JPH01319651A - Free cutting steel having excellent warm workability - Google Patents

Abstract

PURPOSE:To make compatible the warm workability with machinability of the subject free cutting steel by incorporating C, Si, Mn, S, Al, Ca, B and N into a steel, specifying the amounts of N/B (weight ratio), O and Ti+Zr+REM and incorporating specific wt.% of Ca into sulfide inclusions. CONSTITUTION:C; 0.15-0.70%, <=0.35% Si, <=2.0% Mn, <=0.04% S, 0.001-0.100% Al, 0.005-0.01% Ca, 0.004-0.020% B and 0.005-0.02% N are incorporated into a steel; where N/B is regulated, by weight, to 0.5-4.0, O is regulated to <=0.002%, Ti+Zr+REM is regulated to <=0.01% and average 5-50wt.% Ca is incorporated into sulfide inclusions. Furthermore, 0.005-0.05% Pb and 0.01-0.1% B are incorporated thereto. Specific amounts of Ni, Cr and Mo are incorporated thereto as well. By this method, the warm workability and machinability of the free cutting steel can be compatible.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to free-cutting steel with improved warm workability by adjusting inclusions.

[Conventional technology]

Free-cutting steel is preferably used in the manufacture of mechanical structural parts that make up automobiles and various industrial machines in order to increase machining efficiency. On the other hand, it is often desirable to perform the processing steps prior to this, such as rolling and forging, at a warm temperature. Here, "warm processing"
generally means a temperature lower than 800°○, preferably 600°
It is a well-known process in which the temperature ranges from ~700'C to 700'C, and it is possible to perform processing with the lowest deformation resistance next to hot working, and to achieve finishing accuracy close to that of cold working. This is an advantage of warm processing. . However, the free-cutting steel currently used for machine structures is s,
It contains free-cutting elements such as PB, B;, and D, which are harmful to warm workability. Looking at typical S, S mainly exists in the form of MnS inclusions in steel and improves machinability, but MnS stretches for a long time during rolling, impairs isotropy and becomes a base for cracks. . The inventors discovered that BN (hexagonal) inclusions improve the machinability of steel, and disclosed a free-cutting steel containing 0.001 to 0.1% of BN (hexagonal) inclusions. -211350
@). Subsequent research revealed that in order to precipitate the appropriate amount of BN in steel, B: 0.004-0.020%, N: 0.
005 to 0.050%, and N/B (weight ratio): 0.
It was found that the conditions of 5 to 4.0 should be met, and that 0 should be regulated to 0.0015% or less, and the total amount of elements that tend to form nitrides, such as Ti, Zr, and REM, should be regulated to 0.01% or less. Such steel can be used as a free-cutting steel for carburizing and quenching due to its composition in combination with other specific alloy components (Japanese Patent Application No. 63
-15460), ori was confirmed to be useful as a non-thermal free-cutting steel (Japanese Patent Application No. 63-42875@) and proposed it. This time, we obtained new knowledge that BN inclusions are not harmful to the warm workability of steel. [Problem to be solved by the invention 1. Purpose of the present invention (Jl. Utilizing the above new knowledge, it is possible to improve the free-cutting property to a high level by relying mainly on BN inclusions and also with the help of free-cutting inclusions. The objective is to provide a steel that has both free machinability and warm workability, which were previously difficult to achieve, by minimizing inclusions harmful to warm workability. Means for Solving] The free-cutting steel with excellent wet workability of the present invention has a basic composition of C: 0.15 to 0.70%, Si: 0.35% or less, and Mn: 2.0%. Below, S: 0゜040% or more, A〃
:0.00~0.100%, Ca:0.0005
~0.0100%, B:○. 004-0.020%d3 and N:0. ○05-0゜0
Contains 20%, provided that N/B (weight ratio): 0°5~
4. ○ Must be O: 0.002% or less and T i +7
r 10RFM: 0.01% or less, the remainder is substantially composed of Fe, and Ca is present in the sulfide inclusions.
It is characterized by an average content of 5 to 50% by weight. The above-mentioned steel can contain components of the following groups as optionally added elements. o Pb: 0.005-0.050% and Bi
: 0.01-0.10% of one or two types of ox:
: 0.3-5.0%, Cr: 0.1-5°0% Rice Mo: Q, one or two or more of 05-3.0% Pb: 0.005-0.05 % and Bi: 0
．． 01 to 0.10% of one or two types, and Ni
:0.3-5.0%, Cr:0. 1-5.0%
and Mo: 0.05 to 3.0% of one or more types [Function] The functions of the above alloy components and the reason for limiting the composition range thereof are as follows. C: 0.15-0.70% In order to obtain the strength required for structural steel, 0.1
5% or more is required. On the other hand, as the content increases, the deformation resistance increases and warm processing becomes difficult, so 0.70
The upper limit is %. Si: 0.35% or less Added as a deoxidizer or lowers ductility, so 0.3%
Do not exceed 5%. Mn: 2.0% or less Mn is added in an appropriate amount for strength and hardenability, but since it is not desirable for machinability, it is kept at a small amount of 2.0% or less. S: 0.040% or less S, which is used in many free-cutting steels, is also harmful to warm workability, so it is not actively added. However, as these are unavoidable impurities, the formation of sulfide-based inclusions cannot be avoided, so the morphology of these inclusions can be controlled by adding Ca, as will be described later. The acceptable limit is 0.040%. 89: ○, 01 to 0.100% It acts to promote the precipitation of BN by fixing O in the steel and preventing the formation of B oxides, and its presence is required to be 0.001% or more. If the amount is too large, the hardenability and castability will deteriorate and there will be more ground scratches, so it should be kept at 0.100% or less. B: 0.004-0.020%, preferably 00006
