GB2159177A - P-added ferritic stainless steel - Google Patents

Abstract

Ferritic stainless steel consisting essentially of, in % by weight, 0.0050 to 0.0500 of C, 10.00 to 18.00 of CT, up to 0.50 of Si, up to 0.50 of Mn, more than 0.040 but not more than 0.200 of P up to 0.030 of S, up to 0.60 of Ni 0.005 to 0.200 of Sol. Al, and trace to 0.0050 of B, the balance being Fe and impurities, the C, Cr, F, Sol. Al and B content being further balanced so that the point having the % (Cr+50xP) as the abscissa and the % (C+10xB +Sol.Al) as the ordinate may fall within the area of a quadrilateral ABCD of Fig. 1.

Description

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

Ferritic stainless steels have moderate workability and corrosion resistance in spite of the fact that they are relatively inexpensive when compared with austenitic stainless steels, and in consequence, relatively large quantities of ferritic stainless steels are commercially used in the manufacture of durable consumer goods, including kitchen units, and as construction materials. On the other hand, low chromium ferritic stainless steels, including those in accordance with AISI 409 and SUS 410L, are used in large quantities in the manufacture of automobile exhaust gas systems, because of their superior strength and oxidation resistance at an elevated temperature as well as corrosion resistance to low carbon steels. However, commercially available ferritic stainless steels, including those of low chromium, are still expensive when compared with low carbon steels.Accordingly, it is strongly desired to develop more inexpensive ferritic stainless steels.

The invention may provide a novel ferritic stainless steel which can be economically produced and which has excellent formability and secondary workability.

In accordance with the invention there is provided a P-added ferritic stainless steel having excellent formability and secondary workability consisting essentially of, in % by weight, C ; 0.0050 to 0.0500 %, Cr ; 10.00 to 18.00 %, Si ; up to 0.50 Mn up to 0.50 %, P ; more than 0.040% but not more than 0.200%, S ; up to 0.030 %, Ni up to 0.60 %, Sol.AI ; 0.005 to 0.200 %, and B ; trace to 0.0050 %, the balance being Fe and unavoidable impurities, the alloying elements being further balanced so that the point having the percentage represented by the formula (Cr + 50xp) as the abscissa and the percentage represented by the formula (C + 10 x B + Sol.AI) as the ordinate may fall within the area of a quadrilateral ABCD of Fig. 1, wherein the points A, B, C and D have the coordinates (12.0,0.30) , (12.0, 0.005), (22.0, 0.020) and (22.0, 0.30) , respectively.

Some embodiments will now be described with reference to the accompanying drawings in which: Figure 1 is a graphical showing of the relation between the C, Cr, P, Sol. Al and B content in the steel according to the invention; Figure 2 is a graph showing, on 13%Cr-0.03 %C ferritic stainless steels, an effect of the P content on the Yvalue; Figure 3 is a graph showing, on 13%Cr-0.03 %C -0.10 %P ferritic stainless steels, an effect of the Sol. Al content on the Charpy impact value; and Figure 4 is a graphical showing of the results of the test for determining the secondary workability (cup expansion test), on the basis of which results the relation between the C, Cr, P, Sol.Al and B content in the steel according to the invention has been derived.

The ferritic stainless steel embodying the invention is basically characterized by the feature that P, which has been required to be reduced in conventional ferritic stainless steels, is positively added in an appropriate amount in relation to other alloying elements.

Of ferritic stainless steels, 9 species of hot rolled sheets are standardized in JIS G 4304, while 10 species of cold rolled sheets are standardized in JIS G 4305. Regarding the P content of these standardized ferritic stainless steel sheets and strips, the standard prescribes not more than 0.030% of P for two species of SUS 447 JI (Cr ; 28.50 to 32.00%) and SUS XM 27 (Cr ; 25.00 to 27.50%), and not more than 0.040% for other species. On the other hand, a ferritic stainless steel has a crystalline structure of a bodycentered cubic lattice which inherently leads to a reduced toughness of the material. On the other hand, Cr contained in the material in an amount as high as 11% or more, also acts to further reduce the toughness of the material.It is, therefore, believed that regarding impurites, which have been recognized as adversely affecting the toughness of the material, in particular P, the standard prescribes the strict provision of not more than 0.030 (or 0.040) % of P.

It was found that addition of an appropriate amount of P to a ferritic stainless steel improved the pickling performance of the hot rolled material as well as the workability of the cold rolled material.

