RU2472868C2 - Steel for high-strength parts from strips, plates or pipes with excellent deformability, which is especially useful for methods of high-temperature application of coatings - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel has the following composition, wt %: C 0.07 to ≤ 0.15, Al ≤ 0.05, Si ≤ 0.80, Mn 1.60 to ≤ 2.10, P ≤ 0.020, S ≤ 0.010, Cr 0.50 to ≤ 1.0, Mo 0.15 to ≤ 0.25, Ti ≥ 48/14x[N], V 0.05 to ≤ 0.10, B 0.0015 to ≤ 0.0050, iron and common secondary elements of steel, including nitrogen, are the rest. Steel is suitable for high-temperature application of coatings at temperature of above A_c3 (900°C), and part after deformation and at temperature of above A_c3 (900°C) and heat treatment has minimum yield strength after cooling, which is equal to 450 MPa.

EFFECT: steel has high deformability and suitability for high-temperature application of coatings at simultaneous provision of common weldability.

11 cl, 3 dwg, 3 tbl

Description

The invention relates to steel for high-strength parts from tapes, sheets or pipes with excellent deformability and special suitability for high temperature coating methods according to claim 1. In this context, the term "high temperature" means temperatures above A _c3 (about 900 ° C).

A modern method of manufacturing lightweight steel parts for significant resource savings by minimizing weight involves the use of high-strength steels.

This applies, for example, to the production of tinplate or sanitary equipment, chemical instrumentation, the manufacture of equipment for power plants and the automotive industry to reduce specific fuel consumption.

In the automotive industry, parts made of high-strength steels with an anti-corrosion coating are usually used, in most cases zinc. In the other listed applications, enameling is also used along with anti-corrosion coatings.

Semi-finished products, such as tapes or sheets, from ordinary high-strength steels for these applications are mainly obtained by thermomechanical rolling. This implies that these steels at subsequent technological stages will no longer be subjected to heat treatment, as this will lose the mechanical properties acquired during thermomechanical processing.

If these steels are subjected to subsequent heat treatment, in which, for example, a protective coating is applied against corrosion in the form of enamel or coatings of zinc, aluminum or their alloys at temperatures above A _s3 (about 900 ° C), then such steels will lose their original strength . This circumstance also occurs in the corresponding areas subjected to heat during welding.

With repeated heat treatments, such as methods of thermal coating, the crossing of welds in the corresponding area after heat treatment, as well as with usually repeated enamel firing, this phenomenon repeats, as a result of which the material constantly loses its strength.

The loss of strength is shown in Table 1 below for an example of S-420 steel with a thickness of 3.0 mm and 8.00 mm and a minimum yield strength of 420 MPa.

Table 1 Changing the mechanical properties of sheets of steel S-420 after one or two firing Thickness mm Sample Position condition R _p0.2 , MPa R _m , MPa A ₈₀ % 3.0 mm Longitudinal Source 444 569 21.3 state after one 430 521 25.1 annealing after two 405 522 25.9 annealing Transverse Source 502 592 20.9 state after one 453 537 25.1 annealing after two 439 537 23.0 annealing 8.0 mm Longitudinal Source 426 523 29.8 state after one 391 498 31,0 annealing after two 385 494 31.5 annealing Transverse Source 437 538 26.7 state after one 401 505 29.7 annealing after two 395 503 29.2 annealing

In high-strength multiphase steels, such a loss of strength after appropriate heat treatment is even more pronounced, since the initial martensitic phase is lost when heated above the transformation temperature A _c3 in the case when cooling is not regulated and proceeds intensively.

Another problem may arise in high strength steels as a result of the fact that when heated above A _c3, the solubility of hydrogen increases markedly. In this case, with accelerated cooling, hydrogen remains in the structure of the material and can subsequently cause cracking of the material.

For this reason, those steels that have long-term cooling (for example, in a calm air environment) have a hardened structure are in demand.

