JPH02173241A - Case-hardening steel and its production - Google Patents

Abstract

PURPOSE:To efficiently manufacture a case-hardening steel excellent in fatigue strength, etc., by applying carburizing and quenching to a case-hardening steel with a specific composition, subjecting the above steel to spheroidizing annealing in a carburizing atmosphere while specifying temperature, holding time, and cooling velocity, respectively. CONSTITUTION:A case-hardening steel which has a composition consisting of, by weight, 0.10-0.30% C, 0.03-0.50% Si, 0.30-1.80% Mn, 0.30-1.80% Cr, and the balance Fe with inevitable impurities and containing, if necessary, 0.05-0.35% Mo is carburized and quenched. Subsequently, the above steel is held in a carburizing atmosphere at a temp. between Ac1 and Ac1+(Ac3-Ac1)X0.4 for 2-4hr and cooled slowly at 20-30 deg.C/hr cooling rate to undergo spheroidizing annealing (where Ac1 and Ac3 mean the Ac1 transformation point and the Ac3 transformation point of a base phase, respectively). Further, the above steel is held at a temp. between Ac3 and (Ac3+30 deg.C) for a short time and then quenched, by which the case-hardening steel which consists of a base phase composed of martensite containing spheroidal cementite and a carburized layer having a composition consisting of 0.50-1.50% C and the balance Fe with inevitable impurities and a structure composed of martensite containing spheroidal cementite can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a carburized case-hardened steel for mechanical parts such as gears and shafts used in automobiles, civil engineering machines, industrial machines, etc., and a method for manufacturing the same.

[Prior Art] Conventionally, in carburized case-hardened steel commonly used for gears and shafts of automobile parts, the amount of C in the matrix is low (low (0)).
, 15-0.30% by weight (hereinafter weight% is simply referred to as %)
In addition, it was attempted to improve the fatigue strength to some extent by carburizing the surface layer to improve the toughness of the base material and the wear resistance of the surface layer. Therefore, neither the matrix phase nor the carburized layer contains spheroidized cementite, which will be described later, and in its manufacture, only carburizing and quenching was performed.

Such carburized case-hardened steel generally does not have sufficiently high fatigue strength (toughness, wear resistance, etc.). In particular, the fatigue strength is not high enough to meet the recent demands for higher engine output.

Therefore, several proposals have been made to further increase this fatigue strength.
Publication No. 3 discloses a carburizing heat treatment method for forming pseudo-spherical carbides (spheroidized cementite) on the surface of steel materials, and Japanese Patent Application Laid-Open No. 59-35630 discloses a carburizing heat treatment method for forming pseudo-spherical carbides (spheroidized cementite) on the surface of steel materials. A method for heat treatment of gears is disclosed, which includes steps of removing only coarse carbides, performing direct quenching or air-cooling followed by heating quenching, and polishing tooth surfaces.

In the techniques described in these publications, fatigue strength is improved to a certain extent by forming spherical carbides on the surface of the steel material.

[Problems to be Solved by the Invention] However, in the carburized case-hardened steels disclosed in the above-mentioned publications (■) and (■), although spheroidized carbides are formed on the surface of the steel material, cementite in the matrix is formed throughout the steel structure. However, since it did not have a large amount of spheroidized particles, a satisfactory fatigue strength could not yet be obtained.

The reason for this is that it was difficult to spheroidize cementite in the matrix of carburized case-hardening steel, and it was not known that spheroidizing cementite in the matrix improves fatigue strength. It is possible that

In addition, the technique described in the above-mentioned Publication (2) in particular has the disadvantage that the manufacturing process is complicated and the manufacturing cost is too high.

Therefore, the present inventor conducted a detailed study on carburized case-hardened steel, which has a fatigue strength superior to that of conventional steels, and found that by spheroidizing high-hardness cementite with a high C concentration and distributing it in martensite, C in martensitic matrix
It was found that the wear resistance of the martensitic carburized layer increases as the toughness of the matrix increases and the wear resistance of the martensitic carburized layer increases.

The present invention was made against the background of the above-mentioned circumstances, and its main purpose is to provide a carburized case-hardened steel with excellent fatigue strength and an efficient manufacturing method thereof.

