JPS6112826A - Manufacture of inexpensive corrosin-resistant alloy steel sheet having excellent workability - Google Patents

Abstract

PURPOSE:To obtain inexpensively the titled corrosion-resistnt alloy steel sheet having excellent workability by applying hot rolling, winding, soaking, cold rolling, and the final annealing to a steel slab contg. a specified amt. of C, Cr, and P under specified conditions. CONSTITUTION:A steel slab, contg. <=0.05%, by weight, C, 10.0-15% Cr, >=0.04% and <=0.15% P, and the remainder of Fe and inevitable impuriries, is hot-rolled at >=1,100 deg.C to obtain a steel strip, which is wound up at >=700 deg.C and kept at >=600 deg.C for >=2hr. The strip is then pickled without being submitted to diffusion annealing, cold-rolled to the thickness of the final product without being subjected to intermediate annealing, and then applied with final anneaaling wherein the strip is heated at a temp. increasing rate of <=300 deg.C/hr in the temp. range above 300 deg.C to 650-900 deg.C. Besides, a minute amt. of Al, Ti, and Nb can be incorporated into the stee separately or compositely.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention contains 0.05% or less of C and 1% or less of Cr as essential components.
0 to 15%, P in excess of 0.04 to 0.15% is subjected to high-temperature hot rolling and high-temperature winding to cause self-diffusion annealing of the hot-rolled steel strip, and final annealing after cold rolling. The purpose is to provide an inexpensive alloy steel sheet with excellent workability by carrying out this process in a box-type annealing furnace.

Ferritic stainless steel is a well-known corrosion-resistant steel material, but in recent years, high-quality stainless steel has become oriented towards low C and low P. In other words, the current trend is to carefully select low-P raw materials or to perform special P-removal scouring to produce high-quality materials.

However, the present inventors discovered that high-P alloy steel has certain advantages over standard ferritic stainless steel, and attempted to develop a corrosion-resistant alloy steel sheet with high P and lower cost.

According to the present invention, C0.05% or less Cr 1. O, O~1! 5%P
More than 0.040% and less than 0.15%, balance Fe
and unavoidable impurities.
Hot rolling is started at 1 o or above, and the obtained steel strip is 700
℃ or higher, held at a temperature of 600°C or higher for 2 hours or more, then pickled without diffusion annealing, cold rolled without intermediate annealing to the final product thickness, and then heated in a box annealing furnace for 300°C. 'C temperature range 3000C/h
Provided is a method for producing an inexpensive corrosion-resistant alloy steel sheet with poor workability, which comprises final annealing by heating to a temperature range of 650 to 900° C. at a temperature increase rate of less than r.

The corrosion-resistant alloy mentioned here has corrosion resistance equivalent to that of ferritic stainless steel, which exists as a general corrosion-resistant material, but has a chemical composition with a P content of 0.
．． Since the regulation is higher than that of ferritic stainless steel, exceeding 0.40% and below 0.15%, it is possible to directly load ordinary steel blast furnace hot metal with a high P concentration into a converter without special deP treatment during the steelmaking process. It can be manufactured by adding auxiliary raw materials such as Fa-Cr alloy to the alloy.

Therefore, wood corrosion-resistant alloys can be used not only as a substitute for ordinary ferritic stainless steel, but also as a substitute for ordinary steel plates when it is unavoidable to use steel plates or coated steel plates, which are cheaper than stainless steel due to price constraints, even if corrosion resistance is insufficient. It can also be applied to applications where further painting treatment is performed using

Conventionally, ferritic stainless steel sheets, such as 5US410L and 5US430, are produced by hot rolling slabs obtained during the steelmaking process to form steel strips, which are then coiled and cooled, then held at around 800°C for one hour or more. Diffusion annealing is applied to
This is followed by pickling, cold rolling, and short recrystallization annealing in the normal manufacturing process.

