JPH0751727B2 - Method for producing ferritic stainless steel sheet with excellent formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION “Object of the Invention” (Field of Industrial Application) The present invention is a method for producing an 18% Cr ferritic stainless cold-rolled steel sheet that is widely used in kitchen equipment, automobile parts, and other durable consumer products. It is about.

(Prior Art) Conventionally, for kitchen equipment and automobile parts, which have excellent formability in addition to the characteristics such as corrosion resistance, strength, and ductility inherent to stainless steel, and which require a beautiful surface after forming. As a stainless steel sheet, an 18% Cr ferritic stainless steel sheet represented by SUS430 is widely used because it is superior to the austenitic stainless steel in terms of price and resistance to stress corrosion cracking. The steel sheet has a chemical composition in a weight percentage (hereinafter simply referred to as%) of Cr: 10 to 20.
%, C: 0.1% or less, Si: 1.0% or less, Mn: 1.0% or less, N: 0.0
It is generally one containing 500% or less, slab-rolled continuously cast slab or ingot, then hot-rolled,
After that, heat treatment such as batch annealing or continuous annealing for homogenization and softening is performed, followed by cold rolling, annealing, and temper rolling to obtain a product.

However, in recent years, in order to obtain an annealing effect in a short time, Al, T
A continuous annealing method is also performed in which elements such as i, Nb are added.

In order to further shorten the process, there has been proposed a method for omitting hot-rolled sheet annealing in which direct annealing is performed after pickling without performing annealing after hot-rolling. For example, after ferritic stainless steel containing Al is hot-rolled, it is held and heated at 900 to 1100 ° C for 10 minutes and then finish rolling is carried out to secure a ridging reduction r value γ → α
There is JP-A-59-25933 which promotes transformation and precipitation of AlN.

As a technique for improving deep drawability, the C content of SUS430 steel is 0.01%.
Method to reduce Ti to 0.2% (iron and steel) 197
7, vol5, P832), 0.2% Al is added to reduce the cold rolling rate from 80%.
There are Japanese Patent Publications 61-50126 etc. that can increase to 85-95%.

As a measure against ridging, many techniques have been reported in which a large amount of strain during hot rolling is left in order to crush the cast structure that causes the ridging, and recrystallization during annealing is promoted. For example, a hot rolling method in a low temperature region (Japanese Patent Publication No. 45-34016), a high temperature short time annealing method in which Ti and Nb are added (Japanese Unexamined Patent Publication No. 51-149116), and a method of repeating recrystallization including hot rolling (Japanese Unexamined Patent Publication No. 58-199822), and a method of temporarily showing the γ phase during hot-rolled sheet annealing (Japanese Patent Publication No. 48-24611).

In addition, a method of crushing the casting structure by applying a reduction of 20% or more and then reheating and hot-rolling (Japanese Patent Laid-Open No. 232232/1984), hot rolling at least one pass at a strain rate of 150 S ^-1 or more. A method of crushing the cast structure (Japanese Patent Laid-Open No. 62-10217) has been proposed.

(Problems to be Solved by the Invention) When batch-type annealing is performed in the method for manufacturing a ferritic stainless steel sheet described at the beginning of the prior art, a processing time of several tens of hours is required and there is a problem in productivity. , Al, Ti
The cold rolled material produced by continuous annealing for a short time, which is performed by adding such elements as described above, has a drawback in that its characteristics do not reach that of the batch type annealed material. In addition, regarding the above-mentioned JP-A-51-149116,
The raw material cost increases due to the addition of Ti, Nb, etc., and in the technology such as low temperature hot rolling method and repeated recrystallization, there are problems such as limitation of hot rolling conditions, frequent occurrence of steel plate surface flaws, and increase in roll load. In addition, the method of once revealing the γ phase has low productivity because a hard martensite phase is not formed and very slow cooling is required. Regarding the method of crushing the cast structure and then reheating and rolling, there is a decrease in productivity due to an increase in the number of processes, and in the method of rolling at least 1 pass at a strain rate of 150 S ^-1 or more in hot rolling There are problems such as severe restrictions on hot rolling conditions. The present invention was created in order to solve many problems of such conventional methods, does not require expensive additional elements, by setting the composition hot rolling conditions, annealing conditions rationally , Stainless steel's original corrosion resistance, strength,
It is an object of the present invention to provide a method for producing a ferritic stainless steel sheet which is excellent in formability and surface beauty in addition to characteristics such as ductility, is not subject to leasing and roping, and is economical.

