GB2203680A

GB2203680A - Direct production of stainless steel strip having excellent superplasticity and surface properties

Info

Publication number: GB2203680A
Application number: GB08709352A
Authority: GB
Inventors: Kuniaki Osada; Takeya Tohge; Masato Noda
Original assignee: Nippon Yakin Kogyo Co Ltd
Current assignee: Nippon Yakin Kogyo Co Ltd
Priority date: 1987-04-21
Filing date: 1987-04-21
Publication date: 1988-10-26
Also published as: DE3715411A1; DE3715411C2; GB8709352D0; GB2203680B

Description

A DIRECT PRODUCTION PROCESS OF A STAINLESS STEEL STRIP HAVING EXCELLENT

SUPERPLASTICITY AND SURFACE PROPERTIES This invention relates to a direct production process of stainless steel strip having excellent superplasticity and surface properties.

It is already known from for example, Trans.

Quart. A.S.M. 61 (1968), 85 that some kinds of two-phase stainless steel have superplasticity. With a new process employing superplasticity, complicated-shaped objects can be manufactured with less machining that with a conventional process on account of low stress and high ductility in machining resulting from the superplasticity.

Therefore, it is known that in order for two-phase stainless steel to have such superplasticity, it is necessary to give it a fine-grained texture.

In addition, it is reported in Nikkei New Material No. 5, 1986, p.30 that two-phase stainless steel acquires a fine-grained texture provided with superplasticity when it is quenched and solidified into an about 1 mm-thick plate consisting of ferrite phase only, cold-rolled to 80% of the thickness and then annealed at 1050 0 C.

As stated above, conventional product-ion processes have to convert a molten two-phase stainless steel into plate and subject the plate to heat-treatment to give pure ferrite phase or a small amount of austenite remains in ferrite matrix; thus, they have disadvantages:

the heat-treatment has to be made at elevated temperatures, the combination of repeated process is needed and the production yield is low.

According to the present invention there is provided a direct production process of a length of continuous thin two-phase stainless steel strip having excellent superplasticity and surface properties as is casted, characterised by casting molten two-phase stainless steel by means of either single roller or a pair of rollers, continuously quenching and solidifying.

Preferably the strip is of thickness 5 mm or less. The inventors have used the invention using z cast molten SUS 329 J, by continuous quenching and I solidifying on a single roller and a pair of rollers separately to manufacture a thin stainless steel strip of thickness 5 mm or less and to make experiments on superplasticity in order to determine the strain rate sensitivity factor (m), but they failed to make the factor above 0.3. Accordingly, they used a two-phase stainless steel comprising not more than 0.02 Z of carbon, not more than 2.0 Z of silicon, not more than 3.0 Z of manganese, 3 - 10 Z of nickel, 20 - 35 X of chromium, 0.5 - 6.0 Z of molybdenum, 0.08 - 0.3 2 of nitrogen, 0.03 - 2.0 % of either tungsten or vanadium, 0.0005 - 0.01 % of boron, not more than 0.005 % of sulfur and remainder composed substantially of iron, whose processability is referred to in Japanese patent publication No. 59-, 14099, and another two-phase stainless steel containing not more than 2.0 % of copper in.addition to the above composition for the experiment on the superplasticity to determine the strain rate sensitivity factor (m) at elevated temperatures in the same manner as in SUS 329 J 1" As a result, they found that the factor reached 0.3 or more and these steels are preferred in the invention.

Thus, the invention relates to a direct production process of a length of continuous thin two-phase stainless steel strip having ex cellent superplasticity and surface properties as is casted, characteri zed by casting molten two-phase stainless steel by means of either single roller or a pair of rollers, continuously quenching and solidifying the that a small amount of austenite ramains in ferrite matrix.

same, so The above and other objects and features of this invention will appear fully hereinafter from a consideration of the following descrip tion taken in connection with the accompanying drawing wherein one exam ple is illustrated by way of example.

