JPS58113316A - Manufacture of case hardening steel having low heat treatment strain - Google Patents

Abstract

PURPOSE:To obtain a case hardening steel having an extremely low heat strain by carburizing steel at an austenitic region and treating so that structures of the carburized surface and the core part are changed to an austenitic phase and an austenite + ferrite phase respectively before quenching. CONSTITUTION:The steel containing, by weight %, 0.35-0.50 C, <=0.25 Si, 0.3- 1.3 Mn, 0.6-1.5 Cr, 0.02-0.06 sol.Al, 0.006-0.015 N, if necessary 0.05-0.5 V and the balance Fe with inevitable impurities is carburized at the austenitic region. Then the carburized steel is reheated at the stage of cooling after carburization or after cooling to below A1 point so that the core part and the carburized surface part are controlled to the structures constituted of the austenite + ferrite phase and the austenitic phase respectively. This steel is quenched from this state and the case hardening steel having the specified core hardness and low heat treatment strain can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION 1- The present invention relates to a method for producing case hardened steel with extremely low transformation strain and thermal strain, that is, heat treatment strain.

Generally, as case hardening steel, 5Cr415, J Engineering S standard,
5Cr420.80M415. and C content specified as 80M420 etc. is 0.13-0.25%
Steels of 900 to 9 are commonly used.
It is manufactured by carburizing at 50°C and then quenching at around 850°C.

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 hardened 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 after carbon treatment, or by reheating after cooling to below A1 point, the alignment part is adjusted to a structure consisting of austenite + ferrite phase and the steel carburized surface area is composed of austenite phase. When quenched, the 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. It is significantly smaller than when it transforms to martensite, and since quenching from the above structure is done at a relatively low temperature, the generation of thermal strain is also relatively small, and furthermore, the hardness caused by the low temperature quenching in the core part is relatively small. It was discovered that the decrease in the carbon content 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.
I Mn: 0.3-1.3%, cr: 0
．． 6-1.5%.

130/, M: 0.02-0.06%, N: 0
．． 0.006 to 0.015%, and optionally V: 0.05 to 0.5%, with the remainder consisting of Fe and unavoidable impurities (wt%), After carburizing the steel in the austenitic region, during the cooling process after carburizing, or below the A1 point (usually at room temperature)
After being cooled to a temperature of 100%, the steel is reheated to adjust the structure so that the core of the carburized steel is composed of austenite + ferrite phases, and the carburized surface is composed of austenite phases, and then quenched from this state. The present invention is characterized in that it produces a case hardened steel having a predetermined core hardness and less heat treatment strain.

In addition, in this invention, the alignment part consists of austenite + ferrite phase, while the steel carburized surface part consists of austenite phase, which is the equilibrium transformation temperature of the carburized surface part A.
s3 point or Acm point (As3 point if the carburized surface is hypoeutectoid or eutectoid, Acm point if hypereutectoid), Ae, which is the equilibrium transformation temperature of the core, and the bottom of the fish body. It can be formed by holding the temperature at a predetermined temperature within the temperature range of 20 minutes or more.

Next, the reason why the component composition of the steel is limited as described above in the case hardening steel manufacturing method of the present invention will be explained.

(a) C The C component is 0.3%, which is higher than the C content of ordinary case hardening steel: 0.13 to 0.25%, in order to ensure the hardness of the alignment part.
Contains 5-0.50%, but its content is 0.35%
If the hardness is less than 1, the core structure of normal case hardened steel is made of martensite, but the core structure of the case hardened steel of this invention is ferritic martensite (bainite). This makes it difficult to obtain the strength required for mechanical structural steel;
If the content exceeds 0.3%, the machinability, cold workability, and toughness will drop sharply, so the content was set at 0.35 to 0.50%.

(b) 5i Sl is essential for deoxidizing during melting, but 0.
If the content exceeds 25%, the carburizability will deteriorate, and surface grain boundary oxidation will be promoted during the carburizing process, so the upper limit was set at 0.25%. .

(c) Mn Mn is not only essential as a deoxidizing and desulfurizing agent during melting, but also a necessary component to ensure hardenability, but if its content is less than 0.304, the above-mentioned The effect of the Mn component is not fully exerted, and if it is added in excess of 1.3%, cold workability and machinability will rapidly deteriorate. It has been designated as Section 1.3.

(d) Cr The Cr component increases the wear resistance of the carburized surface and
It has the effect of improving the strength and toughness of the core, but if the content is less than 0.6%, the desired effect cannot be obtained, while if the content exceeds 1.5%, the carburized surface part Since the pitting resistance deteriorates due to excessive carburization, the content was determined to be 0.6 to 1.5 inches.

(e) θo4. q M acts as a deoxidizing agent during melting and combines with N to produce 7J
It has the effect of suppressing crystal grain coarsening during carburizing by generating N, but if the content is less than 0.02% in sot and u, the desired effect cannot be obtained;
eot, A1 contains more than 0.0696.
As crystal grains begin to show a tendency to coarsen, AA
, O5 is produced in large amounts, deteriorating machinability, so the content was set at 0.02 to 0.06%.

