JP3087639B2 - Manufacturing method of high hardness austenitic stainless steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-hardness austenitic stainless steel sheet used for a leaf spring or the like, and more particularly, to a product having an arbitrary thickness and hardness from a base steel sheet having a constant thickness. And a method for producing the same by rolling.

[0002]

2. Description of the Related Art Austenitic stainless steels such as SUS304 and SUS301 are cold-rolled to form work-induced martensite and are significantly work-hardened. Therefore, a very high hardness can be obtained, and it is used as a high-strength material such as a high-grade spring material.

[0003] As a general method of controlling the product hardness to a target value by cold rolling, when final finishing cold rolling is performed after annealing, the rolling reduction from the base material thickness to the target thickness is set to the product hardness. There is a method in which cold rolling is performed by changing according to the conditions. However, this method has the following problems.

[0004] 1) Since the rolling reduction differs for each product hardness,
It is necessary to change the base material thickness before final finish cold rolling according to the product hardness, which complicates the manufacturing process.

[0005] 2) The amount of work-induced martensite (hereinafter referred to as α ') generated during cold rolling greatly affects the product hardness, but α' varies greatly depending on the steel composition, rolling reduction, and rolling temperature. Large variation occurs, and it is difficult to hit the target hardness.

As a method for solving the above problem 2), Japanese Patent Application Laid-Open No. 62-199214 discloses a method in which the rolling speed and rolling reduction of each pass are used as operating factors, and the α ′ amount on the entrance side and the exit side in each pass, A method of controlling product hardness based on measurement information of a sheet thickness and a material temperature and rolling information of a rolling speed, a rolling reduction, and a rolling oil temperature is disclosed.

Further, Japanese Patent Application Laid-Open No. 54-81120 discloses a method in which the temperature of a rolling oil is supplied to a mill at a temperature of less than 30 ° C. for the purpose of reducing variation in product hardness and improving rolling efficiency.
Relation of M _S point (martensitic transformation starting temperature) and component composition, correlation with the rolling reduction in M _S point and cold rolling and M _S
There is disclosed a method in which a correlation between a point and a rolling speed is obtained in advance, and a cold rolling is performed by determining a component composition of a material, a cold rolling reduction and a rolling speed according to a target product hardness.

[0008]

However, the cold rolling method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 62-199214 controls the rolling reduction and the rolling speed in order to hit the product hardness to the target value. Therefore, the cold rolling is performed at the expense of the capability of the rolling mill. Therefore, the rolling efficiency is significantly reduced, and the productivity is greatly impaired.

The cold rolling method described in Japanese Patent Application Laid-Open No. 54-81120 can improve the rolling efficiency and the stabilization of the product hardness to some extent, but the steel composition must be changed for each product hardness. In addition, it is not suitable for high-mix, low-volume production, because the material thickness before final rolling must be changed. Further, no study has been made on the rolling temperature that greatly affects the formation of α ′.

[0010] It is an object of the present invention to provide a method for accurately producing a product having various target sheet thicknesses and hardness by rolling without variously changing the sheet thickness of the base steel sheet before final finish rolling. And

[0011]

Means for Solving the Problems The present inventors have conducted various experimental studies on a method of controlling the hardness of austenitic stainless steel during rolling, and have obtained the following findings.

[0012] The hardness of a product greatly varies depending on the amount of α 'generated during cold rolling. However, since the amount of α' varies greatly depending on the material temperature during processing, the material temperature during cold rolling is controlled. By doing so, it is possible to hit the product hardness to the target value even if rolling is performed to the fullest of the rolling capacity, and it is also possible to obtain any product hardness from the same base material thickness before cold rolling. It becomes possible. Therefore, the manufacturing process of the base material to be subjected to the final finish rolling can be greatly simplified.

The present invention has been made on the basis of such knowledge, and the gist of the invention is that "rolling an austenitic stainless steel plate having a constant thickness to a desired product thickness,
A method for producing a high-hardness austenitic stainless steel sheet for providing an arbitrary target hardness, in which a correlation between a rolling temperature, a rolling reduction, and a hardness of an austenitic stainless steel to be rolled is determined in advance, and based on the correlation. High hardness austenitic system characterized in that the number of rolling passes, target rolling temperature, rolling reduction and hardness after each pass are determined and rolled so that the target plate thickness and hardness are obtained in the final pass. Method of manufacturing stainless steel sheet ".

[0014]

FIG. 1 shows an example of a rolling facility for carrying out the present invention. This equipment is a symmetrical equipment centered on a six-stage reverse rolling mill 3. When rolling from the left side to the right side in the drawing, the steel sheet 1 unwound by the unwinding reel 2 is heated to a predetermined temperature by a device 4 for heating by directly energizing the steel sheet from the electrode 5.

