JPH10263621A - Manufacture of cold rolled stainless steel strip for springs excellent in gloss - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a cold rolled stainless steel strip for springs by which the excellent glass of the surface of the strip is obtain even when rolling speed in finish cold rolling of the stainless steel strip for soft springs is raised. SOLUTION: This method is a method for manufacturing the cold rolled strip by which the cold rolled austenitic stainless steel strip is annealed, next, heated to 50-200 deg.C and rolled by one pass at the rolling speed of <=300 m/min and draft of >=22% with the rolling mill the diameters of work rolls of which are <=250 mm. By raising the draft, the strip is finished to high glass and on the other hand, to prevent deviation from the target hardness caused by work hardening at high draft, the strip is heated before rolling.

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 stainless steel strip for springs having good surface gloss.

[0002]

2. Description of the Related Art In general, a stainless steel strip for a spring is formed by subjecting a hot-rolled stainless steel strip to annealing, pickling and flaw removal, and then performing cold rolling, annealing and pickling to obtain a target hardness. In order to provide the steel sheet, it is manufactured by finish cold rolling (sometimes referred to as "temper rolling") at a predetermined draft.

[0003] The hardness of the steel strip is adjusted by the rolling reduction in finish cold rolling. However, since various levels are required depending on the application, the rolling reduction in finish cold rolling is also 8 to 70. % Over a wide range. When it is difficult to obtain the rolling reduction in one pass, finish rolling in a plurality of passes is performed. If the product thickness is small,
Since the work hardening is large, the target thickness cannot be obtained by one cold rolling, so that intermediate annealing is performed during the rolling.

[0004] The cold rolled stainless steel strip for a spring manufactured in this manner is required to be relatively soft, having a Vickers hardness of about 320 or less, since it is subjected to bending depending on the application. In the finish cold rolling of these soft spring stainless steel strips, if the initial temperature of the steel strip is normal temperature,
Since the target hardness can be easily obtained with a reduction ratio (light reduction) smaller than the reduction ratio in ordinary cold rolling, rolling is generally performed in one pass under light reduction.

[0005] However, if the austenitic stainless steel strip that has been annealed and pickled is subjected to finish rolling under light pressure at a rolling reduction of about 15% or less, a roll coating effective for increasing the gloss described below is not formed on the work roll surface. Therefore, the grain boundary erosion grooves generated on the steel strip surface by the pickling remain, resulting in poor gloss, or when an uneven roll coating is generated, the part where the grain boundary erosion grooves remain and the part which has disappeared. However, there is a problem that coexistence causes uneven gloss.

Generally, in cold rolling of stainless steel, the pressure and temperature at the interface between the roll and the material to be rolled are high, and relative slippage occurs, so that very surface oxides of the material to be rolled are transferred to the roll surface. Phenomenon occurs. The oxide is transferred to the roll surface to form a roll coating, and the formation of a uniform, dark brown, dark roll coating is said to be effective for increasing the gloss.

[0007] Steel strips with poor gloss or uneven gloss are not regarded as sound stainless steel products, which are important for aesthetics, and are rejected as defective products, resulting in a significant decrease in yield.

[0008] Poor gloss and uneven gloss become apparent as the rolling speed increases. For this reason, in the finish rolling of the steel strip for soft springs, a method of rolling at a low rolling speed of about 50 m / min or less is used as a means for preventing poor gloss and uneven gloss. Very low speeds of less than a minute may be used.

[0009] Thus, the finish rolling of a soft stainless steel strip for a spring has a very low efficiency as compared with ordinary cold rolling. On the other hand, as a method for obtaining a good surface gloss, for example, Japanese Patent Publication No. 57-13362 discloses a method in which the range of the surface roughness of the work roll used in the pass before the finishing pass and the work roll used in the finishing pass are specified, and the rolling reduction is 18%. A method of reducing the content to ２２22% is disclosed.

However, in this method, gloss is obtained by making the work roll surface roughness different between the pass before finishing and the finishing pass. However, when finish rolling is performed in one pass, the finish rolling before finishing is performed. Since there is no pass, the above method cannot be applied, and the rolling reduction is not more than 22%, so that the roll coating necessary for obtaining good surface gloss is not always generated.

Japanese Patent Application Laid-Open No. 8-174006 discloses a method in which a rolling oil used in the first pass has a lower viscosity than the viscosity of the rolling oil used in the second and final passes. Have been.

However, this method requires two rolling oil tanks, and a rolling equipment equipped with only one rolling oil tank needs to be newly installed, increasing the equipment cost.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and does not require any new equipment, and is suitable even if the rolling speed in the finish cold rolling of a soft spring stainless steel strip is increased. It is an object of the present invention to provide a method for producing a cold rolled stainless steel strip for a spring, which can obtain a high gloss of the steel strip surface.