Fig. 2 graphically shows an effect of P upon the Y value of the cold rolled product, allowing for some variations in the measurement shown by the hatched area. The results shown in Fig. 2 were obtained on cold rolled sheets having a thickness of 0.7mm and a basic composition of 13 %Cr-0.03 %C with varied P content. All the tested sheets were prepared by the same conventional procedure including the steps of hot rolling, annealing of the hot rolled sheet, cold rolling and annealing of the cold rolled sheet. As is well known in the art, the W value is a typical measure representing the ability of the material of being deeply drawn. The greater the "value, especially the more the ""value exceeds 1.0, the better the ability of the material of being deeply drawn.As seen from Fig. 2, with the P content of about 0.025 % normally found in conventional ferritic stainless steels, the 4 value is below 1.0. However, as the P content increases and exceeds 0.075%, the "value becomes higher eventually to 1.4 or more.

As already stated, the pickling performance of a hot rolled ferritic stainless steel is improved by addition of P. As a result, a pickling step can be advantageously carried out using hydrochloric acid normally employed in pickling low carbon steels, instead of using expensive nitric and hydrofluoric acids normally employed in pickling ferritic stainless steels.

The fact that the workability and pickling performance of ferritic stainless steels may be improved by enrichment of P, is very beneficial from the view point of providing inexpensive ferritic stainless steels.

First of all, P itself is a very cheap element. For improving the workability of ferritic stainless steels expensive alloying elements, such as Ti, Nb and Al, have heretofore been employed, inevitably resulting in an increasel f the price of the product. Enrichment of P may be carried out either by adding a suitable P source such as a Fe-P alloy, or by using a P-containing molten pig iron. In the former case an increase of the price of the product is very slight. In the latter case the price of the product can be rather reduced, since the P which has heretofore be n removed is effectively utilized, and thus, a burden of dephosphorisation may be eliminated or reduced.Furthermore, it is possible in the latter case to use as raw materials P-containing iron and chromium ores which have been economically of low value as raw materials for the production of stainless steels because of their high P content. Secondly, the step of pickling the hot rolled material may be carried out using a hydrochloric acid pickling liquid, which is advantageous not only economically but also because of easiness of the procedure.

However, there is no denying the the P in the ferritic stainless steel does frequently adversely affect some properties of the steel. The presence of P frequently impairs the toughness and secondary workability of the ferritic stainless steel.

We found that the adverse effect of P on the toughness of the ferritic stainless steel could be overcome by controlling C and Cr and adding a very slight amount of Sol. Al.

Fig. 3 graphically shows an effect of the Sol. Al content on the toughness (reflected by the Charpy impact value). The results shown in Fig. 3 were obtained on specimens having a basic composition of 13%Cr-0.03 %C -0.10 %P with various Sol.AI content. Each specimen had been prepared by forming a 30kg ingot having the abovementioned basic composition and the particular Sol. Al content, forging it at 1100"C, soaking the forged material at 760 C for 4 hours and cutting off the specimen from the soaked material. The Charpy impact tests were carried out at temperatures of 20"C and 0 C, respectively. It is generally said that the acceptable impact value is at least 5 kgf.micm2. As seen from Fig. 3, while the impact value of the steel containing 0.002% of Sol.Al is nearly zero at the temperatures tested, as the Sol. Al content exceeds 0.005% and approaches 0.010%, the impact value of the steel drastically increases well above the acceptable value of at least 5 kgf.micm2, and with the Sol. Al content of more than about 0.020 % the effect of Sol. Al to improve the toughness tends to be saturated.

By the term "secondary workability" we mean the workability of a deeply drawn material. We have experienced that when a cold rolled sheet of a P-enriched ferritic stainless steel is deeply drawn (a first draw) and then re-stroked (re-striking), or when a deeply drawn P-enriched ferritic stainless steel sheet experiences shock caused by frange cutting, brittle crackings frequently occur in parallel to the direction of the first draw. These crackings are attributed to a reduction in the toughness of the material due to the first draw, and the more likely to occur, the more severe the first draw and the lower the temperature.

The second workability is a property of the material, which is different from the toughness and formability. It should be noted, therefore, that this is frequently a case wherein even if a material has an excellent ability of being deeply drawn as represented by its high w value, it cannot be successfully shaped into the desired final product owing to its poor secondary workability.

Regarding the influence of P on the secondary workability of the P-enriched ferritic stainless steel, a precise operation mechanism is not yet fully understood. We are supposing, however, that whereas P is an element having a tendency to inherently segregate in grain boundaries, the action of P, which has segregated in grain boundaries, to weaken the intergranular bonding force is amplified by the first draw, whereby crackings due to intergranular fracure are likely to occur.

It has been found that the adverse effect of P upon the secondary workability can be overcome by strictly balancing the alloying elements not only individually but also mutually as proposed herein.

