JPH01104717A - Manufacture of ferritic stainless steel sheet excellent in formability - Google Patents

Abstract

PURPOSE:To manufacture a ferritic stainless steel sheet excellent in formability at a low cost by subjecting a ferritic stainless steel having a specific composition consisting of C, N, Cr, Mn, Si, and Fe to hot rolling and continuous annealing under respectively specified conditions. CONSTITUTION:A ferritic stainless steel having a composition which consists of, by weight, 0.0010-0.0070% C, 0.0010-0.0150% N, 15-22% Cr, 0.1-1.5% Mn, >=1.0% Si, and the balance Fe with inevitable impurities and in which C+N is regulated, preferably, to 0.0020-0.0160% is heated to 1000-1200 deg.C, and, after hot rolling is finished at 700-850 deg.C, the resulting steel plate is wound up at 400-600 deg.C. Subsequently, this hot rolled plate is continuously annealed at 750-850 deg.C, which then is subjected to cold rolling and annealing according to the ordinary method. By this method, the ferritic stainless steel sheet requiring no expensive additive elements, excellent in formability and surface attractiveness besides its inherent characteristics, such as corrosion resistance, strength, and suctility, and free from ridging and roping can be obtained.

Description

Detailed Description of the Invention "Object of the Invention" (Industrial Field of Application) The present invention is a method for manufacturing 18% Cr ferritic stainless steel cold-rolled steel sheet, which is widely used in kitchen equipment, automobile parts, and other durable consumer products. It is related to.

(Conventional technology) In addition to the corrosion resistance, strength, and ductility inherent to stainless steel, it has excellent formability and has been used for kitchen equipment and automobile parts, which require a beautiful surface after forming. As a stainless steel plate, 18%C represented by SOS 430
Ferritic stainless thin steel sheets are widely used because they are superior to austenitic stainless steels in terms of cost and resistance to stress corrosion cracking. The chemical composition of the steel plate is % by weight (hereinafter simply referred to as %).
Cr: 10-20%, C: O, 1% or less, Si:
1.0% or less, Mn: 1.0% or less, N: 0.05
00% or less is common, and after blooming a continuously cast slab or ingot, hot rolling it,
After that, it is subjected to batch annealing or continuous annealing heat treatment for the purpose of homogenization and softening, followed by cold rolling, annealing, and temper rolling to produce a product.

However, in recent years, Al,
A continuous annealing method in which elements such as Ti and Nb are added is also used.

Furthermore, in order to shorten the process, a method of omitting hot-rolled sheet annealing has been proposed in which hot-rolled sheet annealing is not performed, and hot-rolled sheet is directly cold-rolled after pickling. For example, after hot-rolling ferritic stainless steel containing Al, it is heated at 900 to 1100°C for 10 minutes, followed by finish rolling to promote the γ-α transformation and the precipitation of HN in order to reduce ridging and secure the r value. Unexamined Japanese Patent Publication No. 1983
-25933 etc.

As a technique for improving deep drawability, there is a method of reducing the amount of C in SUS 430 tA to about o, oi% and adding 0.2% of Ti;
), 0.2%/l was added to increase the cold rolling rate from 80% to 85~
There are special public works such as Sho 61-50126 that increase the rate to 95%.

As a countermeasure against ridging, many techniques have been reported that leave a large amount of strain during hot rolling and promote recrystallization during annealing in order to fracture the cast structure that causes ridging.

For example, low temperature region hot rolling method (Japanese Patent Publication No. 45 34016),
High-temperature short-time annealing method adding Ti and Nb (JP-A-5L-
149116), a method of repeating recrystallization including during hot rolling (Japanese Patent Publication No. 58-199822), a method of once making the γ phase appear during hot-rolled sheet annealing (Japanese Patent Publication No. 48-246)
11) etc.

Another method is to crush the cast structure by applying a reduction of 20% or more, and then reheating and hot rolling (Japanese Patent Laid-Open No. 59-2322
32) At least one pass of hot rolling at a strain rate of 150 S-
'Method of crushing the cast structure by rolling with
10217> etc. have been proposed.

(Problems to be Solved by the Invention) When batch annealing is performed in the manufacturing method of ferritic stainless steel sheets described at the beginning of the prior art, a processing time of several tens of hours is required, which poses a problem in terms of productivity. There is, Af,
Cold-rolled materials manufactured by short-time continuous annealing with the addition of elements such as Ti have a drawback in that their properties are not as good as those of batch-annealed materials. Also, the above-mentioned Japanese Patent Application Laid-Open No. 51-14911
Regarding 6, the raw material cost increases due to the addition of Ti5Nb, etc., and technologies such as low-temperature hot rolling and repeated recrystallization have problems such as restrictions on hot rolling conditions, frequent occurrence of steel plate surface defects, and increased roll load. In addition, the method of making the γ phase appear has low productivity because a hard martensitic phase is not produced and very slow cooling is required.

