JPS5940202B2 - Manufacturing method of ultra-low S1 free-cutting steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing low-Si free-cutting steel, particularly ultra-low-Si free-cutting steel with Si content of 0.005 or less and excellent machinability and cold workability. be.

The most commonly known types of free-cutting steel are sulfur- or lead-added sulfur-free-cutting steel and lead-cutting steel.
a1 Ca free-cutting steel added with special elements such as Ti and Zr, T
i-Ca free-cutting steel, Zr free-cutting steel, etc. have also come to be manufactured and used in recent years.

By the way, several years ago, the use of lead free-cutting steel, which is highly versatile in terms of machining, began to be restricted due to environmental health issues, and as a result, sulfur free-cutting steel has come to be recognized once again.

In this sulfur free-cutting steel (containing composites of other elements such as P), an appropriate amount of MnS is interposed in the steel, and the machinability is improved by the internal notch effect of this MnS. It is as shown in the JIS standard in Table 1.

Hereinafter, the present invention will be described in detail focusing on the above-mentioned sulfur free-cutting steel, but it goes without saying that the present invention is not limited to this and can be similarly applied to other types of free-cutting steel.

Now, the machinability of free-cutting steel is generally evaluated based on tool life,
This is done depending on the amount of tool wear, cutting resistance, cutting processability, etc., but especially in the case of sulfur-cutting steel, in order to improve the above-mentioned rigidity, it is necessary to make the shape of MnS in the steel larger and rounder. is essential.

Further, as a means of controlling the shape of this MnS, it is said that it is effective to suppress the Si content in the steel as low as possible.

On the other hand, the lower the Si content in the steel, the better the cold workability such as cold pressability (cold forging property).

Therefore, when manufacturing free-cutting steel, it is necessary to keep the Si content in the steel to a low level from both machinability and cold workability.
lJ, and maintaining it is positioned as an extremely important technical issue.

Generally, sulfur free-cutting steel is produced by charging (tapping) molten steel that has been refined and melted in a steelmaking furnace such as a converter into a ladle lined with acidic bricks such as chamotte or waxite. eMn-F
After deoxidizing and adjusting the composition by adding ferroalloys such as eS2 and FeP, this molten steel is poured into a mold to form a steel ingot, which is then rolled and subjected to secondary processing. Conventionally, in order to maintain Si in steel at a low level, measures such as (1) use of low-Si alloy iron and (2) reduction of 81 in the steelmaking furnace have been adopted at the above-mentioned steelmaking and molten steel processing stages.

However, even with these countermeasures, Si in steel (Si under assembly)
It is difficult to stably reduce the value (value) to 1010% or less, and in reality there is a variation of 0.006 to 0.026%, much less to achieve and maintain an extremely low level of 0.005% or less. This was not possible in actual operation.

Therefore, in view of the limitations of the above-mentioned methods ① and ②, the inventors of the present invention have conducted various experiments and research, paying particular attention to the molten steel processing environment. It was determined that the contamination of Si from the refractory lining of the ladle used was the cause of inhibiting the reduction of Si in the ladle.

As a result, we have arrived at the present invention, and its gist is that molten steel refined and melted in a steelmaking furnace is made of A1203-8iO2-based refractory (with a SiO2 content of 30% or less), in whole or in part, of the lining refractory wall. ), CaO-Al2O:, -8i02-based refractory (SiO2 content 30% or less), MgO-Al2O
35i02 series refractory (SiO2 content 30% or less), C
aO5102 refractory (SiO2 content 50% or less),
CaO-(VgOS) i02-based refractory (SiO2 content 50% or less), MgO-8i02-based refractory (SiO2 content 6K) In a refractory container composed of one or more selected refractories,
It consists in a method for manufacturing sulfur free-cutting steel, which involves charging and processing.

Refractories are generally classified as acidic, neutral, or basic, and acidic ones often have a high content of SiO2, such as chamotte and waxite-based refractories, and high-alumina refractories SiO2 is also blended in neutral refractories such as aluminum and basic refractories such as dolomite refractories and magnesia refractories.

These refractories do not consist of only a single oxide, but usually exist as a binary or ternary mixture.

As is well known, the crystal structure of each oxide in these mixtures is explained using a binary or ternary phase diagram, but in a multi-component system, it depends on their mixing ratio. Various eutectic compounds exist.

