AU2009238307C1

AU2009238307C1 - Hot-forming steel alloy

Info

Publication number: AU2009238307C1
Application number: AU2009238307A
Authority: AU
Inventors: Devrim Caliskanoglu; Herbert Schweiger; Ingo Siller; Silvia Zinner
Original assignee: Boehler Edelstahl GmbH and Co KG
Current assignee: Voestalpine Boehler Edelstahl GmbH and Co KG
Priority date: 2008-11-20
Filing date: 2009-11-17
Publication date: 2014-03-13
Anticipated expiration: 2029-11-17
Also published as: SI2194155T1; AT506790A4; ZA200908201B; CA2686071A1; US20100150772A1; BRPI0904501A2; ATE487805T1; AT506790B1; ES2353192T3; AU2009238307B2; CA2686071C; AU2009238307A1; EP2194155B1; US20150292067A1; PL2194155T3; DE502009000171D1; EP2194155A1

Abstract

Abstract The invention relates to a hot-forming steel alloy, containing the elements carbon (C), silicon (Si), manganese (Mn), chromium (Cr), molybdenum (Mo), vanadium (V), nitrogen (N) and impurity elements and the rest being iron. 5 In order to achieve a high hardness and an improved toughness of the material in a thermal quenching and tempering even at low cooling rates, it is provided according to the invention that the alloying elements have contents in % by weight of Carbon (C) 0.35 to 0.42 Silicon (Si) 0.15 to 0.29 10 Manganese (Mn) 0.40 to 0.70 Chromium (Cr) 4.70 to 5.45 Molybdenum (Mo) 1.55 to 1.95 Vanadium (V) 0.40 to 0.75 Nitrogen (N) 0.011 to 0.016.