Regarding the method of crushing the cast m weave and then reheating and rolling, there is a decrease in productivity due to the increase in the number of steps, and a method of hot rolling at least one pass at a strain rate of 150 S-' or more etc., there are problems such as severe restrictions on hot rolling conditions. The present invention was devised to solve many of the problems of the conventional method, and does not require expensive additive elements, and by rationally setting composition hot rolling conditions and annealing conditions. In addition to the inherent characteristics of stainless steel, such as corrosion resistance, strength, and ductility, our objective is to provide an economical method for manufacturing ferritic stainless thin steel sheets that have excellent formability and surface beauty, and are free from ridging and roving. purpose.

"Structure of the invention" (Means for solving the problem) In order to achieve the above-mentioned purpose, the inventors have (1)
C in weight%: 0.0010-0.0070X, N: 0
．． 0010~0.0150X, Cr:15~22%,
Mn: 0.1-1.5%, Si: 1.0
% or less, with the remainder consisting of Fe and unavoidable impurities, is heated to 1000-1200°C, then hot-rolled at 700-850°C, and heated to 400°C.
It is rolled up at ~600°C, and then the hot rolled sheet is heated at 750~850°C.
A method for producing a ferritic stainless steel sheet with excellent formability, which comprises continuous annealing at ℃, followed by cold rolling and annealing according to a conventional method.

(2) (C+N): 0.0020 to 0.01
The present invention uniquely proposes a method for manufacturing a ferritic stainless steel sheet with excellent formability, as set forth in claim 1, characterized in that the ferritic stainless steel sheet has an excellent formability of 60%.

(Function) The present invention relates to a method for manufacturing an 18% Cr stainless steel plate based on ultra-low carbon.

First, the reasons for limiting the chemical composition, content, rolling conditions, and heat treatment conditions described in the claims will be explained.

C: 0.0010 to 0.0070% The limitation of Cl is one of the particularly important constituent requirements in the present invention. FIG. 1 shows the relationship between Ci and the T value Δr. In this test, the amount of N was 0.0034% to 0.004%.
While keeping the cl constant at 5%, the cl is 0.0010% to 0.
．． Using steel changed to 0142%, C-shape and formability r
This study investigated the relationship between

The test steel was heated to 1100°C and hot-rolled at 750°C.
It was wound up at 40°C. Thereafter, it was annealed at 800°C and cold rolled at a cold rolling rate of 80%, and the cold rolled sheet was annealed at 800°C.

As is clear from this graph, a decrease in the lower rise Δr was observed as the C content decreased, and a sufficient r value of 1.3 or more was shown when the C content was 0.0070% or less.

Based on this result, the upper limit of C was set at 0.0070%. - Regarding the lower limit, even if the carbon content is extremely low, such as less than 0.0010%, there is a limit to the effect it has on the material quality, as can be seen from the diagram, and that it is possible to obtain extremely low carbon materials for steelmaking. 0.001 because it is difficult and economically disadvantageous to do so.
The lower limit was 0%.

Conventionally, this type of ultra-low carbon ferritic stainless steel
It has been used to improve corrosion resistance and weld toughness, but it has not been used in fields where formability is required. This is one of the important features of the invention.

N: 0.0010 to 0.0150% N hardly impairs corrosion resistance and is an effective element for ensuring strength, so it may be added. However, if it is added in excess of 0.0150%, the moldability will deteriorate significantly, similar to the high C material, so this value was set as the upper limit. - On the other hand, even if the N content is extremely low, the effects of the present invention will not be impaired, but reducing the N content to an extremely low value during steel manufacturing will result in a corresponding increase in cost (so 0.0010%).
was set as the lower limit.

(C+N): 0.0020 to 0.1 60% Figure 2 shows N when C is 0.0070% or less.
A case in which the hot-rolling finishing temperature was set to 750°C as specified in the present invention using steel with different
The relationship with moldability was investigated under two process conditions of 0° C., and the other manufacturing conditions were the same as those described above for the regulation of the amount of C.

