FR2564864A1 - STERILIC FERRITIC STAINLESS STEEL WITH PHOSPHOROUS EXCELLENT FORMABILITY AND SECONDARY OUVRABILITY. - Google Patents

Abstract

THE INVENTION RELATES TO A FERRITIC STAINLESS STEEL WITH EXCELLENT SHAPING ABILITIES AND SECONDARY OPPORTUNITY. ACCORDING TO THE INVENTION, IT CONSISTS ESSENTIALLY IN WEIGHT: C: 0.0050 TO 0.0500, CR: 10.00 TO 18.00, IF: 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.AL: 0.005 TO 0.200, AND B: TRACES UP TO 0.0050, REMAINING FE AND INEVITABLE IMPURITIES, THE ALLOY ELEMENTS BEING MORE BALANCED THAN THE POINT WITH THE PERCENTAGE REPRESENTED BY THE FORMULA (CR 50P FOR THE ABSISSE AND THE PERCENTAGE REPRESENTED BY THE FORMULA (C 10B SOL. AL) FOR L 'ORDERED MAY BE IN THE ZONE OF A QUADRILATER ABCD, OR 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 THE INVENTION APPLIES IN PARTICULAR TO METALLURGY.

Description

The present invention relates to a ferritic stainless steel

  supplemented with phosphorus having excellent formability and secondary workability. Ferritic stainless steels have moderate workability and corrosion resistance despite being relatively inexpensive compared to austenitic stainless steels, and as a result, relatively large amounts of ferritic stainless steels are commercially available for durable consumer goods, including kitchen units, and as materials for

  construction. In addition, ferric stainless steels

  Low chromium ticks, including those in accordance with AISI 409 and SUS 410L, are used in large quantities in the manufacture of automobile exhaust systems due to their high mechanical strength and resistance to high temperature oxidation and their superior corrosion resistance to low carbon steels. However, commercially available ferritic stainless steels, including those with low chromium content, are even more expensive compared to low carbon steels. As a result, it is highly desirable to develop stainless steels

less expensive ferritics.

  The subject of the present invention is a new ferritic stainless steel which can be produced economically and which has an excellent aptitude for shaping and

  excellent secondary workability.

  According to the invention, there is provided a ferritic stainless steel added P, having excellent formability and excellent secondary workability consisting essentially of, in% O by weight

C: 0.050 to 0.0500%

  Cr: 10.00 to 18.00% If: 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.A1: 0.005 to 0.200%, and B: traces up to 0.0050% the rest being Fe and unavoidable impurities, the alloying elements being balanced so that the point having the percentage represented by the formula (Cr + 50xP) for the abscissa and the percentage represented by the formula (C + 10xB + Sol.Al) for the ordinate may be in the quadrilateral zone ABCD of Figure 1, where the 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

  tively. The invention will be better understood, and other purposes, features, details and advantages thereof

  will become clearer during the description

  explanatory text which will follow with reference to the accompanying schematic drawings given solely by way of example and illustrating several embodiments of the invention and in which: FIG. 1 is a graph showing the relationship between the contents of C, Cr, P , Sol.Al and B in the steel according to the invention; FIG. 2 is a graph showing, on ferritic stainless steels at 13% Cr-0.03% C, the effect of the P content on the T value; FIG. 3 is a graph showing, on ferritic stainless steels at 13% Cr-0.03% C-0.10% P, the effect of the Sol.Al- content on the Charpy impact value. ; and FIG. 4 is a graph showing the results of the test to determine the secondary workability (cutting expansion test), results on the basis of which the relationship between the content of C, Cr, P,

  Sol.Al and B in the steel according to the invention was derived.

  The ferritic stainless steel according to the invention is characterized, at the base, by the fact that P, which had to be reduced in conventional ferritic stainless steels, is positively added in an appropriate amount relative to the other alloying elements. . Of the ferritic stainless steels, 9 species of hot-rolled sheet are standardized in the Japanese standard 3JIS G 4304, while 10 species of cold-rolled sheet are standardized in the Japanese standard JIS G 4305. As far as the P content is concerned, of these standard ferritic stainless steel plates and foils, the standard does not prescribe more than 0.030% P for both SUS 447 31 (Cr: 28.50 to 32.00%) and SUS XM 27 (Cr: 25 0 to 27.50%), and not more than 0.040% for

  other species. In addition, a stainless steel ferri-

  It has a crystalline structure of a cubic lattice centered in the body, which inherently leads to reduced material toughness. On the other hand, Cr contained in the material in an amount up to 11% or more, also serves to further reduce the strength of the material. It is therefore believed that with respect to impurities, which have been recognized to adversely affect the strength of the material, particularly P, the prescribed standard is to provide no more than 0.030 (or 0.040) 9 of P. found that the addition of an appropriate amount of P to a ferritic stainless steel improved the stripping performance of the hot-rolled material as well as

  as the workability of the cold rolled material.

