JPS60215710A - Method for adjusting carbon content of molten steel - Google Patents

Abstract

PURPOSE:To obtain molten steel of stable quality by adding a proper amount of molten iron pretreated by dephosphorization and desulfurization to refined and deoxidized molten steel. CONSTITUTION:Enough oxygen is blown into a small amount of slag in a converter from the top in a refining stage to accelerate well the decarburization, dephosphorization and desulfurization of molten steel and to make the quality of the molten steel uniform, and after deoxidizing the molten steel, alloying elements are added. At this time, carbon in the molten steel is consumed by oxidation, so a proper amount of pretreated low P and low S molten iron contg. much carbon is added to the molten steel in accordance with the desired carbon content. By this method the carbon content of the molten steel can be increased to the desired value. Since the deficiency of carbon in the molten steel refined sufficiently by blowing in the converter is supplied by adding the molten iron, molten steel of stable quality is obtd. by simple operation.

Description

[Detailed Description of the Invention] The present invention relates to a method for adjusting the carbon content of molten steel, and in particular, to efficiently adjust the carbon content in molten steel with a small amount of auxiliary raw materials, and to produce molten steel of constant quality with high productivity. It is about how it can be done.

When melting high carbon steel in a converter, it is necessary to maintain a high level of blowstop [C], but in this case as well, the dephosphorization effect in the converter must not be insufficient. To meet this demand, soft blowing methods and double slug methods, which reduce the decarburization effect, are usually used. However, with these methods, ■ There is a large variation in the components after refining, and stable quality (
(1) productivity is low and a large amount of auxiliary raw materials (burnt lime, fluorite, etc.) are consumed, resulting in high refining costs. For this reason, low-phosphorus, low-sulfur, high-carbon steel is surprisingly more expensive than standard low-carbon steel, and furthermore, the variation in the content of elements, including carbon, is relatively large.

By the way, recently, hot metal pretreatment technology has been developed that sufficiently dephosphorizes and desulfurizes the hot metal at the hot metal stage in order to enable slag-free or minimal slag operation by essentially decarburizing and increasing the temperature in the converter. We have achieved certain results.

Under these circumstances, the inventors of the present invention have conducted various studies in an effort to establish a technology that can produce high-carbon steel with particularly stable quality at low cost and with good productivity. As a result, molten steel with a high carbon content can be obtained extremely easily by deoxidizing molten steel with a reduced carbon content by refining in a conventional manner, and then adding an appropriate amount of pretreated hot metal that has been previously treated with dephosphorization and desulfurization. When I realized that this was possible, I completed the invention using strings. That is, the method for adjusting the carbon content of molten steel according to the present invention is to mix an appropriate amount of pretreated hot metal that has been previously descaled and desulfurized to molten steel that has been refined and deoxidized by a conventional method, and to reduce the carbon content of the molten steel. The gist of this is to adjust the amount.

In the present invention, in the refining process using a converter, a relatively small amount of slag is used (or no slag is used) in order to increase productivity and reduce variation in the composition after refining.
After blowing a sufficient amount of oxygen over the top to allow decarburization (and dephosphorization/desulfurization) to proceed sufficiently, thereby keeping the quality of the molten steel (content of elements, blow-off temperature, etc.) as constant as possible, A ferroalloy such as ferromanganese or ferrosilicon or aluminum is added to deoxidize the ferroalloy, and further, a ferroalloy element is added as necessary.

In other words, the phosphorus and sulfur in the molten steel are sufficiently oxidized and removed by the blowing treatment for 12 minutes as described above, and the contents thereof are stabilized at extremely low values, and the blow-off temperature can also be increased.

However, in the blowing process, carbon is also oxidized and consumed as much as possible, so high carbon molten steel cannot be obtained. Therefore, in the present invention, after this molten steel is once deoxidized, an appropriate amount of separately prepared low phosphorus/low sulfur pretreated hot metal is blended in accordance with the target carbon content to adjust the carbon content. In other words, pre-saturated hot metal has not been subjected to excessive oxidation refining treatment and contains a large amount of carbon, so by increasing the amount of nuclear hot metal blended, the amount of carbon in the molten steel after blending can be adjusted arbitrarily. can be increased.

