EP1780293A2 - Procedure for manufacturing of steel starting material by warm deforming - Google Patents

Abstract

Production of an initial material by thermal deformation involves subjecting a manganese-, chromium-, molybdenum-, nickel- and boron-containing steel (I), of specific composition, to controlled cooling from the thermal deformation temperature to give a martensitic-bainitic structure. Production of an initial material by thermal deformation involves subjecting a steel (I) to controlled cooling from the thermal deformation temperature to give a martensitic-bainitic structure. (I) contains (by weight, apart from iron and smelting impurities) 0.08-0.25% carbon, up to 1% silicon, 0.5-2.5% manganese, up to 0.035% phosphorus, up to 0.055% sulfur, 0.1-1.5% chromium, 0.1-0.5% molybdenum, 0.2-1.5% nickel, up to 0.06% aluminum, 0.0010-0.0060% boron, up to 0.040% titanium, up to 0.04% vanadium, up to 0.04% niobium, up to 0.5% copper and up to 0.010% nitrogen.

Description

The invention relates to a method of producing a starting material from steel, for example for producing wire rod and bar steel with high strength and toughness by hot working.

The prior art knows a number of methods for producing steel components with high strength and high toughness. Starting from wire rod or bar steel cold forming and thermoforming processes are known.

In the cold forming, the mechanical properties are set by cold work hardening during forming. To achieve high strength, high degrees of deformation are required. This is at the expense of toughness, so that the cold forming reaches its limits where the toughness of the component as a result of work hardening is no longer sufficient and thus results in an unfavorable strength-toughness ratio.

Thus, in order to achieve high strengths and high toughness, cold forming often involves compensation, i. heating, quenching and tempering. Here, so-called tempered steels according to DIN EN 10083 are used, in which, depending on the thickness of the component, it is possible, via the heat treatment, to set strengths of more than 1000 MPa for fracture constrictions of more than 45%. The ratio of yield strength to strength can be at least 0.8. A disadvantage of this procedure, the high cost of heat treatment and the burden on the environment through the consumption of energy and excipients.

As an alternative to the tempered steels, manganese / silicon dual-phase steels are used to produce cold-formed components of high strength from rolled material State of the art. However, these steels are not suitable for use with required strengths greater than 1000 MPa and high yield ratio above 0.8; They also require to set a certain initial strength and toughness in the starting material, a thermomechanical hot rolling and a parked on this initial strength cold work or strain hardening, so as to set a microstructure of a ferrite matrix with embedded Martensit and Perlitinseln.

High-strength steel components can be produced, starting from hot-rolled starting material, for example wire rod or bar steel, by cold forming and, if appropriate, tempering, also by hot forming.

Also thermoforming parts can be used after forming a heat treatment to adjust the mechanical properties. This is the classic application of tempered steels. However, since they require additional heat treatment, the already mentioned high cost and environmental impact arise. To avoid this heat treatment, hardening from forging heat is known. It eliminates heating to austenitizing temperature and quenching. However, low alloyed steels require a final tempering to provide the required performance properties, especially the necessary toughness.

Another material variant associated with forging heat is the so-called direct-hardening soft-martensitic steels with carbon contents of up to 0.1% and matched chromium, boron and manganese contents, which do not require tempering. These steels contain 0.05% carbon or even 0.10% carbon in the absence of chromium. A disadvantage of these steels is that a high cooling rate is required for adjusting the martensitic microstructure. This requires additional facilities on the Umformaggregat for oil or water quenching, which eat up a part of the cost savings. Furthermore, the high cooling rate causes complex parts or those with large differences in wall thickness tend to delay and the structure and the mechanical properties can be inhomogeneous over the cross section.

A similar development has gone in the past, even produce components with bainitic structure directly from the forge heat. The bainitic structure should avoid the risk of distortion and hardness differences as when setting a weichmartensitischen structure, as for a bainitic structure lower cooling rates sufficient. That's how it describes German Offenlegungsschrift 36 28 264 A1 a method for producing high strength and toughness components, such as truck steering knuckles using a low carbon steel having a carbon content below 0.3%, in which the components, e.g. Forgings are cooled from the deformation temperature to a bainitic structure. The disadvantage of this, however, is that the cooling rate must be selectively controlled depending on the generally locally different cross section of the component. For this complex, the respective component adapted cooling devices are required. Another problem is that these steels achieve high strengths, but only a low yield strength. For applications which require a high yield strength to tensile strength ratio, these steels are therefore unsuitable.

