RU2505618C1 - Low-alloy structural steel with increased strength - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains the following, wt %: carbon 0.15 - 0.20, manganese 1.3 - 1.5, silicon 0.05 - 0.45, phosphorus not more than 0.02, sulphur 0.02 - 0.05, copper not more than 0.25, vanadium 0.03 - 0.055, nitrogen 0.004 - 0.015, and iron and impurities are the rest. Steel has homogeneous fine grained ferrite-pearlite structure with grain grade of 7 - 9 and is characterised by value of carbon equivalent C_equiv ≤ 0.47%.

EFFECT: hot-rolled products manufactured from steel have increased yield point, increased ductility parameter, good weldability, increased processibility, reducing tendency to mechanical ageing, possible use at negative temperatures, possibility of excluding additional heat treatment with maintaining a strength interval.

1 dwg, 2 tbl, 1 ex

Description

The invention relates to ferrous metallurgy, namely to the production of steel with increased strength for the manufacture of hot-rolled automotive components.

Known unalloyed structural steel S355J0 with a minimum yield strength of 355 MPa, containing, wt.% Max:

carbon - not more than 0.22;

silicon - not more than 0.55;

Manganese - not more than 1.60;

phosphorus - not more than 0.03;

sulfur - not more than 0.03;

nitrogen - not more than 0.012;

copper - not more than 0.55;

iron and impurities - the rest. [one]

This steel is the closest to the proposed mechanical properties, composition and purpose and is taken as a prototype.

DIN EN 10025-2: 2005 contains the upper limits of the chemical composition and the obligatory list of elements, but does not indicate the specific limits of their content and additional elements to obtain the required mechanical properties according to DIN and special customer requirements corresponding to operating conditions.

The main technical objective of the invention is to obtain steel with a high yield strength, with an increased ductility index, an increased rate of impact work, the ability to be used at low temperatures, with good weldability, increased machinability, while maintaining a strength interval that is economically feasible in use due to the use of non-deficient alloying elements.

The technical solution to the problem is achieved due to the fact that a modified low-carbon structural, low-alloy steel with increased strength is proposed, containing in wt.%:

carbon 0.15-0.20;

Manganese - 1.3-1.5;

silicon - 0.05-0.45;

phosphorus - not more than 0.02;

sulfur - 0.02 - 0.05;

copper - not more than 0.25;

vanadium - 0.03-0.055;

nitrogen-0.004-0.015

iron and impurities - the rest. Steel symbol S355J0 mod.

The carbon and manganese content provides a predetermined interval of tensile strength (470-630 MPa) and increased ductility (elongation) of at least 20% (instead of at least 16%).

The necessary and sufficient content of vanadium as an element binding nitrogen provides carbonitride hardening in the process y - and the transformation by the dispersion hardening mechanism due to which there is an increase in the yield strength of the material not less than 400 MPa. The increased yield strength allows to reduce the metal consumption in operation and allows to obtain an indicator of impact performance at a temperature from 0 ° C to -40 ° C of at least 80 J.

It was experimentally established that when the manganese content is below the specified limit and the absence of vanadium, the claimed yield strength and impact work are not achieved.

The increased sulfur content leads to improved machinability of steel by cutting.

The ability of the material to weld is estimated by the value of carbon equivalent. To determine the carbon equivalent, the following formula of the International Welding Institute is used, which is given in DINEN 10025-1:

The proposed steel is characterized by a carbon equivalent value SECv ≤0.47%.

The optimal chemical composition, deoxidation method, production technology (hot deformation) of the finished profile make it possible to obtain a homogeneous fine-grained ferrite-pearlite structure with a minimum different grain size not exceeding 3 adjacent numbers. Grain point 7-8-9 in accordance with ASTM E 112 (GOST 5639-82) at the customer's request for grain size not larger than 5 numbers. In addition, the presence of acicular bainitic structure is not observed in the ferrite-pearlite structure (customer requirements: maximum bainite content in the ferrite-pearlite structure is not more than 6%).

The figure 1 presents a photograph of the microstructure of a sample of a hot-rolled profile of a modified steel of one of the melts with a grain size of 8 number according to ASTM E 112 (according to GOST 5639) at 100-fold magnification with an installed scale bar 400 microns long.

Practical implementation example.

