JPS5949301B2 - Ferritic stainless steel with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ferritic stainless steel that has excellent ductility, ridging properties, and secondary workability after press forming.

The term "secondary processing" used here refers to the secondary processing that applies principal strain in a direction different from the principal strain direction of the primary processing, after the ferritic stainless steel sheet has been subjected to processing such as drawing, stretching, bending, and rolling as primary processing. This refers to cases where processing is performed.

For example, ■ bending or curling when bending or curling the outer periphery of a flange that has undergone shrinkage flange processing by drawing.

■ Expanding process when expanding the mouth of a container that has been drawn.

■ This is a burring process when burring is performed on the bottom of a container that has been drawn.

Ferritic stainless steel is widely used in kitchen utensils, automobile parts, etc. because it is cheaper than austenitic stainless steel, has good forming precision, and is free from stress corrosion cracking.

However, it has a higher yield point and lower elongation than austenitic stainless steel or ordinary steel.

Further, expensive austenitic stainless steel is used even in fields where ferritic stainless steel can be used because it is prone to ridging and has poor secondary workability. On the other hand, due to the problem of limited Ni resources worldwide, there is a strong demand for ferritic stainless steel with excellent workability.

There are methods to improve the ductility of ferritic stainless steel, such as making it extremely low in carbon and adding nitride-forming elements such as Ti, but the former significantly deteriorates the zinc properties.
The latter causes problems such as deterioration of secondary workability.

In addition, there are many methods to counteract zinc in ferritic stainless steel, such as low-temperature casting, low-temperature hot-rolling, annealing and cold-rolling using the transformation point, but all of them require some method other than the normal manufacturing process. , it is necessary to change the manufacturing process, which increases the cost of solving problems in the process of developing new technology, which ultimately results in an increase in the cost of the product.

On the other hand, when ferritic stainless steel sheets are used to process kitchen items or automotive items, secondary processing such as bending the edges is often performed after press forming, but cracks often occur during this secondary processing.

As methods for improving this secondary workability, there are methods such as reducing inclusions and precipitates, increasing crystal grain size, and increasing elongation.

For example, if ferritic stainless steel is made to have extremely low carbon, its crystal grains will become coarser and its elongation will improve, making it very suitable for secondary workability. The characteristics that are suitable for the purpose are not necessarily satisfied.

In order to solve such problems, the present inventors attempted many experiments.

As a result, it was concluded that in order to obtain stainless steel with excellent properties as described above, the alloy composition must be controlled within an optimal range. Therefore, we investigated not only the influence of individual alloying elements but also the synergistic effect when combined, and developed a ferritic stainless steel with excellent ductility, rezinc resistance, and secondary workability after press forming. The gist of the invention is C: 0.03% or more, 0.08%
Hereinafter, Sl: 0.4% or less, Mn: 0.5% or less, P: 0.03% or less, s: 0.00s% or less, Ni:
Excellent secondary workability after press forming, consisting of an ingredient range of 0.3% or less, Cr: 15-20%, At2N-containing bush, twice or more and 0.2% or less, and N: less than 0.015%. Made of ferritic stainless steel. Next, the reason for limiting the alloy components will be explained below. If the amount of C is reduced, elongation and secondary workability will improve, but as the crystal grains become coarser, roughness will become worse after processing. In addition, if the amount of C is less than 0.03%, the peeling and zinc properties will deteriorate rapidly, so the lower limit of C should be set at 0.03%.
03%. On the other hand, if the amount of C is increased, elongation deteriorates, tensile strength increases, secondary workability deteriorates, and hardness increases, so the upper limit of C was set at 0.08%. Since Si significantly increases the yield point and tensile strength and impairs toughness and ductility, it is desirable to have a low Si content.

In addition, the reduction in Si makes the crystal grains of the annealed structure of the hot rolled sheet finer, and greatly improves the adhesive zinc properties of the product. In addition, since it is better for the Erichsen value of secondary workability to be low, the Si content was set to 0.4 inches or less. Mn increases tensile strength, increases hardness, decreases elongation, and decreases plastic strain ratio (r value), so 0.
It was set to be 5 inches or less.

Since P is a harmful element and inhibits toughness and hot workability, and also deteriorates secondary workability, it is better to have a lower value, and it is set to 0.03 or less.

Like SP, it is a harmful element and deteriorates hot workability and corrosion resistance.

Further, since A-based inclusions are likely to be generated and the bendability and secondary workability are significantly deteriorated, a lower value is desirable. Therefore, S
was set to 0.008% or less. Ni increases tensile strength.