[Means for Solving the Problems] The first invention for solving the above problems is characterized by having a carburized layer shown below (B) on the surface of the parent phase shown below (A).

(A) In weight%, C: 0.10-0.30%, Si:
0.03-0.50%, Mn: 0.30-1.80
%, Cr: 0.30 to 1.80%, Mo: 0.05 to 0.35% as necessary, the balance consists of Fe and inevitable impurities, and the structure contains spheroidized cementite. The matrix is martensite.

(B) A carburized layer containing C: 0.50 to 1.50% by weight, the remainder consisting of Fe and inevitable impurities, and whose structure is martensite containing spheroidized cementite.

Further, in the second invention, in weight%, c: o, io
~0.30%, Si: 0.03~0.50%, M
n: 0.30-1.80%, Cr: 0.30-1.80
%, optionally containing 0.05 to 0.35% Mo, and the remainder consisting of Fe and unavoidable impurities, the following treatments (A) to (C) are performed in this order. This is a characteristic feature.

(A) Carburize and quench.

(B) In a carburizing atmosphere, Act and Acl+ (Ac3A
ct) After holding in a temperature range between Xo and 4 for 2 to 4 hours, spheroidizing annealing is performed at 20 to b. However, Acl; mother phase Acl
Transformation point, Ac3; Ac3 transformation point of the parent phase.

(C) After being held in a temperature range between Ac3 and (Ac3+30) for a short time, quenching is performed.

[Function] Generally, C is an element that plays an important role in imparting static strength to steel, but if it is added in an excessively large amount, the toughness of the steel will decrease. On the other hand, when the amount of C is insufficient, wear resistance decreases. Furthermore, according to the knowledge obtained by the present inventors, when the overall Ca content is the same, a high-C
Those containing a high concentration of spheroidized cementite distributed in the matrix have higher toughness and wear resistance.

Therefore, the carburized case-hardening steel according to the present invention suppresses the amount of C in the martensite matrix within a predetermined range (0.10 to 0.30%) and spheroidizes the cementite, thereby improving the toughness of the steel matrix. expensive. On the other hand, it has a large amount and a high Ca content (0.50 to 1
．． 50%) cementite is spheroidized and distributed in martensite, so the martensite carburized layer has particularly high wear resistance.

Furthermore, in order to manufacture such a carburized case-hardened steel, it is necessary to perform sufficient spheroidizing annealing after carburizing and quenching to form a large amount of spheroidized cementite in both the matrix and the carburized layer. If a large amount of spheroidized cementite is not formed at this stage, there is a possibility that it will become solid solution in the structure during subsequent reheating and quenching.For the same reason, spheroidization The quenching temperature after treatment should be as low as possible (and the holding time at that temperature should be shortened).

In the method for producing carburized and case-hardened steel according to the present invention, after carburizing and quenching, the steel is processed through a process and heat pattern that satisfies the above conditions, so that the above-mentioned carburized and case-hardened steel can be easily produced. Can be done.

[Specific structure of the invention] The present invention will be explained in more detail below.

First, the reason for limiting the chemical composition in the present invention will be explained.

C: As mentioned above, C is an element necessary to impart a certain static strength to steel, but on the other hand, it deteriorates toughness. In particular, case hardening steel that undergoes carburizing requires a balance between static strength and toughness, and to obtain the minimum static strength,
A C concentration of 0.1% is required. On the other hand, when the amount of C in the martensite matrix exceeds 0.3%, toughness decreases. Therefore, the amount of C in the martensite matrix is set to 0.1θ
It was set at ~0.30%.

On the other hand, a martensitic carburized layer requires at least 0.5% C to obtain the desired wear resistance, but if it exceeds 1.5%, the toughness decreases rapidly. Therefore, the amount of C in the martensitic carburized layer was determined to be 0.5 to 1.5%.

Si: Si is an essential element as a deoxidizer for molten steel, and is also an element necessary to impart a certain static strength to steel. However, if the content is less than 0.03%, the desired deoxidizing effect cannot be obtained, while if it exceeds 0.50%, the formation of Si oxide is significant and the toughness is deteriorated. The content was determined to be 0.03% to 0.50%.