~0.009% N: 0.005~0.020% N/B (weight ratio): 0.5~4.0 The free-cutting steel of the present invention ensures machinability by precipitation of an appropriate amount of BN. . Each lower limit value is the minimum addition amount necessary for that purpose. When B exceeds the upper limit, hot workability decreases, and when N exceeds the upper limit, castability decreases. If the N/B ratio does not reach the above lower limit, hot workability will be poor, and if it exceeds the upper limit, machinability will be impaired. Ca: 0.0005% to 0.0100% Necessary for controlling sulfide inclusions described below, and unless added in an amount of 010005% or more, warm workability will not be good. 0.01
%, ground scratches cannot be ignored. The reason for the impurity regulation is as follows: a″3 boiled. O: 0.0020% or less If ○ exists, B in steel will generate 8203, which will prevent the expected precipitation of BN. Therefore, O must be reduced as much as possible. Ti +7r +REM: 0.01% or less These are N
Since it easily combines with BN and forms nitrides that obstruct and cover the production of BN, only a small amount can be tolerated. As a result of the increased production and use of special steel containing these elements in recent years, care must be taken as the limits may be inadvertently exceeded unless the composition of the raw material scrap is confirmed. The reason for ensuring a certain level of Ca content in sulfide inclusions is that MnS is easily expanded as described above, whereas C
This is because aS is hardly expanded, and as the proportion of CaS increases, it becomes difficult for sulfide inclusions to be expanded as a result of the sulfide inclusions, and the degree to which the inclusions become a base for cracking during warm processing is reduced. This effect is obtained when the Ca content in the inclusions is 5% by weight or more, and is remarkable when the Ca content is 10% or more. Existence of 50% or more is not necessary. By the way, Ca@ in the sulfide inclusions makes each inclusion particle F
Mn was analyzed with PMA (X-ray microanalyzer)
, know the abundance ratio of each element of Ca and S, and calculate Ca /
Calculated as (Mn 10a +S). The effects of the optionally added elements are as follows, and their composition ranges are determined based on the reasons listed below. Pb: 0.005-0.05% and Bi: 0°01
Up to 0.10% of either one type or two types improves chip breakability and contributes to machinability.
It is preferable to use both (,74) because it creates a low melting point eutectic and produces a high effect.However, as mentioned above, it is an element that is unfavorable for warm workability, so it can be used as long as it does not cause any adverse effects. One or more of Ni: 0.3-5.0%, Cr: 0.1-5.○% A3 and Mo: 0.05-3.0%, both of which have high strength. [ Example 1] Based on 355C steel, steel to which the present invention was applied was produced. Its composition is as shown in Table 1. In the table, AJ5 and B are comparative examples, and 1 to 3 are examples. For each glue sample, 10 sulfide inclusions with a length of 5 μm or more are selected, and 6aa
was analyzed by EPMA. The machinability was measured by turning with a carbide tool under the following conditions, and taking the tool life of sample A as 1 and comparing it with that. Tool: Plo Cutting speed: 200 m/m1n -] 3- Life judgment: Flank wear 0.2 During simple warm machining, evaluated based on crack occurrence rate (%) based on J5G pull under end face restraint compression under the following conditions did. Test piece: diameter 20mm x height 30m Processing speed: 1
00mm/sec Pre-processing temperature near 00'C Processing rate - (compression amount/initial height) = 0.8 or more data are listed in Table 6. [Examples 2 to 51 Steels to which the present invention was applied were produced based on the following steels, and the same tests as in Example 1 were conducted. The alloy composition and test results are shown below. Example Base steel Alloy composition Test data 2 5
CR4-20 Table 2 Table 7 3 3CM/120
Table 3 Table 8 Table 4 3NC631 Table 4 Table 9
Table 5 3NCMI20 Table 5 In Table 10, C~
F is a comparative example, and 4 to 15 are examples. Table 6 Table 9 Table 10 [Effects of the Invention] The free-cutting steel of the present invention with excellent warm 1JrlJ properties has BN inclusions that do not impair warm workability responsible for much of the machinability. By controlling the morphology of sulfide-based inclusions so that they do not have a negative effect on warm workability and helping to improve machinability, it is possible to improve machinability and warm workability, which were previously thought to be incompatible. If desired, other machinability-improving elements can be used to roughly improve machinability, or elements that improve strength can be added.Therefore, the present invention , it can be said that it has brought improvements to a large number of steels, mainly the steel types shown in the examples (SC, SCR, SCM, SNC and SNCM), and is typically used in automobile parts, such as rods and steering-related parts. By applying the present invention to the production of these products, it is possible to improve efficiency and reduce 21-1.

