JPH0717945B2 - Manufacturing method of carburizing steel - Google Patents

Description

Detailed Description of the Invention

 Purpose of the invention

[Industrial applications]

The present invention relates to an improvement in a method for producing carburizing steel, and provides a method for producing carburizing steel capable of obtaining a case hardening steel product by performing carburizing treatment while preventing occurrence of so-called abnormal spots.

[Prior art]

In the production of mechanical structural parts that have been subjected to surface treatment by carburization, the structure may become coarse during carburization, and localized giant crystals may be generated. Since the portion where the giant crystals are concentrated can be recognized with the naked eye, it is called an "abnormal spot", which is an undesirable phenomenon. This is because the hardenability becomes too high and the quenching strain becomes large in the region where the local giant crystals exist, and even if the strain is attempted to be corrected, the material may even break. Conventionally, carburizing was performed at a temperature of 920 to 930 ° C,
Recently, in order to shorten the processing time and to obtain a sufficient carburizing depth, a high temperature of 950 to 970 ° C is often used. The occurrence of abnormal spots is an even greater problem when performing this high temperature carburization. The measures that have been taken so far for this problem are:
(1) The composition of the alloy contains a structure refining element (Nb, Ti, etc.), (2) the control of rolling conditions, and (3) a combination of these, but the effect is still satisfactory. Not really. In particular, when the wire is processed and carburized, the usual process is rolling at a relatively high temperature of 1200 ° C or higher, and in some cases 76
Since the order of spheroidizing annealing at around 0 ° C is followed by drawing and carburizing, the processing strain caused by drawing, which is cold working, becomes the driving force for grain coarsening during carburization. Abnormal spots tend to occur.

[Problems to be Solved by the Invention]

The present invention proposes a solution to one of the problems described above, and its object is to provide a carburizing steel such that a case-hardening steel product having a fine grain structure is always obtained even when high temperature carburization is performed. To provide. Structure of the invention

[Means for solving problems]

The method for producing carburizing steel of the present invention is a structural steel listed below, sol.Al: 0.025 to 0.050% and N: 0.017 to 0.025%.
%, With the balance essentially Fe, hot-worked under temperature conditions where the heating temperature is less than 1000 ° C and the finishing temperature is 750 ° C or more and less than 900 ° C, then cold-working It is characterized by Although there is a wide range of steels to which this manufacturing method can be applied, typical ones are structural steels containing the following alloying elements. (SC and SMN) C: 0.07 to 0.61%, Si: 0.15 to 0.35% and Mn: 0.30 to 1.80
% (SCM) C: 0.13-0.48%, Si: 0.15-0.35%, Mn: 0.30-1.00%,
Cr: 0.8 to 1.5% and Mo: 0.08 to 0.45% (SNC) C: 0.10 to 0.40%, Si: 0.15 to 0.35%, Mn: 0.35 to 0.80%,
Ni: 0.9-4.5% and Cr: 0.20-1.00% (SNCM) C: 0.12-0.50%, Si: 0.15-0.35%, Mn: 0.30-1.2%, N
i: 0.40 to 4.50%, Cr: 0.40 to 3.50% and Mo: 0.15 to 0.7
% Chrome steels containing the following alloying elements are also covered by the present invention. (SCR) C: 0.13 to 0.48%, Si: 0.15 to 0.35%, Mn: 0.60 to 0.85% and Cr: 0.7 to 1.2% In addition to the above, if desired, the carburizing steel of the present invention is , Ti: 0.02 to 0.20% and / or Nb: 0.01 to 0.
By adding 10%, it is possible to expect further microstructural refinement effect. Also, if necessary, S: 0.02-0.30%, Se: 0.02-0.30
% And Te: 0.003 to 0.10%, or one or more of them may be added to improve machinability. In the case of cold working, particularly drawing, if a heat treatment (low temperature annealing) at a temperature of A _c1 point or lower is performed prior to that, further favorable results can be obtained.