Brief Description of the Drawing

Fig. I is a vertical cross-sectional view of an apparatus for the embodiment of this invention, which uses a single roller 4 to manufacture a length of quenched thin stainless steel strip 5.

Fig. 2 is a vertical cross-sectional view of another apparatus for the embodiment of this invention, which uses a pair of rollers 9, 9' to manufacture a length of quenched thin stainless steel strip 5 downward.

Figs. 3(a) - (c) are a vertical cross-sectional view of a further apparatus for the embodiment of this invention, which uses a pair of rollers 9, 9' disposed at different levels to manufacture a length of quenched thin stainless steel strip 5.

Fig. 4 is a vertical cross-sectional view of a still further appara- tus for the embodiment of this invention, which uses a pair of rollers 9, 9' to manufacture a length of quenched thin stainless steel strip 5 upward.

Fig. 5 is a 200-fold magnified optical-micrograph showing a fine- grained texture insid a quenched thin-stainless steel strip 5 manufac tured according to this invention.

Fig. 6 is a plan view showing a test piece of stainless steel used in the tensile test.

Fig. 7 is a graph showing the relation between strain rate (sec-1) and elongation (%) of No. 4 stainless steel specimen manufactured by means of a pair of rollers according to this invention.

Detailed Description of the Invention

This invention will be explained in more detail in reference with the accompanying drawing.

First of all, it should be stated here that a two-phase stainless steel comprising not more than 0.02 % of carbon, 3 - 10 % of nickel, - 35 % of chromium, 0.5 - 6.0 % of molybdenum, 0.08 - 0.3 % of nitrogen, 0.0005 0.01 % of boron, 0.03 - 2.0 % of either tungsten or vanadium or the both, not more than 0.005 Z of sulfur, which is referred to in Japanese patent publication No. 59-14099, can give a micro-grained ferrite texture containing less austenite than SUS 329 J 1 when the molten two-phase stainless steel is continuously quenched and solidified on a single roller or a pair of rollers so that a plate of thickness 5 mm or less is manufactured; moreover, the grains are easy to slide on their boundaries, which surely contributes to its excellent superplasticity.

Therefore, in this invention, a thin strip of thickness 5 mm or less is manufactured from a two-phase stainless steel, whose preferred =nposition is 4 as set out in the claims on the basis of this knowledge by directly, continuously quenching and solidifying the molten stainless steel on a single roller or a pair of rollers.

Fig. 1 is a vertical cross-sectional view of an apparatus for the embodiment of this invention, wherein a thin strip 5 is manufactured by casting and quenching a molten stainless steel 2 on a single roller 4.

To begin with, a molten two-phase stainless steel 2 is poured into a tundish 3 from a molten metal vessel 1. Three of the four walls and the bottom of the tundish 3 are made of a refractory. The remainder wall is formed by the surface of the single roller 4. In other words, as far as the tundish 3 is concerned, the single roller 4 serves as one of the four walls to store the molten stainless steel 2. Therefore, as the single roller 4 rotates in the arrow-headed direction in Fig. 1, the molten stainless steel 2 can be quenched and solidified on the single roller 4 in a continuous manner so as to form a continuous thin strip 5 of thickness 5 mm or less according to adjustment.

Fig. 2 is a vertical cross-sectional view of an apparatus for the embodiment of this invention, wherein a pair of water-cooled rollers 9, 9' are used to manufacture a quenched thin stainless steel strip 5.