Mn: Mn is an element necessary for deoxidizing molten steel and is also an effective element for imparting hardenability to steel, but if the content is less than 0.30%, it will not have the desired effect on deoxidizing action. On the other hand, if the content exceeds 1.80%, machinability will decrease, so the content should be reduced to 0.30 to 1.80%.
It was determined that

Cr: Cr is an element effective in imparting hardenability to steel. If the content is less than 0.30%, the desired effect in improving hardenability cannot be obtained, while if the content exceeds 1.80%, Cr oxides will be generated at grain boundaries during carburizing. , reduce fatigue properties.

Therefore, its content was set to 0.30 to 1.80%.

Mo; Mo has the effect of improving the hardenability and toughness of steel materials, but if its content is less than 0.05%, the desired effect cannot be obtained; on the other hand, if it exceeds 0.35% Since machinability deteriorates if it is contained, its content is reduced to 0.
．． 05 to 0.35%.

Next, the spheroidization processing conditions will be explained.

When performing spheroidization treatment, it is necessary to ensure spheroidization in both the matrix and the carburized layer, but if the holding temperature during spheroidization annealing is lower than the ACl transformation point, cementite spheroidization will be delayed and the desired result will not be achieved. On the other hand, if spheroidized cementite is not formed and the temperature exceeds Acl+(Ac3-Act)Xo,4, cementite spheroidization in the matrix becomes difficult.
) XO94.

Furthermore, when the temperature holding time exceeds 4 hours or the cooling rate is less than 20°C/hour, the spheroidized cementite formed in the carburized part becomes coarse;
If the cooling rate is less than 30°C/hour, or if the cooling rate exceeds 30°C/hour, spheroidized cementite is difficult to form.
The holding time was 2-4 hours, and the cooling rate was 20-30°C/hour.

Note that the reason why the spheroidizing annealing is performed in a carburizing atmosphere is to prevent decarburization of the surface portion of the carburized layer.

Next, the quenching conditions will be explained.

After carburizing and quenching the surface of the case-hardened steel to a high C value and spheroidizing cementite by spheroidizing annealing, the structure is an annealed structure containing spheroidized cementite. Therefore, quenching is performed to convert this structure into martensite. Water quenching is suitable for this quenching in order to increase the quenching rate.

During quenching, if the holding temperature is lower than the Ac3 point, ferrite will remain in the matrix. On the other hand, when the holding temperature is 30° C. or more higher than the Ac3 point, or when the holding time is long, spheroidized cementite dissolves in the structure, and a martensitic structure containing spheroidized cementite cannot be obtained. Therefore, the holding temperature is the Ac3 transformation point and (A
The temperature was set at a temperature range between C3+30°C) and held for a short period of time. Note that the holding time was not particularly limited because it depends on the holding temperature and the size of the case hardening steel.

FIG. 4 shows a schematic diagram of the structure of the conventional steel and the structure of the steel of the present invention. As can be seen from the figure, in the steel of the present invention, spheroidized cementite is sufficiently distributed in the carburized layer and matrix.

[Example] Next, an example will be described.

A sample steel having the chemical composition shown in Table 1 was heated at 1250° C. for 1 hour, forged and drawn to a diameter of 100 mm and a diameter of 30 mm, and investigated using the following investigation steps.

First, a steel material with a diameter of 30an was heated to 925°C for 1 hour, cooled in air, normalized, and then turned to a diameter of 2511I11, JIS 3
No. 2ausU notch) Charpy test piece was processed, and Figures 1 to 3 (here, Figure 1 is the heat pattern of carburizing treatment according to the present invention, Figure 2 is the heat pattern of conventional carburizing treatment, The carburizing process was carried out using the heat pattern shown in Figure 3, which shows the carburizing heat patterns used by other companies. Furthermore, it was tempered at 170°C for 1 hour, and then subjected to sheet beaning (φ0.3M).
, 47 m/s.

15 minutes), then a Charpy impact test was conducted.

