JP4106412B1 - Controlled cooling method for steel bars - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for performing various heat treatments by controlling and cooling a steel bar immediately after hot rolling uniformly over its entire length.
SOLUTION: Steel bars are juxtaposed, passed through a cooling zone while being rotated and translated obliquely. The cooling zone has a structure in which a large number of unit fluidized bed tanks that simultaneously cool the entire length of the rod under the same conditions are arranged in parallel. Each unit has three cooling capacities: A) Strong cooling capacity room temperature fluidized bed, B) Air cooling, C) Thermal insulation. The above-mentioned three states are set appropriately for each unit that is held in parallel, and the cooling timing and the cooling intensity are freely adjusted. The cooling pattern enables various heat treatments from annealing to quenching.
[Selection] Figure 1

Description

  The present invention relates to a controlled cooling method after a steel slab is processed into a steel bar by hot rolling.

  In the steel bar rolling, the steel bars immediately after rolling are usually juxtaposed on the cooling table and air-cooled while being translated perpendicularly to the bar axis direction. As a result, it has the same metal structure as that of normalization, and has a medium strength and a stable ductility. When special mechanical properties such as high strength, low strength, high workability, and high ductility are required, the steel bar is subjected to secondary processing such as quenching and tempering or annealing.

For wire rods, the technology of controlled cooling immediately after rolling is substantial and various heat treatments are performed, and much of the secondary processing is omitted, but there are few examples of steel bars.
The reason for this is that there are many products that do not require special heat treatment from the viewpoint of demand, and even if necessary, heat treatment can not be omitted with some degree of performance improvement.
From the technical point of view, the rod diameter is large, so it is difficult to obtain the cooling rate or cooling guidelines necessary for metal structure modification. Even so, it is extremely difficult to devise equipment that can cool the entire length uniformly with the appropriate cooling guideline for steel bars that are parallel-translated to adapt to the large production efficiency (50 to 150 t / h) of the rolling mill. It is done.

  The yield strength of a general-purpose steel bar that occupies the maximum production amount of steel bars is about 300 MPa. With a small diameter (10 to 18 mm), a high-strength product having a pressure of 600 MPa is manufactured by adding an alloy and applying water cooling immediately after rolling. Recently, a high-strength material with a large diameter (22 to 51 mm diameter) is expected. This is one of the objects of the present invention.

This study examines previous cases related to controlled cooling aimed at increasing the strength of steel bars.
Temp Core Method: Non-Patent Document 1
Mainly for low-carbon steel for reinforcing steel, the straight bar immediately after finish rolling is quenched with a powerful water-cooling device, the surface layer is cooled to below the Ms point (temperature at which martensite is generated), quenched, and then air-cooled. A yield strength of about 500 MPa is obtained by tempering the quenched portion by internal self-heating, and 600 MPa is obtained by adding an alloy. The problems are: 1) the larger the rod diameter, the lower the strength and toughness. 2) Since it depends on strong water cooling, the cooling stability is lacking and the quality is difficult to stabilize. If the amount of carbon is increased to increase the strength, the Ms point is lowered, self-thermal tempering is not successful, and the strength obtained is limited. It is hardly manufactured in Japan.

Mist cooling method: Patent Document 1, Patent Document 2
When forming rolled steel bars into a row of bars and translating them in parallel on the cooling table, spray cooling is performed by arranging the mist spray nozzles on the entire surface and translating each bar diagonally while rotating each bar. The direction and tangential direction are devised to be equal. The problem is that spray cooling is basically boiling cooling, and if the amount of water is increased to enhance cooling, film boiling and nucleate boiling are mixed at the working temperature, resulting in uneven cooling, and jetting from above only. The effective surface area is about 1/3, and a large cooling capacity cannot be obtained from these two points, and the average overall heat transfer coefficient is about 150 W / m ² K at most. Therefore, it is suitable for high-strength PC steel rods with a fine pearlite structure of high-carbon low-alloy steel, but it has insufficient cooling capacity for increasing the strength of rebars and producing tempered high-strength steels of medium-carbon steel.

TMCP method: Non-patent document 2
This is a method for obtaining a desired crystal grain size and metal structure by comprehensively combining the three factors of components, rolling conditions and cooling conditions, and aims at high strength by the fine grain effect.
In the above document, 3 μm is obtained by three processes of addition of grain refinement elements (V, Nb), refinement of austenite by low-temperature rolling at 800 ° C. or less, appropriate water cooling for preventing growth of recrystallized grains and controlling transformation structure. It is shown that the strength of 500 to 800 MPa can be increased in a thick plate with fine ferrite having a diameter smaller than that of the main structure.