  Figure 2 graphically shows the effect of P on the value of 7 of the cold-rolled product, allowing certain variations of the measurement shown by the hatched area. The results shown in FIG. 2 were obtained on cold rolled sheets having a thickness of 0.7 mm and a base composition of 13% Cr-0.03,% C with a variable P content. All the test sheets were prepared by the same conventional process including the steps of hot rolling, annealing the hot rolled sheet, cold rolling and annealing of the cold rolled sheet. As is well known, the value of Y is a typical measure of the ability of the material to be deep drawn. The greater the value of X, especially the greater the value of y exceeds 1.0, the better the

  ability of the material to undergo deep drawing.

  As can be seen in FIG. 2, with the P content of about 0.025% normally found in conventional ferritic stainless steels, the value of Y is less than 1.0. However, while the P content increases to more than 0.075%, the value of

  eventually becomes greater than 1.4 or higher.

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

  used for pickling ferritic stainless steels.

  The fact that the workability and pickling performance of ferritic stainless steels can be improved by P enrichment is very beneficial from the point of view of the production of inexpensive ferritic stainless steels. First, P itself is a very cheap item. To improve the workability of ferritic stainless steels expensive alloying elements such as Ti, Nb and A1 have heretofore been employed, inevitably resulting in an increase in the price of the product. The enrichment in P can be carried out either by adding an appropriate source of P such as a Fe-P alloy or by using molten pig iron containing P. In the first case, the product price increase is slight. . In the latter case, the price of the product may be quite small because P which until now was removed is effectively used and so this

  eliminates or reduces the dephosphoric charge

  tion. On the other hand, it is possible in the latter case to use as raw material P-containing chromium and iron mines which have been economically of lower value than raw materials for the production of stainless steels because of their high strength. P content. Second, the pickling step of the hot rolled material can be carried out using a hydrochloric acid pickling liquid, which is advantageous not only economically but also for ease

of the process.

  However, it should not be denied that P in a ferritic stainless steel adversely affects certain properties of the steel. The presence of P frequently hinders fluidity and

  the secondary workability of ferritic stainless steel.

  It has been found that the detrimental effect of P on the strength of ferritic stainless steel can be overcome by controlling C and Cr and adding a very high

slight amount of Sol.Al.

  Figure 3 graphically shows the effect of the soil content. A1 on the solidity (reflected by the impact value of Charpy). The results shown in Figure 3 were obtained on samples having a base composition of 13% Cr-0.03% C-O, 10% P with various sol contents. Al. Each sample was prepared by forming a 30 kg ingot having the above-mentioned base composition and the particular Sol.Al content, forging at 1100 C, quenching the formed material at 760 C for 4 hours. and cutting the sample from the impregnated material. Charpy's impact tests have

  made at temperatures of 20 C and 0 C, respectively

  is lying. The acceptable impact value is generally considered to be at least 49 Nm / cm2. As can be seen in FIG. 3, while the impact value of the steel containing 0.002% of Sol.Al is almost zero at the temperatures tested, while the Sol.Al content exceeds 0.005% and approach of 0.010%, the impact value of the steel increases remarkably well beyond the acceptable value of at least 49 N. m / cm2 and with the Sol.Al content of more than about 0.020, o , the effect of Sol.Al

  to improve the strength tends to saturate.

  By the term "secondary workability" is meant the workability of a deep-drawn material. According to experience, when a cold-rolled sheet of a ferritic stainless steel enriched in P undergoes deep drawing (first stamping) and then undergoes the impact again, or when a ferritic stainless steel sheet enriched in P deep-drawn is subjected to shock caused by cutting edges, cracks frequently occur parallel to the direction of the first stamping. These cracks can be attributed to a reduction in the strength of the material due to the first stretching or stamping, and will occur more likely than the first

  stretching was more important and lower the temperature.

