KR101286948B1 - Steel material, process for producing steel material, and apparatus for producing steel material - Google Patents

Abstract

A method for producing a steel according to one embodiment of the present invention is a method for manufacturing a steel material in which the hardness of a portion of the steel is lower than that of the other portions of the steel W by heat-treating the steel W which has been strengthened. The heat treatment is a heating step of rapidly heating from the skin of the steel (W) to a predetermined depth by induction heating or direct energizing heating, and quenching the steel (W) after the heating step after a predetermined time after the heating step. And a heating temperature in the heating step is equal to or more than the Ac1 transformation point.

Description

STEEL MATERIAL, PROCESS FOR PRODUCING STEEL MATERIAL, AND APPARATUS FOR PRODUCING STEEL MATERIAL}

The present invention relates to a steel, a method for producing a steel, and an apparatus for producing a steel, and more particularly, to a hardness that varies depending on a part.

For example, in secondary processing (heat treatment) from steel materials such as a coiled rolling material (hereinafter referred to as a rolling material), which is a raw material for rods or wires, in order to improve delayed fracture resistance, for example, from the surface layer portion to the center portion. In the meantime, the technique of changing tensile strength (hardness) according to a site | part is provided.

For example, there is known a technique in which ultra-low carbon steel such as pure iron is placed on the surface layer and rolled, such as clad steel, to use decarburization and admiration (see, for example, Japanese Patent Laid-Open No. 4-212570).

Moreover, the technique which uses the decarburization layer heated to Ac1-Ac3 temperature during rolling, and produced in the surface layer (for example, see Unexamined-Japanese-Patent No. 62-267420), or the technique which heat-processes only the surface layer part again after heat processing ( For example, see Japanese Patent Application Laid-Open No. Hei 7-54441).

Moreover, as a means of improving delayed fracture resistance generally, the technique of performing chemical surface treatment, such as plating and nitriding, or the technique of performing the coating material of the nonmetallic material excellent in delayed fracture resistance on the surface is known.

However, the above technique has the following problems. That is, at the stage of a rolling material, after making a decarburization layer by manufacturing clad steel or heating to Ac1-Ac3 temperature, secondary processing (heat processing) is performed, and pretreatment is required. After the secondary processing (heat treatment), only the surface layer portion needs to be subjected to heat treatment or chemical surface treatment again. Therefore, both processes are complicated and control of complicated processing conditions is needed before and after secondary processing (heat processing).

Therefore, an object of the present invention is to provide a steel, a method for producing steel, and an apparatus for producing steel, which vary in hardness depending on the site by a simple treatment.

A method for producing a steel according to one embodiment of the present invention is a method for manufacturing a steel in which a hardness of a portion of the steel is lower than that of other portions of the steel by heat-treating the steel having a high strength. A heating step of rapidly heating the skin from the skin to a predetermined depth by heating or direct energizing heating, and a cooling step of rapidly cooling the steel that has undergone the heating step after a predetermined time after the heating step; Is characterized in that the heating temperature at is equal to or greater than the Ac1 transformation point.

As another aspect, the present invention is characterized in that the time from the heating step to the cooling step is equal to or less than a predetermined time determined by steel grade, wire diameter, heating temperature, and heating time.

In another aspect, the present invention is characterized in that treatment conditions for the heat treatment are determined based on heat transfer characteristics after rapid heating of the surface of the steel.

In another aspect, the present invention is characterized in that the treatment condition of the heat treatment is a time-integrated value of the temperature of the steel, and is determined based on a tempering progress value indicating a tempering progress state of the steel.

In another aspect, the present invention is characterized in that the treatment conditions for the heat treatment include a combination including at least two of frequency, input electrical energy, heating temperature, heating time, and cooling time.

In another aspect, the present invention includes a step of calculating the heat transfer characteristics or the tempering progress value of the steel, wherein the treatment condition of the heat treatment is determined based on the calculated heat transfer characteristics or the tempering progress value. It is done.

In another aspect, the present invention is characterized in that the treatment conditions for the heat treatment are set such that the tempering progress value at the surface layer portion is equal to or greater than 1.5 times the tempering progress value at the center portion.

As another aspect of the present invention, the time from the heating step to the cooling step is set such that the tempering progress value at the surface layer portion is equal to or greater than 1.5 times the tempering progress value at the center portion.

In another aspect, the present invention is characterized in that the steel is linear or rod-shaped.

According to another aspect of the present invention, after the quenching process including heat treatment and cooling treatment is performed on the steel, the heating process and the cooling process are performed once as tempering. It is done.

Another embodiment of the present invention is the steel, which has undergone the heating step and the cooling step, wherein the difference between the hardness near the surface layer portion and the hardness at the center side is 10 HV 50 or more from a position 10% from the surface layer in the radial direction, and JISZ2201. When the tensile test is carried out with a test piece of No. 2, the tensile strength is 1420N / mm 2 or more.

In another aspect, the present invention is a tensile martensite structure in which all cross sections are tempered, the surface layer portion has a hardness of HV380 or less, and a tensile test when a tensile test is performed with a test piece of JISZ2201 No. 2 The strength is 1420 N / mm 2 or more, and the hardness at the center side is more uniform than the surface layer.

In another aspect, the present invention provides a martensitic structure in which all cross sections are tempered, the hardness of the surface layer portion is HV420 or less, and a tensile strength of 1600 N / when the tensile test is performed with the test piece No. 2 of JISZ2201. It is characterized by the above-mentioned, and the hardness of the center side is more uniform than surface layer.

Another embodiment of the present invention is a steel manufacturing apparatus which makes the hardness of a portion of the steel lower than the hardness of the other portions of the steel by heat-treating the strengthened steel, wherein the steel is produced by induction heating or direct current heating. And heating means for rapidly heating from the skin to a predetermined depth and cooling means for quenching the heated steel after a predetermined time after the heating, wherein the heating temperature in the heating step is equal to or higher than Ac1 transformation point. It is a manufacturing apparatus of.

According to another aspect of the present invention, there is further provided a control means for controlling the processing conditions of the heat treatment based on the heat transfer characteristics of the steel or the result of calculating the tempering progress value.

