US3832136A

US3832136A - Double walled tube of high chromium steel

Info

Publication number: US3832136A
Application number: US00273771A
Authority: US
Inventors: T Ohama; K Shinohara
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1971-08-04
Filing date: 1972-07-21
Publication date: 1974-08-27
Anticipated expiration: 1991-08-27
Also published as: DE2238492A1; DE2238492C2; JPS5144887B1; GB1406043A; FR2148249B1; FR2148249A1

Abstract

A DOUBLE WALL TUBE IS COMPOSED OF A TWICE COILED OR ROLLED COPPER-PLATED HIGH CHROMIUM STEEL STRIP ESSENTIALLY COMPOSED OF NOT HIGHER THAN 0.04% C, NOT HIGHER THAN 0.50% SI, NOT HIGHER THAN 0.50% MN, NOT HIGHER THAN 0.50% NI, 10.0-14.0% CR, NOT HIGHER THAN 0.50% AND NOT LOWER THAN C PERCENT X 10 OF TI, AND IRON BALANCE AND THE OUTER SURFACE OF THE TUBE BEING COATED WITH A LEADTIN COATING.

Description

Aug. 27, 1974 TSUYOSHI OHAMA ETAI- 3,832,136

DOUBLE WALLED TUBE OF HIGH CHROMIUH STEEL Filed July 21. 1972 2 Sheets-Sheet 1 FIG. I

C 0.I7 -0.073% Ni 0.I I 024 Si 10.3I -0.66% CI l0.86-I2.I0 Mn O.30-0.55/o Ti-'0.20-0.68%

l I l l l Aug. 27, 1974 TSUYOSHI OHA MA ETAL 3,832,135

DOUBLE WALLED TUBE OF HIGH CHROMIUH STEEL 2 Sheetsi-Sheec 2 Filed July 21. 1972 FIG. 2

O.25-O.45/o

Mn 3 0.28 -O.32 Ti O.36-O.44%

O A V [II SOFTENED ANNEALING STEEL H3OC X 3Mi n.HEATING AND .AVI

AIR COOLING ll I mozmmw A m5:

0 6 Il A Y AA 4 3 2 zo ozod m6 ISZMEZ ymzm; b 1 5515 3m;

United States Patent Oifice 3,832,136 Patented Aug. 27, 1974 3,832,136 DOUBLE WALLED TUBE OF HIGH CHROMIUM STEEL Tsuyoshi Ohama, Ichikawa, and Konosuke Shinohara, Tokyo, Japan, assignors to Nisshin Steel Co. Ltd., Tokyo, Japan Filed July 21, 1972, Ser. No. 273,771 Claims priority, application Japan, Aug. 4, 1971, 46/58,278 Int. Cl. B32b 15/00 U.S. Cl. 29191 2 Claims ABSTRACT OF THE DISCLOSURE A double wall tube is composed of a twice coiled or rolled copper-plated high chromium steel strip essentially composed of not higher than 0.04% C, not higher than 0.50% Si, not higher than 0.50% Mn, not higher than 0.50% Ni, 10.0-14.0% Cr, not higher than 0.50% and not lower than C percent X of Ti, and iron balance and the outer surface of the tube being coated with a leadtin coating.

'BACKGROUND' OF THE INVENTION Field of the Invention This invention relates to a double wall tube of high chromium steel and a process for production thereof. More particularly, this relates to a double wall tube of high chromium steel which comprises a double wall tube of high chromium steel with a single phase ferrite similar to a plain carbon steel in point of mechanical properties and a lead-tin coating on the outside of the tube.

Description of the Prior Art Heretofore, a copper-plated double wall tube has been used in place of a copper tube, as a feeding tube for the brake fluid of car, a feeding tube for gas, a radiator tube for a heat exchanger and the like. This copper-plated double Wall tube may be produced by cold-rolling a plain carbon steel to form a thin strip of several hundreds microns in thickness, plating both surfaces with copper to form a copper layer of several microns thick, annealing the thin steel strip thus plated, rolling the resulting tube material to produce a tube and brazing the copper plating. The double wall tube thus obtained is preferable from the point of view of formability and cost. However, this double wall tube prepared from a plain carbon steel is not of sufiicient corrosion resistance under the recent severe corrosive environment, and its durability is extremly short in certain severe environments. For the purpose of improving the durability, there have been recently employed a treatment of plating the outer surface with zinc or a rubber lining, but no satisfactory treatment has yet been found. I

