JP2003320439A - Method for manufacturing carburization steel slab having small heat treatment strain variation, and cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for a constant strain carburization steel slab for a machining component in which variation in a dimensional change by heat treatment of the carburization steel for hollow shaft components for an automobile or the like is suppressed, dimensional accuracy is excellent and shape correcting by way of cutting or polishing after the heat treatment can be omitted, and to provide a constant strain carburization steel slab. <P>SOLUTION: In the slab manufacturing method for continuous casting of the carburization steel containing the carbon content of less than 0.5 mass % to be used for the hollow shaft components which are subjected to plastic work accompanied by plastic fluidity to the axial direction, then an axis insertion part is punched from the center part into a predetermined shape, the magnetic flux density of the M-EMS strength during the continuous casting is controlled to 30 to 100 μT to set the average molten steel flow rate in a mold to 2 cm/s to 15 cm/s, and the area ratio of the slab center segregation zone to be used for the hollow shaft components is lessened to continuously cast the constant strain carburization steel slab. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hollow shaft component used for heat treatment of an axially symmetric shape or a cyclically symmetric shape such as a gear and an inner race in a constant velocity joint, which is suitable for manufacturing a machined component having a small strain variation. Regarding

[0002]

2. Description of the Related Art Conventionally, since heat treatment such as carburizing and quenching is generally performed at the final processing stage of parts,
The generated heat treatment strain causes deterioration of the outer diameter roundness and end face flatness. Since dimensional changes (hereinafter referred to as “heat treatment strain”) due to heat treatment cannot be physically avoided, each manufacturing company is focusing on reducing heat treatment strain or variation.

When heat-treating a part manufactured by plastic working such as forging, the shape by plastic working can be determined for the required dimensional accuracy from the dimensional change occurring during heat treatment. In other words, in order to realize the required dimensional accuracy, the problem of the amount of heat treatment strain can be compensated by setting the plastic deformation shape and designing the forging die shape. However, if there is a large variation in the heat treatment strain, the dimensional changes in each direction may become non-uniform and the outer diameter roundness may deteriorate, and twisting and bending may occur. Design becomes impossible. Therefore, it is a major issue to reduce the variation in heat treatment strain.

In recent years, from the viewpoints of environmental impact and cost reduction, there is an increasing demand for polishing-free after-heat treatment of precision parts for automobiles, and a low heat treatment strain itself or a small variation of heat treatment strain is small. Demand for constant heat treated strain material is expected to grow.

By the way, the constant strain in this specification means
It means that the variation in heat treatment strain is small and is almost constant. Therefore, the above-mentioned constant strain material refers to a material with a small variation in strain due to heat treatment.

By making the continuous casting mold circular when the isotropic crystal isotropic, it is possible to obtain a material having a substantially circular slab solidification structure pattern. This makes it possible to manufacture fastener parts such as bolts, torsion bars, gears, shafts, parts for heat treatment such as inner races in constant velocity joints, which are suitable for constant strain processing.
No. 84 publication. Further, it is known that by obtaining a square solidification pattern with a square mold, variations in heat treatment strain are lower than with a rectangular mold product. The mold shape in continuous casting is generally rectangular.

As described above, each manufacturing company makes the mold shape isotropic, that is, the equiaxed crystal shape is isotropic, to reduce the variation in heat treatment strain.
This is because most automotive precision parts have an axisymmetric shape.

The applicant of the present invention is currently performing continuous casting in a large-section rectangular mold having a size of 380 × 490 mm. Therefore, the solidification pattern is rectangular, and variations in heat treatment strain are greater than those of the circular mold and square mold described above. There was a big problem.

Reduction of equiaxed crystal ratio Further, in a region mainly composed of equiaxed crystals which is a central segregation zone of a continuous cast piece, an area ratio of equiaxed crystals, that is, a cross-sectional area of a bloom in a transverse section by continuous casting. As a result of investigating the effect of the ratio of the area of the central segregation zone to the heat treatment strain during carburizing and quenching, we obtained the knowledge that the smaller the quenching strain amount, the smaller the area ratio of equiaxed crystals, the shape, etc. The invention for reducing the axial crystal ratio is disclosed in JP-A-11-131.
No. 184 gazette.

[0010] By the way, to explain the cause of the variation in heat treatment strain, it is clear in past literatures that the volume change caused by heat treatment correlates with the C concentration of the steel material. It is considered that the variation greatly affects the variation in heat treatment strain.

Further, as a result of investigating the relationship between the solidification structure and the C concentration based on many experimental data including the examples, it was revealed that the C concentration differs between the equiaxed crystal region and the columnar crystal region. .

