JPH01149917A - Manufacture of steel ingot with very slight segregation of phosphorus - Google Patents

Abstract

PURPOSE:To manufacture a steel ingot with very slight segregation of phosphorus by adding Ca to molten steel before solidification to fix S and O, adding a specified amt. of a rare earth element and immediately solidifying the molten steel. CONSTITUTION:The amts. of S, O and P as impurities in molten steel are reduced to <=about 0.008% S, <=about 0.005% O and <=about 0.01% P before solidification and about 0.002-0.023% Ca is added to the molten steel to fix S and O. A rare earth element is then added by an amt. satisfying an inequality 0.05<[wt.% REM]/[wt.% P]<=4 (where wt.% REM is the concn. of the rare earth element in the steel and wt.% P is the concn. of phosphorus in the steel) in accordance with the concn. of P in the molten steel and immediately the molten steel is solidified. A steel ingot with very slight segregation of phosphorus is obtd. with the existing equipment.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing a steel ingot with extremely low phosphorus segregation, and in particular aims to effectively prevent the segregation of phosphorus that is inevitably mixed into molten steel. It is something.

(Prior art) Recently, it has been confirmed that ultra-low sulfurization improves ductility, anisotropy, and lamellar tear resistance in steel plates for thick plates, and low-temperature toughness has been improved in low-temperature steel plates used as pipeline steel plates due to low phosphorus content. known to improve. As described above, reducing the impurity concentration has become a major trend in order to improve the quality of steel materials. However, sulfur and phosphorus tend to segregate during the solidification process of the rope, so
Reducing segregation is a major technical challenge. This is because sulfur and phosphorus cause harmful effects such as causing brittle fracture in steel materials or propagating cracks at their segregation sites.

For this reason, efforts have been made to reduce the above impurity elements by improving not only the hot metal pretreatment process but also the refining process such as flux injection in the converter and Ca addition, as well as optimizing the casting temperature, electromagnetic induction stirring, Particularly in continuous casting, efforts are being made to reduce their segregation by reducing the distance between rolls and controlling the solidification process such as light reduction casting.

Note that the above-described segregation reduction method attempts to reduce the segregation of sulfur, phosphorus, etc. all at once, and there has been no conventional method that attempts to reduce them individually.

(Problems to be solved by the invention) Improvements in the refining process such as the hot metal pretreatment process, flux injection in the converter, and Ca addition, as well as optimization of the casting temperature, electromagnetic induction stirring, reduction of the roll spacing, and light weight. With regard to sulfur, it is possible to achieve a sufficiently low sulfurization through reduction casting, etc., and the concentration of sulfur in the segregated portion is sufficiently reduced to a level where there is no problem.

However, with regard to phosphorus, it is impossible to hope for a satisfactory reduction in the concentration, so at present it cannot be said that the concentration in the segregated areas after solidification has been sufficiently reduced.

The present invention advantageously solves the above-mentioned problems by using existing equipment without modifying or improving various casting equipment required to reduce segregation. The purpose is to propose a manufacturing method that can obtain continuously cast pieces (the same shall apply hereinafter).

(Means for Solving the Problems) In the present invention, when solidifying molten steel to produce a steel ingot, Ca is first added to the molten steel before solidification to fix S and O, and then P in the molten steel is fixed. Depending on the concentration, the following formula 0.05 < (w
tχREM) / [tχP]≦4 wtχRU here
M: Concentration of rare earth elements in steel tχP: A method for producing a steel ingot with extremely low phosphorus segregation, which is characterized by adding rare earth elements in an amount that satisfies the concentration of phosphorus in steel, and solidifying immediately thereafter.

Here, the rare earth elements (hereinafter abbreviated as REM) are:
In particular, La, Ce+Pr, Nd, etc. are suitable.

First, the background to the elucidation of this invention will be explained.

Conventionally, the addition of REM to molten steel was carried out in Japanese Patent Publication No. 53-12333.
As disclosed in Japanese Patent Publication No. 54-15523, this method has been used to form REM sulfides during the solidification process. This means that S as an impurity is M
This is to prevent the formation of so-called A-based inclusions called nS, where stress concentration occurs and causes cracks.

