Steel for machine structural use excellent in friability of chips

Abstract

Disclosed is a free cutting steel for machine structural use having excellent chip-breakability. The steel consists essentially of, by wt. %. C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Ca or Ca+Mg; 0.0005-0.02%, Ti: 0.002-0.010% and / or Zr: 0.002-0.025%, O: 0.0005-0.010%, and the balance of impurities and Fe. At least five MnS inclusion particles having averaged particles sizes of 1.0 μm or more exists per mm2 per 0.01% of S-content in the steel. The steel satisfies the condition that, in the microscopic fields, (area[μm2] / aspect ratio)≧10, and that the the area percentage of Ca-containing sulfide inclusions containing at least 1.0 wt. % of Ca is in the range of 15-40% of the area of all the sulfide inclusions.

Description

TECHNICAL FIELD [0001] The present invention concerns a steel for machine structural use having excellent chip-breakability at machining with cemented carbide tools. The steel for machine structural use of the invention is characterized by configuration of sulfide inclusions in the steel. [0002] In the specification the term “Ca-containing sulfide inclusion” means the inclusion of the structure formed by a core inclusion mainly consisting of CaO, and another inclusion mainly consisting of sulfides and surrounding the core. In regard to the MnS inclusion the phrase “finely dispersed” means that the inclusion particles are finer than the MnS inclusion particles in the conventional steel, and that they are homogeneously dispersed throughout the steel without either coagulation or concentration. The “aspect ratio” is defined as the value given by dividing the longest diameter by the shortest diameter of the inclusion particles observed on the surface formed by cutting a steel sample alo...

AI Technical Summary

Benefits of technology

[0009] The object of the invention is to provide, on the basis of the above mentioned our discovery, a free cutting steel for machine structural use which facilitates automation of machining by controlling the configuration of the sulfide inclusion particles so that the good tool lives and improved chip breakability may be enjoyed.

[0014] Silicon is used as a deoxidizing agent at steelmaking and becomes a component of the steel. Si is useful because it enhances hardenability of the steel. The effect may not be expected at a small amount less than 0.01%. Si also increases the activity of sulfur, and a large amount of Si causes the same problem as that of a large amount of carbon, namely, formation of Ca-containing sulfide inclusion may be prevented. Also, a large amount of Si damages the resilience of the steel, which results in tendency of cracking at plastic processing. The addition amount of Si must be, therefore, up to 2.5%. Mn: 0.1-3.5%

[0020] Phosphor, which is inevitable as an impurity in the steel, is harmful to the resilience, and therefore, should not be contained in an amount exceeding 0.2%. However, P is a component-which improves the machinability, particularly, the properties of the finished surface. This effect may be observed at a content of 0.001% or more.

[0023] Chromium and molybdenum enhance hardenability of the steel and addition of a suitable amount or amounts are recommended. Excess addition will damage the hot workability of the steel and cause cracking. With consideration of the costs of addition, the respective upper limits are set to 3.5% for Cr and 2.0% for Mo. Copper makes the matrix of the steel dense and heightens the strength. Because addition of Cu in a large amount is not favorable from the view points of both the hot workability and the machinability, addition amount should be up to 2.0%. Though nickel also enhances the hardenability like chromium and molybdenum, it is unfavorable element as far as the machinability is concerned. Taking this and the costs of addition into account, the upper limit is set to 4.0%. Boron enhances the hardenability even at a small amount of addition. In order to obtain this effect, boron must be added in an amount of 0.0005% or more. Addition of B exceeding 0.01% is unfavorable due to lowered hot workability. One or both of Nb: up to 0.2% and V: up to 0.5%

[0024] Niobium is useful for preventing coarsening of crystal grains at high temperature. Because the effect of addition saturates as the Nb-content increases, it is recommended to add it in an amount up to 0.2%. Vanadium combines with carbon and nitrogen to form the carbonitride, which makes the crystal grains fine. The effect saturates at a content exceeding 0.5%.

[0033] The reason why the present free cutting steel for machine structural use exhibits excellent chip breakability is considered to attribute to the mechanism that, at turning in machining, the sulfide inclusion forms a melted film on the surface of the tool to minimize the curl diameter of the chips. The melted film of the sulfide inclusion exhibits so high lubricating effect that it may be useful for minimizing the curl diameter. EXAMPLES

Problems solved by technology

Recently, however, due to increasing significance of environmental problems, free cutting steels containing no lead are often demanded.

The technical problem common in the lead-free free cutting steels is breakability of chips at machining.

As is well known, in the automated machining not only tool lives but also the chip breakability is important, because lower chip breakability may cause entangling of the chips with the tools or works, or conveying troubles in chip conveyers, and thus, results in obstruction of automation.

At present, however, the achieved chip breakability is not satisfactory, because the fluctuation of the improvement is significant and it is difficult to ensured substantially constant chip breakability.

However, it was further discovered that existence of fine sulfide inclusion particles is not sufficient and that it is necessary to form sulfide inclusion films having a smaller friction coefficient with the chips on the surface of the tools.

If the sulfide films of smaller friction coefficient with the chips are formed on the surfaces of the tools, the films give the effect of decreasing “curl diameter” of the chips formed by machining, and as the results, the chips may be easily broken.

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