JPH0753287B2 - Method for cold rolling metastable austenitic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is intended to reduce variations in hardness of rolled products and to make the mechanical properties of the rolled products match a target value under a target plate thickness. The present invention relates to a cold rolling method for stable austenitic stainless steel.

[Prior art] Metastable austenitic stainless steels such as SUS304 and SUS301 are often obtained by annealing and then cold rolling to obtain high strength materials. That is, in such a metastable austenitic stainless steel, a part of the austenite phase is transformed into a hard martensite phase by cold rolling, so that the austenite phase is work hardened and the hard martensite phase is induced (this (Processing-induced martensite is hereinafter referred to as α '), and strength properties such as hardness, proof stress, and tensile strength can be significantly increased. Moreover, since these high-strength materials have excellent corrosion resistance and heat resistance in addition to high-strength characteristics, they are widely used as spring materials and steel belts and vehicle materials.

In the production of such high-strength cold-rolled materials of metastable austenitic stainless steel, it is necessary for the materials for the same application to have stable and constant mechanical properties even if the production opportunities are different. It is desirable that mechanical properties that meet the target values required for various applications be obtained by appropriate control of cold rolling. However, the amount of α ′ that has a great effect on the mechanical properties of this cold rolled material is
Since it is greatly affected by the rolling temperature and other factors, it is difficult to control the α'value required to obtain the desired mechanical properties. In particular, the thickness of cold-rolled products is often set, and it is difficult to control the target amount of α'when the set thickness is satisfied. In the conventional cold-rolling technology for metastable austenitic stainless steel, it cannot be said that the technology for controlling the amount of α'in the cold-rolled material so as to hit the target mechanical properties has been established.

For example, in the cold rolling method of this kind of material, most of the technologies related to the strengthening method of steel (for example, JP-A-48-4062).
No. 4, JP-A-49-115929, JP-B-49-16011, JP-A-54-120223 to 120225, etc.), the mechanical properties of cold-rolled material are stably hit with target values. Almost no technology related to the rolling method has been recognized, and the method of controlling the temperature of rolling oil during rolling proposed in JP-A-55-61303, and further, JP-A-54- 8112
While controlling the rolling oil temperature as proposed in Japanese Patent No. 0, the composition of the material, the cold rolling rate and the rolling rate were determined from the correlation between the Ms point and the cold rolling rate and the correlation between the Ms point and the rolling rate. Only the method of cold rolling is recognized.

[Problems to be solved by the invention]

The present invention has a problem that cannot be achieved by the conventional cold-rolling technology for metastable austenitic stainless steels. That is, the final product of cold-rolled material obtained by rolling while adjusting the amount of martensite induced during cold rolling is obtained. The challenge is to match the mechanical properties with the target mechanical properties, and for this reason, the following problems that have not been solved in the past are solved.

As mentioned above, when using metastable austenitic stainless steels such as SUS304 and SUS301 as high-strength materials,
It is necessary to maintain a high strength by cold rolling, but the strength is strongly affected by the presence of work-induced martensite (α ') phase in addition to the work hardening of the austenite phase, and the hardness is solid solution strengthened. C and N content of elements, rolling rate,
It is substantially determined by the amount of α '. The amounts of C and N are fluctuation values determined at the time of steel making, and the rolling ratio is a fluctuation value determined by the plate thickness before rolling and the product plate thickness (set value). Further, the amount of α'is significantly influenced by the material temperature during each pass rolling in addition to the component (γ stability) and the rolling rate (and the rolling distribution per each pass). The material temperature is the atmospheric temperature during rolling, the roll temperature,
In addition to factors such as rolling oil temperature, it also varies depending on the current rolling conditions such as rolling speed and amount of reduction per pass. Therefore, in order to obtain the target α ′ amount and thus to obtain the target hardness, it is necessary to control all of these numerous factors of variation individually, because the factors of each other interact with each other. It contains a very difficult problem.

