KR920009990B1 - P-added ferritic stainless steel having excellent formability and secondary workability - Google Patents

Abstract

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Description

P-added ferritic stainless steel with excellent formability and secondary processability

1 is a graph showing the relationship between C, Cr, P, So1.A1 and B content of steel according to the present invention.

치에 대한 P함량의 효과를 표시하는 그래프.2 is made of 13% Cr-0.03% C ferrite stainless steel,

Graph showing the effect of P content on values.

3 is a graph showing the effect of So1.A1 on the Charpy impact value in 13% Cr-0.03% C-0.10% P ferritic stainless steel.

4 shows the results of tests for determining secondary machinability (expansion test) based on the results derived from the relationship between the contents of C, Cr, P, So1.A1 and B of steel according to the present invention. The graph to display.

The present invention relates to a P-added ferritic stainless steel having excellent moldability and secondary workability.

Ferritic stainless steels have reasonable workability and corrosion resistance in comparison with austenitic stainless steels, although they are relatively inexpensive. Low chromium ferrite stainless steels, including those according to AISI 409 and SUS 410L, on the other hand, are used in large quantities in the production of vehicle exhaust systems due to their high strength and high temperature oxidation resistance. Used. However, commercially available ferritic stainless steel, including low chromium stainless steel, is still much more expensive than low carbon steel. Therefore, there is a strong demand for developing a more inexpensive ferrite stainless steel.

It is an object of the present invention to provide a novel ferritic stainless steel which is economically manufactured and has excellent swellability and secondary workability.

According to the invention the balance between Fe and the unavoidable impurities is at% by weight, C; 0.0050 to 0.0500%, Cr; 10.00-18.00%, Si; Up to 0.50%, Mn; Up to 0.50%, P; 0.040% or more but 0.200% or less, S; Up to 0.030%, Ni; Up to 0.60%, Sol. Al; 0.005 to 0.200% and B; It is composed of a small amount to 0.0050%, the alloy element is further represented by the formula (C + 10 × B + Sol. Al) as the longitudinal coordinates of Figure 1, and by the formula (Cr + 50 × P) as the abscissa. The point with the percentage indicated is of the quadrilateral ABCD (where points A, B, C and D have coordinates (12.0, 0.30), (12.0, 0.005), (22.0, 0.020) and (22.0, 0.30), respectively). P-added ferritic stainless steel is provided with excellent formability and tertiary workability that are balanced to fall within the area.

The ferritic stainless steel according to the present invention is basically characterized by the active addition of P in an appropriate amount in relation to the remaining alloying elements, which has been required to be reduced in conventional ferritic stainless steels.

Nine kinds of hot rolled sheets of ferritic stainless steel are standardized in JIS G 4304, while ten kinds of cold rolled plates are standardized in JIS G 4305. As for the P content of these standardized ferritic stainless steel sheets and strips, the standard is 0.030% or less of P for two kinds of SUS 447 JI (Cr; 28.50 to 32.00%) and SUS XM 27 (Cr; 25.00 to 27.50%). For other types, it is specified as 0.040% or less.

Ferrite stainless steels, on the other hand, have a body-centered cubic lattice structure which inherently causes reduced toughness of the material. On the other hand, Cr contained in the material in an amount of 11% or more also acts to further reduce the toughness of the material.

It is therefore believed that, with respect to impurities that were perceived to have an adverse effect on the toughness of the material, in particular the standards in P set a strict specification of 0.030 (or 0.040)% of P.

It is known that the addition of an appropriate amount of P to ferritic stainless steel improves the workability of the cold rolled material as well as the acid wash performance of the hot rolled material.

치에 대한 P의 영향을 그래프로 표시한다.2 shows a cold rolled product in which any change in measurement is indicated by the shaded area

Graph the effect of P on the values.

치는 디이프 드로우잉(deep drawing)되는 물질의 능력을 나타내는 전형적인 크기이다.The results shown in FIG. 2 were obtained on a cold rolled sheet with a thickness of 0.7 mm and a base component of 13% Cr-0.03% C with a varying P content. All the tested plates were prepared by the same conventional procedure including the steps of hot rolling, annealing of hot rolled plates, cold rolling and annealing of cold rolled plates. As is well known in the field

The teeth are typical sizes that indicate the ability of a material to be deep drawn.

치가 클수록

치가 1.0을 초과할수록 디이프드로우잉 되는 물질의 능력이 좋아진다. 제2도에서 보는 바와 같이 종래의 페라이트 스텐레스강에서 통상적으로 보이는 약 0.025%의 P함량에서

치는 1.0이하이다.

The larger the value

If the value exceeds 1.0, the ability of the material to be deep-drawn is better. As shown in FIG. 2, at a P content of about 0.025%, which is commonly seen in conventional ferritic stainless steels.

