KR101249164B1 - Method for manufacturing continuous casting of low carbon martensitic stainless steel having excellent surface quality - Google Patents

Abstract

Provided is a method for producing cast steel of low carbon martensitic stainless steel having excellent surface quality.

This manufacturing method is a method of producing a cast piece by continuously casting martensitic stainless steel in which the primary delta ferrite phase is solidified with delta ferrite and then austenite phase is formed.

In the continuous casting, the primary cooling rate of the cast steel up to the temperature at which the formation of the austenite phase is completed and the secondary cooling rate of the cast steel after the formation of the austenitic phase are completed are controlled.

According to the present invention, it is possible to manufacture a healthy cast by preventing the surface cracks of the cast steel and linear defects due to hot rolling in advance of low-carbon martensitic stainless steel.

Martensitic stainless steel, continuous casting, surface cracks, ship defects, cooling rate

Description

Method for manufacturing continuous casting of low carbon martensitic stainless steel having excellent surface quality}

The present invention relates to a method for producing cast steel of low carbon martensitic stainless steel. More specifically, when the low carbon martensitic stainless steel is manufactured by continuous casting, cracks are prevented from the surface of the cast steel and the linear defects generated during hot rolling can be prevented. The present invention relates to a method for producing cast steel of excellent low carbon martensitic stainless steel.

Generally, disc brakes for motorcycles require wear resistance and corrosion resistance. Suitable materials include low carbon martensitic stainless steel.

13Cr-0.2C 420J1 steel containing a large amount of carbon or 420J2 steel with 13Cr-0.3C has high temperature dependence of hardening hardness, so to use it as a motorcycle disc brake, it needs to be additionally processed after hardening. Not only does it lead to an increase in cost, but also a decrease in corrosion resistance due to the chromium deficient zones formed around the chromium carbonitride, which is created as a side effect of the annealing process.

In order to compensate for these problems, when using 16Cr-0.3C steel, hardness can be secured only by quenching treatment. In this case, strict heat treatment standards are required, and recently, low carbon martensitic stainless steel is mainly used.

In addition, martensitic stainless steel has an austenite structure or a mixed structure of austenite and ferrite at a high temperature, and when slowly cooled, the phase transformation into a ferrite structure is caused by diffusion, but when rapidly cooled so that diffusion does not occur, the martensite structure becomes phase transformation into a martensite structure. do.

Particularly, martensitic stainless steels for disc brakes containing 12% Cr and (C + N) of about 0.07% are solidified to austenite after solidification begins with ferrite when the solidification starts after steelmaking. After the transformation, the ferrite is transformed into ferrite again and the cooling is completed. The ferrite structure has a martensite and ferrite abnormal structure in which residual austenite which is not plastic transformation into ferrite is transformed.

However, when martensitic stainless steel has a mixed structure of austenite and ferrite, there is a problem in that hot workability is lowered due to cracking at the austenite / ferrite interface.

In addition, since 12% Cr-0.07% (C + N) steel rapidly causes martensite transformation during the solidification process, the surface cracks are liable to occur during the rolling process, and the hot workability may decrease depending on the reheating temperature during hot rolling. Cracks are also likely to occur.

In order to prevent such cracks, in the past, when the continuous casting of martensitic stainless steel is slowly cooled, the grinding may be performed, but this method has a problem of causing a decrease in the error rate and an increase in manufacturing cost.

The crack that determines whether it can be manufactured is a linear defect detected after pickling in a hot rolling process. Since these defects are fatal defects in the cast steel and the surface quality of stainless steel, where the surface quality is important, the price increase factor is needed because additional processes are required to remove the defects such as property tax and grinding. It is becoming. In order to prevent such ship defects, various studies have been made from the continuous casting process to the hot rolling and annealing process.

In the hot rolling process, in order to prevent such defects, it is important to suppress ferrite phase precipitation in the surface layer of the slab in the heating furnace. To this end, when martensitic stainless steel casts are cracked in a heating furnace, they are reheated to 20 ~ 200 ℃ lower than the boundary temperature between the austenitic single phase region, the ferrite phase, and the austenitic two phase region to maintain cracking for more than 2 hours and less than 10 hours. After rolling, a method of rolling is proposed (Japanese Patent Laid-Open No. 6-78567).