For example, as in Japanese Patent Laid-Open No. 55-61303, it is not possible to hit the amount of α'necessary to obtain the target hardness by controlling only the temperature of the rolling oil. In the method disclosed in JP-A-54-81120, it is difficult to accurately hit the target amount of α ', since the material temperature directly related to the amount of α'formed is not taken into consideration. It is possible to control the material temperature before rolling, but even if the material temperature before rolling is simply controlled, the deformation heat generated during each pass rolling (depending on the rolling speed, the amount of reduction per pass, the rolling speed, etc.) Deformation heat is not related to the amount of deformation and material strength, and is also related to the amount of α'generation.) Therefore, even if only the material temperature is controlled, the target hardness can be accurately determined. It is practically difficult to hit the target, and it is also difficult to stabilize the mechanical properties.

An object of the present invention is to establish a technique for controlling the amount of α'during cold rolling which is influenced by such a number of factors so as to reach a target amount of α'and thus a cold rolled material having a target hardness. Is.

[Means for solving problems]

According to the present invention, as a cold rolling method for achieving the above object, a rolling mill is used when cold rolling an as-annealed steel strip of metastable austenitic stainless steel to a target plate thickness by multi-pass cold rolling. A method for producing a cold-rolled product having a target hardness and a target plate thickness that are set with the rolling speed and the amount of reduction in each pass of the steel strip passing through
Use the previously determined correlation between the amount of work-induced martensite (α ') corresponding to the target hardness of the cold rolled product and the rolling rate as the control target pattern. Of the α'amount, sheet thickness and material temperature, and the rolling information of rolling speed, reduction amount and rolling oil temperature in each pass, are input to a computer, and the input information is calculated by the computer. In addition, the rolling speed and the reduction amount in the next pass rolling necessary to approximate the target pattern value to the above target pattern value are calculated, and the rolling speed and the reduction amount in the next pass rolling are calculated based on this calculation signal. Cold-rolled material of target hardness and target thickness by repeating this control until the final pass to obtain cold-rolled material of metastable austenitic stainless steel. To provide a method.

In carrying out the present invention, the influence of the components on the mechanical properties of the metastable austenitic stainless steel cold rolled product,
Quantitative understanding of the effect of cold rolling rate and the effect of α ′ amount, the effect of the component (γ stability) on the α ′ amount produced during each pass rolling, the effect of the cumulative rolling reduction up to that pass ，
Quantitatively grasp the influence of the rolling conditions (material temperature during rolling, rolling speed and rolling amount / pass) of that pass,
Then, the α'amount of the steel strip during the cold rolling process is continuously measured on the inlet side and the outlet side of the rolling mill, and the material temperature directly related to the production amount of α'phase is also continuously measured on the inlet side and the outlet side of the rolling mill. It is necessary to do at least.

These items will be described below. Note that FIG. 4 shows an example of a control flow when the method of the present invention is carried out.

The following formula (1) shows that the inventors of the present invention use the SUS301 having different amounts of C and N and γ stability to determine the influence of the component, rolling rate and α ′ amount on the hardness (Hv) by X-ray diffraction. It is a mathematical expression of the results of the examination by the investigation of the microscopic organization.
As an example of this investigation, Fig. 1 shows the relationship between the amount of α'and the hardness when the rolling rate is changed. The relationship in FIG. 1 is almost expressed by the equation (1).

Hv = [{90 + 1300 (C% + 1 / 2N%) ^1/2 } ・^0.06 + 100] × Vα ′ + 1800 × {0.020 × ^0.35 + [8.7 × 10 ^-4 +0.04 (C% + 1 / 2N%)] × Vα ' ^1/2 -0.00628} ^0.36 x (1-Vα') ...
(1) where Hv: hardness, Vα ′: volume ratio of work-induced martensite (α ′): equivalent strain amount due to rolling, CR: rolling rate, where 100% rolling rate is 1.

Equation (1) shows that the hardness Hv of the cold rolled material is determined by the C, N content, rolling rate and α'content. Therefore,
Once the target product hardness is set, the C and N contents are known, and the rolling ratio is determined, it is possible to calculate by calculation how much α'a of the product cold rolled material is.