The value is less than 1.0.

치는 최후에는 1.4또는 그 이상까지 높아진다.However, when P content increases and exceeds 0.075%

The final value is raised to 1.4 or higher.

As already mentioned, the pickling performance of hot rolled ferritic stainless steel is improved by the addition of P. As a result, the pickling step can advantageously be carried out with the use of hydrochloric acid commonly used for pickling of conventional steel, instead of the expensive nitrogen and hydrofluoric acid typically used for pickling of ferritic stainless steel.

The fact that the workability and pickling performance of ferritic stainless steels are improved by the addition of P is very beneficial in terms of providing inexpensive ferritic stainless steels. Above all, P itself is very inexpensive. In order to improve the processing performance of ferritic stainless steel, expensive alloying elements such as Ti, Nb and Al have been used to inevitably increase the manufacturing cost. The increase in P is accomplished by adding a suitable P resource, such as a Fe-P alloy, or by using P-containing molten pig iron.

In the former case, the increase in manufacturing cost is very small. In the latter case, the value of the product is rather reduced since P, which was previously removed, is effectively used, and thus the burden of removal of phosphorus is excluded or reduced. Moreover, in the latter case, it is possible to economically use inexpensive chromium ores and iron containing P as raw materials for the production of stainless steel because of their high P content. Secondly, the pickling step of the hot rolled material is carried out with the use of hydrochloric acid pickling solution, which is advantageous not only economically but also because of the ease of the process.

However, P in ferritic stainless steel sometimes does not deny the adverse effect on certain properties of the steel. The presence of P sometimes overcomes the toughness and secondary workability of ferritic stainless steels.

We found that the inverse effect of P on the toughness of ferritic stainless steels was overcome by the control of C and Cr and the addition of very small amounts of Sol.Al.

Figure 3 graphically shows the effect of Sol. Al content on the toughness (as reflected by Charpy impact value).

FIG. 3 shows the results obtained in samples with 13% Cr-0.03% C-0.10% P base components with various Sol.Al contents. Each sample was prepared by forming a 30 kg ingot with the basic ingredients and the specific Sol.Al content described above, forging at 1100 ° C. for 4 hours at 760 ° C. and cutting the sample from the immersed material. .

Charpy impact tests were performed at temperatures of 20 ° C. and 0 ° C., respectively. Acceptable impact values are generally said to be at least 5 kgf.m / cm 2. As can be seen in FIG. 3, the impact value of steel containing 0.002% Sol.Al is close to zero at the test temperature, and the impact value of steel is at least 5 kgf when Sol.Al exceeds 0.005% and approaches 0.010%. The acceptability of m / cm 2 increases thoroughly well, and the effect of Sol.Al to improve toughness with a Sol.Al content of about 0.020% or more tends to be saturated.

The term "secondary machinability" refers to machinability of deep draw material. When the deep draw of the P-added ferritic stainless steel is blown (first drawn) and then re-hit, or when the deep-draw P-added ferritic stainless steel is impacted by the cutting of the flange, the weak fragility is reduced. Frequently occurs parallel to the direction of stretching.

These cracks are attributed to the decrease in the toughness of the material due to the first stretching, and the higher the first stretching, the more likely the temperature is to occur. Secondary workability is a property of a material different from toughness and formability.

치에 의하여 표시된 바와 같은 디이프 드로우잉 되는 우수한 능력을 가진 물질일지라도 그의 빈약한 2차 가공성 때문에 바라는 최종의 제품으로 충분하게 성형되지 않는 경우가 빈번히 있는 것에 주의하여야 한다.Therefore, the material is high

It should be noted that even a material with a good ability to deep draw as indicated by the teeth is often not sufficiently formed into the desired final product due to its poor secondary processability.

The precise mechanism of action on the effect of P on the secondary workability of P-added ferritic stainless steels is not yet fully understood. However, since P is inherently a tendency to segregate into grain boundaries, the action of P segregated into grain boundaries is increased by the first stretching to weaken the grain boundary binding force, resulting in cracks due to grain boundary fracture. It seems to be easy to do.

It has been found that the adverse effect of P on secondary processing performance can be overcome by strict balancing of the combined combustion as well as individually, as suggested here.

4 is a graph showing the results of a test (expansion test) for determining secondary machinability, based on the results derived from the relationship between C, Cr, P, Sol.Al and B contents of the steel according to the present invention. to be. The results shown in FIG. 4 show that the cold rolled sheet having a thickness of 0.7 mm and various components prepared by the same process as in the prior art including the steps of hot rolling, annealing of hot rolled sheet, cold rolling and annealing of cold rolled sheet. Obtained from. Each plate to be tested was deep drawn with a draw ratio of 2.0 into a cup with an outer diameter of 27.0 mm extended by a conical punch to break at a temperature of 0 ° C. The destruction of the cup was tested to see if it was soft or fragile.