Another problem in such steels is that the presence of a dense protective coating, such as enamel, prevents the escape of hydrogen from the material. In this case, there is a danger of the coating bouncing (the formation of “fish scales”).

“Fish scales” are defective enamel areas that do not provide complete protection for the steel substrate. In this regard, of great importance in the enameling of steels is, in particular, the great resistance of the enameled part to the formation of "fish scales."

It is believed that the reason for the formation of "fish scales" is that during the enameling process, the steel surface comes into contact with moisture from the atmosphere of the furnace and enamel slip.

As a result of the reaction between water and the steel surface, atomic hydrogen is formed, which diffuses into the steel during the firing process.

After enamel burning at about 900 ° C and subsequent cooling, the solubility of hydrogen in steel decreases, hydrogen tends to stand out from the steel at the steel / iron interface, and recombination with the formation of molecular hydrogen.

Such a reaction is accompanied by an increase in volume, while locally high pressure can be created, which in the end becomes so large that it exceeds the adhesion strength of the combined enamel / steel material and pieces of hardened enamel bounce off in the form of a crescent (“fish scales”).

A number of steels are known from the prior art that are resistant to the formation of "fish scales" to produce cold or hot rolled sheets. Due to the need for special properties for deep drawing, such steels are usually smelted in the form of steels of low strength with a low content of penetration elements (for example, EP 0386758 B1). This method is based on the fact that resistance to the formation of "fish scales" is achieved by the fact that during cold rolling along the grain boundaries along which atomic hydrogen is distributed, crushed cementite (hydrogen traps) is released and thus the formation of "fish scales" is prevented.

High-strength, well-deformable steels suitable for high-temperature types of processing, such as enameling, are not known to date. The requirements for high-strength steel, which must be observed, taking into account the indicated areas of application, can be formulated as follows:

- greater strength of the material of the part after deformation and after heat treatment at temperatures above 900 ° C,

- resistance to the formation of "fish scales" after enameling,

- good deformability,

- general good weldability,

- good weldability by high-frequency induction and laser methods in the manufacture of pipes,

- good galvanizing ability of the part.

The basis of the invention is the creation of cheap steel for the manufacture of high-strength parts from tapes, sheets or pipes, which has excellent deformability and is suitable for high-temperature coating methods while ensuring overall weldability, in particular, high-frequency induction welding.

According to the invention, this problem is solved by means of steel of the following composition, wt.%:

FROM 0.07 to≤0.15 Al ≤0.05 Si ≤ 0.80 Mn 1.60-≤2.10 P ≤0.020 S ≤0.010 Cr 0.50 to≤1.0 Mo 0.10 to≤0.30 Ti _min 48 / 14x [N] V 0.03-≤0.12 AT 0.0015-≤0.0050 rest iron and ordinary associated steel elements.

The high-strength steel according to the invention is an improved steel that can harden dispersively in air or in another medium with comparable cooling gradients. Steel is particularly suitable for high-temperature coating methods, such as enameling or kicking, as well as for processing at temperatures above 900 ° C and differs in that it does not decrease its strength during cooling after coating, on the contrary, even increases due to the improvement effect her. During a wide series of tests, experts unexpectedly found that, due to the composition of the alloying elements according to the invention, steel was first produced that has both excellent enameling ability and resistance to the formation of fish scales, and at the same time high strength as a result of improved processing during enamel or galvanizing .

The creation of such cheap steel is achieved, in particular, due to the fact that with a low carbon content excellent deformability is achieved in the cold initial “soft” state, which is of particular importance when using parts obtained by deep drawing, for example, for sanitary purposes for water heaters, in boiler building, in chemical instrument making or in the manufacture of automobile bodies.

In addition, the relatively low carbon equivalent provides excellent overall weldability. In particular, the weldability by high frequency induction welding, used, for example, in pipe manufacturing, is excellent, since the chromium content is relatively low and there are no undesired chromium carbide precipitates in the weld.

