JPS61127816A - Hot rolling method of austenitic stainless steel - Google Patents

Abstract

PURPOSE:To eliminate the need for an annealing stage after hot rolling by dividing a hot finish rolling group to the first half and second half, executing ordinary high-speed hot rolling with the finishing mills of the first half and rolling a steel at a specific draft or above with the finishing mills of the second half. CONSTITUTION:A heating zone or holding zone is preliminarily installed between the rolling stands in the hot finish rolling group and the hot rough rolling is executed in the stage of subjecting an austenitic stainless steel to hot rolling. The steel is then hot-rolled by the finishing mills in the stage before the heating zone or holding zone and is held at >=1,000 deg.C within 10 minutes in the heating zone or holding zone. The steel is succession hot-rolled at >=20% draft by the finishing mills in the rear stage and the temp. on the outlet side of the final hot finishing mill is maintained at >=950 deg.C. The steel plate right after the end of the final hot finish rolling is quickly cooled down to <=600 deg.C and is thereafter coiled.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a hot rolling method for austenitic stainless steel, mainly 18% CR-8% Ni system, and particularly relates to an annealing process after hot rolling (so-called hot rolling). The present invention relates to a hot rolling method that makes it possible to omit plate annealing.

Conventional technology Conventionally, in the production of austenitic stainless steel thin sheets, composition-adjusted molten steel is formed into a slab by continuous casting or ingot-blubber rolling, and the slab is hot-rolled to form a hot-rolled sheet. In the continuous annealing-pickling line, softening annealing (hot-rolled sheet annealing) and descaling are performed using a pickling method to remove scale attached to the surface, followed by cold rolling to form a thin sheet of the required thickness. Normally, the product is then subjected to final annealing and pickling to produce a cold-rolled thin steel sheet product. The purpose of annealing the hot-rolled sheet after hot rolling in this conventional general method is to equalize the mechanical properties and reduce the in-plane anisotropy of the cold-rolled product, and at the same time to reduce the in-plane anisotropy of the cold-rolled product. The objective is to soften the hot rolled sheet in order to facilitate processing during rolling, and for this purpose heat treatment at a high temperature of 1000° C. or higher is usually required. In such high-temperature heat treatment for annealing hot-rolled sheets, a large amount of thermal energy is consumed, and the line speed of the continuous annealing-pickling line is determined by the annealing. Therefore, if such hot-rolled sheet annealing can be omitted, it is expected that great benefits will be obtained in terms of energy saving and productivity improvement.

Recently, several proposals have already been made for omitting the annealing step for hot rolled sheets in the manufacturing process of thin sheets of austenitic stainless steel. Typical examples include, for example, Special Publication No. 5B-56642
There is a method of increasing the hot rolling end temperature simply for the purpose of making the hot rolled sheet soft, as shown in No. 1983, or as shown in, for example, JP-A-59-13028.
There is a method to improve the material quality of cold-rolled steel by lowering the biting temperature during hot finish rolling to leave the hot-rolled structure intact without considering the softening of the hot-rolled steel sheet.

Problems to be Solved by the Invention As mentioned above, the method for omitting the annealing process of hot-rolled austenitic stainless steel sheets has been developed in such a way that the workability in the subsequent cold rolling process and the material quality of the cold-rolled product are Both aspects have not been comprehensively studied, and each has advantages and disadvantages, and has not been put into practical use. In other words, of the two methods mentioned above, the former method is only intended to soften the hot-rolled sheet and improve workability in the cold rolling process, and to reduce in-plane anisotropy by making the material uniform, especially the material. On the other hand, the latter method was considered only in terms of material quality, and did not improve workability (cold-rollability) in the cold rolling process by softening the hot-rolled sheet. In addition, in hot-rolled sheets in which annealing after hot rolling is omitted, there is generally less Cr-removal layer directly under the surface scale layer, resulting in poor descaling performance in the pickling process, and as a result, scale remains easily even after pickling. , there is a problem that adversely affects the surface quality of cold-rolled sheets.
The reality is that this point has not been solved in the conventional methods described above.

This invention was made in view of the above circumstances, and even when the annealing process for hot-rolled sheets is omitted, it facilitates cold rolling in the subsequent process, and at the same time improves the in-plane anisotropy of the cold-rolled product. The purpose of the present invention is to provide a method for hot rolling austenitic stainless steel, which makes it possible to significantly reduce the earring ratio (that is, significantly reduce the earring ratio) and to significantly improve the descaling properties of hot rolled sheets. It is something to do.

