JPH0637684B2 - Method for producing steel in which Ti oxide is uniformly and finely dispersed - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention has a Ti oxide useful for improving the toughness, surface properties and workability of welds dispersed in steel with good reproducibility, uniformly and finely. It relates to a method for manufacturing steel having a tensile strength of 70 kgf / mm ² or less.

(Prior Art) Recently, steel containing Ti oxide is used in various fields such as thick plates, steel pipes, and thin plates. For example, in the field of thick plates, as illustrated in JP-A-61-79745, steel containing Ti oxide is very effective in improving the toughness of large heat input welds, and requires low temperature toughness. Application to high strength steel is promising. Further, in the field of thin plates, as disclosed in JP-A-62-107026, in a steel containing Ti oxide, the Ti oxide is finely and uniformly dispersed as compared with an oxide of Al, so that the surface properties and Its use is expanding due to its excellent workability.

The main method of dispersing Ti oxide in steel is to add Ti to molten steel that does not substantially contain a strong deoxidizing element such as Al. Compared with the method of directly adding and mixing Ti oxide into molten steel, this method has less problems of floating and agglomeration of oxide, and enables relatively uniform dispersion. However, it is difficult to control the amount and degree of dispersion of Ti oxide in the steel by simply adding Ti or a master alloy containing Ti to the molten steel. As a result, in the steel in which the Ti oxide is dispersed by deoxidation, for example, there is a party problem that causes fluctuations in toughness in the plate thickness direction.

(Problems to be Solved by the Invention) Although the steel in which the Ti oxide is dispersed in the steel has excellent properties even by the conventional method, the properties are further improved by further uniform fine dispersion of the Ti oxide. It seems that there is room for improvement, and for that purpose it is necessary to establish a new Ti oxide dispersion method with excellent reproducibility.

(Means for Solving the Problems) In the method of dispersing Ti oxide in steel by Ti deoxidation, the inventors of the present invention combine the steps from deoxidation of molten steel to solidification and the combination of the amounts of elements related to Ti oxide. Etc. in detail, simply
Compared to the case of adding Ti as a final deoxidizing element in steel containing no Al, a Ti oxide is uniformly and finely dispersed, and it has come to invent a technique capable of finely dispersing the gist thereof. For steel with a tensile strength of 70 kgf / mm ² or less, perform preliminary deoxidation of molten iron, add alloys, adjust the composition, and
Of 0.006% or less, the molten steel temperature is changed from 1560 to 1640.
After holding at ℃ for 10 minutes or more, Ti is 0.005 ~ as final deoxidation.
It is a method for producing a steel in which Ti oxide is uniformly and finely dispersed, which is characterized in that it is added within a range of 0.020%, and is steeled and solidified within 30 minutes.

The details will be described below. First, the inventors of the present invention did not substantially contain Al melted in a vacuum melting furnace and added Ti as a final deoxidizing element (hereinafter, steel manufactured in this manner
The relationship between the oxygen content of the deoxidized steel) and the content and size of Ti oxide contained in the steel ingot was investigated. As a result, the amount and size of Ti oxide contained in the steel ingot mainly correlates with the amount of oxygen in the steel ingot, and the amount of oxygen in the steel ingot is 30 to 1
It was found that the best Ti oxide dispersion state was obtained when the amount was about 00 ppm. However, it is difficult to control the oxygen content even in the vacuum melting method, which is considered to be more suitable for controlling the components than other melting methods, and the oxygen content in the steel ingot largely varies depending on the ordinary method. For example, in FIG. 2, C = 0.05 to 0.1%, Si = 0.05 to 0.20%, Mn = 0.5 to 1.
Ti-deoxidized steel containing about 5% is used for 150-300 by the vacuum melting method.
It is the figure which showed the fluctuation | variation of the oxygen analysis value at the time of casting in a kgf ingot. It was found that the amount of oxygen fluctuated within the range of about 15 to 200 ppm despite the fact that the dissolved raw material was kept constant, which eventually resulted in variations in the material properties. It was also found that the oxygen content in the steel ingot is closely related to the oxygen concentration in the molten steel before Ti addition.

Then, the present inventors have examined the melting process in more detail, and have found that the amount of oxygen in the molten steel, and thus the amount of oxygen in the steel ingot, is related to the melting process, particularly the temperature of the molten steel. FIG. 1 shows the results of investigation of changes in oxygen concentration in molten steel when the temperature of the molten steel was kept as constant as possible after preliminary deoxidation and component adjustment. The amount of oxygen in the molten steel and the amount of oxygen in the steel ingot after solidification are almost in a one-to-one relationship, but when variations are taken into consideration, the oxygen amount of 30 to 100 ppm, which is the optimum range, is definitely obtained in the steel ingot. In order to achieve this, the oxygen content in the molten steel before solidification is 40 to 90.
It must be contained within a narrower range of ppm. As shown in the results of FIG. 1, it is clear that the molten steel temperature needs to be maintained in the range of 1560 to 1640 ° C. in order to fall within this oxygen amount range.