Five to ten cups were prepared in one particular component of the steel to be tested, and the percentage (fragility failure rate) of the cups that had been expanded to break and broken fragile was determined.

This test is referred to as the expansion test. If all cups with special components to be tested have not been subjected to fragility fractures in expansion tests, ie the fragility fracture rate is zero, then the secondary machinability of steel with such components can be said to be practically satisfactory.

In FIG. 4, the percentage represented by the formula (C + 10 × B + Sol.Al) as the ordinate of the component of the steel being tested and the percentage represented by the formula (Cr + 50 × P) as the abscissa are indicated in the coordinates. In these formulas, Cr, P, C, B and Sol.Al represent the percentage of each element of the steel.

4 shows a clear correlation between secondary workability of cold rolled material and the composition of steel. More specifically, at a point that falls within the area of the left side of the straight line connecting point B ((12.0, 0.005) and C (22.0, 0.020) and the line connecting point C (22.0, 0.020) and D (22.0, 0.30). It can be seen in Figure 4 that fragility fractures in the components indicated were not observed in the expansion test except for steels P ₁ and P ₂ .

4 further shows that Cr and P act to impair secondary processability, while C, B and Sol.Al act to enhance secondary processability.

It is believed that the steel P ₁ and P ₂ silver behavior is due to the content of special alloying elements. Incidentally, the steels P ₁ and P ₂ have components as indicated in the first table below. We make it unreasonably excessive secondary workability of Cr steel P ₁ because of his very low carbon content of 0.0035%, despite the presence of a good amount of Sol.Al and does not represent a satisfactory secondary workability of steel by hand P ₂ Because it contains one amount (18.60%), even a relatively low P content is believed to be damaged. Therefore, it is necessary for the purpose of the present invention to define the upper limit of Cr and the lower limit of C.

[Table 1]

Based on the expansion test results and considerations above, we define the interrelationships between a minimum of 0.005% of C and a maximum of 18.00% of Cr, as well as several composite elements as graphically shown in FIG.

The precise mechanism of action of C, B and Sol.Al to improve secondary processability is not yet accurately understood. But we think Regarding C and B, they themselves are probably segregated into grain boundaries to harden the grain boundaries or prevent P from being harmful to secondary processing performance in the segregation of grain boundaries. As regards Sol.Al, since it acts to inhibit the precipitation of Cr carbide, the consumption as Cr carbide is reduced to C and conversely dissolves or dissolves at the binding boundary, so that the amount of C is increased so that C is secondary to the above-described gyrone. Improve workability

The reason for the numerical limitation of the individual alloy elements is now explained. C should be at least 0.0050%. If it is unduly low, the desired secondary workability is not achieved as described above with respect to steel P ₁ . However, excessively high C unduly hardens the material with unsatisfactory moldability and adversely affects the weldability of the material. In order to avoid these inconveniences, it is required to set an upper limit of C 0.0500%.

A lower limit of 10.00% for Cr is required to achieve the desired level of corrosion resistance. The line AB in FIG. 1 is determined from the lowest possible content for P and the lower limit for Cr. Excessively high Cr harms the toughness as well as the secondary processability of the material as described above for steel P ₂ . For this reason, the condition for Cr is set to 18.00%.

Si acts to improve the oxidation resistance of the material at elevated temperatures. However, excessively high Si unduly hardens the material, so the upper limit for Si is set at 0.50%.

Mn is an element that improves the toughness of the weld portion of the material and the hot workability of the material. However, at more than 0.50% of Mn such effects tend to be saturated and products become expensive.

For this reason, the upper limit for Mn is set at 0.50%.

S is a detrimental factor that adversely affects the hot workability and corrosion resistance of the material, so the lower the content of S, the better.

The allowable upper limit for S is set to 0.30%.

Ni has a beneficial effect in order to improve the toughness of the ferrite material. However, the high content of Ni makes the product expensive for the purposes of the present invention. Thus, an upper limit of 0.60% for Ni as defined in conventional standardized ferritic stainless steels applies an upper limit of 0.60% for Ni of the alloy according to the invention as an upper limit for Ni of the alloy according to the invention.

The content of P is crucial for the purposes of the present invention.

For 0.040% or less of P, special treatment for the removal of P in the converter which causes an increase in manufacturing cost or the removal of the beginning of P from pig iron is required.

In addition, the effect of the addition of P, i.e., improved pickling performance and formability, is not obtained. Therefore, 0.040% or more of P is required. However, excessively high P adversely affects the toughness, thermal processing performance and secondary processing performance of the material.

The inverse effect of such P is reduced by strictly balancing other alloying elements according to the invention, but we set an upper limit here for 0.20% of P.

Al acts as a deoxidizer for steel, reducing the oxygen content of the steel and making the process for cleaning the steel.