The blown molten steel to which molten pig iron is mixed needs to be deoxidized in advance as described above. If molten metal is mixed with molten steel before deoxidation, the carbon in the molten metal will mix with the oxygen in the molten steel. The reaction causes boiling (foaming), and the molten metal scatters due to the bumping phenomenon, and the accuracy of adjusting the carbon amount also decreases. Further, alloying elements may be added as necessary at any time, and may be added at an appropriate time before or after blending the hot metal. The timing of mixing hot metal with molten steel can be determined arbitrarily depending on the situation at the site, such as adding it to the molten steel in the converter that has finished deoxidation treatment, or adding it to the molten steel flow while being tapped from the converter. Alternatively, it is possible to select an appropriate method of adding it during the process after tapping the steel and before transporting it to an ingot factory or a continuous casting factory. However, the most preferable time schedule is
Hot metal is kept on standby (in a fixed or movable container with heat retention function) during the molten steel transport route to the ingot factory or continuous casting factory, and then sent to the ingot factory or continuous casting factory. The most common method is to blend and mix the hot metal. When determining the blending amount of hot metal, it is easiest to prepare the carbon content of the molten steel and the carbon content of the hot metal in advance, and then calculate the target carbon concentration according to the amount of molten steel. However, in addition to this, it is also possible to first measure the amount of carbon after blending a certain amount of hot metal, and then add the amount of hot metal that is insufficient to finely adjust the amount of carbon.

The present invention is constructed as described above, and its effects can be summarized as follows.

(1) Increasing the blow end [C] in converter blowing means:
In other words, it is equivalent to stopping the blowing in the middle, and it is difficult to adjust the blow-up [C] amount and blow-off temperature, making it difficult to obtain a product of constant quality.However, in the present invention, the converter blowing can be sufficiently progressed. Since this method makes up for the lack of carbon by mixing hot metal, the operation is simple and molten steel of stable quality can be obtained.

(2) In order to maintain the blowout (C) at a high level and to sufficiently progress dephosphorization and desulfurization, it is essential to adopt the double slag method as mentioned above, and the consumption of flux such as burnt lime. As the amount increases, productivity also decreases, but in the present invention, since the blowing progresses sufficiently, efficient conditions can be set, and therefore the 7 lux basic unit can be reduced and productivity can also be increased. . Incidentally, when comparing the burnt lime consumption and production time between the conventional double slag method and the method of the present invention, for example, in the former, the burnt lime consumption is 50 kg/(}n) and the production time is 60 minutes/(charge). However, in the latter case, it was possible to reduce the amount of burned lime consumed to 23 kg/(}n) and shorten the production time to 47 minutes/(=M-di).

(3) In the conventional high carbon steel melting process, in order to adjust the blowout [C], blowstopping and refining are required at least twice as shown in Fig. 1, which results in a large loss of thermal energy. In the present invention, as shown in FIG. 2, there is no intermediate drop in molten steel temperature and there is little temperature change, so there is little loss of thermal energy and temperature adjustment is easy.

Next, examples of the present invention will be shown.

Example 1 Converter molten steel (deoxidized) with the composition shown in Table 1 and dephosphorized and
Desulfurized hot metal was combined in the process after being tapped in a converter and before being sent to an ingot factory to prepare high carbon steel having the composition shown in the table. For comparison, a high carbon steel having the composition shown in the same table was prepared by adjusting the stopper [C] using the double slug method.

As is clear from Table 1, the molten steel obtained by the present invention after combining has no superior color in terms of composition compared to conventional high carbon steel obtained by the double slag method. Furthermore, in the present invention, the amount of carbon can be easily adjusted by adjusting the blending amount of dephosphorization/desulfurization hot metal, thereby reducing the variation in carbon amount from batch to batch, and reducing the flux during converter blowing. It was confirmed that the basic unit can be reduced and productivity can be increased.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing temperature changes from converter blowing to ingot making according to the conventional method and the method of the present invention.

Claims (1)

[Claims] A method for adjusting the carbon content of molten steel, which comprises adding an appropriate amount of pretreated hot metal that has been previously dephosphorized and desulfurized to molten steel that has been refined and deoxidized by a conventional method.