RU2250272C1 - Ferrite stainless steel - Google Patents

Abstract

FIELD: steel making.

SUBSTANCE: invention relates to steels for pipe-welding industry, whose produce is appropriate in food-processing industry, chemical industry, agriculture, and motor car construction, for example in manufacture of teet cup cartridges and automotive muffler parts. Steel contains following components, wt %: carbon 0.01-0.05, silicon 0.01-0.8. manganese 0.1-0.8. chromium 13.0-18.0, titanium 0.05-0.5, aluminum 0.01-0.1, calcium 0.001-0.02, zirconium 0.005-0.035, nitrogen 0.001-0.025, boron 0.0003-0.005, barium 0.001-0.1, magnesium 0.001-0.02, rare-earth metals 0.001-0.05, nickel 0.1-0.95, vanadium 0.01-0.35, molybdenum 0.01-0.5, tungsten 0.01-0.3, and iron - the balance.

EFFECT: increased plasticity and yield limit thereby contributing to increased pipe-welding productivity.

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Description

The invention relates to the field of metallurgy, in particular to ferritic stainless steels, and can be used in pipe welding production, the products of which are applicable in the food, chemical, agricultural and automotive industries, for example, in the manufacture of sleeves for milking cups and car silencer parts,

Known compositions of ferritic stainless steels used both in sheet and in pipe welding.

Currently, there is a problem in that all known ferritic stainless steels due to the high carbon content and low content of alloying elements are not sufficiently ductile and have a low yield strength. It is practically impossible to use them in pipe welding production, for example, in the manufacture of sleeves for milking cups and car silencer parts, since spherical crimping with thinning and bending of these pipes leads to increased rejects.

Known ferritic stainless steel 08X18T1, GOST 5632-72, containing in wt.%:

carbon no more than 0.8

silicon no more than 0.8

manganese not more than 0.7

chrome no more than 17.0-19.0

titanium 0.6-1.0

Impurities:

sulfur no more than 0,025

Phosphorus no more than 0,035

iron - the rest

This steel is used for the production of welded pipes in the sugar and automotive industries.

However, the known steel has several disadvantages: the tendency to increase the grain of ferrite during welding, low viscosity and ductility. Known steel is alloyed with one stabilizing element titanium (0.6-1.0%). The titanium content necessary for stabilization must not be exceeded, otherwise an oxide film will form that impairs the viscosity, surface quality and deformability of the steel when welding pipes. When the titanium content is more than 0.75%, titanides are formed, causing steel embrittlement as a result of precipitation hardening during heat treatment.

In addition, titanium captures, together with other non-metallic inclusions, form large arachnid inclusions at the ferrite grain boundary, which greatly reduce the ductility of steel, and also contribute to the production of cracks and cuts during the production of welded pipes by spherical crimping with thinning of the sleeves of the teat cups and during bending of parts muffler car.

Stainless steel was selected for the prototype (see patent RU No. 2017856, class C 22 C 38/32, 1994 B.15), containing the following components, in wt.%:

carbon 0.01-0.08

silicon 0.1-0.8

manganese 0.1-0.8

chrome 13.0-18.0

titanium 0.05-0.5

aluminum 0.01-0.1

calcium 0.001-0.02

zirconium 0.005-0.035

nitrogen 0.001-0.025

boron 0.0003-0.005

barium 0.001-0.1

magnesium 0.001-0.02

REM 0.001-0.05

iron rest

However, the proposed composition of the known steel is intended to produce sheet metal with good formability, increased crack resistance and polishability during secondary deformation after deep drawing of products. The use of this composition of steel for the production of welded pipes by mechanized argon-arc welding with a two-arc plasmatron and their subsequent processing by spherical crimping with thinning and flexible, in particular for the liners of the milking cup and for the details of the silencer of the car, is practically impossible due to increased rejects in cracks, cuts and etc.