Newly soluble Al (Sol.Al) contributes to suppress the adverse effects of product toughness and secondary processability. In order to suppress such beneficial effects of Sol.Al, at least 0.005% of Sol.Al is required. However, for more than 0.200% of Sol. Al, such effects tend to saturate on the one hand, and on the other hand, become cornered about the clogging of the nozzle in the technically troublesome step.

For these reasons we set an upper limit of 0.200% for Sol.Al.

B effectively works to improve the secondary processability of the material, even at very small amounts.

The result of B is sufficient if the corresponding amounts of C and Sol.Al are appropriate for the particular Cr and P content to achieve the desired secondary processability.

For optimal secondary machinability we want at least 0.0005% of B. However, the upper limit for B is set at 0.0050% because B tends to impair the formability of the product.

N is not very critical for the purposes of the present invention. It inevitably enters during the fabrication of the steel and may be contained in the ferritic stainless steel according to the invention in amounts ranging from 0.0050% to 0.50%, as seen in conventional ferritic stainless steels.

For the purposes of the present invention the alloying elements must be individually controlled within their respective ranges as described above, in addition C, B and Sol.Al, depending on the specific Cr and P content, are represented by the formula (C + 10 × B) as the ordinate. The point with the percentage indicated by + Sol.Al) and the percentage indicated by the formula (Cr + 50 × P) as the abscissa should be balanced to fall within the area of the quadrilateral of FIG.

Characteristic aspects and advantageous results of the invention are further illustrated by the following working and control examples.

Molten steels with the chemical constituents shown in Table 2 were prepared.

Hot rolled strips of 3.2 mm thickness were prepared from each molten steel. The hot rolled strip is stripped by pickling, then cold rolled to a thickness of 0.7 mm without any intermediate annealing and then heated at a temperature of 820 ° C. for 1 minute and cooled in the atmosphere. received.

치와 2차 가공성에 대하여 시험되었다.

치는 식,

=(r₀+2r₄₅+r₉₀)/4에 의하여 계산되었다.The prepared steel sample

Tooth and secondary workability were tested.

Expression,

Calculated by = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4.

Here, r ₀ , r _45, and r ₉₀ are Rankford values measured along the directions of 0 °, 45 °, and 90 ° respectively with respect to the direction of rolling.

For the determination of the secondary processing performance, it was carried out at a temperature of 0 ° of the in vitro test described above with reference to FIG. 4 and the results were evaluated as in FIG.

[Table 2]

[Table 3]

I found the following:

Steel Nos. 1 to 7 have a fairly high r value indicating the excellent ability of deep drawing without breaking fragility in expansion tests reflecting their satisfactory secondary workability.

Control steel number 8 contains an excess amount of P as defined herein and has a percentage of excess (Cr + 50 × P) above the range indicated in FIG.

치는 상당히 높지만 이 강철의 2차 가공성은 불만족하다.As a result,

Level is quite high, but the secondary machinability of this steel is not satisfactory.

치는 낮고, 디이프 드로우잉의 빈약한 능력을 표시한다.Control steel number 9 has a low P content and as a result

Is low, indicating poor ability of deep drawing.

Control steel number 10 contains 0.0028% of C lower than that specified here. As a result, the secondary machinability of this steel is insufficient.

Control steel number 11 includes alloying elements in amounts which fall individually within the respective ranges specified here.

However,% (Cr + 50P) is larger than the range shown in FIG. 1 and as a result its secondary machinability is unsatisfactory.

Control steel number 12 contains 0.003% of Sol.Al than specified herein, and% (C + 10B + Sol.Al) is too low for its 20.59% of (Cr + 50P).

As a result, secondary machinability is unsatisfactory.

Control steel No. 13 also contains lower amounts of Sol.Al than specified here, and% Cr and% (Cr + 50P) are too high, resulting in poor secondary workability of the steel.

Claims (2)

The balance between Fe and unavoidable impurities is from% by weight: C: 0.005 to 0.0500%, Cr: 10.00 to 18.00%, Si: 0.50%, Mn: 0.50%, P: 0.040% or more or 0.200% Hereinafter, S: 0.030%, Ni: 0.60%, Sol.Al: 0.005 to 0.200%, and B: minute amount to 0.0050%, and the alloy element is further represented by the formula (C + 10) as the ordinate of FIG. Points with percentages represented by × B + Sol.Al and percentages represented by the formula (Cr + 50 × P) as abscissa are quadrilateral ABCD (where points A, B, C and D are coordinates (12.0,0.30) P-added ferritic stainless steel with excellent formability and secondary formability, characterized by being balanced to fall within the areas of), (12.0, 0.005), (22.0, 0.020) and (22.0, 0.30). The method of claim 1 wherein the percentage of B is from 0.0005 to 0.0050%.

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