Means to Solve the Problems In order to achieve the above objectives, we have softened the hot-rolled austenitic stainless steel sheet, improved the descaling properties, and improved the material quality of the cold-rolled sheet (especially reduced in-plane anisotropy). ), and as a result of intensive experiments and research, we divided the hot finishing mill group, which usually consists of 6 to 7 stands, into the first half and the second half, and the first half hot finishing mill After performing normal high-speed hot rolling, it is heated and held at a temperature of 100°C or higher for less than 10 minutes, and then rolled at a reduction rate of 20 inches or higher in a hot finishing mill in the latter half, and then rolled at the exit of the final finishing mill. Temperature 9
By maintaining the temperature at 50°C or higher, and then rapidly cooling the hot-rolled steel sheet immediately after hot finish rolling to 600°C or higher before winding it, the hot-rolled steel sheet becomes softer and the descaling property is improved. They discovered that it is possible to simultaneously reduce the in-plane anisotropy of a rolled sheet, and came up with the present invention.

Therefore, in the method of the present invention, when hot rolling austenitic stainless steel, a heating zone or a heating zone is installed between rolling mill stands in a group of finishing mills,
After the austenitic stainless steel slab is hot-rough rolled, it is hot-rolled in a hot finishing mill on the preceding stage of the heating zone or insulating zone, and then heated to a temperature of 1000°C or higher for 10 minutes in the heating zone or insulating zone. Then, hot rolling is carried out at a reduction rate of 20% or more using a hot finishing mill on the later stage, and the temperature at the exit of the final finishing mill is maintained at 950°C or higher, and further, the copper immediately after the final finishing rolling is The band is heated to 600℃
It is characterized by being rapidly cooled and then rolled up.

DETAILED DESCRIPTION OF THE INVENTION First, the findings that formed the basis for the method of this invention will be explained.

The inventors of the present invention obtained the following knowledge from the results of investigation on hot rolled sheets in which the annealing process of hot rolling was omitted.

(,) In order to soften the hot-rolled sheet, it is necessary to raise the finishing temperature of hot finish rolling.

(b) If hot rolling is performed under normal conditions and the annealing step is omitted, scale residue and black skin tend to occur in the pickling line, making it impossible to increase the line speed.

(C) When hot rolling is performed under normal conditions and the annealing process is omitted, the in-plane anisotropy of the cold rolled sheet is greater than that of the current material which is annealed.

(d) After hot rough rolling, the in-plane anisotropy of the cold rolled sheet can be improved by adjusting the grain size of the sheet bar.

As a result of further experiments and research based on these basic findings, we first found that a Cr-depleted layer was formed just below the surface scale layer by heating and holding at a temperature of 1000°C or higher for a short time in the middle of hot finish rolling. is promoted, which is advantageous for improving descaling properties, and recrystallization is promoted and the texture of the hot rolled sheet is randomized, which is advantageous for improving the in-plane anisotropy of the cold rolled sheet. The rolling reduction rate after heating and holding is 20% or more, and the final finish rolling machine exit temperature is 950.
It has been found that softening of the hot-rolled sheet can be achieved by adjusting the temperature to 600°C or lower, and that quenching to 600°C or lower immediately after final finish rolling (before winding) is also advantageous for improving descaling properties. By combining these conditions, even if the annealing process for hot-rolled sheets is omitted, the descaling properties are excellent, the workability in cold rolling is good, and the in-plane anisotropy after cold rolling is improved. This made it possible to obtain hot-rolled sheets with low hardness.

Next, the reason for limiting each hot rolling condition will be explained based on experimental results.

SUS, a typical austenitic stainless steel
A continuous cast slab of 304 steel type O with a thickness of 200 m was heated at 1240°C for 1 hour, hot rough rolled into a sheet bar with a thickness of 2° to 5Qw, and then immediately hot-rolled in 7 baths to a hot rolled plate with a thickness of P4wm. And so. During this hot finish rolling, heating and holding were performed under various conditions during the hot finish rolling. The hot-rolled sheet thus produced was subjected to descaling treatment by shit blasting and pickling without hot-rolled sheet annealing. Pickling was carried out in 20% H2SO4 at 80 °C.
The test was carried out under the conditions of immersion for 0 seconds, followed by 50 seconds of immersion in 20 ml (I(NO, +HF)).After pickling, the hot rolled sheet was examined for the presence or absence of scale remaining, and the descaling performance was evaluated. Furthermore, the hot-rolled sheet after pickling was cold-rolled according to a conventional method to obtain a cold-rolled sheet with a thickness of 0.70, which was then final annealed to produce a cold-rolled sheet product.The material properties, especially the in-plane anisotropy, were investigated. Ta.