By the way, the shape of the solidified structure is greatly influenced by the shape of the continuous casting mold. As a result, since the continuous cast material by the rectangular mold is the bloom of the slab 2 having a rectangular cross section,
As shown in FIG. 1B, the equiaxed crystal 5 at the center of the outer columnar crystal 6 has a rectangular shape. Therefore, since the C concentration of the solidified structure of the round steel material 4 of the product varies depending on the location, the dimensional change occurring during heat treatment becomes non-uniform in the circumferential direction, which is one of the major factors affecting the variation in heat treatment strain. Conceivable.

That is, as shown in FIG. 1A, since the circular continuous cast piece 1 formed by the circular mold has a circular cross section, the equiaxed crystal 5 at the center of the outer columnar crystal 6 also has a circular cross section. In the round steel material 3 which is a circular product after rolling, the equiaxed crystal 5 also has a concentric circular shape. Therefore, the difference in heat treatment strain in the circumferential direction is small, and the equiaxed crystal 5 after heat treatment is also a substantially perfect circle.

On the other hand, as described above, as shown in FIG. 1B, the continuous cast piece 2 by the rectangular mold is manufactured by rolling the bloom of the rectangular cross section to produce the round steel material 4 having the circular cross section. Therefore, due to the solidification structure of the bloom, the equiaxed crystal 5 having a rectangular cross section also appears in the round steel product 4 of the circular product after rolling. There is a problem that a difference in heat treatment strain occurs in the circumferential direction and the roundness of the equiaxed crystal after heat treatment deteriorates.

[0015]

The problem to be solved by the present invention is to carburize precision parts for automobiles, particularly CVJ inner races and hollow shaft parts such as gears, having a C content of less than 0.5% by mass. In the heat treatment of steel materials, it is possible to suppress the heat treatment dimensional change, that is, the variation of heat treatment strain, the dimensional accuracy is good, and in some cases, the processed parts that can omit the shape correction process such as the cutting process or the polishing process after the heat treatment. The object of the present invention is to provide a manufacturing method for obtaining the same and a steel slab for constant strain carburization for this hollow shaft part.

In a carburized steel material having a C content of 0.5% by mass, the absolute value of the variation in the C concentration in the steel material becomes large, and even if the variation in the heat treatment strain is reduced, the cutting step or polishing after the heat treatment is performed. It is difficult to omit a shape correction process such as a process.

[0017]

Means and principles of the present invention for solving the above problems will be described below. First,
A means for making the shape of the equiaxed crystal 5 isotropic will be examined.
As described above, it was considered that the shape of the equiaxed crystal 5 had a great influence on the variation in the heat treatment strain, and thus the shape of the equiaxed crystal 5 was made isotropic, that is, squared. The variation was not very good.

Next, the equiaxed crystal ratio and the C concentration variation are reduced, that is, electromagnetic stirring in the mold (hereinafter referred to as "M-EM").
"S". ) Consider means for optimizing strength.

It was found that the effect of improving the shape of the equiaxed crystal 5 was thin as described above, but as a result of further study, the area of the equiaxed crystal 5 is more uneven than the shape of the equiaxed crystal 5 in the heat treatment strain. It was found to be important for the reduction of This is largely due to the difference in C concentration between the equiaxed crystal 5 and the columnar crystal 6.

Most precision parts for automobiles have a hollow shaft shape, and after the cold forged part 7 is forged, the axial center part of the cold forged part 7 is punched out as a punched punch piece 8.
From the actual machine examination and the CAE analysis, it was found that how much the equiaxed crystal 5 remains in the cold forged part 7 after punching greatly affects the heat treatment strain variation.

As shown in FIG. 2, when using a rectangular mold,
The equiaxed crystal 5 remaining in the cold-forged part 7 after punching is formed by forming a small expansion coefficient portion 9 and a large expansion coefficient portion 10 in the equiaxed crystal 5, as shown in FIG. The portion 5 is not axisymmetric. When the cold forged part 7 is heat-treated and the heat-treated part 12 is applied, a difference in the heat treatment strain is generated corresponding to the difference in C concentration between the equiaxed crystal 5 and the columnar crystal 6. As a result, the heat-treated part 12 after heat treatment has a difference in the vertical axis length 13 and the horizontal axis length 14, and the circularity of the equiaxed crystal 5 deteriorates.

From the above, as a means for reducing the variation in heat treatment strain, the M-EMS strength during continuous casting is reduced, the equiaxed crystal ratio is reduced, and the equiaxed crystal 5 occupying in the cold forged part 7 is reduced. By reducing the ratio of the portion, it is intended to reduce the variation in the heat treatment strain in the heat treated component 12.

