JPH0791579B2 - Method for manufacturing case-hardening steel in which crystal grains do not coarsen during carburizing heat treatment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel that is machined into machine parts such as automobiles and industrial machines, and in particular to steel that is carburized and heat treated to become parts.

[0002]

2. Description of the Related Art Automobiles and industrial machine parts made of case-hardening steel are hot or cold-formed and cut, and then usually heated in a carbon atmosphere of about 920 ° C to 930 ° C for a long time. It is used by diffusing carbon over the surface over time to strengthen the surface. In recent years, in order to reduce the cost, it has been required to carry out a carburizing heat treatment at a high temperature in a short time, but a problem in the carburizing heat treatment at a high temperature is that abnormal coarse grains are generated. Coarse grains increase heat treatment strain,
Gears cause noise during use.

It is well known that it is effective to disperse fine precipitates in steel in order to suppress the generation of coarse grains in the carburizing heat treatment. For example, JP-A-2-85.
Japanese Patent No. 342 describes a method of using fine NbN. Further, in order to enhance the effect of preventing coarsening of crystal grains at high temperature, it is effective to make dispersed precipitate particles finer, and JP-A-63-162812 discloses that the solidification rate after casting is controlled. Method for finely dispersing TiN in steel.

[0004]

With these inventions,
Although case-hardening steel that does not easily generate coarse grains at high temperatures has come into practical use, the fact that coarse grains cannot be prevented by high-temperature carburizing heat treatment cannot be prevented depending on the processing conditions. The present invention utilizes the control effect of crystal precipitates by oxides to realize a more effective and stable prevention of coarse grains, and the production of case-hardening steel in which the crystal grains do not coarsen during carburizing heat treatment at high temperature. It is intended to provide a method.

[0005]

As described in the prior art, in order to prevent the coarsening of crystal grains that occurs during the carburizing heat treatment, a stable precipitate or crystallized substance is finely dispersed at the carburizing heat treatment temperature. However, the present inventors have found that MnS particles that have not been used to prevent the generation of coarse particles during the carburizing heat treatment are actually effective for preventing the generation of coarse particles.

Further, fine oxides produced during deoxidation of steel or oxides precipitated during cooling after casting of steel,
By optimally utilizing it as a crystal precipitation nucleus of MnS, M
The present invention has been completed by discovering that nS can be finely dispersed in steel and, as a result, the effect of preventing the formation of coarse particles becomes large. As a result of diligent studies by the inventors of the present invention on the control method of crystal precipitates using oxides, using the steels of the basic components shown in Table 1, the amount of dissolved oxygen before Al deoxidation that affects the size of MnS particles Regarding the effect of the deoxidized Al amount, the results shown in FIG. 1 were obtained. Figure 1 shows the results for an as-cast 10 kg steel ingot. From the figure, the amount of dissolved oxygen before deoxidation is 10 to 1
It can be seen that relatively fine MnS is crystallized when the content is 60 ppm and the Al content is 0.005% to 0.06%.

Further, steels having the basic components shown in Table 1 were melted under various deoxidizing conditions and cooling conditions after casting, samples were cut out from ingots having different MnS sizes, and normalized at 900 ° C. for 2 hours. After that, 70% cold working was added, and heating and quenching was performed at 1000 ° C. for 1 hour to examine the former austenite structure. The result is shown in FIG. It can be seen from FIG. 2 that the finer the MnS, the smaller the average grain size of the reheated austenite structure and the more difficult it is for coarse grains to occur.

[0008]

[Table 1]

Further, as a result of conducting similar research on the deoxidizing elements of Ti and Zr, the inventors have found a method for controlling crystal precipitates that can be realized by the current steel refining and casting equipment, and completed the present invention. That is, the gist of the present invention is as follows. (1) C: 0.10 to 0.30% by weight, Si: 0.05 to 0.5
0%, Mn: 0.30 to 2.00%, S: 0.02 to 0.2
0%, N: 0.002-0.020%, and Al: 0.005-0.050%, Ti: 0.005-
When producing 0.030% steel containing one or two kinds, deoxidation treatment by adding one or more kinds of C, Si, Mn simple substance or alloy to the crude molten steel discharged from the steelmaking furnace , And / or deoxidation by vacuum degassing treatment to adjust the dissolved oxygen amount in the molten steel to 10 to 150 ppm, and then Al, Ti or A
1. A method for manufacturing case-hardening steel, in which crystal grains do not coarsen during carburizing heat treatment, which is characterized by deoxidizing one or two of a 1-alloy and a Ti-alloy.

