JP5125893B2 - Continuous casting method of ferritic stainless steel - Google Patents

Description

  The present invention relates to a continuous casting method of ferritic stainless steel, and more particularly to a continuous casting method of ferritic stainless steel for preventing generation of ridging by electromagnetic stirring.

  Ferritic stainless steel sheets are widely used in kitchen appliances and home appliances because they are excellent in corrosion resistance, can retain beautiful luster for a long time, and are relatively inexpensive. Usually, this ferritic stainless steel sheet is made of a steel plate after melting Cr-containing steel using a converter, electric furnace, etc., vacuum refining, making it a steel piece by continuous casting or ingot casting, etc. However, when used, it is often subjected to secondary processing such as press processing. However, when ferritic stainless steel sheet is subjected to secondary processing, surface irregularities (wrinkles) called ridging are generated on the steel sheet surface, which not only impairs the appearance of the surface but also causes the origin of minute cracks. Therefore, it is necessary to remove irregularities by means such as polishing, which leads to a decrease in product productivity and an increase in cost.

  This ridging is caused by the coarse dendrite solidification structure developed inside the steel slab at the time of casting, and the effect remains in the crystal structure after becoming a steel sheet. This is a phenomenon caused by a difference in behavior, and is particularly likely to occur in ferritic stainless steels in which the phase transformation does not occur in the manufacturing process after casting and the crystal structure tends to be coarse.

  Measures for preventing ridging include a method of making the solidified structure fine equiaxed or a method of making the structure fine by rolling-recrystallization. As the former method, a precipitate such as TiN is generated at the time of solidification, or a method of performing electromagnetic stirring at the time of continuous casting, and as the latter method, a strong reduction is performed at the time of hot rolling, or a cold rolling is performed. A method for increasing the number of times of rolling down is known. However, if precipitates such as TiN are generated excessively, or if rolling is reduced during rolling or the number of times of rolling is increased, surface flaws tend to occur and surface quality is degraded.

On the other hand, as a method for preventing ridging by electromagnetic stirring, for example, in Patent Document 1, when casting a ferritic stainless steel molten steel by a continuous casting method, the superheating temperature of the molten steel in the intermediate vessel is 15 to 25 ° C, and A method has been proposed in which an electromagnetic stirrer is installed at a position 1.5 m to 3.0 m below the slab hot water surface during casting, and the unsolidified phase of the slab is stirred at a stirring thrust of 60 mmHd or more. Patent Document 2 describes that when continuously casting ferritic stainless steel, the casting temperature of the molten steel is set to (solidification point + 25 ° C) or higher, and the solidification in the slab is performed under a stirring thrust of 45 mmHd to 130 mmHd. There has been proposed a method in which a part is electromagnetically stirred and the direction of stirring is changed in forward and reverse directions at intervals of 10 to 30 seconds.
JP-A-52-47522 JP 54-125132 A Okano et al .: "Iron and Steel", 61 (1975) 69

  However, in the method described in Patent Document 1, it is difficult to control the superheated temperature of the molten steel in the intermediate container to 15 to 25 ° C, and the molten steel is solidified in the nozzle that supplies the molten steel to the mold because the superheated temperature is low. There is a risk. Further, in the method described in Patent Document 2, the flow of molten steel in the mold is greatly disturbed by changing the stirring direction in the forward and reverse directions, and non-metallic inclusions and the like cannot be levitated and removed, which may reduce the surface quality. .

  An object of the present invention is to provide a ferritic stainless steel continuous casting method that can easily control the molten steel temperature by electromagnetic stirring and can prevent ridging without deteriorating the surface quality.

As a result of studying a continuous casting method of ferritic stainless steel for preventing ridging by electromagnetic stirring, the present inventors have found the following.
i) If the composition of ferritic stainless steel is optimized and the molten steel flow velocity at the solidification interface in the mold is within the range of 5 to 60 cm / sec by electromagnetic stirring, the molten steel temperature can be easily controlled and the surface quality The generation of ridging can be prevented without lowering.

  The present invention has been made on the basis of such knowledge, in mass%, C: 0.01% or less, Si: 0.03-0.3%, Mn: 0.1-0.5%, P: 0.05% or less, S: 0.01% or less , N: 0.005 to 0.015%, Cr: 20 to 25%, Ti: 0.2 to 0.5%, and the steel flow rate at the solidification interface in the mold by electromagnetic stirring The present invention provides a continuous casting method of ferritic stainless steel, characterized in that casting is performed in a range of 5 to 60 cm / sec.

