JP2000265214A - Cold rolled steel sheet manufacturing method with excellent flatness of stamped parts - Google Patents

Abstract

(57) [Summary] [PROBLEMS] As a material for a gear or a plate as a transmission part of an automobile, the punched part has excellent dimensional accuracy, and a heat treatment step for ensuring the hardness of the punched part is unnecessary. In addition, it is possible to obtain a steel sheet having uniform dimensional accuracy after punching over the entire length of the coil. SOLUTION: C: 0.05 to 0.4%, Mn: 0.1 to 0.4%.
1 to 1%, Si: 0.01 to 1%, P: 0.01 to 0.2%, Cr: 0.01 to 1% (weight percent)
When hot rolling a steel containing 0.1% by weight or more of Si, P and Cr, the hot rolling is terminated at a temperature of Ar ₃ or more, and (Ar ₃ -50 ° C) and the average cooling rate between the temperatures of 550 ° C
Winding is performed at the above temperature, and thereafter, cold rolling of 70% or less is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet having excellent flatness of a stamped part, and more particularly to a method for producing a gear or a plate as a transmission part of an automobile, which has a high dimensional accuracy. Excellent, and does not require a heat treatment process to ensure the hardness of the stamped parts.
The present invention relates to a method for producing a cold-rolled steel sheet having excellent flatness of a punched part, capable of obtaining a steel sheet having uniform dimensional accuracy after punching over the entire length of a coil.

[0002]

2. Description of the Related Art Gears and plates used as transmission parts of automobiles are subjected to heat treatment such as quenching or aging precipitation of the punched parts to secure hardness after a steel plate is punched into a predetermined shape by a parts manufacturer. It is manufactured by applying.

However, in recent years, in order to reduce the manufacturing cost, there has been a demand for the development of a steel sheet capable of securing the hardness by cold rolling instead of these heat treatments. However,
In the case of securing the hardness by such cold rolling, there may be a case where a large warpage occurs in the part after punching. For this reason, it is necessary to press-temper the stamped part, and it may be difficult to correct the shape of the part even when the stamping is performed.

[0004] Under such circumstances, there has been a strong demand for the development of a steel sheet having excellent flatness of a part after punching while being cold-rolled.

Conventionally, with respect to a technique of omitting heat treatment for securing hardness after punching of gears and plates as transmission parts of an automobile, a hot rolled sheet structure is formed of hard bainitic ferrite or bainite as a main phase. Japanese Patent Laid-Open No. 8-295 describes a method for producing a high-strength steel sheet for precision punching.
No. 927. Hereinafter, this technique is referred to as Conventional Technique 1.

[0006] Further, as for the technology for improving the strength while securing the dimensional accuracy of parts, a high carbon steel sheet in which the strength is enhanced by adding Cu or V instead of a quenching treatment which increases the thermal strain, and a method of manufacturing the same are disclosed. And Japanese Patent Application Laid-Open No. 4-254546. Hereinafter, this technique is referred to as Conventional Technique 2.

[0007]

However, the prior art 1 has the following problems. That is, prior art 1 relates to precision punching after cold rolling, that is, control of the form of a punched surface such as sag and shear surface ratio, and mentions improvement in dimensional accuracy of a punched part. Absent. Also,
In order to ensure the strength in the low-temperature transformation phase such as bainite, if there is variation in the winding temperature during hot rolling, in the coil longitudinal direction, or, in the case of wide materials, the material fluctuation in the coil width direction becomes large, The dimensional accuracy of the stamped parts after cold rolling varies.

The prior art 2 has the following problems. That is, the prior art 2 requires heat treatment at a tempering temperature for aging precipitation of Cu and V.
Heat treatment for securing the hardness of the stamped part cannot be omitted.

As described above, a method of manufacturing a steel sheet which is excellent in dimensional accuracy of a part after punching a cold-rolled thin steel sheet and which does not require a heat treatment step for securing hardness of the punched part, and The method of manufacturing a steel plate whose accuracy is uniform over the entire length of the coil is as follows:
It has not been proposed yet.

[0010] Accordingly, an object of the present invention is to provide, as a material for a gear or a plate as a transmission component of an automobile, excellent dimensional accuracy of a stamped part and no heat treatment step for ensuring the hardness of the stamped part. Steel plate manufacturing method,
Another object of the present invention is to provide a method for manufacturing a steel sheet having uniform dimensional accuracy after punching over the entire length of the coil.