Like this RI! Although M has continued to attract attention due to its relationship with S, the inventors focused on the relationship between REM and P and completed this invention.

Regarding the relationship between REM and P, no thermodynamic data have been reported, so the ability of REM to produce phosphide has not been clarified at all. Therefore, as shown in Table 1, the inventors added La, Ce, and Mitsushi metal (La: 30%, Ce: 50%, Nd: 20%
) were added individually, and the solidified steel ingot cross section was subjected to EPMA
I subjected it to inclusion analysis using

The analysis results thus obtained are also listed in Table 1.

As is clear from the table, it was observed that REM phosphides were precipitated in the steel ingot. From this, it can be said that RE
It is thought that when M is added, REM phosphides are formed in the molten steel and the solid solute Pe content is reduced, thereby reducing the P concentration in the final solidified zone and, in turn, reducing P segregation.

By the way, it has been known that REM and 0 and REM and S have strong chemical affinity and form compounds in molten steel. The formation of sulfides by the addition of REFI mentioned above takes advantage of this property.

Therefore, it is presumed that if REM is directly added to molten steel, REM oxide or REM sulfide will be generated, making it difficult to generate REM phosphide.

As a result of repeated studies to avoid this problem, the inventors found that the molten steel first had 0. Ca, which has a greater affinity for S than REM, is added to fix O and S, and then R
By adding EM, effective utilization for REMO dephosphorization was achieved.

(Function) Regarding impurity components in molten steel, S is 0.0% before addition of Ca.
If it is more than 0.08%, when Ca is added, inclusions will accumulate significantly in the precipitation height of the steel ingot or in the inclusion accumulation area around 1/4 thickness of the continuously cast piece, impairing the mechanical properties of the steel material. S needs to be 0.008% or less.

Similarly, if the content of 0 before Ca addition is more than 0.005%, oxide inclusions may occur in the precipitated high part of the steel ingot or in the continuously cast piece.
AI! , it is necessary to perform deoxidation treatment and the like to reduce 0 to 0.005% or less.

If Ca is less than 2%, Ca oxides and Ca sulfides are difficult to form, and the REM added later is
REM is consumed in the production of EM oxide and 178M sulfide.
0.00 because it no longer contributes effectively to the production of phosphides.
It is desirable to contain 2% or more. On the other hand, S is 0.00
8% and 0 is 0.005%, Ca oxide (Cab
), and the amount of Ca required to generate Ca sulfide (CaS) is 0.023%, so Ca is 0.023%.
% or less.

Furthermore, if the P content before REQ addition is more than 0.01%, 0.
Although it is necessary to add a large amount of REM exceeding 0.04%, RE
M is an expensive element, and adding a large amount will increase the cost of steel products, so P is kept at 0.01% or less in REM.
It is desirable to suppress the amount of addition.

If 12EM is less than 0.001%, it is difficult to generate REM phosphide, so it is desirable to set it to 0.001% or more. On the other hand, as mentioned above, if it exceeds 0.04%, it will increase the cost of steel products, so it is desirable to suppress it to 0.0/1% or less.

Furthermore, R1? In order to investigate the relationship between the amount of M and the concentration of P,
The following experiment was conducted.

100 kg high frequency vacuum induction melting furnace Te C: 0.05~
0.08%, Si: 0.10-0.25%, Mn
: 1.0-1.5%, P: 0, 01-0.05%,
S: less than 0.001 to 0.005%, AI
:0.02~0.05% and O:0.001~0.0
Various steels with a composition of 0.5% are melted, and Ca is added to the molten steel.
was contained in an amount of 0.003 to 0.007%, and then Mitsushmetal was contained in an amount of 0.002 to o.oos%. Note that for some steels, Ca and REM were not added for comparison.

Regarding the steel ingot obtained by pouring the molten steel into a sand mold, the parts shown in Figure 2 were subjected to line analysis using a macro analyzer, and the segregation rate Pmax/Po (the value obtained by dividing the maximum concentration by the average concentration) of P was calculated. did. These results are shown in Table 2. Note that if the segregation rate is less than 4, there will be no practical problem.