  Secondary workability is a property of the material, which is different from strength and formability. It should be noted therefore that there is frequently a case where even if a material has an excellent ability to undergo deep drawing as represented by its high value of Y, it can not be successfully configured to the desired end product of

  made of its bad secondary workability.

  With regard to the influence of P on the work-

  secondary unit of ferritic stainless steel enriched in P, the precise mechanism is not yet understood. It is assumed, however, that while P is an element tending to inherently induce segregation at the grain boundaries, the action of P, which has segregated at the grain boundaries, weakens the binding force between the grains, is amplified by the first stretching or stamping, and thus cracks can occur due to a fracture between the grains.

  It has been found that the detrimental effect of P on the work-

  Secondary ability could be overcome by strictly balancing the alloying elements not only individually but also mutually as proposed here. Figure 4 is a graph showing the results of the test to determine the secondary workability (cutting expansion test) on the basis of which the relationship between the contents of C, Cr, P, Sol.Al and B in the steel according to the invention. The results shown in FIG. 4 were obtained on rolled sheets

  0.7 mm thick and having various composi-

  These were prepared by the same conventional process including hot rolling, annealing of hot rolled steel, cold rolling and annealing of cold rolled steel. Each test sheet was deep-drawn at a draw ratio of 2.0 in a section having an external diameter of 27.0 mm, which was expanded to fracture at a temperature of 0 ° C. means of a conical punch. The fracture of the cut was examined to see if it was ductile or brittle. For a particular steel composition to be tested, 5 to 10 sections were prepared which were expanded to fracture and the percentage of the sections which had suffered the embrittlement fracture (percent fracture by embrittlement) was determined. The test will be called the cut expansion test. If all the sections having the particular composition to be tested have not undergone a brittle fracture in the section expansion test, that is to say if the percentage of embrittlement fracture is zero, it can be considered that the secondary workability of steel having this composition is

practically satisfactory.

  In FIG. 4 is shown the composition of the steels tested on a coordinate system with the percentage represented by the formula (Cr + 50xP) for the abscissa and the percentage represented by the formula (C + 10xB + Sol.A1) for the ordered. In these formulas, Cr, P, C, B and Sol.Al indicate the percentages of the respective elements in the steel. Figure 4 shows that there is a clear correlation between the composition of steel and

  the secondary workability of the cold rolled material.

  More particularly, it can be seen in FIG. 4 that with the composition represented by a point falling in the area above the straight line connecting points B (12.0, 0.005) and C (22.0, 0.020) and the left side of the straight line connecting the points C (22.0, 0.020) and D (22.0, 0.30), no brittle fracture was observed in the expansion test of the cutting except for Pl and P2 steels. Figure 4 further reveals that Cr and P serve to impair secondary workability while C, B and Sol.Al serve to improve secondary workability. Exceptional behavior of P1 and P2 steels is believed to be attributed to the content of particular alloying elements. On the other hand, the P1 and P2 steels had the compositions indicated in Table 1 below. It is believed that the P1 steel, because of its very low carbon content of 0.0035%, has no satisfactory secondary workability despite the presence of a significant amount of Sol. Al, while the secondary workability of P2 steel is hindered even by its relatively low P content because it

  contains an unduly excessive amount of Cr (18.60%).

  Consequently, it is necessary, in the context of the invention, to prescribe the lower limit of C as well

that the upper limit of Cr.

  Table 1 Chemical composition (% by weight) C Si Mn P S

PI 0.0035 0.29 0.18 0.082 0.006

P2 0.0370 0.08 0.23 0.048 0.005

Cr Ni Sol.Al N B

- 12.09 0.07 0.035 0.0070 tr.

18.60 0.13 0.023 0.0250 tr.

  Based on the results of the expansion test

  above, as well as their considerations, the mutual relation between certain alloying elements is prescribed as shown graphically in Figure 4 and C minimum of

0.005% and maximum Cr of 18.00%.

  The precise mechanism by which C, B and Sol.Al.

  serve to enhance the secondary workability is not

  still totally understood. But we suppose the following.

  With respect to C and B, in themselves they would probably separate at the grain boundaries, thereby reinforcing grain boundaries or preventing PR, which is harmful to secondary workability, from separating at the boundaries of

  grains. With regard to Sol.Al, as it serves to

  the precipitation of the Cr carbide, C consumed as Cr carbide, is reduced and in turn the amount of C which is dissolved or separates at the grain boundaries increases, so C serves according to the mechanism

  mentioned above, to improve secondary workability.