The microstructure of steel consists of grains of ferrite, carbides and carbonitrides of titanium and zirconium and chromium carbide type Cr ₂₃ C ₆ , located mainly along the grain boundaries. It should be noted that when the chromium carbides in the known steel form a carbide network along the grain boundaries during primary crystallization and heat treatment, a sharp decrease in the ductility and yield strength of the steel occurs. The grain size of ferrite - the main metal of welded pipes, determined by its primary crystallization, should be no larger than the fifth ball according to GOST 5639-82, and in the welding zone no larger than the second ball, but in practice when welding pipes due to the low welding speed, the value grain in the welding zone was one point, which led to increased rejects in the production of parts of milking equipment and silencers of cars, as well as to a decrease in the quality of pipes during tests for flattening and distribution by cone.

In addition, the well-known steel, having low strength, fluidity and ductility, cannot resist the stresses arising during crimping with thinning and flexible, which forced the use of expensive chromium-nickel steel 12X18H10T for the above products. Moreover, the composition of the known steel did not allow to have a high thermal effect of the power of the welding arc when welding pipes and to increase the welding speed of pipes 42 × 2 mm more than 0.8 m / min, and pipes 43 × 1.5 mm more than 1.5 m / min from - due to lack of penetration, cracks and other defects, which negatively affects productivity.

The objective of the invention was to develop the composition of ferritic stainless steel, which allows to improve the quality of welded pipes by increasing ductility and yield strength, as well as to increase the productivity of pipe welding.

This object is achieved by the fact that in the known composition of ferritic stainless steel containing carbon, silicon, manganese, chromium, titanium, aluminum, calcium, zirconium, nitrogen, boron, barium, magnesium, rare-earth metals and iron, nickel, vanadium, molybdenum are additionally introduced and tungsten in the following ratio of components, in wt.%:

carbon 0.01-0.05

silicon 0.1-0.8

manganese 0.1-0.8

chrome 13.0-18.0

titanium 0.05-0.5

aluminum 0.01-0.1

calcium 0.001-0.02

zirconium 0.005-0.035

nitrogen 0.001-0.025

boron 0.0003-0.005

barium 0.001-0.1

magnesium 0.001-0.02

REM 0.001-0.05

nickel 0.1-0.95

vanadium 0.01-0.35

molybdenum 0.01-0.5

tungsten 0.01-0.3

iron rest

Steel may contain impurities, in wt.%: Sulfur up to 0.25, phosphorus up to 0.35 and copper up to 0.3.

The invention consists in the following.

The composition of the proposed steel in order to improve the quality of welding pipes by increasing their strength and ductility properties introduced 0.1-0.95% of Nickel. It forms a solid solution with ferrite, changing the crystal lattice of ferrite and strengthening interatomic bonds. Nickel in ferritic stainless steel in the above amounts promotes the appearance of γ-α transformations. It does not flow to the end, which nevertheless gives a noticeable increase in the yield strength without reducing plasticity. Exceeding the nickel content of more than 0.95% leads to the appearance of austenite in the steel structure, which adversely affects the quality of the pipes.

An additional introduction of 0.01-0.35% vanadium in the proposed steel is based on the formation of carbides, carbonitrides and nitrides, which are released in a finely dispersed state and are evenly located inside the grains and harden the steel. Vanadium increases the Ac ₃ point _of steel, which favorably affects the welding of pipes. Vanadium grinds the grain of ferrite of the base metal and the weld, contributing to the production of steel with a fine-grained structure during primary crystallization, which increases the resistance of steel to shear deformation during crimping with thinning and bending of welded pipes in the manufacture of parts of milking equipment and car silencers.

The introduction of vanadium into steel below 0.01% does not lead to a noticeable increase in the properties of steel, more than 0.35% impairs the relaxation stability of steel and increases the pollution of steel with vanadium nitrides.