Therefore, in the case of alumina-silica-based refractories, for example, single oxides (hereinafter referred to as single crystals) such as At203 and SiO The taste crystals and the crystals of the eutectic compound (3At203.2Si02: mullite in the above example) exist in a mixed state.

That is, considering SiO2, under a mixed system, Si
In addition to existing as a single crystal of O2, 3AL20
It exists as eutectic compounds such as 3.2SiO7 and 3CaO-8iO2.

However, 5i02, which has a high SiO2 content of 75 to 80% and exists essentially as a single crystal, such as chamotte and waxy refractories that have been used in the past, has extremely oxidizing activity (in this specification, activity Since the Mn level is high, especially in sulfur free-cutting steel (when it comes into contact with 0.70% slightly molten steel, it is easily reduced to Si by itself, and 5102+2
A reaction shown by Mn-+81+2Mn0 occurs to oxidize Mn, and Si is released into the steel, eventually suppressing the production of MnS.

On the other hand, the activity of SiO2 in the eutectic compound is small, and it is extremely rare for it to be released into molten steel as free Si.

In the end, reducing the SiO2 activity in refractories is
This study was conducted with the main focus on reducing the SiO2 activity in refractories, as this was found to be extremely important in achieving this goal.

However, if even a small amount of SiO2 single crystal exists in the refractory, when its absolute amount is compared with the amount of Si in molten steel,
In terms of chemical equilibrium, it is inevitable that the amount of Si increases on the refractory side, which ultimately acts to increase the amount of Si in the molten steel.

Therefore, in the present invention, it was decided to use a refractory having a composition in a region where SiO2 single crystals are substantially absent.

Also, as is clear from the phase diagram itself, the melting point of the eutectic compound is generally low, and the composition should not be such that the refractory will melt at the handling (operating) temperature of molten steel.

For this reason, in the present invention, the refractory according to the present invention is specified as a refractory having a composition in which substantially no single crystal of 5i02 is present in the treatment temperature range, and at least one selected from the group consisting of these refractories is specified. We decided to form a fireproof container using one type of refractory material.

In consideration of actual operation, this processing temperature range specifically refers to a temperature range of 1460'C to 1760C.

Next, the range that satisfies the conditions of the present invention regarding the multi-component refractories will be explained.

Figures 1 to 3 show phase diagrams for a binary system, and Figures 4 to 6 show phase diagrams for a ternary system. The white circles in the figures mean eutectic compounds, and their compositions are appended to each.

Note that in FIGS. 4 to 6, isothermal lines are omitted to avoid complication.

First, in FIG. 1, the Si02 content in the region where 3A403.2SiO2 is formed is about 30%, and A403-
It can be seen that in the case of 8i02 type (that is, alumina type), if the composition contains more than 30% of SiO□, 5t02 single crystals will be present and the LSiO2 activity will increase, which is disadvantageous for the present invention.

Therefore, in this system, the SiO2 content must be 30% or less.

To the same effect, in the CaO-8i02 series shown in Fig. 2, the SiO2 content must be 50% or less, and furthermore, in the MgO-8i02 thread (magnesia series) shown in Fig. 3, the SiO2 content must be 50% or less.
2 content must be 60% or less.

In the CaO-A4035in2 system shown in Fig. 4, each eutectic compound Ca(l S 102-CaOA403 ・28102
-3A/! , it must satisfy the composition in the area below the line connecting 203 and 2S102,
Although the limit content of SiO3 varies depending on the content ratio of CaO and At203, it should be at least 50% or less, and particularly preferably 30% or less.

In the CaO-MgO5in2 system (dolomite system) shown in Figure 5, each eutectic compound CaO-8iO2-Ca0.2SiO
It must satisfy the composition in the region below the line connecting 2Mg()SiO2, and the limiting content of 5i02 varies depending on the content ratio of CaO and MgO, but it should be at least 50% or less. be.

In the MgOA4035i02 system shown in Figure 6, almost each eutectic compound MgOMgO-8iO2-2.2A403.5S
It must satisfy the composition in the region below the line connecting i 02-3A403/2SiO3, and the content of SiO2 changes depending on MgO and At203, but it must be at least 60% or less. In particular, it is desirable to set it to 30% or less to be sure.