Under the conditions of the present invention, when (C+N)l is 0.0160% or less, the r value is high and exhibits excellent formability, so the upper limit is set to 0.0.
It was set to 160%. Although there is no particular problem with the lower limit from the viewpoint of material quality, the lower limit was set at 0.0020% from the economical point of view of steelmaking operations.

If the hot-rolling finishing temperature is too high, the cumulative strain in the ultra-low carbon hot-rolled sheet will be reduced, and the effect of improving material quality by promoting recrystallization in the next annealing process will be reduced, resulting in a decrease in Δr and an increase in Δr. become.

It is no exaggeration to say that the synergistic effect of the adoption of the ultra-low carbon components of the present invention and the lowering of the hot rolling finishing temperature has resulted in the improvement in material quality as shown in FIG. 2.

Cr: 15-22% The steel of the present invention is targeted at 18% Cr stainless steel of the SOS 430 series. If Cr% is less than 15%, the corrosion resistance will be poor, and if it is added in excess of 22%, the expected effect will not be achieved, but on the other hand, the cost will increase and the steel sheet of the present invention will not be useful in the field of application, so the lower limit and This is set as the upper limit.

Mn: 0.1 to 1.5% Mn is effective as a deoxidizing element and a solid solution strengthening element, but if it is added in excess of 1.5%, the r value will decrease, so 1.
The upper limit was set at 5%. On the other hand, if it is less than 0.1%, the hot workability of the steel will be significantly reduced, so the lower limit was set at 0.1%.

Si: 1.0% or less Si is effective as a deoxidizing element and a reinforcing element, so it is good to add an appropriate amount, but if it is added in excess of 1.0%, formability and ductility will decrease, and weldability will also decrease. Therefore, the upper limit is set to 1.
- Set to 0%. Although the effect of the present invention is hardly affected even if the content of Si is small, a range of 0.2 to 0.6% is practically preferable.

Next, we will discuss limitations on process conditions.

Heating temperature for hot rolling: 1000-1200°C The heating temperature for hot rolling is low temperature heating. At slab heating temperatures exceeding 1200°C, the grain growth of ferrite is significant because the steel type is extremely low-order, making it extremely difficult to refine the structure in hot-rolled sheets, and the r-value and ridging properties of cold-rolled sheets deteriorate. do. Therefore, the upper limit was set at 1200°C. The lower limit is 1000℃
The reason for this is that if the temperature is lower than this, it becomes difficult to secure the hot rolling finishing temperature (lower limit of 700° C.) specified in the present invention in view of the temperature drop during rolling.

Hot rolling finishing temperature near 00~850℃ Figure 3 shows the hot rolling finishing temperature and the final product using both steel B shown in Table 1, which is the steel of the present invention, and steel E, which has a high C content as a comparison material. The relationship with the r value was investigated. O if you give a review
,OO35%C,,0,0040%N steel and 0.0
Both 126%G and 0.0042%N steel containing 1100
After heating to 660°C to 980°C, hot rolling was carried out at 6 temperatures between 660°C and 980°C.
Continuous annealing was performed at 0°C, cold rolling was performed to a thickness of 0.7 mm at a cooling rate of 80%, and continuous annealing of the cold rolled sheet was performed at 800°C. In the steel of the present invention, the r value indicating deep drawability increased as the hot rolling finishing temperature decreased, and at 850° C. or lower, a value of around 1.4 indicating high deep drawability was obtained.

It can be seen that Δr decreases as the finishing temperature decreases, and at 850° C. or lower, it becomes a value that causes no practical problems. Based on this result, the upper limit was set to 850°C or less.

Moreover, the upper limit of this finishing temperature is thought to be controlled by the conditions that are sufficient to accumulate enough strain energy to cause complete recrystallization in the subsequent continuous annealing of the hot rolled sheet, and this condition is exactly 850°C. It would correspond to For the lower limit, r
Low-temperature finishing is preferable from the viewpoint of material properties such as value and Δr, but when extremely low-temperature finishing is applied, the descaling performance decreases as the temperature of the steel sheet decreases, and scale remains on the surface of the steel sheet.
Significant deterioration of surface properties such as scale flaws occurs,
Further, since problems such as an increase in the load on the rolling mill would occur, the lower limit was set at 700°C in consideration of both the quality of the steel plate and the operation. In addition, although the r value and Δr value of E steel with high Cf are slightly improved as the hot rolling finishing temperature is lowered, the improvement effect is clearly smaller than that of the steel of the present invention, especially when 7 is 1.0
It can be seen that Δr is around 0.8 before and after, indicating that the moldability is extremely poor.