In order to replace the tempered steels and associated heat treatment, the so-called AFP steels, i. precipitation-hardening ferritic-pearlitic steels developed (for example according to DIN EN 10267). These obtain their mechanical properties through a controlled cooling from the thermoforming temperature and the excretion of carbonitrides of the elements titanium, vanadium and niobium. These steels are less prone to distortion than the blacksmiths site or bainite. Compared to tempered steels, however, they have a lower yield strength and lower toughness. At strengths of 800 to 1000 MPa only yield strengths of up to 600 MPa are achieved. For applications in the high load range, which require strengths around 1000 MPa at yield strengths above 750 MPa, the conventional AFP steels are therefore unsuitable.

A further development of the AFP steels is in the direction of tempered steels with higher strength and higher yield strength with good toughness. Today, improved alloying concepts are currently in view of optimum precipitation of carbonitrides by size and composition.

That's how it describes European Patent Application 1 408 131 A1 a low carbon precipitation hardening ferritic-perlitic steel having 0.12 to 0.45% carbon, 0.10 to 1.00% silicon, 0.50 to 1.95% manganese, 0.005 to 0.060% sulfur, 0.004 to 0.050% aluminum, 0.004 to 0.050% titanium, to 0.60% chromium, to 0.60% niobium, 0.10 to 0.40% vanadium, and 0.015 to 0.040% nitrogen, balance including iron due to melting. This steel needs to develop its mechanical properties only from its forming temperature of 950 to 1250 ° C with a cooling rate of at least 0.2 ° C / s, for example, to be cooled in still air. In order to optimally control the precipitation of the carbonitrides, however, the analysis specifications and defined parameters during heating to the forming temperature and during cooling must be strictly adhered to.

The invention is directed to a method with which a high strength and high toughness and a high ratio of yield strength to strength can be achieved without a heat treatment.

According to the invention can be in a steel with 0.08 to 0.25% carbon, up to 1% silicon, 0.5 to 2.5% manganese, up to 0.035% phosphorus, to 0.055% sulfur, 0.1 to 1.5 % Chromium, 0.1 to 0.5% molybdenum, 0.2 to 1.5% nickel, to 0.06% aluminum, 0.0010 to 0.006% boron, each to 0.04% vanadium, niobium and titanium, up to 0.5% copper and up to 0.010% nitrogen, the remainder being iron, including any impurities caused by melting, by adjusting a martensitic-bainitic structure by mere hot working and controlled cooling.

genügen. Die genannten Elemente, vorzugsweise Titan, werden zum Abbinden von Stickstoff genötigt. Dies ist erforderlich, damit das Bor härtbarkeitssteigernd wirksam ist.The contents of titanium, vanadium and niobium should be the condition $$(\%\mathrm{Ti})+\left(\%V\right)+\left(\%\mathrm{Nb}\right)=0.001\mathrm{to}0.2\%$$

suffice. The said elements, preferably titanium, are required for the setting of nitrogen. This is necessary for the boron hardenability enhancing effect.

The alloy composition and the cooling rate adjust the mechanical properties. On cooling from the deformation temperature of about 1000 to 1300 ° C, a bainitic-martensitic mixed structure, the proportion of ferrite and perlite should not exceed 10% in total. Cooling from the forming heat with gas, water or oil is possible but not required; In order to adjust the bainitic-martensitic microstructure, cooling on or with air is sufficient. A cooling with moving air is to be preferred, as this ensures the preferred minimum cooling rate of 0.3 ° C / s. The use of static or moving air is preferable to other refrigerants, since the environment is then not contaminated by vapors, no additional auxiliaries such as oil or gas and no disposal units such as filters, tanks and catch basins are required. The cooling rate should be at least 0.3 ° C / s in the temperature range between about 1000 and 610 ° C. The steel then has not only high toughness after cooling from the final temperature of hot working to room temperature, but also high strength. The ratio of yield strength to strength is also high.

The inventively cooled from the deformation heat starting material is readily suitable for cold forming. By strain hardening, tensile strengths of more than 1200 MPa can be achieved at yield strengths above 1050 MPa. The ratio of yield strength to strength is above 0.85. The high toughness is evident in fracture necking values of above 40% and elongations at break above 12%. The mechanical properties are therefore better than those of conventional steels or dual-phase steels.

This results in almost the properties of tempered steels, without the need for costly heat treatment.

The inventively cooled from the deformation heat starting material is also suitable in turn as a starting material for hot forming. In such - second - hot working again arise the original mechanical properties without the need for quenching in water or oil when the cooling conditions of the invention are met. Compared to forged martensite, the tendency to warp is lower because of the milder deterrent conditions. In comparison to the bainitic steels and the usual AFP steels, higher strengths and, in particular, significantly higher yield strengths result. Since precipitation hardening by carbonitrides is not strength-determining for the primary material according to the invention, a larger window results in the setting of the analysis and in particular in the conditions of thermoforming in comparison to newer AFP steels.