In general, the inner surface of a tube is hardly subjected to corrosion, but the outer surface of a tube is often subjected to corrosion. For example, outer surface of a brake tube of car is exposed to outside atmosphere water and mud and therefore, corrosion proceeds from the plated layer and rubber lining through the copper coating to the iron portion so that the life of the tube is not satisfactorily long. Particularly, even when the outer surface of the tube is plated with a lead-tin plating layer, the corrosion resistance against chlorine ions is not effectively improved since the natural electrode potential of the lead-tin alloy is nobler than that of iron and, therefore, the iron is subjected to corrosion in spite of the presence of the lead-tin plating layer on the outer surface of the tube.

There have been recently used clad materials of plain carbon steel and stainless steel plated With copper which have fairly high corrosion resistance and mechanical strength. Double Wall tubing using the stainless steel clad material solves the problems of corrosion resistance and durability to some extent, but is not yet satisfactory. The clad material is usually expensive due to complicated manufacturing processes.

It has been also recently proposed to manufacture a duplex roll tube by using an inexpensive and corrosion proof stainless steel as the base material for the tube. However, conventional stainless steels are generally hard and do not have such easy workability as plain carbon steels or stainless steel clad material and therefore, conventional stainless steel can not be used for double wall tubes. In other words, when a conventional relatively hard stainless steel sheet or strip is wound up twice, shaped and brazed, the outer layer and inner layer of the double wall tube can not closely contact each other but form gaps since the stainless steel is hard and poor in bending workability and further has a buckling property. In an extreme case, the adjoining sheet surfaces are separate from each other and each surface is separately brazed. Thus, any satisfactory double wall tube is not obtained.

In addition, a stainless steel itself is not easy to plate with a brazing metal such as copper, zinc, solder and the like.

SUMMARY OF THE INVENTION According to the present invention, there is provided a double wall tube of high chromium steel produced by shaping, as by rolling or coiling a copper-plated high chromium steel strip essentially consisting of not higher than 0.04% C, not higher than 0.50% Si, not higher than 0.50% Mn, not higher than 0.50% Ni, 10.014.0% Cr, not higher than 0.50% and not lower than C percent X 10 of Ti, and iron balance and then coating the outer surface of the tube with a lead-tin coating.

An object of this invention is to provide a double wall tube having high corrosion resistance under severe conditions.

Another object of this invention is to provide a double wall tube free from transformation of structure such as hardening, embrittlement and loss of strength upon heating treatment in brazing.

A further object of this invention is to provide a double wall tube readily adaptable to brazing.

Still another object of the present invention is to provide a double Wall tube composed of a stainless steel of high chromium containing a specified combination of components giving excellent workability.

Still further object of the present invention is to provide a method for producing the double wall tube of high chromium.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph showing a relation between contents of C and Ti and transformation of steel structure when a chromium steel containing titanium is heated at 1150 C.; FIG. 2 is a graph showing a relation between chromium contents and mechanical properties.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to solve the above-mentioned problems in prior art, the present inventors have now succeeded in manufacturing an inexpensive and highly corrosion proof (even under severe conditions) double wall tube of high chromium steel by selecting the amounts of C, Si, Mn, Ni and Cr components at low level as far as possible so as to lower the yield strength, tensile strength and hardness to those of a plain carbon steel, adding titanium thereto for the purpose of preventing the steel from hardening and embrittling after brazing treatment and thereby producing a chromium series stainless steel having a single phase of ferrite, coating the resulting chromium series stainless stainless steel with copper, shaping the stainless steel thus copper-plated into a double wall tube and coating the outer surface of the resulting double wall tube with a lead-tin coating.

Contents of components in the high chromium steel used, as tube material for the double wall tube in the present invention are selected by the reason stated below.

Carbon is an important component affecting yield strength, tensile strength, and hardness and moreover, is an effective element to form austenite, but when the carbon content is higher, the steel becomes hard and formability for forming the double wall tube is lowered. Therefore, the carbon content should be not higher than 0.04%. At a carbon content higher than 0.04%, the heat treatment in brazing results in the formation of an austenite system and therefore a hard martensite is apt to form at normal temperature. This formation is not desirable from the mechanical property point of view, and the titanium content should be increased so as to prevent such formation. Therefore, the upper limit of the carbon content is set at 0.04%.