An additional introduction to the composition of the proposed steel 0.01-0.5% molybdenum and 0.01-0.3% tungsten contributes to the formation of carbide and carbonitride phases, contributing to the grinding of ferrite grains. Molybdenum and tungsten, dissolving in ferrite, increase the energy of interatomic bonds and the resistance to elastic deformation of the lattice of a solid solution of ferrite, strengthen it and increase thermal stability. The properties of ferrite, additionally hardened by molybdenum and tungsten, do not depend on how the steel was cooled - quickly or slowly, which is very useful when welding pipes. At the same time, molybdenum and tungsten, being a part of the strengthening carbide phase of the type (Cr, Mo, W, V, Fe, Si) ₂₃ C ₆ , increase the strength of interatomic bonds in it, reduce its ability to coagulate and inhibit the formation of a carbide network, which reduces brittleness and increases the ductility of welded pipes.

In addition, the proposed steel has reduced the content of the main element that affects the quality of pipe welding - carbon from 0.08% to 0.05%, which also reduced the tendency of steel to embrittlement in welded joints. The generally accepted carbon content of up to 0.08% has an adverse effect on pipe welding, as it extends the crystallization interval and enhances the tendency of steel to form hot and cold cracks. Due to the decrease in carbon content, the intensity of the formation of a carbide network of chromium carbide type (Cr, Mi, Mo, W, Fe, Si) ₂₃ C ₆ and its amount in a solid solution of ferrite and at the grain boundaries due to its dissolution by titanium, zirconium, and vanadium nitrides decreases, which improves metal quality and improves weldability of pipes.

Additional complex introduction of Nickel, vanadium, molybdenum and tungsten and the proposed ratio of the components of the proposed ferritic stainless steel contribute to improving the quality of welded pipes due to:

- hardening of a solid solution of ferrite by increasing the lattice parameter of doped ferrite from 2.86 Å to 2.94 Å;

- the formation of complex dispersed carbides and carbonitrides containing, in addition to chromium, titanium, zirconium, also vanadium, tungsten and molybdenum;

- creating the conditions for the development of predominantly intragranular segregation and the release of finely dispersed rounded carbides, carbonitrides and nitrides both in the grain body and along the boundaries when cooling steel from the welding temperature and during heat treatment;

- difficulties in coagulation of the hardening carbide and carbonitride phases due to a decrease in diffusion mobility and a decrease in diffusion and self-diffusion coefficients;

- obtaining a finer structure of the base metal, the metal of the weld seam and the heat-affected zone after welding of pipes due to additional alloying and increase the welding speed;

- slowing down the rate of softening of steel, which helps to reduce the depth and width of the zone of softening after welding of pipes.

It should be noted that the joint introduction of nickel, vanadium, molybdenum and tungsten into the proposed ferritic stainless steel within the ranges indicated above, as well as a decrease in carbon content to 0.05% made it possible to increase the pipe welding speed by 1.5 times and increase the thermal effect of power welding arc by increasing the current strength of the arc. This allowed to reduce the surface volume of the molten metal and reduce the heterogeneity of the weld metal. In this case, there are no defects in the welds, the ferrite grains of the weld structure are oriented along the traces of the dendritic structure, and the grain size is significantly reduced.

An example of obtaining a substance

In 000 NPIF “Alloy” in production conditions they produce ferritic stainless steel, containing in wt.%:

chrome metal GOST 5905-67 - 16.5

ferrosilicon GOST 1415-70 - 0.5

ferromanganese GOST 4755-70 - 0.5

ferrotitanium GOST 4761-67 - 0.6

nitrated manganese TU 14-5-59-75 - 0.1

electrode battle - 0.02

GOST 11070-74 aluminum - 0.1

Ferrosilicobarium TU 14-5-160-84 - 0.05

GOST 849-70 nickel N-1 - 0.5

ferrovanadium GOST 4760-84 - 0.2

ferromolybdenum GOST 4759-79 - 0.3

ferro-tungsten GOST 17293-71 - 0.15

Ligature RZM FSZO TU 14-5-136-8 - 0.15

silicocalcium GOST 4762-71 - 0.03

ferroboron FB-V TU 14-5-14-72 - 0.01

ferrosilicon zirconium FSTsR-50 TU 14-5-3-77 - 0.05

ferrosilicon magnesium alloy TU 14-05-134-86 - 0.03

armco iron - rest

Steel was smelted in an induction furnace with a main lining. In this case, armco iron, chromium, electrode battle, nitrated manganese, nickel, ferromolybdenum and ferro-tungsten were introduced directly into the filling together with a slag mixture consisting of freshly calcined lime, hydrofluoric state and magnesite.