  The problem with this method is that, when applied to steel bars, drastic strengthening of rolling mill strength and forced cooling after rolling are essential. In contrast to the latter, the method of controlling cooling by water cooling while straightly moving the steel bar is too long for the travel distance. Incidentally, since the thick plate has a large cross-sectional area, the traveling speed is low, and the problem of space is not large for the required cooling time of about 100 seconds. As another implementation method, if the above-described mist cooling method is applied after the low-temperature rolling, a strength of 600 to 700 MPa is expected by suppressing the growth of recrystallized grains, but a high strength premised on the formation of martensite. The material is difficult due to lack of cooling capacity.

  Although there is no direct relationship with the rolling and cooling of steel bars, Patent Document 3 discloses that the normal temperature fluidized bed has substantially larger cooling capacity than the lead bath quenching, and the adjustment of the cooling capacity is achieved by intermittent contact with the fluidized bed. It is disclosed that it is effective for the patenting of wire rods and the quality is superior to that of the conventional method. Although an appropriate application method for a straight wire is disclosed, various problems are newly expected to be applied to a steel bar and a row of bars crossing a cooling zone in translation, but there is no suggestion.

Japan Iron and Steel Institute, Steel Technology Flow 4: Controlled Rolling / Controlled Cooling, P.151 Published Patent Publication No. Hei 1-234527 Published Patent Publication No. 61-26730 Supervised by Nippon Steel Co., Ltd., NIPPON STEEL MONTHRY 2007,6, p.7 ~ 8 Patent 3914953

  The problem to be solved is that in a mass-produced steel bar mill, the parallel translation of steel bars has the appropriate cooling strength and adjustability for the purpose of giving various heat treatment effects to the steel bars by controlled cooling. There is no cooling method. An object of the present invention is to solve this problem.

  In order to solve the above problems, the present invention includes a room temperature fluidized bed having a strong cooling capacity in a method in which the rod row of the preceding example is rotated in parallel while passing through the cooling zone and passes through the cooling zone. The most important feature is that the unit tanks that are simultaneously processed in parallel are configured to form a cooling zone, each unit tank is individually opened and closed to induce intermittent cooling, and cooling can be freely adjusted.

1st invention is the control cooling method of the steel bar immediately after hot rolling, and cuts the steel bar which goes straight after passing through the final rolling by a predetermined length, moves it in the direction perpendicular to the bar axis, and makes it horizontally parallel at a predetermined interval. In a method of forming a row of rods in parallel and forcibly cooling each rod by rotating around the axis of the rod and translating obliquely with respect to the axis of the rod while passing through the cooling zone, 1) 2) The fluidized bed tank has a structure in which a large number of unit fluidized bed tanks parallel to the rod axis are closely paralleled. 3) The unit tank is mainly a) parallel to the rod axis, and a row of bars. Comprises a water-cooled side wall having an opening through which b can be translated, b) a row of air holes provided in the center of the bottom surface and parallel to the rod axis, and c) fluidized sand. Forming a fluidized bed that surrounds and cooling, and in a closed state, the fluidized sand is deposited to air-cool the steel bar, ) By setting the opening and closing of the blowing of the unit tank individually, it is a control method of cooling a steel bar, characterized in that to adjust to a predetermined cooling rate in the entire length the same conditions for steel bar passing.

  The second invention is the method for controlling and cooling a steel bar according to the first invention, wherein a ceiling that can be opened and closed is provided above each unit tank, and the ceiling is closed and kept warm in a state where the air blowing is stopped.

  The third invention is a low-medium-carbon low-alloy steel hot steel bar having a nominal diameter of 22 to 51 mm, which avoids the occurrence of pearlite transformation by the method of the first or second invention, and the surface temperature is higher than the Ms point of the steel type. A steel bar controlled cooling method characterized by quenching by quenching to a low temperature and then performing air cooling or heat insulation to perform self-thermal tempering.

  In the fourth invention, a hot steel bar having a nominal diameter of 22 to 51 mm, which is a low-medium carbon low alloy steel, is finish-rolled at 850 ° C. or less, and the surface temperature is increased to 700 ° C. by the method of the first invention or the second invention. A steel bar controlled cooling method characterized by suppressing the growth of recrystallized grains by cooling at a rate of s or more and maintaining fine ferrite having a metal structure of 3 μm or less in diameter.