Specifically, the apparatus is constructed substantially of a molten metal vessel 1, whose bottom is provided with a downwardly extending nozzle 6, an inclined refractory plate 7, whose upper surface is in contact with the lower end of the nozzle 6, a pair of rollers 9, 9" disposed under the lower end of the inclined plate, and a pair of plates each slidably disposed on each side of a pair of rollers 9, 9', for damming up the molten stainless steel 2 poured between a pair of rollers 9, 9', so that the molten stainless steel 2 can be casted downward from an interstice formed where a pair of rollers 9, 9' are closest to each other, wherein a notch 8 is provided to one side of the lower end of the nozzle 6 facing the lower end of the inclined.plate 7, the distance between where the lower end of the nozzle 6 is brought into contact with the inclined plate 7 and'the lower end of the inclined plate 7 is sufficient so that the molten stainless steel 2 flowing out of the notch 8 of the nozzle 6, spreading out like an unfolded fan, can form a steady laminar flow with uniform flow distribution extending over the inclined plate 7 before it arrives at the lower end of the inclined plate 7, and the distance between the lower end of the inclined plate 7 and where a pair of rollers 9, 9' close most to each other is fixed so that the molten stainless steel 2 dripping from the lower end of the inclined plate 7 does not cause any turbulence hydromechanically on the surface and inside of the molten stainless steel 2 stored for a while on a pair of rollers 9, 9'.

A production process of a continuous thin two-phase stainless steel strip 5 by the use of the above apparatus will be described as follows:

Since the upper surface of the inclined refractory plate 7 and the lower end of the pipe 6 are in contact with each other and a nozzle 8 is provided to one side of the lower end of the nozzle 6 facing the lower end of the inclined plate 7, as stated above, the molten stainless steel 2 flows out of the notch 8, spreading out like the unfolded fan, and forms a steady laminar flow with uniform flow distribution extending over the inclined plate 7 before the molten stainless steel 2 gets to'the lower end of the inclined plate 7. The molten stainless steel 2, having dripped from the lower end of the inclined plate 7, is stored for a while in an upwardly opened space formed by a pair of water-cooled rollers 9, 91, of which the line connecting their centers is disposed parallel to the lower end of the inclined plate 7, and a pair of plates, each slidably disposed on each side 6f a pair of rollers 9, 9', and then casted downward from a gap formed where a pair of rollers 9, 9' close most to each other so as to form the quenched thin stainless steel strip 5.

Figs. 3(a) - (c) are a vertical cross-sectional view of a further apparatus for the embodiment of this invention, wherein a pair of water cooled rollers 9, 9', disposed at different levels, are used for the pro duction of the quenched thin stainless steel strip 5. Specifically, a nozzle 6 for the molten stainless steel 2, whose width is made equal to or less than the length of a pair of rollers 9, 9', is brought into contact with-a pair of rollers 9, 9' so that the molten stainless steel 2 can be fed between them and casted therefrom upon having been quenched on their rotating surface.

Fig. 4 is a vertical cross-sectional view of a still further appara- tus for the embodiment of this invention, wherein a pair of water-cooled rollers 9, 9', disposed at the same level, are used for the production of the quenched thin stainless steel strip 5. Specifically, a nozzle 6 6 for the molten stainless steel 2, whose width is made equal to or less than the length of a pair of rollers 9, 9', is brought into contact with them from downward thereof so that the molten stainless steel 2 can be fed therebetween and casted upward therefrom upon having been quenched-on their rotating surface.

A thin stainless steel strip produced in the above mention embodi- ment of this invention proves to have fine-grained texture, as shown by a micrograph (x200) in Fig. 5, in which some austenite is in sight on and inside ferrite grains.

In the above apparatuses shown in Figs. 1, 2, 3(a) - (c) and 4, the thickness of stainless steel strip can be changed by changing the diameter, material, rotation speed of the roller or rollers; additionally, as far as a pair of rollers system is concerned, it can be changed by changing the gap and cooling rate thereof as well. In any case, however, in a thin stainless steel strip of thickness 5 mm. or less, there can be found a texture with a small amount of austenite depositing on or inside ferrite grains. Two-phase stainless steels, which are related in the

Claims

following examples and of which the ccmposition is clairred respectively

in Claims
2 and 3, have excellent superplasticity and surface properties as cast.