Resistance to the formation of "fish scales" of steel is achieved according to the invention by the addition of chromium and vanadium, while finely dispersed precipitates of chromium and vanadium carbides or carbonitrides and titanium nitrides in the structure of hardened steel form hydrogen traps in which atomic hydrogen formed during enameling can be held without harm to enameling.

An alloy based on Mn, Cr, Mo, V, and B provides an improvement in steel due to a cooling gradient comparable to cooling in air as a result of optimal shift of relative transformation points.

It is believed that the nitrogen present in the steel according to the invention is completely bound by additional titanium additives to form titanium nitrides, preventing the release of boron nitride and, therefore, ensuring the effectiveness of the boron additive. For this, according to the invention, it is necessary to add at least a stoichiometric amount of titanium relative to the nitrogen content.

According to an optimal embodiment of the invention, galvanizing steel contains a small amount of silicon ≤0.30%, which makes it possible to galvanize, for example, for use in the automotive industry.

Improved steels are known from the prior art in which, upon cooling in air, for example, after heat treatment of a part, hardening occurs and, therefore, the required material properties are achieved.

If, after hot rolling, the steel cools at least in certain areas in air so quickly that the effect of air quenching occurs, then cold deformability can be achieved by subsequent softening annealing, for example, in a bell annealing furnace or by homogenizing annealing. Alternatively, cold deformability can be maintained after hot rolling, provided that the tightly wound roll is slowly cooled, including in a special heat-insulated bell furnace.

After cold deformation or shaping, an improved condition can be achieved again by subsequent heat treatment.

Under cold deformation here should be understood the following technological options:

a) the direct manufacture of the relevant parts from hot tapes by deep drawing, etc., followed by possible processing for improvement;

b) further processing to obtain pipes using appropriate drawing and annealing processes. The pipes themselves are then deformed to obtain parts, for example, by flexible deformation due to high internal pressure, etc., and then an improvement is carried out;

c) additional processing of the hot tape to obtain a cold tape with subsequent processes of annealing and training. After that, the cold tape is treated with deep drawing, etc., as described in paragraph a) or b).

The following table 2 shows the indicators obtained on samples of steel according to the invention for hot and cold rolled sheets or strips and pipes obtained from them.

table 2 Change in the mechanical properties of steel according to the invention after enameling 1.5mm cold tape R _p0.2 , MPa R _m , MPa A ₅ % Condition after delivery: soft 339 494 35.1 After enameling 490 770 12.1 4.6mm hot tape R _p0.2 , MPa R _m , MPa A ₅ % Condition after delivery: soft 336 528 33,4 After enameling 475 740 12,2

To determine the ability to enamel, a metal removal test was carried out by etching and resistance to the formation of "fish scales" on sheets of steel according to the invention, as well as on comparative steels of increased strength of three grades.

The test results of the steel according to the invention for the ability to enamel compared with other steel grades of increased strength are shown in table 3. Tests of sheets for pickling by etching and resistance to the formation of "fish scales" was carried out taking into account the standard EN 10209.

When testing the resistance to the formation of fish scales, along with the Ferro 2290 test frit for cold tape, test enamel for boilers was also used.

Table 3 Comparison of enameling results Test Preset value Steel according to the invention, 1.5 mm thick sheet H420LAD comparative steel, 2.5mm thick sheet Comparative steel, MS 1200, 1.5 mm thick sheet Comparative steel TRIP NXT800, sheet thickness 1.0 mm Removal 20 - 50 g / m ² 45 25 49 212 pickling Fish scale test, test enamel for boilers Not found Not detected (see figa) Over 30 (see fig. 1b) More than 30 (see figs) Testing is not possible Fish scale test, Ferro RTU 2290 Not found Not found Over 30 Over 30 Testing is not possible

The test results show that the TRIP NHT800 comparative steel has a pick-off during pickling, significantly exceeding the specified values, as a result of which there is no need to conduct a test for the formation of "fish scales".

The pick-off during pickling of two other comparative steels was within the required values, however, there was no resistance to the formation of “fish scales”.