By the way, as a concern in reducing the M-EMS strength during continuous casting, deterioration of quality, specifically, center segregation of carbon concentration, center porosity and increase of inclusions can be mentioned. Therefore, it is necessary to formulate the optimum M-EMS strength that can realize high quality and reduction of heat treatment strain variation.

Generally, in order to obtain a high equiaxed crystal ratio for the purpose of improving the internal properties of the cast slab, JP-A-57-75271, JP-A-53-45627, and JP-A-2001-2001.
As disclosed in Japanese Patent No. 138018, it is the mainstream to carry out M-EMS. That is, increasing the M-EMS strength to promote the molten steel flow in the mold to suppress the formation of columnar crystals 6, that is, to increase the equiaxed crystal ratio has a better effect on the quality, but is described above. As described above, it is not a good idea to reduce the variation in heat treatment strain.

Therefore, in the means of the present invention, M-EM which is optimum for both quality and heat treatment strain variation.
Setting the S intensity is important. Therefore, the inventors of the present invention have measured equiaxed crystal ratio and quality in a cast piece, that is, center segregation of C concentration, center porosity, based on a lot of experimental data in which M-EMS strength is changed, including examples described later. Investigation of inclusions, and further, heat-treated part 12 manufactured by plastic working from continuous cast pieces by this rectangular mold and heat-treated
The heat treatment strain variation was measured. as a result,
The M-EMS strength is set to a magnetic flux density of 30 to 100 μT, and the average molten steel flow rate in the mold is set to 2 cm / s to 15 cm / s to suppress the formation of equiaxed crystals 5 and, at the same time, cause variations in the C concentration of the columnar crystals 6. It has been found that the heat treatment strain can be reduced, and as a result, the heat treatment strain variation and quality suitable for the heat treatment component 12 can be obtained.

Further, as an important finding, it has been found that the variation of the C concentration in the region of the columnar crystals 6 can be significantly reduced under the above manufacturing conditions. As described above, since the hollow shaft part is manufactured by punching out the region of the equiaxed crystal 5 of the cold forged part 7, most of the cold forged part 7 is composed of columnar crystals 6. Actually, the influence of the C concentration variation in the columnar crystals 6 on the heat treatment strain cannot be ignored. Bloom of a rectangular cross section is rolled to measure the C concentration of a slab with a circular cross section at the measurement position shown in (a) of FIG. 3, and the result is shown in the graph of (b). As shown in FIG. 3B, it can be seen that the C-segregation in the region of the columnar crystals 6 can be significantly reduced when the M-EMS intensity is set to 100 μT or less compared to the current state of the magnetic flux density of 250 μT. Therefore, the M-EMS intensity is set to the magnetic flux density 3
The average molten steel flow velocity in the mold is set to 0 to 100 μT and 2 cm / s.
By setting it to be 15 cm / s, the C segregation in the region of the columnar crystals 6 can be greatly reduced, and as a result, it is effective in reducing the variation in the heat treatment strain of the heat treated part 12 after the heat treatment of the cold forged part 7. understood.

That is, the means of the present invention for achieving the above object is, in the invention of claim 1, performing a plastic working accompanied by a plastic flow in the axial direction, and thereafter punching out a shaft core insertion portion from a central portion. In the method for producing a slab by continuous casting of carburizing steel having a carbon content of less than 0.5% by mass, which is used for a hollow shaft part produced by shape processing, the average molten steel flow velocity in the mold during continuous casting is 2 cm / s to 15 cm. / S (desirably 5 cm / s to 15 cm / s) is a method for producing a cast slab for constant strain carburizing steel.

According to the second aspect of the present invention, the carbon content used in the hollow shaft part manufactured by performing the plastic working accompanied by the plastic flow in the axial direction and then punching the shaft core insertion portion from the central portion is 0. In the slab manufacturing method by continuous casting of steel for carburizing less than 0.5 mass%, the average molten steel flow velocity in the mold during continuous casting is set to 2 cm / s to 15 cm / s (desirably 5 cm / s to 15 cm / s), A slab production method by continuous casting of steel for carburizing constant strain, characterized in that the area ratio of the segregation zone of the slab used in the hollow shaft part is reduced.