(2) C: 0.10 to 0.30% by weight, Si: 0.05 to 0.5
0%, Mn: 0.30 to 2.00%, S: 0.02 to 0.2
0%, N: 0.002-0.020%, and Cr: 0.20-2.00%, Mo: 0.10-0.5.
0%, one or more of Ni: 0.30 to 4.00%, and Al: 0.005 to 0.050%, Ti: 0.005.
When producing 0.030% steel containing one or two kinds, deoxidation treatment by adding one or more kinds of C, Si, Mn simple substance or alloy to the crude molten steel discharged from the steelmaking furnace , And / or deoxidation by vacuum degassing treatment to adjust the dissolved oxygen amount in the molten steel to 10 to 150 ppm, and then Al, Ti or A
1. A method for manufacturing case-hardening steel, in which crystal grains do not coarsen during carburizing heat treatment, which is characterized by deoxidizing one or two of a 1-alloy and a Ti-alloy.

(3) In addition to the above-mentioned component (1) or (2), Zr: 0.010 to 0.070% by weight, C
e: 0.010 to 0.070%, Hf: 0.010
When producing steel containing 0.070% of one or more, C, Si and Mn were added to the crude molten steel discharged from the steelmaking furnace.
After deoxidizing by adding one or more of the above simple substance or alloy, and / or vacuum degassing, the dissolved oxygen content in the molten steel is adjusted to 10 to 150 ppm,
Manufacture of case-hardening steel that does not coarsen crystal grains during carburizing heat treatment, which is characterized by complex deoxidizing one or more of Al and Ti and one or more of Zr, Ce, and Hf, or alloys. Method.

(4) In addition to the method described in (1), (2) or (3) above, during carburizing heat treatment characterized in that the average cooling rate from the freezing point after casting to 1000 ° C. is 20 ° C./min or more. A method for manufacturing case-hardening steel in which crystal grains do not coarsen. The reasons for limiting the components and conditions of the present invention will be described below. C: C is an element effective as a deoxidizer for adjusting the amount of dissolved oxygen in molten steel, and C is a strengthening element of steel,
If it is less than 0.10%, it is insufficient to improve the strength of the steel, and if it exceeds 0.30%, the toughness of the steel is lowered.

Si and Mn: Si and Mn of the present invention control the amount of oxygen in molten steel as a weak deoxidizing element. That is,
It is effective for controlling the amount of oxygen before the final deoxidation with Al. Further, the oxides of Si and Mn are finely dispersed in the steel as a complex oxide of Si and Mn, or a complex oxide of Al and Si, and Al and Mn to form MnS crystal precipitation nuclei. MnS
To form an oxide as a crystal precipitation nucleus of
5% or more and Mn must be 0.30% or more. Si,
Mn is a solid solution strengthening element of steel, but when Si is added in an amount of more than 0.50%, a grain boundary oxide layer is easily formed during the carburizing heat treatment, which deteriorates the fatigue properties of the final part.
When Mn is added in excess of 2.00%, the toughness is significantly reduced.

It should be noted that Si and Mn form a sufficient amount of complex oxide whether added before or after deoxidation with Al, Ti or Zr, and act as crystal precipitation nuclei for MnS, TiN and the like. Cr: Cr is an element added for improving the strength. If it is less than 0.20%, it is not sufficient for improving the strength, and if it exceeds 2.00%, the toughness is lowered.

S: S is 0. to crystallize fine MnS in an amount sufficient to prevent the generation of coarse grains during the carburizing heat treatment.
02% or more is required. However, S exceeding 0.20% forms a large amount of sulfides, which brings about great anisotropy in mechanical properties, and is therefore limited. N: N has the effect of precipitating fine Al and Ti nitrides and enhancing the effect of preventing crystal grain coarsening at high temperatures.
If the N content is less than 002%, a sufficient effect of preventing coarse grains cannot be expected, and if it exceeds 0.020%, coarsening of crystal grains is prevented, but toughness decreases.

Mo: Mo is effective for improving strength and toughness or reducing the grain boundary oxide layer. If it is less than 0.10%, this effect is insufficient, and if it exceeds 0.50%, the toughness is rather deteriorated. Ni: Ni is an element that improves strength and toughness like Mo. It also has the effect of improving fatigue strength. If Ni is less than 0.30%, the effect is small, and if it exceeds 4.00%, the cost becomes too high.

Al, Ti and Zr: Al, Ti and Zr are finely dispersed in the steel as oxides and serve as crystal precipitation nuclei of fine MnS particles which are effective in preventing the generation of coarse grains during carburizing heat treatment. In addition, it also prevents precipitation of coarse particles during the carburizing heat treatment by precipitation as a nitride. The dissolved oxygen content in the molten steel stage before addition of Al, Ti, and Zr is 150 ppm or less, and A
l, Ti and Zr content ranges of 0.005 to
0.060%, 0.005-0.060%, 0.010
At 0.070%, MnS particles are finely dispersed.
Al, Ti less than 0.005%, or 0.010%
If Zr is added in an amount of less than less than 0%, the oxide serving as a crystal precipitation nucleus will be insufficient, and if more than 0.06% of Al and Ti and more than 0.07% of Zr are added, MnS will be finely crystallized on the oxide. Hard to do.