Further, from the results shown in FIG. 1, it is clear that if the molten steel temperature is too high, the fluctuation of the oxygen content during holding is large and it becomes difficult to stabilize the Ti oxide content and the degree of dispersion. Further, in this case, as the holding time becomes longer, the oxide is aggregated and coarsens and floats, so that the Ti oxide amount tends to gradually decrease.
Therefore, the size of the Ti oxide in the steel ingot tends to be coarse, which is not preferable. On the other hand, when the molten steel temperature is as low as 1540 to 1560 ° C, it is difficult to secure the required amount of oxygen in the molten steel, and as a result, the amount of Ti oxide becomes insufficient. Further, if the temperature of the molten steel is too low, there is a concern that it may hinder the operation, which is a problem.

Furthermore, the result of FIG. 1 shows that the molten steel temperature is simply 1560 to 1640.
It also shows that the use of only ° C is not sufficient. That is, it is clear from FIG. 1 that the oxygen amount fluctuates greatly when the holding time is short even in this temperature range. For the purpose of a method with good reproducibility, it is obvious that even if the amount of oxygen is within the target range, it is not preferable if the amount of oxygen varies with time. From FIG. 1, it is necessary that the holding time after the component adjustment is 10 minutes or more.

As shown in Fig. 3, the fluctuation of the oxygen content in the steel ingot when melted according to the above conditions is much smaller than that by the normal method of Fig. 2, and most of them are within the oxygen content range of 30 to 100 ppm. The effect of the present invention is clear.

From the above, in order to ensure the amount of Ti oxide in the steel ingot with good reproducibility and to finely disperse it, the oxygen content in the steel ingot, and therefore in the molten steel before Ti addition, should be reproducible within the optimum range. It is necessary to make a good hit, and for that purpose the molten steel temperature after composition adjustment
It was concluded that the temperature should be kept as much as possible at a constant temperature in the range of 1560 to 1640 ° C. and the holding time should be 10 minutes or more.

(Action) From the above, preliminary deoxidation of molten iron in Ti deoxidized steel, it is essential to maintain the molten steel temperature after composition adjustment at 1560 to 1640 ° C. for 10 minutes or more, but various limitations are further imposed on the present invention. It is necessary, and the reason is described.

The present invention contains Ti oxide, and has an effect irrespective of steel type in steel types expected to have effects on various properties thereof,
In high-strength steel having a tensile strength of more than 70 kgf / mm ² , the effect of Ti oxide cannot be expected, or in some cases, it is harmful. For example, it is only in the case of steel with low hardenability that the generation of intragranular ferrite by Ti oxide can be promoted to improve the toughness of the heat-affected zone of welding, and the high tensile strength exceeding 70 kgf / mm ² In the steel, even if it contains Ti oxide, the hardenability of the steel is too high, so that the formation of intragranular ferrite cannot be expected, and in some cases, there is a risk of deterioration of toughness due to the oxide. Therefore, the present invention is limited to steels having a tensile strength of 70 kgf / mm ² or less, which can be clearly expected to have improved properties due to an increase in the amount of Ti oxide and fine dispersion.

When Al is added, stable Al oxides are generated and the formation of Ti oxides is hindered, so positive addition is not preferable, of course, but even when mixed as an impurity element, 0.00
If it exceeds 6%, problems will occur in the formation of Ti oxide, so Al
Must be limited to 0.006% or less.

Ti is naturally an essential element in forming Ti oxide, and
005% addition is required. However, if it exceeds 0.020%, even if other conditions are within the scope of the present invention, the Ti oxide becomes coarse, which may adversely hinder the toughness and workability. Therefore, 0.005 to 0.020% of The range was set. Ti to add
Either of pure Ti or a master alloy containing Ti may be used.

Further, it is preferable that the steel is tapped and solidified immediately after the addition of Ti. That is, when the staying time after the addition of Ti until the tapping time becomes long, immediately after the addition of Ti, even if a fine Ti oxide is contained in the molten steel, agglomeration coarsening, and further oxidation due to levitation This is not preferable because it causes a decrease in the quantity. In the present invention, the amount of Ti oxide in the steel ingot does not cause a large change in the degree of dispersion and does not impose a load on the process, so that the staying time after Ti addition until tapping is within 30 minutes. Limited

Since the Ti oxide after solidification is very stable, it can be used by adjusting the desired shape and strength by any method after being tapped and solidified by the method of the present invention. That is, as-cast steel ingot or continuous cast slab, as hot-rolled, after hot-rolling quenching and tempering, hot-press rolling direct quenching and tempering, and other heat treatment, and further using a suitable method such as hot forging. Even if the shape and strength are adjusted, the effect of Ti oxide is not diminished.

