JPS6041687B2 - Manufacturing method of case hardened steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized by the fact that there is little variation in quality without annealing or normalizing treatment after rolling, and even in subsequent high-temperature surface hardening treatment such as carburizing treatment or carbonitriding treatment. The present invention relates to a method for producing case-hardened steel for mechanical structures, which has stable characteristics with few abnormal structures such as coarse grains or mixed grains.

Low carbon Scr used for case hardening among mechanical structural steels specified in JISG4051, 4103, etc.
42O, SCM42噸, etc. are easily affected by the thermal history during hot rolling processing, and there is a problem that the crystal grains tend to become coarse during subsequent high-temperature heat treatment such as carburizing or carbonitriding. After being subjected to annealing or normalizing treatment after rolling, it is used as a material for various mechanical structural parts.

However, in recent years, from the perspective of energy saving, it has been considered to omit the annealing or normalizing treatment after hot rolling, and at the same time rolling at a relatively low temperature to suppress variations in quality after rolling. The use of so-called controlled rolling is being considered, but it is possible to reduce the variation in quality after rolling and to reduce the coarsening of crystal grains and mixed grains during high-temperature heat treatment such as carburizing or carbonitriding after forming mechanical structural parts. Controlled rolling conditions that do not generate abnormal structures Yes/Not yet established.

'Therefore, the present inventors have attempted to establish controlled rolling conditions that do not require post-rolling annealing or normalizing treatment and that provide stable quality characteristics when manufacturing case-hardened steel for machine structures. As a result of conducting experiments, we found that the above technical problems can be solved all at once by controlling the heating temperature of the material, the final finishing temperature, and the cooling rate after rolling.

That is, the present invention has (1) a weight ratio of C: 0.13 to 0.23%, Si: 0.
15-0.35%, Mn: 0.60-1.00%, Cr
:0.90~1.20% and as necessary MO:0.15
950~1 steel consisting of ~0.35% and the balance essentially Fe
100. After raising the temperature to C, the final finishing temperature is 800℃.
2000C/M after continuous hot rolling at temperatures above ℃
A method for manufacturing case-hardened steel for machine structures, characterized in that the temperature is lowered to 600°C at a cooling rate of less than 100°C.

(2) The method for producing case hardened steel for machine structures according to claim 1, wherein the area reduction rate in the temperature range of 800 to 920° C. during hot rolling is 50% or less.

By adopting the manufacturing method of the present invention, the variation in hardness after rolling is suppressed to ±2 or less in terms of HRB hardness, resulting in extremely homogeneous rolling, and even in the subsequent high-temperature heat treatment (carburizing or carbonitriding treatment), there is little coarsening of crystal grains. JISGO
No.55L with grain size number. 5 or more stable fine grain steel can be produced. The case-hardening steel for machine structures that is the object of the present invention is chromium steel (G4) specified by JIS.
lO4), chromium molybdenum steel. Next, the reasons for limitations in each step of the production method of the present invention will be described below.

Heating temperature of material: 950-1100°C If the temperature during rolling heating is too high, crystal grains will grow and a rolled steel material with fine crystal grains will not be obtained, and furthermore, subsequent high-temperature heat treatment (carburizing or carbonitriding) will occur. This is not preferable because it causes generation of coarse crystal grains and mixed grains.

In detailed experiments conducted by the present inventors, the rolling heating temperature was 1100°C.
It was confirmed that the crystal grains would hardly become coarse by controlling the final finishing temperature and cooling rate if the temperature did not exceed 1100°C, so the temperature was limited to 1100°C or less. Further, if the temperature during rolling and heating is too low, an excessively large rolling blade will be required during rolling, so the rolling heating temperature was set at 950° C. or higher. Final finishing temperature: 8000C to 9500C If the final finishing temperature in rolling is too low, a rolled material having fine crystal grains can be produced, but on the other hand, the crystal grains tend to become coarse in the subsequent high temperature heat treatment.