  In the fifth invention, a hot steel bar having a diameter of 22 to 51 mm of high carbon low alloy steel is cooled by the method of the first invention or the second invention, and the minimum temperature of pearlite transformation is 570 ° C. or more and the maximum temperature is 640 ° C. or less. It is a high-strength PC steel bar characterized in that the patenting process is performed by induction.

  The sixth invention is a hot rolling steel bar having a nominal diameter of 22 to 51 mm of medium and high carbon low alloy steel, finish-rolled at 850 ° C. or less, cooled to 640 to 680 ° C. by the method of the first invention or the second invention, and then kept warm. Then, a pearlite transformation is induced, and a pseudo-annealing process is performed, which is a controlled cooling method for steel bars.

The steel bar controlled cooling method of the present invention uses a fluidized bed at room temperature, and therefore has a very high cooling capacity. As a result, 1) quenching is facilitated, and 2) the amount of hardenable alloy can be reduced.
Secondly, the cooling capacity and the cooling timing can be freely adjusted by intermittent cooling, and various heat treatments are facilitated.
Third, there is an advantage that it can be attached to an ordinary mass production steel bar mill and heat treatment can be added at low cost.

It can be used to increase the strength of high-carbon low-alloy steel PC steel bars by patenting.
It can be applied to the production of high-strength steel bar that has been tempered (quenched and tempered) of medium-low carbon steel.
This makes it possible to manufacture deformed steel bars for reinforcing bars with high strength weldability by reducing the grain size of low carbon steel.
Enables the production of pseudo-annealed steel bars for medium and high carbon low alloy steel.

The purpose of applying appropriate controlled cooling to the steel bar immediately after rolling and obtaining the heat treatment effect to improve the mechanical properties of the product is a relatively difficult method, relatively inexpensive, non-redundant equipment and low cost. Realized the method.
FIG. 1 (plan view) is a schematic diagram illustrating the entire example of equipment for carrying out the method of the present invention. The steel bar 1 that has passed through the finish rolling, cut to a predetermined length, traveled straight on the roller gang 2 and arrived at the side of the fluidized bed tank 3 serving as a cooling bed, was sequentially squeezed out in the orthogonal direction, and parallel to the bar. Column 4 is formed. The row 4 undergoes a skew translation while rotating individually via a loop conveyor 5 and a skew belt 6 and passes through the tank 3 to undergo a predetermined cooling process. The rod after the controlled cooling is pushed up from the tank and carried out by the carrying-out roller gunk.

  FIG. 2 (side view) is an explanatory view of the transfer mechanism of the rod row 4. The steel bar 1 on the tilted rail 8 is projected at a predetermined interval by a partition claw 7 provided on the outer periphery of the conveyor 5 below a parallel number of loop conveyors 5 running in the translational direction (perpendicular to the bar axis). A bar array 4 is formed and then transferred onto a parallel number of skew belts 6. Both the conveyor 5 and the belt 6 travel through the fluidized bed tank 3. The conveyor 5 naturally runs at a speed synchronized with the rolling pitch. The belt 6 is stretched horizontally and obliquely with respect to the rod axis direction. The traveling path of the conveyor may be reversed below the fluidized bed tank.

  The individual steel bars 1 are subjected to a rotation force and a thrust force by the traveling of the oblique belt 6 and the traveling of the conveyor 5 accompanied by the restraining by the partitioning claws 7, and are skewed while rotating. At this time, if the speeds of the conveyor and the belt coincide with each other, neither rotation nor skew occurs. The skew angle is not constant and depends on both speed ratios. This rotation has the effect of leveling the influence of the tangential non-uniform distribution of heat transfer coefficient in the fluidized bed cooling of steel bars. The skew disperses the local abnormal cooling caused by the contact between the steel bar and the structure such as the conveyor. In addition, the uniformity of cooling in the rod axis direction and tangential direction is improved. This is one of the essential mechanisms of the present invention.

  FIG. 3 (side view) is a diagram illustrating the main points of the fluidized bed tank 3. The tank 3 has a structure in which a large number of unit fluidized bed tanks 10 having a length capable of accommodating bar steel and a width close to the interval between the row of bars are arranged in parallel in parallel with the rod axis. The unit fluidized bed tank 10 is mainly parallel to the rod axis and has a water-cooled side wall 11, an opening 12 that translates and penetrates the row of rods 4 provided on the side wall 11, and an air flow that extends parallel to the rod axis and covers the entire length of the tank. It consists of a bottom structure provided with a row of holes 13 or slit nozzles, a ceiling 16 that can be opened and closed, and a deposited layer of about 300 mm thick of about 0.1-0.7 mm diameter zircon sand 14 in the bath. Those skilled in the art can easily understand the size, arrangement, and constant pressure of the nozzle in order to form a fluidized bed having a uniform tank length.

