CZ2018364A3 - Bainitic steel with increased contact-fatigue resistance - Google Patents

Abstract

Bainitic steel with increased contact-fatigue resistance and containing C, Si, Mn, Cr, Ni, Mo, B, Nb, Al and Ti, wherein the steel consists of: 0.15 to 0.25% by weight C, 1.3 to 1.8% Si, 1.7 to 2.2% Mn, at most 1.5% Cr, at most 1.5% Ni, 0.20 to 0.50% Mo and further comprising the following elements having micro-alloying properties: 0.001 to 0.005% by weight of B, 0.02 to 0.06% by weight of Nb, at most 0.04% by weight of Al, at most 0.04% by weight of Ti, the remainder up to 100% weight is iron and unavoidable impurities.

Description

CZ 2018-364 A3

Bainitic steel with increased contact-fatigue resistance

Technical field

The present invention is in the field of processing of iron alloys and relates to bainitic steel with increased contact-fatigue resistance, especially intended for wheels of rail vehicles.

BACKGROUND OF THE INVENTION

In the operation of rail vehicles, there is a complex load of materials at the point of contact of the wheels and rails, causing a combination of their adhesive, abrasive and contact-fatigue wear. The critical limit state of the wheel material in terms of operational safety is the surface initiation of fatigue cracks, which are further developed into typical contact-fatigue defects of the running surfaces. Further development of these defects results in peeling of the material from the surface. In a greater extent of material damage transversal branching of surface-initiated cracks occurs, followed by radial development of these cracks and, to a limited extent, to operational wheel fractures. Suppressing the initiation of surface cracks or slowing down their further development is therefore a fundamental question of the safety of the material used under the given operating conditions. Existing standardized wheel materials are categorized according to static strength, guaranteed ductility and impact strength, and the static strength increases with increasing carbon content, for example from 0.48% C for Bis steel with strengths ranging from 600 to 720 MPa to 0.65% C for B6 steel with strength between 920 and 1050 MPa. These are steels with a limited content of alloying elements, for example for the variant with the highest strength R8T the maximum content of carbon is 0.6%, manganese 0.80%, silicon 0.40% and the sum of chromium, molybdenum and nickel content is at most 0.5 %. All strength variants are steels with pearlitic microstructure, with different proportion of proeutectoid ferrite according to strength category. Increasing the contact-fatigue resistance is achieved by microalloying or by heat treatment to reduce the intermellar distance of the perlite, which leads to an increase in the fatigue limit, but in principle does not make it possible to increase the toughness. The high carbon content of these types of steels causes a high sensitivity to thermal load, where it is not possible to prevent undesirable turbidity of the surface layers in contact slippages, which cannot be eliminated in real operation. The carbon content is also limiting in terms of weldability, ie in the application it limits the possibility of renovating profiles, for example. tram wheels. Recent findings show that pearlitic steels have, in principle, limited possibilities in terms of further enhancing wear resistance and fatigue damage under given conditions; solving the problem therefore requires developing alternative steels on a different structural basis.

Steel with a bainitic structure solves increased resistance to contact fatigue. Bainitic carbide steels have a significantly higher wear rate compared to pearlitic steels, which has a positive effect based on the fact that accumulated fatigue damage is continually removed, thus suppressing the extensive development of fatigue cracks. However, materials of this type have limited possibilities in terms of toughness. The current development of bainitic steels is oriented towards carbide-free bainitic steels, especially steels with very fine plates of bainitic ferrite and carbon-enriched austenite.

