Rolls for metal shaping

Details Rolls for metal shaping Rolls for metal shaping

1997-08-29

2001-01-09

Bryant, David P. (Department: 3726)

Roll or roller

Concentric layered annulus

Specific composition

Reexamination Certificate

active

06171222

ABSTRACT:

This invention relates to improved rolls for metal shaping and to the manufacture of such rolls. The rolls of the invention have particular application in the hot rolling of metals, such as of steel. However, it is to be understood that the rolls of the invention also have application in the cold rolling of steel and other metals, and as continuous casting support rolls or thin strip caster work rolls.
Rolls, for the hot rolling of steel and other metals, can be made from a wide variety of cast irons, cast steels and forged steels. However, cast irons of the types considered below have generally been used.
In hot rolling, the temperature of feed material to be rolled may reach 1000° C. or more. The material of which the rolls are made must be hard enough to resist wear, strong enough to maintain dimensional shape and tough enough to resist cracking and fracture. The material also must have sufficient resistance to thermal shock in order to resist thermal cracking, usually referred to as “fire cracking”.
Resistance to thermal shock is of paramount importance in hot rolling at the high temperatures that are met in the hot rolling of steel. The effect of such temperatures is particularly severe in the event of a rolling mill stalling or being stopped during a rolling pass, as the hot feed material then remains in contact with the stationary roll. With such contact, conventional cast rolls will usually suffer severe cracking, resulting in the need to remove the roll for re-machining or in the roll being scrapped.
Conventional alloy mottled cast irons, containing considerable amounts of both carbides and graphite, are generally used for the production of rolls for the rolling of hot metal, particularly long steel products such as billets, bars, sections, wires, rods and tubes. Typical of the mottled cast irons used are nodular and indefinite chill irons. The mottled cast irons are designed to combine a reasonable resistance to both wear and thermal fatigue (thermal shock). However, a compromise between these main properties is involved, since a change in the balance between carbides and graphite will inevitably lead to an improvement in one of the main properties at the expense of the other. Thus, for example, nodular cast irons with high carbide content and, hence, less graphite, will have enhanced surface hardness with superior resistance to wear, but will have inferior fire-crack resistance. One, generally undesired, property of rolls manufactured from mottled cast irons is that, because of the decrease in solidification rate from the surface into the depth of the roll, there is an associated decrease in hardness from the surface towards the centre of the roll.
Conventional chromium alloyed white cast irons are also generally used for the production of rolls for the rolling of hot metal, particularly flat steel products such as plate or strip. Typically these white irons have in excess of 15 wt % chromium, in excess of 2 wt % carbon, together with other alloying elements such as nickel, molybdenum, manganese and silicon. The white cast irons contain a substantial amount of a hard chromium-rich eutectic carbide phase, separated by a martensitic matrix in which further very fine alloy carbides may have been precipitated. The white cast irons are chosen because of their wear resistance, that is imparted by the chromium-rich carbides, the high strength martensitic matrix and strengthening of the matrix by fine carbide precipitates. This microstructure provides desirable high hardness, typically about HV500, and high resistance to wear in most wear situations. However, the white cast irons have poor thermal shock resistance. Thus chromium white cast iron rolls, for hot metal rolling products, fire-crack readily during hot rolling of steel. Chromium white cast irons do not normally exhibit a decrease in hardness from the surface into the depth of the roll.
The present invention is directed to providing improved ferrous alloy rolls, which have very high resistance to wear, and which also have high resistance to thermal shock, in particular to fire-crack formation.
The conventional materials for rolls for hot metal rolling are based on the Fe—C system. They are dependent upon the presence of carbon as carbides and graphite in the case of the mottled cast irons, and at least as chromium-rich carbides in the case of the chromium white cast irons. The present invention, in contrast, can be based on the Fe—C—B system, but more typically is based on the Fe—B system or the Fe—B—C system with carbon, if present, being at a low level, typically not in excess of 1.0 wt %. However if strong carbide forming elements, such as molybdenum, vanadium, titanium and niobium, are included in the alloy composition then the carbon level may be in excess of 1.0 wt % provided that the level of strong carbide forming elements is such that the excess carbon is bound by these elements in carbide or carbo-boride phases. The carbon content of the matrix would then remain low.
The fracture toughness, thermal shock resistance and wear resistance of the cast ferrous alloys is largely determined by the volume fraction of hard phases, which in turn is a function of the content of both boron and carbon and carbide and boride forming elements, and also the interstitial boron and carbon content of the matrix. The boron content of the matrix is always low because the solubility of boron in ferrite and austenite is low. However the solubility of carbon in austenite and therefore the carbon content of the martensitic matrix can be as high as approximately 2 wt % unless the carbon is bound in some other phase.
It is overriding in the design of satisfactory alloy compositions that the carbon content of the matrix is kept to a low enough level for sufficient fracture toughness or thermal shock resistance to be achieved for the application in question. For hot metal rolls the preferable level of carbon in the matrix is less than 0.3 wt % and can be significantly lower in some applications.
The present invention provides a roll for metal shaping, the roll having at least a peripheral surface layer of a sufficient thickness of an iron-base cast alloy having from 1 to 20 wt % chromium, from 0.5 to 3 wt % boron, up to 1.0 wt % carbon or higher if substantial amounts of strong carbide forming elements such as molybdenum, vanadium, titanium, tungsten and niobium are present, optional alloying additions as detailed in the following and, apart from incidental impurities, a balance of iron.
The roll of the invention is found to be suitable for the hot rolling of metals, such as steel. As a consequence of the required iron-base cast alloy present in at least a peripheral surface layer, but able to comprise substantially the entire roll body, the roll has both a very high resistance to wear and to thermal shock. However, the physical properties of the invention also render it suitable for the cold rolling of metals, such as flat and/or long products of for example steel, and for use as a continuous casting roller for a variety of metals.
The roll, in one form of the invention, is comprised substantially solely of such an iron-base cast alloy. That is, the roll may be of unitary, monolithic construction, and preferably produced in a single casting operation. However, in an alternative form, the roll may have an outer shell of the iron-base cast alloy, providing the sufficient surface layer, and a core of another ferrous alloy with which the shell is made integral.
The iron-base alloy can be substantially free of carbon, with carbon present effectively only as an incidental impurity. However, as indicated, carbon can be present at up to 1.0 wt %, or higher if substantial amounts of strong carbide forming elements such as molybdenum, vanadium, titanium, tungsten and niobium are present. Preferably carbon does not exceed 0.6 wt %, and may for example be present at from 0.1 to 0.6 wt %, such as from 0.1 to 0.3 wt %. The boron content preferably is not less than 0.5 wt %, and most preferably is from 0.5 to 2.5 wt %, such as from 1 to 2.5 wt %.

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