na12, and a finishing rolling mill row 23 consisting of rolling mills 413 to &16. are arranged in series. 24 is an intermediate cooling zone provided between the intermediate rolling mill row 22 and the finishing rolling mill row 23, and the flow rate of cooling water supplied to the rolled material in this cooling zone 24 is controlled by a control device 2 including a process computer.
9. 30.31.3
2 is a thermometer that measures the surface temperature of the rolled material, and 25 is a rear cooling zone provided between the finishing rolling mill row 23 and the cooling bed 26. 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 strongly cooled in an intermediate cooling zone 24 to bring the surface layer below the Ms point. A quenched structure such as martensite is formed, and then, by adjusting the recuperation time in the reheating process of the surface layer between the intermediate cooling zone 24 and the finishing rolling mill row 23, the temperature in the center is reduced to below 1 point Ar and becomes pearlite. Transformation is caused, and the transformation of the untransformed structure (austenite structure) in the cross section of the rolled material is completed before finish rolling. As a result, as for the structure before the start of rolling in the finishing rolling mill row 23, as shown in the CCT curve modeling the temperature pattern of the surface layer and center of the rolled material in FIG. 2, the surface layer of the rolled material is martensite. With such a quenched structure, a ferrite/pearlite structure is obtained in the central part, and a mixed structure including an intermediate structure is obtained in the other cross-sectional areas. Of course, the same result can be obtained even if the intermediate cooling zone 24 is provided between the rough rolling mill 21 and the intermediate rolling mill row 22. The reason why the structure is adjusted to the above structure during such a hot continuous rolling process, especially before finish rolling, is that among these structures, the layered pearlite structure that is the most difficult to form into spherules is produced in the next finish rolling mill row 2.
By the rolling process in step 3, the plate-like or rod-like carbide is divided or accelerated to become a pre-structure in which a spheroidized structure can easily be formed, and the quenched structure in the surface layer of the rolled material is affected by the strain energy caused by the rolling process. This is because, by increasing the number of particles, it becomes a pre-tissue in which it is easier to form a spheroidized structure. Therefore, specifically, after heating a billet of carbon steel or alloy steel containing 0.03 to 1.5% + 1% C, which is the raw material for steel bars or wire rods, it is subjected to rough rolling or rough rolling and intermediate rolling. The rolled material is forcibly cooled using a coolant such as water in an intermediate cooling zone, but it is desirable that the cross-sectional structure of the rolled material before forced cooling is in an austenitic state from the viewpoint of uniformity of the structure after transformation. However, even if a part of the structure such as ferrite exists in the cross section, the effects of the present invention are not impaired in any way. When determining the cooling water flow rate and reheating time required to obtain the effects of the present invention, it is important to note that the length of the intermediate cooling zone to be used varies depending on the required amount of cooling water, and that the temperature at the center of the rolled material is It should be considered that the reheating time required to reduce the temperature to below Ar1 point is mainly controlled by physical factors such as the amount of temperature drop in the intermediate cooling zone, the diameter of the abrasive material, and the cross-sectional circular temperature distribution. There is a need. Of course, if the length of the intercooling zone and the subsequent recuperation zone (distance between the intermediate cooling zone and the next stand) are too large, the distance between rows of rolling mills or between rolling mills will become long, which will cause layout problems. However, if it is too short, it will not be possible to obtain the necessary temperature drop and recuperation time, and it will be difficult to install a looper, etc., which is normally placed between the intermediate cooling zone and the next stand. To avoid equipment problems, the length of the intercooling zone and the distance between the intercooling zone and the next stand (recuperation zone) are usually set at 10% each.
It is appropriate to set the distance to about 15 m. When using such equipment, the temperature of the surface layer of the rolled material is kept below the Ms point, and during the subsequent reheating process, the center part is heated to A.
The flow rate of cooling water necessary to bring about the pearlite transformation at a temperature below the r1 point is as shown in FIG. 3 with respect to the diameter of the rolled material. However, this figure is the result obtained by temperature analysis using the experimental formula for the cooling zone cooling capacity, which was understood in detail in the actual machine, and A
Strictly speaking, the r□ point differs depending on the steel type, cooling rate, etc.
