Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-02-12
2003-11-04
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S334000, C148S547000, C148S651000, C148S648000
Reexamination Certificate
active
06641681
ABSTRACT:
TECHNICAL FIELD
The invention relates to a steel product of a steel material having a new chemical composition and microstructure. The invention also relates to the manufacturing of the material as well as its use.
BACKGROUND OF THE INVENTION
High demands as far as toughness and wear resistance are concerned are raised on materials which shall be used for the manufacturing of cold work tools. This generally is true e.g. for tools for cutting, punching, bending, and deep drawing of metal plates or sheets; tools for pressing metal powders; and cold rolling rolls. A steel which today is used for cold rolling rolls, e.g. for cold rolling of steel strips, has the nominal composition 0.73 C, 1.0 Si, 0.60 Mn, 5.25 Cr, 1.10 Mo, 0.50 V, balance iron and unavoidable impurities. Rolls made of that material normally has a hardness of 58-60 HRC in the use condition, when the roll is through hardened. It is a problem with this material that the material has a tendency to crack in the through hardened condition of the material, which may cause a total failure. Moreover, the wear resistance is not quite satisfactory. Powder metallurgy manufactured steels on the other hand, which contain high contents of vanadium, satisfy high demands as far as toughness as well as wear resistance are concerned, but are expensive. It is conventional to design cold rolling rolls made of compound materials, wherein a wear resistant outer material, which usually consists of a high alloyed steel, is united through casting or in any other mode with a core made of a tougher material, which usually is less alloyed In this way it is possible to obtain a roll with good wear resistance and toughness. On among several disadvantages is that the manufacturing is expensive. Therefore it exists a demand of materials which do not require powder metallurgy manufacture or compounding technique but which nevertheless satisfy the demands which are raised on cold work steels, among them toughness and wear resistance.
BRIEF DISCLOSURE OF THE INVENTION
The purpose of the invention is to address the above problems and provide a new steel material which can be employed for cold work tools, particularly for cold rolling rolls, and which has a satisfactory toughness, hardenability, and wear resistance. In the first place the invention aims at providing a material for solid working rolls and/or for supporting rolls for cold rolling of steel strips. “Solid” is this context means rolls which do not consist of compound materials. This and other objectives of the invention can be achieved by a chemical composition, which is a characterising feature of the invention, in combination with a microstructure of the steel which also is a characterising feature.
The chemical composition and the microstructure of the steel of the invention are stated in the appending patent claims and will be commented more in detail in the following. If not otherwise is mentioned, always weight-% are referred to.
The structure of the steel product of the invention has a hardness in the order of 250 HB in the soft annealed condition and a hardness of 30-50 HRC in the tough hardened condition, and a microstructure which contains 5-12 vol-% MC-carbides, at least about 50 vol-%, preferably at least about 80 vol-%, having a size which is larger than 3 &mgr;m but smaller than 25 &mgr;m, preferably smaller than 20 &mgr;m. Preferably at least 90 vol-% of the precipitated carbides of MC-type have a size which is larger than 3 &mgr;m but smaller than 25 &mgr;m, preferably smaller than 20 &mgr;m. This material is suited to be subjected to cutting type of work in connection with the manufacturing of the tool. In the condition of use the finished product, i.e. the tool, e.g. the roll, has a surface hardness amounting to 60-67 HRC, which can be afforded by through hardening or induction hardening followed by tempering, wherein the microstructure in the hardened and tempered material consists of tempered martensite containing 5-12 vol-% MC-carbides, of which at least 50 vol-%, preferably at least about 80 vol-% have a size which is larger than 3 &mgr;m but smaller than 25 &mgr;m, preferably smaller than 20 &mgr;m. Preferably also in this case at least about 90 vol-% of the MC-carbides have a size which is larger than 3 &mgr;m but smaller than 25 &mgr;m, preferably smaller than 20 &mgr;m. Prior to tempering, the martensite contains 0.50-0.70 weight-% C. Size in this text means the longest extension of the carbide particle in any direction in a studied section of the material.
For the achievement of the said carbide dispersion in the matrix of the steel, a number of techniques which may be known per se can be employed for the production of steel ingots, from which the steel product is manufactured. In the first place the so called spray forming technique is recommended, which is also known as the OSPREY-method, according to which an ingot rotating about its longitudinal axis successively is established therein that molten metal in the form of drops is sprayed against the growing end of the ingot which continuously is being manufactured, wherein the drops are caused to solidify comparatively fast once they have hit the substrate, however not as fast as in connection with powder manufacturing and not as slow as in connection with conventional manufacturing of ingots or in connection with continuous casting. Another technique which possibly might be employed is ESR-remelting (Electro Slag Remelting), in the first place for the manufacturing of products of larger sizes, i.e. with diameters from Ø 350 mm up to 600 mm.
As far as the various alloying elements in the steel are concerned, the following applies.
Carbon shall exist in a sufficient amount in the steel in order, on one hand, together with vanadium and possibly existing niobium to form 5-12 vol-% MC-carbides, where M substantially is vanadium, and on the other hand to exist in solid solution in the matrix of the steel in an amount of 0.50-0.70 weight-%. Suitably, the content of carbon that is dissolved in the matrix of the steel is about 0.60%. The total amount of carbon in the steel, i.e. carbon that is dissolved in the matrix of the steel plus that carbon that is bound in carbides, shall be at least 1.0%, preferably at least 1.1%, while the maximum content of carbon may amount to 1.9%, preferably max 1.7%.
According to a first preferred embodiment of the invention, the steel contains 1.4-1.7 C, preferably 1.45-1.65 C, nominally about 1.5 C, in combination with 3-4.5 V, preferably 3.4-4.0 V, nominally about 3.7 V in order to provide a total content of MC-carbides amounting to 8-12, preferably 9-11 vol-% MC-carbides, in which vanadium partly can be replaced by the double amount of niobium.
According to a second preferred embodiment, the steel contains 1.1-1.3 C, nominally about 1.2 C, in combination with 2.0-3.0 V, nominally about 2.3 V in order to provide a total content of MC-carbides amounting to 5-7 vol-%, preferably about 6 vol-% MC-carbides, in which the vanadium partly can be replaced by the double amount of niobium.
According to all embodiments, the hardened, martensitic matrix of the steel contains 0.50-0.70% C prior to tempering.
Silicon, which partly can be replaced by aluminium, shall, together with possibly existing aluminium, exist in a total amount of 0.5-2.0%, preferably in a an amount of 0.7-1.5%, suitably in an amount of 0.8-1.2% or in a nominal amount of about 1.0% in order to increase the carbon activity in the steel and hence contribute to the achievement of an adequate hardness of the steel without creating brittleness problems because of dissolution hardening at too high contents of silicon. The aluminium content, however, must not exceed 1.0%. Preferably, the steel does not contain more than max 0.1% Al. Manganese, chromium, and molybdenum shall exist in the steel in a sufficient amount in order to afford the steel an adequate hardenability. Manganese also has a function to bind those residual amounts of sulphur, which can exist in low contents in the steel, by forming manganese sulphide. Manganese theref
Rydell Bo
Sandberg Odd
Nixon & Vanderhye
Uddeholm Tooling Aktiebolag
Yee Deborah
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