The rolling ratio is determined by the product sheet thickness (set value) and the original sheet thickness before annealing (variable value). Moreover, if the rolling rate is determined, the original sheet thickness as annealed is inevitably determined rather than the product sheet thickness. That is, as shown in FIG. 4, when the components (C, N content), the target hardness of the product, and the product thickness are obtained as product information, the original thickness before rolling as-annealed is determined. When the rolling rate is determined by this, the α'amount of the product cold rolled material is determined from the equation (1).

Next, the relationship between the rolling rate and the amount of α'is patterned. That is, the required α'amount and rolling rate of the cold rolled material product are determined from the above equation (1), but the pattern of the relationship between the α'increase and the rolling reduction in each pass up to this is determined. (FIG. 4 shows an example of a pattern in which the horizontal axis represents the cumulative rolling reduction (rolling rate) and the vertical axis represents the α ′ amount). The pattern of the amount of α'and the rolling rate becomes the control target value.

The rolling condition in each pass is controlled so as to approach the pattern of this control target value. The rolling speed and the reduction amount are adopted as the control targets, and the amount of operation of this rolling speed and the reduction amount (operation In determining the amount, the amount of α'in the next pass obtained from the process information of the previous pass and the cumulative reduction are determined so as to be as close as possible to the target value of the pattern.
That is, feedback control is performed. The process information required for this feedback control is the measured values of the α'amount on the inlet side and the outlet side of the rolling mill in each pass, the same plate thickness measurement value, and the same material temperature measurement value. Rolling condition values such as speed, rolling reduction, rolling oil temperature, etc.

Fig. 4 shows the flow of this feedback control. First, the detection part of this feedback control is the α'amount measuring unit 1 which measures the α'amount of the steel strip entering and leaving the rolling mill 12. A steel strip data measuring unit including a material temperature measuring unit 2 that also measures the material temperature and a plate thickness measuring unit 3 that also measures the sheet thickness, and a rolling process that includes a rolling speed measuring unit 4 and a rolling oil temperature measuring unit 6. It consists of a condition measuring unit.

Here, the electrical resistance method, the X-ray diffraction method, and the like are known for the measurement of the α ′ amount in steel, but they are not necessarily suitable for the online measurement as in the present invention. For this reason, a probe-type martensite detector using electromagnetic induction is used,
It is convenient to measure while supporting the tip of the probe on the surface of the moving steel strip in a non-contact manner. The details of this martensite detector are described in Japanese Patent Application No. 61-41466 (Japanese Patent Application Laid-Open No. 62-200262).
Japanese patent publication). The detected values for each pass from the steel strip data measuring unit and the rolling condition measuring unit are input to the computer 9 via the amplifier 7 and the A / D converter 8. The computer 9 compares the previously recorded α ′ amount and the pattern target value of the rolling rate with this input calculated value, and adjusts the rolling speed so that the calculated α ′ amount of the next pass is as close as possible to the pattern target value. A control signal is output to the controller 13 and the reduction amount controller 14. By repeating this for each pass, the target α ′ amount and the target plate thickness are hit at the end point.
In other words, by controlling the rolling speed and rolling amount in each pass on the way, the α ′ amount and rolling ratio in each pass do not deviate from the trajectory of the pattern target, the rolling of each pass is repeated and the final cooling is repeated. The amount of α'needed to obtain the target hardness of the rolled material and the target plate thickness are matched.

In this control, how is the process information (detected values from the above-mentioned steel strip data measuring unit and rolling condition measuring unit) obtained in each pass involved in determining the rolling conditions (rolling speed and reduction amount) in the next pass? Is an important point. This will be explained below.

(A). The amount of α'generated during each pass rolling The amount of α'newly generated during each pass rolling (denoted as ΔVα ') is the amount of α'after each pass rolling [denoted as (Vα') ₂ ].
And the amount of α'before each pass rolling [denoted as (Vα ') ₁ ], and ΔVα' is (Vα ') ₁ , material temperature before each pass rolling (denoted as T _in ), each primarily involved in rolling reduction at the time of pass rolling (referred to as d _i), it is expressed by the following equation (2).