Here, it was assumed that Ar1 point to 550°C. In addition, this cooling water flow rate is such that the intercooling zone and recuperation zone are each 15 m in length, and the extraction temperature is set to the minimum temperature necessary for austenitization (
~800℃), when the rolling speed is set to the slowest possible operating speed (0.1m/5e at the Nα1 stand of the rough rolling mill row)
e) is the minimum flow rate required to carry out the present invention. If this cooling water flow rate is expressed with respect to the diameter of the rolled material, the following equation (1) is obtained. Here, W: Cooling water flow rate (rn''/hr) D: Rolled material diameter (m+=) Note that the larger the cooling water flow rate in the intermediate cooling zone, the greater the temperature drop in the cooling zone. This is advantageous in terms of operation because it can shorten the reheating time required to bring about pearlite transformation by keeping the temperature at the center below Ar1 point in the reheating process after strong cooling, but if it is too high, the cooling equipment and The required power is large, which is not practical. The maximum practical flow rate in the intermediate cooling zone is 1 liter per meter of cooling zone.
oom'/hr, so here we set the required maximum flow rate to 1500 rn' taking into account the length of the intercooling zone mentioned above.
/hr. Next, the required minimum recuperation time will be explained. As mentioned above, by slowing down the rolling speed and increasing the cooling time,
If the flow rate of cooling water is increased, the amount of temperature drop can be increased, so the reheating time is shortened to bring the temperature of the center of the rolled material below the Ar□ point and transform it into pearlite in the reheating process after strong cooling. Become. Therefore, by setting the rolling speed to the slowest possible operational level, the cooling time is maximized, and in the reheating process after cooling using the maximum flow rate in the intermediate cooling zone, the central part of the rolled material is reduced to A r 1 point or less. The required minimum recuperation time is the time such that: This required minimum recuperation time is shown in FIG. 4 with respect to the diameter of the rolled material, and can be expressed as in the following equation (2). Trmin=0.00015D2-0.002D+0.
60 ・=(2) Here, Trmin: Required minimum recuperation time (see) D: Diameter of rolled material (+=m) In addition, in the case of the above-mentioned intermediate cooling zone length, this value depends on the operating speed of the rolling The slowest possible speed is 0.1 m/5ec) at the Nα1 stand of the rough rolling mill row, and the maximum practical flow rate (100 rn'/hr per 1 m of cooling zone).
The temperature is calculated using the empirical formula for the cooling zone cooling capacity, which has been determined in detail using actual equipment, such as the one described above, which takes the recuperation time such that the temperature at the center of the rolled material becomes below the Ar1 point during the forced recuperation process when using the This is a value calculated by analysis. On the other hand, the upper limit of the reheating time is determined from the equipment length (the length of the intercooling zone and the recuperation zone) and the lowest rolling speed. each length of 15 m), and the rolling speed is set to the slowest possible operating speed (0.1 m/5 e in the No. 1 stand of the rough rolling mill row).
c), the diameter of the rolled material is as shown in FIG. 5, and can be expressed as the following equation (3). However, since the length of the cooling zone used was adjusted by the flow rate of cooling water, the maximum length of the recuperation zone was set to 25 m. Trmax=0,015D”...
(3) Here, Tra+ax: upper limit of reheating time (se
e) D: Diameter of rolled material (,n,) In the above example, the temperature of the surface layer of the rolled material is set to below Ms point, and in the subsequent reheating process, the temperature of the central part is set to below Ar1 point to cause pearlite transformation. Although an example is shown in which strong cooling is carried out in an intermediate cooling zone provided between the intermediate rolling mill row 22 and the finishing rolling mill row 23, rough rolling It goes without saying that cooling equipment may be provided between the mill row 21 and the intermediate rolling mill row 22 or between each rolling mill. According to the above embodiment, the rolled material obtained by finish rolling has a structure that easily becomes spheroidized in the entire cross section. In the present invention, for a rolled material having such a pre-structure, it is preferable to perform appropriate temperature control immediately after finish rolling to obtain a spheroidized structure directly over the entire cross section of the rolled material. That is, immediately after finish rolling, the rolled material has an Ac1 point to A
c. A part of the carbide is dissolved in the austenite by heating to the temperature range of point c, and in the subsequent slow cooling process, the solid solution carbon is precipitated with the remaining carbide as a nucleus and grown to directly obtain a spheroidized structure. . In addition, when heated to a temperature range exceeding the Ac point, the amount of carbide dissolution into austenite increases, and the carbide precipitation nuclei disappear in the subsequent slow cooling process, making it difficult to spheroidize. , Ac less than 1 point makes it impossible to dissolve carbides into austenite. In the slow cooling process, the temperature range up to at least 650℃ is
It is preferable to perform slow cooling at a cooling rate of 0° C./hr or less. FIG. 6 shows an example of a continuous hot rolling line for this purpose.