1 is an explanatory diagram schematically showing a configuration of a heat treatment apparatus according to a first embodiment of the present invention.
2 is an explanatory diagram schematically showing a heat treatment step of the embodiment.
3 is a side view illustrating the configuration of the PC steel bar in the embodiment.
4 is a table showing the components of the PC steel bar in the embodiment.
5 is a graph showing a relationship between the number of coil turns and a wire diameter in the heat treatment apparatus of the embodiment.
6 is a table showing heat treatment conditions of the embodiment and the conventional example.
7 is a graph showing the relationship between the elapsed time and the temperature distribution by the electrothermal analysis of the PC steel bar of the embodiment.
8 is a graph showing the relationship between the cooling time and temperature distribution by electrothermal analysis of PC steel bars of the embodiment.
9 is a graph showing the relationship between the hardness of the PC steel bar and the distance in the radial direction of the embodiment.
10 is a graph showing the temperature distribution from the surface layer portion to the center in the tempering process in the same embodiment.
11 is a graph showing the efficiency of the N parameter in the same embodiment.
12 is a graph showing the relationship between the ratio of the N parameter between the surface and the center, the heating process and the cooling time in the same embodiment.
It is a graph which shows the cross-sectional hardness distribution of the PC steel bar obtained by the heat processing which concerns on the same embodiment, and the conventional heat processing.
14 is a graph showing the sectional hardness distribution in the axial direction of the PC steel bar manufactured by the heat treatment of the embodiment.
FIG. 15 is a table showing components of plural kinds of PC steel bars used in the heat treatment of the embodiment. FIG.
Fig. 16 is a table showing the results of the delayed fracture test of the PC steel bar W by the heat treatment of the same embodiment and the conventional heat treatment.
17 is a graph showing the results of the same delay fracture test.
18 is a side view illustrating a configuration of the PC steel bar W1 of the embodiment.
It is a side view which shows the structure of the notch of PC steel bar W1 of the same embodiment.
20 is a table which shows the result of the delayed fracture test of the PC steel bar by the heat treatment of the same embodiment and the conventional heat treatment.
21 is a graph showing the results of the same delay fracture test.
22 is a graph showing the relationship between the depth of the notch and the break time formed on the PC steel bar W1 of the embodiment.
It is explanatory drawing which shows roughly the structure of the heat processing apparatus in other embodiment of this invention.
It is a side view which shows the structure of the mold release PC steel bar which concerns on 2nd Embodiment of this invention.
It is a table which shows the component of the mold release PC steel bar in the same embodiment.
26 is a table showing the heat treatment conditions of the heat treatment and the comparative heat treatment according to the embodiment.
FIG. 27 is a graph showing simulation results by electrothermal analysis of elapsed time and temperature change in heat treatment of a release PC steel bar according to the embodiment; FIG.
It is a perspective view which shows the structure of the spring steel wire which concerns on 3rd Embodiment of this invention.
It is a table which shows the component of the spring steel wire in the same embodiment.
30 is a table showing heat treatment conditions in the heat treatment and the comparative heat treatment according to the embodiment.
Fig. 31 is a graph showing simulation results by electrothermal analysis of elapsed time and temperature change in heat treatment according to the embodiment.
32 is a graph showing the cross-sectional hardness distribution of spring steel wires after treatment in the heat treatment and the comparative heat treatment according to the embodiment.
33 is a graph showing the relationship between the hardness and the fatigue limit of the heat treatment material.
34 is a table showing the results of the rotation bending fatigue test of the comparative heat treatment material.
35 is a graph showing the relationship between the distance of inclusions and the number of times of durability from the surface layer of the comparative heat treatment material.
36 is a table showing a rotation bending fatigue test result of a spring steel wire by heat treatment according to the embodiment.
37 is a graph showing the cross-sectional hardness, residual stress, and stress amplitude of the spring steel wire by the heat treatment according to the embodiment and the spring steel wire by the comparative heat treatment material.
38 is a table showing a component example of a spring steel wire according to another embodiment.
It is a perspective view which shows the structure of the bolt which concerns on 4th Embodiment of this invention.
40 is a table showing the components of the bolt according to the embodiment;
41 is a table which shows the heat treatment conditions in the heat treatment and the comparative heat treatment according to the embodiment.
FIG. 42 is a graph showing simulation results by electrothermal analysis of elapsed time and temperature change in heat treatment of bolt Wb in the embodiment. FIG.
43 is a graph showing the cross-sectional hardness distribution of bolts obtained by heat treatment and comparative heat treatment according to the embodiment.
Fig. 44 is a table showing the results of the delayed fracture test of the bolts obtained by the heat treatment according to the embodiment and the comparative heat treatment.
45 is a graph showing the cumulative breaking probability and breaking time of bolts obtained by the heat treatment according to the embodiment and the comparative heat treatment.

EMBODIMENT OF THE INVENTION Hereinafter, 1st Embodiment of this invention is described with reference to FIGS. 1 is a conceptual diagram of a heat treatment apparatus 10 according to the present embodiment. 2 is a flowchart of a manufacturing process of the PC steel bar in the present embodiment.

As shown in FIG. 1, the heat treatment apparatus 10 as an example of the manufacturing apparatus of steel materials includes the pinch roll 11 (conveying means) which conveys PC steel bar W which is an example of steel materials, the quenching heating coil 12 and quenching cooling jacket 13 which are quenching means. And a pinch roll 14 (conveying means), a tempering heating coil 15 as a heating means for high frequency induction heating, a cooling jacket 16 as a cooling means, and a pinch roll 17 (conveying means) are arranged along a straight conveying path. It has a function of heating and cooling while conveying PC steel bar (steel material) W along a conveyance path | route.

The PC steel bar W to be processed is, for example, a solid round bar as shown in FIG. 3, and is continuously transported along the axial direction. PC steel bar W is comprised, for example including the component shown in FIG. 4, but is not limited to this.

The tempering heating coil 15 has a function of high frequency induction heating of the PC steel bar W passing through. The tempering heating coil 15 is set to the appropriate number of coil turns in accordance with the wire diameter or the conveying speed. Although the number of turns of the tempering heating coil 15 in this embodiment is 6 turns, for example, it is not limited to this. Moreover, as a comparison object, the prior art which used the coil of 17 turns is illustrated. 5 shows the relationship between the general number of coil turns, the wire diameter, and the conveyance speed.

In tempering by high frequency induction heating, the PC steel rod W itself generates heat, but the depth of the heating part can be adjusted by the combination of the coil turns and frequency of the tempering heating coil 15, frequency, input electric energy, heating temperature, heating time, and cooling time.

The cooling jacket 16 has a function of cooling by spraying a cooling liquid on the PC steel bar W passing through.

The distance from the tempering heating coil 15 to the cooling jacket 16 is set to 500 mm or less, for example. In the conventional processing apparatus, this distance is about 1900 mm, but in this embodiment, the distance between them was set short in order to shorten the time from heat processing to cooling processing.