After complete melting, ferrosilicon, ferromanganese, aluminum, and ferrotitanium were introduced. When the metal was heated to a temperature of 1610 ° C, silicocalcium, ferrovanadium, ferrosilicosirconium, an REM ligature, and a ferrosilicon magnesium alloy (according to known technology) were introduced before casting.

Similarly, four more compositions of the proposed steel with different contents of components and compositions were prepared (table 1).

Metal was siphoned into molds. Further redistribution of the ingots into cold rolled coil sheets was carried out at the Zaporizhstal plant using the technology adopted for ferritic stainless steels. Rolls for pipe welding were delivered with the surface in the rolling state, heat treatment, and pickling. Heat treatment mode: quenching - 850 ° С, cooling in water.

Table 1 presents the compositions of the tested ferritic stainless steels.

Table 1 Components Content in wt% I II III IV V Carbon 0.008 0.01 0,03 0.05 0.06 Silicon 0.08 0.1 0.4 0.8 0.9 Manganese 0.08 0.1 0.4 0.8 0.85 Chromium 12.5 13.0 15,5 18.0 18.5 Titanium 0.04 0.05 0.3 0.5 0.55 Aluminum 0.008 0.01 0.05 0.1 0.12 Calcium 0,0008 0.001 0.01 0.02 0,022 Zirconium 0,036 0,035 0.02 0.005 0.004 Nitrogen 0,0009 0.001 0.012 0,025 0,026 Boron 0,0002 0,0003 0.004 0.005 0.006 Barium 0,0009 0.001 0.05 0.1 0.11 Magnesium 0,0009 0.001 0.01 0.02 0,021 REM 0,0009 0.001 0.02 0.05 0.051 Nickel 0.09 0.1 0.46 0.95 0.96 Vanadium 0.009 0.01 0.18 0.35 0.36 Molybdenum 0.009 0.01 0.25 0.5 0.51 Tungsten 0.009 0.01 0.15 0.3 0.31 Sulfur 0.01 0.012 0.018 0,025 0,026 Phosphorus 0,036 0,035 0.02 0.012 0.01 Copper 0.001 0.01 0.15 0.3 0.31 Iron rest rest rest rest rest

The pipes were welded from cold-rolled coiled steel with a thickness of 1.5-2 mm at the Novomoskovsk Pipe Plant at the mill ADS 20-76 in a special chamber by argon-arc welding with a two-arc plasmatron of the NIAT design. Before welding, the tube preform was preheated through a high-frequency inductor at a temperature of 200-300 ° C. Gas protection during welding and the cooling seam was made with pure argon. The first arc of the plasma torch heated and melted the edges, and the second melted the metal. The welding speed and the current of the first and second arcs of the plasma torch were recorded with a H-340 recorder.

WELDING MODES Diameter and thickness of pipe, mm Welding Speed, m / min Welding current, A Voltage 1st arc 2nd arc 1st arc 2nd arc 42 × 2 0.8 70-100 100-130 15-18 16-20 42 × 2 1,2 100-130 120-160 15-18 17-20 43 × 1.5 1,5 90-120 120-150 15-18 17-20 43 × 1.5 2.0 100-130 150-180 15-18 17-20

To relieve the internal stresses of the weld in the mill line, the first heat treatment was carried out using a ring multi-coil inductor by heating to a temperature of 800 ± 20 ° С followed by jet water cooling. The second heat treatment was carried out after calibration and straightening of pipes in continuous electric furnaces of the СРО 8/100 type at temperatures of 810-860 ° С with a pipe speed of 0.7 m / min.

Mechanical testing of pipes was carried out according to GOST 10006-80 and GOST 8695-75. The mechanical properties of the weld metal were determined on longitudinal flat samples with a working part width of 1.2 mm. The percentage of increase in the outer diameter of the welded pipe when testing for distribution by a cone was determined by 10 samples on nozzles 100 mm long by introducing a conical mandrel having a 1:10 inclination or an inclination angle of 60 °, until a crack appears on the distributed part of the nozzle.