  As shown in FIG. 3A, the sand floats by the injection of compressed air, and forms a fluidized bed 15 swelled by about 100 to 150 mm. The running level of the oblique belt 6 is higher than the height of the stationary sand and lower than the fluidized bed height. The steel bar 1 passes through the fluidized bed 15 and is cooled by contact. When the blowing is stopped, the sand is deposited and the bar is left unattended. When the ceiling is opened, it is in an air-cooled state (FIG. 3B), and when the ceiling is closed, it is in a state close to heat retention (FIG. 3C). That is, the unit fluidized bed has three kinds of cooling ability and can be switched by blowing and opening / closing the ceiling.

In normal fluidized bed cooling, the cooling capacity is controlled by the fluidized bed temperature. Therefore, although a heating device, a cooling device, and a control device are attached, in the present invention, only the cooling device is attached and the others are unnecessary. The fluidized bed is always maintained at “room temperature”. (In the present invention, “normal temperature” means normal temperature at work and is defined as 100 ° C. or lower.) The first reason for setting the fluidized bed to normal temperature is that the temperature difference from the material to be treated is large and the cooling capacity is maximum. The second is that temperature control is easy, the equipment is simple and energy saving, and the third is that a mechanism capable of freely adjusting the cooling capacity has been devised. That is, it is a system in which unit fluidized bed tanks are arranged in parallel and three states are individually set as appropriate.

The cooling capacity in each state is as follows.
Cooling capacity Q = heat transfer coefficient α × (bar temperature θ−ambient temperature θs)
State A: Heat transfer coefficient α1≈1100 (W / m ² K), ambient temperature θs1≈100 ° C.
Condition B: α2 ≒ 110 θs2 ≒ 100 ℃
Condition C: α3 ≒ 50 θs3 ≒ 300 ℃

In the controlled cooling, the cooling timing and cooling strength must be set appropriately in accordance with the target heat treatment. In the present invention, the following adjustment method is adopted. That is, each unit fluidized bed tank is set to one of the above three states from the upstream side. For example, in the case of quenching self-tempering and patenting, the following is qualitative.
Quenching self-thermal tempering: AAAAAAAAAAACCCCCCCCCC
Patenting: AAAAAABBABABBCCCCCCCCCC

A specific method for maintaining the fluidized bed at room temperature is to construct the unit fluidized bed tank with a water-cooled wall, desirably arrange fins on the wall, and increase the contact area between the fluidized sand and the wall to positively sand. Cool down.
The value of the heat transfer coefficient between the bar and the fluidized bed is close to that between the fluidized bed and the water cooling wall. Therefore, when the effective surface area of the water cooling wall in one unit tank is made 10 times or more the surface area of the rod in the tank, the temperature of the fluidized bed is 1 from the rod temperature (900 to 500 ° C.) to the water cooling wall temperature (about 30 ° C.) side. : 10 or more biased and easily maintained at 100 ° C. or less. If the side wall area is insufficient, water-cooled walls are also provided in the perpendicular direction.

The value of the heat transfer coefficient α of the fluidized bed is almost constant and cannot be freely adjusted when the design conditions are determined. Accordingly, the cooling strength is adjusted by intermittent contact between the rod and the fluidized bed as described above. The intermittent width, that is, the width of the unit tank is preferably about the pitch of the partition claws. Even if it is large, it should be 4 times or less of the pitch. The larger the value, the rougher the digital control of the present invention.

The progress of cooling by the fluidized bed is analyzed. The value of the heat transfer coefficient α was found to be 800 to 1200 W / m ² K due to the large influence of the material diameter in the previous experiment. In many literatures, it is shown as about 800-1500. α = 1160 was used to analyze the cooling rate of steel bars with diameters of 20, 32, and 50 mm. Since the rod diameter is large and the α value is large, the temperature distribution in the cross section is non-uniform, so the calculation was performed by an analytical solution using a Bessel function. The results are shown in FIG. The cooling rate and the internal / external temperature difference depending on the rod diameter are found, and it is possible to read a guideline at which timing the unit fluidized bed tank should be closed with respect to a desired cooling pattern.