There are no particular restrictions on the type of fireproof container that uses a refractory that satisfies the above compositional conditions, but considering the current steelmaking processing system, there are particularly noticeable be effective.

However, when considering application to a ladle, for example, installing a refractory having the above composition on the entire surface of the refractory wall inside the ladle would result in an unnecessarily high burden.

Therefore, in order to investigate a refractory-lined structure that would be practically satisfactory, we used waxite bricks with high SiO2 activity (SiO2 Ni 8%, A403: 20%) and alumina bricks with low SiO2 activity (Si02:16'AA4, 03:6 types of ladle refractory walls are constructed by combining 82% and 82% less in appropriate ratios, and molten steel blown and melted in a converter is charged into these ladles.
Deoxidation and component adjustment were performed using ferroalloys such as eMn, FeS2, and FeP, and actual tests were conducted to produce sulfur free-cutting steel.

Table 2 shows the analytical values of the chemical composition (assembled composition) of the free-cutting steel thus obtained.

(However, the average value (X) other than the Sj group) Figure 7 shows the specific structure of the ladle fireproof wall used in this manufacturing test (phase "U" in Table 2 1 to ■). Bricks with hatchings in the figure indicate parts using alumina bricks that satisfy the refractory composition of the present invention, and bricks with dots indicate parts using waxite bricks that do not satisfy the compositions of the present invention.

As is clear from the results in Table 2, when the refractory that satisfies the conditions of the present invention is applied to at least 65% (specifically, a small area or more of the bottom area of the ladle side (■ to ■)), Si in the steel The average content is 0.006% or less, and the maximum content is 0.009%.
% or less, and the minimum value is 0.003% or less, which maintains an extremely low level compared to the conventional type (I), and the variation is also very small.
% or more, it turns out that free-cutting steel with an extremely low Si level of 0.005% or less can be stably obtained.

In addition, from the viewpoint of the Si level in the steel, 100% refractory material that satisfies the conditions of the present invention (■υ is better, but after use, it is necessary to remove the slag and metal that have formed and solidified at the bottom of the ladle. Furthermore, considering the cost of the refractory, it is more preferable to use the refractory (V) in which the above refractory is applied to the bottom of the ladle, and the rest is made of conventional refractories.

Now, the sulfur free-cutting steel obtained through the manufacturing test was taken as a sample, and the steel according to the present invention (component: C: 0.0
8%, Mn: 1.15%, P: 0.072%,
S: 0.308%, S word 0-003%, ladle structure; 2nd
Table M) and conventional method (component: C: 0.09%
, Mn: 1.13%, P: 0.075%, S: 0.
310%, Si: 0.008%, ladle structure; Table 2 I)
The results of a comparative test of machinability and cold pressability are shown in FIGS. 8 and 9.

From Figure 8, the number of pieces cut is 10 for both tool wear amount and finished surface roughness.
00 or more, the conventional material gradually increases, but the inventive material shows almost no change, which proves that the inventive material has excellent machinability.

Furthermore, from FIG. 9, it can be seen that the material of the present invention, regardless of whether it is unbound or bundled, has less cracking even at a high compression rate and has high cold compressibility compared to the conventional material.

As explained above, according to the present invention, it is possible to stably produce ultra-low Si free-cutting steel with a Si content of 0.01% or less, particularly 0.005% or less, by relatively easy means. This provides the remarkable effect that a free-cutting steel with excellent machinability and cold workability can be obtained.

[Brief explanation of the drawings] Figures 1 to 6 are state diagrams, Figure 7 is a schematic diagram showing a structural example of a fireproof wall in a ladle, Figure 8 is a graph showing machinability test results of free-cutting steel, FIG. 9 is a graph showing the results of a synchronous cold pressure test.

Claims (1)

[Claims] 1. All or part of the force of the lining refractory ζ A1203-8
iO2-based refractory (S t 02 content 30% or less),
CaOAl 203 S t 02 series refractory (S t
02 content 30% or less), Mg0-A i203-
S i 02 series refractories (SiO2 content content 3 king figures (SiO3 content 50% or less), Ca 0-Mg
O-8 i 02 series refractory (SiO2 content content 5
Ultra-low Si free machining characterized by processing by charging molten steel melted in a copper furnace into a refractory container made of one or more refractories selected from the group consisting of Method of manufacturing steel.