Silicon is an element capable of forming ferrite and suppressing the formation of martensite. When the silicon content is too high, the steel becomes hard, as in the case of carbon content, so that the silicon content is kept as low as possible, and the upper limit is 0.50%.

Manganese is an element capable of forming austenite and is also necessary as a deoxidizer, but a higher content of manganese accelerates the formation of a martensite system. The upper limit is 0.50%.

Nickel is an important element for improving corrosion resistance as is chromium, but can form austenite and therefore, high nickel content accelerates the formation of a martensite system and furthermore, results in high cost. Therefore, the upper Ni limit is set at 0.50%.

Chormium is an element capable of forming ferrite and is important for maintaining corrosion resistance, but at a content of less than the corrosion resistance is not sufficiently maintained while at a content of higher than 14% the yield strength, tensile strength and hardness of the steel become so high that the bending workability is lowered. Therefore, the content range of chromium is set between 10.0 and 14.0%

Titanium is an element capable of forming ferrite and and also can form a stable compound with carbon and nitrogen. Therefore, titanium is an element effective for complete ferrite formation. For formation of complete ferrite the content of titanium should be not lower than 4 10 times the content of carbon, but when the Ti content is higher than 0.50%, the steel is hardened so that the optimum range of Ti content is not higher than 0.50% and not lower than 10 times the C. percent. In this content range of Ti the sensitivity of intergranular corrosion can be suppressed upon heat treatment in the brazing step.

As a brazing material for the steel plate used for a double wall tube according to the present invention, there may be used a metal coating layer such as copper, zinc, solder and the like.

Particularly, copper plating is easily applicable to the steel material used in the present invention. It is preferred to employ a copper plating layer as the brazing material for a usual usage.

When the copper-coated steel is annealed, the Vickers hardness becomes about and this softness is almost the same as that of plain carbon steel, and furthermore, when the steel is shaped into a double wall tube and heated for hazing, there may be obtained a double wall tube of high chromium having high corrosion resistance and no void at the bonding layer.

The corrosion resistance is further improved by applying a lead-tin coating to the outer surface of the copperplated double wall tube.

The double wall tube of high chromium itself according to the present invention show a good corrosion resistance even if the coating on the outer surface of tube is not provided, but a complete corrosion resistance against chlorine ion is not expected when the double wall tube is used as a brake tube for a car and a freezing inhibitor such as calcium chloride is scattered on a wet road in winter. Under such a severe condition as above, the corrosion resistance can be maintained by plating the outer surface of the copper-plated double wall tube with lead-tin.

When a copper-plated double wall tube is exposed to a corrosive atmosphere containing chlorine ion, the copper is corroded with the chlorine ion and the corrosion rapidly reaches the steel portion. The natural electrode potential of the high chromium steel used in the present invention is more base than that of copper and there easily occurs pitting corrosion when exposed to a corrosive environment. On the contrary, when a lead-tin coating is applied to the outer surface of the copper-plated double wall tube, the direct attack of chlorine ion is effected onto the lead-tin layer and this corrosion proceeds only slowly. Furthermore, natural electrode potential of lead-tin is more base than that of the high chromium steel and therefore, as far as the lead-tin coating is present on the surface of the double wall tube, the high chromium steel portion nobler than lead-tin is not subjected to corrosion. In case of plain carbon steel, the iron is more base than lead-tin and therefore, the iron is still corroded in spite of the presence of lead-tin on the surface and the lead-tin coating is not so effective.

According to the present invention, the lead-tin coating on the copper plating of the outer surface of the copperplated double wall tube can improve the corrosion resistance in a corrosive atmosphere containing chlorine ion, to a great extent.

The following example is given for illustrating the present invention, but should not be construed as restricting the present invention.