Fig. 4 is a graphical showing of the results of the test for determining the secondary workability (cup expansion test), on the basis of which results the relation between the C, Cr, P, Sol. Al and B content in the steel according to the invention has been derived. The results shown in Fig. 4 were obtained on cold rolled sheets having a thickless of 0.7mm and various compositions, which had been prepared by the same conventional procedure including the steps of hot rolling, annealing of the hot rolled sheet, cold rolling and annealing of the cold rolled sheet. Each sheet to be tested was deeply drawn with a draw ratio of 2.0 into a cup having an external diameter of 27.0mm, which was expanded to fracture at a temperature of 0 C by means of a conical punch. The fracture of the cup was examined whether it was duc tile or brittle.On one particular composition of steel to be tested 5 to 10 cups were prepared and expanded to fracture, and a percentage of the cups, which had undergone brittle fracture (% brittle fracture) was determined. This test will be referred to as the cup expansion test. If all the cups having the particular composition to be tested have not undergone brittle fracture in the cup expansion test, that is, if the % brittle fracture is zero, it can be said that the secondary workability of the steel having that composition is practically satisfactory.

In Fig. 4, the composition of the tested steels is shown in a coordinate system with the percentage represented by the formula (Cr+50xP ) as the abscissa and the percentage represented by the formula (C +10xB +Sol.AI) as the ordinate. In these formulea, Cr, P, C, B and Sol. Al stand for percentages of the respective elements in the steel.Fig. 4 reveals that there is a clear co-relation between the composition of the steel and the secondary workability of the cold rolled material.More specifically, it can be seen from Fig. 4 that with the composition represented by a point falling within the area above the straight line connecting points B (12.0, 0.005 ) and C (22.0, 0.020 ) and left side of the straight line connecting points C (22.0, 0.020 ) and D (22.0, 0.30) , no brittle fracture has been observed in the cup expansion test, except for Steels P1 and P2. Fig. 4 further reveals that Cr and P act to impair the secondary workability, while C, B and Sol. Al serve to enhance the secondary workability.

It is believed that the exceptional behavors of Steels P1 and P2 should be attributed to the content of particular alloying elements. Incidentally, Steels P1 and P2 had the compositions as indicated in Table 1 below. We believe that Steel P1, because of its very low carbon content of 0.0035 %, does not exhibit a satisfactory secondary workability in spite of the presence of a fair amount of Sol. Al, whereas the secondary workability of Steel P2 is impaired even by its relatively low content of P because it contains an unduly excessive amount of Cr (18.60 %). Accordingly, it is necessary for the purposes of the invention to prescribe the lower limit of C as well as the upper limit of Cr.

TABLE 1 Chemical Composition (% by weight) C Si Mn P S Cr Ni Sol.AI N B P, 0.0035 0.29 0.18 0.082 0.006 12.09 0.07 - 0.035 0.0070 tr.

P2 0.0370 0.08 0.23 0.048 0.005 18.60 0.13 - 0.023 0.0250 tr.

Based on the above-mentioned cup expansion test results and considerations thereon, we prescribe the mutual relation between some alloying elements as graphically shown in Fig. 4 as well as C mini mum 0.005 % and Cr maximum 18.00 %.

The precise operation mechanism by which C, B, and Sol. Al act to enhance the secondary workability is not yet exactly understood. But we are supposing as follows. Regarding C and B, they themselves would presumably segregate in grain boundaries, thereby to strengthen the grain boundaries or to pre vent P, which is harmfull to the secondary workability, from segregating in the grain boundaries. Regard ing Sol.AI, since it serves to suppress precipitation of Cr carbide, C consumed as Cr carbide is reduced and in turn the amount of C, which is dissolved or segregates in the grain boundaries increases, which C acts, according to the above-mentioned mechanism, to enhance the secondary workability.

The reasons for the numerical restrictions of the individual alloying elements will now be described.

C should be at least 0.0050 %. If it is unduly low, the desired secondary workability will not be achieved, as demonstrated hereinabove regarding Steel P1. However, an excessively high C not only ren ders the material unduly rigid, leading to an unsatisfactory formability, but also adversely affects the weld ability of the material. To avoid these inconveniences, it is required to set the upper limit for C 0.0500%.

The iower limit of 10.00% for Cr is required to achieve a desired level of corrosion resistance. The line AB in Fig. 1 is determined from this lower limit for Cr and the lowest possible content for P. An exces sively high Cr impairs the toughness as well as the secondary workability of the material, as hereinabove demonstrated regarding Steel P2. For this reason the upper limit for Cr is set 18.00 %.