(Example) In Table 1, the production results of Ti deoxidized steel slabs of various components (slab thickness 300 mm) produced by continuous casting, and the Ti oxidation observed in the observed cross-section 1 mm ² at various positions of the slab at that time were observed. The results of investigating the number of objects and the particle size are shown. The number of Ti oxides and the particle size were determined by image analysis processing of characteristic X-rays of Ti and oxygen by a computer (CMA apparatus).

Steels 1 to 8 were produced by the method of the present invention, and despite the fact that the steels of various chemical compositions were spread, fine pieces of slab such as top piece, middle piece, and bottom piece were produced regardless of the slab position.
40 or more Ti oxides / mm ² are obtained, and the number of Ti oxides varies little depending on the slab position, and a more uniform and fine distribution of Ti oxides is achieved compared to the conventional method. Therefore, the effect of the present invention is clear.

On the other hand, Steels 9 to 15 are comparative steels, all of which are out of the scope of the present invention, so that the number of Ti oxides in the slab is smaller than that of Steels 1 to 8 or coarse oxides are recognized, or Whether the dispersion degree varies greatly depending on the slab position,
The optimum distribution of Ti oxide is not obtained. That is, steel 9
Since it contains Al, Ti oxide is not sufficiently formed. Steel 10
Has a Ti content exceeding the range of the present invention, and since the oxygen content is also high, the number of Ti oxides is large, but coarse Ti oxides having a particle size of more than 3 μm are recognized, and an appropriate dispersion is obtained. It is hard to say the state. Steel 11 is not preferable because the molten steel temperature after the composition adjustment and before the addition of Ti is too high, so that the number of oxides in the slab fluctuates greatly and there are many coarse oxides. Conversely steel 12
Since the molten steel temperature is too low, the number of Ti oxides is small,
The effect of Ti oxide cannot be expected enough. Like Steel 11, Steel 13 has a high molten steel temperature and a short holding time, so that a preferable dispersed state is not obtained. Since the molten steel temperature of Steel 14 is within the range of the present invention, the number of oxides is secured, but since the holding time until Ti addition is too short, the number of oxides varies greatly depending on the slab position, and the characteristics are reproduced. It is thought that there is a problem with sex. Steel 15 is suitable for the conditions up to the addition of Ti, but after the addition of Ti, the staying time until tapping is too long, so Ti oxide floats from the molten steel, and the final number of Ti oxides is small. We cannot secure the quantity.

As is clear from the above examples, when all the requirements of the manufacturing method of the present invention are satisfied, as shown in Steels 1 to 8 in Table 1, regardless of the steel type and the position of the slab, 40 pieces / mm ² It is clear that the above fine dispersion of the Ti oxide and the state in which the Ti oxide distribution varies little depending on the slab position are achieved.

(Effects of the Invention) According to the present invention, fine dispersion of Ti oxide can be stably obtained in various steel types. As a result, further improvement of various characteristics improved by Ti oxide, for example, improvement of toughness of welded portion, surface property of steel sheet, improvement of workability, and improvement of reproducibility of the characteristics can be expected, and its ripple effect is large. . Therefore, the industrial effect of the present invention is extremely remarkable.

[Brief description of drawings]

Figure 1 shows the effect of molten steel temperature and the elapsed time after the composition adjustment on the change in the amount of oxygen in molten steel in the vacuum melting process, and Figure 2 shows the amount of oxygen in the steel ingot during the normal process. FIG. 3 is a diagram showing a variation range of the above, and FIG. 3 is a diagram showing a variation range of the oxygen amount in the steel ingot when the melting process is controlled by the method of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yoshiyuki Senjiiwa 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa Inside the 2nd Technical Research Laboratory, Nippon Steel Corp. (56) Reference JP 61-284517 (JP, A) Japanese Patent Laid-Open No. 62-1811 (JP, A)

Claims (1)

[Claims] 1. A steel having a tensile strength of 70 kgf / mm ² or less is subjected to preliminary deoxidation of molten iron, addition of an alloy, and adjustment of composition, and Al
After making the molten steel 0.006% or less, the molten steel temperature is 1560 to 1640 ℃
After holding for 10 minutes or more, Ti is 0.005 ~ as the final deoxidation.
A method for producing a steel in which Ti oxide is uniformly and finely dispersed, which comprises adding 0.020% in a range, and tapping and solidifying within 30 minutes.