A heat treatment furnace 27 is arranged to heat and slowly cool the rolled material that has passed through the finishing mill 23. Figure 7 shows a 30φ steel plate (545) made from a 1550 billet.
In the rolling process to C), after forced water cooling in the intermediate cooling zone 24 and water cooling, the in-plane structure of the rolled material is transformed into martensite, pearlite, etc. throughout the rolled material in the reheating process until reaching the finishing rolling mill row 23. After finishing, finish rolling is carried out, and immediately after that, it is heated to 740 ° C. in the heat treatment furnace 27, and the relationship between various cooling rates and the obtained hardness is shown when slowly cooling from this temperature range to 650 ° C. It is something that As can be seen from the figure, when the cooling rate is 50° C./hr or less, the hardness decreases significantly. In other words, if the cooling rate is within these ranges, precipitation of carbides and
Growth occurs and a spheroidized tissue is obtained. On the other hand, when the cooling rate exceeds approximately 50° C./hr, the hardness remains at a relatively high level. This is because the structure contains granular carbide or layered pearlite, and a good spheroidized structure cannot be obtained at such a cooling rate. Furthermore, if slow cooling is terminated in a temperature range higher than about 650° C., a structure containing layered pearlite tends to form in the subsequent cooling process, and a good spheroidized m weave cannot be obtained. Note that although Fig. 7 shows the case where the structure before finish rolling in the center of the rolled material is ferrite + pearlite, the above cooling rate range is also applied to the case where the structure in the surface layer of the rolled material is quenched. During the slow cooling process (≦50°C/hr), carbides precipitate and grow to obtain a spheroidal structure. In this way, even without performing spheroidizing annealing on a separate line, if proper temperature control is performed after finish rolling, the granular carbide structure and other structures adjusted during the rolling process can be directly transferred to the entire cross section of the rolled material. It becomes possible to obtain a good spheroidized tissue within a short time.

[Left below]

(Example) Next, an example of the present invention will be shown. Example 1 In the hot continuous rolling line shown in Fig. 1, steel type 548
After heating the 1550 billet C to 900°C, it is continuously hot rolled, and the temperature of the surface layer of the rolled material (diameter 46φ) is kept below the Ms point (approximately 350'C) in the intermediate cooling zone in front of the finishing rolling mill row. In order to
The temperature is 0°C/sec. After forced water cooling, Nα of finishing mill row
13 During the reheating process of the surface layer of the rolled material for about 12 seconds before reaching the rolling mill, the temperature at the center of the rolled material becomes below the Ar1 point (approximately 550°C), and the pearlite transformation is completed before finish rolling. Moreover, the outermost surface temperature at this time was reheated to about 450° C., and then finish rolling was performed to finish the barbed steel with a diameter of 38, followed by air cooling to room temperature. As shown in Figure 8, the structure at the center of the finished material is such that the plate-like or rod-like carbides of the layered pearlite structure, which is the most difficult to form into spheroids, are divided by the finish rolling process and form a spheroidized structure. It has become a pre-organization that can be promoted. This steel bar is processed through a heat treatment line (not shown in Fig. 1) at 740°C.
After reheating to .degree. C., the temperature was kept constant for 1.5 hours, and the temperature was slowly cooled to 680.degree. C. at a cooling rate of 30.degree. C./hr, followed by air cooling. The structure after heat treatment was a spheroidized structure Nα2 in the surface layer as shown in FIG. 9, and a spheroidized structure of about Nα3 was obtained in the center as shown in FIG. In this way, even if the processing time is significantly reduced compared to the conventional method, a spheroidized structure of 99 Na 3 or more can be obtained over the entire cross section of the rolled material, and the hardness is Hv 180 or less within the entire cross section of the rolled material, which is different from the conventional spherical structure. Values on the same level as annealed material were obtained. Example 2 Similar to Example 1, steel type 5Cr with the chemical composition shown in Table 1 was used.