Further, since it enlarges the crystal grains of the annealed structure of the hot rolled sheet and deteriorates the adhesive zinc properties of the product, it is set at 0.3% or less. Cr is an extremely effective element for oxidation resistance and corrosion resistance, but if added in large amounts, the elongation and impact value deteriorate significantly, so it is recommended to add 15 to 2
It was set to 0.

At improves oxidation resistance and suppresses quench hardenability.

By adding at least twice the amount of At as the amount of N, the yield point is reduced, the elongation is increased, and the r value is improved, and deep drawability is improved. Furthermore, At is effective in refining crystal grains and rust resistance, and improves zinc resistance. However, when added in large amounts, secondary processability deteriorates. Therefore, the range of At was set to At7, which was 2 or more and 0.2 or less.
Since N is an austenite-forming element and has the same effect as C, a lower content is preferable in order to improve ductility.

Since the steel of the present invention has At added, aluminum nitride precipitates, which is one of the causes of deterioration in secondary workability, so a lower value is desirable. Further, since N coarsens the crystal grains of the annealed structure of the hot rolled sheet, the lower the N content, the better the product's adhesive zinc resistance. Therefore, N was set to be less than 0.015. The present invention will be explained in detail by giving examples below.

Ferritic stainless steel having the components shown in Table 1 is melted, and 0.7 m
A thick steel plate was manufactured and the material properties were measured. The result is
The excellent properties of the present invention, such as ductility, rezinc resistance, and secondary workability, are expressed in easy-to-understand symbols as shown in
Shown in the table. The symbols are classified into three categories, superior or inferior, and the range is determined according to each characteristic based on the results of many studies, as shown in Table 1. The mark ○ indicates that the characteristics are excellent. Ductility is JI
Expressed as the elongation in the direction of the Sl3 B tensile test piece,
The zinc resistance was expressed as the value of a JISS No. tensile test piece, which was pulled by 20%, and the amount of zinc appearing on the surface was measured using a roughness meter. In addition, as a method to evaluate secondary workability, 0.7tr
After cold rolling a product with an thickness of 30% at a rolling reduction rate, the total length of the crack when closely bent so that the bending ridgeline is parallel to the rolling direction (usually referred to as C direction bending) is calculated as the total length of the crack relative to the length of the ridgeline. Expressed as a percentage.

It is clearly seen that all three properties of Invention Steels 1, 2, 3, and 4 are excellent.

Comparative steels 5 to 11 are good in one of the three properties but inferior in the others.

In any case, not all characteristics are excellent, such as being inferior to all three. SUS43O, which is a representative ferrite stainless steel, is shown at the bottom, and although it has good secondary workability, its ductility and zinc resistance are not as good as the steel of the present invention.

However, normally in SUS43O, Mn-Sulph
Those containing inclusions such as ide and Mn-Silicate have poor secondary processability.

The influence of the chemical components in Table 1 will be explained in detail.

As for the influence of C, a low C material such as Comparative Steel 5 has excellent ductility and secondary workability, but has extremely poor rezinc resistance.

Comparative Steel 6 is a high C material and has excellent zinc resistance but poor secondary workability. C of the invention steels 1, 2, 3 and 4 is 0
．． It is in the range of 0.03 to 0.08%, and has excellent properties in terms of ductility, rezincability, and secondary workability. S has a large effect on secondary workability, and comparative steel 9 and SUS43O 12 are almost the same in components other than S, but comparative steel 9 has a high S content and therefore has extremely poor secondary workability.

All of the steels of the present invention have an S value of 0.00 s or less, and have excellent secondary workability. The influence of S content on secondary processability will be explained in more detail. C: 0.050%, Si:
0.30z, Mn: 0.40%, P: 0.025%, N
i: 0.10%, Cr: 16.50%, Az: 0.07
Ferritic stainless steel paste consisting of 0%, N: 0.0080, At/N: 8.75,
Steels with Sf: 0.002 to 0.015% were melted in vacuum, and steel plates with a thickness of 0.7 tran were manufactured by known hot rolling, cold rolling and annealing, and secondary workability was investigated. As a result, as shown in Figure 2, the S content was 0.00s
% or less, the cracking in the C-direction close bending after 30% rolling is a factor of 10 or less, and when the S content exceeds 0.00 s%, the cracking increases rapidly. The present inventor has determined that when processing kitchen utensils or automobile parts, in order to prevent cracks from occurring during secondary processing after press forming, cracks in the C-direction close bending after 30-inch rolling should be 10 inches or less. It makes it clear that something is necessary. Therefore, S in the steel of the present invention
The content needs to be 0.00s% or less. A
Comparing the influence of t on Inventive Steel 2 and Comparative Steel 10 or Conventional Steel 12, Inventive Steel is superior to both Comparative Steel and Conventional Steel in both ductility and zinc resistance. At combines with N to form A7N, which reduces solid solution N in the steel and improves ductility.