However, since the above method cannot prevent surface defects in steel grades having high carbon content and requires investment in a heating furnace, it is difficult to completely remove the linear defects by the above-described method.

Looking at the method of preventing the surface defects of the cast during continuous casting, a method of securing the surface quality of the cast by controlling the molten steel flow by applying an electromagnetic field in the mold has been proposed and some practical use. For example, a method (CAMP-ISIJ 9 (1996), P206) of applying an electromagnetic field to a mold to control molten steel flow in the mold and at the same time raising the bath surface temperature has been put into practical use. However, these methods are a technique applied to carbon steel during high-speed casting, there is a limit in preventing defects occurring in the surface portion of the cast steel in the case of stainless steel that strictly requires the surface quality of the cast due to the problem of not completely solve the instability of the hot water.

In the manufacturing method of martensitic cast steel made of 13Cr-0.2C 420J1 steel or 13Cr-0.3C 420J2 steel containing a large amount of carbon, it is possible to reduce the surface defects of the cast steel by controlling the cooling rate in the secondary cooling zone during continuous casting. However, in the case of low-carbon martensitic steel, the transformation temperature of martensite is different, and since it does not go through a solid reaction during the solidification process, a more appropriate control method is required.

The present invention has been made to improve the above-mentioned problems. In the continuous casting without additional equipment, cracks and hot rolling occurring on the surface of the slab of low carbon martensitic stainless steel by appropriately controlling the cooling rate of the slab in the secondary cooling zone. It is an object of the present invention to provide a continuous casting method that can prevent the occurrence of line defects.

In the present invention for achieving the above object, in the method for producing a cast cast by continuously casting martensitic stainless steel in which the austenite phase is formed after the primary delta ferrite (δ-ferrite) solidified with delta ferrite, Castings of low carbon martensitic stainless steel with excellent surface quality to differentially control the primary cooling rate of the cast steel to the temperature at which the austenite phase is formed during continuous casting and the secondary cooling rate of the cast steel after the formation of the austenitic phase is completed. It relates to a manufacturing method.

In addition, the primary cooling rate is characterized in that faster than the secondary cooling rate.

In addition, in the continuous casting method of the present invention, the primary cooling rate of the cast steel up to the temperature at which the austenite phase is formed is maintained in the range of 200 ~ 500 ℃ / min, the secondary of the cast after completion of the austenite phase formation The cooling rate can be maintained by operating at 50 ° C / min or less.

In addition, the low carbon martensitic stainless steel is characterized in that it comprises 0.1% or less of carbon.

In addition, the cast steel of the present invention is characterized in that the primary delta ferrite (δ-ferrite) phase starts to solidify at the liquidus temperature and completes solidification with delta ferrite, and then the austenite phase is transformed along the interface of the ferrite phase.

As described above, according to the method for producing cast steel of low carbon martensitic stainless steel having excellent surface quality according to the present invention, the surface crack of the cast steel during continuous casting of low carbon martensitic stainless steel or the linear defect due to hot rolling is prevented in advance. Thereby, there exists an effect which can manufacture a healthy cast.

Hereinafter, the present invention will be described in detail.

According to the present invention, in the case of manufacturing low-carbon martensite steel by continuous casting, it is possible to prevent the solidification crack of the casting during continuous casting by investigating the correlation between the occurrence position of linear defects during hot rolling and the surface crack of the casting. Can be.

The metallurgical analysis of surface cracks in cast steel during continuous casting is as follows. The mechanism that acts depends on the plastic mismatch between two phases, interfacial segregation of impurity elements, the phase fraction between two phases, the distribution shape of minor phases, and the deformation direction.