According to the third aspect of the invention, the carbon content used in the hollow shaft component manufactured by performing the plastic working accompanied by the plastic flow in the axial direction and then punching the shaft core insertion portion from the central portion is 0. In the method for producing a slab by continuous casting of steel for carburizing less than 0.5 mass%, M- at the time of continuous casting
EMS (electromagnetic stirring in the mold) strength of magnetic flux density 30-100
The average molten steel flow velocity in the mold is controlled to be μT and the average molten steel flow velocity is 2 cm / s to 15 cm.
cm / s (desirably 5 cm / s to 15 cm / s) is a method for producing a slab by continuous casting of steel for constant strain carburizing.

According to the fourth aspect of the present invention, the carbon content used in the hollow shaft part manufactured by performing the plastic working accompanied by the plastic flow in the axial direction and then punching the shaft core insertion portion from the central portion is 0. In the method for producing a slab by continuous casting of steel for carburizing less than 0.5 mass%, M- at the time of continuous casting
EMS (electromagnetic stirring in the mold) strength of magnetic flux density 30-100
The average molten steel flow velocity in the mold is controlled to be μT and the average molten steel flow velocity is 2 cm / s to 15 cm.
cm / s (desirably 5 cm / s to 15 cm / s), and reducing the area ratio of the segregation zone at the center of the slab used for the hollow shaft component by continuous casting of constant strain carburizing steel. It is a piece manufacturing method.

According to the fifth aspect of the present invention, after the plastic working accompanied by the plastic flow in the axial direction is performed, the shaft core insertion portion is punched out from the central portion, and the carbon content for the hollow shaft part having a predetermined shape is 0.
The steel product for constant strain carburizing steel produced by the means according to any one of claims 1 to 4, wherein the area ratio of the central segregation zone in the carburizing steel of less than 5% by mass is reduced.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. The steel material having the chemical composition shown in Table 1 is melted,
By continuous casting, a rectangular mold was used to cast a bloom having a rectangular cross section. Table 2 shows the M-EMS strength (μT) in order to change the central segregation zone of the slab during continuous casting, that is, the equiaxed crystal ratio of the ratio of the equiaxed crystal to the cross section of the slab and the C% variation. It changed as follows. Each M-EMS intensity (μT)
The average molten steel flow velocity in the mold at 0.
3 cm / s, 16.5 cm / s, 25 cm / s, 33c
It was m / s and 42 cm / s.

[0033]

[Table 1]

Thereafter, each was rolled into a circular cross section, and a macro test was carried out on the cross sections of these rolled materials, and the proportion of equiaxed crystals in the cross section of the rolled material was shown as the equiaxed crystal ratio by the area ratio. . The measurement of the equiaxed crystal region is about 20% according to the method for testing the macrostructure of steel specified in JIS G 0553.
It was corroded in the HCI solution for about 30 to 40 seconds, and the area ratio of equiaxed crystal regions was measured. Furthermore, a Daliko was taken from the diameter of the rolled material, the C concentration at each place was investigated, and the C% in the columnar crystal region was measured.
The variation (σ) was calculated. Table 2 shows the above measurement results.

[0035]

[Table 2]

After cutting these steel materials to a length of 45 mm, they are cold forged in a process as shown in FIG. 4 to form a cold forged component 7 having a height of 30 mm as shown in the drawing, and Φ25 mm of the equiaxed crystal part 5 was punched out as a punching punched piece 8. Then, carburizing and quenching treatment was performed.

After that, as shown in FIG. 5, one component is L / 4 each of the heights L of (a) in the height direction at three positions (a), (b) and (c) and is evenly distributed in the circumferential direction of (b). Divided into a, b, c,
The roundness of the outer diameter was measured at a total of 12 locations of 4 locations of d. The results are also shown in Table 2.

As a result, the strength of M-EMS was set so that the magnetic flux density was 30 to 100 μT or less, and the average molten steel flow velocity in the mold was 2 c.
By setting m / s to 15 cm / s, the quality was good and the dimensional change due to heat treatment, that is, the outer diameter roundness after heat treatment by carburizing and quenching could be reduced to a required level.

[0039]

As described above, in the method of the present invention, in continuous casting of a continuous slab having a rectangular cross section, the M-EMS strength during casting is properly set, and the average molten steel flow velocity in the mold is properly set. By doing so, the central segregation zone area ratio can be reduced, and C segregation in the columnar crystal region can be suppressed. By this method, the hollow shaft component manufactured by using the cast slab can be reshaped after carburizing and quenching. The present invention exerts an excellent effect which has never been obtained, such as the carburizing steel which does not need to be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing changes in equiaxed crystals when a continuous cast slab is rolled into a round steel product, where (a) shows a circular mold and (b) shows a rectangular mold. Indicates.