  The reasons for the numerical restrictions of

  Individual alloy elements will now be described.

  C must be at least 0.0050%. If undesirably low, the desired secondary workability can not be obtained as was demonstrated above with respect to the P1 steel. However, an excessively high C content not only renders the material unduly rigid, leading unsatisfactory formability, but also adversely affects the weldability of the material. To avoid these drawbacks, it is necessary to establish the upper limit of C at 0.0500%. The lower 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 of Cr and the lowest possible content of P. An excessively high content of Cr hinders the solidity as well as the secondary workability of the material as one demonstrated above with respect to steel P2. For this reason, the limit

  Cr is established at 18.00%.

  If used to improve the oxidation resistance of the material at a high temperature. But the upper limit Si is set at 0.50% because an excessively high Si content renders the material unduly rigid. Mn is an element that improves the hot workability of the material and the strength of the welded areas of the material. However, with more than 0.50% Mn, such effects tend to saturate and the product becomes expensive. For these reasons, the upper limit of Mn is set at 0.50% S is a harmful element, which adversely affects the corrosion resistance and hot workability of the material and thus, it is preferred that the content of S as low as possible. The upper limit

  permissible S is now 0.030%.

  Neither has a beneficial effect to improve the strength of ferritic materials. But a high Ni content makes the product expensive contrary to the purpose of the invention. As a result, the upper limit of 0.60% Ni as prescribed with conventional standard ferritic stainless steels is now adopted as the upper limit for Ni in

alloys according to the invention.

  The P content is critical in the context of the invention. With no more than 0.040% P, a preliminary removal of P from pig iron or special treatment for the removal of P from the converter is required, which leads to an increase in manufacturing prices. In addition, the effect of P enrichment, i.e., the best pickling performance and the best workability, do not occur. As a result, more than 0.040% of P. is required. However, an excessively high P content adversely affects the strength, hot workability and secondary workability of the material. Although these detrimental effects of P can be reduced by strictly balancing the other alloying elements according to the invention,

  the upper limit of P is now set at 0.200%.

  A1 serves as a deoxidizer in a steelmaking process to reduce the oxygen content in the steel and to clean the steel. On the other hand, acid-soluble A1 (Sol.Al) helps to eliminate the harmful effects of P on the solidity and the secondary workability of the product. To enjoy this beneficial effect of Sol.Al, one needs at least 0, 005%, of Sol.Al. However, with more than 0.200% Sol.Al, such an effect has a tendency to saturate on the one hand and a technological problem may arise on the other hand as regards the plugging of the tubes in the casting step. For these reasons,

  sets the upper limit of Sol.Al at 0.200,%.

  B, even in a very small amount, serves effectively

  to improve the secondary workability of the material.

  To obtain the desired secondary workability, a trace of B may be sufficient provided that the quantities of C and Sol.Al that cooperate are appropriate with respect to the particular content of Cr and P. For optimal secondary workability, it is preferred to less 0.0005% of B but the upper limit of B is set at 0.0050% because B tends to impair the ability to

formatting of the product.

  N is not very critical in the context of the invention. It inevitably occurs in the product during the steelmaking process and may be contained in the ferritic stainless steel according to the invention in an amount of from 0.0050% to 0.05% as it appears in conventional ferritic stainless steels. In the context of the invention, the alloying elements must be individually controlled in the respective ranges prescribed above and moreover, depending on the particular content of Cr and P, it is necessary to balance C, B and Sol.Al of so that the point having the percentage represented by the formula (Cr + 50xP) for abscissa and the percentage represented by the formula (C + 1OXB + Sol.Al) for ordinate can fall in the quadrilateral zone

ABCD of Figure 1.

  The important features and the advantageous results of the invention will be better described by the

  following examples of work and control.

  Molten steels having the chemical compositions shown in Table 2 were prepared. From each molten steel, a hot rolled strip having a thickness of 3.2 mm was prepared. A piece of the hot rolled strip was pickled and then cold rolled to a thickness of 0.7 mm without any intermediate annealing and then subjected to a finishing anneal comprising the steps of baking at a temperature of 820 ° C.

  for one minute, allowing to cool in the air.

  The steel samples thus prepared have been

  tested for their y value and secondary workability.