Si serves to improve the oxidation resistance of the material at an elevated temperature. But the upper limit for Si is set 0.50% since an excessively high Si renders the material unduly rigid.

Mn is an element, which improves the hot workability of the material and the toughness of weld zones of the material. With more than 0.50 % of Mn, however, such effects tend to be saturated and the prod uct becomes expensive. For these reasons the upper limit for Mn is set 0.50%.

S is a harmful element, which adversely affects the corrosion resistance and hot workability of the material, and thus, the lower the content of S the more we prefer. The allowable upper limit for S is now set 0.030%.

Ni has a beneficial effect to improve the toughness of the ferritic materials. But a high content of Ni renders the product expensive contrary to the purpose of the invention. Accordingly, the upper limit of 0.60 % for Ni as prescribed with conventional standardized ferritic stainless steels is now adopted as the upper limit for Ni in the alloys according to the invention.

The content of P is critical for the purpose of the invention. With not more than 0.040% of P, a preliminary removal of P from pig iron or a special treatment for removal of P in the converter is required, leading to the increase in the manufacturing costs. In addition, the effects of the enrichment of P, that is the improved pickling performance and formability, are not enjoyed. Accordingly, more than 0.040% of P is required. However, an excessively high P adversely affects the toughness, hot workability and secondary workability of the material. Although such adverse effects of P may be reduced by strictly balancing the other alloying elements in accordance with the invention, we now set the upper limit for P 0.20%.

Al acts as a deoxidizer in a steel making process to reduce the oxygen content in the steel and to clean the steel. Further, acid soluble Al (Sol.AI ) contributes to suppress the adverse effects of P in the toughness and secondary workability of the product. To enjoy such beneficial effect of Sol.AI, at least 0.005 % of Sal.AI is required. However, with more than 0.200% of Sol.AI, such an effect tends to be saturated on the one hand, and a technologicai problem may be posed on the other hand regarding clogging of nozzles in the casting step. For these reasons we set the upper limit for Sol.Al 0.200%.

B, even with a very small amount, effectively acts to improve the secondary workability of the material.

To achieve the desired secondary workability a trace of B can be sufficient, provided that the amounts of the cooperating C and Sol.AI are appropriate in relation to the particular Cr and P content. For an optimum secondary workability we prefer at least 0.0005 % of B. But the upper limit for B is set 0.0050 %, since B tends to impair the formability of the product.

N is not very critical for the purpose of the invention. It inevitably comes into the product in the course of the steel making process and may be contained in the ferritic stainless steel in accordance with the invention in an amount ranging from 0.0050% to 0.05 h as it appears in the conventional ferritic stainless steels.

For the purpose of the invention, the alloying elements must be individually controlled with the respective ranges as prescribed above, and in addition, depending on the particular Cr and P content, the C, B and Sol. Al must be balanced so that the point having the percentage represented by the formula (Cr+50,tsP ) as the abscissa and the percentage represented by the formula ( C+10 x B +Sol.AI) as the ordinate may fall within the area of a quadrilateral ABCD of Fig. 1.

Characteristic features and advantageous results of the invention will be further described by the following working and control examples.

Molten steels having chemical compositions indicated in Table 2 were prepared. From each molten steel a hot rolled strip having a thickness of 3.2mm was prepared. A piece of the hot rolled strip was descaled by pickling, and thereafter cold rolled to a thickness of 0.7mm without any intermediate anneal, and then subjected to a finish anneal comprising the steps of even heating at a temperature of 820 C for one minute and allowing to cool in air.

The steel specimens so prepared were tested for the Y value and the secondary workability. The γ value was calculated in accordance with r0 + 2 r45 + r90)/4 wherein the r0, r45 and r5" are Lankford values measured along the directions of 0 , 45 , and 90 relative the direction of rolling, respectively. For the determination of the secondary workability, the cup expan sion test, mentioned above with reference to Fig. 4, was carried out at a temperature of 0 C, and the results were evaluated as in Fig. 4.