Patent EP 0804623 B1 discloses an inventive carbide-free bainitic steel for the production of rails with increased wear resistance and at the same time contact-fatigue resistance comprising 0.05 to 0.50% C; 1.00 to 3.00% Si and / or Al; 0.5 to 2.5% Mn; 0.25 to 2.5% Cr; max 3% Ni; max. 0.025% S; max 1.00% W; max 1.00% Mo; max 3.00% Cu; max 0.10% Ti; max. 0.50% V and max. 0.005% B. The rest is Fe and impurities. Continuous cooling from rolling temperatures results in a carbide-free structure with increased contact-fatigue strength, ductility and fracture toughness, as well as similar or better wear resistance compared to heat treated pearlitic rails. CN 106191665 discloses the invention of bainitic steel and a manufacturing process for rail wheels

Vehicles with increased strength, toughness and resistance to thermally induced cracks containing 0.10 to 0.40% C; 1.00 to 2.00% Si; 1.00 to 2.50% Mn; 0.20 to 1.00% Cu; 0.0001 to 0.035% B; 0.10 to 1.00% Ni; 0.020% P max and 0.020% S max. The remainder consists of Fe and impurities. The process comprises a heat treatment process ensuring a targeted carbide-free structure of the individual parts of the wheel profile. Patent EP 2614171 B1 discloses so-called super-bainitic high carbon steel, particularly useful for armor plating and a process for its manufacture. For the same application, the steel disclosed in EP 2310545 B1 is a high carbon variant of bainitic steel with a carbon content of 0.6 to 1.1%, in which the superbainitic structure is achieved isothermally at very long heating times, which is on wheels of rail vehicles unrealistic. Patent EP 1538231 represents the invention of microalloyed bainitic steel for wheels of rail vehicles with high resistance to fatigue and contact fatigue, containing a maximum of 1.2% Si, a maximum of 1.5% Mn and a number of micro-steels, which are , Ca, B and N.

The previously known bainitic structural-based materials are preferably developed for rail profiles, where their chemical composition takes into account the corresponding possibilities of the forming regime, respectively. heat treatment including cooling conditions. The steels, primarily intended for the production of wheels of rail vehicles, by their chemical nature require a complex heat treatment to achieve the required structure and thus the mechanical parameters of the material.

It is an object of the present invention to provide a new steel suitable for rail vehicle wheels which, by its chemical composition in combination with the profile forming process, provides an increase in contact-fatigue resistance in comparison with existing standard materials in terms of suppression of contact-fatigue defect Increased safety against contact shock.

SUMMARY OF THE INVENTION

This object is achieved by the invention, which is a bainitic steel with increased contact fatigue resistance and containing C, Si, Mn, Cr, Ni, Mo, B, Nb, Al and Ti, wherein the principle of the invention is that the steel contains 0.15 up to 0.25 wt% C, 1.3 to 1.8 wt% Si, 1.7 to 2.2 wt% Mn, not more than 1.5 wt% Cr, not more than 1.5 wt% Ni, 0.20 to 0.2 wt% 0.50% Mo, and further comprises the following elements having micro-alloying properties, namely 0.001 to 0.005% B, 0.02 to 0.06% Nb, at most 0.04% Al, at most 0.04% Those, and the remainder up to 100% by weight are iron and unavoidable impurities.

The present invention achieves a new and higher effect by providing a specific microstructure consisting of bainitic ferrite, martensite and stabilized austenite, in a combination of a high strength base and a deliberately increased proportion of untransformed austenite, which transforms under tension to martensite, carbon-stabilized austenite increases the strain hardening of this complex structure under operational load. Due to the contact-fatigue load, the heterogeneity of the structure is promoted, whereby a controlled phase change of the blocks of carbon-enriched austenite to martensite induces continuous strengthening of the surface layer. The present bainitic steel has the possibility to control the wear sensitivity at higher hardness compared to standard pearlitic steel, which regulation is based on the continuous hardening of the surface depending on the nature of the load. As the dynamic (impact) component increases, the strain hardening increases.

The bainitic steel prepared in this way has a yield strength Rm of at least 1200 MPa, a yield strength R _P0 of 2 of at least 800 MPa, an elongation of A of at least 16%, a KU toughness of at least 25 J and a dynamic fracture toughness of KCdyn of at least 80 MPa. conditions corresponding to a normal contact pressure of 950 MPa and a longitudinal chute of 2% at least 60% lower compared to standard wheel steel

-2GB 2018 - 364 A3 rolling stock of the highest strength category.

