CS257822B1 - High strength weldable steel "cold" - Google Patents

Abstract

High-strength weldable steel, which has a very good combination of strength and plastic properties and at the same time is characterized by good resistance to brittle fracture at low atmospheric temperatures. The steel contains in % mass 0.04 to 0.12% C, 0.5 to 1.9% Mn, 0.05 to 0.65% Si, traces up to 0.020% P, traces up to 0.015% S, the total content of Cu + Cr + Ni + Mo is less than 0.6%, 0.015 to 0.090% Al, 0.015 to 0.200% Ti, 0.015 to 0.150% V and 0.006 to 0.030% N. The advantages of steel stand out especially if a continuous casting method is used in its production and the well-known controlled rolling technology is applied. Steel can be advantageously used in the production of dynamically stressed parts produced by cold forming, for welded parts of structures operating at low atmospheric temperatures. It is especially advantageous for load-bearing parts of transport equipment.

Description

The invention relates to a high strength weldable steel with a fine-grained structure which, in addition to its high yield strength, exhibits excellent brittle fracture resistance at low atmospheric temperatures and good plasticity which predisposes it to cold forming.

Modern constructional 2-weldable microalloyed steels achieve high strength parameters and with consistent application of controlled rolling also good resistance to brittle fracture at low atmospheric temperatures. Similarly, it is possible to achieve a good combination of strength and plastic properties, where two-phase steels are progressively hired. Steels which, besides having a high yield strength and good brittle fracture resistance, while having excellent plasticity, are not solved.

This gap is filled by the steel according to the invention, which comprises in percent by weight 0.04 to 0.12% C, 0.5 to 1.9% Mn, 0.05 to 0.65% Si, to 0.02 % P, up to 0.010% S, in total Cu + Cr + Ni + Mo up to 0.6%, 0.015% to 0.090% Al, 0.006 to 0.030% N, 0.015 to 0.200% Ti, 0.015 to 0.150% V.

Advantages of this steel arise especially when the steel is continuously cast and rolled by controlled rolling technology. The solidification of the steel results in TiN particles which, upon rapid cooling of the melt in the crystallizer of the continuous casting apparatus, are dispersed and evenly distributed, with a particle size of less than 1.0 µm, sometimes below 0.1 µm. These dispersed particles are practically insoluble in the solid solution and thus function as an austenite grain growth inhibitor. This positively affects not only the fine-grained steel, but also its metallurgical weldability.

The combination of aluminum, titanium, vanadium and nitrogen in the steel is determined so as to refine the austenite grain in the controlled rolling process and obtain an optimum level of precipitation hardening. The low sulfur content and the modification of the sulfur inclusions by titanium, the low carbon content and the suppression of the formation of perlite by the chemical composition guarantee high plastic properties.

The steel according to the invention is characterized by a minimum yield strength of 420 MPa, a minimum strength of 580 to 720 MPa, a minimum ductility λ = 22% and a minimum contraction Z = 75%. At the same time, the steel has a minimum notched toughness at -50 ° C of 27 J.cm determined in the notched toughness test on 10 x 5 x 55 mm samples with a 2 mm Charpy V-notch.

In addition, the steel has the advantage that the cold fracture test does not give rise to cracks even at a 180 ° bend on a mandrel with a diameter equal to half the sheet depth.

To further elucidate the subject matter of the invention, examples of two melts produced in oxygen converters are given, in which the off-furnace desulphurisation is applied in a ladle followed by continuous casting of the gates.

The slabs are rolled on a warm wide-band mill to a thickness of 4 to 8 mm. A controlled rolling process with these main technological parameters is applied. The heating temperature of the gates is 1220 ± 10 ° C, the rolling temperature is 830 ± 10 ° C, the rolling temperature is 585 ± 15 ° C, the total removal in the control order is 65 to 75%. After controlled rolling, the sheet is characterized by a fine-grained homogeneous ferritic structure.