An example of the processing conditions of the heat processing in this embodiment is shown in FIG. This processing condition was discovered by the applicant using the temporal change of the heating temperature pattern and the tempering characteristics of the steel at the time of high frequency induction heating according to the principle described below. By continuously performing heat treatment under these treatment conditions, it is desirable to produce a steel bar W having a low hardness layer at the surface layer portion in one temper, having a uniform hardness distribution at a certain depth, and having a strength level of 1420 N / mm 2 or more. It becomes possible.

The processing conditions of this embodiment are the frequency of 50 Hz, the quenching heating temperature of 1000 degreeC, tempering heating temperature of 805 degreeC, tempering heating time 0.17s, and time from tempering heating to cooling 0.63s. The PC steel bar used here is a narrow diameter PC steel bar with a diameter d (common name) of 7.1 mm, and the tempering heating temperature is adjusted so that the tensile strength may be about 1440 N / mm 2.

The treatment conditions of the prior art to be compared are a frequency of 9.5 kHz, a quenching heating temperature of 1000 ° C, a tempering heating temperature of 603 ° C, a tempering heating time of 0.59s, and a time of tempering heating to cooling of 3.48s. This conventional product is also a narrow diameter PC steel bar with a diameter d (common name) of 7.1 mm, and the tempering heating temperature is adjusted so that the average tensile strength of all cross sections is about 1440 N / mm 2. In addition, the component of this embodiment and a conventional product are made into the same component.

That is, the tempering heating temperature of this embodiment is set higher than the conventional products made into comparison object, and the time from tempering heating to cooling is set shorter than the conventional products.

In addition, regarding the tempering temperature, heating to a tempering temperature above Ac1 transformation point (727 ° C) is conventionally considered to be "impossible because it becomes quenched". However, in this embodiment, rapid heating of the surface using high frequency induction heating is performed. By controlling the rapid cooling immediately after the end of overheating, it was found that it is possible to prevent quenching by combining rapid cooling even at a temperature higher than the Ac1 transformation point.

In the heat treatment apparatus 10 which concerns on this embodiment, based on the principle mentioned later, the feed rate by the pinch roll 11 etc., the heating temperature by the tempering heating coil 15, a heating time, the distance of the tempering heating coil 15, the cooling jacket 16, etc. are mentioned. By appropriately setting and adjusting, PC steel bar W which has desired hardness distribution can be obtained.

Hereinafter, the operation of the heat treatment apparatus 10 having the above configuration will be described with reference to the flowchart of FIG. 2. The continuous wire W0, which is drawn in the material process and is a continuous linear or rod-shaped steel, is continuously conveyed from the left side to the right side in FIG. 1 by the pinch roll 11 (transport means). The conveyed continuous wire W0 is rapidly heated to the quenching temperature by induction heating by the quenching heating coil 12 in the quenching step, and then quenched by the quenching coolant jet by the quenching cooling jacket 13 and continuously quenched.

The continuous wire W0 subjected to the quenching treatment is heated while passing through the tempering heating coil 15. The continuous wire W0 heated at a predetermined tempering temperature is conveyed to the cooling jacket 16 to which the cooling liquid is continuously injected, and rapid cooling is started by the cooling jacket 16. Passing through the cooling jacket 16, the entire length of the continuous wire W0 is cooled and the tempering heat treatment is completed. W0 is carried out by the pinch roll 17. After the heat treatment is completed, the continuous wire W0 (PC steel bar W) is processed and inspected to be a finished product (PC steel bar W).

Next, the principle for determining the processing conditions in the heat treatment will be described.

Below, the N parameter value (tempering progress value) which is a new parameter used as a reference which sets a process condition is demonstrated.

Fig. 7 shows simulation results by electrothermal analysis using a finite element model (FEM) of elapsed time and temperature change from the start of heating of any cross section of the steel in continuous heat treatment to 0.8 s. In six places from the surface to the center, the elapsed time is shown on the horizontal axis and the temperature on the vertical axis, respectively. In addition, since heating time is 0.17s here, 0.17s of the first (left side of a graph) is heating time among the 0.8s in a graph, and 0.63s of the right side of a graph is cooling time.

As a general tempering parameter, the Larson-Miller parameter P = T × (A + logt) [T: temperature (K), A: constant, t: time (h)] that is established when heated for a long time Known. The larger the value of this parameter P, the more the tempering has progressed (low hardness).

On the other hand, when the high frequency induction heating is for short time heating, for example, when the rapid heating process and the rapid cooling process in the high frequency heat treatment as shown in Fig. 7 are established, any N value can be set as a parameter. The N value (N parameter value) to define is time integration of the temperature of PC steel bar W, and is represented by following formula (1).

[1]

…(1)

... (One)

Here, T: temperature (° C), t: time (s), and t0: heat treatment time (s).

In other words, this N parameter value refers to the area enclosed below each curve in the graph. The larger the value of the N parameter value, the more the tempering has progressed (low hardness).

According to FIG. 7, when t0 is 0.8, that is, 0.8s from the start of heating, and the cooling time is 0.63s, when comparing the N parameter values represented by the area, the surface layer portion is larger than the N parameter value at the center portion. Moreover, at the depth of about 2 mm or more, the difference of N parameter hardly changes. Therefore, when it cools at this time, the surface layer part as shown in FIG. 9 is low hardness, and the state which has a substantially uniform hardness distribution from the low hardness layer end part to a center part can be implement | achieved. In addition, in the example of this embodiment, t0 was 0.8, but it is not limited to this, A suitable value can be applied according to various conditions by a line diameter, steel grade, etc.

8, the graph which shows the relationship between the cooling time and temperature distribution by the electrothermal analysis of the PC steel bar using FEM is shown. It is shown every 0.3s that r (distance from center) / R (radius) is shown on the horizontal axis, and the temperature is shown on the vertical axis. Analysis conditions were shape; solid round bar, radius; 3.65 mm, material: S40C, heating layer depth 0.154 mm, heating time 0.17 s, cooling time 0.63 s, initial temperature 20 degreeC. In addition, t shows the time from the start of heating corresponding to FIG. 7, and has shown the analysis result from 0.17s which starts cooling to 0.8s which completes cooling. Therefore, t = 0.8s corresponds to the cooling time 0.63s.

As shown in FIG. 8, heat caused by heat generation at the surface layer portion is transmitted to the center or the outside of the PC steel rod with time. In the initial stage of heating, the surface portion shows a high temperature distribution and the central portion exhibits a low temperature distribution. However, as time passes, the surface and center temperatures become uniform. At the time point of t = 0.8s, the surface layer temperature Ts = 430 ° C. and the center temperature = 429.1.