FIG. 1 shows the relationship between the heating holding temperature in the middle of hot finish rolling and the descaling property of the hot rolled sheet in this experiment. From Figure 1, the heating holding temperature in the middle of hot finish rolling is 100
It can be seen that when the temperature is 0° C. or higher, the descaling properties are extremely good. However, heating and holding at a temperature of 1000° C. or more for 10 minutes or more increases the amount of scale loss, lowers the yield, and is not preferable from the viewpoint of energy saving. Therefore, in the present invention, the temperature is maintained at 1000° C. or higher for a short time of 10 minutes or less in the middle of hot finish rolling. In order to maintain it in this way,
1 between the intermediate rolling mill stands in the hot finishing mill group
It is sufficient to install a heating zone equipped with a heating means for heating the steel plate, or a heating zone that only keeps the steel plate warm so that the heat does not escape. Furthermore, there is no particular lower limit to the holding time at this temperature of 1000° C. or higher, but in order to fully exhibit the effect, it is usually preferable to hold the temperature for 10 seconds or more. In addition, the upper limit of the holding temperature in the middle of hot finish rolling as mentioned above is not particularly limited, but it is usually set at 1 from the viewpoint of scale loss or energy cost.
The temperature is preferably 200°C or less.

On the other hand, regarding the in-plane anisotropy of the product sheet after cold rolling, heating and holding during the middle of hot finish rolling is extremely effective. It was confirmed that retention can sufficiently reduce the earring rate.

Next, the second half of hot finish rolling, that is, 1000 rolls as described above.
The reduction ratio in finish rolling after heating and holding for at least 10 minutes above °C and the finishing temperature of final finish rolling affect the softening of the hot rolled steel sheet and, in turn, the workability in cold rolling. That is,
If the rolling reduction ratio after heating and holding is less than 20%, the strain introduced by rolling will be difficult to release even if the finish rolling temperature is increased, and the material will become hard. The rolling reduction ratio was set to 2C1 or more. On the other hand, even if the rolling reduction in the latter half of hot finish rolling after heating and holding is 20 inches or more, if the finish rolling end temperature is less than 950°C, recrystallization will not be completed and the hot rolled sheet will become hard. Finish rolling end temperature (
The final finish rolling mill exit temperature (1111I temperature) was 950°C or higher.

The cooling process after finishing hot rolling affects the descaling properties of the hot rolled sheet and the surface quality after pickling. That is,
If carbides precipitate during the cooling process after finishing hot rolling, the grain boundaries will become sensitized and undergo intergranular corrosion during the pickling process, causing rough skin. If the temperature is rapidly cooled to below 600'C during the process, precipitation of Cr carbides can be suppressed and roughness of the skin due to pickling can be prevented. By rapid cooling, the scale is easily broken and the descaling performance in the pickling process can be improved.
It was decided to rapidly cool the film to below 00°C and then wind it up. Incidentally, such rapid cooling is preferably carried out at a cooling rate of about 1.3 liters or more, for example, by water cooling or blast cooling.

As mentioned above, in the middle of the hot finish rolling after the hot rough rolling, the temperature K is kept at 1000°C or more for 10 minutes or less, and the rolling reduction rate in the latter half of the hot finish rolling is kept at 20% or more, and the final heat is By setting the exit side temperature of the finish rolling mill to 950°C or higher, and then rapidly cooling the steel plate immediately after finish rolling to 600°C or lower before winding it, descaling can be achieved even if the current hot-rolled plate annealing is omitted. It is possible to obtain a soft hot-rolled sheet that has good descaling properties in the process and excellent cold rollability in a cold rolling mill. Furthermore, even if this hot-rolled sheet is cold-rolled without hot-rolled sheet annealing, 1.
By maintaining the temperature at 000° C. or higher, recrystallization is sufficiently promoted and the hot-rolled texture is randomized, making it possible to obtain a cold-rolled sheet with a small earring ratio and little in-plane anisotropy. In addition, by heating and holding in the middle of hot finish rolling,
The finishing rolling temperature in the second half can be increased, and therefore it is also possible to increase the finishing rolling reduction ratio in the second half to obtain a hot rolled steel sheet that is thinner than before.

EXAMPLE Using a continuous cast slab of SUS 304 steel with a thickness of 200 m1, experimental rolling was carried out according to the process shown in FIG. 2, and the properties of the hot-rolled sheet and the cold-rolled product sheet were investigated. Table 1 shows the main hot rolling conditions. In addition, the slab heating before hot rough rolling is 1250
℃X l hr, and hot-rolled sheets were pickled at 80℃ for 20
%H2SO4 for 50 seconds, then 20 cm (HNO
3+HF) 50 seconds in the mixed acid solution! ! Pickled.