For the method according to the invention, a steel which contains at least 0.10% carbon, 0.3% silicon, 1% manganese, 0.2% chromium, 0.2% nickel, 0.2% molybdenum, 0.0015% is particularly suitable. Boron, 0.014% titanium, single or side by side.

Furthermore, the steel - individually or next to each other - also in each case at most 0.24% carbon, 2% manganese, 0.020% phosphorus, 0.045% sulfur, 1.4% chromium, 1.4% nickel, 0.4% molybdenum, 0 , 05% aluminum, 0.038% titanium, 0.02% vanadium, 0.02% niobium, 0.3% copper, 0.005% boron and 0.010% nitrogen.

In one embodiment, a steel refined by the LD process was hot rolled into 15 mm diameter wire, cooled from the rolling heat of accelerated air, and then cold drawn to a final diameter of 14 mm.

The steel was made 0.205% carbon 0.56% silicon 1.62% manganese 0.011% phosphorus 0.01% sulfur 0.54% chrome 0.32% molybdenum 0.22% nickel 0.03% aluminum 0.0038% boron 0.036% titanium 0.002% vanadium 0.002% niobium 0.0044% Nitrogen, rest iron including contaminants due to melting.

The structure and the mechanical properties of the wire after cooling from the rolling heat at a cooling rate of 2 ° C / s using accelerated air and after cold drawing are shown in the table below. properties As rolled Drawn Tensile strength R _m [MPa] 1180 1230 Yield strength R _{p 0.2} [MPa] 805 1070 Fracture Z [%] 53.6 42 Elongation at break A5 [%] 14 14 Yield ratio 0.72 0.86 Structure shares [%] edge core martensite 35 to 50 30 to 55 bainit rest rest

The above data show that the inventive method results in a material that is characterized both in hot-formed and cold-worked state by a high strength and toughness and a high yield ratio and can be produced inexpensively and environmentally friendly because of the elimination of a heat treatment.

Claims (9)

Process for the production of semi-finished material by hot-forming, in which a steel comprising the following constituents in percent by weight 0.08 to 0.25% carbon up to 1% silicon 0.5 to 2.5% manganese up to 0.035% phosphorus up to 0.055% sulfur 0.1 to 1.5% chrome 0.1 to 0.5% molybdenum 0.2 to 1.5% nickel up to 0.06% aluminum 0.0010 to 0.0060% boron up to 0.040% titanium up to 0.04% vanadium up to 0.04% niobium up to 0.5% copper up to 0.010% nitrogen Residual iron, including any contaminants due to melting, is adjusted from the deformation temperature by a controlled cooling to a martensitic-bainitic structure. A method according to claim 1, characterized by the use of a steel with in each case at most 0.24% carbon, 2% manganese, 0.020% phosphorus, 0.045% sulfur, 1.4% chromium, 1.4% nickel, 0.4% molybdenum, 0.05% aluminum, 0.038% titanium, 0.02% vanadium, 0.02% niobium, 0.3% copper, 0.005% boron and 0.010% nitrogen, singly or side by side. A method according to claim 1 or 2, characterized by the use of a steel having at least 0.10% carbon, 0.3% silicon, 1.0% manganese, 0.2% chromium, 0.2% molybdenum, 0.2% nickel, 0.0015% boron , 0.014% titanium individually or next to each other. Method according to one of claims 1 to 3, characterized in that the contents of the steel of titanium, vanadium and niobium of the condition $$(\%\mathrm{Ti})+\left(\%V\right)+\left(\%\mathrm{Nb}\right)=0.001\mathrm{to}0.2\%$$

suffice. Method according to one of claims 1 to 4, characterized in that the steel is set on cooling to a martensitic-bainitic structure with at most 10% ferrite and perlite. Method according to one of claims 1 to 5, characterized in that the steel is cooled in the range between the forming temperature and 610 ° C with at least 0.3 ° C / s. Method according to one of claims 1 to 6, characterized in that the starting material is cold worked. Use of a starting material according to claims 1 to 7 for the manufacture of high-strength wire, automotive chassis parts and suspensions, wheel carriers, wishbones, steering and wheel journal, crankshafts, connecting rods, bearings, stabilizers and connecting elements. Use of a starting material according to the methods of claims 1 to 8 for the manufacture of forgings.