JPS626614B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing case-hardened steel with extremely low transformation strain and thermal strain, that is, heat treatment strain. In general, case hardening steels include SCr415 and JIS standards.
Steels with a C content of 0.13 to 0.25%, such as SCr420, SCM415, and SCM420, are often used, and these steels are usually carburized at 900 to 950°C and then quenched at around 850°C. It is manufactured by. However, in the production of conventional case-hardened steel, the core quenching treatment after carburizing involves quenching not only the carburized surface but also the core completely from the austenitic phase, so the structure after quenching is martensitic. As a result, the transformation strain remaining in the case hardening steel becomes extremely large. Therefore, in order to reduce the transformation strain, it may be possible to prevent the core structure after quenching from becoming a martensitic structure, but in this case, especially if the C content in the core is Since the content is as low as 0.13 to 0.25%, the core hardness rapidly decreases and the fatigue strength also decreases, making it impossible to put it to practical use in mechanical structures. Further, in manufacturing the conventional case hardened steel, since quenching is performed from a high temperature as described above, thermal strain is also generated. Therefore, from the above-mentioned viewpoint, the present inventors conducted research to obtain a case hardened steel with less heat treatment strain. During the cooling process, or by reheating after cooling to below _A1 point, the steel core is adjusted to a structure consisting of austenite + ferrite phases, and the steel carburized surface is composed of austenite phases, and then quenched from this state. , since austenite, which is a part of the austenite + ferrite structure in the core that makes up the main part of the steel, only needs to transform into martensite or bainite, the generation of transformation strain will be avoided when the entire conventional austenite phase turns into martensite. The size is significantly smaller than that in the case of transformation, and since quenching from the above-mentioned structure state is quenched at a relatively low temperature, the occurrence of thermal strain is also relatively small. They found that this can be prevented by increasing the C content. This invention was made based on the above findings, and includes C: 0.35 to 0.50%, Si: 0.25% or less, Mn: 0.3 to 1.3%, Cr: 0.6 to 1.5%, sol.Al:
0.02 to 0.06%, N: 0.006 to 0.015%, further containing V: 0.05 to 0.5% as necessary, and the remainder consisting of Fe and unavoidable impurities (weight% above). After carburizing the steel in the austenitic region, the core of the steel after carburizing becomes an austenite + ferrite phase, either during the cooling process after carburizing or by reheating after cooling to below _A1 point (usually room temperature). and the carburized surface part is adjusted to a structure composed of austenite phase,
By quenching from this state, case hardened steel having a predetermined core hardness and less heat treatment strain is produced. In this invention, the steel core consists of austenite + ferrite phases, while the steel carburized surface consists of an austenite _phase . In the case of eutectoid or eutectoid, Ae ₃ points, in case of hypereutectoid, Acm point) or higher, and at a specified temperature within the temperature range of Ae ₃ points or less, which is the equilibrium transformation temperature of the core, for 20 minutes or more. It can be formed by holding. Next, in the method for producing case hardened steel of this invention,
The reason why the composition of the steel is limited as described above will be explained. (a) C The C content is higher than the C content of normal case hardening steel: 0.13-0.25% in order to ensure the hardness of the steel core.
Contains 0.35-0.50%, but its content is 0.35%
If the hardness is less than 1, the core structure of normal case hardened steel is martensite, but the core structure of the case hardened steel of this invention is ferrite + martensite (bainite). If the content exceeds 0.50%, the machinability, cold workability, and toughness will decrease rapidly. Therefore, its content is 0.35
It was set at ~0.50%. (b) Si Si is essential for deoxidizing during melting, but if it is contained in excess of 0.25%, not only will carburizability deteriorate, but surface grain boundary oxidation during carburizing will also occur. The upper limit was set at 0.25%. (c) Mn Mn is not only indispensable as a deoxidizing and desulfurizing agent during melting, but also a necessary component to ensure hardenability. The effects of the ingredients are not fully exerted, and on the other hand,
If the content exceeds 1.3%, the cold workability and machinability will deteriorate rapidly, so the content was set at 0.30 to 1.3%. (d) Cr The Cr component has the effect of increasing the wear resistance of the carburized surface and improving the strength and toughness of the core, but if its content is less than 0.6%, the desired effect will not be achieved. On the other hand, if the content exceeds 1.5%, the carburized surface portion will be excessively carburized and the pitting resistance will deteriorate, so the content was set at 0.6 to 1.5%. (e) sol.Al Al acts as a deoxidizing agent during melting and combines with N to form AlN, which has the effect of suppressing grain coarsening during carburizing. .Al
If the content of sol.Al is less than 0.02%, the desired effect cannot be obtained; on the other hand, if the content of sol.Al exceeds 0.06%, the crystal grains tend to coarsen, and Al ₂ O ₃ It started to be produced in large quantities,
Since the machinability deteriorates, the content should be reduced from 0.02 to
It was set at 0.06%. (f) N N combines with Al to form AlN and has the effect of suppressing crystal grain coarsening during carburizing, but if its content is less than 0.006%, the desired effect of suppressing crystal grain coarsening is ensured. On the other hand, if the content exceeds 0.015%, the cold workability will deteriorate rapidly, so the content was set at 0.006 to 0.015%. (g) V V has austenite+ core after carburizing treatment.
In the process of holding at a predetermined temperature for a predetermined time to create a ferrite structure and carburized surface part to an austenite structure, V carbonitrides are precipitated as finely dispersed V carbonitrides in the core, which has the effect of improving the hardness of the core. Therefore, it is included as necessary, especially when high hardness is required for the core part, but if the content is less than 0.05%, the desired hardness improvement effect cannot be obtained, whereas if the content exceeds 0.5%, the desired effect of improving hardness cannot be obtained. Even if it is, the effect of further improving the hardness does not appear and the state becomes saturated, so the content was determined to be 0.05 to 0.5%, taking economic efficiency into account. Incidentally, when the steel according to the present invention contains one or more of Ca, P, and S, the free machinability is further improved without any deterioration of the properties. Next, the method of the present invention will be specifically explained using examples. Example: Steels having the compositions shown in Table 1 were manufactured by ordinary melting and casting methods, and from these steels the shape shown in the front view in Figure 1a and in the side view in Figure 1b was produced. Outer diameter with: 55mmφ x inner diameter: 35
C-ring specimens of mmφ x thickness: 10mm x notch opening width: 6mm were cut out, and then these specimens were charged into a gas carburizing furnace, furnace temperature: 930°C, carbon potential: 0.75%, processing time. : Carburizing was carried out for 6 hours, and the structure was adjusted during the cooling process after carburizing, or by reheating after cooling to below ₁ point A, and adjusting the structure under the same temperature and time conditions shown in Table 1. , by immediately quenching in oil at 60°C.
Case hardened steels 1 to 13 of the present invention, comparative case hardened steels 1 to 4, and conventional case hardened steels 1 and 2 were manufactured, respectively. Comparative case hardening steels 1 and 2 have the same composition as inventive case hardening steel 4, but were manufactured under extremely high temperature conditions during structure adjustment, and comparative case hardening steel 3 was , which also has the same composition as the present invention case hardening steel 4, but was manufactured under conditions where the retention time during structure adjustment was shortened. Therefore, both comparative case hardening steels have the structure defined in the present invention. It has been hardened without having any moreover,
Comparative case hardening steel 4 has a composition in which the C content is low and outside the range of the present invention. In addition, conventional case hardening steels 1 and 2 are JIS/
It has a composition equivalent to SCr420 and SCM420. Next, the amount of displacement of the notch opening in the various case-hardened steel specimens obtained as a result was measured and shown in Table 1. Table 1 also shows the Vickers hardness of the core of the thick wall portion of the sample. As is clear from the results shown in Table 1, comparative case hardening steel 1 whose manufacturing conditions were outside the scope of the present invention.
~4 and conventional case hardening steels 1 and 2 are inferior in either displacement or hardness, whereas the present invention