9, the cross-sectional hardness of the PC steel bar W is measured, and the position in the radial direction on the horizontal axis and the Vickers hardness on the vertical axis are shown. Hardness corresponds to tensile strength, and the surface layer part realizes the state which has a low hardness (low intensity | strength) and a substantially uniform hardness distribution from the low hardness layer end part to a center part.

Furthermore, according to FIG. 8, since it is in a crack state after 0.63 s, it turns out that the longer the time to cooling becomes, the more the difference of the N parameter value of a surface layer part and a center part disappears.

10 shows N parameter values and Vickers hardness after 0.8 s from the start of heating of the PC steel bar W. FIG. It can be seen that the N parameter value and the Vickers hardness are symmetrical and well matched, and the hardness can be summarized by the N parameter.

In summary, since the tensile strength of the PC steel bar W is determined by the N parameter value that is the tempering progression from the surface to the center, it can be seen that a certain range exists to satisfy the specification. In addition, it is understood that a steel material having desired characteristics can be obtained by controlling the time to temperature distribution and cooling so that the difference between the N parameter values between the surface layer portion and the center portion as large as possible within this range.

That is, by utilizing temporal change of the heating temperature pattern at the time of high frequency induction heating and tempering characteristics of the steel, in the continuous heat treatment, a single temper has a low hardness layer at the surface layer portion and uniform hardness distribution from a certain depth. The manufacturing method of the steel material which has tensile strength, for example, the strength of 1420 N / mm <2> or more can be implement | achieved.

In general, it is known that the lower the tensile strength, the better the delayed fracture resistance. That is, the PC steel bar W having a low hardness portion on the surface is a PC steel bar having both excellent delayed fracture resistance and predetermined tensile strength, and such PC steel bar W can be obtained by the above method.

That is, in high frequency tempering, the depth of a heat generating part can be adjusted by selecting an appropriate coil, frequency, input electric energy, heating temperature, heating time, and cooling time, and the pattern calculated by simulation can be implemented. Therefore, by adjusting the time to cooling in high frequency heating, only the surface layer part can be made low hardness, satisfying the specification of the tensile strength of the whole PC steel bar. In addition, the external heating of the radiation method such as furnace heating other than the high frequency heating does not increase the temperature rise in a short time and continues to heat slowly. Therefore, due to the tempering characteristics of the steel, it is not possible to make a difference in strength in the radial direction. The case has a short time such as 1 s or less, so that there is an advantage that the internal hardness other than the surface layer portion is made uniform.

On the basis of this N parameter, by using a temperature distribution unique to the high frequency heat treatment (difference of the N parameters), it is possible to adjust the hardness distribution of the surface layer portion with one temper and to satisfy the standard value of the tensile strength of the steel bar. High frequency heat treatment line can be realized.

For example, desired good surface layer softening can be achieved by setting the N parameter of the surface layer to be 1.5 times or more of the central N parameter.

In addition, N parameter can be used suitably with respect to steel materials of 100 kg / mm <2> grade or more, and the steel of the normal steel of C: 0.1 mass%-0.5 mass% is preferable from limitation of tempering temperature. As a range of the diameter for which N parameter acts as a principle, the range of 5 mm-40 mm is preferable, for example.

That is, the N parameter uses tempering of high strength steel, and overshoot by rapid heating peculiar to high frequency induction heating and rapid transition to crack heating furnace by heat transfer of steel. This is because it is a range where the desired surface layer softening can be achieved (the N parameter of the center and the surface layer is 1.5 times or more), and it is difficult to temper the whole to be uniform and within the standard strength. However, this invention is not limited to this range, It is applicable even if it is high frequency temporal phase and larger wire diameter or more exceeding this range.

In addition, this suggests a time constraint. That is, as shown in Fig. 11 showing the efficiency of the N parameter, it is understood that in the continuous heating line, cooling in a short time considering the state where the whole is tempered, for example, time of 0.8 s or less in Fig. 8 is preferable. Can be. That is, since the overshoot time is very short, when the N parameter value of the surface layer portion is not 1.5 times or more of the central portion, tempering proceeds to the parameters of Rason and Miller, as is well known, and therefore takes 10 s or more in industrial continuous heat treatment. If discarded, high strength cannot be ensured, resulting in a time constraint of the N parameter.

11 shows the relationship between the N parameter and the Vickers hardness. Since the N parameter value and hardness are determined by the equation shown in the figure, select the appropriate coil, frequency, input electrical energy, heating temperature, heating time, and cooling time, and estimate the N parameter value by simulation to obtain the target hardness. You can get it.

12 shows the N parameter ratios of the center and the surface layer portion in the PC steel bar W and the elapsed time from the start of heating. From the start of heating to about 1 s, the state of the parameter value was 1.5 times or more, and only the surface layer part had low hardness. In addition, the PC steel bar W obtained by the heat treatment method is a full-section tempered martensite structure.

13, the cross-sectional hardness distribution of PC steel bar W obtained by the said heat processing method is shown. In FIG. 13, the horizontal axis shows the distance from the surface layer, and the vertical axis shows the Vickers hardness. As shown in FIG. 13, while the cross-sectional hardness distribution of the conventional PC steel bar was uniform, it was confirmed that the PC steel bar according to the present invention had a low hardness near the surface layer but a uniform hardness at the center portion.

14, the cross-sectional hardness distribution of the PC steel bar manufactured by the heat processing of this embodiment is shown. For six locations with different distances from the surface layer, the Vickers hardness is shown on the vertical axis and the distance in the axial direction from the reference point on the horizontal axis, respectively. As shown in FIG. 14, it was confirmed that the hardness distribution of the PC steel bar W was almost constant in the axial direction.

Subsequently, as the PC steel bar W, the delayed fracture test results when the heat treatment was performed by the heat treatment method under the conditions shown in FIG. It is shown in FIG. In addition, among the steel grade A, the steel grade B, the steel grade C, and the steel grade D shown in FIG. 15, the steel grades A, B, and C used round PC rods, and the steel type D used the release PC steel rod. FIG. 17 is a graph showing the test results shown in FIG. 16, in which the breaking time is taken on the vertical axis and the cumulative breaking probability on the horizontal axis, and the longer the breaking time is, the better the delayed fracture resistance is.

The delayed fracture test was performed by applying a load of 1420 × 0.7 N / mm 2 while being immersed in a 20% NH ₄ SCN solution kept at 50 ° C.

As shown in FIG. 17, it was confirmed that various steel bars subjected to the heat treatment according to the present embodiment were superior in delayed fracture resistance than steel bars subjected to normal heat treatment.