[0011]

Means for Solving the Problems The present inventors have intensively studied to achieve the above object. As a result, the following various findings were obtained. Deterioration of dimensional accuracy after punching of a steel sheet as it is cold-rolled is caused by the structure of the hot-rolled sheet. That is, in the structure having bainitic ferrite, the structure in the coil width direction becomes non-uniform, resulting in a difference in residual stress after cold rolling, and as a result, the dimensional accuracy of the stamped part is deteriorated. In a structure having a low-temperature transformation phase, a low-temperature transformation phase such as bainite is
Since the volume fraction has a strong cooling rate dependency, the volume fraction differs in the coil width direction. As a result, a difference occurs in the residual stress after the cold rolling, and the dimensional accuracy of the stamped part is deteriorated. In order to prevent the dimensional accuracy of stamped parts from deteriorating due to the mechanism described above, polygonal ferrite is positively generated in the hot-rolled sheet structure, thereby preventing the formation of bainitic ferrite and low-temperature transformation phase. It is effective to control. Prior to hot rolling, heating the entire rough bar after rough rolling, or, or heat retention, by heating the edge of the rough bar, the leading and trailing ends of the rough bar on the side where the finishing rolling stand enters. By controlling the temperature difference and the temperature difference in the width direction to be low, it is possible to obtain a steel sheet having excellent dimensional accuracy after punching the steel sheet as it is cold-rolled uniformly over the entire length of the coil.

The present invention has been made based on the above-mentioned findings, and the invention according to claim 1 is characterized in that C: 0.05 to 0.4%, Mn: 0.1 to 1%, Si: 0.01 From 1%, P: 0.01 to 0.2%, Cr: 0.01 to 1% (or more by weight), and the total amount of Si, P and Cr is 0.1% by weight or more. When hot rolling steel, Ar
_{When 3} or more, hot rolling is completed, and from the rolling completion temperature (Ar ₃ −
(50 ° C.) at an average cooling rate of 20 ° C./sec or less, winding at a temperature of 550 ° C. or more, and then performing cold rolling of 70% or less.

According to a second aspect of the present invention, prior to hot rolling the steel described in the first aspect, the entire rough bar after the rough rolling is heated or heat-retained or the rough bar is heated. By heating only the edge, the temperature difference between the leading end and the trailing end of the coarse bar at the entrance side of the finishing rolling stand is within ± 30 ° C, and the temperature difference in the width direction of the coarse bar is within ± 15 ° C. In particular, it has features.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the composition of steel in the present invention will be described. In addition, the addition amount% of the component composition
Are all% by weight.

C (carbon): C is required to be at least 0.05% in order to impart necessary strength to gears, plates, and the like as transmission parts of an automobile. However, if the addition exceeds 0.4%, the load during cold rolling increases, and the productivity decreases. Therefore, in the present invention, the addition range of C is limited to the range of 0.05 to 0.4%.

Mn (manganese): Mn must be at least 0.1% in order to impart necessary strength to gears, plates, and the like as transmission parts of an automobile. However, if it is added in excess of 1%, the material greatly fluctuates due to fluctuations in the winding temperature during hot rolling. As a result, a uniform material cannot be obtained over the entire length of the coil. Therefore, in the present invention, the addition range of Mn is limited to the range of 0.1 to 1%.

Si (silicon), P (phosphorus) Cr (chromium): Si, P, and Cr are all ferrite-forming elements. In order to generate polygonal ferrite,
0.01% or more must be added, and the total amount of these three elements must be 0.1% or more. But,
Excessive addition of Si and Cr causes an increase in cost, and excessive addition of P causes grain boundary embrittlement. Therefore, in the present invention, the addition range of Si and Cr is limited to the range of 0.01 to 1%, and the addition range of P is 0.01 to 0.2%.
%, And the total added amount of Si, P, and Cr is limited to 0.1% or more.

The steel in the present invention contains 0.1% or less of Nb (niobium), 0.5% or less of Ti (titanium), 0.1% or less of V (vanadium) for securing strength. And, in order to improve ductility, Ca (calcium) may be added at 0.01% or less. In order to improve corrosion resistance, Mo (molybdenum), Ni (nickel), Cu
(Copper) may be added in an amount not to exceed 1%.

Next, the reasons for limiting the manufacturing conditions in the present invention will be described.