  In the continuous casting method of the ferritic stainless steel of the present invention, it is preferable that the steel further contains Cu: 0.3 to 0.8% and Ni: 0.1 to 0.8% by mass%.

  The method of the present invention makes it possible to produce a ferritic stainless steel sheet that is easy to control the molten steel temperature by electromagnetic stirring and that does not cause ridging without deteriorating the surface quality. In addition, since the solidified structure after casting becomes fine equiaxed crystal by the method of the present invention, it is possible to reduce the load on the reduction rate and the number of reductions in the subsequent rolling process.

  Details of the present invention will be described below.

1) Composition (“%” below represents “% by mass”)
C: 0.01% or less
If the C content is 0.01% or less, the solidification temperature will increase, the solidification temperature will rise, and it will be easy to cause overcooling to promote equiaxed crystallization of the solidified structure, prevent the formation of Cr carbides, and improve the corrosion resistance. The effect of increasing the amount is remarkably expressed. Therefore, the C content is 0.01% or less.

Si: 0.03-0.3%
Since Si is an element useful for reduction and deoxidation of Cr oxide generated during refining, the amount is set to 0.03% or more. On the other hand, when the Si content exceeds 0.3%, the workability of the steel sheet deteriorates. Therefore, the Si content is 0.03 to 0.3%.

Mn: 0.1-0.5%
Since Mn is an element useful for deoxidation and strengthening, its amount is set to 0.1% or more. On the other hand, if the amount of Mn exceeds 0.5%, the amount of precipitation of MnS increases, resulting in deterioration of pitting corrosion resistance or an increase in cost. For this reason, the amount of Mn is made 0.1 to 0.5%.

P: 0.05% or less
If the P content exceeds 0.05%, the toughness, hot workability and corrosion resistance are significantly deteriorated. For this reason, the amount of P is set to 0.05% or less, but the smaller the amount, the better.

S: 0.01% or less
When the amount of S exceeds 0.01%, as with P, toughness, hot workability and corrosion resistance deteriorate significantly. For this reason, the amount of S is set to 0.01% or less, but is preferably as small as possible.

N: 0.005 to 0.015%
N precipitates as TiN at the time of solidification, and is an element useful for preventing the generation of ridging by finely equiaxing the solidified structure. In order to obtain such an effect, the N amount needs to be 0.005% or more. On the other hand, if the amount exceeds 0.015%, precipitates are excessively generated and surface defects are generated, resulting in a decrease in surface quality and a significant decrease in toughness. For this reason, the N amount is set to 0.005 to 0.015%.

Cr: 20-25%
Cr is an element useful for ensuring the corrosion resistance and oxidation resistance of ferritic stainless steel. In order to obtain such effects, the Cr content needs to be 20% or more. On the other hand, when the amount exceeds 25%, so-called brittleness at 475 ° C. is likely to occur, and the toughness is significantly reduced. For this reason, the Cr content is 20 to 25%.

Ti: 0.2-0.5%
Ti precipitates as TiN during solidification, and is a useful element for preventing ridging by fine equiaxed crystallization of the solidified structure. Also, it forms a carbide by combining with C, and it has workability, corrosion resistance and toughness. It also has an effect of improving. In order to obtain such effects, the Ti content needs to be 0.2% or more. On the other hand, when the amount exceeds 0.5%, Ti oxides and nitrides are excessively generated to generate surface defects, which deteriorates the surface quality. Therefore, the Ti amount is set to 0.2 to 0.5%.

  The balance is Fe and inevitable impurities, but it is preferable to further contain Cu: 0.3 to 0.8% and Ni: 0.1 to 0.8% for the following reason.

Cu: 0.3-0.8%
Cu is an element that improves the corrosion resistance of ferritic stainless steel. In order to obtain such effects, the Cu content needs to be 0.3% or more. On the other hand, if the amount exceeds 0.8%, a low melting point Cu compound is formed in the hot rolling process (at the time of heating or rolling), surface defects are generated, and the surface quality is deteriorated. For this reason, the amount of Cu is made 0.3 to 0.8%.

Ni: 0.1-0.8%
Ni is an element that improves the corrosion resistance of ferritic stainless steel. In order to obtain such effects, the Ni content needs to be 0.1% or more. On the other hand, if the amount exceeds 0.8%, depending on the C content, an austenite phase is generated in the hot rolling process, and a two-phase structure of ferrite + austenite is formed, and the corrosion resistance deteriorates. Therefore, the Ni content is 0.1 to 0.8%.