Smelting of the declared steel grade is carried out at CJSC Omutninsky Metallurgical Plant in a steelmaking unit. In the SPA, steel of the main composition is smelted, containing carbon, manganese, silicon, iron and inevitable impurities; after heating to 1620-1640 ° C, it is released into the steel pouring ladle. Deoxidation of steel by aluminum is carried out at a drain from the steelmaking unit into the ladle, components for deoxidation are introduced into the bottom zone of the ladle at the optimal ratio [Mn] / [Si] ≤3. A sufficiently deep deoxidation of steel by secondary aluminum is carried out to obtain optimal conditions for the ascent of the formed large aluminum oxides. Additionally, oxygen is controlled (not more than 0.001% on the proposed steel grade).

After the release of the heat from the spa, furnace slag is removed from the steel pouring ladle. During out-of-furnace treatment with a metal purge with argon, lime-alumina slag is induced by additives of lime and aluminum-containing material. Get branded content of the basic elements (carbon, manganese, silicon) and deoxidated refining "white" slag.

Vanadium is introduced in the form of lumpy ferroalloy to achieve the target concentration. Then the metal is heated to a temperature guaranteeing a predetermined overheating of the metal above the liquidus temperature of the steel in the intermediate ladle during casting at the continuous casting machine, taking into account the existing heat losses and subsequent alloying with sulfur. Sulfur is introduced into steel by flux-cored wire using a tribamer after thickening the slag with magnesite powder.

Casting in the continuous casting machine is carried out with the protection of the metal from secondary oxidation in a "under the level" way. Exposure of the metal surface in the bucket (sparking) is not allowed.

The billet is rolled in hot rolling mills according to technological instructions and rolling schemes of OMZ CJSC. Get a hot-rolled profile for the manufacture of the door hinge of a Volkswagen car.

They made seven heats with the proposed composition of the modified steel, two heats 8 and 9 with a low manganese content and without vanadium. The resulting chemical composition in comparison with the prototype are shown in table 1. The claimed invention corresponds to swimming trunks 1 to 7.

The mechanical properties and structure of the steel were evaluated in the laboratory of control tests of OMZ CJSC. The mechanical properties were tested on a QUASAR 250 25-ton tensile testing machine; the hardness tests were carried out on a TSh-2M hardness tester according to the Brinnel method. The results of the study of the mechanical properties of the known and proposed steel, as well as the carbon equivalent are shown in table 2. Tests of the impact at low temperatures were carried out on samples of 3 experimental melts. An example of calculation is presented for one of the heats:

Pilot tests according to the criteria of strength, ductility and impact performance were carried out by EDSCHA, a manufacturer of door hinges for cars of various sizes. Products with the specified parameters satisfied the consumers of the company. There was a proposal to replace the S355 JO steel grade of all supplied hot-rolled sections with the S355 JO mod as the most satisfying operating conditions.

Thus, the proposed chemical composition allows to obtain steel with a high yield strength, with an increased ductility index, good weldability, increased machinability, reduced tendency to mechanical aging, the possibility of use at low temperatures, the possibility of eliminating additional heat treatment, while maintaining an interval of strength that is economically feasible in use due to the use of non-deficient alloying elements in the manufacture of hot-rolled cars beat components.

table 2 steel grade Tensile strength, MPa Yield Strength, MPa Relative extension, % Work Impact, J Hardness, HB at at at 0 ° C -20 ° C -40 ° C one 630 497 23.5 107 - - 187 2 630 460 27 112 - - 170 3 615 469 26 111 128 112 170 four 624 458 26 105 - - 187 5 617 414 26 136 104 83 179 6 627 464 28 107 125 121 179 7 630 474 25 133 - - 179 8 576 384 32 67.6 - - - 9 600 395 22 47.0 - - - Of the proposal
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S355J0 mod 470-630 ≥400 ≥20 ≥80 160-220 Replaces S355J0 470-630 ≥355 ≥16 ≥27 not standardized

Information sources

1. DINEN 10025-2: 2005.

Claims (1)

Low-carbon structural low-alloy with high strength steel containing carbon, silicon, manganese, phosphorus, sulfur, nitrogen, copper, iron and impurities, characterized in that it additionally contains vanadium in the following ratio of components, wt.%:
carbon 0.15-0.20 manganese 1.3-1.5 silicon 0.05-0.45 phosphorus no more than 0,02 sulfur 0.02-0.05 copper no more than 0.25 vanadium 0.03-0.055 nitrogen 0.004-0.015 iron and impurities rest,
while it has a uniform fine grain structure with ferritoperlitnuyu grain score 7-9 and characterized by the carbon equivalent C _ECV ≤0,47%.

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