[Action]

The present invention, AlN is involved in the occurrence of abnormal spots,
It is based on the knowledge obtained by the inventor for the first time. That is, under the condition that hot rolling is performed by heating to a high temperature of about 1200 ° C., which was typical in the production of conventional carburizing steel, AlN is almost solid-solved in the matrix, and it is It was found that when a precipitate was formed with a decrease in the temperature of, a small number of the previously precipitated AlN particles became nuclei and grew into large particles, which caused local enlargement of austenite grains. Therefore, in the present invention, with the intention of not causing the phenomenon of solid solution of AlN → precipitation → grain growth, AlN precipitates and is processed while being dispersed as fine particles, and the austenite crystal grains are locally enlarged. The cause was removed. (If there are many AlN nuclei and the amount of new precipitation is small,
It will be easily understood that the nucleus does not grow. ) It was found that 200 ppm or more of AlN in steel is necessary to realize this intention. The lower limits of sol.Al:0.025% and N: 0.017% are the amounts necessary to form this AlN. Al
If it is present in a large amount, the cleanliness will be impaired, and N will cause the generation of blowholes. Therefore, 0.050% for Al and 0.025% for N
The upper limit of is set. Within this range, better results are obtained when N is somewhat richer than the equivalent. As for hot working conditions, the heating temperature of less than 1000 ° C and the finishing temperature of 750 ° C or more and less than 900 ° C are
It was chosen to take advantage of N's behavior.

Example 1 A rolled material of SCM420 steel having the following composition is heated to raise the temperature, C: 0.21% Cu: 0.12% Si: 0.28% Ni: 0.07% Mn: 0.80% Cr: 1.16% P: 0.013% Mo: 0.16% S: 0.021% In the process, what was kept at various temperatures for 30 minutes was water-cooled.
The amount of AlN deposited was examined for each material, and the results shown in FIG. 1 were obtained. From now on, AlN will precipitate once at 800-1000 ° C.
It was found that almost solid solution occurs at 00 ℃ or higher, so to avoid solid solution of AlN, hot rolling should be performed from a relatively low heating temperature of 1050 ℃ or less, especially less than 1000 ℃. The same steel is kept for 10 hours in the heating temperature range of high temperature (1150 ° C or higher) or low temperature (1050 ° C or lower) according to the pattern shown in Fig. 2 and is water-cooled at a lower finishing temperature equivalent temperature for 30 minutes. Was heat treated. The results of examining the occurrence of abnormal spots for each sample are shown in FIG. From the data in Figure 2, when the heating temperature is high, the finishing temperature is
It can be seen that abnormal spots occur below 850 ° C, and when the heating temperature is low, the abnormal spots can be avoided unless the finishing temperature is lower than 750 ° C.

Example 2 SCR420 steel having the following composition was prepared by using C: 0.20% Ni: 0.05% Si: 0.20 Cr: 1.09 Mn: 0.78 Mo: 0.02 P: 0.015 sol.Al:0.029 S: 0.009 N: 0.019 Cu: 0.10. O: 0.0013 The wire rod was rolled under the following conditions. Abnormal spots were observed in the comparative example having a high heating temperature. The above No. 2 and No. 3 rolled materials were processed according to the steps of wire drawing (WD ₁ ) -low temperature annealing (650 ° C) -wire drawing (WD ₂ ). No abnormal spots were observed in the combinations in which the reduction rate of WD ₁ was within 20% and the reduction rate of WD was within 30%. The wire rod obtained in this way is processed into a roller for a bearing, and the temperature is 970 ℃.
It was carburized by a high temperature gas carburizing method for 6 hours to obtain a case-hardening steel product. A roller with no abnormal spots was obtained. EFFECTS OF THE INVENTION The carburizing steel produced according to the present invention is abnormal not only in the manufacturing process but also in the subsequent carburizing process, especially when carburizing at high temperature, which causes local enlargement of austenite crystal grains. There are no spots. Therefore, a case-hardened steel product having no defects can be obtained without causing an abnormally large quenching strain. The present invention relates to various mechanical parts, such as gears for automobiles,
It is especially meaningful when applied to the manufacture of handle racks, roller bearings, etc.