Verification by laboratory prototyping of whether the manufacturing method of high-strength PC steel rod disclosed in Patent Document 2 can be replaced or improved by intermittent cooling (switching between states A and B) of the room temperature fluidized bed of the present invention. did. The steel type of the test material is 0.7% C-0.8% Si-1.2% Mn-0.7% Cr-0.03% Mo. In the following, all the% indications of ingredients are mass%.
The shape and size of the test material is a rolled steel bar having a diameter of 32 mm and a length of 800 mm, which is heated to 950 ° C. 6 was intermittently immersed in a normal temperature fluidized bed of width 100 mm × length 1 m and cooled. 3 seconds immersion 3 seconds air cooling is repeated to cool to 600 ° C, left to cool, and the start of transformation exothermic temperature rise is confirmed, 2 seconds immersion 4 seconds air cooling is continued 6 times, and the temperature rise temperature is 630 ° C or less. Then it was air cooled. The minimum temperature was 570 to 600 ° C. Simply bending the bar up and down allowed bending and was constantly rotated with fire chopsticks. The five samples, which were relatively well worked, had a tensile strength of 1270-1370 MPa and a squeezing of 33-37%, which gave somewhat better performance than the disclosed data. This experiment confirmed that the method of the present invention can be easily patented. Moreover, the steel grade normally used for the product is 0.6 to 0.8% C, 0.5 to 1.5% Si, 0.5 to 1.5% Mn, 0.1 to 0. It is appropriately microalloyed based on .8% Cr, and there is no particular obstacle in applying the present invention. As shown in Document 2, the relationship between components, cooling strength and obtained strength is well known.

  A high-strength reinforcing bar was manufactured by the same experimental method as above. The target strength is 800 MPa or more in yield stress, 950 MPa or more in tensile strength, and the target elongation is 8% or more. This product is currently manufactured and marketed with a wire rolling / controlled cooling method with a small diameter of 13 mm. The purpose is to expand the product to a diameter of 22-51 mm.

The steel type of the test material is 0.18% C-0.8% Si-1.5% Mn-0.9% Cr-0.03% Mo which is the same as the current wire product, 36 mm diameter x 800 mm A long threaded steel bar was heated to 900 ° C., immersed in a room temperature fluidized bed having a width of 100 mm × a length of 1 m for 20 to 50 seconds, and then air-cooled. Strength was insufficient until the immersion time was 20 seconds, and the target values were obtained with a yield strength of 860 MPa and a tensile strength of 970 MPa after 30 seconds. When the immersion time was 40 seconds or more, the strength was further improved, but the elongation was 5% or less and the ductility was insufficient. The metal structure is mainly composed of ferrite and pearlite when the immersion time is short, and tempered martensite is mainly used at the appropriate time. Contains a small amount. By the method of the present invention, it was proved that a deformed steel bar for high-strength reinforcing bars having a mixed structure mainly composed of quenching and tempering can be produced in the rolling process. It can be easily understood that for the 50 mm diameter, the alloy may be increased to some extent. The relationship between the ingredients, quenching and tempering conditions, and strength is obvious to those skilled in the art.

  A spring steel pseudo-annealing material was manufactured by the same experimental method as above. The target strength is that the tensile strength is 1000 MPa or less and the metal structure is pearlite. The purpose is to improve the peelability, and therefore it is necessary to have a low strength and a pearlite structure. The spheroidized annealed material is sticky and is not good for peeling. The steel type of the test material is 0.6% C-1.5% Si-0.8% Mn-0.7% Cr-0.10% V, and a rolled bar steel of 26 mm diameter × 800 mm length is 870 ° C. Then, immersion and air cooling were repeated at intervals of 3 seconds in a room temperature fluidized bed of width 100 mm × length 1 m, and after the surface temperature reached 680 ° C., air cooling was performed. The strength obtained was 950 to 990 MPa, but it was predicted to be a limit at a diameter of 20 mm. Presumed to be warm.

An example of implementation prediction will be described. Regarding the increase in strength by ultrafine grain refinement, not only the addition of refinement elements, but also refinement of austenite grains by low-temperature rolling at 850 ° C. or lower, refinement of ferrite grains derived therefrom, Therefore, controlled cooling that suppresses the growth of recrystallized grains is necessary. This is so-called TMCP steel. A cooling rate of 5 ° C./second or more is sufficient. The above is a well-known matter. The cooling rate is easily obtained by the method of the present invention. That is, from the analysis result of FIG. 4, the cooling rate of 9 ° C./s is obtained even in the central portion of the 50 mm diameter rod by continuing the state A (fluidized bed cooling).
The relationship among ingredients, rolling temperature, austenite grain size, ferrite grain size, ferrite area ratio, and strength is well known in addition to Non-Patent Document 2, and strengthening becomes clear when the ferrite grain size is 3 μm or less. The ferrite area ratio is strongly related to the elongation. If the ratio is 50% or more, an elongation of 10% or more can be obtained.