On the other hand, a steel material with a diameter of 1100I was heated to 925°C for 5 hours, cooled in air, and normalized, and then the gear test piece shown in Fig. 5 (shape: two-handed gear, module = 2.0, number of teeth: 33, pitch diameter) was prepared. =66), and the carburizing treatment shown in Figs. 1 to 3 was carried out. Furthermore, we retempered the strawberries at 170℃ for 1 hour.
Shot peening treatment was performed, and fatigue strength was investigated using a power circulation gear fatigue tester. In addition, the fatigue strength is 107
Evaluation was made based on the strength (fatigue limit) at which no fracture occurred in the test. Regarding tooth surface fatigue, the degree of damage to the tooth surface of gears that did not break in 107 cycles was visually observed.

For static bending test, 30aui diameter forged material was heated to 925℃.
After heating for 1 hour, air-cooling, normalizing, and processing into the static bending test piece shown in FIG. 6, carburizing treatment was performed using the heat pattern shown in FIGS. 1 to 3. Furthermore, tempering was performed at 170℃ for 1 hour, shot pinning treatment was performed, and 102/S
The static bending strength was investigated at strain rates of . Note that the static bending strength was evaluated using the crack initiation load.

Regarding the carburizing depth (depth from the surface at the position of hardness Hv550), a steel material with a diameter of 301n11 was heated to 925°C for 1 hour, cooled in air and normalized, then turned to a diameter of 25mo+, and
Measurements were taken after carburizing using the heat patterns shown in Figures 3 to 3.

The above measurement results are shown in Table 1. In Table 1, each measured value is the result of performing the processing shown in the heat patterns in the same table. The tooth surface fatigue was evaluated as follows: ○: No damage at all, Δ: Slight damage, ×: Significant damage.

As is clear from Table 1, the steel of the present invention has no tooth surface fatigue and has excellent wear resistance. It also has higher static bending strength and Charpy impact value than conventional steel, and has good toughness.

The fatigue limit is approximately 30% higher than that of conventional steel, and the fatigue strength is greater (improved).

Furthermore, among the comparative steels, those subjected to the carburizing treatment shown in Fig. 1 have superior wear resistance, toughness, and fatigue properties compared to those carburized by the conventional method shown in Fig. It can be seen that the steel manufacturing method is superior.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide carburized case-hardened steel with excellent fatigue strength and the like and an efficient method for manufacturing the same.

[Brief explanation of the drawing]

FIG. 1 is a heat pattern diagram of carburizing treatment according to the present invention,
Figure 2 is a heat pattern diagram of carburizing treatment using the conventional method;
The figure is a heat pattern diagram of carburizing treatment in another company's patent, Figure 4 is a comparative illustration of the structures of conventional steel (a) and steel of the present invention (b), Figure 5 is a schematic diagram of a gear test piece, and Figure 6 is a diagram showing a static bending test piece. (b)

Claims (2)

[Claims] (1) A carburized case-hardening steel characterized by having the following carburized layer (B) on the surface of the parent phase shown below (A). (A) In weight%, C: 0.10-0.30%, Si: 0
．． 03-0.50%, Mn: 0.30-1.80%, C
r: Contains 0.30 to 1.80%, Mo
: Contains 0.05 to 0.35%, the remainder consists of Fe and unavoidable impurities, and its structure is a matrix of martensite containing spheroidized cementite. (B) Contains C: 0.50 to 1.50% in weight%,
The carburized layer is composed of the remainder Fe and unavoidable impurities, and its structure is martensite containing spheroidized cementite. (2) In weight%, C: 0.10-0.30%, Si: 0
．． 03-0.50%, Mn: 0.30-1.80%, C
r: Contains 0.30 to 1.80%, Mo
Manufacture of carburized case-hardened steel, which is characterized in that the following treatments (A) to (C) are performed in this order on case-hardened steel containing 0.05 to 0.35%, the balance being Fe and unavoidable impurities. Method. (A) Carburize and quench. (B) In a carburizing atmosphere, Ac1 and Ac1+ (Ac3-A
c1) x 0.4 for 2 to 4 hours, and then slowly cooled at a cooling rate of 20 to 30°C/hour to perform spheroidizing annealing. However, Ac1: Ac1 transformation point of the matrix, Ac3
; This is the Ac3 transformation point of the parent phase. (C) After being held in a temperature range between Ac3 and (Ac3+30) for a short time, quenching is performed.