[Table] Light-hardened steels 1 to 13 all have small displacements;
It not only has less heat treatment distortion, but also
It shows a Vickers hardness of 320 or higher, which indicates that it can be fully used for practical use in mechanical structures. In addition, FIG. 2 shows the case hardening steel 4 of the present invention.
The changes in displacement and hardness are shown when the temperature during structure adjustment, which is the quenching temperature, is varied. In addition,
The hardness of the carburized surface in Figure 2 is 0.05 lower than the surface.
It is at a depth of mm. As shown in Figure 2, the temperature during structure adjustment, which is the quenching temperature, was set at _three points Ae on the carburized surface and at the core.
When the temperature is within the range of Ae ₃ points, the carburized surface part is austenite phase and the core part is austenite +
Since the structure is composed of ferrite phase,
It is clear that it is possible to secure enough hardness for practical use in mechanical structures with relatively little thermal strain. As described above, according to the present invention, it is possible to produce case hardened steel that has extremely small heat treatment distortion and has carburized surface hardness and core hardness that are sufficiently practical for use in mechanical structures. It is.

[Brief explanation of the drawing]

Figure 1 shows a C-ring specimen, and Figure a is a front view;
FIG. 2B is a side view, and FIG. 2 is a diagram showing the relationship between the quenching temperature, the opening displacement amount, and the hardness of the C-ring specimen.

Claims (1)

[Claims] 1 C: 0.35-0.5%, Si: 0.25% or less, Mn: 0.3-1.3%, Cr: 0.6-1.5%, sol.Al: 0.02-0.06%, N: 0.006-0.015%, After carburizing the steel in the austenitic region of the steel, the steel has a composition (by weight % or more) containing Fe and unavoidable impurities, and then the carburized surface structure is changed to the austenite phase, and the steel core structure is changed to the austenite phase. A method for producing case-hardened steel with little heat treatment strain, which is characterized by adjusting the structure to an austenite + ferrite phase and quenching from this state. 2 Contains C: 0.35 to 0.5%, Si: 0.25% or less, Mn: 0.3 to 1.3%, Cr: 0.6 to 1.5%, sol.Al: 0.02 to 0.06%, N: 0.006 to 0.015%, and further, V: 0.05 to 0.5%, and the rest is Fe and unavoidable impurities (wt%). After carburizing the steel in the austenitic region, the steel carburized surface structure changes to the austenitic phase. , and a method for producing case hardened steel with little heat treatment strain, characterized by adjusting the steel core structure to an austenite + ferrite phase and quenching from this state.