The invention will be understood more readily with reference to the following examples; however, these examples are intended to illustrate the invention and are not to be construed to limit the scope of the in vention.

Example Table 1 shows the composition of various stainless steels used in the examples of this invention (No.
3 - 7) and the comparative examples therefor (No. 1 and 2). Table 2 shows the casting condition (roller type, roll diameter, amount of circulating cooling water for rollers strip thickness and cooling rate), maximum elongation and strain rate sensitiv ity factor m at 1000 C of stainless steels listed in Table 1.

In advance of the experiment for the determination of the maximum elongation, a test piece such as shown in Fig. 6 is prepared so that the drawing direction is in conformity with the direction perpendicular-to the casting direction and the jaw interval falls 5.0 mm. Each test piece is drawn at a constant strain rate of 5.0 x 10-4_ 5.0 x 10-3 sec-1 after 7 having been kept for 5 minutes at elevated temperatures. Fig. 7 shows the relation between strain rate (sec-') and elongation (%) of No.
4 stainless steel specimen at 1000 C. Table 3 shows the relation between Z strain rate (sec -1) and deformation resistance (kgImm2) of the same specimen iat 1000 OC.

Table 3 Strain rate (sec-')
5.00x10-4 8.33x10-4 1.67x10-3 3.33x10-3 5.00x10-3 Deformation (kglmm 2 0.28 0.40 0.54 0.63 0.79 resistance The strain rate sensitivity factor m of the specimen can be deter- mined by putting its measurement results in the following equation, cr - K where a stands for deformation resistance (kglmm 2), K is a constant and is a strain constant (sec-'), whereby m is determined as 0.51. Table 2 shows the strain rate sensitivity factor (m) of all other specimens de termined in the same way, together with their maximum elongation.

Meanwhile, according to a report titled Superplasticity and Super- plastic Forming Process in Mat. Sci. & Tech., 1, 925 (Nov. 1985), it is made clear that the superplasticity of micro-grained texture shows a strain rate sensitivity factor of 0.3 or more. Therefore, it becomes evident from Tab16 2 that five specimens No. 3 - No. 7, which were all manufactured according to this invention, have superplasticity.

The superplasticity is accounted for that crystallites torn by strain in the progress of earlier plastic deformation crystallize again by therm--.

al energy in the progress of the further deformation and grow up to micro crystallites capable of causing superplasticity.

As stated above, according to this Invention, a continuous thin stainless steel strip of thickness 5 mm or less, having excellent super plasticity and surface properties as Is casted, can be manufactured easily at lower cost by casting molten two-phase stainless steel on either single roller or a pair of rollers so that the molten metal is quenched and solidified.

Table 1 No. c si Hn p S Ni Cr MO Cu N B v W Comparative 1 0.028 0.61 0.54 0.025 0.005 5.01 23.70 1.48 1.50 0.114 Examples 1 2 0.031 0.72 0.52 0.026 0.006 4.96 24.48 1.51 - 0.099 3 0.009 0.60 0.29 0.023 0.0007 4.68 24.00 2.04 1.44 0.113 0.0011 0.08 0. 19 4 0.008 0.60 0.34 0.020 0.001 5.55 27.38 3.06 0.28 0.110 0.0030 - 0.19 Examples 0.013 1.51 0.60 0.021 0.001 5.85 24.50 2.00 - 0.080 0.0009 0.13 - of Invention
6 0.009 0.61 0.55 0.020 0.0009 6.65 26.01 3.40 0.61 0.122 0.0010 0.08 0. 15
7 0.010 0.57 0.60 0.022 0.001 6.01 25.01 3.00 - 0.101 0.0009 - 0.14 Table 2 1 OD Casting condition Max. Strain rate No. Roller diameter Amount of circulating Strip thickness Cooling rate Elongation Sensitivity 1000 c (%) - Roller type cooling water (Rlom) (0m) (clace) factor (m) at Comparative 1 single roller 600 280 0.
8 10 3 105 80.0 0.22 Examples 2 a pair of rollers 400 320 1.2 10 10 4 63.0 0.18 1 3 single roller 600 480 0.8 10 5 106 311.0 0.33 4 a pair of rollers 400 320 1.2 2 4 404.0 0.51 10 Examples a pair of rollers 400 320 2.0 10 % 102 210.0 0.33 of Invention 3.8 10 2 10 4 313.0 0.44 6 single roller 600 480 7 a pair of rollers 400 480 4.
9 10 3 10 5 307.0 0.31 - CLAIMS (1) A direct production process of a length of continuous thin two-phase stainless steel strip having excellent superplasticity and surface properties as cast characterized by casting molten two-phase stainless steel by means of either single roller or a pair of rollers, continuously quenching and solidifying.