In addition, it has the effect of making crystal grains finer, and as a result, the adhesive and zinc properties of the product are improved. All of the steels of the present invention have an AUN of 2
It contains at least 0.2 At and has excellent ductility and ridging properties.

Comparing the influence of N between Inventive Steel 2 and Comparative Steel 11, the Inventive Steel is superior in both re-zinc resistance and secondary workability.

N beams deteriorate the zinc properties, and those with A7 contain AtN.
precipitates and deteriorates secondary workability.

All of the steels of the present invention have a low N content and are excellent in both properties. There is an appropriate range for secondary workability in terms of the relationship between C content and N content, which is the shaded range in Figure 3, where the C content is 0.03% or more and 0.08% or less, and the N content is 0.03% or more and 0.08% or less. 0
It is appropriate that it is less than 15% and C16.5N≦0.14. Regarding the influence of Si, inventive steels 1 and 2 and comparative steel 7
and 8, the three characteristics and other general characteristics are shown in FIG.

Low Si lowers the yield point, decreases tensile strength, increases elongation, improves re-zink, improves secondary workability, decreases elongation at yield point, improves Erichsen value, and decreases hardness. Greatly improves many characteristics necessary for sex.

However, it does not affect the crystal grain size of the product, and when the Si is reduced, there is no problem with roughening of the product's surface after processing, which is advantageous. As described above, Si has a large influence on the processing characteristics of ferritic stainless steel, and low-Si materials greatly improve these characteristics.

As a rough guide to the properties of ferritic stainless steel, which has excellent workability, the yield point is low, 28kv/1 or less, the elongation is 33% or more, the higher the r value, the better the zinc, 8μ or less, and 30% rolling process. The subsequent C-direction close bending crack is less than a factor of 10, and the Erichsen value is more than 10 mm.

There are other problems such as spring bags and rough skin after processing, and considering the hardness and crystal grain size, it is clear from Figure 1 that Si
The amount will be 0.4 factor or less. Inventive steels 1, 2, 3, and 4 all have Si of 0.4 or less, and are excellent in ductility, rezinc resistance, and secondary workability.

The influence of Si content on secondary workability and elongation will be explained in more detail.

C: 0.050% Mn: 0.40, P: 0.02
5chi, S: 0, 007%, Ni: 0.10%, Cr: 1
6.50%, Az: 0.070% N: 0.0080%,
A/7'N: Based on 8.75 ferritic stainless steel, a steel with Si changed to 0.02 to 1.01 was vacuum melted and made into a 0.7 m thick steel plate in the same manner as above. Secondary workability and elongation were investigated. As a result, as shown in FIG. 4, the St content needs to be 0.4% or less. P also deteriorates secondary processability. C:0
．． 050chi, Si: 0.30%, Mn: Q. 4Q%,
s: 0.007%, Ni: 0.10%, Cr:
16.50 section, Az: 0.070 chi, N: 0.0080
Based on ferritic stainless steel with At/N: 8.75, steel with P content varied from 0.006 to 0.049% was vacuum melted and made into a 0.7 mm thick steel plate in the same manner as above. , the secondary processability was investigated. As a result, as shown in FIG. 5, the P content needs to be 0.03% or less. Regarding the influence of other components, Ni has a slight influence on ductility and zinc resistance, so the content of Ni in the steel of the present invention is 0.3% or less in both cases.

As described above, the present invention provides a ferritic stainless steel that has excellent ductility, zinc resistance, and secondary workability after press forming by adjusting the composition of the ferritic stainless steel.

In addition, the present invention only adjusts the components during melting, and other steps are only part 1.
-1 is sufficient.

Therefore, it greatly contributes to cost reduction.

[Brief explanation of the drawing]

Figure 1 shows the effect of S on each property of ferritic stainless steel.
Figure 2 shows the influence of S on secondary workability. Figure 3 shows C and N with good secondary workability.
4 is a diagram showing the influence of P on secondary workability and elongation, and FIG. 5 is a diagram showing the influence of P on secondary workability.

Claims (1)

[Claims] 1 C: 0.03% or more and 0.08% or less, Si: 0.4
% or less, Mn: 0.5% or less, P: 0.03% or less, S
: 0.008% or less, Ni: 0.3% or less, Cr: 15
~20%, Al: more than twice the N content (%) and less than 0.2%, N: less than 0.015%, balance Fe and unavoidable impurities. Ferritic type with excellent secondary workability after press forming. stainless steel.