  Table 1 shows an example of equipment design according to the method of the present invention. The overall configuration of the equipment for controlling and cooling steel bars with diameters of 22, 32, and 51 mm on an actual production scale becomes clear, and it can be read that it is relatively simple and compact and is not difficult to implement.

In the steel bar controlled cooling method of the present invention, each steel bar can be uniformly cooled over its entire length, and the timing and cooling strength can be freely adjusted. Various heat treatments from pseudo annealing to quenching can be applied on an actual production scale.
This can be implemented by partially modifying the existing steel bar cooling table.

It is explanatory drawing (plan view) of the equipment which implements the control cooling method of the steel bar of this invention. It is explanatory drawing (side view) of the mechanism which carries out skew translation of a steel bar. It is a figure (longitudinal sectional view) explaining the principal part of a fluidized bed tank. It is an analysis figure of the cooling rate of a steel bar.

Explanation of symbols

1: Steel bar 3: Fluidized bed tank 4: Rod row 5: Loop conveyor 6: Skew belt 7: Partition claw 9: Unit fluidized bed tank 11: Side wall 12: Opening 13: Blow hole array 14: Zircon sand 15: Fluid Floor 16: Ceiling

Claims (6)

This is a method for controlling cooling of steel bars immediately after hot rolling, in which straight steel bars after passing through the final rolling are cut at a predetermined length, sequentially moved in the direction perpendicular to the bar axis, and arranged in parallel in parallel at predetermined intervals. And forcibly cooling by passing through a cooling zone while rotating the individual rods around the rod axis and translating obliquely with respect to the rod axis, and 1) the cooling zone is constituted by a normal temperature fluidized bed tank 2) The fluidized bed tank has a structure in which a large number of unit fluidized bed tanks parallel to the rod axis are closely paralleled. 3) The unit tank is mainly a) parallel to the rod axis so that the row of bars can be translated. A water-cooled side wall having an opening; b) a row of air holes provided in the center of the bottom surface and parallel to the rod axis; and c) fluidized sand. 4) a fluidized bed that surrounds a bar steel that passes through the air hole in the air hole state. In the closed state, the fluidized sand is deposited and the steel bar is cooled by air. By setting the opening and closing of the air individually controlled cooling method steel bars and adjusting to a predetermined cooling rate in the overall length the same conditions in response to a variety of heat treatment content to bars passing.   The steel bar controlled cooling method according to claim 1, wherein a ceiling that can be opened and closed is provided above each unit tank, and the ceiling is closed to keep warm in a state where air blowing is stopped.   A hot steel bar having a nominal diameter of 22 to 51 mm of low-medium-carbon low-alloy steel is a temperature whose surface temperature is lower than the Ms point of the steel type while avoiding the occurrence of pearlite transformation by the method described in claim 1 or 2. A method for controlled cooling of bar steel, characterized by quenching and quenching, followed by air cooling or heat insulation to perform self-thermal tempering.   A hot steel bar having a nominal diameter of 22 to 51 mm of low-medium-carbon low-alloy steel is finish-rolled at 850 ° C. or lower, and a surface temperature of 5 ° C./s or higher up to 700 ° C. by the method according to claim 1 or 2. A controlled cooling method for steel bars, characterized in that the growth of recrystallized grains is suppressed by cooling at a rate of 5 mm, and the microstructure is maintained as fine ferrite having a diameter of 3 μm or less.   A hot steel bar having a diameter of 22 to 51 mm of high carbon low alloy steel is cooled by the method described in claim 1 or 2 to induce a minimum temperature of pearlite transformation to 570 ° C or higher and a maximum temperature to 640 ° C or lower. And then performing a patenting process.   A hot steel bar having a nominal diameter of 22 to 51 mm of medium and high carbon low alloy steel is finish-rolled at 850 ° C. or lower, cooled to 640 to 680 ° C. by the method described in claim 1 or claim 2, and then kept warm. A steel bar controlled cooling method characterized by inducing pearlite transformation and performing pseudo annealing.

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