JP4162865B2 - Constant strain steel used for CVJ and other machine structural products - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a constant strain steel for use in the manufacture of parts such as CVJ for automobile parts which are finished by precision cold forging and are not polished thereafter.
[0002]
[Prior art]
The CVJ of automobile parts is finished by precision cold forging so as to reduce costs by reducing the number of processes, so that subsequent polishing is not performed. For this reason, in ingots or continuous cast pieces having a rectangular cross section of steel to be subjected to cold forging, the initial solidification structure is different between the inside and outside of the macro pattern of the cast steel material. This is a major problem that causes a variation of 0.02% difference between the inside and outside. This difference in strain between the inside and outside of the pattern poses a serious problem for parts that are not subjected to finish polishing or cutting after cold forging. On the other hand, it is conceivable that the cross-sectional shape of the continuous cast piece is a medium-diameter round continuous cast piece or a square continuous cast piece. However, this increases the cost by newly installing or remodeling the equipment. Furthermore, there is a problem of deterioration of center segregation due to the shape during casting. That is, there is a knowledge that the round shape is larger in the positive segregation and negative segregation near the final solidification position than in the slab having a rectangular cross section. The medium-diameter round slab or square slab has a problem that the forging ratio is low, and the structure is not sufficiently refined and homogenized.
[0003]
[Problems to be solved by the invention]
The problem to be solved by the present invention is to adopt a slab having a rectangular cross-sectional shape as it is, and to maximize the center segregation suppressing effect that the positive segregation and the negative segregation near the final solidification position are small. Constant strain capable of suppressing the product strain after heat treatment due to the shape difference while taking advantage of the rectangular cross-section slab, which can improve the quality by reducing the strain difference inside and outside the macro pattern It is to provide steel.
[0004]
[Means for Solving the Problems]
The means of the present invention for solving the above-mentioned problems is that, in the invention of claim 1, in a steel bar formed by rolling or forging from a large cross-sectional bloom having a short side of 320 mm or more by continuous casting or ingot casting. The constant strain steel used for CVJ and other products for mechanical structures, wherein the area ratio of the core portion which is a negative segregation portion in the cross section of the steel bar is 25% or less.
[0005]
The operation of the above means will be described. For example, in a CVJ inner race, which is an automobile part manufactured by cold forging from φ36 mm sample steel, as shown in FIG. Heat treatment strain, that is, the deviation between the balls in each groove in the drawing, for example, the ball 3 and the ball 6, the ball 4 and the ball 7, shown in FIG. The deviation of the distance between the ball 5 and the ball 8 is evaluated by a method of evaluating the cross sections a, b, c, d, and e, and the evaluation results are shown in FIG. 2, (a) is a pattern showing the measurement positions of the five points a, b, c, d, and e in the longitudinal direction of the groove surface 1 and the projection surface 2 of the cold forged product of the CVJ inner race shown in FIG. (B) is a pattern diagram which shows the measurement position in the top view of the cold forged goods of the CVJ inner race shown in FIG. As shown in FIG. 3, when the value of σ / μm of the amount of change in the P method is σ = 30 μm with respect to the spec: σ = 30 μm, 4μm, sample steel B product macro aspect ratio 1.02-1.16 and core area ratio 23.2-23.27% 5.5μm, sample steel C steel product macro aspect It is 4.9 μm when the ratio is 1.40 to 1.58 and the core area ratio is 19.5 to 28.4%, and the round continuous cast material of the comparative example is 10.69 × 9.63 mm (aspect ratio). In the case of the elliptical core of 1.09), it was 4.7 μm. The A steel material and the C steel material of the sample steel clear the specifications even at 6σ, which is the same evaluation result as the round continuous cast material of the comparative example.
[0006]
Next, as shown in FIG. 4, in the metal flow investigation which is the CVJ inner race forging analysis result, the sample steel has a 100.25 mm × 7.07 mm rectangular core block rolled steel material and 10.46 mm × 9.63 mm. CVJ inner race cold-forged material in FIG. 1 for a round cast material with an elliptical core of 10 mm, a rolled material with a large square core of 10.25 mm square, and a rolled material with a small square core of 7.07 mm square The metal flow on the groove surface 1 and the projecting surface 2 of the sample steel is 10.25 mm × 7.07 mm, and the distribution width of the projecting surface 2 is 8.02 mm. 9.84 mm, the distribution width of the groove surface 1 is 1.27 mm in the round cast material, and the distribution width of the projection surface 2 is 1.96 mm, and the distribution width of the groove surface 1 is in the large square core block rolled material. Is a protrusion at 8.84mm 2 distribution width is 10.55Mm, distribution width of the projection surface 2 distribution width of Mizomen 1 in slabbing material small square core with 3.53mm is 4.49mm.
[0007]