Looking at the manufacturing process here, diffusion annealing is performed with the aim of making the steel a homogeneous metal structure. In other words, an alloy containing pure Fe and more than about 11.5% Cr has a single ferrite phase from room temperature to high temperature, but 5U
Ferritic stainless steels such as S41OL and 5US430 contain C, which is mixed into raw materials and during steel manufacturing.
Since it contains austenite-forming elements such as NMn, an austenite phase appears in addition to a ferrite phase when exposed to high temperatures. This austenite phase contains a lot of C and N.
However, when heat treated at around 800'C for 2 hours or more and slowly cooled, the supersaturated solid solution C and N precipitate as Cr carbides and Cr nitrides. Therefore, 5U
The metallographic structure of hot-rolled steel strips such as S41OL and 5US430 is a mixed structure of a ferrite phase and a phase in which C and N are supersaturated in solid solution. In order to eliminate the present phase and obtain a structure consisting of ferrite and uniform precipitates, the above-mentioned diffusion annealing was performed. If this diffusion annealing is omitted, not only will the workability of the product decrease, but also edge breakage will occur during cold rolling, which will impede manufacturability.

The hot-rolled strip of steel used in the method of the present invention also exhibits a metallographic structure similar to that of 5US430 when hot-rolled in the same manner as conventional methods.
By high-temperature coiling, solid solution C and N in the austenite phase generated during hot rolling are precipitated as carbides and nitrides, softening the material itself, causing so-called self-diffusion annealing, which is similar to the above-mentioned diffusion annealing. It is intended to have a similar effect.

Next, the reasons for limiting the composition of the steel used in the steel of the present invention will be described.

C: If it is too high, the martensite phase partially generated in the hot rolled state becomes hard, impairing the toughness and ductility of the material in the hot rolled state. In addition, since diffusion annealing is not performed, if the martensitic phase becomes too hard, it will harm the toughness, workability, and weldability after cold rolling annealing. Therefore, the upper limit is set at 0.05%.

The lower limit of 10% for Cr is the minimum amount necessary to maintain corrosion resistance. Further, a higher Cr content is advantageous in terms of corrosion resistance, but if it is too high, toughness decreases and costs increase, so the upper limit is set to 15%.

P: If it is less than 0.04%, preliminary deP in blast furnace hot metal or special deP treatment in a converter is required, and the advantage of producing inexpensive steel with excellent workability and corrosion resistance is lost. Since the effect of improving workability due to enrichment of C cannot be obtained, the lower limit is set to an amount exceeding 0.04%. Moreover, if it exceeds 0.15%, it is unfavorable in terms of toughness and hot workability, and the workability of the product also deteriorates, so the upper limit is set to 0.15%.

In addition, this application uses trace amounts of AI and T, which are generally added to provide processability and corrosion resistance, as components other than essential components.
i, Nb may be contained alone or in combination without any problem. Furthermore, it is assumed that Si, Mn, P, and S may also be included as a matter of course according to the JIS standards for stainless steel.

Next, the reason why hot rolling is started at 1100° C. or higher is to minimize hot rolling in the temperature range near the nose of the γ loop. After winding at 700°C or higher, the temperature is kept at 600°C or higher for 2 hours or more to precipitate C1N, which is supersaturated in solid solution in the matrix, as carbides and nitrides. This is to make the rolled steel strip softer and to improve the workability of the cold rolled steel strip. At a temperature below 600"C, the above-mentioned material change does not occur, so there is no point in holding it. The reason for this can be understood from Figures 1 and 2.
The figure basically shows 13%Cr, 0.02%C10,08%
After hot rolling and winding the corrosion-resistant alloy steel containing P, 6
00 '(:!) After holding the temperature for 2 hours, only descaling was performed without diffusion annealing, and finish annealing was performed on the cold rolled sheet obtained by one cold rolling at a heating rate of 12
The effect of the hot rolling start temperature on elongation was investigated when the annealing was performed in a box-type annealing furnace at a slow rate of 0° C./hr. From this figure, there is a tendency for the elongation to improve as the hot rolling temperature increases, and the elongation improves when the hot rolling start temperature is 1100° C. or higher. In addition, Fig. 2 basically shows 13%Cr, 0.02%C10,
Corrosion-resistant alloy containing 08% P was extracted at 1250°C, 95
After hot rolling under 0°C finishing conditions, and then holding at a temperature up to 600″C for a certain period of time, only descaling was performed without diffusion annealing, and the cold rolled sheet obtained by one cold rolling was finished. The influence of the holding time up to 600° C. after hot-rolling on the r value and elongation was investigated when annealing was performed in a box-type annealing furnace with a slow heating rate of 120° (!/hr). From this figure, it can be seen that holding the temperature up to 600°C for a long time after hot-rolling and winding improves the r value and elongation rather than rapidly cooling it. Use a heat retention pit.