The described examples of a particular embodiment do not in any way limit the scope of protection given in the definition, but merely illustrate the nature of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

A steel according to the invention was prepared with the composition of the reference melt shown in Table 1, the remainder to 100% being iron and impurities.

Table 1

element C Si Mn Cr Mo Ni AI (B) Nb Ti wt. % 0.20 1.52 1.81 0.12 0.42 0,087 0.040 0.003 0,038 0.001

The prepared steel has the parameters given in Table 2, which is compared to the ER9 steel according to the requirements of EAN 13262, where R _p o, 2 is the yield strength, Rm is the yield strength, A is the ductility, KU is toughness, KCdyn is dynamic fracture the tensile strength measured and M _p is contact - fatigue wear.

Table 2

Rp0.2 [MPa] Rm [MPa] A [%] KU [J] KC _dy n [MPa. In m Steel according to the invention 806 1228 17.6 29 92 ER9 steel according to EN 13262 + A1 min. 580 900-1050 min. 12 13 43 ¹ '

^υ value from direct product measurement

When measuring the contact fatigue wear M _{p of the} reference samples made of steel according to the invention under conditions corresponding to a normal contact pressure of 950 MPa and a longitudinal slip of 2% at a load time of 1 million cycles, M _p = 0.12 g / km. M _p = 0.34 g / km was found for reference samples made of ER9 steel.

The following two additional examples of reference melts having a chemical composition according to the invention (shown in Tables 3, 5) show the parameters listed in Tables 4, 6.

Example 2

Table 3

element C Si Mn Cr Mo Ni AI (B) Nb Ti wt. % 0.198 1.44 1.98 0.18 0.48 0,042 0,028 0.004 0,034 0.012

-3GB 2018 - 364 A3

Table 4

Rp0.2 [MPa] Rm [MPa] A [%] KU [J] KC _dy n [MPa. s / m Steel 2 according to the invention 876 1288 18.4 39 87

When measuring the contact-fatigue wear M _{of p} reference samples made from steel according to the invention with a chemical composition according to Table 3 under conditions corresponding normálovému contact pressure of 950 MPa and a longitudinal slip of 2% at the time of a load of 1 million cycles has been detected value M _p = 0.116 g M _p = 0.34 g / km was found for reference samples made of ER9 steel.

Example 3

Table 5

element C Si Mn Cr Mo Ni Al (B) Nb Ti wt. % 0.21 1.56 2.01 0.38 0.41 0.013 0.012 0,0042 0.040 0,023

Table 6

Rp0.2 [MPa] Rm [MPa] A [%] KU [J] KCdyn [MPa. s / m Steel 3 according to the invention 920 1354 16 35 97

When measuring the contact fatigue wear M _{p of the} reference specimens made of the steel of the invention with a chemical composition according to Tab.5 under conditions corresponding to a normal contact pressure of 950 MPa and a 2% longitudinal chute of 1 million cycles, M _p = 0.104 g M _p = 0.34 g / km was found for reference samples made of ER9 steel.

Industrial applicability

The invention is intended for use in the field of transport, in particular for the manufacture of wheels of rail vehicles for railway and tramway transport.

PATENT CLAIMS

Claims (1)

1. Bainitic steel with increased contact-fatigue resistance and comprising C, Si, Mn, Cr, Ni, Mo, B, Nb, Al and Ti, characterized in that it comprises: -4GB 2018 - 364 A3 0.15 to 0.25% by weight of C, 1.3 to 1.8% Si, 1.7 to 2.2% by weight of Mn, not more than 1.5% by weight of Cr, 5 not more than 1.5% by weight of Ni, 0.20-0.50% Mo, and further comprises the following elements having micro-alloying properties: 0.001 to 0.005% by weight of B, 0.02 to 0.06 wt% Nb, not more than 0.04 wt% Al, not more than 0.04 wt% Ti, the remainder up to 100 wt% are iron and unavoidable impurities.