The test results for the formation of "fish scales" are shown in figa-1C.

On figa shows the steel according to the invention, "fish scales are absent.

Figures 1b and 1c show the results of a fish scale test using test enamel for boilers.

The change in the mechanical properties of the steel according to the invention during enameling compared to other high-strength steels is shown in the following figures. For comparative steel NHT800, it was impossible to obtain data after enameling, because due to excessive pick-up of the metal by etching, the steel was unsuitable for enameling.

These results are shown in FIGS. 2 and 3.

Figure 2 shows the change in yield strength R _p0,2 compared with the required minimum yield strength after enameling with a firing temperature of 900 ° C.

Figure 3 shows the change in tensile strength R _m compared with the required minimum tensile strength after enameling with a firing temperature of 900 ° C.

The advantages of the steel according to the invention can be formulated as follows:

- high strength of the material also after heat treatment at temperatures above 900 ° C,

- resistance to the formation of "fish scales" after enameling parts,

- noticeably increased strength of the finished part and, therefore, the possibility of use in a lightweight structure due to the reduction of thickness compared to traditional steel for enameling,

- very high weldability of steel, in particular, also a high-frequency pulsed method in the manufacture of pipes,

- excellent deformability of the steel in the cold state and, therefore, the ability to obtain complex parts,

- guaranteed steel galvanizing ability,

- suitability for applying non-metallic protective coatings to it.

The following are typical values for hot and cold rolled sheets and pipes in the state after soft annealing of steel according to the invention.

R _el or R _p0,2 310-430 MPa R _m 450-570 MPa A ₅ ≥23%.

In the state after heat treatment, for example, after enameling or galvanizing at temperatures above 900 ° C, the following mechanical indicators are achieved, for example:

R _el or R _p0,2 450-600 MPa R _m 700-850 MPa A ₅ ≥12%.

Steel according to the invention can be used for a wider range of hot or cold rolled strips, sheets, or for the manufacture of welded or seamless pipes.

For cold rolled or cold formed products, the thickness range or wall thickness range may be, for example, 0.5-4 mm. Corresponding values for hot-rolled or hot-formed products are from about 1.5 to 8 mm.

Claims (11)

1. High-strength well-deformable steel, suitable for high-temperature coating at temperatures above A _s3 (900 ° C) for the manufacture of parts from tapes, sheets or pipes, having the following composition, wt.%:
FROM from 0.07 to ≤0.15 Al≤0.05 Si≤0.80 Mn from 1.60 to ≤2.10 P≤0.020 S≤0.010 Cr 0.50 to ≤1.0 Mo from 0.15 to ≤0.25 Ti≥48 / 14 · [N] V 0.05 to ≤0.10 AT from 0.0015 to ≤0.0050 iron and ordinary associated steel elements, including nitrogen rest 2. Steel according to claim 1, characterized in that the carbon content in it is from 0.08 to ≤0.10%. 3. Steel according to claim 1, characterized in that the silicon content in it is ≤0.30%. 4. Steel according to claim 1, characterized in that the manganese content in it is from 1.80 to ≤2.0%. 5. Steel according to claim 1, characterized in that the chromium content in it is from 0.70 to ≤ 0.80%. 6. Steel according to claim 1, characterized in that the titanium content in it is from 0.02 to ≤0.03%. 7. Steel according to claim 1, characterized in that the boron content in it is from 0.0025 to ≤0.0035%. 8. A part made of high strength well deformable and suitable for high temperature coating at temperatures above A _s3 (900 ° C) of steel according to any one of claims 1 to 7, in the form of a tape, sheet or pipe, which after deformation at temperatures above A _c3 (900 ° C) is heat-treated and has a minimum yield strength after cooling of 450 MPa. 9. Detail according to claim 8, characterized in that the heat treatment includes enameling with single or multiple firing. 10. Detail according to claim 8, characterized in that the heat treatment includes applying a metal coating. 11. Detail according to claim 10, characterized in that the coating is applied by galvanizing.

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