2) Molten steel flow velocity at the solidification interface in the mold by electromagnetic stirring: 5-60cm / sec
As disclosed in Patent Documents 1 and 2, a technique for preventing the occurrence of ridging by finely equiaxing the solidified structure using electromagnetic stirring has been studied conventionally, and serves as a driving force for so-called stirring thrust. It is considered to be more effective as the applied magnetic field intensity is increased. However, if the magnetic field application intensity is simply increased, the flow of molten steel becomes intense and may involve non-metallic inclusions, mold powder, etc., reducing the surface quality. It is difficult to improve the surface quality together. Therefore, the present inventors have examined that if the molten steel flow velocity at the solidification interface in the mold is controlled to 5 to 60 cm / sec instead of the magnetic field applied strength, the solidified structure can be obtained without causing the inclusion of non-metallic inclusions or mold powder. It was found that fine equiaxed crystallization can be prevented and generation of ridging can be prevented. This is because when the molten steel flow rate is less than 5 cm / sec, the effect of washing the solidification interface is reduced, nonmetallic inclusions, mold powder, etc. are likely to adhere to the solidification interface, and when the molten steel flow rate exceeds 60 cm / sec, This is because non-metallic inclusions, mold powder, and the like are involved and the surface quality is lowered.

  Here, the flow rate of molten steel at the solidification interface in the mold can be controlled by controlling casting conditions such as the magnetic field application intensity of electromagnetic stirring, the immersion nozzle discharge flow rate, the molten steel throughput, and the mold width. The molten steel flow velocity at the solidification interface in the mold can be obtained by a method described in Non-Patent Document 1 that is calculated from the inclination angle of the dendritic dendrites of the solidified structure. Moreover, the control becomes possible if the relationship between the above casting conditions and the molten steel flow velocity is obtained in advance.

  Hot metal was inserted into the converter for decarburization and refining, and after finishing decarburization and degassing using VOD, Al was added to deoxidize. Next, Ti was added, and the resulting molten steel was placed in a ladle and transported to a continuous casting facility. Therefore, slabs were manufactured by casting molten steel from a ladle into a continuous casting mold. The components of the obtained slab are as shown in Table 1. During continuous casting, the molten steel flow velocity at the solidification interface in the mold was controlled as shown in Table 1 by electromagnetic stirring.

  The 220 mm thick slab thus produced was roughly rolled into a rough bar by reverse rolling to a thickness of 35 mm, and then hot rolled to a thickness of 4.0 mm with a tandem finish rolling mill to produce a hot rolled steel sheet. The hot-rolled steel sheet was continuously annealed at 950 ° C., then pickled, cold-rolled to a thickness of 1.0 mm, and continuously annealed at 900 ° C. to produce a cold-rolled steel sheet. And the ridging characteristic and surface quality were evaluated with the following method with respect to the obtained cold-rolled steel plate.

  Ridging characteristics: Two JIS 13B tensile test pieces specified in JIS Z 2201 are collected from cold-rolled steel sheets in parallel with the rolling direction, and one side of the test piece is polished with No. 600 abrasive paper, and the strain is 20%. The waviness height (ridging unevenness) of the polished surface at the center of the test piece was measured with a roughness meter. The average waviness height of the two was less than 5 μm without ridging (◯), 5 to 15 μm was ridging small (Δ), and more than 15 μm was ridging large (×).

  Surface quality: The surface of the cold-rolled steel sheet is visually observed, and 2 or less of the ridge-like surface defects observed per 1000 m of the cold-rolled steel sheet are good (○), 3-5 pieces are acceptable (△), 6 More than one piece was judged as defective (x), and the case of ○ was regarded as the present invention. Here, the ridge-like surface defects caused by precipitates and the like are evaluated, and surface defects caused by ridging are excluded.

  The results are shown in Table 1. It can be seen that the cold-rolled steel sheets Nos. 1 to 7 produced by the method of the present invention have excellent ridging characteristics and surface quality.

Claims (2)

  In mass%, C: 0.01% or less, Si: 0.03-0.3%, Mn: 0.1-0.5%, P: 0.05% or less, S: 0.01% or less, N: 0.005-0.015%, Cr: 20-25%, Casting steel with a composition containing Ti: 0.2-0.5% and the balance consisting of Fe and inevitable impurities so that the molten steel flow velocity at the solidification interface in the mold is in the range of 5-60 cm / sec by electromagnetic stirring. A method for continuous casting of ferritic stainless steel.   2. The method for continuous casting of ferritic stainless steel according to claim 1, wherein the steel further contains Cu: 0.3 to 0.8% and Ni: 0.1 to 0.8% by mass%.