Further, heating and holding during the hot finish rolling was performed using an electric furnace, and temperature measurement was performed using both a thermocouple and a radiation temperature wire. The finished plate thickness of the cold-rolled product is 0.71111, and the final annealing is 11.
The test was carried out at 00°C and 11ml/g. In Table 1, 41-mm 15 was carried out using the method within the condition range of this invention (method of the present invention), and 41-mm 15 was carried out by the method within the condition range of this invention (method of the present invention), and 41-mm 15 was carried out by the method within the condition range of this invention (method of the present invention), and although heating and holding was performed in the middle of hot finish rolling, These conditions were carried out using a method (comparative method) that is outside the scope of this invention. Furthermore, for comparison, according to the current method, hot-rolled sheet annealing was performed at TT without heating and holding in the middle of hot finish rolling.
An example conducted under the conditions of oo'cxt- is shown in Table 1, 24.
&25.

Table 2 shows the results of investigating the characteristics of the hot-rolled sheets and cold-rolled product sheets in the above examples. In addition, in Table 2, the descaling performance evaluation is as follows: "O" indicates that there is no scale remaining after pickling and the descaling performance is good, "Δ" indicates that there is some scale remaining, and "X" indicates that there is a lot of scale remaining. Each case is shown below. The earring ratio (dull) was expressed by he in the following equation, where h+ is the height of the peak of the ear in the drawing test and ht is the height of the valley.

he=((h+ht)/(ht+hz)/z)xt
o.

Table 2 As shown in Table 2, the descaling property of the hot-rolled sheet obtained by the method of the present invention is different from that of the hot-rolled sheet obtained by the comparative method, which does not meet the conditions of the present invention, where scale remains. It is clear that this method is extremely superior, and is comparable to the current method of annealing hot-rolled sheets. In addition, the hardness of the hot-rolled sheet is softer than that produced by the comparative method, and is almost the same as that produced by the current method, making it easy to roll without edge cracking in the next step of cold rolling. did it.

Furthermore, the elongation of the cold-rolled product sheet is hardly affected by the hot-rolling conditions because finish annealing is performed after cold rolling, and the elongation is equal to or higher than that obtained by the current method, and is not shown in Table 2. However, there was almost no difference in 0 and 2% yield strength and tensile strength compared to those made by the current method.

Furthermore, the earring ratio, which is an evaluation value of the in-plane anisotropy of cold-rolled product sheets, is much lower when using the method of the present invention than when using the comparative method, and may even be lower than when using the current method. It is clear that the in-plane anisotropy is extremely small.

In addition, in the method of the present invention and the comparative method, when the heating holding time in the middle of hot finish rolling is 10 minutes, the relationship between the heating holding temperature and the earring ratio of the cold rolled product sheet is shown in FIG.

As is clear from FIG. 3, the earring ratio is significantly lowered by setting the heating holding temperature in the middle of hot finish rolling to 1000° C. or higher according to the method of the present invention.

Effects of the Invention As is clear from the above examples, the hot rolling method of the present invention makes it possible to produce a hot rolled sheet with extremely excellent descaling properties in the pickling process even if hot rolled sheet annealing is omitted. you can get
Therefore, it is possible to reduce manufacturing costs by saving energy and increasing line speed, and since recrystallization is sufficiently performed and a soft hot-rolled sheet is obtained, there is no occurrence of edge cracks during cold rolling. It can be easily cold-rolled, and since the in-plane anisotropy of cold-rolled products is reduced, the earrings produced during deep drawing are also smaller, contributing to improved yields during processing. The effect of this can be obtained.

[Brief explanation of the drawing]

Figure 1 shows the heating holding temperature in the middle of hot finish rolling and the hot rolled plate 5.
A correlation diagram showing the relationship with descaling properties, Fig. 2 is a process diagram showing the process in the example, and Fig. 3 shows the relationship between the heating holding temperature in the middle of hot finish rolling and the earring ratio of the cold rolled product sheet in the example. FIG.

Claims (1)

[Claims] When hot rolling austenitic stainless steel, a heating zone or a heating zone is installed between rolling stands in a group of hot finishing mills, and after hot rough rolling an austenitic stainless steel slab, the heating zone is Alternatively, hot rolling is performed in a finishing rolling mill located before the heating zone, and then heated to a temperature of 1000°C or higher in the heating zone or the heating zone.
Hold the roll for less than 0 minutes, and then use the finishing mill on the later stage to roll it for 20 minutes.
% or more, and ensure the final hot finishing mill outlet temperature is 950°C or higher, and then quickly cool the steel plate immediately after the final hot finishing rolling to a temperature of 600°C or lower. A method for hot rolling austenitic stainless steel characterized by winding.