Description

AUSTRALIA FB RICE & CO Patent and Trade Mark Attorneys Patents Act 1990 BOHLER EDELSTAHL GMBH & CO KG COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Hot-forming steel alloy The following statement is a full description of this invention including the best method of performing it known to us:- Hot-forming steel alloy The invention relates to a hot-forming steel alloy with high toughness and at the same time a great hardening depth or improved martensitic through-hardening capability with a thermal quenching and tempering of products such as, for example, die-casting dies or extrusion dies and the like. 5 A thermal quenching and tempering of a part, e.g., of hot-forming steel, to adjust a high material hardness at operating temperatures of the part up to 550*C and more, essentially means heating the material to a temperature at which it has a cubic face-centered atomic structure or an austenitic structure, followed by a forced cooling to obtain a martensitic structure and a subsequent tempering treatment, optionally multiple times, at 10 temperatures of generally more than 500*C. During the tempering, on the one hand the stresses in the material formed during the cooling and structural transformation are reduced at least in part, and on the other hand the material hardness is increased or a so called secondary increase in hardness is achieved due to carbide deposits. A transformation of an austenitic structure into a martensitid structure, as one skilled in 15 the art is aware, calls for a minimum cooling rate of the material, because this transformation takes place as a diffusionless flip-over process of the atomic structure due to a markedly high subcooling. Lower cooling rates lead to the formation of a bainite or pearlite structure. The properties of a material depend on the chemical composition thereof and on the fine 20 structure thereof adjusted by a thermal treatment and produce therefrom a specific property profile of a part. In other words: the chemical composition of a material and the intensity of the cooling or the heat dissipation from the surface during the hardening of the part determine the fine structure in the region of the surface and, due to the rewarming from the interior of the 25 part, the structural development depending on the distance from the part surface. The respective local fine structure determines the material properties of the thermally quenched and tempered material locally present. I A For reasons of increasingly economic production of the products, hot-forming materials for die cast molds and the like are subject to increasing stresses through shortened press sequence times and increased casting pressures. Furthermore, complex geometries of the mold cavities are provided to an increasing extent, so that much 5 higher total stresses of the material are present overall. These total stresses can cause tool failure due to stress cracks, fire cracks, coarse fracture, corrosion and erosion, so that materials with a high hardness and strength as well as high toughness and ductility at the same time are required. However, these required properties depend on the chemical composition of the alloy and the tempered properties of the same resulting 10 therefrom. Cr-Mo-V steels have long been used for hot-forming tools, wherein the steel types X38 CrMoV 51 and X38 CrMoV 53 according to DIN steel iron list material no. 1.2343 and material no. 1.2367, as also given in the list, are "highly resistant to softening" and suitable for "tools with large dimensions." 15 Material no. 1.2343 is used for "highly stressed tools, dies and presses." The above materials have a high hardening depth and a deep-reaching tempering quality to required hardness values between 50 and 55 HRC. However, their toughness properties are low, which can be a disadvantage for the wearing qualities of die casting molds. 20 With a material no. 1.2343, a considerable increase in the material toughness after a quenching and tempering treatment can be achieved by a reduction of the provided silicon content from 0.90 to 1.20% by weight to a concentration of about 0.2% by weight, but high cooling rates during hardening are necessary for this, which often cannot be achieved. 25 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application. 2 Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 5 The aim of the invention is now to create a hot-forming steel of the type mentioned at the outset, which forms a largely complete martensitic fine structure during a forced cooling from the austenite range even at low cooling rates, after which a high hardness and improved toughness of the material are achieved through a targeted 10 tempering treatment. This aim is achieved with a hot-working steel alloy in that the alloying elements have contents in % by weight of Carbon (C) 0.35 to 0.42 Silicon (Si) 0.15 to 0.29 15 Manganese (Mn) 0.40 to 0.70 Chromium (Cr) 4.70 to 5.45 Molybdenum (Mo) 1.55 to 1.95 Vanadium (V) 0.40 to 0.75 Nitrogen (N) 0.01 Ito 0.016 20 the rest being iron (Fe) and impurity elements. The advantage of the alloy composition according to the invention is to be seen essentially in that the elements overall, in particular the elements silicon, molybdenum, vanadium and nitrogen, are coordinated with one another within narrow limits in terms of transformation kinetics so that a desired strength and hardness with a high toughness 25 of the material can be achieved with a thermal quenching and tempering with a reduced cooling rate during hardening. 3 It is thus possible either to achieve greater penetration depths of a martensitic hardness structure that is favorable for the mechanical properties of the part with a given cooling rate or advantageously to use a lower cooling rate during a hardening and to thus minimize the hardening strains in the die-casting die often provided with an 5 engraving or with a negative mold of the cast part. This is particularly important because a so-called vacuum hardening of molded parts is being used to an increasing extent, wherein a heating in vacuum also takes place for reasons of avoiding oxidations and decarburization of the processed surface of the workpiece or the mold during an austenitization, after which a forced cooling is carried out with a nitrogen gas flow. For 10 this type of hardening of a part, an alloy with the chemical composition according to the invention has proven to be particularly useful. Another important increase in the toughness properties of the tempered and quenched material can be achieved when the hot-working steel alloy has maximum concentrations of one or more elements in % by weight of 3A Phosphorus (P) 0.005 Sulfur (S) 0.003 Nickel (Ni) 0.10 Tungsten (W) 0.10 5 Copper (Cu) 0.10 Cobalt (Co) 0.10 Titanium (Ti) 0.008 Niobium (Nb) 0.03 Oxygen (0) 0.003 10 Boron (B) 0.001 Arsenic (As) 0.01 Tin (Sn) 0.0025 Antimony (Sb) 0.01 Zinc (Zn) 0.001 15 Calcium (Ca) 0.0002 Magnesium (Mg) 0.0002. The above elements can form either precipitations or compounds, which are enriched in particular at the grain boundaries and suddenly reduce the toughness properties of the material from a concentration limit or they cause grain boundary occupancies, which likewise have an unfavorable effect. 20 Through a chemical composition of the material according to the invention adjusted within very narrow limits according to a preferred embodiment of the same, the hot forming steel alloy contains one or more of the alloying elements in % by weight Carbon (C) 0.37 to 0.40 Silicon (Si) 0.16 to 0.28, preferably 0.18 to 0.25 4 Manganese (Mn) 0.45 to 0.60, preferably 0.50 to 0.58 Chromium (Cr) 4.80 to 5.20, preferably 4.90 to 5.10 Molybdenum (Mo) 1.50 to 1.90, preferably 1.65 to 1.80 Vanadium (V) 0.45 to 0.70, preferably 0.52 to 0.60 5 Nitrogen (N) 0.012 to 0.015 the rest being iron (Fe) and impurity elements. By means of this alloy according to the invention, which is characterized by particularly narrow limits in the chemical composition and which makes particular demands on a smelting technology, it is possible to achieve high toughness values of the material even 10 with low cooling rates in the thermal quenching and tempering process with high material hardnesses. The invention is described in more detail below based on test results. As a help, the test results are summarized in Fig. 1. Fig. I shows: Impact strength values of the material after a thermal quenching and 15 tempering as a function of the cooling parameters in the hardening treatment. Alloys with a chemical composition according to the invention and according to DIN material no. 1.2343 with standardized and with reduced Si contents and according to DIN material no. 1.2367, as given in Table 1, were examined after a thermal quenching and tempering treatment for a material hardness of 44 HRC with different cooling parameters 20 X during hardening. The value that characterizes the parameter X is thereby calculated as follows: Cooling parameter [X] corresponds to the time [in sec.] for a cooling from 800*C to 500*C divided by 100. The alloying elements of the materials listed in Table I are cited below, wherein the rest 25 represents the content of iron and accompanying elements and impurity elements. Alloy composition in % by weight 5 Material no. C Si Mn P S N Cr Mo Ni V 1.2343 0.39 1.11 0.41 0.021 0.023 - 5.28 1.26 0.21 0.38 1.2343 So 0.38 0.21 0.39 0.022 0.019 - 5.34 1.30 0.16 0.40 1.2367 0.38 0.40 0.47 0.029 0.021 - 5.00 2.98 0.20 0.61 W 350 0.39 0.19 0.51 0.004 0.001 0.013 4.91 1.69 0.06 0.53 Fig. I shows that with a cooling parameter up to approx. X = 12 the material no. 1.2343 with Si contents reduced to approx. 0.20% by weight in a thermally quenched and tempered state to a material hardness of 44 HRC has the highest toughness measured according to Charpy V. However, as a result with increasing cooling parameter X, the 5 toughness values drop sharply to a low level. The materials no. 1.2343 with standardized Si contents and no. 1.2367 have a lower toughness with a quenched and tempered hardness of 44 HRC, but have a remarkable through-hardening capability, which is documented by only slightly reduced toughness values as a function of the cooling parameter. 10 Although at high cooling rates or in the range of the cooling parameter X to 13, a test alloy W 350 according to the invention shows slightly lower toughness values at room temperature in the state quenched and tempered to 44 HRC compared to the So material no. 1.2343 (Si : 0.2% by weight), the toughness of the material remains essentially unchanged at superior high values even with reduced cooling rates or higher cooling 15 parameters. 6

