JPS6358904B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ferritic stainless steel that has excellent stretchability and secondary workability. More specifically, the present invention has superior stretchability than SUS 430 LX, which is said to have the best formability among conventional general-purpose ferritic stainless steels, and has good toughness (secondary strength) even after deep drawing. The object of the present invention is to provide a stainless steel having a ferrite single phase structure in an annealed state that exhibits (sub-processability). Stainless steel can be roughly divided into two types: ferritic stainless steel represented by SUS 430 and austenitic stainless steel represented by SUS 304.
Compared to austenitic stainless steel, it has characteristics such as no period cracking after forming and low susceptibility to stress corrosion cracking, and since it does not contain expensive nickel, it is inexpensive and has the advantage of saving resources. , are widely used mainly in durable consumer goods. On the other hand, however, ferritic stainless steels are generally inferior in material properties such as formability, corrosion resistance, and weldability compared to austenitic stainless steels, so their uses are limited in some respects.
In particular, in terms of moldability, it is difficult to apply to applications that require severe press molding due to poor stretchability and secondary workability, and improvements in this aspect have been strongly desired. For example, SUS 430, a typical ferritic stainless steel, contains 16.00 to 18.00% Cr, up to 0.12% C, up to 0.75% Si, up to 1.00% Mn,
Although it is a steel containing up to 0.040% P and up to 0.030% S, its stretchability and deep drawability are both poor, and its press formability is poor. In order to improve this, attempts have been made to add and adjust various elements. Among them, SUS 430 LX is said to have the most formability.
16.00~18.00% Cr, up to 0.030% C, 0.75
% Si, up to 1.00% Mn, up to 0.040% P, up to 0.030% S, and between 0.10 and 1.00% Ti or Nb, SUS 430
It is characterized by a lower C value and the addition of Ti or Nb. This SUS 430 LX is SUS
Compared to 430, formability, especially deep drawability, has improved. However, this is not always sufficient;
Problems with the forming surface are still being pointed out, such as insufficient stretch formability and the occurrence of vertical cracks during secondary processing after deep drawing. When adding Ti as in this SUS 430 LX, or when adding Al to improve the Ti yield or purify the steel,
When the amount of Ti and Al added increases, Ti carbonitrides and Al oxides are exposed as linear scratches on the plate surface after cold rolling, resulting in the production of stainless steel, which is characterized by its beautiful surface texture. Since it causes a fatal defect, special manufacturing methods and processes are required for its manufacture, which impairs manufacturability. Therefore, it is desired that the addition of Ti and Al be as small as possible from the viewpoint of manufacturability. In addition, as stainless steels with a lower Cr content than the SUS 430 series, we also offer SUS 410 series, SUS 420 series, and SUS 420 series that do not contain any special alloying elements.
There are SUS 429 series, which are SUS 410, SUS 410 S, SUS 410 L, and SUS depending on their C content.
420 J ₁ , SUS 420 J ₂ , SUS 429, and SUS 429 J ₁ , these stainless steels are classified as martensitic stainless steels or heat-resistant steels in terms of their structures and uses, and cannot be subjected to severe press forming. In fact, they have poor press moldability. Furthermore, AISI 409 has a lower Cr content than SUS 410 and is classified as a ferritic stainless steel. This is 10.50-11.75%
Cr, up to 0.08% C, up to 1.00% Si, 1.00%
Mn up to 0.045%, S up to 0.045%, 6×(%C)~
It contains 0.75% Ti. This AISI 409 is
It was originally developed as an exhaust gas muffler material for automobiles, with emphasis on heat resistance and oxidation resistance. Therefore, even if it exhibits a certain degree of formability and corrosion resistance, it is not sufficient and cannot be applied to applications that require secondary processing after stretch forming or deep drawing. Something similar to this is