From this, it can be seen that a small rectangular shape is better than a large square core. That is, it can be seen that the portion indicated by the black circle in FIG. 4 which is the corner portion of the core portion is smaller in the small rectangular shape than in the large square shape in the core portion, so that the entry into the groove surface 1 and the projection surface 2 is smaller. It can be seen that the cross-sectional core is effective in reducing variation in heat treatment strain. From FIG. 3, the sample steel A has a core area ratio of 24% or less and a P method variation of 4.4 μm. This is all when cold forged into a CVJ inner race. When the amount of change in the law is acceptable and the portion of the arrow below the dotted line indicating the area ratio of 25% in FIG. 3 is the region of the present invention and the area ratio of the core portion is 25.0% or less, an excellent It can be seen that a strained steel material is obtained. That is, since the area ratio of the core portion which is a negative segregation portion having a low C% in the cross section of the bar steel after the block rolling is 25% or less, the steel material having an area ratio of this core portion of 25% or less is used. For example, when cold forging into a CVJ inner race, it is possible to reduce the entry of negative segregation parts into the product by easily collecting and removing the low C% part of the core part as a poncas part. Can be reduced, that is, constant strain can be obtained.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be shown through the following examples. A rectangular bloom of cold forging steel by continuous casting with an aspect ratio of 1.2 or more, for example, a shape including 490 mm × 380 mm and an aspect ratio of 1.29 including a strong reduction pass with a reduction rate of 15% or more per pass from the short side. Roll down by rolling on a schedule. Thereafter, without using a special pass schedule, the obtained slab or steel slab is further subjected to, for example, φ170 without rolling or forging, for example, by a three-roll planetary mill (hereinafter referred to as “PSW”). , Rolled into. Then, it is finish-rolled to a round bar for cold forging, for example, φ40. The core area ratio of the macro pattern of this round bar is 25% or less, and when this round bar is made into a product such as CVJ for automobile parts by cold forging, constant strain with uniform heat treatment strain after heat treatment Cold forged products.
[0009]
【Example】
By continuous casting, a rectangular bloom of 490mm x 380mm shape and 1.29 aspect ratio of cold forging steel SCR420H at superheat (SH) 29 ° C was created, heated in a heating furnace, and reduced per pass from the short side. Split rolling is performed with a pass schedule including a pass under high pressure at a rate of 15% or more. Thereafter, it was reheated and rolled to φ40 to obtain a steel bar product. By this method, a constant strain steel material for cold forging having an area ratio of 25% or less of the core portion was obtained. When this steel material is used to make products such as CVJ inner races, even if some of the corners of the core part enter into the cold forged product, the core part enters slightly when finishing the product because it is a very small amount of metal flow. However, it could be removed as a poncus part, and a product with a constant strain was obtained in each part.
[0010]
【The invention's effect】
As described above, a cold-forged product such as CVJ is obtained by cold forging, and as a constant strain steel used for finishing CVJ parts without polishing thereafter, the present invention is a continuous cast material or ingot having a large rectangular cross section. The material is made into a bar steel by ingot rolling or forging, and the area ratio of the core part, which is the negative segregation part of the cross section of this bar steel, is 25% or less. A uniform strain steel is obtained. Therefore, variation in product shape after heat treatment can be suppressed. Therefore, there is no need to newly install or modify the continuous casting equipment, and the existing continuous casting equipment for rectangular slabs can be used as it is, and therefore it is inferior to the round continuous cast material without particularly increasing the cost. It is possible to carry out cold-forged products such as CVJ by maintaining constant strain steel without any defects, and to maintain the excellent positive and negative segregation near the final solidification position, which is a feature of rectangular cast pieces. The present invention can provide a constant strain steel for use in the manufacture of parts that are finished by precision cold forging and thereafter not polished, such as CVJ for automobile parts, and the present invention is superior in the past. It is effective.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a CVJ inner race product partially cut by cold forging.
2 is a diagram showing a pattern for evaluating heat treatment strain by the P method of the CVJ inner race product shown in FIG. 1, (a) is an elevation view of the groove surface 1 and the projection surface 2 at five locations; The pattern which shows the measurement position of the amount of change after heat treatment after cold forging and after heat treatment is shown, (b) is a mimetic diagram of the pattern which shows the measurement position before heat treatment in the top view of the CVJ inner race product, and after heat treatment. .
FIG. 3 is a graph showing a P method evaluation result evaluated according to the pattern of FIG. 2;
FIG. 4 shows the shape of the core, which is a negative segregation part of a steel bar for cold forging after the rolling or forging of a rectangular or square continuous casting or ingot, or cold forging after the rolling of a round continuous casting. It is a schematic diagram of the metal flow in which each core part in the steel material for use shows entry into the groove surface 1 or the projection surface 2 of the CVJ inner race product shown in FIG.
[Explanation of symbols]
1 Groove surface 2 Projection surface 3 Ball 4 Ball 5 Ball 6 Ball 7 Ball 8 Ball

Claims (1)

In a steel bar obtained by performing continuous rolling or forging from a rectangular large cross-section bloom having a short side of 320 mm or more by continuous casting or ingot casting, the area ratio of the core part which is a negative segregation part in the cross section of the steel bar is 25% or less A constant strain steel used for CVJ and other mechanical structural products.

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