Rolling end temperature: When rolling is performed in the ferrite region, the structure is uneven in the thickness direction, and the dimensional accuracy of the stamped part after cold rolling is deteriorated. Therefore, in the present invention, the rolling end temperature is limited to Ar ₃ points or more.

Average cooling rate between the temperature at the end of rolling and (Ar ₃ -50 ° C.): If the cooling rate during the transformation from austenite to ferrite is high, bainitic ferrite is formed and the stamping after cold rolling is performed. The unloading accuracy deteriorates. Therefore, in the present invention, the average cooling rate between the rolling end temperature and the temperature of (Ar ₃ -50 ° C.) is set to 20 ° C./s.
Limited to ec or less.

Winding temperature: At a winding temperature of less than 550 ° C.,
The formation of a low-temperature transformation phase cannot be avoided. Therefore,
In the present invention, the winding temperature is limited to 550 ° C. or higher.

Cold Rolling Rate: In the present invention, the hardness required as a material for gears and plates as transmission parts of an automobile must be ensured by cold rolling. However, when the cold rolling reduction exceeds 70%, uniformity of the rolling strain in the coil width direction cannot be ensured.
Therefore, in the present invention, the cold rolling reduction is limited to 70% or less.

Temperature at the end of the row of finishing stands: If the temperature at the side of the row of the finishing stands fluctuates in the longitudinal direction and width direction of the coil, the recrystallization behavior of austenite during the finish rolling is different. Variations occur in the structure of the hot-rolled sheet such as the polygonal ferrite particle size and the pearlite volume ratio in the coil longitudinal direction and the width direction. This variation in the structure is reflected in the rolling strain introduced by the cold rolling, so that the punching dimensional accuracy after the cold rolling varies. In order to solve this problem, on the side where the finishing rolling stand row enters, the entire coarse bar is heated, or heat is maintained, or the edge of the coarse bar is heated,
The temperature difference between the leading end and the rear end of the coarse bar on the side of the finishing rolling stand row was within ± 30 ° C., and the temperature difference in the width direction of the coarse bar was within ± 15 ° C.

The heating of the coarse bar is performed by induction heating, direct energization, or a burner. For stabilization by heat retention, winding or rewinding into a coil box, a tunnel furnace, or the like is used.

In the present invention, the effect of the present invention is not lost even if rolling is performed by skin pass after hot rolling or cold rolling. After the hot rolling, the cementite is annealed for spheroidizing the cementite, so that the effect of the present invention is not lost.

Further, the smelting of steel may be carried out in either a converter or an electric furnace, and may be carried out using a thin slab. In this case, rough rolling can be omitted. As a rolling method, besides starting rolling after normal slab heating, direct-feed rolling, which starts rolling directly after melting and casting, may be performed.

[0028]

Next, the present invention will be described in more detail by way of examples.

A steel having the chemical composition of (1) to (17) shown in Table 1 was melted, and then hot-rolled and cold-rolled according to the manufacturing conditions shown in Table 2 to obtain a sheet thickness. A cold-rolled thin steel sheet having a thickness of 1.2 mm and a thickness tolerance of 0.05 mm was manufactured. The thin steel sheet produced in this manner was leveled by a leveler. Then, from this thin steel sheet, a diameter of 10
A 0 mm disc-shaped test piece was punched out, and the flatness of the test piece was evaluated.

As shown in FIGS. 1 and 2, the flatness was measured at an opening of 1.35 mm × 105 mm and a length of 200 m.
A jig in which a square gap of m was formed was prepared, the test piece was dropped into the gap, and the passing condition was evaluated. Table 2 shows the results.

[0031]

[Table 1]

[0032]

[Table 2] In Table 2, ○ indicates that the test piece passed through the gap, and x indicates that the test piece was caught on the way and did not pass through the gap.

As is clear from Table 2, all of the test pieces of steels 1 to 17 according to the method of the present invention passed through the gap, indicating that they had excellent flatness. On the other hand, after the end of rolling, (A)
r ₃ -50 ° C.)
It was found that none of the test pieces made of the comparative steels 2 ′, 12 ′, and 13 ′ outside the scope of the present invention passed through the gap and had poor flatness.