Claims (4)

[Claims] (1) C: 0.15-0.70%, Si: 0.35% or less, Mn: 2.0% or less, S: 0.040% or less, Al
:0.001~0.100%, Ca:0.0005~0
．． 0100%, B: 0.004-0.020% and N
: Contains 0.005 to 0.020%, provided that N/B (
weight ratio): 0.5 to 4.0, O: 0.002% or less, and Ti+Zr+REM: 0.01% or less, with the remainder consisting essentially of Fe, with sulfide-based intervention. A free-cutting steel with excellent warm workability, characterized by containing an average of 5 to 50% by weight of Ca. (2) C: 0.15-0.70%, Si: 0.35% or less, Mn: 2.0% or less, S: 0.040% or less, Al
:0.001~0.100%, Ca:0.0005~0
．． 0100%, B: 0.004-0.020% and N
: Contains 0.005 to 0.020%, provided that N/B (
weight ratio): 0.5 to 4.0, and in addition, Pb:
0.005-0.050% and Bi: 0.01-0.
Contains 10% of either one or two, O: 0.0
02% or less, and Ti + Zr + REM: 0.01% or less, the remainder is substantially composed of Fe, and the sulfide inclusions contain an average of 5 to 50% by weight of Ca. A free-cutting steel with excellent warm workability. (3) C: 0.15-0.70%, Si: 0.35% or less, Mn: 2.0% or less, S: 0.040% or less, Al
:0.001~0.100%, Ca:0.0005~0
．． 0100%, B: 0.004-0.020% and N
: Contains 0.005 to 0.020%, provided that N/B (
weight ratio): 0.5 to 4.0, and in addition, Ni:
0.3-5.0%, Cr: 0.1-5.0% and Mo
: Contains one or more of 0.05 to 3.0%,
O: 0.002% or less, and Ti+Zr+REM: 0.
01% or less, with the remainder substantially consisting of Fe, and has excellent warm workability, characterized by an average content of 5 to 50% by weight of Ca in sulfide inclusions. Free-cutting steel. (4) C: 0.15-0.70%, Si: 0.35% or less, Mn: 2.0% or less, S: 0.040% or less, Al
:0.001~0.100%, Ca:0.0005~0
．． 0100%, B: 0.004-0.020% and N
: Contains 0.005 to 0.020%, provided that N/B (
weight ratio): 0.5 to 4.0, and in addition, Pb:
0.005-0.050% and Bi: 0.01-0.
10% of any one or two types and Ni:0
．． 3-5.0%, Cr: 0.1-5.0% and Mo:
Contains 0.05 to 3.0% of one or more kinds, O
: 0.002% or less, and Ti+Zr+REM: 0.0
1% or less, the remainder is substantially composed of Fe, and has excellent warm workability, characterized by an average content of 5 to 50% by weight of Ca in sulfide inclusions. Free-cutting steel.