The same can be said of the ferritic stainless steel described in US Pat. No. 3,455,681. In fact, the Olsen Cup test values shown in this US patent specification cannot be said to be sufficient. As mentioned above, the purpose of the present invention is to improve the basic material disadvantage of conventional ferritic stainless steels, which is that they are unsuitable for applications that require secondary processing after stretch forming or deep drawing. be. For this purpose, the present inventors have conducted extensive research and examination based on the basic policy of examining the Cr content preferable for moldability, reducing impurities, and adding a small amount of Ti. As a result, after reducing C and N, which are interstitial solid solution elements, to below a predetermined value, we added an appropriate amount of Ti to fix these C and N, purifying the matrix, and of molten elements
If the contents of Cr, Si, Mn, Ti, P, and Al are controlled within the range of mutually related amounts, it will be possible to achieve a It has also been found that stainless steel with a ferrite single phase structure can be obtained in the annealed state, which exhibits extremely good stretchability and secondary workability. That is, in the weight% of the present invention, Cr: 11.0 to 16.0% C: 0.03% or less (preferably 0.02% or less), N: 0.02% or less (preferably 0.01% or less), C+N: 0.04% or less (preferably 0.03% or less) % or less), Si; 0.5% or less, Mn; 0.5% or less, P; 0.025% or less, S; 0.01% or less, Al; 0.1% or less, O; 0.01% or less, Ti; 5 (%C + %N) ~ 0.30%, and α = 0.03 (% Cr) + 0.2 (% Si) + 0.03% (% Mn) + 3.3 (% P) + 1.0 (% solid solution Ti) + 1.0 ( % solid solution Al) However, (% solid solution Ti) = (%Ti) - {4 (%C) +
The content of Cr, Si, Mn, P, Ti and Al is such that α expressed by the formula 3.4(%N)} is 0.8 or less, and the balance is Fe and unavoidable impurities in the annealed state. Provided is a ferritic stainless steel having a phase structure and excellent stretchability and secondary workability. The details of the steel of the present invention will be explained below. Conventionally, the improvement of formability in ferritic stainless steel has mostly been based on improving the plastic strain ratio (γ value: Rankford value), which is an index of deep drawability. but,
Processability in forming processes such as press forming is the first priority.
If you place importance on precision, not only deep drawing processing but also
Formability must be evaluated when complex forming processes such as overhanging and deep drawing/overhanging complex processes are performed, and the γ value alone is not sufficient. In other words, there is a limit to the actual improvement in moldability by improving deep drawability alone, and overall moldability cannot be improved without improving stretchability. In the present invention, from such a viewpoint, emphasis is placed on stretchability, and the work hardening index (n value) is used as an evaluation index of stretchability. Work hardening index (n
It is well known that n value) is an index for suitably evaluating the overhang property, but in ferritic stainless steel, the influence of the components on this n value was investigated, and the content of the components was specified based on the results. has never seen such an example. For your reference, Figure 1 shows the relationship between the Erichsen value and n value of the Erichsen test, which is the oldest and most popular test method for evaluating formability, especially stretch formability (each value is shown below). (including test values). As seen in FIG. 1, there is a correlation between the n value and the Erichsen value, and materials with a high n value also have a high Erichsen value. In the present invention, this n value was adopted as an index for strictly investigating stretch formability, and the influence of various elements was investigated with the aim of improving the n value of ferritic stainless steel, which was conventionally thought not to change much. The details of the present invention will be described based on the test results. Table 1 shows the chemical composition range of the sample materials. Steels in the range shown in Table 1 were melted in a high-frequency vacuum melting furnace, and plates with a thickness of 0.8 mm were produced according to the steps shown in FIG. 2 and used for testing.