In addition, it is shown from FIG. 17 that the upper part of the plot (that is, the longer break time) is superior in delayed fracture resistance. That is, in FIG. 17, it turns out that the PC steel bar processed on condition of this embodiment is more excellent in delayed fracture resistance than a conventional product.

Moreover, about the steel grade which was not broken also in the said delayed fracture test, the sample provided with the notch 20 was produced, and the delayed fracture test was done. The structure of PC steel bar W1 which provided the notch 20 is shown to FIG. 18, 19. FIG.

Subsequently, as the PC steel bar W1, when two kinds of steel types having different components shown in C and D shown in FIG. 15 are applied, the heat treatment is performed by the heat treatment method under the same conditions as the PC steel bar W shown in FIG. The delayed fracture test results are shown in FIG. 20. In addition, this cumulative breaking probability is shown in FIG.

The delayed fracture test was performed by applying a load of 1420 × 0.8 N / mm 2 while being immersed in a 20% NH ₄ SCN solution kept at 50 ° C. FIG. 21 shows the results shown in FIG. 20 in terms of the breaking time on the vertical axis and the cumulative breaking probability on the horizontal axis. The longer the breaking time, the better the delayed fracture resistance.

22 shows the relationship between the depth of the notch 20 formed in W1 and the breaking time. Even if the PC steel bar W1 provided with the notch 20, it was confirmed that the steel rod by this heat treatment is superior in delayed fracture resistance than the steel rod by the normal heat treatment. Average break time on the vertical axis and notch 20 depth on the horizontal axis. It turns out that the delayed-destructive improvement effect by this embodiment is falling rapidly from 0.4 mm in depth. Since the diameter of the PC steel bar W1 used this time was 7.2 mm, the effect of this embodiment was confirmed from the surface layer to 10% of the sample radius, ie, about 0.36 mm from the surface layer.

According to the PC steel bar, the heat processing method of a PC steel bar, and a heat processing apparatus which concern on this embodiment, the following effects can be acquired.

By fusing the surface heating by high frequency induction heating and the tempering performance of steel, the PC steel bar excellent in delayed fracture resistance can be obtained by a simple process. That is, by using temporal change and tempering characteristics of the temperature pattern at the time of heating, it is possible to obtain a steel material having different hardness depending on the site with a simple treatment in one temper satisfying the predetermined treatment condition.

By high frequency induction heating, it discovered the conditions which can make the surface layer soft enough which can apply the temperature more than 720 degreeC which was not applied because it is generally quenched. In addition, quenching tempering heat treatment by rapid high-speed heating by high frequency can obtain high strength and high toughness compared with the heat treatment by conventional furnace heating.

In addition, by using the simulation results as described above, by finding the heat treatment conditions based on the tempering characteristics and the heat transfer characteristics, appropriate conditions corresponding to all steel materials can be easily found.

In addition, this invention is not limited to the said embodiment itself, In an implementation step, a component can be modified and actualized in the range which does not deviate from the summary. For example, the specific processing conditions can be appropriately changed according to the steel grade of the steel material to be subjected, the required strength standard and hardness distribution, the specifications of the apparatus, and the like. Moreover, the process conditions of the heat processing to set are not limited to the above, either.

In addition to the heat treatment apparatus 10 of the first embodiment as in the heat treatment apparatus 101 shown in FIG. 23, the detection means 21 for detecting various kinds of information on the steel material to be treated and the finite element method are used in FIGS. Calculating means 22 for calculating simulation results as shown, control means 23 for controlling CPU 23 and other control means 23 for adjusting the respective conditions of the apparatus 101 according to the simulation results, and adjusting means 24 for adjusting various settings according to the control of the control means 23; And heat treatment conditions may be controlled based on the information of the corresponding steel materials. In this case, for example, the time from tempering to cooling can be controlled by adjusting the speed of the feed mechanisms of the pinch rolls 11, 14, 17 and the position of the cooling jacket 16. The control means 23 may also be configured to control other heat treatment conditions such as heating temperature and frequency. Also in this case, the same effects as in the first embodiment can be obtained. In addition, you may make it user input about the information of steel materials.

[Second Embodiment]

EMBODIMENT OF THE INVENTION Hereinafter, 2nd Embodiment of this invention is described with reference to FIGS. 24-27. In addition, since it is the same as that of the said 1st Embodiment except the point which made the steel material used as the process object as the release PC steel bar Wc, common description is abbreviate | omitted.

In addition, the heat treatment according to the present embodiment is a surface softening treatment, and the steel material (here, the release PC steel bar Wc) by this surface softening treatment is a surface softening material, and a heat treatment as a comparative example to be compared. Steel bars are called comparative heat treatment materials.

In this embodiment, as shown in FIG. 24, as a steel material, the release PC steel bar Wc which has the same spiral groove continuous to the surface layer was made into the processing object. The manufacturing apparatus and the manufacturing process were processed similarly to 1st Embodiment using the manufacturing process shown in FIG. 2 using the heat processing apparatus 10 shown in FIG. That is, since this embodiment only differs in the shape, component, diameter, etc. of the steel material to be processed, heat treatment conditions are determined by the same principle and the same parameters as in the first embodiment.

In this embodiment, although the case where the mold release PC steel bar Wc containing the component shown in FIG. 25 is used as a process object is not limited to this.

26, the heat processing conditions of the release PC steel bar Wc which concerns on this embodiment, and the heat processing conditions of the comparative heat processing made into a comparative object are shown. The heat treatment conditions of the release PC steel bar Wc of this embodiment (surface softening material) are frequency 50 Hz, quenching heating temperature 1000 degreeC, tempering heating temperature 805 degreeC, tempering heating time 0.17s, and time from tempering heating to cooling 0.63s. have. The release PC steel bar Wc used here is a release PC steel bar with a diameter db (common name) of 7.1 mm, and the total cross-sectional tensile strength is adjusted to be about 1400 N / mm 2.

Comparative processing conditions were a frequency of 9.5 kHz, a quenching heating temperature of 1000 ° C., a tempering heating temperature of 603 ° C., a tempering heating time of 0.59 s, and a time of tempering heating to cooling of 3.48 s. This comparative heat treatment material is also a release PC steel bar with a diameter of 7.1 mm, and is adjusted so that the total cross-sectional average tensile strength is about 1400 N / mm 2. In addition, the component of the release PC steel bar Wc of the surface softening material of this embodiment, and the component of a comparative heat treatment material are made into the same component.