  The value of y was calculated according to = (r0 + 2 r45 + r90) / 4 where ro, r45 and r90 are the Lankford values measured along the directions of 00, 45 and 90 with respect to the rolling direction, respectively . To determine the secondary workability, the section expansion 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.

Table 2

  Steel N Class C If Mn P S Cr Ni Sol.Al N B Cr + 50P C + l0B4ol.Al Remain Fe and 1A 0.0468 0.35 0.20 0.104 0.005 10.45 0.06 0.045 0.0082 tr. 15.65 0, 0918 impurities Fe and 2A 0.0121 0.09 0.17 0.089 0.006 11.29 0.10 0.012 0, 0120 tr. 15.74 0.0241 impurities 0.0220 0.27 0.23 0.181 0.001 12.43 0.07 0.058 0.0250 0.0010 21.48 0.0900 Fe and 3 A 0.0220 0.27 0 , 0.181 0.001 12, 430.0.07 impurities Fe and 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 impurities Fe and A 0.0450 0.30 0.36 0.092 0.004 16.72 0.30 0.120 0.0087 tr. 21.32 0.1650 Fe and impurities Fe and 6A 0.075 0.18 0.12 0.053 0.007 16.85 0.07 0.010 0.0210 0.0042 19.50 0.0895 impurities Feet 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 Impurities Fe and 8 B 0.0308 0.29 0.24 0.250 0.005 11.34 0.07 0.023 0.0103 tr. 23.84 0.0538 impurity Fe and 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 impurities B 0.0028 0, 48 0.19 0.084 0.008 12.59 0.10 0.063 0.0143 tr. 16.79 0.0658 Fe and Fe and 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 imuet Fe and 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 Impurities Fe and 13 B 0.0342 0.36 0.27 0.062 0.006 19.70 0.10 0.004 0.0080 tr. 22.80 0.0382 Impurities A: Steel according to the invention B: Steel Control Co

Table 3

  Steel Class Workability Value * Secondary NO

1 to 1.50

2 to 1.42

3 to 1.49 O

4 to 1.37 O

5 to 1.65 Q

6 to 1.21 Q

7 to 1.34

8 B 1.32

9 B 0.87 Q

B 1.45

11 B 1.51

12 B 1.43

13 B 1.33

  A: steel according to the invention B: control steel *: cut expansion test O: fragility fracture percentage: O @: fragility fracture percentage: less than 50% *: fragility fracture percentage:

not less than 50%.

We can reveal the following.

  The NOs 1 to 7 steels have fairly high values indicating their excellent deep drawing capability, without embrittlement fracture in the sectional expansion test reflecting their

  satisfactory secondary workability.

  Control steel N 8 contains P in an amount of that prescribed herein, and also a percentage of (Cr + 50P) exceeding the range shown in Figure 1. As a result, the secondary workability of this steel is not satisfactory although the value of r is quite high. The N 9 control steel has a low P content and accordingly its r value is low, indicating

  its poor ability to undergo deep drawing.

  The N 10 control steel contains 0.0028% of-C, which is less than what is prescribed here. As a result,

  the secondary workability of this steel is insufficient.

  Control steel N 11 contains alloying elements at the amounts individually within the respective ranges prescribed herein. But the percentage (Cr + 50P) is in excess of the range indicated in FIG. 1, resulting in secondary workability.

unsatisfactory.

  The N 12 control steel contains 0.003% Sol.Al, which is lower than that prescribed here, and the percentage (C + 10B + Sol.Al) is very low for its, 59% of (Cr + 50). As a result, secondary workability

is not satisfactory.

  The control steel NO 13 also contains Sol.Al in an amount less than that prescribed here and% Cr and o (Cr + 50P) are too high. Therefore,

  the secondary workability of steel is bad.

R E V E N D I CA T I 0 N S

  1. Ferritic stainless steel added with P having excellent aptitude for shaping and secondary workability, characterized in that it consists essentially of, in% in 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. Al: 0.005 to 0.200%, and B: traces up to 0.0050%, the remainder being Fe and unavoidable impurities, the alloying elements being further balanced so that the point having the percentage represented by the formula ( Cr + 50 x P) for the abscissa and the percentage represented by the formula (C + 10xB + Sol.Al) for the ordinate can be in the area of a quadrilateral ABCD, where the 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.

  2. Steel according to claim 1, characterized in that the percentage of B is from 0.0005 to 0.0050%.