Claims

1. A hot-forming steel alloy, comprising alloying elements carbon (C)., silicon (Si), manganese (Mn), chromium (Cr), molybdenum (Mo), vanadium (V), nitrogen (N) and impurity elements and the rest being iron, with the proviso 5 that the alloying elements are present in the following ranges in % by weight, based on the total weight of the alloy, of: Carbon (C) 0.35 to 0.42 Silicon (Si) 0.15 to 0.29 Manganese (Mn) 0.40 to 0.70 10 Chromium (Cr) 4.70 to 5.45 Molybdenum (Mo) 1.55 to 1.95 Vanadium (V) 0.40 to 0.75 Nitrogen (N) 0.011 to 0.016.

2. The hot-forming steel alloy according to claim 1, comprising maximum 15 concentrations of one or more elements in % by weight, based on a total weight of the alloy, of: Phosphorus (P) 0.005 Sulfur (S) 0.003 Nickel (Ni) 0.10 20 Tungsten (W) 0.10 Copper (Cu) 0.10 Cobalt (Co) 0.10 Titanium (Ti) 0.008 Niobium (Nb) 0.03 25 Oxygen (0) 0.003 Boron (B) 0.001 7 Arsenic (As) 0.01 Tin (Sn) 0.0025 Antimony (Sb) 0.01 Zinc (Zn) 0.001 5 Calcium (Ca) 0.0002 Magnesium (Mg) 0.0002.

3. The hot-forming steel alloy according to claim 1 or 2, comprising one or more of the alloying elements in % by weight, based on the total weight of the alloy, of: 10 Carbon (C) 0.37 to 0.40 Silicon (Si) 0.16 to 0.28 Manganese (Mn) 0.45 to 0.60 Chromium (Cr) 4.80 to 5.20 Molybdenum (Mo) 1.50 to 1.90 15 Vanadium (V) 0.45 to 0.70 Nitrogen (N) 0.012 to 0.015 the rest being iron (Fe) and impurity elements.

4. The hot-forming steel alloy according to claim 3, wherein the alloy comprises the alloying elements silicon (Si), manganese (Mn), chromium 20 (Cr), molybdenum (Mo) and Vanadium (V) in % by weight, based on the total weight of the alloy, of: Silicon (Si) 0.18 to 0.25 Manganese (Mn) 0. 50 to 0.58 Chromium (Cr) 4.90 to 5.10 25 Molybdenum (Mo) 1.65 to 1.80 Vanadium (V) 0.52 to 0.60 8

5. A hot-forming steel alloy as substantially hereinbefore described with reference to the examples, excluding comparative examples. 9