As the heating device 4, there are direct current heating, induction heating, reducing gas heating, etc., but the equipment cost is low, the heating efficiency is good, there is no problem of the installation space, and the temperature of the material to be rolled can be instantaneously set. It is desirable to use a direct current heating device capable of changing the temperature to the above range.

The heating temperature of the heating device 4 can be controlled based on the temperature information of the heated steel sheet obtained by the temperature sensor 6 so that the steel sheet reaches the target temperature.

Immediately before entering the rolling mill 3, the heated steel sheet is supplied with rolling oil by a nozzle (not shown), and is rolled to a target thickness by the rolling mill. The rolled steel sheet is cooled by a cooling device 7 'and wound up by a take-up reel 2'. The rolled steel sheet after rolling is unwound from the reel 2 ′, heated by the heating device 4 ′, and rolled in the second pass. In this manner, the product is repeatedly rolled a plurality of times and finished to a target thickness.

The hardness after rolling is greatly affected by the rolling temperature. That is, the production amount of α ′ varies greatly depending on the rolling temperature, and the production amount increases as the temperature decreases. Therefore, a target product hardness can be obtained by appropriately controlling the temperature during rolling. The rolling temperature is not limited because it varies depending on the thickness of the base steel sheet, the number of passes, and the target product hardness. However, if the temperature exceeds 200 ° C., heat scratches may occur, so the rolling temperature is preferably 200 ° C. or less.

Next, the correlation between the rolling temperature, the rolling reduction, and the hardness will be described.

FIG. 2 shows the results of an examination of the relationship between deformation resistance and hardness when SUS301 is rolled at room temperature with various rolling reductions, and the relationship between deformation resistance and hardness is shown. It turns out that there is a very good correlation. If the deformation resistance value at the end of the final rolling pass can be predicted, the product hardness can be obtained in reverse from FIG.

Therefore, the present inventors have described the deformation resistance.
As a result of investigating and examining the relationship between the rolling temperature and the rolling reduction in detail, it was found that the deformation resistance can be accurately estimated by the following equation.

[0022]

(Equation 1)

[0023]

(Equation 2)

[0024]

(Equation 3)

[0025]

(Equation 4)

[0026]

(Equation 5)

[0027]

(Equation 6)

Here, σ: deformation resistance ε: equivalent strain [ε = 1.15 ln (outside plate thickness / base metal plate thickness)] T: material temperature A ₁ , a ₂ , a ₃ , b ₁ , b ₂ , c ₁ , c ₂ , c ₃ , d ₁ , d ₂ , η ₁ , η
₂ and B are coefficients, which are obtained in advance by regression of the amount of martensite formed in a multi-pass rolling test under light pressure and the results obtained in a tensile test of a rolled material. Although these vary depending on the components, Table 1 shows the results obtained by regression on SUS301 steel plate, which is a typical austenitic stainless steel.

[0029]

[Table 1]

The above σ _A is the deformation resistance of the austenite phase,
σ _M is the deformation resistance of the martensite phase, σ ₀ is the initial deformation resistance of the austenite phase, and M is the martensite generation rate.

Thus, by knowing the material temperature during rolling and the equivalent strain, it is possible to accurately know the deformation resistance after one pass from equations (1) to (6). That is, rolling temperature,
Determining the equivalent strain (reduction ratio) allows prediction of the deformation resistance (hardness).

Therefore, the rolling temperature, rolling reduction, and hardness can be arbitrarily set for each pass in the rolling pass schedule, so that the thickness of the base steel sheet and the target product thickness are kept constant to give various hardnesses. It becomes possible. Furthermore, the rolling speed can be set to the highest rolling speed in all passes, and high-efficiency rolling can be realized.

The method of determining the number of passes, the target rolling temperature, rolling reduction, and hardness for each pass will be described below.

In the method of the present invention, the target product hardness can be obtained by controlling the rolling temperature, no matter how many times the number of passes is set within the capacity range of the rolling mill. The smaller the number of passes, the better. That is, the number of passes is determined by the rolling reduction of each pass, and the rolling reduction may be determined from the allowable maximum rolling load and the maximum torque of the rolling mill.

The rolling temperature, rolling reduction, and hardness in each pass are determined by a rolling analysis simulator incorporating the above formulas (1) to (6) into a generally known cold rolling load formula. This simulator is a model that also takes into account temperature calculations such as the amount of temperature decrease due to cooling during rolling and the amount of heat generated during processing.
When the target hardness and the rolling reduction are input, a rolling temperature satisfying the conditions is calculated, and a heating temperature required before rolling is displayed.
For example, the rolling temperature, rolling reduction, and hardness in each target pass can be determined as follows.