1. melting

Chemical composition by weight: 0.11% C, 0.58% Mn, 0.125% Si, 0.016% P, 0.012% S, 0.038% Cu, 0.042% Cr, 0.034% Ni, 0.028% Mo, 0.034% Al, 0.125% Ti, 0.024% V, 0.010% N.

a) Thickness plate 4 mm. The mean ferrite grain size was 4.2 µm. Achieved mechanical properties of the steel in the longitudinal direction relative to the rolling direction: Re = 510 MPa, Rm = 621 MPa, A ^ = 26%,

Z = 78%; in the transverse direction: Re = 536 MPa, Rm = 634 MPa, λ = 23%, Z = 76%. The cold fracture test did not give a 180 ° bend on the 2 mm diameter mandrel, even on samples taken in the transverse direction. The plastic properties are improved by annealing in the range of 720 to 950 ° C as the strength properties decrease. Specifically, at an annealing of 760 ° C for 240 seconds, the steel has a longitudinal direction of the rolling direction: Re = 440 MPa, Rm = 590 MPa,?

Z = 81 4.

b) 5 mm thick sheet. Mean ferrite grain size 3.7 µm. Achieved mechanical properties of the steel in the longitudinal direction with respect to the rolling direction: Re = 485 MPa, Rm = 594 MPa, A $ = 25 4, Z = 79 4; in the transverse direction: Re = 506 MPa, Rm = 595 MPa, A ^ = 23 4, Z = 77 4. Notched toughness determined on 10 x 5 x 55 mm samples with a 2 mm Charpy V-notch reaches in the longitudinal direction at - 20 [deg.] C. 125 [mu] m.cm ^-2 , at -50 [deg.] C 72 [mu] m ^-2 , at -70 [deg.] C 32 J.cm ² . In the transverse direction at -20 ° C it is 72 J.cm ^-2 , at -40 ° C it is 35 J.cm ^-2 . During the cold fracture test according to ČSN 42 0401, there were no 180 ° bending cracks on a 2 mm diameter mandrel, even on samples taken in the transverse direction.

2. melting

Chemical composition by weight: 0.10 4 C, 1.37% Mn, 0.28 4 Si, 0.013 4 P, 0.009 4 S, 0.045 Cu, 0.058% Cr, 0.021 4 Ni, 0.019 4 Mo, 0.033 4 Al, 0.022 4 Ti, 0.052 4 V, 0.012 4 N. The 8 mm thick sheet has a mean ferrite grain size of 3.1 µm. Mechanical properties achieved in the longitudinal direction: Re = 505 MPa, Rm = 605 MPa, λ = 27 4, Z = 77 4; notch toughness determined on 10 x 5 x 55 mm specimens with a 2 mm Charpy V-notch reaches 150 J.cm ^-2 at -20 ° C, at -50 ° C 125 J.cm ^-2 , at -80 ° C 45 J.cm ^-2 .

In the transverse direction: RE = 520 MPa, Rm = 608 MPa, λ = 24%, Z = 75%, notched toughness at -20 ° C is 68 J.cm at -50 ° C is 37 J.cm. The cold brittleness test according to CSN 42 0401 did not produce a 180 ° C bend on the 8 mm diameter mandrel, even on samples taken in the transverse direction.

The steel can be advantageously applied where there are demanding requirements on the strength properties of the material and its brittle fracture resistance, on dynamically stressed structures, especially the supporting parts of the transport devices. At the same time, since the steel has extra plasticity, it can be applied to cold-forming technologies.

Claims (1)

OBJECT OF THE INVENTION High strength weldable cold forming steel having excellent brittle fracture resistance, excluding iron containing by weight 0,04 'to 0,12% C, 0,5 to 1,9% Mn, 0,05 to 0,65% Si, traces up to 0.020% P, traces up to 0.015% S, total content of Cu + Cr + + Ni + Mo is less than 0.6%, characterized by containing 0.015 to 0.090% Al, 0.015 to 0.200% Ti, 0.015 up to 0.150% V and 0.006 to 0.030% N.