According to detailed experiments by the inventors, the final finishing temperature was 800.
It was confirmed that by controlling the subsequent cooling rate, the occurrence of abnormal structures due to coarsening of crystal grains in the high-temperature heat treatment after rolling can be almost prevented, as long as the temperature does not fall below 'C. The temperature was limited to 800°C or higher. On the other hand, if the final finishing temperature exceeds 950° C., flaws are likely to occur on the surface of the steel material during the period from final finishing to winding in the case of wire rods, and during transportation after finishing in the case of steel bars.

Cooling rate after rolling: 200'C/Min up to 600°C
Below: If the cooling rate after rolling is too fast, a uniform microstructure will not be obtained, resulting in variations in hardness of the rolled material, and it will also cause grain coarsening in the subsequent high-temperature heat treatment, which is not preferable.

Detailed experiments by the present inventors have shown that by lowering the temperature in the temperature range up to 600°C after the end of rolling at a slow cooling rate of 200°C/Min or less, a nearly uniform microstructure can be obtained after high-temperature heating. Because of this, the cooling rate after rolling was limited to 200° C./Min or less. As described above, by controlling the heating temperature, finishing temperature/degree, and cooling rate during rolling, the objective can almost be achieved. By performing hot rolling on the material, the effect of the method of the present invention becomes more noticeable in the saw layer. The features of the method of the present invention will be explained in detail with reference to Examples. [
Example 1 A sample material of SCM42O was CO. 2O%, SlO. 23%, MnO. 73%,
Crl. O2%, MOO. 2l%, as a sample material of Scr42O. 2O%, SlO. 25%, MnO. 7
2%, Crl. A copper material having a chemical composition of 5% O was melted to produce a billet with a square cross section of 110 TrOn.

Using the above-mentioned BUILT, the effects of heating temperature, end-of-rolling temperature, cooling rate, and area reduction rate of the material were investigated.

Effect of rolling heating temperature 110 Tr$L When hot rolling a square billet, 900
Bars with a diameter of 30 TIrm were manufactured by heating to various temperatures of ~1200° C. and hot rolling.

The rolling end temperature was adjusted to 800°C or higher in all cases, and the cooling rate thereafter to 600°C was also adjusted to 200'C/Min or lower. The variation in hardness of the manufactured bars was investigated. The results are shown in Table 1. As shown in the same table, when the rolling heating temperature is high, the hardness of the rolled material is high and the variation is large, but at a temperature of 1100°C or less, the variation in hardness is extremely small and a homogeneous rolled material can be obtained. ing.

However, when the heating temperature is 900°C, it is insufficient for rolling, so it was confirmed that the suitable heating temperature is between 950 and 1100°C. Next, the rolled material was subjected to high-temperature heat treatment (930°C x 511r), and the coarsening tendency of crystal grains was investigated. The results are also listed in Table 1. As seen in the same table, it is clearly recognized that the crystal grains of the material heated and rolled at a temperature of 1150° C. or higher become coarser due to subsequent high temperature heating. On the other hand, a material heated and rolled at a temperature not exceeding 1100°C will not produce any coarse grains even during the subsequent high-temperature heat treatment.
It has fine crystal grains of O7 or more. As described above, it has been confirmed that the heating temperature during rolling has a large effect on the homogenization of the hardness of the rolled material and on the coarsening of crystal grains during high-temperature heating. Effect of end-of-rolling temperature 1S
During hot rolling of the CM42-11CyD0n square billet, it was heated to various temperatures of 1,000 to 1,100°C and rolled, and the end-of-rolling temperature was also controlled to produce bars with a diameter of 3 mm.

The cooling rate up to 600°C after rolling is all 200°C.
The temperature was adjusted to be below ℃/Min. Table 2 shows the results of examining the variation in hardness of the manufactured bars.

As seen in the same table, it is shown that the end-of-rolling temperature does not have much effect on the hardness of the rolled material and the variation in hardness.

Next, the above-mentioned material was subjected to high temperature heat treatment (930'C x 5llr) corresponding to carburizing treatment, and the coarsening tendency of crystal grains was investigated.

The results are also listed in Table 2. As seen in the same table, when the end-of-rolling temperature is lower than 800°C, there is a tendency for crystal grains to coarsen in the subsequent high-temperature heat treatment, whereas when the end-of-rolling temperature is lower than 800°C, In all cases, the grain size number is NO. 7
This shows fine crystal grains.