[0014]

Means for Solving the Problems The present inventors prepared an austenitic stainless steel strip which had been subjected to cold rolling, annealed and pickled, and then subjected to one-pass finish cooling under various conditions of reduction and rolling speed. As a result of observing the steel strip surface after rolling and the surface of the work roll used for rolling in detail,
The following findings were obtained.

(1) When the rolling reduction is low and the rolling speed is high, grain boundary erosion grooves generated by pickling remain on the surface of the steel strip after rolling, and the luster before rolling (after pickling). There is not much difference and it is remarkably inferior. On the other hand, the surface of the work roll is almost in the same state as it is immediately after polishing, and the generation of roll coating generated by ordinary rolling is not recognized.

(2) With respect to the rolling reduction and the rolling speed at which the gloss becomes poor, the rolling reduction is increased by keeping the rolling speed at a certain value, or the rolling speed is maintained by keeping the rolling reduction at a certain value. As the amount decreases, strips or linear portions (irregularity) having different gloss appear on the surface of the steel strip in parallel with the rolling direction. On the other hand, uneven roll coating is generated on the work roll surface in the width direction.

(3) If the rolling reduction is further increased or the rolling speed is further reduced, a uniform and glossy steel strip surface can be obtained. Also, a uniform black-brown dark roll coating is formed on the work roll surface.

(4) As described above, the quality of gloss on the surface of the rolled steel strip due to the generation of roll coating depends on the rolling reduction and the rolling speed.

(5) Therefore, in the finish cold rolling of the austenitic stainless steel strip annealed and pickled, 1
In order to improve the gloss of the surface of the steel strip by passing, it is effective to increase the rolling reduction so as to generate a uniform and thick roll coating on the work roll surface.

(6) If the rolling reduction is increased, the hardness exceeds the allowable upper limit due to work hardening, and the hardness cannot be kept within a target range. Therefore, as a means for suppressing the work hardening, before the finish cold rolling, Preferably, the steel strip is heated to a predetermined temperature.

(7) The rolling reduction at which a good gloss is stably obtained at a rolling speed of 300 m / min or less depends on the type of steel strip, sheet thickness,
Although it depends on the annealing and pickling conditions, it should be at least 22%.

The present invention has been made based on such findings, and the gist of the invention is that "austenitic stainless steel cold-rolled steel strip is annealed, then heated to 50 to 200 ° C, and the work roll diameter is 250 mm. This is a method for producing a cold-rolled steel strip which is subjected to one-pass finishing cold rolling at a rolling speed of 300 m / min or less and a rolling reduction of 22% or more by the following rolling mill, wherein the material to be rolled is heated in advance, the rolling reduction and finishing Determine the relationship between the hardness of the steel strip after cold rolling and determine the heating temperature and rolling reduction within the above range so that the hardness after finish cold rolling is the target value, and perform finish cold rolling. Method for producing stainless steel cold-rolled steel strip for spring with good gloss "
It is in.

In a broad sense, the present invention anneals an austenitic stainless steel cold-rolled steel strip,
This is a method for producing a cold-rolled steel strip which is heated to 00 ° C. and subjected to one-pass finish cold rolling at a rolling speed of 300 m / min or less and a rolling reduction of 22% or more by a rolling mill having a work roll diameter of 250 mm or less.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limitation in the present invention will be described below. 1) Rolling speed and rolling reduction (300 m / min or less, 22% or more) The reason for setting the rolling speed to 300 m / min or less is that even if the rolling reduction at which high-density roll coating is generated is 300 m / min.
If the amount exceeds the limit, the amount of rolling oil introduced into the interface between the roll and the steel strip surface increases, and a large number of minute pit defects called oil pits are generated on the steel strip surface, so that a target level of gloss cannot be obtained. That's why. The lower limit of the rolling speed is not limited, but if the rolling speed is too slow, the production efficiency is reduced.
/ Min or more is desirable.

The following tests were conducted to examine the effects of rolling speed and rolling reduction on surface gloss. That is, an austenitic stainless cold-rolled steel sheet (SUS304) having a thickness of 1.0 mm is subjected to one-pass finish rolling at room temperature by changing the rolling speed and rolling reduction with a work roll having a diameter of 130 mm at various temperatures. The surface gloss was observed. The rolling oil used in this rolling was a mineral oil-based neat oil used for ordinary cold rolling of stainless steel sheets.

FIG. 1 is a diagram showing the observation results of surface gloss. As can be seen from the figure, in the region where the rolling reduction is small and the rolling speed is relatively high (A in FIG. 1), the roll coating is not generated and the grain boundary erosion grooves on the rolled material surface remain, so that the gloss is poor. Become. On the other hand, in a region where the rolling reduction is large and the rolling speed is relatively low (C in FIG. 1), uniform and sound roll coating is generated, so that the gloss is good. In addition, since uneven roll coating is generated in the intermediate region (B in FIG. 1), uneven gloss is generated on the surface of the rolled steel strip.