Coiling temperature: 400-600℃ Figure 4 shows hot-rolled sheets obtained by heating the slab to 1100℃, finishing hot rolling at 740℃, and changing the coiling temperature.
The graph shows the relationship between the r value and the Δr value obtained by continuous annealing at a constant temperature of 0.degree. C. and then cold rolling annealing, and the coiling temperature.

Invention steel B and comparative steel E were compared. This chart shows that for Steel B, when the coiling temperature exceeds 600°C, the cumulative amount of strain in the hot rolled sheet becomes smaller, the driving force for recrystallization during annealing becomes smaller, and the r value increases from 1.45 to 1. It can be seen that the value has decreased to 0. This is the reason why the upper limit was set at 600°C, and the lower limit was set at 400°C because when cooling the hot rolled sheet on the runout table, controlling the stop temperature below this temperature is necessary because the heat transfer coefficient of the steel sheet increases rapidly. be difficult, and
The reason for this is that the deformation resistance increases as the temperature of the hot-rolled sheet decreases, making it difficult to wind it using normal equipment and making it difficult to wind it tightly.

In addition, when examining the influence of the winding temperature on Em, the improvement effect shown below is smaller than that of the present invention (T is 1.1, Δ
It can be seen that the improvement effect on r was only up to 0.8, and the moldability was significantly inferior to that of the present invention.

Continuous annealing of hot-rolled sheets at 50-850°C The significance of hot-rolled sheet annealing for SO5430 steel containing normal C content is to cause recrystallization to destroy the hot-rolled texture and to improve cold rollability. The two points are to separate the hard phase generated by the transformation of the γ phase into ferrite + carbide, but if the ultra-low carbon steel of the present invention is the basis, only the former requirement needs to be controlled. become. However, if the annealing temperature of this hot-rolled sheet exceeds 850°C, the crystal grains will become coarser, and if the structure before cold rolling becomes coarser, the r value of the cold-rolled annealed material will decrease, Δr will increase, and ridging will increase. The upper limit was set at 850° C., resulting in significant deterioration of deep drawability. On the other hand, if continuous annealing is performed at less than 700°C, an unrecrystallized structure remains, the original effect of hot-rolled sheet annealing cannot be expected, and formability deteriorates, so 700°C was set as the lower limit.

Next, cold rolling is carried out according to a conventional method, and the cold rolled sheet is annealed, but this final step has no special characteristics and may be appropriately processed according to a conventional method. Therefore, the cold rolling rate, annealing conditions, etc. will be omitted here.

The reason for the numerical limitations on chemical composition, content, rolling conditions, and heat treatment conditions has been described above, but if we summarize the relationship between the particularly important composition and hot rolling finishing temperature, the specified range as shown in Figure 5 is optimal. It turns out that. In the end (C+
N) ffi needs to be 0.0160% or less due to the material quality, but it is clear that it is preferably within the range of 0.0020 to 0.0160% from the viewpoint of steel manufacturing, while the hot rolling finishing temperature is The upper limit of 850°C and the lower limit of 700°C are determined from the viewpoint of surface flaws, rolling mill load, etc., and the range of appropriate conditions shown in the diagram is determined.

Although Aβ can be added as a deoxidizing element up to a normal level, there is basically no need to add it, and there is no need to increase the amount of A1 added in order to improve the material as in the prior art. High addition of Af tends to lead to an increase in alumina-based inclusions, and this point is also one of the features that the steel of the present invention is superior to conventional steels.

This invention can of course be applied not only to the process of hot rolling a slab after reheating, but also to processes such as HCR, HDH that do not cool down to room salt after casting, or to process strips whose solidified structure is fine-grained. It can also be applied to slab materials made of thin cast pieces made by casters and block casters, so it can be applied to an extremely wide range of applications.

(Example) The following Table 1 shows nine types of A-D steel whose composition and content are within the range specified by the present invention, and comparative material E-1 steel whose composition and content are outside the range. Using 18% Cr stainless steel, the slab heating temperature was 1100°C, and the hot rolling finishing temperature was 74°C.
Although a hot rolled sheet was produced at 0°C and a coiling temperature of 520°C, the hot rolled sheet was then continuously annealed at a temperature of 800°C, cold rolled at a cold rolling rate of 80%, and briefly annealed at 800°C. The chemical composition and characteristics are shown, and the process conditions are within the range defined by the present invention.