27, the simulation result of the heat treatment elapsed time and temperature change relationship of the mold release PC steel bar Wc in this embodiment is shown. Here, the relationship between elapsed time and temperature according to the distance from the surface is shown.

As can be seen from FIG. 27, it is understood that the area of the oblique line portion in the figure is larger in the surface layer portion than in the central portion. That is, in this surface layer part, it will be in the state maintained at high temperature for a long time, and hardness will fall largely.

Also in this embodiment, the effect similar to the said 1st Embodiment can be acquired. That is, by fusion of surface heating by high frequency induction heating and tempering performance of steel, a release PC steel bar excellent in delayed fracture resistance can be obtained by a simple treatment. That is, by using temporal change and tempering characteristics of the temperature pattern at the moment of heating, it is possible to obtain a release PC steel bar having different hardness depending on the site with a simple treatment in one temper satisfying the predetermined treatment condition. In addition, quenching tempering heat treatment by rapid high-speed heating by high frequency can obtain high strength and high toughness compared with the heat treatment by conventional furnace heating.

[Third embodiment]

EMBODIMENT OF THE INVENTION Hereinafter, 3rd Embodiment of this invention is described with reference to FIGS. 28-37. In addition, since it is the same as that of the said 1st Embodiment except the point which used the spring steel wire Ws as an example of the steel material to be processed, common description is abbreviate | omitted.

In addition, the heat treatment which concerns on this embodiment is surface softening, the steel material by this surface softening (here spring steel wire Ws) is a surface softening material, and the heat processing as a comparative example compares heat treatment, and the spring steel wire by this comparative heat treatment is called a comparative heat treatment material. It is called.

In this embodiment, the steel material to be processed is set to the spring steel wire Ws shown in FIG. 28. The heat processing apparatus and the manufacturing process were the same as that of 1st Embodiment, and while using the heat processing apparatus 10 shown in FIG. 1, it processed in the manufacturing process shown in FIG. That is, this embodiment only differs in the component, diameter, etc. of the steel material to be processed. Therefore, heat treatment conditions are determined using the same principle and the same parameters as in the first embodiment.

In this embodiment, although the case where the spring steel wire Ws comprised including the component shown in FIG. 29 is used as a process target is illustrated, it is not limited to this.

30, the heat processing conditions of the heat processing which concerns on this embodiment, and the comparative heat processing as a comparative example are shown. The heat treatment conditions of the surface softening treatment according to the present embodiment are a frequency of 50 Hz, a quenching heating temperature of 950 ° C, a tempering heating temperature of 789 ° C, a tempering heating time of 0.4s, and a time of 2.6s from tempering heating to cooling.

The spring steel wire Ws used here is a spring steel wire of diameter ds = 12.0mm, and is adjusted so that the total cross-sectional tensile strength may be about 1900 N / mm <2>.

The comparative heat treatment conditions as a comparative example are frequency 9.5 Hz, quenching heating temperature 950 degreeC, tempering heating temperature 495 degreeC, tempering heating time 1.7s, and time from tempering heating to cooling 11.1s.

The comparative heat treatment material is also a spring steel wire having a diameter of 12.0 mm, and is adjusted so that the total cross-sectional average tensile strength is about 1900 N / mm 2. In addition, the spring steel wire Ws of this embodiment and a comparative heat treatment material are made into the same component.

31, the simulation result of the heat treatment elapsed time of the spring steel wire Ws and temperature change relationship in this embodiment is shown. Here, the relationship between elapsed time and temperature according to the distance from the surface is shown.

As can be seen from FIG. 31, it is understood that the area of the oblique line portion in the figure is larger in the surface layer portion than in the center portion. That is, in this surface layer part, it will be in the state maintained at high temperature for a long time, and hardness will fall largely.

32 shows the distribution of the distance from the surface layer and the hardness. The hardness [HV0.3] is shown on the vertical axis, and the distance [mm] from the surface layer is shown on the horizontal axis. In FIG. 32, the hardness distribution of the spring steel wire (comparative heat treatment material) with which the heat processing as a comparative example was performed, and the spring steel wire Ws (surface softening material) processed by the heat processing conditions of this embodiment are shown, respectively.

In the comparative heat treatment material, even when the distance from the surface layer changes, the hardness hardly changes. On the other hand, in the spring steel wire Ws which heat-treated according to this embodiment, it turns out that it changes so that hardness may become large as distance from a surface layer becomes large near surface layer. That is, Hv <500 is set in the range of 1 mm from the surface layer.

In addition, as shown in FIG. 33, in general, when hardness [Hv]> 500, a premature breakage starting from an inclusion becomes easy to occur when a fatigue test is performed. Therefore, in the spring steel wire Ws (surface softening material) which softened the surface layer by processing on the heat processing conditions of this embodiment, a fatigue characteristic improves.

34 shows the relationship between the distance from the surface layer to the inclusions and the number of times of durability as a result of the rotation bending fatigue test of the comparative heat treatment material as the comparative example. 35 shows graphs of the relationship between the distance from the surface layer to the inclusions and the number of times of durability in the comparative heat treatment material. 34 and 35, in the comparative heat treatment material, the tendency of the endurance frequency increases as the distance from the surface layer to the inclusion is increased.

36 shows the relationship between the distance from the surface layer to the inclusions in the spring steel wire Ws and the number of times of durability as a result of the rotational bending fatigue test of the spring steel wire Ws (surface softening material) treated according to the present embodiment.

The results of the rotation bending fatigue test all exemplify the case where the rotation bending fatigue test is performed after shot peening. Test conditions were made into the stress amplitude of 700 MPa, and 2000 rpm. The test was performed up to 10 million times. "> 1000" in a graph shows that it was not broken after 10 million times.

When comparing FIG. 34 and FIG. 36, in the spring steel wire Ws (surface softening material) of this embodiment, compared with the spring steel wire (comparative heat treatment material) of a comparative example, the durability number is largely increasing, and the fatigue characteristic is improving. It can be seen that. In addition, in the spring steel wire Ws (surface softening material) of this embodiment, it is recognized that all the breaks generate | occur | produce from a surface layer as a starting point.

37 is a graph showing the distribution of cross-sectional hardness, residual stress, and stress amplitude. The distance [mm] from a surface layer is shown on a horizontal axis. Hardness [HV0.3], stress amplitude [MPa], and residual stress [MPa] are shown on the vertical axis.

All of the residual stress distributions have the same distribution, and the maximum compressive stress was exhibited in the vicinity of 0.1 mm from the surface layer, and the tensile stress was shown in 0.2 mm or more.