Ce, Hf: When Ce and Hf having a large specific gravity are added to form an oxide with Al and Ti, the flocculation and floating of the oxide are prevented, and the distribution of the size and number of crystal precipitates depends on the casting time. Less susceptible to. To expect this effect 0.
010% or more is required, but the addition of more than 0.070% is costly, so this is limited. One of the important points in the present invention is to finely disperse a suitable oxide as crystal precipitation nuclei. However, when a strong deoxidizing element that forms an oxide is added to the crude molten steel containing dissolved oxygen in excess of 150 ppm, the oxide becomes coarse and is less likely to become a crystal precipitation nucleus. Also, the amount of dissolved oxygen in molten steel is 1
If it is less than 0 ppm, the required oxide is not sufficiently formed.
Therefore, first, one or two of C, Si alloy, and Mn alloy is used.
Deoxidation treatment is performed by adding at least one species and / or vacuum degassing treatment, and the amount of dissolved oxygen in molten steel is set to 10 to 15
After adjusting to 0ppm, Al, Ti, Zr or Al alloy,
It is suitable to perform final deoxidation with one or more of Ti alloy and Zr alloy.

Deoxidation treatment by adding one or more of C, Si and Mn alone or alloys, and deoxidation by vacuum degassing treatment are for the purpose of adjusting the amount of dissolved oxygen. Does not limit Moreover, since the other components have almost no effect on the dissolved oxygen amount, the timing of addition is not limited. Further, by setting the average cooling rate from the freezing point after casting to 1000 ° C. to 20 ° C./min or more, MnS and TiN that are crystallized during solidification or after solidification are refined, and the effect of preventing the formation of coarse grains during carburizing heat treatment is achieved. Is further enhanced.

In the method of the present invention, a steel to which Nb is further added for strengthening, a steel to which Ca, Bi and Pb are added for improving machinability, and other purposes such as rust prevention and salt resistance improvement are claimed. It is also an effective method for steel containing elements not listed in the range. Examples will be described below.

[0021]

EXAMPLES Example 1 After melting 20 kg of crude molten steel in a vacuum melting furnace and adjusting the components shown in Table 2, as shown in Table 3, Al, Ti, Z
r Deoxidation was performed by changing the amount of oxygen before deoxidation, and cast into a steel ingot.
The average cooling rate from the freezing point to 1000 ° C after casting is 30
A mold was used such that the temperature was ° C / min. The cooled steel ingot was heated to 1200 ° C. and formed into a steel bar having a diameter of 30 mm, which was used as a test material.

In order to reproduce the structure at the time of carburizing heat treatment, this test material was used as a cylindrical sample having a diameter of 10 mm and a height of 15 mm, cold-pressed by 50%, and then reheated from 950 ° C to 1100 ° C for 1 hour and quenched. Then, the old austenite crystal grains were observed. Table 3 shows the results of generation of coarse particles. Here, the occurrence of coarse particles means the appearance of particles having a diameter three times or more the average particle diameter of the matrix.

From Table 3, it is clear that coarse particles are not generated during the high temperature carburizing heat treatment at 1000 ° C. when the oxygen content before deoxidation of Al, Ti or Zr is 10 to 150 ppm.

[0024]

[Table 2]

[0025]

[Table 3]

Example 2 After melting 20 kg of crude molten steel in a melting furnace, C, Si, M
n or vacuum degassing to pre-deoxidize and adjust other components to adjust the dissolved oxygen content to 100ppm or less, then deoxidize by changing the addition amount of Al, Ti, Zr ,
Steel ingots having the components shown in Tables 4 to 6 were cast. A mold was used such that the average cooling rate from the solidification point to 1000 ° C after casting was 30 ° C / min. The cooled steel ingot was heated to 1200 ° C. and formed into a steel bar having a diameter of 30 mm, which was used as a test material.

This test material was heat-air-cooled at 750 ° C. for 8 hours, air-cooled, and then made into a cylindrical sample having a diameter of 10 mm and a height of 15 mm, which was cold-rolled by 50%. Subsequently, in order to reproduce the carburizing heat treatment, reheating to 1000 ° C. for 1 hour and quenching were performed, and generation of coarse particles was examined. The results are shown in Table 6. As shown in Table 6, the steel of the component of the present invention does not generate coarse grains in the 1000 ° C. carburization reproduction experiment.

Coarse grains are not generated in the steel of Comparative Steel No. 25 in Table 6, but coarse grains are generated when 0.025% of N is added as described in the reason for limiting N. Even without it, the toughness decreases.