“Structure of the Invention” (Means for Solving Problems) In order to achieve the above-mentioned object, the present inventors have (1) in% by weight, C: 0.0010 to 0.0070%, N: 0.0010 to 0.0150%, Cr : 15 to 22%, Mn: 0.1 to 1.5%, Si: 1.0% or less, the balance is Fe and unavoidable impurities, and the ferritic stainless steel is heated to 1000 to 1200 ° C.
Formability characterized by finishing hot rolling at 700 to 850 ° C, winding at 400 to 600 ° C, continuously annealing the hot rolled sheet at 750 to 850 ° C, and cold rolling and annealing according to a usual method. Of excellent ferritic stainless steel sheet.

(2) (C + N): 0.0020 to 0.0160% The method for producing a ferritic stainless steel sheet having excellent formability according to claim 1 characterized in that the content is 0.0020 to 0.0160%.

(Operation) The present invention relates to a method for producing an 18% Cr-based stainless steel sheet based on an extremely low carbon.

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

C: 0.0010 to 0.0070% Limitation of the C content is one of the particularly important constituent features in the present invention. FIG. 1 shows the relationship between the C amount, the value and Δr. In this test, the relationship between the C content and the formability r was investigated by using steel in which the C content was changed from 0.0010% to 0.0142% while the N value was kept substantially constant from 0.0034% to 0.0045%. .

The above-mentioned values are average Rankford values, and JIS No. 5 tensile test pieces were sampled from the product sheet in the rolling direction (r ₀ ), the rolling right-angle direction (r ₉₀ ), and the rolling 45 ° direction (r ₄₅ ), respectively. It is the value obtained from the following formula from the respective r ₀ , r ₉₀ , and r ₄₅ values.

= (R ₀ + r ₉₀ + 2r ₄₅ ) / 4 Also, Δr is a value showing anisotropy and is a value obtained by the following equation.

Δr = (r ₀ + r ₉₀ −2r ₄₅ ) / 2 The supplied steel was heated to 1100 ° C., hot rolling was completed at 750 ° C., and wound at 540 ° C. After that, annealing was performed at 800 ° C, cold rolling was performed at a cold rolling rate of 80%, and the cold rolled sheet was annealed at 800 ° C.

As is clear from the chart in Fig. 1 above, a decrease in the increase Δr was observed with a decrease in the C content, and the C content was 0.0070.
A value of 1.3 or more was shown as a sufficient value at less than%. From this result, the upper limit of C was set to 0.0070%. On the other hand, regarding the lower limit, even if extremely low carbonization such as less than 0.0010%, there is a limit to the effect on the material as shown in the chart, and it is difficult to obtain an extremely low carbonaceous material on steelmaking Therefore, 0.0010% is set as the lower limit because it is economically disadvantageous.

Conventionally, this type of ultra-low carbon ferritic stainless steel is
It has been used for improving corrosion resistance and toughness of welded parts, but it has not been used in fields requiring formability. This is one of the important features of the present invention.

N: 0.0010 to 0.0150% N hardly impairs the corrosion resistance, and is an element effective for ensuring the strength and may be added. However, if added in excess of 0.0150%, the formability deteriorates remarkably as in the high C material, so this value was made the upper limit. On the other hand, even if N is extremely lowered, the effect of the present invention is not impaired, but extremely low N during steelmaking
However, the lower limit is set to 0.0010% because the increase in cost will increase the cost.

(C + N): 0.0020 to 0.0160% Fig. 2 defines the hot rolling finish temperature in the present invention when C is set to 0.0070% or less and N is used for steel.
The relationship between the moldability was examined under the two-level process conditions of 750 ° C. and 880 ° C., which was outside the scope of the present invention, and other manufacturing conditions were the same as those described in the above C content regulation. It is the same. In the case of the conditions of the present invention, when the (C + N) content is 0.0160% or less, the r value is high and excellent moldability is exhibited, so the upper limit is 0.0160%.
%. The lower limit is not a problem in terms of material, but 0.0020% was set as the upper limit because of the economical efficiency 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, the effect of material improvement due to the promotion of recrystallization in the annealing in the next step will be reduced, and will decrease and Δr will increase. Become.

