JPH06297089A - Method for continuously casting low carbon sulfur and sulfur complex type free cutting steel - Google Patents

Abstract

PURPOSE:To execute the stable operation and to enable the secureness of a good cast slab by suitably adjusting basicity of powder and beforehand adding MnO, at the time of casting a low carbon and high oxygen steel. CONSTITUTION:A sulfur fry cutting steel or a sulfur complex free cutting steel containing <=0.15wt.% C, <=0.03wt.% Si, <0.003wt.% Al, 0.015-0.025wt.% Total O is continuously cast. Then, the powder having >=1.0 basicity (CaO/SiO2 wt.% ratio) is added on the molten steel in a mold. Further, by adding the powder having >=1.0 basicity (CaO/SiO2 wt.% ratio) and <=6wt.% MnO in the molten steel in the mold, the absorption of (MnO) and (FeO) from the molten steel is restrained. By this method, i.e., by suitably adjusting the basicity of the powder and beforehand adding MnO, the execution of the stable operation and the secureness of the good cast slab can be obtd..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the structure of powder used in continuous casting of sulfur free-cutting steel or sulfur composite free-cutting steel.

[0002]

2. Description of the Related Art C≤0.15%, Si≤0.03%, M
n 0.80 to 1.30%, P 0.040 to 0.080
%, S0.200 to 0.350%, Al <0.003
%, T-O 0.015 to 0.025%, low carbon sulfur free cutting steel (JIS standard SUM22, etc.) and low carbon sulfur lead free cutting with 0.10 to 0.300% Pb added. Steel (JIS standard SUM22L, etc.) is used for parts requiring excellent machinability.

In these steel types, the C content is kept low,
The O content is increased without adding a deoxidizing element that forms a hard oxide inclusion such as Si or Al. Mn is MnS
P and N are added to improve the machinability.

These steel types are characterized by low carbon and high oxygen concentrations, and the steel contains a large amount of free oxygen and oxides mainly composed of MnO and FeO.

These free oxygen and oxides are useful along with sulfides such as MnS for improving the machinability of steel materials, but in the production of bloom billets by continuous casting, bubbles during solidification Harmful effects such as generation and melting of refractory materials.

Regarding the continuous casting method for this steel type, regarding iron and steel 67 (1981) 8, P1041 regarding continuous casting, regarding iron and steel 67 (1981) 2, A85 regarding oxygen control and improvement of the material of the immersion nozzle. Stated.

[0007]

Since low carbon sulfur and sulfur composite free-cutting steel do not contain deoxidizing elements such as Si and Al,
The molten steel in the tundish contains about 50 to 100 ppm of free oxygen (Of) and 100 to 250 ppm of total oxygen (TO).

For this reason, bubble defects such as pinholes are likely to occur in the bloom billet slab due to the CO reaction in the solidification process.

Further, the present steel type is a peritectic reaction steel having C ≦ 0.15% or less, which is characterized in that depletion and vertical cracks due to depletion are likely to occur.

According to the knowledge of the inventors, in continuous casting of low carbon sulfur and sulfur composite free-cutting steel, when MnO and FeO in the molten powder sampled from the mold were analyzed, it was found that the concentration in the powder before casting was higher than that in the powder. Is also getting higher.

This phenomenon is caused by (MnO) and (Fe in molten steel.
It is considered that O) floats in the mold during casting and is absorbed into the molten powder.

Concentration of MnO and FeO raises the oxidation potential of the powder, and therefore it is a problem that CO bubbles are generated by reacting with C in the powder according to the following formulas [1] and [2].

[0013]

Embedded image (MnO) + C = CO (g) + Mn ... [1]

[0014]

Embedded image (FeO) + C = CO (g) + Fe ... [2]

Further, the increase of MnO and FeO in the powder lowers the melting point and viscosity of the molten powder and causes non-uniform inflow as will be described later, so that heat removal in the mold is changed to cause slab surface defects such as depletion. This is a problem because it increases the number and causes breakouts.

Needless to say, the viscosity of the powder is the most important characteristic for maintaining a uniform inflow between the mold and the slab, and excessive reduction of the viscosity during casting is an operational problem. Nakano et al., Iron and Steel 69 (1983), S103.
In 6, the powder viscosity (η) for ensuring stable heat removal and performing sound solidification is given by the formula [3], which can be uniformly organized by the casting speed (Vc).

[0017]

[Chemical formula 3] η · Vc = 2.0 to 3.0 ... [3]

In the above equation [3], for example, the casting speed is Vc =
When 0.75 m / min, the viscosity is η = 2.7-
This indicates that it needs to be adjusted to 4.0 poise.

[0019]

The present invention solves the above-mentioned problems. That is, the basicity (%) of the powder in the mold used for continuous casting of the low-carbon sulfur or sulfur composite free-cutting steel.
By setting Ca Ο /% Si Ο ₂ ) to 1.0 or more,
It suppresses the absorption of (MnΟ) and (FeΟ) from the molten steel during casting.

Further, the powder having the basicity of 1.0 or more contains MnO in a range of 6% or less in advance, thereby suppressing absorption of (MnO) and (FeO).

According to the measurement of the inventors, the concentration of MnΟ and FeΟ in powder during the casting, the base of the additive powder as shown in FIG. _{1 (% CaΟ /% SiΟ 2} ) that increases the lower the There was found.