Therefore, in order to suppress the surface crack of the cast steel, it is necessary to reduce the amount of segregation of solute elements and to form the phase fraction between the two phases appropriately. For this purpose, the solidification structure of low carbon martensitic stainless steel should be identified. 1 is a graph showing a state diagram of a low carbon martensitic stainless steel according to the present invention, Figure 2 is a graph showing a cooling transformation curve of a low carbon martensitic stainless steel according to the present invention. 1 and 2, it can be seen that the solidification structure of the low carbon martensitic stainless steel according to the present invention.

Step 1: During solidification, the primary delta ferrite phase is formed at liquidus temperature and solidification is completed as delta ferrite phase as the temperature drops.

Step 2: After the solidification is completed, the austenite phase is formed through δ → γ solid state transformation, and the delta ferrite phase disappears.

Step 3: After the transformation to the austenite phase, ferrite is transformed along the austenite grain boundary near 700 ° C, and has a shape surrounding the austenite.

Step 4: At 300 ~ 400 ° C, the austenite has the abnormal structure of martensite and ferrite, which is not transformed into ferrite.

Therefore, the temperature range in the solidification section is an on-tool tool in which the delta ferrite phase is generated and grown. When the cooling rate is increased, the size of the solidification structure of the delta ferrite phase initially formed is finely formed, which has the advantage of suppressing surface cracks. On the other hand, there is a disadvantage that the solute diffusion is delayed.

On the other hand, when the cooling rate is slowed, the solute is diffused, so that the content of solutes concentrated in the liquid phase is reduced, while the coagulation structure size of the delta ferrite phase is coarse. In addition, when the cooling rate is slowed to control the austenite phase, the formation of the austenite phase increases, so that segregation of solute elements is increased, which causes surface cracks. On the contrary, if the cooling speed is increased, the formation of the austenite phase is reduced, thereby suppressing surface cracks.

In conclusion, the present inventors confirmed through experiments that the formation of the delta ferrite phase and austenite phase and the solute element segregation of impurities in the cast during the continuous casting can be controlled by controlling the cooling conditions during the solidification process, the experiment The present invention has been completed based on the results.

That is, the method for producing a cast steel of the present invention is a method of producing a cast cast by continuously casting martensitic stainless steel in which the austenite phase is formed after the primary delta ferrite phase is solidified with delta ferrite,

This is achieved by differentially controlling the primary cooling rate of the cast steel up to a temperature at which the formation of the austenite phase is completed during the continuous casting and the secondary cooling rate of the cast steel after the formation of the austenite phase is completed. At this time, the primary cooling rate is characterized in that the higher than the secondary cooling rate.

On the other hand, in the process of controlling the cooling rate during continuous casting may cause problems in addition to the prevention of surface cracks can be set the limit and range of the cooling rate. Therefore, in the present invention, the primary delta ferrite (δ-ferrite) phase is solidified with delta ferrite, and then a continuous cast of martensitic stainless steel in which the austenitic phase is formed to produce a cast casting, the liquid line during the continuous casting Control the primary cooling rate of the cast steel from the temperature to the temperature at which the austenite phase is completed in the range of 200 ~ 500 ℃ / min, and the secondary cooling rate of the cast after the completion of the austenite phase formation to 50 ℃ / min or less Can be controlled.

Hereinafter, the reason for setting the cooling rate will be described in detail.

When the primary cooling rate of the cast steel from the liquidus temperature to the temperature at which the austenite phase is formed is more than 500 ℃ / min, the time for the segregation of the solute element remaining between the dendrite during the continuous casting is less Surface cracks in cast steel may occur. On the other hand, when the primary cooling rate of the cast steel is less than 200 ℃ / min there is a problem that the coarsening of the delta ferrite phase formed initially. In addition, in order to control the cooling rate below 200 ° C / min, the quantity of mold cooling and secondary cooling should be reduced during continuous casting. As a result, as the heat transfer of the cast is delayed during casting, the strength of the cast solidification layer is lowered. Bulging may occur, resulting in deterioration of operation and quality. Therefore, the primary cooling rate of the cast steel from the liquidus temperature to the temperature at which the austenite phase is formed is preferably maintained in the range of 200 ~ 500 ℃ / min.