able 2

Steel No. Class C Si Mn P S Cr Ni col.Al N B Cr + 50P C + 10B + sol.Al Balance 1 A 0.0468 0.35 0.20 0.@04 0.005 10.45 0.0@ 0.045 0.0082 tr. 15.65 0.0918 Fe and impurities 2 A 0.0121 0.09 0.17 0.089 0.006 11.29 0.10 0.012 0.0120 tr. 15.74 0.0241 Fe and impurities 3 A 0.0220 0.27 0.23 0.181 0.001 12.4@ 0.07 0.058 0.0250 0.0010 21.48 0.0900 Fe and impurities 4 A 0.0318 0.42 0.22 0.067 0.010 14.58 0.06 0.033 0.0060 tr. 17.93 0.0648 Fe and impurities 5 A 0.0450 0.30 0.36 0.0@2 0.004 16.72 0.30 0.120 0.0087 tr. 21.32 0.1650 Fe and impurities 6 A 0.0@75 0.18 0.12 0.053 0.007 16.85 0.07 0.010 0.0210 0.0042 19.50 0.0895 Fe and impurities 7 A 0.0089 0.25 0.23 0.085 0.005 17.50 0.06 0.052 0.0069 0.0035 21.75 0.0959 Fe and impurities 8 B 0.0@09 0.29 0.24 0.250 0.005 11.34 0.07 0.023 0.0103 tr. 23.84 0.0538 Fe and impurities 9 B 0.0283 0.10 0.28 0.027 0.010 11.03 0.06 0.037 0.0081 tr. 12.38 0.0653 Fe and impurities 10 B 0.0028 0.48 0.19 0.084 0.008 12.59 0.10 0.065 0.0145 tr. 16.79 0.0658 Fe and impurities 11 B 0.0359 0.23 0.23 0.151 0.006 16.32 0.08 0.029 0.0121 tr. 23.87 0.0649 Fe and impurities 12 B 0.0085 0.28 0.20 0.078 0.005 16.69 0.06 0.003 0.0230 tr. 20.59 0.0115 Fe and impurities 13 B 0.0@42 0.56 0.27 0.062 0.006 19.70 0.10 0.004 0.0080 tr. 22.80 0.0382 Fe and impurities A Steel according to the invention B : Control steel TABLE 3 Steel Class r value Secondary * No. workability 1 A 1.50 0 2 A 1.42 0 3 A 1.49 0 4 A 1.37 0 5 A 1.65 0 6 A 1.21 0 7 A 1.34 0 8 B 1.32 9 B 0.87 0 10 B 1.45 O 11 B- 1.51 12 B 1.43 13 B 1.33 A ; Steel according to the invention B;Control steel +; Cup expansion test O; ; % Brittle fracture:0 O; ; % Brittle fracture:less than 50 % @; % Brittle fracture:not less than 50 % The following results were found: Steels Nos. 1 to 7 have fairly high γ values indicating their excellent ability of being deeply drawn, and do not undergo brittle fracture in the cup expansion test reflecting their satisfactory secondary workability.

Control steel No. 8 contains P in an amount in excess of that prescribed herein, also has % of (Cr+50P ) in excess of the range shown in Fig. 1. As a result, the secondary workability of this steel is unsatisfactory, although the Value is fairly high.

Control steel No. 9 has a low P content, and in consequence, its γ value is low, indicating its poor ability of being deeply drawn.

Control steel No. 10 contains 0.0028% of C, which is lower than that prescribed herein. As a result the secondary workability of this steel is insufficient.

Control steel No. 11 contains alloying elements in amounts individually falling within respective ranges prescribed herein. But the % (Cr+50P ) is in excess of the range shown in Fig. 1, resulting in its unsatisfactory secondary workability.

Control steel No. 12 contains 0.003 % of Sol. Al, which is lower than that prescribed herein, and the % (C+10B +Sol. Al) is too low for its 20.59% of (Cr+50). As a result, the secondary workability is unsatisfactory.

Control steel No. 13 also contains Sol.AI in an amount less than prescribed herein, and the % Cr and % (Cr+50P) are too high. As a result, the secondary workability of this steel is poor.

Claims (3)

1. P-Added ferritic stainless steel consisting essentially of, in % by weight, C ; 0.0050 to 0.0500 %, Cr ; 10.00 to 18.00 %, Si ; up to 0.50 %, Mn ; up to 0.50 %, P ; more than 0.040% but not more than 0.200%, S ; up to 0.030 %, Ni ; up to 0.60 Sol.AI ; 0.005 to 0.200 %, and B ; trace to 0.0050 %, the balance being Fe and unavoidable impurities, the alloying elements being further balanced so that the point having the percentage represented by the formula (Cr+50xP ) as the abscissa and the percentage represented by the formula ( C+1OxB +Sol.AI) as the ordinate may fall within the area of a quadrila teral ABCD of Fig. 1, wherein the points A, B, C and D have the coordinates (12.0, 0.30) , (12.0, 0.005 ) (22.0, 0.020 ) and (22.0, 0.30) , respectively.

2. P-added ferritic stainless steel according to claim 1 wherein the percentage of B is from 0.0005 to 0.0050 %.

3. P-added ferritic stainless steel according to claim 1 and substantially as any described and exemplified herein.