Among the comparison materials, E steel has an unspecified C content, F steel has an unspecified N content, G@ has an unspecified Si content, and H steel has an unspecified Mnf content.
In Im, (C+N) exceeds the specified value. It can be seen that in steel E where C exceeds 0.0070%, the lower value decreases to 1.08 and Δr increases to 0.81. The amount of N decreases to 1.06 in 0.0185% steel like F steel, and it decreases to 1.18 in 5iil when it reaches <1.25% like G steel. are doing. Furthermore, it can be seen that in steels such as H steel, where the Mn content exceeds 1.5%, the Mn content decreases to 1.09.

Regarding C and N, even if the content of each is within the range specified by the present invention, if (C+N)I exceeds the range specified (for example, in the case of 1), the lower value is 1. .21
It can be seen that the steel materials A to D of the present invention are far superior in each characteristic of T and Δr, which indicate formability, compared to these comparative materials.

The ridging properties of all the steel materials in Table 1 were around 20 .mu.m, and there was no significant difference between the inventive material and the comparative material, and it was within a range that did not cause any practical problems.

The following Table 2 shows an example in which a final cold-rolled annealed sheet was obtained using the A steel, 0w4, and D steel shown in Table 1 under the process conditions specified in the present invention, and the manufacturing conditions and the thin sheet are shown in Table 2. This is a list of product characteristics.

Tests 11hl~3.9~11.16~18 all satisfy the range of chemical composition and addition amount specified in the present invention, and also satisfy the process conditions, and the T value is approximately 1.3 or more. , Δr are about 0.4 or less, which proves that excellent formability can be obtained even without the addition of Nb or Ti and even under continuous annealing conditions of hot-rolled sheets.

On the other hand, those which are outside the specified range of the present invention are those subjected to high-temperature slab heating of lll[L4, high-temperature hot-rolled finished materials such as 5.12.19, high-temperature coiled materials such as RLH6.13.20, etc. , high-temperature hot-rolled plate annealing materials such as 7.14.21, Arashi 8.
Even if the amount of chemical components added is within the specified range, such as low-temperature hot-rolled plate annealed materials such as No. It is clearly shown that the quality is significantly inferior.

"Effects of the Invention" As explained in detail above, when the method of the present invention is used, as long as the chemical composition and content range specified in the claims and the process conditions at the predetermined treatment temperature are complied with, Aj, Nb, Ti 18% Cr ferritic stainless steel sheet that does not require additions such as, and has superior press formability than conventional steel without being subject to severe restrictions such as extreme low-temperature hot rolling finish or strong reduction per bath. can be manufactured.

As mentioned above, the present invention does not require the addition of special elements, so it is resource-saving and economical, and since it is made of ultra-low carbon steel, it is a steel plate with better corrosion resistance than SUS 430 steel. Manufacture is possible. Note that this invention does not only apply to the process of hot rolling after reheating the slab (HCR, HDR).
It can also be applied to processes that do not cool down to room temperature after casting, such as
Also, coagulation! Since it can be applied to slab materials made of thin slabs made of strip casters and block casters made of fine-grained iJl weaves, it can be said that this is an excellent invention that can be applied to a very wide range of applications.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between the amount of C in steel and the 7, Ar value of the product. Figure 2 is a chart showing the relationship between (C+N)'Hk in the steel and r and Ar value of the product. Figure 3 is a chart showing the relationship between the finishing temperature of hot rolling and the 7, Ar value of the product. Figure 4 is a chart showing the relationship between the coiling temperature of the steel plate after hot rolling and the 7, Ar value of the product. Figure 5 shows the range of appropriate conditions in the relationship between the amount of (C+N) in the steel material and the hot rolling finishing temperature. In addition, the ○ marks in the above-mentioned FIGS. 2 to 4 indicate the composition of the present invention,
The symbol ``*'' indicates a value due to a composition, content, or process condition that is outside the specified range of the present invention.

Claims (2)

[Claims] (1) C: 0.0010-0.0070%, N: 0.0010 in weight%
~0.0150%, Cr: 15-22%, Mn: 0.1
Ferritic stainless steel containing ~1.5% Si, 1.0% or less of Si, and the balance consisting of Fe and unavoidable impurities is heated to 1000 to 1200°C, and then hot rolled at 700 to 850°C. , 400～
The hot-rolled sheet is rolled up at 600°C, and then heated at 750-850°C.
1. A method for producing a ferritic stainless steel sheet with excellent formability, which comprises continuously annealing the sheet, followed by cold rolling and annealing according to a conventional method. (2) (C+N): 0.0020 to 0.0160% A method for manufacturing a ferritic stainless steel sheet with excellent formability as set forth in claim 1.