It is no exaggeration to say that the synergistic effect of the use of the ultra low carbonaceous component of the present invention and the lowering of the hot rolling finishing temperature resulted in the improvement of the material as shown in FIG.

Cr: 15-22% The present invention steel is intended for 18% Cr stainless steel of SUS430 system. If Cr% is less than 15%, the corrosion resistance is poor, and 22%
If added in excess, the expected effect will not be obtained, and on the other hand, the cost will increase and the usefulness of the steel sheet of the present invention in the field of application will be lost.

Mn: 0.1-1.5% Mn is effective as a deoxidizing element and a solid solution strengthening element,
If the content exceeds 1.5%, the r-value decreases, so 1.5% was made the upper limit. On the other hand, if it is less than 0.1%, the hot workability of steel is significantly deteriorated, so 0.1% was made the lower limit.

Si: 1.0% or less Si is effective as a deoxidizing element and a strengthening element, so an appropriate amount may be added, but if it is added in excess of 1.0%, the formability and ductility decrease and the weldability also decreases, so the upper limit is 1.0. %. Even if the Si content is small, the effect of the present invention is hardly affected, but in the practical range, 0.2 to 0.6% is preferable.

Next, limitation of process conditions will be described.

Heating temperature for hot rolling: 1000 to 1200 ° C The heating temperature for hot rolling is low temperature heating. At a slab heating temperature of over 1200 ° C, the grain growth of ferrite is remarkable due to the extremely low carbon type of steel, making it extremely difficult to refine the structure of the hot-rolled sheet and degrading the r-value and ridging property of the cold-rolled sheet. To do. Therefore, the upper limit was set to 1200 ° C. The lower limit is set to 1000 ° C., because it is difficult to secure the hot rolling finishing temperature (the lower limit is 700 ° C.) specified in the present invention at a lower temperature than this in view of the temperature drop during rolling.

Finishing temperature of hot rolling: 700 to 850 ° C. FIG. 3 shows the results of the hot rolling finishing temperature and the final product by using both B steel shown in Table 1 of the present invention and E steel having a high C content as comparative materials. The relationship with the r value is investigated. 0 in detail.
0035% C, 0.0040% N-containing steel, 0.0126% C, 0.0042%
Both N-containing steels were heated to 1100 ° C and then between 6 ° C and 980 ° C.
After hot rolling at the stage temperature, air cooling and winding at 500 ℃, continuous annealing at 800 ℃, and cold rolling at 80%
It was cold rolled to a thickness of mm and continuously annealed at 800 ° C. In the steel of the present invention, the value showing the deep drawability increased with the decrease in the hot rolling finish temperature, and at 850 ° C or lower, the value showing the high deep drawability of about 1.4 was obtained. It can be seen that Δr decreases with a decrease in the finishing temperature, and is below a value at which there is no practical problem at 850 ° C. or lower. From this result, the upper limit is 850 ℃
Below. The upper limit of this finishing temperature is considered to be governed by the conditions sufficient to accumulate sufficient strain energy to cause complete recrystallization in the subsequent hot-rolled sheet continuous annealing,
This condition would correspond exactly to 850 ° C. Regarding the lower limit, low temperature finishing is a preferable direction from the viewpoint of material such as r value and Δr, but at extremely low temperature finishing, descaling property decreases with temperature drop of the steel sheet, and scale remains on the steel sheet surface, Since surface quality deterioration such as scale flaws will occur significantly, and problems such as an increase in load on the rolling mill will occur, the lower limit was set to 700 ° C in consideration of both the quality of the steel plate and the operation. . In the case of E steel having a high C content, the value and Δr value are slightly improved as the hot rolling finishing temperature is lowered.
Compared with the steel of the present invention, the improvement effect is obviously small,
It can be seen that the formability is remarkably inferior, with 1.0 and Δr around 0.8.