EXAMPLE A ferrite stainless steel containing Cr 11.26%, Ti 0.32%, Mn 0.41%, Si 0.35%, and C 0.034% was coldrolled and cut to produce a steel strip of 0.335 mm. thick and 27.5 mm. wide and both sides of the resulting high chromium stainless steel strip were activated after pretreatment and copper-plated by using a plating solution containing 200 g./l. of copper sulfate and 50 g./l.

of sulfuric acid to form copper coating of 5 microns The ratio of Ti -to C suitable for preventing transformathick on the whole surface of the steel strip. The resulttion is higher than 10 as is clear from FIG.'1. ing copper coating was tightly adhered to the steel. (2) The above mentioned steel strips are plated with The resulting copper-plated steel strip was continuously copper as a brazing material, shaped into a double wall subjected to roll-processing to produce a double wall tube 5 tube by roll-processing, heat-treated at a temperature of 4.76 mm. in diameter. Any burn of copper on the higher than melting point of the brazing material to core spindle for determining the inside diameter of the finish brazing. For the purpose of con-firming formability tube was not observed at all. and properties of the double wall tube after brazing, The resulting copper-plated double wall tube was subthere are compared mechanical properties of the steel jected to brazing at 1130 C. in a furnace and there was 10 strip itself and those of the steel strip after heat treatment obtained a double wall tube free from voids. as shown in Table 2. Further, the resulting double wall tube was soaked in As is clear from Table 2, the mechanical properties an alloy melting bath of 75% lead and 25% tin at 300 of high chromium steel strip used in this invention is C. for 10 seconds to produce a lead-tin coating of 8 very close to those of a plain carbon steel strip and the microns thick on the outer surface of the copper-plated workability is excellent and further, the properties are double wall tube. not changed even after heat treatment.

A copper-plated double wall tube coated with lead-tin (3) FIG. 2 shows the effect of the composition of of the present invention was compared with a coppersteel upon mechanical properties before and after heat plated double wall tube without a lead-tin coating with treatment with respect to the high chromium steel conrespect to a rust resistance test (refer to Table 3 infra). taining Ti used in the present invention. FIG. 2 indicates The test was carried out by corrodkote test in accordthat the chromium content should be lowered as far as ance with JIS-D-OZOI. The copper-plated double wall possible without losing minimum corrosion resistance. tube of high chromium steel without a lead-tin coating It is preferable to lower Si, Mn and Ni contents as was better than a double wall tube of plain carbon steel, far as possible. but whole surface of the former was covered with rust (4) The high chromium steel strip used for the douin 200 hours. On the contrary, no rust was formed on ble wall tube of this invention has a far better corrosion the copper-plated double wall tube of high chromium resistance than plain carbon steels which have been used steel with a lead-tin coating at all in 500 hours. for conventional double wall tube and the corrosion re- As the result of a continuous salt spraying test in acsistance is not deteriorated by heat treatment upon cordance with JISZ-2371, the copper-plated double wall brazing. tube with a lead-tin coating formed no rust in 800 hours Results of the rust resistance test are shown in Table 3. while the copper-plated double wall tube without a lead- As stated above, the copper-plated double wall tube tin coating formed rust on the whole surface in 500 with a lead-tin coating has mechanical properties similar hours under the same atmosphere as above. This results to those of plain carbon steel, and the mechanical propindicate that the copper-plated double wall tube of high erties are hardly changed by brazing. Further, the double chromium steel with a lead-tin coating according to the wall tube according to the present invention has high present invention has an excellent corrosion resistance corrosion resistance, formability and bending workeven in a corrosive atmosphere containing chlorine ion. ability.

The properties of the high chromium steel and the double wall tube according to the present invention are compared with those of conventional ones as shown below.

. TABLE 1 (1) Some examples of components of the high chromium steel strip used in this invention and an example Element of components of a plain carbon steel strip are illustrated Type C 51 M11 P 5 Ni Cr Tl in Table 1. Plain carbonsteel 0.07 0.25 0.01 0.02

The high chromium steel used in the double wall tube gffi figfig g; has rtic vention:

i lady low ,contents of carbon f and No.1 0.023 0.35 0.30 0.010 0.000 0.20 11.28 0.30 the titanium content 1s lowered as far as possible without No.2 0. 025 0.32 0.30 0. 018 0.010 0.21 11.00 0. 37 causing transformation upon heat treatment in brazing. M20 0 31 0 28 M20 12'25 TABLE 2 Steel strip after softening annealing 1,l30 C.X3 min. heating and air cooling Yield Tensile Elon- Hard- Yield Tensile Elon- Hardstrength strength gation ness strength strength gation ness Type (kg/mm!) (kg/mm!) (percent) (HV) (kg/mm!) (kg/mm!) (percent) (HV) Plain carbon steel 19 a4 49 88 20 30 45 90 High chromium steel in this invention:

No.1 17 40 42 10 37 40 103 No.2 17 40 41 100 10 88 39 10s No. 3 20 41 as 112 19 39 as