[0029]

[Table 4]

[0030]

[Table 5]

[0031]

[Table 6]

Example 3 After refining 100 tons of crude molten steel in an oxygen converter, Si alloy and Mn alloy were added, and the amount of dissolved oxygen was set to 100 ppm or less as shown in Table 7 by a vacuum degassing apparatus. After adding other alloying elements, final deoxidation was performed with Ti. No. 33
Steel was cast by a continuous casting machine having a casting cross-sectional size of 350 × 560 mm, and finally rolled into a steel bar having a diameter of 100 mm (reduction ratio 25.0). No. 34 steel has a casting cross section size of 16
It is cast on a 2 x 162 mm continuous casting machine and finally has a diameter of 37.
It was rolled into a steel bar of mm (reduction ratio: 24.4). No. 33, 3
The average cooling rates from the freezing point to 1000 ° C during casting of No. 4 were 10 ° C / min and 52 ° C / min, respectively. No.
Samples with a diameter of 20 mm were cut out from steel bars 33 and 34, and 12
50% hot working was added at 00 ° C. Then 900 ° C
Normalized for 1 hour at room temperature and allowed to cool, then used as the test material at 950 ° C
To 1100 ° C. for 1 hour and quenched to examine old austenite crystal grains. As a result, as shown in Table 7, it is understood that the method showing the invention (3), that is, the coarse grain generation temperature of the No. 34 steel is higher than that of the No. 33 steel.

[0033]

[Table 7]

[0034]

As described above, in the steel produced by the method of the present invention, the crystal grains do not become coarse during the carburizing heat treatment at high temperature, and the carburizing heat treatment can be performed at high temperature in a short time.

[Brief description of drawings]

Figure 1: MnS circle equivalent diameter in as-cast 10kg steel ingot
It is a figure which shows the influence of the dissolved oxygen amount before Al deoxidation and the amount of deoxidized Al on (the diameter of the circle at the time of replacing the observed area of MnS with a circle).

FIG. 2 is a diagram showing a relationship between a circle-equivalent diameter of MnS particles in a slab and austenite grain size when the slab is cold-worked by 70% and then reheated to 1000 ° C. for 1 hour.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/00 301 N 38/14

Claims (4)

[Claims] 1. C: 0.10 to 0.30% by weight, Si: 0.05 to 0.5
0%, Mn: 0.30 to 2.00%, S: 0.02 to 0.2
0%, N: 0.002-0.020%, and Al: 0.005-0.050%, Ti: 0.005-
When producing 0.030% steel containing one or two kinds, deoxidation treatment by adding one or more kinds of C, Si, Mn simple substance or alloy to the crude molten steel discharged from the steelmaking furnace , And / or deoxidation by vacuum degassing treatment to adjust the dissolved oxygen amount in the molten steel to 10 to 150 ppm, and then Al, Ti or A
1. A method for manufacturing case-hardening steel, in which crystal grains do not coarsen during carburizing heat treatment, which is characterized by deoxidizing one or two of a 1-alloy and a Ti-alloy. 2. C: 0.10 to 0.30% by weight, Si: 0.05 to 0.5
0%, Mn: 0.30 to 2.00%, S: 0.02 to 0.2
0%, N: 0.002-0.020%, and Cr: 0.20-2.00%, Mo: 0.10-0.5.
0%, one or more of Ni: 0.30 to 4.00%, and Al: 0.005 to 0.050%, Ti: 0.005.
When producing 0.030% steel containing one or two kinds, deoxidation treatment by adding one or more kinds of C, Si, Mn simple substance or alloy to the crude molten steel discharged from the steelmaking furnace , And / or deoxidation by vacuum degassing treatment to adjust the dissolved oxygen amount in the molten steel to 10 to 150 ppm, and then Al, Ti or A
1. A method for manufacturing case-hardening steel, in which crystal grains do not coarsen during carburizing heat treatment, which is characterized by deoxidizing one or two of a 1-alloy and a Ti-alloy. 3. In addition to the component of claim 1 or 2, Zr: 0.010 to 0.070% by weight, Ce: 0.01
When producing a steel containing one or more of 0 to 0.070% and Hf: 0.010 to 0.070%, C, Si, or Mn alone or in the crude molten steel discharged from the steelmaking furnace Deoxidation treatment by adding one or more alloys, and / or
Alternatively, deoxidation is performed by vacuum degassing treatment to adjust the dissolved oxygen content in the molten steel to 10 to 150 ppm, and then one or more of Al and Ti and one or more of Zr, Ce, and Hf alone. Alternatively, a method of manufacturing case-hardening steel in which crystal grains do not coarsen during carburizing heat treatment, which is characterized by complex deoxidation with an alloy. 4. The method according to claim 1, 2 or 3, wherein the average cooling rate from the freezing point after casting to 1000 ° C. is 20 ° C.
/ Min or more, a method for producing case-hardening steel in which crystal grains do not coarsen during carburizing heat treatment.