As can be seen from this graph, since the hardness is high in the comparative heat treatment material and the toughness is low in the case of the same stress amplitude, premature breakage occurs based on inclusions within the range of 0.2 to 1.0 mm from the surface layer where the residual stress is tensile.

On the other hand, in the spring steel wire Ws of this embodiment which is a surface softening material, since the hardness of a surface is low and toughness is high, the generation | occurrence | production of a fatigue crack starting from an interference | inclusion is suppressed also in the range close to the said surface layer of about 0.2-1.0 mm. For this reason, the endurance frequency improves significantly.

Also in this embodiment, the effect similar to the said 1st Embodiment can be acquired. That is, by fusing the surface heating by high frequency induction heating and the tempering performance of steel, the spring steel wire excellent in delayed fracture resistance can be obtained by a simple process. That is, by using temporal change and tempering characteristics of the temperature pattern at the time of heating, it is possible to obtain a spring steel wire having different hardness depending on the site with a simple treatment by one tempering satisfying the predetermined treatment condition. In addition, quenching tempering heat treatment by rapid high-speed heating by high frequency can obtain high strength and high toughness compared with the heat treatment by conventional furnace heating.

Moreover, according to the spring steel wire Ws manufactured by this embodiment, by softening in the range of 0.2-1.0 mm from residual surface layer which is tensile, the effect which makes it difficult to produce the early break which originates from an inclusion can be acquired.

In addition, in this embodiment, although the component shown in FIG. 29 was shown as an example, it is not limited to this. As another example, the steel grade E-the steel grade I containing the component as shown in FIG. 38 are mentioned, for example. Also in these steel materials, it is possible to soften the vicinity of the surface layer by the same treatment as described above, to obtain high strength and high toughness, and to prevent premature breakage based on inclusions. In addition, the lowermost part of the table | surface of FIG. 38 has shown the each component contained in the illustrated spring steel wire in the range.

[Fourth Embodiment]

EMBODIMENT OF THE INVENTION Hereinafter, 4th Embodiment of this invention is described with reference to FIGS. 39-45. In addition, since it is the same as that of the said 1st Embodiment except the point which used the bolt Wb as an example of the steel material to process, common description is abbreviate | omitted.

In addition, the heat treatment which concerns on this embodiment is surface softening treatment, the bolt Wb by this surface softening treatment is surface softening material, the heat processing as a comparative example is comparative heat treatment, and the bolt by this comparative heat treatment is called comparative heat treatment material.

In this embodiment, the bolt Wb shown in FIG. 39 was made into the process object. The manufacturing apparatus and the manufacturing process are the same as that of 1st Embodiment, and while using the manufacturing apparatus shown in FIG. 1, it processed by the manufacturing process shown in FIG. That is, in this embodiment, since only the shape, component, diameter, etc. of the steel material to be processed differ, the heat treatment conditions are determined by the same principle and the same parameters as in the first embodiment.

In this embodiment, although the case where the bolt Wb containing the component shown in FIG. 40 is used as a process object is not limited to this.

41 shows heat treatment conditions of the bolt Wb in the heat treatment and the comparative heat treatment according to the present embodiment. The heat treatment conditions of this embodiment (surface softening material) are frequency 50 Hz, quenching heating temperature 1000 degreeC, tempering heating temperature 780 degreeC, tempering heating time 0.15s, and time from tempering heating to cooling 0.61s.

The bolt Wb used here is a bolt with a diameter of db = 7.1 mm and the total cross-sectional tensile strength is adjusted to be about 1600 N / mm 2.

Comparative heat treatment conditions were a frequency of 9.5 Hz, a quenching heating temperature of 1000 DEG C, a tempering heating temperature of 480 DEG C, a tempering heating time of 0.61 s, and a time of tempering heating to cooling of 3.50 s. This comparative heat treatment material is also a bolt of 7.1 mm in diameter, and is adjusted so that the total cross-sectional average tensile strength is about 1600 N / mm 2. In addition, the component of the bolt Wb of the surface softening material of this embodiment, and the component of a comparative heat processing material are made into the same component. In addition, any heat treatment shall be performed before rolling a screw to the bar material Wb1 of a bolt.

42 shows simulation results of the relationship between the elapsed time of heat treatment of the bolt Wb and the temperature change in the present embodiment. Here, the relationship between elapsed time and temperature according to the distance from the surface is shown.

As can be seen from FIG. 42, it is understood that the area of the oblique portion is larger in the surface layer portion than in the center portion. That is, in this surface layer part, it will be in the state maintained at high temperature for a long time, and hardness will fall largely.

43, the hardness distribution of the bolt (comparative heat treatment material) processed by the comparative heat treatment conditions as a comparative example, and the bolt Wb (surface softening material) processed by the heat treatment conditions of this embodiment, respectively. The hardness [HV0.3] is shown on the vertical axis, and the distance [mm] from the surface layer is shown on the horizontal axis.

In FIG. 43, even when the distance from the surface layer of the comparative heat treatment material changes, the hardness hardly changes. On the other hand, in the bolt Wb (surface softening material) which performed the heat processing which concerns on this embodiment, it turns out that it changes so that hardness may become large as distance from a surface layer becomes large in the vicinity of a surface layer. That is, Hv <500 is set in the range of 1.0 mm from the surface layer.

In the bolt Wb (surface softening material) treated under the heat treatment conditions of the present embodiment, the delayed fracture characteristic is improved as compared with the comparative heat treatment material.

44, the table which compared the delayed fracture test result of the bolt Wb (surface softening material) of this embodiment and a comparative heat treatment material is shown. Here, the delayed fracture test was performed after screw rolling on the entire length of the bar material Wb1 of the bolt Wb. The depth of the screw was 0.7 mm. The conditions of the delayed fracture test were made to be immersed in a 20% NH ₄ SCN solution, and the test temperature was 50 ° C., and a tensile strength of 1530 N / mm 2 × 0.7 was applied to the threaded portion. The test was carried out up to 200 hours using the load carrying method and the constant strain method. ">200" shows the thing which does not break even after 200 hours.

45 is a graph showing the relationship between the cumulative breaking probability and breaking time of the surface softener and the comparative heat treatment material of the present embodiment. A cumulative breaking probability [%] is shown on the horizontal axis and a breaking time [h] is shown on the vertical axis.

44 and 45 show that the comparative heat treatment material breaks at 30 to 130 hours, whereas the surface softener does not break even after 200 hours or more in all samples. Therefore, improvement of delayed fracture characteristic is recognized by performing surface layer softening process.