In addition, when the secondary cooling rate of the cast steel after the completion of the austenite formation exceeds 50 ℃ / min, the amount of formation of martensite phase is increased, so that the surface cracks of the cast steel may increase as the plastic mismatch between the two phases increases. Therefore, it is preferable to maintain the secondary cooling rate of the cast steel after completion of austenite formation at 50 ° C / min or less.

In addition, the method for producing a cast piece of martensitic stainless steel of the present invention can be targeted to low carbon martensitic stainless steel based on containing C of 0.1% or less.

Hereinafter, the present invention will be described in more detail with reference to Examples.

[Example]

Using a low carbon martensitic stainless steel of the composition as shown in Table 1 below, the continuous casting cast was prepared under the production conditions shown in Table 2. Investigation of the surface crack occurrence degree of the produced continuous casting cast, the results are described in detail in the following, while the comprehensive results are shown in Table 2 below.

Chemical composition of the test material (% by weight) Cr Ni Si Cu Mn P S C N 12.2 0.3 0.4 0.3 0.4 0.030 0.003 0.055 0.020

division Cooling rate (℃ / min) Performance
Surface cracking degree Primary Secondary Inventory 1 400 40 Good Inventory 2 300 30 Good Inventory 3 500 50 Good Comparative Example 1 1000 60 Bad Comparative Example 2 700 80 Bad Comparative Example 3 100 40 Bad Comparative Example 4 175 100 Bad

Primary cooling rate: The average cooling rate from the liquidus temperature to the temperature at which the austenite phase is formed.

Secondary cooling rate: Average cooling rate from the completion of the austenite phase to the casting completion.

In the invention examples (1 to 3) satisfying the conditions of the present invention it was possible to ensure a good quality that the surface crack does not occur on the surface of the continuous casting cast and hot-rolled coil. On the contrary, in Comparative Examples (1 to 4), the primary or secondary cooling rate was out of the conditions of the present invention, and segregation of solutes occurred in the solidification, resulting in fine surface cracks of the cast steel. Due to the crack, the scale was excessively formed during hot rolling, causing linear defects on the hot rolled coil.

In the case of Comparative Examples 1 and 2, the primary and secondary cooling rates were faster than the conditions of the present invention, but there was no bulging during casting. This occurred.

In the case of Comparative Example 3, the first cooling rate is slower than the conditions of the present invention, the bulging of the cast is generated, and the hunting surface of the casting during the casting (hunting) occurred badly. In the case of Comparative Example 4, the first cooling rate is slower than the conditions of the present invention, and the second cooling rate is faster than the conditions of the present invention, and cracks of the surface layer are severe, so that grinding is necessary several times to make a healthy cast.

As can be seen from the above examples, by controlling the cooling rate in the secondary cooling zone during continuous casting, it is possible to obtain a continuous casting cast quality of low carbon martensitic stainless steel as well as to enable stable continuous casting operation. Confirmed.

1 is a graph showing a state diagram of a low carbon martensitic stainless steel according to the present invention.

2 is a graph showing a cooling transformation curve of the low carbon martensitic stainless steel according to the present invention.

Claims (4)

In the method of producing a cast piece by continuously casting martensitic stainless steel in which the austenite phase is formed after the primary delta ferrite phase is solidified with delta ferrite, In the continuous casting, the primary cooling rate of the cast steel up to the temperature at which the formation of the austenite phase is completed and the secondary cooling rate of the cast steel after the formation of the austenitic phase is completed, The primary cooling rate is higher than the secondary cooling rate, The primary cooling rate of the cast steel up to the temperature at which the austenite phase is formed is maintained in the range of 200 to 500 ° C./min, and the secondary cooling rate of the cast steel after the austenite phase is completed is 50 ° C./min or less Process for producing performance cast steel of low carbon martensitic stainless steel with excellent surface quality. delete delete The method of claim 1, The low carbon martensitic stainless steel is produced by weight of the low carbon martensitic stainless steel having excellent surface quality, characterized in that it comprises more than 0% to 0.1% of carbon.

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