Winding temperature: 400-600 ℃ Fig. 4 shows that the slab is heated to 1100 ℃, the hot rolling is finished at 740 ℃, and the hot-rolled sheet obtained by changing the winding temperature is continuously annealed at a constant temperature of 800 ℃. And then the value obtained by cold rolling annealing, Δ
It shows the relationship between the r value and the winding temperature. The comparison was made between the invention steel B and the comparative steel E steel. From this chart B
In steel, when the coiling temperature exceeds 600 ℃, the cumulative amount of strain in the hot-rolled sheet becomes smaller, the driving force for recrystallization during annealing becomes smaller, and the value decreases from 1.45 to 1.0. .
This is the reason why the upper limit of 600 ℃ is set, and the lower limit of 400 ℃ is to control the stop temperature below this temperature when cooling the hot-rolled sheet on the runout table with water in the normal hot rolling. It becomes difficult due to the rapid increase of the heat transfer coefficient of the steel sheet.
The reason is that the deformation resistance accompanying the temperature drop of the hot-rolled sheet becomes large, making it difficult to wind with ordinary equipment at the time of winding and making it difficult to wind tightly.

In addition, considering the effect of the coiling temperature for E steel,
The improvement effect of is smaller than that of the present invention.
Also, it was found that the improvement effect was only up to 0.8, and the moldability was significantly inferior to that of the present invention.

Continuous annealing of hot-rolled sheet: 750-850 ℃ The significance of hot-rolled sheet annealing in SUS430 steel containing normal C content is to recrystallize to destroy the hot-rolled texture and to improve cold-rolling property. There are two points of separating the hard phase generated by the transformation of the γ phase into ferrite + carbide. However, when the ultra low carbon steel of the present invention is the basic, it is necessary to control only the former requirement. become. However, if the annealing temperature of this hot-rolled sheet also exceeds 850 ° C, the crystal grains become significantly coarse, and if the pre-cold-rolling structure becomes coarse, the value of the cold-rolled annealed material will decrease, and Δr will increase and ridging will increase. As a result, the deep drawability deteriorates significantly, so the upper limit is 850 ° C.
And On the other hand, when continuously annealed at a temperature lower than 700 ° C, an unrecrystallized structure remains and the original action of hot-rolled sheet annealing cannot be expected, and the formability deteriorates, so the lower limit was 700 ° C. Then, cold rolling is performed according to a conventional method to anneal the cold rolled sheet, but this final step has no characteristics and may be appropriately treated by a usual method. Therefore, the cold rolling rate, annealing conditions, etc. are omitted here.

The reasons for limiting the numerical values in the chemical composition and content, rolling conditions, and heat treatment conditions have been described above. The most important composition is summarized in the relationship of hot rolling finishing temperature, and the specified range as shown in Fig. 5 is optimal. It turns out that After all (C +
The amount of N) needs to be 0.0160% or less in terms of the material, but it is clear that it is preferably in the range of 0.0020 to 0.0160% from the viewpoint of steel making, while the hot rolling finishing temperature is 850 ° C, which is the upper limit from the material viewpoint.
However, the lower limit of 700 ° C will be specified from the viewpoints of surface flaws, rolling mill load, etc., and the range of appropriate conditions shown in the chart will be determined.

Although Al can be added as a deoxidizing element to a normal level, it is basically not necessary to add it, and it is not necessary to increase the amount of Al added for improving the material as in the prior art. Addition of high Al easily causes increase of alumina inclusions,
This is also one of the features that the steel of the present invention is superior to the conventional steel.

Incidentally, the present invention is not only applicable to the process of hot-rolling after reheating the slab, but of course it can be applied to a step of not cooling to room temperature after casting such as HCR and HDR, or a strip caster in which the solidified structure is finely divided, Since it is also applicable to the thin cast slab material by block casters, it can be applied in an extremely wide range.

(Examples) The following Table 1 shows nine types of A to D steels within the composition and content ranges specified in the present invention, and comparative materials E to I steels in which any of the components is out of the range. 18% Cr stainless steel is used to produce hot-rolled sheets at a slab heating temperature of 1100 ° C, hot-rolling finishing temperature of 740 ° C and water-cooling temperature of 520 ° C by water cooling by a conventional method, and then the continuous annealing temperature Performed at 800 ℃, cold rolling rate 80%
The following shows the chemical composition and characteristics of a product cold-rolled at 80 ° C. and annealed at 800 ° C. for a short time, and the process conditions are those within the range specified in the present invention.

Among the comparative materials, the C content of E steel is out of specification, the N content of F steel is out of specification, the Si content of G steel is out of specification, and the Mn content of H steel is
In steel I, (C + N) exceeds the specified value. It can be seen that in the E steel with C exceeding 0.0070%, the value decreased to 1.08 and Δr increased to 0.81. The amount of N is
The value of 0.0185% like F steel is 1.06, but when the amount of Si reaches 1.25% like G steel, the value is 1.18.
Has fallen to. Also, it can be seen that the value of Mn is decreased to 1.09 in the case where the amount of Mn exceeds 1.5% like H steel.