[Table] Figures 3 to 8 organize and plot the relationship between changes in the content or correlation amount of each element and the n value. However, the n values are n 0 , n 45 , n 90 in the ₀ ° direction, 45° direction, and _{90 °} direction with respect to the rolling direction of the test material.
were measured and calculated according to the formula n=(n ₀ +2n ₄₅ +n ₉₀ )/4. FIG. 3 shows the relationship between the n value and the amount of Cr. As is clear from this figure, the n value decreases as Cr increases. Figure 4 shows the sample material with Cr constant at 13% and the content of other impurity elements at the same level.
The n value indicates the relationship Ti/(C+N). This Figure 4 shows an interesting relationship in that as Ti/(C+N) increases until Ti/(C+N) reaches about 7, the n value increases as well, but once Ti/(C+N) exceeds about 12, it begins to decrease. There is. That is, Ti/(C+N) shows a maximum value in the range of 7 to 12 for the n value. Ti is C
It is thought that this matrix purification effect provides an effect of increasing the n value by fixing N and TiC as TiC and TiN. On the other hand, an excess of Ti is required.
This shows that Ti has the effect of lowering the n value as solid solution Ti. FIG. 5 shows the relationship between the n value and the amount of (C+N) for test materials with Ti/(C+N) of 7 to 12. This figure 5 shows that the increase in the amount of (C+N) is n
This indicates a rapid decrease in the value. In Figures 6 and 7, the (C+N) amount is 0.02%.
Below, Ti amount is 0.10-0.25%, Ti/(C+N) is 6
The relationship between the P content and the n value and the relationship between the amount of solid solution Al and the n value are shown for the sample materials No. 1 to 16, respectively. From these figures, the n value is determined by P and solid solution.
It is clear that it decreases as the amount of Al increases,
In particular, it can be seen that even a slight increase in P has a large effect on the n value. FIG. 8 shows the relationship between the n value and α for all the sample materials in the composition range shown in Table 1. The plot marked with a black circle in the figure is (C+N) 0.04%.
These are the results for a sample material with a composition of Ti/(C+N)5. This parameter α is the substitution type solid solution element Cr, Si, Mn, P, Ti and Al.
This is a comprehensive evaluation of the influence of the content of % solid solution Ti) + 1.0 (% solid solution Al) However, (% solid solution Ti) = (%Ti) - {4 (%C) + 3.4 (%N)} (% by weight) Defined by the formula be done. The coefficient of each component in this equation represents the influence of each component on the n value as a relative ratio. The results shown in FIG. 8 show that there is a clear correlation between this parameter α and the n value, and as α becomes smaller, the n value becomes larger. In addition, for steel with a composition of (C + N) 0.04% and Ti / (C + N) 5, which is indicated by a black circle in the figure,
It can be seen that in order to obtain a sufficiently high n value (0.24 or more) compared to conventional ferritic stainless steel, it is necessary to set α to 0.8 or less. Furthermore, Figure 9 shows the γ value, which is an index of deep drawability.
The relationship with Ti/(C+N) is shown. From this figure,
It can be seen that the γ value is improved by adding Ti, and becomes higher as Ti/(C+N) becomes larger. The present invention has determined the range of the content of each component based on several findings as seen in the above test values, and has thereby improved the pressability of ferritic stainless steel, which has conventionally been a concern. Although the poor moldability has been improved and this has been further improved, the range of the amount of each component is still determined for several reasons. The reasons for limiting the content of each component are summarized as follows. The reason why Cr is set to 11.0 to 16.0% is that if the Cr content is less than 11.0%, a ferrite single phase structure may not be obtained even after annealing, and sufficient corrosion resistance may not be obtained. Furthermore, as the amount of Cr increases, the overhang property decreases, so the upper limit is set at 16.0%. The reason for keeping C below 0.03% is that (C+N)
This is to keep it below 0.04%. Further, an increase in C is unfavorable from the viewpoint of overhang properties and structure, and more preferably 0.02% or less. The reason why N is set to 0.02% or less is due to the same reason as for C, and it is more desirable that N be 0.01% or less. The reason for keeping (C+N) below 0.04% is that (C+N)
This is because the overhang property decreases due to an increase in N). Also, as (C+N) increases, the amount of Ti that should be added increases.
Since the amount is large, it is preferably as small as possible, and more preferably 0.03% or less. Setting Ti to 5×(C+N) to 0.30% is a very important point of the present invention. Ti amount is (C+N)
If the amount is less than 5 times, the effect of improving stretchability is not sufficient, and it is also unfavorable from the viewpoint of deep drawability and corrosion resistance. In addition, excessive Ti dissolves in the matrix and has a negative effect on stretchability, and an increase in Ti is undesirable from the viewpoint of surface quality, manufacturability, economic efficiency, etc., and the upper limit is set at 0.30%. The reason why Si is set to 0.50% or less is that Si is a substitutional solid solution element and is not favorable for overhanging properties. The reason why Mn is set to 0.50% or less is that, like Si, it is unfavorable for the overhang properties. It is very important for the present invention that P be 0.025% or less. Conventionally, the influence of P on moldability, particularly stretchability, has not been studied, and the present invention is the first to regulate the amount of P from the viewpoint of moldability. That is, P
Since even a slight increase in % has a large negative effect on the overhang properties, it is preferable to keep it as low as possible, and the upper limit is set at 0.025%. Also, more preferably
0.015% or less. The reason for setting Al to 0.10% or less is that solid solution Al has a negative effect on stretchability, so a low value is preferable, but if it is too low, the amount of oxygen in the steel will increase, resulting in a large number of nonmetallic inclusions, which will have a negative impact on formability. Therefore, the content should be 0.10wt% or less, which satisfies both. In addition, S and O are also unfavorable in terms of corrosion resistance and formability, and are 0.01% or less and 0.01%, respectively.
The following are desirable. Next, an example will be given and explained. Table 2 shows the chemical components of the invention steel and comparative steel. This is done by normal hot rolling and cold rolling.
The product plate was 0.8mm thick. Table 3 shows its mechanical properties and various moldability test values.