After heating the 1550 billet of 420 to 930°C, it is continuously hot rolled, and the temperature of the surface layer of the rolled material (diameter 46φ) is lowered to below the Ms point (about 380°C) in the intermediate cooling zone before the finish rolling mill row. For this purpose, forced water cooling was performed for 6 seconds at a cooling water flow rate of 260 rn'/hr. After forced water cooling, during the reheating process of the surface layer of the rolled material for approximately 10 seconds until it reaches the Nα13 rolling mill in the finishing mill row, the temperature at the center of the rolled material reaches the Ar□ point (approximately 57
0°C) and pearlite transformation is completed before finish rolling.
Further, the outermost surface temperature at this time was reheated to about 440° C., and then finish rolling was performed to finish the steel into 38φ+IP steel, which was then air cooled to room temperature. In this case as well, the central part of the rolled material had a granular structure in which plate-like or rod-like carbides of layered pearlite were separated by the finish rolling process. After reheating this steel bar to 770℃ in a heat treatment line,
The temperature was maintained at a constant temperature for hr, and the mixture was gradually cooled down to 680°C at a cooling rate of 30°C/hr, and then air-cooled. Even though the processing time is significantly reduced compared to the conventional method, as is clear from the structure of the surface layer shown in FIG. 11 and the structure of the center part shown in FIG. A somewhat spheroidized tissue was obtained. Also. The hardness was also approximately Hv140 within the entire cross section, which was the same level as the conventional spheroidized annealed material. Comparative Examples 1 and 2 Comparative Example 1 uses the same steel type 5450 as in Example 1, and Comparative Example 2 uses the same steel type S as in Example 2.
38 of each similar 1150 billet per Cr420
In the hot continuous rolling process for φ steel bars, in order to avoid suitable cooling structures such as quenched structures in the surface layer of the rolled material, the surface temperature of the rolled material at the end of cooling in each intermediate cooling zone is higher than the Ms point. When cooling was carried out so as to remain within the Ar1 point, during the reheating process of the surface layer of the rolled material up to the Nα13 rolling mill in the finishing mill row, the temperature at the center did not drop to the Ar1 point and remained in the austenite region. Therefore, even if the same reheating heat treatment as in Example 1.2 is performed after finish rolling,
In all steel types, no spheroidized structure was obtained in most of the cross-sectional area except for a small part of the surface layer of the rolled material. [1 side] - In the hot continuous rolling line shown in Fig. 6, steel type S45C (Ac1 point ~ 720°C) with the chemical composition shown in Table 1 was used.
After heating the 1550 billet to 900 °C, it is continuously hot rolled, and the temperature of the surface layer of the rolled material (diameter 45φ) is lowered to the Ms point (approximately 350 °C) or lower in the intermediate cooling zone in front of the finishing rolling mill row. For this purpose, forced water cooling was performed for 5 seconds at a cooling water flow rate of 280 rn'/hr. The cooling rate of the outermost surface at this time is approximately 50
It is 0°C/see. After forced water cooling, Nc of finishing mill row
During the reheating process of the surface layer of the rolled material for about 12 seconds before reaching the i13 rolling mill, the temperature at the center of the rolled material falls below the Ar□ point (approximately 550°C) and undergoes pearlite transformation, until the start of finish rolling. The untransformed set in the cross section represented by the center of the rolled material! The transformation of (austenite) was completed, and the outermost surface temperature of the rolled material at this time was reheated to about 450'C. Subsequently, finish rolling was performed to produce a 30φ steel bar. In this case, the plate-like or rod-like carbides of the layered pearlite structure, which is the most difficult to form into spherules, represented by the center of the rolled material, are broken up by finish rolling and exhibit a granular structure, and the quenched structure of the surface layer of the rolled material is the result of finishing rolling. Due to the increase in strain energy due to processing, etc., the formation of spheroidized structures was easy for each structure. Immediately after finish rolling, the rolled material (30φ steel bar) is heated in the heat treatment furnace 27 to Ac, the intended temperature of 740°C, and then heated to 640°C from this temperature.
The temperature range up to 50°C was slowly cooled at a cooling rate of about 30°C/hr, and then air cooling was performed. The obtained structure is a spheroidized structure Nα2 in the surface layer as shown in FIG.