(2) A direct production process of a length of continuous thin two-phase stainless steel strip according to Claim 1, characterized in that said molten two-phase stainless steel comprises not more than 0.02 of carbon, not more than 2.0 % of silicon, not more than 3.0 % of manganese, 3 - 10 % of nickel, 20 - 35 2 of chromium, 0.5 - 6.0 % of molybdenum, 0.08 - 0.3 % of nitrogen, 0.03 - 2.0 % of either tungsten or vanadium, 0.0005 - 0.01 % of boron, not more than 0.005 % of sulfur and remainder composed substantially of iron.

(3) A direct production process of a length of continuous thin two-phase stainless steel strip according to Claim 1, characterized in that said molten two-phase stainless steel comprises not more than 0.02 % of carbon, not more than 2.0 % of silicon, not more than 3.0 % of manganese, 3 - 10 2 of nickel, 20 - 35 % of chromium, 0.5 - 6.0 % of molybdenum, 0.08 - 0.3 % of nitrogen, 0.03 - 2.0 % of either tungsten or vanadium, 0.0005 - 0.01 % of boron, not more than 0.005 % of sulfur, not more than 2.0 % of copper and remainder composed sub stantially of iron.

(4) A direct production process of a length of cqntinuous thin two-phase stainless steel strip according to claim 1, 2 or 3 characterized in that said continuous thin two-phase stainless steel strip has a thickness of 5 mm or less.

(5) -A direct production process of a length of continuous thin two-phase stainless steel strip according to Claim 1, 2, 3 or 4 characterised in that said molten two-phase stainless steel is continuously poured into a tundish to the full, said single roller is rotated in the direction from the bottom of said tundish to the free surface of said molten stainless steel stored in said tundish and said molten stainless steel is casted by means of said single roller, continuously quenched and solidified.

(6) A direct production process of a length of continuous thin two-phase stainless steel strip according to Clairn 1, 2, 3 or 4 characterized in that said molten stainless steel is caused to flow out of a notch provided to one side of the lower end of a nozzle in contact with an inclined refractory plate, spreading out like an unfolded fan, and form a steady laminar flow with uniform flow distribution extending over said inclined plate before getting to the lower end of said inclined plate, and then continuously fed into an upwardly opened space formed by 9 pair of water-cooled parallel rollers thereunder of which the line connecting both the centers is disposed substantially parallel to the lower end of said inclined plate and a pair of plates each slidably disposed on each side of said rollers so as not to give any turbulence to the same kind of molten stainless steel already stored in said upwardly opened space, and finally casted downward from an interstice formed where said rollers close most to each other so as to be quenched and solidified; said nozzle extending downwardly from a molten metal vessel to said inclined plate, said notch facing toward the lower end of said inclined plate, and said rollers rotating in opposite direction with each other so as to put said molten stainless steel therebetween and send the same therefrom downward.

