JP2005248317A - Method for producing low nitrogen middle carbon boron steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a melting method for obtaining a boron steel for driving shaft of a car, excellent in quenchability by minimizing the development of boron nitride in molten steel. <P>SOLUTION: S content in the molten steel is desulfurized into ≤0.005 mass% and also, N content in the molten steel is made to be ≤120 ppm by ladle-refining the molten steel melted in an electric furnace with the ladle and further, the molten steel is degassed with an RH degassing apparatus and furthermore, after making the N content into ≤60 ppm with the denitrification by adding Ti to make the contained nitrogen into titanium nitride, the boron steel containing 0.0005-0.005 mass% B, is refined by adding B. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a low nitrogen medium carbon boron steel in which the formation of boron nitride in the medium carbon boron steel is suppressed.

  Conventionally, there is boron steel as a material mainly used for drive shafts related to automobiles. This boron steel is made of steel suitable for drive shafts by adding boron as a steel component to improve the hardenability of the steel. However, in this boron-added boron steel, if free-N is present in the steel, boron is easily combined with nitrogen to form a nitride composed of boron nitride, and if the boron nitride increases in the steel, the hardenability of the steel is sufficient. The problem that it cannot be secured arises. Therefore, usually, a method of adding about 0.01 to 0.05% of Ti and fixing free-N is adopted (for example, see Patent Document 1), but this makes the austenite crystal grains coarse. The amount of AlN to be suppressed is reduced and mixed grains or coarse grains are formed at the time of carburizing, and there is a problem that adversely affects the mechanical properties, fatigue characteristics, and strain amount at the time of heat treatment (see, for example, Patent Document 2 and Patent Document 3). .

  Therefore, in order not to generate boron nitride in the steel, it is necessary to solve the problem of adding about 0.01 to 0.05% of the above Ti and to perform a low nitrogen operation in refining the molten steel. .

Japanese Patent Laid-Open No. 2-145745 JP-A-9-217144 JP-A-10-130720

  The problem to be solved by the present invention is a melting method in which boron nitride is not generated in the steel in order to produce boron steel having excellent hardenability, and boron nitride can be generated in the molten steel as much as possible. An object of the present invention is to provide a method for melting boron steel that is excellent in hardenability by low nitrogen operation.

  Molten steel is melted in an electric furnace, and the resulting molten steel is refined in a ladle to desulfurize the S content in the molten steel to 0.005% by mass or less and increase the N content in the molten steel during refining. In addition to degassing using a degassing apparatus such as RH, for example, denitrification is performed by adding titanium to titanium nitride.

  In the denitrification using the above RH degassing apparatus, S is reduced to 0.005% or less before degassing with the RH degassing apparatus as described above in order to denitrify faster. After denitrification to 90 ppm or less, preferably 80 ppm or less using an RH degasser, Ti is added before B is added, finally B is added, and finally the nitrogen content is 60 ppm or less. , Cast into boron steel.

  The principle of the above means will be described. Since the denitrification reaction is a reaction at the interface of the molten steel, the less the surface activation element S, the faster the denitrification. Therefore, the S content in the molten steel is desulfurized to 0.005 mass% or less in advance in ladle refining. The desulfurized molten steel is further denitrified by a vacuum degassing method such as an RH degasser. In this case, in addition to this denitrification by degassing, by adding Ti, Ti is combined with N to be denitrified by becoming TiN. By adding B in the final stage after this denitrification, it becomes difficult to form BN nitride, and a molten steel which is boron steel with sufficiently ensured target hardenability can be obtained.

  Typical steels to which the melting method according to the present invention can be applied are based on carbon steel for machine structure (SC) and alloy steel for machine structure (SMn, SCr, SCM, SNC, SNCM), and B and It is boron steel added with Ti. Note that boron steel in which B and Ti are added to steel whose components are improved based on these JIS standards is also applicable.

  The present invention smelts the molten steel melted in an electric furnace to desulfurize the S content in the molten steel to 0.005 mass% or less, and the N content in the molten steel during refining is 120 ppm or less, After degassing with an RH degassing apparatus and adding Ti to nitrogen containing titanium nitride to denitrify the N content to 60 ppm or less, B is added at the final stage, so that B becomes BN Boron steel that can be refined as boron steel containing 0.0005 to 0.005% by mass of the necessary B without being taken into the formation of the steel, and has sufficient hardenability for drive shafts of automobiles is obtained. Has an excellent effect.

  The best mode for carrying out the present invention will be described. For example, the component of the hot water which melted the molten steel shown in Table 1 with the electric furnace is shown. Furthermore, the molten steel is deoxidized by ladle refining in a ladle and the S content in the molten steel is desulfurized to 0.005% by mass or less, and the increase in the N content in the molten steel during refining is 120 ppm. The hot water shown in Table 2 is obtained by suppressing to the following. Since the management of N varies depending on various factors such as temperature, type and amount of alloying elements, slag components and amount, and the area of the interface with hot water, the amount of N is originally measured for each refining process. The operating time should be managed, but it is only necessary to know the operating time when the N content is 120 ppm empirically for each steel type and finish the refining by that time.

  Subsequently, the molten steel shown in Table 2 is subjected to RH degassing for 30 minutes by an RH degassing apparatus to obtain hot water shown in Table 3, and Ti and further S and B are added to this hot water. This hot water is easy to denitrify because the S content is reduced to 0.005% by mass or less. When 15 minutes have elapsed after the start of the degassing treatment, the N content is 80 ppm as shown in Table 3. We were able to reduce to.

  Further, when 15 minutes have elapsed, Ti is added to denitrify the nitrogen contained in the molten steel to titanium nitride, and finally the N content is 55 ppm as shown in Table 4 and the N content. 60 ppm or less was achieved. Then, by adding B, it refine | purifies to boron steel containing 0.0005 to 0.005 mass% of B. In this embodiment, S is added in consideration of machinability. However, when the alloying element of S is added, it may be added at the end of the degassing step so as not to inhibit desulfurization. In the examples of the present invention, it was added together with B.

  Next, the produced molten steel shown in Table 4 was cast as a slab by continuous casting to obtain an automotive part having improved hardenability, for example, boron steel which is a steel material suitable for a drive shaft.

  As a comparative example, the case where the sulfur content in the molten steel exceeds 0.005% by mass and the desulfurization is completed is shown. As shown in Table 5, the components of the hot water in the comparative example at the time of steel removal from the electric furnace are the same as those of the hot water in the examples.

  In the hot water before degassing after ladle refining, as shown in Table 6, the S content is 0.007% by mass, the N content is 120 ppm or less, and the N content is almost the same as in the examples. The same, but desulfurization is insufficient.

  For this reason, in the degassing after ladle refining, it dropped to 80 ppm in 15 minutes and completed the degassing in 30 minutes in total, but in the comparative example, as shown in Table 7, it reached 92 ppm in 15 minutes. However, it could only be denitrified, and it took 45 minutes to reduce the N content before Ti addition to the same level as in the example, and as a result, it took more time for denitrification than in the example.

Claims (1)

  The molten steel melted in an electric furnace is smelted in a ladle to desulfurize the S content in the molten steel to 0.005% by mass or less and the N content in the molten steel to 120 ppm or less. After degassing with a gas device and adding Ti to nitrogen content to make titanium nitride, the nitrogen content is reduced to 60 ppm or less, and then B is added. A method for refining boron steel containing 0.005% by mass.