Even in the center, as shown in FIG. 14, there was a spheroidized structure with a spheroidization group wtHa of about 3. In this way, by appropriately controlling the temperature after continuous hot rolling, a direct spheroidized structure with a spheroidized structure Nα3 or more can be obtained over the entire cross section of the rolled material, and the hardness is also less than HV180 within the entire cross section, which is different from conventional offline rolling. A value similar to that of the spheroidized annealed material was obtained in offline processing. Example 4 As in Example 3, steel type S with the chemical composition shown in Table 1
A 1550 billet of Cr420 (Ac, point ~725°C) is heated to 930°C, then continuously hot rolled, and the temperature of the surface layer of the rolled material (diameter 45φ) is brought to the Ms point in an intermediate cooling zone in front of the finishing rolling mill row. (approximately 380℃) or less
Forced water cooling was performed for 6 seconds at a cooling water flow rate of rr//hr. After forced water cooling, during the reheating process of the surface layer of the rolled material for about 10 seconds until it reaches the &13 rolling mill in the finishing mill row, the temperature at the center of the rolled material is below Ar1 point (approximately 570°C). Pearlite transformation occurs, and the transformation of the untransformed structure (austenite) in the cross section represented by the center of the rolled material is completed by the start of finish rolling, and the outermost surface temperature of the rolled material at this time is approximately 440°C.
It was reheated to ℃. Successively finish rolling to 30φ
Finished with dedicated steel. In this case as well, the plate-like or rod-like carbides of the layered pearlite structure, which is the most difficult to form into spherules, represented by the center of the rolled material, are separated by finish rolling and exhibit a granular structure, and the quenched structure of the surface layer of the rolled material is the same as the finish rolling. Due to the increase in strain energy caused by rolling, each structure had become easily spheroidized. Immediately after finish rolling, the rolled material (30φ steel bar) is heated to 770°C, which is the Ac1 point or higher, in the heat treatment furnace 27, and then
The temperature range up to 50°C was slowly cooled at a cooling rate of about 300C/hr, and then air cooling was performed. The obtained tissue is the 15th.
As shown in Figure 16 (surface layer of rolled material) and Figure 16 (center of rolled material), even in low-alloy steel, due to appropriate temperature control after continuous hot rolling, the whole cross section of the rolled material has a zo-spheroidized structure Nα2.
A directly spheroidized structure with a degree of hardness can be obtained, and the hardness is Hv
The value was about 140, which is the same level as that of the conventional offline spheroidized annealed material, which was obtained by offline processing. Comparative Example 3 For the same steel type 545C as in Example 3, 15
After the continuous hot rolling process of 50 billet to 30φ bar steel, it is heated to 740℃ immediately after finishing rolling, and from this temperature it is heated to 65℃.
When the sample was cooled at a rate of 80°C/hr in the temperature range down to 0'C, a structure containing granular carbide and layered pearlite was obtained, and a good spheroidized structure could not be obtained. Comparative Example 4 The same steel type 5Cr420 as in Example 4 was similarly hot-continuously rolled into a 1550 billet 30φ steel bar, heated to 770°C immediately after finish rolling, and then rolled in a temperature range from this temperature to 650°C. When the sample was cooled at a cooling rate of 80'C/hr, as in Comparative Example 3, the structure contained granular carbide and layered pearlite, and a good spheroidized structure was not obtained. Although the above embodiments have been described with respect to a steel bar, the present invention can be applied to a wire rod wound into a coil after finish rolling, and the same effects can be obtained. In addition, not only the steel types shown in Table 1, but also 0.03 to 1.5
% C can be similarly applied to other carbon steels and alloy steels. (Effects of the Invention) As described in detail above, according to the present invention, when manufacturing steel bars and wire rods by continuous hot rolling, a pre-structure in which a spheroidized structure is easily formed in the subsequent heat treatment process is rolled, Moreover, it can be realized over the entire cross section of the rolled material.