ΔVα ′ = (Vα ′) ₂ − (Vα ′) ₁ = {As · _si2 ^Bs / (1 + As · _si2 ^Bs )} − (Vα ′) ₁ (2) where As is the Ni equivalent of steel It is a constant determined by (Nieq) and is given by As = −0.405 × Nieq + 8.82, where Nieq = Ni% + 0.35Si% + 0.5Mn% + 0.65Cr% + 12.6 (C% + N%).

Bs is a constant of 1.14, and _si2 is the equivalent strain amount due to rolling shown in the following equation (3). _si2 = {(Vα '/ Vγ) ₁ / As} ^{1 / Bs} + k ・ d _i・ ・
・ (3) where Vγ is the amount of austenite, which is (1-V
It is represented by α ').

However, k depends on the material temperature before each pass rolling and is represented by k = ab−T _in (4). a and b are constants and are represented by 3.1 and 0.025, respectively.

This k is an index for standardizing the rolling strain d _i in each pass rolling in relation to the material temperature, because the amount of α ′ generated in each pass rolling depends on the material temperature in each pass rolling. The effect of γ stability on the material temperature dependence of k is negligible.

FIG. 2 shows the reduction ratio / pass (d) in each pass and the change in the amount of α'when the rolling speed is changed. It is clear from Fig. 2 that the amount of α'is significantly affected by the rolling amount and rolling speed in each pass. Therefore, (2) above
~ The amount of α'after each pass is calculated by the equation (4), and the rolling amount and rolling speed of the next pass are controlled so that the calculated value is as close as possible to the pattern target value of the amount of α'and the rolling rate. It should be done.

(B). Material temperature change during rolling Next, it is necessary to estimate the material temperature change during each pass rolling and the material temperature change after each pass rolling and before the next pass rolling in order to determine the rolling conditions during the next pass rolling. These are the heat of deformation of the γ phase and the α ′ phase as heat generation, the heat of transformation from the γ phase to the α ′ phase, the cooling with rolling oil as the heat absorption, and the natural heat dissipation between each rolling pass. Determined by Of the heat generated, the deformation heats of the γ phase and α ′ phase are basically the deformation heats that deform the γ phase and α ′ phase, and these are the deformations unique to each pass rolling of the γ phase and α ′ phase. It is considered that it depends on the required strength, the amount of γ phase and the amount of α ′ phase, and the amount of deformation by each pass rolling, but it can be processed in relation to the hardness and the amount of deformation during each pass rolling. The heat of transformation from the γ phase to the α'phase can be treated with b · ΔVα '(b is a constant). Therefore, the heat generation amount (ΔT _up ) is ΔT _up = f (Hv, d, ΔVα ′) = a · (Hv · d) + b · ΔVα ′ ··· (5) However, a and b vary depending on the plate thickness. It is a coefficient.

On the other hand, cooling of the rolling oil as an endothermic component is represented by = (k '/ S) · (T in -T oiL).

Where T _in is the material temperature before each pass rolling, T _oiL is the oil temperature, S is the rolling speed (m / min), and k ′ is a coefficient that varies depending on the strip thickness.

Also, the natural heat dissipation between each rolling pass is expressed as = a + (b-C * S) * _Tout .

However, a, b and C are constants and T _out is the material temperature (° C) immediately after each pass rolling.

From these results, the change in material temperature during each pass rolling (Δ
T) is given by ΔT = a · (Hv · d) + b · ΔVα ′-(k ′ / S) · (T _in −T _oiL ) ... (6), and between each rolling pass Natural heat dissipation temperature (Δ
T _DOWN ) is represented by ΔT _DOWN = a + (b−C · S) · T _out ··· (7). Therefore, the material temperature at the inlet side (T
_{IN (N)} ) is given by T _{IN (N)} = T _in + ΔT _up −ΔT _DOWN ··· (8).