FIG. 2 is a schematic diagram of a crystal structure of a round steel product by a rectangular mold, a punching image of a cold forging part shaft part and a diagram showing a state where an equiaxed crystal part remains, (a) is a schematic side view of a crystal structure of a round steel product,
(B) is a schematic plan view of the crystal structure of the round steel material, (c) is a side view showing the punching image of the shaft part of the cold forged part cold-forged the round steel material of (a), (d) is the cold forged part It is a top view which shows the equiaxed crystal part residual state of a shaft part.

FIG. 3 is a diagram showing the C concentration in each part of a round steel material obtained by rolling a bloom in a rectangular mold, (a) is a diagram showing measurement positions, and (b) is a graph showing measurement results.

4A is a side view of a round steel material rolled from a bloom by a rectangular mold, FIG. 4B is a schematic side view of a center of an equiaxed crystal portion of a cold forged part before punching, and FIG. 4C is a punching thereof. It is a typical side view after.

FIG. 5 is a view showing strain measurement points of a cold forged part in which the center of the equiaxed crystal part shown in FIG. 4 is punched, and (a) is a height direction,
(B) is a figure which shows the circumferential direction.

[Explanation of symbols]

1 Circular mold slab 2 Slab by rectangular mold 3 round steel 4 round steel 5 equiaxed 6 columnar crystals 7 Cold forging parts 8 punched pieces 9 Expansion coefficient small part 10 Expansion coefficient 11 punching part 12 Heat treated parts 13 Vertical axis length 14 Horizontal axis length

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) // C23C 8/22 C23C 8/22 (72) Inventor Undangame Taiwa Nakajima, Himeji City, Hyogo Prefecture 3007 Sanyo Special Steel Co., Ltd. (72) Inventor Shinichi Kitade One letter in Nakajima, Shikoma-ku, Himeji City, Hyogo Prefecture 3007 Sanyo Special Steel Co., Ltd. F term (reference) 4E004 AA09 MB12 NB02 NC04 4E087 AA08 BA02 CA11 CB03 HA01 HA25 HA31 HA82 HB01 4K028 AA01 AB01 AB06

Claims (5)

[Claims] 1. A carbon content of less than 0.5% by mass used in a hollow shaft part manufactured by performing a plastic working accompanied by a plastic flow in an axial direction and thereafter punching a shaft core insertion portion from a central portion to produce a predetermined shape. In the slab production method by continuous casting of carburizing steel, the average molten steel flow velocity in the mold during continuous casting is set to 2 cm / s to 15 cm / s. 2. A carbon content of less than 0.5% by mass used for a hollow shaft part manufactured by performing a plastic working accompanied by a plastic flow in an axial direction and thereafter punching a shaft core insertion portion from a central portion to produce a predetermined shape. In the method for producing a slab by continuous casting of carburizing steel, the average molten steel flow velocity in the mold during continuous casting is set to 2 cm / s to 15 cm / s to reduce the area ratio of the slab center segregation zone used for the hollow shaft part. A method for producing a slab by continuous casting of steel for carburizing constant strain, which comprises: 3. A carbon content of less than 0.5% by mass used in a hollow shaft component manufactured by performing a plastic working accompanied by plastic flow in an axial direction and thereafter punching a shaft core insertion portion from a central portion to produce a predetermined shape. In the method for producing a slab by continuous casting of carburizing steel, the M-EMS (electromagnetic stirring in the mold) strength during continuous casting is controlled to a magnetic flux density of 30 to 100 μT, and the average molten steel flow velocity in the mold is 2 cm / s to 15 cm / A slab production method by continuous casting of steel for constant strain carburization, characterized in that 4. A carbon content of less than 0.5% by mass used in a hollow shaft part manufactured by performing a plastic working accompanied by plastic flow in an axial direction and thereafter punching a shaft core insertion portion from a central portion to produce a predetermined shape. In the method for producing a slab by continuous casting of carburizing steel, the M-EMS (electromagnetic stirring in the mold) strength during continuous casting is controlled to a magnetic flux density of 30 to 100 μT, and the average molten steel flow velocity in the mold is 2 cm / s to 15 cm / s, and reducing the area ratio of the center segregation zone of the slab used for the hollow shaft part, a method for producing a slab by continuous casting of steel for constant strain carburization. 5. A carburizing steel having a carbon content of less than 0.5% by mass for a hollow shaft part which is formed into a predetermined shape by punching out a shaft core insertion part from a central part after performing a plastic working accompanied by a plastic flow in an axial direction. 5. The slab of constant strain carburizing steel produced by the method according to any one of claims 1 to 4, wherein the area ratio of the central segregation zone is reduced.