(7) A direct production process of a length of continuous thin two-phase stainless steel strip according to Claim 1, 2, 3 are characterized in that said molten stainless steel is fed into between a pair of water- cooled parallel rollers each disposed at different levels from a nozzle of which the front end is in contact with said rollers and of which width is made equal to or less than the length of said rollers and casted from an intersticeof said rollers so as to be quenched and solidified; said rollers rotating in opposite direction with each other so as to put said molten stainless steel tberebetween and send the same therefrom to an arbitrary direction.

(8) A direct production process of a length o " f continuous thin twophase stainless steel strip according to Claim 1, 2, 3 or 4 characterized in that said molten stainless steel is fed into betweeen a pair of water cooled parallel rollers each disposed at the same level from a guide nozzle, of which the upper end is in contact with the lower side of said rollers and of which width is made equal to or less than the length of said rollers and casted from an interstice of said rollers so as to be quenched and solidified; said rollers rotating in opposite direction with each other so as to put said molten stainless steel therebetween and send the same therefrom upward.

il 9. A process according to claim 2 or 3 wherein the steel additionally includes phosphorus, preferably up to 0.06%, more preferably 0.04%, preferably more than 0.01%, more preferably more than 0.02%.
10. A two phase stainless steel strip made by a method substantially as hereinbefore described with reference to the accompanying drawings.
11. A two phase stainless steel strip made by a method according to claim I and substantially as bereinbefore described in the Example herein.
12. A two phase stainless steel strip made by the method of any preceding claim.
13. A two phase stainless steel comprising not more than 0.02% carbon, not more than 2.0% silicon not more than 3% manganese, 3-10% nickelf 20-35% chromium, 0.5-6.0% molybdenum, 0.08-0.3% nitrogen, 0.03-2.0% of tungsten and/or vanadium, 0.0005-0.01% boron, not more than 0.005% sulphur, the remainder being substantially iron.
14. A steel according to claim 13 wherein a small amount of austenite remains in and/or on the ferrite matrix and a strain rate factor in excess of 0.3 is displayed.
15. A steel according to claim 13 or 14 wherein the steel additionally includes not more than 2% copper.
16. A steel according to claim 13, 14 or 15 wherein the steel is in the form of a strip of thickness 5 mm or less.
17. A steel according to claim 13, 14, 15 or 16 wherein the steel additionally includes phosphorus.
18. A steel according to claim 17 wherien the amount of phosphorus is up to 0.6%. preferably up to 0.4% and wherein the amount of phosphorus is preferably more than 0.01%, more preferably 0.02%.

12
19. A two phase stainless steel according to claim 13 substantially as hereinbefore described.
20. Apparatus for use in the process of claim 1 including a tundish, means to supply molten metal thereto, a roller at one side of the tundish to rotate in a direction towards the surface of the metal to cast the metal thereon and quench and solidify the metal.
21. Apparatus for use in the process of claim 1 including an inclined refractory plate, a nozzle to release molten metal to flow down the plate, a pair of rollers defining a nip disposed below the plate with the nip below the lower end of the plate so as to cast the metal and quench and solidify the metal.
22. Apparatus for use in the process of claim 1 including a pair of parallel rollers at different levels with a nozzle by which molten metal can be supplied to the rollers having its front end in contact with the rollers so as to cast the metal on the rollers and quench and solidify the metal.
23. Apparatus for use in the process of claim 1 including a pair of parallel rollers and a nozzle to feed molten metal upwardly to the rollers, the nozzle end contacting the ro.llers so as to cast the metal on the rollers and quench and solidify the metal.

Publibhed 1988 at The Patent office, state House, 8W1 High H01born, London WC1R 4TP. Further copies May be obtained from The Patent Office, Sales Branch, St Mw7 Cray, Orpington, Kant BR5 3RD. Printed by multiplex techniques ltd, St Ma17 Cray, Rent. ro L r 1/87.

---- --- - --.Y ---V, Wnu MnD or"j. 1'rInTea DY JIMUITApiex wonniques ltd, St Mar7 Cray, Kent. CorL 1/87.