JPH03183721A - Calcium treatment of molten steel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a calcium treatment method for molten steel containing A!l and S, which renders non-metallic inclusions present in the molten steel harmless, This makes it possible to produce bloom billets of good quality with few inclusion defects in continuous casting.

(Conventional technology) In continuous bloom billet casting for rods and wire rods, the cross-sectional size is smaller than in continuous slab casting, so there are restrictions on the installation space for the immersion nozzle in the mold, and it is difficult to control the amount of molten steel injected. Casting has been carried out using small-diameter submerged nozzles in order to improve properties. Recently, aiming to reduce costs by omitting the blooming process, which is a post-process of continuous casting,
Furthermore, continuous billet casting has been developed that uses a small cross-sectional size and a small diameter immersion nozzle.

Molten steel for rods and wire rods generally needs to be deoxidized by method A, but in this case, 8fhOs, which is a deoxidation product, is unavoidably present and grows (enlarges) on the inner wall of the immersion nozzle during casting. This causes nozzle clogging and becomes an impediment to stable casting, as well as Al12 peeled off from the inner surface of the immersion nozzle.
0. Grains (AR20s that have become agglomerated to a large size) may be the cause of inclusion defects in the slab. This tendency is particularly noticeable in the case of small-section casting, that is, casting using a small-diameter immersion nozzle. In order to establish a small-section casting technology, preventing Affi20s from adhering to the inner surface of a small-diameter immersion nozzle was an important technical issue.

^ff120 to the inner surface of the immersion nozzle. Measures to prevent adhesion include ■ technology to remove R20 from molten steel, and ■ AjhOs.
Techniques are being considered to modify the inclusions into particles that are less likely to adhere to the inner surface of the nozzle. Of these, ■ is explained in detail in pages 11 to 15 of the 126th and 127th Nishiyama Memorial Technical Lectures on High-Purity Steel, published by the Japan Iron and Steel Institute in November 1988, and there are also various examples in actual industrial processes. A method that combines the following elemental technologies has been adopted. However, among these methods, there are almost no techniques that have a sufficient ability to remove AQ203. This is because the behavior of Rin 203 is based on complex physicochemical phenomena, and furthermore, because ALOs are extremely fine, it is difficult to separate them from molten steel.

On the other hand, details regarding (1) are described in "Calcium Steel" published by Maruzen Co., Ltd. in April 1981, pages 81 to 83. According to it, the mainstream reforming technology is to add Ca to molten steel and convert high melting point Ant's to low melting point 12Ca.
This is a method of reforming to O.7AU20s, which creates A20. This prevents the adhesion of However, even with this method, sufficient modification is often not achieved in the case of molten steel for rods and wires. The reason for this is that molten steel rods and wire rods contain 0.01% by weight or more of S, and for this reason, the addition of Ca improves the A-texture 203 and at the same time increases the Ca content.
This is because CaS grows and adheres to the inner surface of the immersed nozzle, inducing nozzle clogging. Although there is a proposal in Japanese Patent Application Laid-open No. 7322/1983, it is not sufficient. Furthermore, CaS composite inclusions peeled off from the inner surface of the nozzle cause product inclusion defects.

The inventors of the present invention conducted various studies in order to deal with such problems, and filed an application on November 22, 1989 entitled "Continuous casting method for bloom and billet by calcium treatment" (hereinafter referred to as
We have invented a method for suppressing the formation of CaS by controlling the ratio of total Ca concentration/child center concentration in molten steel according to the O concentration in molten steel, as in the previous application.

(Problems to be Solved by the Invention) The present invention is a further development of the method of the earlier application, and focuses on the generation of CaS during the reaction process, and presents a method for stably suppressing the generation of CaS to a low level. It is something to do. As a result, product inclusion defects (for example, defects in ultrasonic flaw detection, etc.) can be significantly improved, and nozzle clogging can be completely prevented.

(Means for Solving the Problems) The gist of the present invention is that when calcium is added to molten steel containing 0.100% by weight or less, and 0.150% or less, By controlling the addition rate of calcium according to the C content according to the following formula, the generation of CaS can be controlled as much as possible.
The direction of the rod and wire rod is to refer to the calcium concentration of molten steel, 1vc, ≦-25 [0C] + 35 in the formula [! kC]; C content in molten steel (wt%) VC&:
Calcium Addition Rate (for g/a+in/1 on-steel) The details of the present invention will be described below.

The present inventors further developed the method described in the previous application and
We sought a method to suppress the generation of the result,
In the vicinity of the Ca addition position in the molten steel, that is, when Ca is added in the iron-coated Ca wire, the wire immersed in the molten steel melts, and in the area where Ca is supplied into the molten steel,
It has become clear that this is due to the fact that the Ca concentration is extremely high compared to other molten steel parts. If a region with a high Ca concentration is formed in this way, there will be excess Ca in excess of that used for the modification of ^X, 03, and the excess Ca will contribute to the generation of CaS. This result was obtained by analyzing inclusions in molten steel sampled from the vicinity of the C8 addition point in the molten steel.