Also, in Table 1, No. For steel No. 8, when hot rolling was performed, the entire rough bar or the edge of the rough bar was heated, and each of the unheated bars was subjected to 55% cold rolling to obtain a sheet thickness. 1.2m
m, a sheet width of 1200 mm, and a sheet thickness tolerance of 0.5 mm were produced. Next, each of the front end, the center and the rear end in the longitudinal direction of the thin steel sheet manufactured in this manner was cut into six strips in the sheet width direction, and each strip was leveled by a leveler. Next, ten disc-shaped test pieces having a diameter of 100 mm were punched out from each strip, and the flatness of the test pieces (A, B) was evaluated (Examples 1 and 2 of the present invention).

In addition, No. For steel _No. 8, the entire rough bar or the edge of the rough bar was heated during hot rolling, but the heating conditions (ΔT ₁ ) and (ΔT ₂ ) were outside the range of the present invention. , Board thickness 1.2mm, board width 1200m
m, a cold-rolled thin steel sheet having a sheet thickness tolerance of 0.5 mm was manufactured, and the flatness of the test piece (C) was evaluated in the same manner as in the present invention (Comparative Example 1).

Further, No. Regarding steel No. 8, when hot rolling was performed, heating of the entire coarse bar or heating of the edge of the coarse bar was performed under the heating conditions within the range of the present invention. Cold-rolled to a thickness of 1.2
A cold-rolled thin steel plate having a thickness of 1200 mm, a width of 1200 mm and a thickness tolerance of 0.5 mm was manufactured, and the flatness of the test piece (D) was evaluated in the same manner as in the present invention (Comparative Example 2).

The results are shown in Table 3.

[0038]

[Table 3] As is clear from Examples 1 and 2 of the present invention in Table 3, the temperature difference between the leading end portion and the rear end portion of the coarse bar and the temperature difference in the width direction of the coarse bar on the side of the finishing rolling stand row are within the range of the present invention. When it is within, it was found that the flatness of the punched test piece was uniformly obtained over the entire length of the coil.

However, as is apparent from Comparative Example 1 in Table 3, the temperature difference between the leading end and the trailing end of the coarse bar and the temperature difference in the width direction of the coarse bar on the side of the row of the finishing rolling stands are not significant. When it was outside the range of the invention, it was found that the flatness of the punched test piece could not be obtained uniformly over the entire length of the coil.

Further, as is apparent from Comparative Example 2 in Table 3, when the cold rolling reduction is 80% out of the range of the present invention, the leading and trailing ends of the coarse bar on the side where the finishing rolling stand row enters. Even if the temperature difference between the part and the temperature difference in the width direction of the coarse bar is within the range of the present invention, since the rolling strain in the width direction due to the cold rolling becomes non-uniform, the flatness of the punched test specimen stably excellent. Was not obtained.

[0041]

As described above, according to the present invention, as a material such as a gear or a plate as a transmission part of an automobile, the dimensional accuracy of the punched part is excellent and the hardness of the punched part is ensured. The heat treatment step is unnecessary, and a useful effect that a dimensional accuracy after punching can be obtained over the entire coil length can be obtained.

[Brief description of the drawings]

FIG. 1 is a front view showing a test jig for flatness of a test piece.

FIG. 2 is a plan view showing a test jig for flatness of a test piece.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shoji Shioya 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Inventor Yasuyuki Takada 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Katsutoshi Ito 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term (in reference) 4K037 EA05 EA06 EA11 EA15 EA23 EA27 EB06 EB08 EB09 FA02 FB10 FC03 FC04 FC07 FD02 FD03 FE02 FM01

Claims (2)

[Claims] 1. C: 0.05 to 0.4%, Mn: 0.1 to 1%, Si: 0.01 to 1%, P: 0.01 to 0.2%, Cr: 0.01 Hot rolling at a temperature of Ar ₃ or more when hot rolling steel containing from 1% to 1% (or more by weight) and having a total amount of Si, P and Cr of 0.1% by weight or more. The average cooling rate between the rolling end temperature and the temperature of (Ar ₃ -50 ° C.) is set to 20 ° C./sec or less, and 550 ° C.
A method for producing a cold-rolled steel sheet excellent in flatness of a punched part, wherein the steel sheet is wound at the above temperature and then subjected to cold rolling of 70% or less. 2. When hot rolling the steel according to claim 1, the entire rough bar after rough rolling is heated or heat-retained, or only the edge of the rough bar is heated, The temperature difference between the leading end and the rear end of the coarse bar within the finishing roll stand row entry side is within ± 30 ° C, and the temperature difference in the width direction of the coarse bar is ± 15 ° C.
The method for producing a steel sheet according to claim 1, wherein: