US3306735A

US3306735A - Hardenable forged-steel roll material

Info

Publication number: US3306735A
Application number: US391954A
Authority: US
Inventors: Nemoto Tadashi; Yaegashi Toshio
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1963-08-28
Filing date: 1964-08-25
Publication date: 1967-02-28
Anticipated expiration: 1984-02-28

Description

United States Patent Ofifice 3,306,735 Patented Feb. 28, 1967 2 Claims. (51. 75-126) The present invention relates to hardenable forgedsteel roll materials and is intended to provide an improved roll material having a high hardness and an excellent wear resistance.

According to the present invention, the hardenable forged-steel roll material comprises from 1.3% to 1.6% by weight of carbon, 4% to 6% by weight of chromium, 4% to 6% by weight of molybdenum, 4% to 6% by weight of tungsten, 3.5% to 6% by weight of vanadium, 3.5% to 6% by weight of cobalt, not more than 0.6% by weight of silicon and not more than 0.6% by weight of manganese, the balance including iron and a small amount of impurities.

In general, high-carbon high-chromium alloy steels are employed as materials for Sendzimir mill cold-working rolls. This kind of alloy steel, when quenched from a high temperature, obtains a martensitic base structure with chromium carbides of extreme hardness dispersed therein and has previously been recommended as a roll material which has a high wear resistance. Most recently, however, as the market grows more active, rolled sheet steel is increasingly required to have higher dimensional accuracy and improved surface luster for an extended period of time, and the conventional high-carbon highchromium alloy steels used as a roll material give only an insufficient wear resistance in attempting to meet such requirements.

In an effort tosatisfy the above requirements, a roll material has been developed in accordance with the present invention which has a highly improved Wear resistance and extreme hardness and which comprises as major constituents from 1.3% to 1.6% by weight of carbon, 4% to 6% by weight of chromium, 4% to 6% by weight of molybdenum, 4% to 6% by weight of tungsten, 3.5% to 6% by weight of vanadium, 3.5 to 6% by weight of cobalt, not more than 0.6% by weight of silicon, and not more than 0.6% by weight of manganese, the balance being substantially iron.

The reasons why such ranges of content are prescribed will next be explained for the respective major constituents.

Carbon forms carbides which are critically important in giving Wear resistance. The presence of carbon in a proportion of 1.3% or less by weight, however, gives only insufficient hardness and wear resistance while 1.6% or more by weight of carbon impairs the workability and the hardenability of the alloy and increases the danger of quenching crack.

Chromium combines with carbon to form carbides of extreme hardness in the form of M c and M C and thus plays an important role in improving the wear resistance of the product roll. However, as the proportion of chromium is reduced below 4% by weight, the amount of carbide formed is rapidly decreased. On the other hand, over 6% by weight of chromium gives chromium carbides as primary crystals in excess amount and in massive form and thus causes exfoliation or firecracks on the roll in use. This makes it necessary to coordinate the chromium content with the amounts of the other alloying elements.

Molybdenum, like chromium, forms carbides, part of which dissolves in the base structure to form a solid solution effective to afford substantial strength and heat resistance. This effect is important particularly because, in the use of the Sendzimir mill, the roll surfaces are instantaneously subjected to elevated temperatures, and thus are required to have a base structure strengthened enough to withstand such temperatures. The amount of molybdenum, however, is critical and with a content thereof of 4% or less by weight the effect is drastically reduced. As the content is increased, the amounts of molybdenum and molybdenum carbides in the base structure are also increased to give a noticeably improved wear resistance, but as the Mo content exceeds 6% by weight the workability is materially reduced and the hardenability markedly impaired.

Tungsten has substantially the same effects as M0 except that it is much more effective in improving the wear resistance of the roll. However, as the W content exceeds 6% by weight, a larger proportion of tungsten is dissolved in the base structure to form a solid solution which makes the roll brittle.

As for vanadium, it forms carbides, during ingot-making, in advance of any other carbide-forming element and is thus highly effective in improving the wear resistance of the roll. However, a content of vanadium of lower than 3.5% by weight gives a reduced wear resistance and also an insufiicient quench hardness. On the other hand, a V content in excess of 6% by weight causes excessive absorption of carbon in the vanadium carbides, thereby hampering the formation of other carbides so that a reduced hardenability and an extremely limited hot workability result.

Cobalt is dissolved for the most part in the base structure to form a solid solution and thus is effective in improving the wear resistance of the roll. However, this effect is not substantial with a C0 content of less than 3.5 by weight. On the other hand, as the Co content exceeds 6% by weight, the quenching temperature required to give the hardness desired is shifted to the higher side so that the size of the crystal grains is uselessly increased while rendering the quenching conditions diificult.

The present invention will be further described with reference to the accompanying drawings, in which:

FIG. 1 is a graphical representation of the relationship between the quenching temperature and the hardness obtained of a roll material according to the present invention; and

FIG. 2 illustrates the relationship between the annealing temperature and the hardness obtained of the material.

Reference will first be made to FIG. 1, which illustrates the hardness of the inventive alloy obtained after quenching at different temperatures and that obtained after subsequent annealing at 570 C. for a one hour period. As observed, the hardness increases with the quenching temperature and the material quench-hardened at a temperature of 1200 C. and above is secondarily hardened by the annealing at 570 C. FIG. 2 illustrates the relationship between the annealing temperature and the hardness obtained when the annealing is effected after the quenching at 1200 C. As observed, the secondary hardening takes place in the vicinity of 550 C. This characteristic is substantially the same as that of high speed steel in general.

The inventive alloy used in these experiments had a composition including 1.51% by weight of carbon, 0.23% by weight of silicon, 0.6% by weight of manganese, 5.28% by weight of chromium, 5.01% by weight of molybdenum, 5.01% by weight of vanadium, 4.86% by weight of cobalt, and 4.88% by weight of tungsten, the balance being iron.

When the alloy was held at a temperature of from 1200 C. to 1225 C. for 30 minutes, the crystal grains did not grow in a coarse manner. Also, by annealing at about 570 C., an annealed martensitic structure was obtained in which fine carbides were dispersed, and which exhibited a hardness of 65 Rockwell C. It has been found that in practice rolls made of the inventive alloy have a service life two or three times as long as that of rolls made of any conventional high-carbon high-chromium alloy steel.

What is claimed is:

1. In a process of cold-rolling metals by means of a rolling mill having rolls which contact the metal, the im provement in said process comprising contacting said metals with said rolls, said rolls consisting essentially of 1.3 to 1.6% by weight of carbon, 4 to 6% by weight of chromium, 4 to 6% by weight of molybdenum, 4 to 6% by weight of tungsten, 3.5 to 6% by weight of vanadium, 3.5 to 6% by weight of cobalt, not more than 0.6% by weight of silicon, not more than 0.6% by weight of manganese, the balance being substantially iron and a small amount of impurities.

2. In a process of cold-rolling metals by means of a rolling mill having rolls which contact the metal, the improvement in said process comprising contacting said 4 metals with said rolls, said rolls consisting essentially of 1.51% by Weight of carbon, 5.28% by weight of chromiurn, 5.01% by weight of molybdenum, 4.88% by weight of tungsten, 5.01% by weight of vanadium, 4.86% by weight of cobalt, 0.23% by weight of silicon, 0.6% by weight of manganese and the balance iron.

References Cited by the Examiner UNITED STATES PATENTS 2,147,121 2/1939 Emrnons et al. 75-126 2,230,687 2/1941 Houdremont et al. 75-126 2,662,010 12/1953 Ahles 75-126 2,983,601 5/1961 Fletcher 75126 2,996,376 8/1961 Nehrenberg 75-126

Claims

1. IN A PROCESS OF COLD-ROLLING METALS BY MEANS OF A ROLLING MILL HAVING ROLLS WHICH CONTACT THE METAL, THE IMPROVEMENT IN SAID PROCESS COMPRISING CONTACTING SAID METALS WITH SAID ROLLS, SAID ROLLS CONSISTING ESSENTIALLY OF 1.3 TO 1.6% BY WEIGHT OF CARBON, 4 TO 6% BY WEIGHT OF CHROMIUM, 4 TO 6% BY WEIGHT OF MOLYBDENUM, 4 TO 6% BY WEIGHT OF TUNGSTEN, 3.5 TO 6% BY WEIGHT OF VANADIUM, 3.5 TO 6% BY WEIGHT OF COBALT, NOT MORE THAN 0.6% BY WEIGHT OF SILICON, NOT MORE THAN 0.6% BY WEIGHT OF MANGANESE, THE BALANCE BEING SUBSTANTIALLY IRON AND A SMALL AMOUNT OF IMPURITIES.