The test of the welded joint for static bending was performed according to GOST 6996-66 on 5 samples of each heat. It is characterized by a bending angle at which the first crack is formed in the stretched zone, which develops during the test.

Table 2 presents the characteristics of indicators and properties of the metal of welded pipes.

Analyzing the data of table 2, we conclude that the optimal content of components in ferritic stainless steel for welding pipes during their subsequent processing by spherical crimping with thinning and flexible is in the range (wt.%):

Carbon 0.01-0.05

Silicon 0.1-0.8

Manganese 0.1-0.8

Chrome 13.0-18.0

Titanium 0.05-0.5

Aluminum 0.01-0.1

Calcium 0.001-0.02

Zirconium 0.005-0.035

Nitrogen 0.001-0.025

Boron 0.0003-0.005

Barium 0.001-0.1

Magnesium 0.001-0.02

REM 0.001-0.05

Nickel 0.1-0.95

Vanadium 0.01-0.35

Molybdenum 0.01-0.5

Tungsten 0.01-0.3

Iron rest

When the composition of the proposed ferritic stainless steel is less than the lower limit of nickel, vanadium, molybdenum and tungsten, no noticeable effect is seen in improving the quality of pipe welding, as well as in increasing the productivity of their manufacture.

When the composition of the steel components is within optimal limits, the quality of the welded pipes improves, which allows to increase the performance of tests for distribution by cone, flattening and bending, which made it possible to produce parts of milking equipment and silencer from them by spherical crimping with thinning and bending. In addition, the optimal composition allowed to increase the productivity of pipe welding, yield strength and elongation. If the components of the composition exceed the permissible limits (see table 2), then we have a decrease in the properties and characteristics of the indicators, since they themselves begin to contribute to an increase in deformation resistance and embrittlement.

The use of the proposed steel composition allows, due to the improvement of the quality of welded pipes, to manufacture parts of milking equipment and automobile silencer. We have an additional advantage in increasing the productivity of manufacturing welded pipes, yield strength and elongation of the weld.

Industrial testing of welded pipes of 42 × 2 mm in size from the proposed composition of ferritic stainless steel was carried out at the Rezekne milking equipment factory. From the pipes by spherical crimping with thinning according to the serial technology existing at the plant, 1,500 pieces of milking cup sleeves were made. Crack marriage was only 0.5%. In the manufacture of these parts from chromium-nickel austenitic steel 12X18H10T, marriage reached 5%.

Testing the manufacture of 1000 muffler parts No. 2103-1201048 / 50 from pipes with a size of 45 × 1.5 mm showed that bending defects are reduced by 30% compared to the production of these parts from 08X18T1 steel.

Thus, the use of the proposed ferritic stainless steel in pipe welding, the products of which are used for the manufacture of parts of milking equipment and the automotive, sugar and other industries, including the production of cartridges of milking cups and silencer parts for cars obtained from welded pipes by spherical crimping with thinning and flexible, gives a significant economic effect and is in great demand in the above industries.

Claims (1)

Ferritic stainless steel containing carbon, silicon, manganese, chromium, titanium, aluminum, calcium, nitrogen, zirconium, boron, magnesium, barium, rare-earth metals and iron, characterized in that it additionally contains nickel, vanadium, molybdenum and tungsten in the following ratio components, wt.%: carbon 0.01-0.05 silicon 0.1-0.8 manganese 0.1-0.8 chrome 13.0-18.0 titanium 0.05-0.5 aluminum 0.01-0.1 calcium 0.001-0.02 zirconium 0.005-0.035 nitrogen 0.001-0.025 boron 0.0003-0.005 barium 0.001-0.1 magnesium 0.001-0.02 REM 0.001-0.05 nickel 0.1-0.95 vanadium 0.01-0.35 molybdenum 0.01-0.5 tungsten 0.01-0.3 iron rest