As mentioned above, even if the heating temperature during rolling is controlled to 1100'C or less, if the rolling end temperature is lower than 800'C, the grains will coarsen in the subsequent high temperature heat treatment, so the rolling end temperature will be 800'C or less. It was confirmed that the score must be C or higher. Effect of cooling rate SCM42O. ．． During hot rolling of a 110Tf0n square billet of SCr42O steel, it was heated to a temperature of 1050°C and rolled to a diameter of 30WfL, and then the temperature range from the end of rolling temperature to 600J0C was rolled at 100°C/M.
The diameter of
? A rolled bar was manufactured.

Note that the rolling end temperature was 850'C in all cases. Table 3 shows the results of examining the variation in hardness of the manufactured bars. As shown in the same table, the rolled material with a cooling rate of 250°C/Min after rolling has a large variation in hardness and a high average hardness.
It was confirmed that all the materials rolled at a speed lower than 0°C/m'1n had very little variation in hardness and had stable quality characteristics.

Next, the above-mentioned material was subjected to high-temperature heat treatment (930°C x 511r) corresponding to carburizing treatment, and the coarsening tendency of crystal grains was investigated. The results are also listed in Table 3. As shown in the same table, the rolled material with a cooling rate of 250°C/Min after rolling has SCM42O. It is shown that coarse crystal grains are generated in both SCr42O and the subsequent high-temperature heat treatment, but no coarse crystal grains are observed in any of the rolled materials whose cooling rate after rolling is 200'C/Min or less. It shows. As mentioned above, it was confirmed that the cooling rate after rolling has a large effect on the variation in the hardness of the rolled material and on the coarsening of crystal grains during high temperature heating, so it was confirmed that it is necessary to slowly cool the material at a cooling rate of 200°C/Mjn or less. . Effect of area reduction rate, SCM42
O. ．． SCr42OlIIu billet (110? square)
During hot rolling, the rolling is heated to a temperature of 1000'C, the hot rolling end temperature is 800'C or higher, and the diameter is 30m! FL bars were manufactured.

In this example, the area reduction rate was varied in the temperature range of 920 to 800°C, and
The temperature was lowered at a cooling rate of 0C/Min or less. The produced bar material is subjected to high temperature heat treatment (930
℃ x 5 hours) to investigate the coarsening tendency of crystal grains.

The results are shown in Table 4. As shown in the same table, in rolled materials with an area reduction rate of 60% in the rolling heating temperature range of 920 to 800°C, the grains tend to become coarser during subsequent high-temperature heating, but the maximum The grain size is NO. It is a fine grain of 5 or more. On the other hand, the area reduction rate is 5
In the rolled materials with 0% or less, no coarsening of crystal grains was observed even after subsequent high-temperature heating, and in both cases the grain size number was NO. It shows a fine grain size of 7 or more.

As mentioned above, the area reduction rate during rolling has a large effect on the coarsening of crystal grains, so
It was confirmed that it is preferable to control the area reduction rate to 50% or less in the temperature range of .

Claims (1)

[Claims] 1. C: 0.13 to 0.23%, Si: 0.1 in weight ratio
5-0.30%, Mn: 0.60-1.00%, Cr:
95% steel consisting of 0.90~1.20% and the balance essentially Fe
After raising the temperature to a temperature of 0 to 1100°C, hot rolling is performed at a final finishing temperature of 800°C or higher and 950°C or lower,
A method for producing case-hardened steel for machine structures, characterized in that the temperature is subsequently lowered to 600°C at a cooling rate of 200°C/min or less. 2 C: 0.13-0.23%, Si: 0.1 in weight ratio
5-0.30%, Mn: 0.60-1.00%, Cr:
After heating the steel consisting of 0.90 to 1.20%, Mo: 0.15 to 0.30%, and the remainder substantially Fe to a temperature of 950 to 1100°C, the final finishing temperature is 800°C or more and 950°C or less. Perform hot rolling processing, followed by 200℃/min
A method for producing case hardened steel for mechanical structures, characterized by cooling down to 600°C at the following cooling rate: 3. A method for producing case hardened steel for machine structures according to claim 1 or 2, characterized in that the area reduction rate in the temperature range of 800 to 920°C during hot rolling is 50% or less. .