The number of passes to obtain the final product hardness is determined.

Assume a target hardness for each pass.

As an initial condition, it is assumed that the hardness of one pass is (target hardness of final product−175) / (number of passes) +175.

175 is the hardness of the base steel sheet before rolling. Next, it is assumed that the rolling reduction is maximized in the initial pass in consideration of work hardening and is reduced as the pass progresses.

When the above assumptions are completed, the rolling temperature for achieving the target hardness of each pass is calculated by a rolling analysis simulator incorporating the deformation resistance formulas of the above formulas (1) to (6). Calculate the rolling load and torque when rolling is performed at the rolling reduction determined in the above.

If the calculated rolling load and torque exceed the determined rolling mill capacity, the target hardness and rolling reduction are corrected.

By repeating the above, the rolling reduction and rolling temperature of each pass are determined.

Using a rolling mill having the specifications shown in Table 2, a SUS301 steel plate having a base material thickness of 0.6 mm was used as a base material, and a target thickness of 0.3 mm and a target hardness of Hv400 were used. Table 3 shows an example obtained by calculating rolling conditions in each pass by pass rolling.

As a result of rolling by the rolling mill having the specifications shown in Table 2 by the rolling schedule shown in Table 2 in the pass schedule shown in Table 3, the hardness of the final product was 398, which was almost the target value.

[0045]

[Table 2]

[0046]

[Table 3]

As described above, even when the rolling is performed with the highest rolling efficiency, the final product hardness can be hit to the target value by accurately controlling the material temperature. Also,
By setting the heating temperature optimally using the above analysis model even from the same base material plate thickness, the final product hardness can be arbitrarily changed.

[0048]

EXAMPLE A SUS301 steel plate having a chemical composition shown in Table 4 and having a thickness of 0.6 mm and a width of 700 mm was used.
v400, 460, and 520 were set, and rolling was performed by a rolling mill having the specifications shown in Table 2 on the rolling line shown in FIG.

[0049]

[Table 4]

As an example of the present invention, rolling was performed at the number of rolling passes, rolling reduction in each pass, and rolling temperature shown in Table 5 obtained by the above-described method. As a comparative example, ordinary cold rolling was performed under the rolling conditions shown in Table 5. In the cold rolling, when the target hardness increases, the rolling reduction also needs to be increased. Therefore, the base material plate thickness must be increased.
As shown, steel plates of three different thicknesses were used.

After rolling, the Vickers hardness and the thickness of each steel sheet were measured, and the results are shown in Table 5.

[0052]

[Table 5]

As shown in the example of the present invention in the same table, by controlling the material temperature during rolling, the base material was cold-rolled from a thickness of 0.6 mm to a target thickness of 0.3 mm, and the target three A steel sheet almost satisfying the product hardness was obtained. From this, it is understood that an arbitrary product hardness can be obtained from the base material having the same thickness before cold rolling according to the present invention. Further, in the present invention, the rolling speed is cold-rolled at the maximum rolling speed in all passes, and the rolling speed is not restricted, so that the productivity is remarkably improved.

In the comparative example in which the steel sheet was cold-rolled without heating, the thickness of the base material had to be changed according to the target product hardness, so that the rolling speed was lower and the number of passes was lower than in the present invention. And the rolling efficiency deteriorates. Further, it can be seen that the final product hardness in the comparative example also has a large variation with respect to the target value.

[0055]

According to the present invention, it is possible to obtain an arbitrary product hardness from the same rolled base material sheet thickness, and it is possible to greatly reduce the number of rolling passes. In addition, since the rolling speed can be increased without restriction, the rolling efficiency is significantly improved, and the productivity can be significantly improved. Further, since the variation in the product hardness is small, the yield can be remarkably improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a rolling line used in the present invention.

FIG. 2 is a diagram showing a correlation between deformation resistance and hardness of a steel sheet.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2, 2 'reel 3 Rolling machine 4, 4' heating device 5, 5 'electrode 6, 6' Temperature sensor 7, 7 'cooling device

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-128108 (JP, A) JP-B-7-102378 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 37/00 B21B 3/02 C21D 8/02

Claims (1)

(57) [Claims] 1. A method for producing a high-hardness austenitic stainless steel sheet which is rolled from an austenitic stainless steel sheet having a constant thickness to a target product sheet thickness to give a predetermined target hardness. The correlation between the rolling temperature, rolling reduction, and hardness of the series stainless steel is determined in advance, and based on the correlation, the number of rolling passes so that the target thickness and hardness are achieved in the final rolling pass, and rolling of each target pass A method for producing a high-hardness austenitic stainless steel sheet, comprising determining a temperature, a reduction ratio, and hardness after each pass and rolling.