Also in this embodiment, the effect similar to the said 1st Embodiment can be acquired. That is, by fusing the surface heating by high frequency induction heating and the tempering performance of steel, the bolt which is excellent in delayed fracture resistance can be obtained by a simple process. That is, by using temporal change and tempering characteristics of the temperature pattern at the time of heating, it is possible to obtain bolts having different hardness depending on the site by a simple treatment with one tempering satisfying the predetermined processing conditions. In addition, quenching tempering heat treatment by rapid high-speed heating by high frequency can obtain high strength and high toughness compared with the heat treatment by conventional furnace heating.

In addition, in this embodiment, although the component shown in FIG. 40 was shown as an example, it is not limited to this.

In addition, this invention is not limited to each said embodiment itself, In an implementation step, a component can be modified and actualized in the range which does not deviate from the summary. For example, specific processing conditions can be changed as appropriate depending on the shape, steel type, component, required strength standard and hardness distribution, specification of the apparatus, and the like of the steel material to be subjected. Moreover, the process conditions of the heat processing to set are not limited to the above, either.

As an example, in the first to fourth embodiments, the PC steel bar, the release PC steel bar, the spring steel wire, and the bolt are exemplified, respectively. The concept of the present invention is not limited to these exemplified steel grades, and is widely applicable to other kinds of steel materials.

In addition, in each embodiment, although the case where tempering was performed to the surface softening by performing the tempering treatment on the steel high-strength by quenching process, in addition, using the steel high-strength by the process of drawing, stretching, carburizing, etc. Also good.

Moreover, various inventions can be formed by suitable combination of the some component disclosed by said each embodiment. For example, some components may be deleted from all the components shown in the embodiment. In addition, you may combine suitably the component which concerns on other embodiment.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the steel material, the manufacturing method of steel materials, and the manufacturing apparatus of steel materials from which hardness differs according to a site | part by a simple process.

Claims (15)

A method of manufacturing a steel material having a martensite structure in which the entire cross section is formed by subjecting a hardened steel material to tempering to lower the hardness of a portion of the steel than that of the other portions of the steel,
The tempering treatment includes: a tempering heating step of heating the surface of the steel to a predetermined depth or more by an Ac1 transformation point by induction heating or direct energizing heating; And a cooling step of cooling the steel that has passed through the tempering heating step after a predetermined time elapses after the tempering heating step. The method for producing steel according to claim 1, wherein the time from the heating step to the cooling step is equal to or less than a predetermined time determined by steel grade, wire diameter, heating temperature, and heating time. The method for manufacturing steel according to claim 1 or 2, wherein the treatment conditions for the tempering treatment are determined based on heat transfer characteristics after rapid heating of the surface of the steel. The steel fabrication method according to claim 1 or 2, wherein the processing conditions of the tempering treatment are time integral values of the temperature of the steel, and are determined based on a tempering progress value indicating a tempering progress state of the steel. Way. The method for producing steel according to claim 1 or 2, wherein the treatment conditions for the tempering treatment include a combination comprising at least two of frequency, input electric energy, heating temperature, heating time, and cooling time. The method of claim 4, further comprising the step of calculating the heat transfer characteristics or the tempering progress value of the steel material,
The processing conditions of said tempering treatment are determined on the basis of said calculated heat transfer characteristics or said tempering progress value. The method for producing steel according to claim 4, wherein the processing conditions for the tempering treatment are set such that the tempering progress value at the surface layer portion is equal to or greater than 1.5 times the tempering progress value at the center portion. The steel manufacturing method according to claim 4, wherein the time from the heating step to the cooling step is set such that the tempering progress value at the surface layer portion is equal to or greater than 1.5 times the tempering progress value at the center portion. The method of claim 1, wherein the steel is linear or rod-shaped. The steel material according to claim 1 or 2, wherein after the quenching treatment including heat treatment and cooling treatment is performed on the steel, the heating step and the cooling step are performed once as tempering. Manufacturing method. As said steel material which passed through the said heating process and the said cooling process of Claim 1 or 2,
The hardness difference near the surface layer portion and the hardness at the center side from the position of 10% from the surface layer in the radial direction are HV50 or more, and the tensile strength when the tensile test is conducted with the test piece No. 2 of JISZ2201 is characterized by being 1420 N / mm 2 or more. Steel. As said steel material which passed through the said heating process and the said cooling process of Claim 1 or 2,
The whole cross section is composed of a tempered martensite structure, the hardness of the surface layer portion is HV380 or less, and the tensile strength when the tensile test is conducted with the test piece No. 2 of JISZ2201, and the tensile strength is 1420 N / mm 2 or more,
The hardness of the center side is more uniform than the surface layer. As said steel material which passed through the said heating process and the said cooling process of Claim 1 or 2,
The whole cross section is composed of a tempered martensite structure, the hardness of the surface layer portion is HV420 or less, and the tensile strength when the tensile test is conducted with the test piece No. 2 of JISZ2201, and the tensile strength is 1600N / mm 2 or more,
The hardness of the center side is more uniform than the surface layer. An apparatus for producing steel, in which the entire cross section is made of martensitic structure, in which the hardness of a portion of the steel is lower than that of the other portions of the steel by subjecting the quenched steel to tempering.
Tempering heating means for heating from the skin of the steel to a certain depth by more than the Ac1 transformation point by induction heating or direct energizing heating;
And a cooling means for cooling the steel heated by the tempering after a predetermined time elapses after the tempering heating. 15. The method according to claim 14, wherein the time from the end of the processing in the heating means to the start of the processing in the cooling means is equal to or less than a predetermined time determined by steel grade, wire diameter, heating temperature, and heating time,
The tempering treatment conditions include at least two of heat transfer characteristics after rapid heating of the surface of the steel, a tempering progress value indicating a tempering progress state of the steel, a frequency, an input electric energy, a heating temperature, a heating time, and a cooling time. Determined on the basis of the combination,
The processing conditions of the tempering treatment are set so that the tempering progress value of the surface layer portion is equal to or greater than 1.5 times the tempering progress value of the center portion,
The time from the heating step to the cooling step is set to a predetermined time or less determined by steel grade, wire diameter, heating temperature, heating time,
The steel is linear or rod-shaped;
A conveying means for continuously conveying the steel material along a predetermined path passing through the heating means and the cooling means, wherein the processing condition includes a distance and a conveying speed in a conveying path,
And a quenching means for performing heat treatment and cooling treatment on the steel,
The heating means and the cooling means are provided one each downstream of the quenching means in the conveying path,
And control means for controlling the processing conditions of said tempering treatment based on the heat transfer characteristics of said steel or the result of calculating said tempering progress value.

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