SJ (molten steel tapped from a 180-ton converter is alloyed and deoxidized by RH treatment, C: 0.02-0.10%
.

Si: 0.06-0.25%, Mn: 0.8
~1.0%, P: 0.02-0.03%, S:
Less than 0.001, full to 0.002%, Al: 0.0
Molten steel with a composition of 2 to 0.04% and O: 0.001 to 0.003% is obtained, and further Ca is added in the form of a CaFe alloy at the end of the RH treatment to adjust the Ca concentration and degree to 0.002 to 0.00.
It was set at 5%. Thereafter, the molten steel in the continuous casting tandy was made to contain 0.002 to 0.02% Mitshu metal.

In addition, for some steels, Ca, RE
No M was added. The cross section of the continuously cast slab C thus obtained was subjected to analysis using a macro analyzer, and the segregation rate of semi-macro segregation observed at the center of the thickness of the slab was calculated. The results obtained are shown in Table 3.

Regarding experiments 1 and 2 above (wt%REM) / [w
t%P] and the segregation rate Pmaχ/Po is shown in FIG. 1.

As is clear from the figure, (wt%REM) / Cw
Good results with a segregation rate of less than 4 were obtained in the range of t%P〉0°05. In other words, 0.05< (wt
%REM)/(wt%P), phosphorus segregation can be suppressed. On the other hand, REM phosphide produced by the addition of REM is a compound with an atomic ratio of REM and P of 1:1, so from the relationship of atomic weight, it is
Adding REM in an amount about 4 times as much as P is sufficient for formation of phosphorus. Therefore, the relationship between the amount of REM and the P concentration should satisfy the following formula (% formula %).

(Example) Steel having the composition shown in Table 4 was melted in a 100 kg high-frequency vacuum induction melting furnace, and the molten steel was made to contain Ca and then Mitshu metal. Regarding the steel ingot obtained by casting the molten steel into a sand mold, the parts and macroscopic areas shown in Figure 2 are as follows.
It was subjected to line analysis using an analyzer, and the segregation rate of P was calculated.

Table 4 also shows the Ca, REM content, and segregation rate.

As is clear from the table, the conforming example A according to the present invention is
Comparative example A or Ca without adding Ca and REM
Compared to Comparative Example B in which REM was added without addition, phosphorus segregation was significantly reduced.

The molten steel tapped from a 180-ton converter was alloyed and deoxidized by R1 (processing) to obtain molten steel having the composition shown in Table 5, and at the end of the R1+ process, Ca was added in the form of CaFe0. After that, the molten steel in the continuous casting tandy was made to contain Mitsushi metal.The continuous casting slab C obtained in this way
The cross section was analyzed using a macro analyzer, and the segregation rate of semi-macro segregation observed in the center of the steel thickness was calculated.

The amounts of Ca and REM added and the segregation rate are also listed in Table 5.

As is clear from the table, the conforming example A according to the present invention is
Segregation rate is small. In addition, in Comparative Example A, in which REM was added to the tandem dish without Ca addition, REM was consumed by S and O, and (wt% REM )/(wt%
No improvement in P segregation is seen, reflecting that P)<0.05.

(Effects of the Invention) According to the present invention, a steel ingot with extremely low phosphorus segregation can be obtained by simply adding Ca and then REM to molten steel using existing equipment without requiring modification of the casting equipment. Can be done.

[Brief explanation of the drawing]

Figure 1 shows the segregation rate Pmax /Po (value obtained by dividing the maximum concentration by the average concentration) and [&4t%REM) / [w
t%P] FIG. 2 is a diagram showing the parts of the steel ingot prepared in Experiment 1 that were subjected to macro analyzer analysis. Figure 1 Cwt%REM/[WtgaP]

Claims (1)

[Claims] 1. When producing a steel ingot by solidifying molten steel, first add Ca to the molten steel before solidification to fix S and O, and then calculate the following equation according to the P concentration in the molten steel. 0.05<[wt%REM
]/[wt%p]≦4 where wt%REM: Concentration of rare earth element in steel wt%p: Rare earth element is added in an amount that satisfies the concentration of phosphorus in steel, and is then immediately solidified. A method for manufacturing steel ingots with extremely low phosphorus segregation.