(From the above knowledge, we came up with a method to control the Ca addition rate as a means of preventing the formation of a high Ca concentration region near the Ca addition point in molten steel, and conducted an iron-coated Ca wire addition experiment on a scale of 120 tons and The appropriate range of Ca addition was clarified by inclusion analysis of molten steel sampled from the vicinity of the Ca addition site.

Figure 1 shows the results, and shows the C content and C content of molten steel.
The appropriate range was shown in relation to the addition rate. In the figure, the straight line A corresponds to Vca = -25C]+35, but when Vc->-25[C]+35, the generation of CaS becomes noticeable, which is not preferable. That is, VCa≦−25[0C
]+35, the ratio of inclusions compounding CaS is 5% or less, whereas when Vca>-25 C]+35, the ratio increases to 30% or more. The inclusion ratio in which CaS is combined was determined by randomly extracting 100 inclusions from the steel and conducting a composition analysis using an X-ray microanalyzer. The reason why the straight line A can increase the VCa at low [96C] is that the O content in the molten steel is high at low [96C] and the conditions are such that it is difficult for CaS to form.

On the other hand, in the range of VCa≦−25 [0c1◆35,
If the VCa, that is, the Ca addition rate, is too slow, the processing time will become longer and the temperature will drop to a temperature at which continuous casting is not possible, which is undesirable.As a guideline for actual operation, a high [zero C] solidification temperature is undesirable. Considering the low point, VCa≧-15 [kC]
It needs to be +15.

From the above, it should be controlled within the range of VCa≦-25[0C)+:+S, and the more preferable range is -15
[Desk] ◆15≦VCa≦-25 [Desk 1◆35.

Note that the Ca source added in the present invention is not particularly limited, and VCa may be set within an appropriate range depending on the Ca content of each additive. As a Ca source, iron solution Nc
A wire, Ca-51 alloy, etc. can be used. Zarani C
The method of supplying a is also not limited, and can be applied to a wire addition method, a blowing method using an immersion lance, etc. Furthermore, molten steel to which the present invention can be applied [zero Cl is 0.01 to 1.0
It is 0%.

When [96C] becomes 1% or more, ^figs in the molten steel decreases, and the necessity of Ca treatment becomes weak.

In the present invention, the ranges of A2 and S in steel are defined for the following reasons.

Sentence A is necessary for adjusting the grain size of steel, but it is 0.10
Even if it is added in excess of 0%, the crystal grain size adjustment effect will be saturated, so the upper limit is set at 0.100%.

S is necessary from the perspective of imparting machinability to steel bar wire rod products used for machine structural steel or cold forging steel, etc.
If the content exceeds 0.150%, the mechanical properties will deteriorate, so the upper limit is set at 0.150%.

When the present invention is combined with the method of the prior application, the effect of producing CaS becomes more pronounced.

(Example) Examples of the present invention are shown below.

When producing 120 ton/heat rods and wires using a converter and degassing equipment, converter slag is removed at the stage of tapping the steel from the converter to the ladle, and then the steel is heated in the ladle. After forming a non-oxidizing slag and performing composition adjustment and dehydrogenation, a wire with an outer diameter of 13
mmφ iron-coated Ca wire (Ca content in filling 9
2% by weight> was added. In the examples, a total of 1o heat was conducted, but the Ca addition conditions were as shown in the Example column of Table 1.
And so. The molten steel after Ca treatment has a slab cross-sectional size of 162 mm.
A curved billet continuous casting machine with a diameter of 162 mm was used to produce a billet in the rod and wire direction.

On the other hand, as a comparative example, V as shown in Table 1.

Five heats adjusted independently of the present invention were also cast using the same continuous casting machine.

The billet obtained from each heat was 40 mm in diameter.
After rolling into a steel bar, an ultrasonic flaw detection test was performed on the steel bar product to determine the ultrasonic flaw detection failure rate due to large hard inclusions.

The results are also shown in Table 1, and the comparative example has an average defective rate of -0,
In contrast, the average defective rate of the embodiment of the present invention was -0.
, 11%, which was an extremely low level, and inclusion defects were significantly improved. Further, in the examples of the present invention, no nozzle clogging occurred at all, and there was only a small amount of nozzle deposits.

(Effects of the Invention) As described in detail above, the method of the present invention suppresses the generation of CaS during Ca treatment to a low level even in molten steel when rods and wire rods containing A2 and S are used.
O・ALl□0. The technology to modify it has been established. This has made it possible to cast small cross-sections of molten steel rods and wires without clogging the nozzle. Furthermore, the small cross-section slab obtained by the present invention is of good quality with few inclusion defects, and is extremely useful for the steel industry.

[Brief explanation of drawings]

Figure 1 shows the relationship between molten steel C content and Ca addition rate.
2 is a drawing showing a preferable range in which generation of CaS can be prevented. 4 others

Claims (1)

[Claims] 1. When adding calcium to molten steel containing Al; 0.100% by weight or less, S; 0.150% by weight or less,
A calcium treatment method for molten steel characterized by controlling the addition rate of calcium according to the C content in molten steel according to the following formula V_C_a≦−25[%C]+35 where [%C]; C content in molten steel (% by weight) V_C_a; Calcium addition rate (g/min/ton steel).