The heat treatment time required to obtain the target spheroidized structure within the entire cross section of the rolled material can be significantly shortened compared to the conventional method, and this has extremely large effects in improving heat treatment productivity and saving energy. In particular, if appropriate temperature control is performed online immediately after finish rolling, a spheroidized structure can be obtained directly, and the spheroidizing annealing process itself, which conventionally took a long time offline, can be omitted. Therefore, the effects of process simplification and energy saving are even more remarkable.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of a hot continuous rolling line used for carrying out the present invention, Fig. 2 is a diagram showing temperature patterns of the surface layer and center of the rolled material, and a schematic OCT curve, and Fig. 3 is a diagram showing a schematic OCT curve. A diagram showing the relationship between the minimum cooling water flow rate during cooling and the diameter of the rolled material. Fig. 4 is a diagram showing the relationship between the minimum recuperation time and the diameter of the rolled material, Fig. 5 is a diagram showing the relationship between the maximum recuperation time and the diameter of the rolled material, and Fig. 6 is a diagram showing the relationship between the maximum recuperation time and the diameter of the rolled material. The figure which shows the other example of a line. FIG. 7 is a diagram showing the relationship between various cooling rates in temperature control after continuous hot rolling and the hardness of the obtained rolled material, and FIGS. 8 to 10 are metal structures of the rolled material in Example 1 of the present invention. This is a microscopic photograph showing. Figure 8 shows the granular structure (x4000) in the center of the rolled finished material, and Figure 9 shows the spheroidized structure (x4000) in the surface layer of the rolled material after heat treatment.
Figure 10 shows the spheroidized set N&(X400) at the center of the rolled material after heat treatment. 11 and 12 are micrographs showing the metallographic structure of the rolled material after heat treatment in Example 2 of the present invention, and FIG. 11 shows the spheroidized set r&(X400) of the surface layer of the rolled material, Figure 12 shows the spheroidized structure at the center of the rolled material (X 400
), and FIGS. 13 and 14 are microscopic photographs showing the metallographic structure of the rolled material obtained by temperature control immediately after hot continuous rolling in Example 3 of the present invention, and FIG. FIG. 14 shows the spheroidized structure (X 400) in the surface layer of the rolled material, FIG. 14 shows the spheroidized structure (X 400) in the center of the rolled material, and FIGS. These are micrographs showing the metallographic structure of the rolled material obtained by temperature control immediately after continuous rolling. FIG. 15 shows the spheroidized structure (X400) in the surface layer of the rolled material, and FIG. 16 shows the spheroidized structure (X400) in the center of the rolled material. Spheroidized tissue (X 400) is shown. 1 to 16...Rolling mill, 20...Heating furnace, 21...
Rough rolling mill row, 22... Intermediate rolling mill row, 23... Finishing rolling mill row, 24... Intermediate cooling zone, 25... Later cooling zone, 26... Cooling bed, 27... Heat treatment Furnace, 29...
Control device, 30-32... thermometer. Patent Applicant Kobe Steel Co., Ltd. Representative Patent Attorney Nao Nakamura fx K jl Heat MPA (5eC) (Expression time (sec) Fig. 1 Fig. 12 Fig. 13 Fig. 14 Figure 15 Figure 16

Claims (1)

[Scope of 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. By adjusting the reheating time after strong cooling until the rolled material reaches the next rolling mill,
A method for manufacturing steel bars and wire rods by continuous hot rolling, characterized by lowering the temperature of the center of the rolled material to a temperature below Ar_1 point in the reheating process to cause pearlite transformation. (2) In the strong cooling, the cooling water flow rate w (m^3/hr) between rolling mill rows and/or between rolling mills is calculated as the diameter of the rolled material D (mm).
for 1500.0≧w≧3050.0/D-0.95D+8
The reheating time Tr is equal to or more than the minimum recuperation time Trmin (sec), and the maximum recuperation time Trmax (
sec) The method according to claim 1, wherein the method is adjusted in the following range. Trmin=0.00015D^2-0.002D+0
．． 60Trmax=0.015D^2 (3) 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 strengthened to below the Ms point. By adjusting the reheating time after strong cooling until the rolled material reaches the next rolling mill,
In the reheating process, the temperature of the center of the rolled material is lowered to below the Ar_1 point to cause pearlite transformation, and immediately after finish rolling, the center of the rolled material is
After heating to a temperature range of _1 point to Ac_3 point, 50℃/h
A method for manufacturing steel bars and wire rods by continuous hot rolling, characterized by slow cooling at a cooling rate of r or less.