On the other hand, MnO was added to a predetermined powder to increase the MnO content, and the viscosity of the powder at 1,300 ° C. was measured for four kinds of powders. As a result, as shown in FIG. It was found that the viscosity of the powder also decreased with the increase of the MnO content.

Also in low carbon sulfur or sulfur composite free cutting steel, the lower the basicity of the powder, as shown in FIG.
The amount of MnO and FeO absorbed during casting is large, and as is clear from FIG. 2, the viscosity decreases as the MnO concentration increases.

As described above, even if the powder viscosity before casting is set within an appropriate range, there is a problem that the powder has a low basicity, the viscosity is lowered during casting and the powder falls outside the appropriate range.

In order to prevent the decrease of the viscosity, in molten steel (Mn
It is essential to suppress the concentration of MnO in the powder due to the absorption of (O) within a predetermined range, and it is necessary to keep the MnO concentration in the powder during casting within 6%.

The reason for this is that M in the powder during casting
This is because if the nO concentration is within 6%, the rate of decrease in viscosity can be suppressed within 30%, which is not a problem in actual operation.

From the above viewpoint, in the present invention, the basicity (% CaO /% SiO ₂ ) of the powder of the sulfur composite free-cutting steel is 1.
It is defined as 0 or more.

The relationship shown in FIG. 1 can be considered to indicate the relationship between the basicity of powder and the solubility of (MnO).

As is apparent from FIG. 1, the basicity is 1.0
If MnO is contained in the powder in the range of 6% or less in advance, and if the content ratio of other components is adjusted so that the powder viscosity during casting becomes an appropriate value, the MnO content in the molten steel (Mn
O) can be suppressed and Mn in the powder can be suppressed.
It is possible to secure a substantially constant and proper viscosity in which the O concentration hardly changes before and during casting.

[0030]

According to the present invention, in continuous casting of sulfur composite free-cutting steel which is a high oxygen steel, in molten steel (MnO) or (FeO)
Since it can suppress the absorption of
It is possible to prevent generation of CO bubbles due to the reaction of oxides such as O and FeO with C in the powder.

At the same time, the melting property (viscosity) of the powder during casting can be prevented from deteriorating, and the inflow amount of the powder can be kept constant, so that depletion and breakout can be avoided.

[0032]

EXAMPLES Examples will be described in detail below.

In a 270T converter, sulfur composite free-cutting steel was smelted, and a 4-strand curved continuous casting machine with a radius of curvature of 12 mR
Bloom having a slab cross-sectional size of 350 mm × 560 mm was cast at a casting speed Vc = 0.75 m / min.

The composition of molten steel in the tundish is 0.08.
% C-1.02% Mn-0.069% P-0.325%
S-90 ppm Of-157 ppm T.I. O and S
There was no i or Al.

The chemical composition and viscosity of the powder used before and during casting are shown in Table 1 together with comparative examples.

In the present invention 1, since the basicity of the powder before casting is 1.09, (MnO) and (F
There is little absorption of eO).

Therefore, the decrease in viscosity is small and η before casting =
The value of 3.5 poise is η = 2.7 poise during casting, which is within the proper range given by the formula [3].

Further, in the present invention 2, since the basicity of the powder is set to 1.06 and 4.5% MnO is added in advance, there is almost no absorption of (MnO) during casting, so that the viscosity of No change is observed and it is good.

On the other hand, in the case of Comparative Example 1 and Comparative Example 2, since the basicity is lower than 1.0, (Mn during casting)
The amount of O) absorbed is large and the viscosity is greatly reduced.

Therefore, in Comparative Example 1, the viscosity before casting must be set higher than the proper range, and in Comparative Example 2, even if the viscosity is set within the proper range before casting, the viscosity during casting is still high. Is a problem because it is off by a large amount.

From the above, the surface properties of the obtained slab were extremely good in the case of carrying out the present invention.

[0042]

[Table 1]

[0043]

EFFECTS OF THE INVENTION When casting low-carbon high-oxygen steels such as low-carbon sulfur and sulfur composite free-cutting steels, the basicity of the powder is properly adjusted, and MnO is added in advance to stabilize the stability. It is possible to carry out operations and secure good slabs, and its application effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the basicity of powder and the concentrations of MnO and FeO.

FIG. 2 is a diagram showing a relationship between MnO concentration in powder and viscosity.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Ken Sugawara 12 Nakamachi, Muroran City Muroran Works, Nippon Steel Co., Ltd. Steelworks (72) Inventor Takashi Yoshimitsu 1808-3 Hachiya, Buzen, Fukuoka Prefecture

Claims (2)

[Claims] 1. C is 0.15% by weight or less and Si is 0.0
3% by weight or less, Al less than 0.003% by weight, Tota
A powder having a basicity (CaO / SiO ₂ weight% ratio) of 1.0 or more when continuously casting a sulfur free-cutting steel or a sulfur composite free-cutting steel containing 10% by weight or more and 0.025% or less by weight. Continuous casting method for low-carbon sulfur and sulfur composite free-cutting steels, characterized by adding to the molten steel in the mold. 2. C is 0.15% by weight or less and Si is 0.0
3% by weight or less, Al less than 0.003% by weight, Tota
When continuously casting a sulfur free-cutting steel or a sulfur composite free-cutting steel containing 10% by weight or more and 0.025 or less by weight, the basicity (CaO / SiO ₂ weight% ratio) is 1.0 or more. And a powder containing MnO in an amount of 6% by weight or less is added onto the molten steel in the mold to continuously cast low carbon sulfur and sulfur composite free-cutting steel.