FIG. 3 shows changes in the rolling reduction / pass (d) at each pass and the material temperature (referred to as the entering material temperature) at the roll entrance side during the next pass rolling when the rolling speed is changed. . As shown in FIG. 3, the incoming material temperature at the time of the next pass rolling is significantly affected by the rolling amount and rolling speed at each pass. Therefore, the material temperature after rolling and the amount of α'in any pass are measured, the entering material temperature at the time of the next pass is calculated by the above equations (5) to (8), and the calculated value is used to calculate the above (2). The amount of α ′ after each pass rolling can be estimated by calculation according to the equation (4). More specifically, by substituting the equation (8) into the equation (4) and using the equation (2), ΔVα ′ of the next path can be estimated by calculation. This makes it possible to guess even after many passes.

Actually, based on the material temperature and the amount of α'measured after each pass rolling, what rolling condition (reduction amount and rolling speed) will be used to calculate what the amount of α'in the next pass will be. Then, of these calculated values, the calculated values of the rolling conditions (reduction amount and rolling speed) for the next pass that are as close as possible to the pattern target values of the α ′ amount and rolling rate are selected, and rolling is performed under these conditions. By performing predictive controlled rolling and repeating this control until the final pass, it is possible to obtain a product material having an α ′ amount close to the target value.

When performing this control, the target pattern of the α'amount and rolling rate is set so that it can be obtained under standard rolling conditions in the rolling facility. In addition, you can choose rolling conditions,
That is, the amount of reduction per pass and the upper and lower limits of rolling speed are set so as to meet the equipment specifications. Then, the rolling reduction is performed in increments of 1%, for example, and the rolling speed is repeatedly calculated at intervals of 5 m / min, for example. An arithmetic value that obtains the α ′ amount closest to the target pattern and is close to standard rolling conditions Select the conditions, set it to roll with this, and repeat rolling until the target product thickness is reached.

As described above, according to the present invention, when high strength material products such as springs, steel belts and vehicles are manufactured by cold rolling from metastable austenitic stainless steel,
A cold-rolled material with the target mechanical properties can now be manufactured accurately and stably with the target plate thickness, and the quasi-mechanical properties that greatly change due to fluctuations in the amount of process-induced martensite produced. It seems that there is a great deal of contribution to the progress of precision cold rolling technology for stable austenitic stainless steel.

[Brief description of drawings]

Fig. 1 shows the relationship between α'value and hardness when changing the rolling ratio in cold rolling of SUS301, and Fig. 2 shows the rolling reduction / pass (d) and rolling speed at each pass rolling. Fig. 3 is a diagram showing changes in the amount of α'when changed, Fig. 3 is a diagram showing changes in rolling reduction / pass at each pass rolling, and changes in inlet side material temperature when the rolling speed is changed. It is a control flow chart of an invention method.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C21D 7/02 E 7217-4K

Claims (1)

[Claims] 1. When cold rolling an as-annealed steel strip of a metastable austenitic stainless steel to a target strip thickness by multi-pass cold rolling, the rolling speed at each pass of the steel strip passing through the rolling mill is A method for producing a cold-rolled product having a target hardness and a target plate thickness set by using the reduction amount as an operation factor, wherein the amount of work-induced martensite (α ′ corresponding to the target hardness of the cold-rolled product is Amount) and the rolling ratio obtained in advance as the control target pattern, and the measurement information obtained by measuring the α ′ amount, the plate thickness and the material temperature on the inlet and outlet sides in each pass, In addition, by inputting rolling information such as rolling speed, reduction amount and rolling oil temperature in each pass into a computer, the input information is calculated in the computer and compared with the target pattern value to approximate the target pattern value. To calculate the rolling speed and reduction amount in the next pass rolling required for the purpose, control the rolling speed and reduction amount in the next pass rolling based on this calculation signal, and repeat this control until the final pass to obtain the target hardness and A cold rolling method for metastable austenitic stainless steel, characterized in that a cold rolled material having a target thickness is obtained.