Next, the reason why the rolling reduction is 22% or more is as follows.
This is because, as shown in FIG. 1, 22% or more is required to obtain stable and good gloss at a rolling speed of 300 m / min or less. The preferred rolling reduction per pass is about 25 to 35%, and the upper limit may be any rolling reduction that can be rolled in one pass, and is not particularly limited because it depends on the type of steel.

2) Work roll diameter (250 mm or less) Since a stainless steel strip for a spring generally has a large thickness of 0.5 mm or less and has high deformation resistance, a stainless steel rolling mill for a small diameter work roll is used as a cold rolling mill. It is suitable. If the diameter of the work roll of the finish rolling mill exceeds 250 mm, the rolling load increases, and the number of passes is required, which is not suitable for efficient finish cold rolling. In addition, the oil film thickness interposed between the roll and the rolled material increases, and it becomes difficult to obtain good gloss. For this reason, the diameter of the work roll is set to 250 mm or less. The lower limit is not particularly limited, but if the diameter is too small, a flat plate shape cannot be obtained.

3) Heating temperature (50 to 200 ° C.) In the present invention, in order to eliminate the grain boundary erosion grooves formed on the steel strip surface in the pickling process and obtain a good glossy steel strip surface, sound roll coating is performed. Is rolled at a rolling reduction of 22% or more, which is generated. However, when the rolling reduction is increased in the finish rolling of the steel strip for a soft spring, the hardness exceeds the allowable upper limit due to work hardening. Therefore, in order to suppress the work hardening, the steel strip is preliminarily heated before the finish cold rolling.

The heating temperature of 50 ° C. or more is set at 50 ° C.
When the steel strip temperature exceeds 200 ° C., seizure flaws called heat scratches occur during rolling when the steel strip temperature exceeds 200 ° C. .

As a heating device for the steel strip, an electromagnetic induction heating device or the like is suitable. However, when the heating temperature does not need to be 100 ° C. or higher, a method of immersing the steel plate in a coiled hot water bath is also effective.

4) Relationship between Heating Temperature, Reduction Ratio and Hardness In the present invention, the relationship between the heating temperature of the material to be rolled, the reduction ratio and the hardness after finish cold rolling is determined in advance, and The finish temperature and the cold rolling are determined by determining the heating temperature and the rolling reduction within a range defined so that the hardness of the steel becomes the target value.

The hardness of the steel strip after cold rolling is closely related to the heating temperature and the rolling reduction of the material to be rolled, and the relationship among the hardness, the heating temperature and the rolling reduction is obtained in advance. The heating temperature and reduction required to obtain the target hardness after the finish cold rolling can be known.

The following tests were conducted to determine these relationships. An austenitic stainless cold rolled steel sheet (SUS304) having a thickness of 1.0 mm was heated to a heating temperature of 25%.
~ 200 ° C variously, with a work roll of 100 mm diameter, finish rolling at a constant rolling speed of 50 m / min,
The hardness of the rolled steel strip was measured. Mineral oil-based neat oil was used as the rolling oil. FIG. 2 shows the measurement results.

As for the heating temperature and the rolling reduction for obtaining the target hardness, the relationship between the rolling reduction and the hardness is previously determined for each heating temperature as shown in FIG. 2, and the target hardness is obtained when the rolling reduction is 22% or more. The final heating cold rolling is performed by determining the heating temperature and the rolling reduction to be performed. However, the heating temperature and the rolling reduction need to be in the range of 50 to 200 ° C. and 22% or more as described above.

5) Annealing The cold rolled steel strip before the finish cold rolling is annealed is as follows.
This is for softening in order to give a target hardness in the finish cold rolling, and may be performed at a temperature of about 1100 ° C. which is usually performed.

6) Austenitic stainless steel cold-rolled steel strip The target austenitic stainless cold-rolled steel strip is not particularly limited as long as it has a normal component composition for springs.
-CSP or the like is preferable.

[0038]

EXAMPLES The effects of the present invention will be described below based on examples. Table thickness 3.0mm, width 7 of the component composition shown in Table 1.
00mm hot rolled austenitic stainless steel strip (SU
S304) was subjected to annealing, descaling, pickling, and flaw removal, and then was subjected to one-time intermediate annealing by a six-stage cold rolling mill having a work roll diameter of 80 mm to perform cold rolling.
The sheet thickness after cold rolling was 0.6 mm.