(f) N N combines with M to form fiJ, N, and has the effect of suppressing crystal grain coarsening during carburizing, but when its content is 0.0
If the content is less than 0.06%, the desired crystal grain coarsening suppressing effect cannot be ensured, while if the content exceeds 0.015%,
Since cold workability rapidly deteriorates, the content of O, OO was determined to be 6 to 0.015%.

(b)) ■ ■ After carburizing, the core part becomes an austenite + ferrite structure, and the carburized surface part is held at a predetermined temperature for a predetermined time to make an austenite structure. ■It precipitates as carbonitride and has the effect of improving the hardness of the core, so it is included as necessary when high hardness is required for the core, but the content is 0.0
If it is less than 5%, the desired hardness improvement effect cannot be obtained, while if it is less than 0.
Even if the content exceeds 5%, no further hardness improvement effect will be obtained and the condition will be saturated, so considering economic efficiency, the content should be set at 0.05 to 01'5%. Established.

Note that 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.

EXAMPLES Steels having the compositions shown in Table 1 were produced by the usual melting method and casting method, respectively. From these steels, the front view is shown in FIG. Outer diameter with the shape shown in the figure: 55. lφ×inner diameter: 35ilIφX
Thickness: Lolong x Notch opening width: 6 C-ring specimens of III were cut out, and then these specimens were charged into a gas carburizing furnace, furnace temperature: 93°C, carbon potential: 0.75.
%, treatment time: Carburizing was performed under the conditions of 6 hours, and during the cooling process after carburizing, or once cooled to below the A1 point and then reheated, under the conditions of temperature i and time shown in Table 1. The case hardened steels 1 to 13 of the present invention were prepared by adjusting the structure and immediately quenching in oil at 60°C. Comparative case hardening steel l~4.
and conventional case hardened steel 1.2 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 microstructure adjustment was shortened. Therefore, all of the comparative case hardening steels do not have the microstructure defined in the present invention. It has been hardened in its original state. moreover,
Comparative case hardening steel 4 has a composition in which the C content is low and outside the range of the present invention.

Still, conventional case hardening steels 1 and 2 are J-S and 5Cr4, respectively.
It has a composition corresponding to SCM 20 and SCM 420.

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 steels 1 to 4 and conventional case hardening steels 1 and 2, whose manufacturing conditions were outside the scope of the present invention, had either displacement or hardness. In contrast, case hardening steels 1-13 of the present invention not only have a small amount of displacement and little heat treatment strain, but also exhibit a Vickers hardness of 320 or more, making them suitable for use in mechanical structures. It can be seen that it is fully usable for practical use.

Further, FIG. 2 shows changes in the amount of displacement and hardness when the temperature at the time of structure adjustment, which is the quenching temperature, was variously changed for case hardening steel 4 of the present invention. The hardness of the carburized surface in FIG. 2 is at a position 0.1 depth below the surface.

As shown in Fig. 2, when the temperature during structure adjustment, which is the quenching temperature, is within the range of the Ae3 point of the carburized surface and the A03 point of the core, the carburized surface has an austenite phase. Since the core has a structure composed of austenite and ferrite phases, it is clear that a hardness sufficient for practical use as a mechanical structure can be secured 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, in which (a) is a front view and (a) is a front view.
b) is a side view, and FIG. 2 is a diagram showing the relationship between the quenching temperature and the opening displacement amount and hardness of the C-ring specimen. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo Procedural Amendment (Imported) October 27, 1980 Director of the Patent Office Kazuo Wakasugi 2 Title of Invention Process for producing case-hardened steel with low heat treatment distortion 3 , Representative of the person making the amendment: Norihiro Kumagai 4, Agent 7 Contents of the amendment As shown in the attached sheet, the content of the amendment (1) The statement "075%" on page 9, line 12 of the specification, rO, 75%,” he corrected. (21 specification, page 12, lines 9-10 r O, 1i
Correct the statement +m J to r 0.05 m* J. (3) Table 1 on page 11 of the specification is amended as shown in the attached sheet. that's all

Claims (1)

[Claims] C: 0.35-0.50%, Si: 0.25%
Below, Mn: 0.3 to 1.3%, Cr: Q, 6
~1.5%+eot, AL:0.02~0.06%, N
: Contains 0.006 to 0.015%, further contains V : 0.05 to 0.5% as necessary, and the remainder is Fe.
After carburizing a steel having a composition (more than % by weight) consisting of and unavoidable impurities in the austenite region of the steel, the structure of the carburized surface part of the steel is adjusted to an austenite phase, and the structure of the alignment part is adjusted to an austenite + ferrite phase. A method for manufacturing case-hardened steel with little heat treatment strain, which is characterized by quenching from this state.