【table】

[Table] As is clear from the results in Table 3, the n value of the steel of the present invention is significantly higher than that of conventional ferritic stainless steel, and the γ value is also equal to or higher than that of SUS 430 LX. There is. In addition, the Erichsen value, which is an index of stretchability in model formability tests, was over 11, which is much better than conventional steel. On the other hand, the conical cup value, which is an index of deep drawability, is also superior to conventional steel. Further, the results of the secondary workability test are shown in FIG. This secondary workability was evaluated by a falling weight test on the toughness after deep drawing.
A disk of mmφ was squeezed into a cup with an outer diameter of 27 mmφ, with the ears removed, and this was used as a test material.As shown in Figure 11, this test cup 1 was placed on its side and a weight was applied toward its edge. 2 was dropped and evaluated based on the energy required to cause a crack. As is clear from FIG. 10, all of the steels of the present invention have excellent secondary workability. Figure 12 shows the steel of the present invention, SUS 410, and SUS
430 pitting potential (VC′ ₁₀ , 200ppmCl ⁻ , 40°C). As can be seen from the figure, the steel of the present invention exhibits a pitting potential that is superior to SUS 410 and almost equivalent to SUS 430.

[Brief explanation of drawings]

Figure 1 is a relationship diagram between n value and Erichsen value, Figure 2
The figure shows the manufacturing process of the sample material, Figure 3 shows the relationship between the n value and Cr content, Figure 4 shows the relationship between the n value and Ti/(C+N), and Figure 5 shows the relationship between the n value and (C+N). Figure 6 is a diagram showing the relationship between n value and P quantity, Figure 7 is a diagram showing the relationship between n value and P amount.
Figure 8 is a diagram of the relationship between n value and α, Figure 9 is a diagram of the relationship between γ value and Ti/(C+N), and Figure 10 is the secondary workability test. This is a result diagram showing the relationship between the energy and temperature required to cause cracking in the drop weight test after deep drawing, No. 11
The figure is a model diagram of the falling weight test in Figure 10, and Figure 12 is a diagram showing the pitting corrosion potential of the sample material. 1...Test cup, 2...Weight.

Claims (1)

[Claims] In 1% by weight, Cr: 11.0 to 16.0%, C: 0.03% or less, N: 0.02% or less, C+N: 0.04% or less, Si: 0.5% or less, Mn: 0.5% or less, P; 0.025% or less, S: 0.01% or less, Al: 0.1% or less, O: 0.01% or less, Ti: 5 (%C + %N) ~ 0.30%, and α = 0.03 (%Cr) + 0.2 (%Si) +0.03 (%Mn) +3.3 (%P) +1.0 (%Solution Ti) +1.0 (%Solution Al) However, (%Solution Ti) = (%Ti) - {4(%C)+
The content of Cr, Si, Mn, P, Ti, and Al is such that α expressed by the formula 3.4(%N)} is 0.8 or less, and the balance is Fe and unavoidable impurities. A ferritic stainless steel with a phase structure and excellent stretchability and secondary workability. 2. The ferritic stainless steel according to claim 1, wherein C is 0.02% or less. 3. The ferritic stainless steel according to claim 1 or 2, wherein N is 0.01% or less. 4. The ferritic stainless steel according to claim 1, 2 or 3, wherein (C+N) is 0.03% or less.