Furthermore, in the present invention, the conditions for final annealing are specified for the following reasons. In other words, the temperature increase rate in the temperature range of 300°C or higher is 300'C/hr or less.
In the temperature range below 300°C, recovery and recrystallization of the material cannot occur, and the heating rate can be set arbitrarily; however, in the temperature range above 300°C, the influence of the heating rate on workability increases,
Since the effect of improving workability is not sufficient at a heating rate exceeding ℃/hr, the F limit is set to 300' (!/hr. Also, the maximum annealing temperature is set to 650°C or higher and 900°C or lower).
0'C! Recrystallization is insufficient at temperatures below 900°C.
If the temperature exceeds 900'C, the crystal grains will become coarser and the surface quality of the product after processing will deteriorate, so the upper limit is set at 900'C. Note that the holding time at the annealing temperature may be arbitrary.

3. Example 1 A slab with a thickness of 200 mm was made using steel with the chemical composition shown in Table 1, and was hot-rolled to 3.0 m under the conditions shown in Table 2.
A steel strip with a thickness of I looked into it. The elongation, r value, Erichsen value which is a model formability test value, and CCV (the smaller the value, the better the deep drawability) of these steel plates are shown in Table 2 together with the hot rolling conditions.

From Table 2, as seen in method 1.2.3 of the present invention, the hot rolling start temperature is 1180°C or more, the first winding temperature is 750°C or more, and the temperature after 9th winding is 600'C, which is maintained for more than 2 hours. For example, even without diffusion annealing, the elongation, r value, Erichsen value, and CCv are good, and it is clear that the workability is excellent. In contrast to the method of the present invention, comparative method l
, 2 starts hot rolling at 1050°C, winds up at a temperature of 650°C or less, and then rolls from 0 to 0 at a temperature of up to 600°C.
This case was held for 5 hours, and then cold rolled and box-shaped annealed without diffusion annealing. These comparative methods 1.
Regarding each characteristic value of No. 2, no improvement in processability was observed compared to the method of the present invention.

Example 2 A slab with a thickness of 200cm was made using steel with the chemical composition shown in Table 1, and the hot rolling start temperature shown in Table 3 was 121O ℃, 1 coiling temperature was 800℃, and the temperature after coiling was 600℃ or more. 2.
A steel strip with a thickness of 3.0 mm is obtained by hot rolling for 8 hours, followed by descaling, cold rolling once to a thickness of 0.7 am without diffusion annealing, and final annealing by box annealing. The influence of the heating rate of final annealing on the workability of cold-rolled steel strips in a furnace was investigated. The results are also shown in the same table. Elongation, r value, Erichsen value, CC■ of these steel plates
are also shown in Table 3.

From Table 3, as seen in the methods 1 to 4 of the present invention, hot rolling is performed by the method of the present invention, and the subsequent final annealing is performed at 60 to 2.
If carried out according to the method of the present invention in which the temperature is raised at 50° C./hr, held at 1320° C. for 4 hours, and then cooled in a furnace, all of the characteristic property values are improved and workability is improved. On the other hand, in the comparative method I, hot rolling was carried out exactly the same as in the method of the present invention, and then the final annealing was carried out at a temperature of 400°C/hr.
This is the case where the sample was held for 4 hours and cooled in the furnace. The characteristic value of this comparative method 1 is lower than that of the present invention.

& Sumire Mamedate] According to the present invention, a corrosion-resistant alloy steel sheet having good properties can be produced using inexpensive raw materials and without special dephosphorization treatment.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the elongation rate of the steel plate and the hot rolling start temperature and coiling temperature. Figure 2 shows the relationship between the elongation rate and r value of the steel plate and the
The relationship between the cells held up to 0°C is shown. Figure 1: Shuken Kaihaku Gohana (0C) Figure 2

Claims (1)

[Claims] A steel slab containing C 0.05% or less, Cr 10.0 to 15%, P more than 0.040% and 0.15% or less, and the balance Fe and unavoidable impurities is heated at 1100°C. Hot rolling is started with the above steps, and the obtained steel strip is rolled up at a temperature of 700°C or higher, held at a temperature of 600°C or higher for 2 hours or more, then pickled without diffusion annealing, and intermediately annealed to the final product thickness. It is then cold-rolled in a box-type annealing furnace at a temperature of 300°C or higher.
A method for producing an inexpensive alloy steel sheet with excellent workability, which comprises performing final annealing by heating to a temperature range of 650 to 900°C at a temperature increase rate of 0°C/hr or less.