[0039]

[Table 1]

Next, the cold-rolled steel strip was annealed at 1100 ° C. in a continuous atmospheric annealing furnace, immersed in an alkali molten salt at 500 ° C., and then pickled with a nitric hydrofluoric acid solution. First, a part of the cold-rolled steel strip that had been pickled was used, and the work roll diameter was 130
Finish cold rolling was performed using a 6-mm cold rolling mill with various heating temperatures and reduction ratios, and the hardness after rolling was measured to determine the relationship between the heating temperature, the reduction ratio, and the hardness. FIG. 3 is a diagram showing the relationship among the determined heating temperature, reduction ratio, and hardness.

Next, in the above-mentioned 6-high rolling mill, the heating temperature was set as shown in Table 2.
The reduction rate required to obtain the target hardness of 280 or 310 (Vickers hardness) when the temperature is 50 to 200 ° C. shown in FIG. In each condition shown in the following, one pass of finish cold rolling was performed.

[0042]

[Table 2]

The rolling oil used had a viscosity at 40 ° C. of 7
It was a mineral oil-based neat oil of cSt, and the surface roughness of the work roll was 0.10 μm in center line average roughness Ra. The heating of the steel strip was performed by immersing it in a hot water tank in the form of a coil up to 90 ° C., and at 150 ° C. and 200 ° C. in an electric heating furnace.

As a conventional method, as shown in Table 2, the reduction rate was 10 to 20% without heating before cold rolling.
And the rolling speed was as low as 30 to 80 m / min.

After the finish cold-rolled steel strip is degreased and washed,
The glossiness in the direction parallel to the rolling direction of the surface was measured, the gloss unevenness was visually evaluated, and the hardness was measured. The gloss was measured at an incident angle of 60 degrees according to the method specified in JISZ8741, and the hardness was measured by a Vickers hardness meter. Table 2 shows the results of measuring the gloss of the steel strip surface after degreasing and washing, the results of visually evaluating the unevenness of gloss, and the results of measuring the Vickers hardness of the steel strip.
Shown in

When the rolling reduction of A-2 in Table 2 was 10%, even at a low rolling speed of 50 m / min, grain boundary erosion grooves remained on the sheet surface, and the gloss was extremely poor. Further, the production state of the roll coating was also examined, but only a very light color was produced. Further, the rolling speed of A-1 is set to 3
Even when reduced to 0 m / min, uniform surface gloss was not obtained.

When the rolling reduction of B-1 was 15%, even at a low speed of 50 m / min, the roll coating became non-uniform and uneven gloss remained. The rolling reduction of C-1 and C-2 is 2
In the case of 0%, when the rolling speed is reduced to 30 m / min, the gloss becomes good.
Until the m length was rolled, there was uneven gloss. This is considered to be because a certain amount of time is required until uniform roll coating is performed at a rolling reduction of 20% and a rolling speed of 50 m / min. Further, the rolling speed of C-3 is set to 80 m.
/ Min, uneven glossiness occurred over the entire length of the steel strip. In other words, even a slight speed difference greatly changes the surface properties of the steel strip, which is problematic in obtaining a stable and good surface gloss, and the rolling efficiency is very low.

When the rolling reduction is 25% or more according to the present invention, even if the rolling speed is increased to 280 m / min, a uniform and dark brown roll coating is formed to obtain a good glossy steel strip surface. However, when the steel strip was not heated in advance, the hardness of the steel strip exceeded the upper limit of the target value. Further, when the rolling speed was 300 m / min or more, the surface gloss of the steel strip deteriorated, and the target glossiness could not be obtained.

[0049]

According to the production method of the present invention, it is not necessary to introduce new equipment, and the rolling reduction speed is increased by increasing the rolling reduction in the finish cold rolling to produce a uniform and dense roll coating. Even if it is performed, a stainless steel strip for a spring having good and uniform surface gloss can be easily obtained.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the rolling reduction rate and the rolling speed, which affects the quality of the surface gloss of a rolled material.

FIG. 2 is a diagram showing a relationship among hardness, rolling reduction, and rolled material heating temperature in SUS304 cold rolling.

FIG. 3 is a diagram showing the relationship among the determined heating temperature, reduction ratio, and hardness.

Claims (1)

[Claims] 1. An austenitic stainless steel cold rolled steel strip is annealed and then heated to 50 to 200 ° C., and a rolling mill having a work roll diameter of 250 mm or less has a rolling speed of 300 m / min or less and a rolling reduction of 22% or more. A method for producing a cold-rolled steel strip which is subjected to one-pass finish cold rolling in which the relationship between the heating temperature of the material to be rolled, the rolling reduction, and the hardness of the steel strip after finish cold rolling is determined in advance. A method for producing a stainless steel cold-rolled steel strip for a spring having a good gloss, comprising determining a heating temperature and a rolling reduction within the above-mentioned range so that the hardness after rolling becomes a target value and performing finish cold rolling.