Regarding C and N, even if the respective contents are within the range specified in the present invention, if the (C + N) amount exceeds the specified range (for example, in the case of I steel), 1.21 It can be seen that the steel materials A to D of the present invention are far superior to the comparative materials in terms of the formability and the Δr characteristics.

The ridging properties of all the steel materials in Table 1 were around 20 μm, and there was no great difference between the material of the present invention and the comparative material, and it was within the range of no practical problem.

The following Table 2 shows an example in which the final cold rolled annealed sheet was obtained by using the A steel, C steel, and D steel of the above Table 1 under the process conditions specified in the present invention. This is a list of characteristics of thin plate products. Test No. 1-3, 9-11, 16
~ 18, all satisfy the chemical composition and the range of the addition amount specified in the present invention, and also satisfy the process conditions,
The value is 1.3 or more, Δr is 0.4 or less, and Nb or
It has been proved that excellent formability can be obtained without adding Ti or under the continuous annealing condition of the hot rolled sheet.

In contrast, No. 4 high temperature slab heating outside the specified range of the present invention, No. 5, 12, 19 etc. high temperature hot rolled finish, N
High temperature coiling material such as o.6,13,20, No.7,14,21
For steel materials such as high-temperature hot-rolled sheet annealed materials such as No. 8 and low-temperature hot-rolled sheet annealed materials such as No. 8 and 15 whose process conditions are out of the range even if the addition amount of chemical components is within the specified range. It is clearly shown that the moldability is remarkably inferior because it is difficult to optimize the structure.

"Effects of the Invention" As described in detail above, when the method of the present invention is used, as long as the chemical composition and the range of the content specified by the claims and the process conditions at a predetermined processing temperature are observed, Al, Nb, Ti 18% Cr ferritic stainless steel sheet that has superior press formability to conventional steel without the need for additions such as the above, without being subjected to severe restrictions such as extreme low temperature hot rolling finish and strong reduction per pass. Can be manufactured.

Since the present invention does not require the addition of a special element as described above, it is a resource-saving, economical, and ultra-low carbon type, so that even if compared to SUS430 steel, the production of a steel sheet with more excellent corrosion resistance Is possible. The present invention can be applied not only to the process of hot-rolling after reheating the slab, but also to the process of not cooling to room temperature after casting such as HCR and HDR, and also the strip caster and block caster in which the solidified structure is made fine. It can be said that the present invention is an excellent invention that can be applied to an extremely wide range because it can be applied to a slab material of a thin cast piece.

[Brief description of drawings]

FIG. 1 is a table showing the relationship between the amount of C in steel and the Δr value of the product, and FIG. 2 is the amount of (C + N) in the steel and Δ of the product.
Fig. 3 is a chart showing the relationship with the r value, Fig. 3 is a chart showing the relationship between the hot rolling finishing temperature and the product's Δr value, and Fig. 4 is the rolling temperature of the steel sheet after hot rolling and the product's Δr value. FIG. 5 shows the range of appropriate conditions in the relationship between the amount of (C + N) in the steel material and the hot rolling finish temperature. The circles in FIGS. 2 to 4 are the compositions of the present invention,
The mark ● indicates the content or the process condition, and the mark ● indicates the value depending on the composition, the content, or the process condition outside the specified range of the present invention.

Claims (2)

[Claims] 1. By weight%, C: 0.0010 to 0.0070%, N: 0.0010 to 0.0150%, Cr: 15 to 22%, Mn: 0.1 to 1.5%, Si: 1.0% or less, the balance being Fe and unavoidable. Ferritic stainless steel, which consists of static impurities, is heated to 1000-1200 ℃, and then
Molding characterized by finishing hot rolling at 700 to 850 ° C., winding at 400 to 600 ° C., continuously annealing the hot rolled sheet at 750 to 850 ° C., and cold rolling and annealing according to a usual method. Method for producing a ferritic stainless steel sheet having excellent properties. 2. A method for producing a ferritic stainless steel sheet having excellent formability according to claim 1, wherein (C + N): 0.0020 to 0.0160%.