[Brief description of drawings]

FIG. 1 is a graph showing the behavior of precipitation-solid solution with respect to the temperature change of AlN in SCM steel. FIG. 2 is a graph showing the state of occurrence of abnormal spots when the same steel is heat-treated in a pattern corresponding to various heating temperatures and finishing temperatures in accordance with hot rolling conditions.

Claims (10)

[Claims] 1. C: 0.07 to 0.61%, Si: 0.15 to 0.35% and M
n: 0.30 to 1.80% and sol.Al:0.025 to 0.050% and
Steel containing N: 0.017 to 0.025%, the balance being essentially Fe, with a heating temperature of less than 1000 ° C and a finishing temperature of 750 ° C.
A method for producing carburizing steel, which comprises hot working while suppressing solid solution and precipitation of AlN in steel by working under a temperature condition of not less than 900 ° C., and then cold working. 2. The method according to claim 1, which includes heat treatment at a temperature of A _c1 point or less. 3. C: 0.13 to 0.48%, Si: 0.15 to 0.35%, Mn: 0.
Steel containing 30 to 1.00%, Cr: 0.8 to 1.5% and Mo: 0.08 to 0.45%, as well as sol.Al: 0.025 to 0.050% and N: 0.017 to 0.025%, with the balance being essentially Fe. , Heating temperature is 10
By working under the temperature condition of less than 00 ° C and finishing temperature of 750 ° C or more and less than 900 ° C, hot working while suppressing solid solution and precipitation of AlN in steel, and then cold working A method for producing steel for carburizing. 4. The method according to claim 3, which includes heat treatment at a temperature of A _c1 point or less. 5. C: 0.10 to 0.40%, Si: 0.15 to 0.35%, Mn: 0.
Steel containing 35 to 0.80%, Ni: 0.9 to 4.5% and Cr: 0.20 to 1.00% as well as sol.Al: 0.025 to 0.050% and N: 0.017 to 0.025% with the balance being essentially Fe. , Heating temperature is 10
By working under the temperature condition of less than 00 ° C and finishing temperature of 750 ° C or more and less than 900 ° C, hot working while suppressing solid solution and precipitation of AlN in steel, and then cold working A method for producing steel for carburizing. 6. The production method according to claim 5, which includes heat treatment at a temperature of A _c1 point or less. 7. C: 0.12-0.50%, Si: 0.15-0.35%, Mn: 0.
30 ~ 1.2%, Ni: 0.40 ~ 4.50%, Cr: 0.40 ~ 3.50% and M
O: 0.15-0.7% plus sol.Al: 0.025-0.050% and N:
By processing steel containing 0.017 to 0.025% and the balance being essentially Fe under the temperature conditions where the heating temperature is less than 1000 ° C and the finishing temperature is 750 ° C or more and less than 900 ° C. Hot working while suppressing solid solution and precipitation of AlN,
Then, a method for producing carburizing steel, which comprises cold working. 8. The method according to claim 7, which includes heat treatment at a temperature of A _c1 point or less. 9. C: 0.13-0.48%, Si: 0.15-0.35%, Mn: 0.
60 ~ 0.85% and Cr: 0.7 ~ 1.2% plus sol.Al:0.025
〜0.050% and N: 0.017〜0.025%, the balance is steel consisting essentially of Fe under the temperature condition that heating temperature is less than 1000 ℃ and finishing temperature is 750 ℃ or more and less than 900 ℃. A method for producing carburizing steel, which comprises hot working while suppressing solid solution and precipitation of AlN in the steel, and then cold working. 10. The method according to claim 9, which includes heat treatment at a temperature of A _c1 point or less.