KR20240056258A - Martensitic stainless steel with excellent primary carbide quality and method of manufacturing the same - Google Patents

Abstract

The martensitic stainless steel cast according to an example of the present invention has, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6. to 15.0%, Ni: 0.01 to 0.3%, the remainder includes Fe and inevitable impurities, and the area fraction of primary carbides having an area of 2000㎛ ² or more in the central segregation area is 2.5% or less. You can.

Description

Martensitic stainless steel with excellent primary carbide quality and manufacturing method thereof {MARTENSITIC STAINLESS STEEL WITH EXCELLENT PRIMARY CARBIDE QUALITY AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a cast of martensitic stainless steel with excellent primary carbide quality, stainless steel, and a method for manufacturing the same. More specifically, the present invention relates to a method of manufacturing the same, which is economical and high hardness by controlling the number of primary carbides and omitting the forging process. , Martensitic stainless steel that can be used in highly corrosion-resistant ceramics and automobile parts, and its manufacturing method.

As the standard of living improves, the application of martensitic stainless steel with high hardness, high strength, and high corrosion resistance is increasing in household cutlery and industrial band saws. These uses require high hardness, high strength, wear resistance, and corrosion resistance, and to secure high hardness and high strength, a tempered martensite structure created through strengthening heat treatment and tempering is utilized. This tempered martensite structure is a very hard structure that can be obtained by forming an austenite phase, a high-temperature stable phase, through an annealing structure (ferrite + fine chromium carbide) through strengthening heat treatment and tempering processes, and then quickly cooling it. The higher the carbon content, the more high hardness martensite structure can be secured. Meanwhile, in order to secure wear resistance properties, a certain percentage of carbides can be retained or precipitated after strengthening heat treatment.

Since the residual chromium carbide has the effect of reducing the chromium content of the matrix, compensation of corrosion resistance corresponding to the residual carbide is necessary for excellent corrosion resistance. In addition, esophagus, which requires corrosion resistance and wear resistance, contains 0.4 to 0.6% carbon content and 13.5 to 15.5% Cr content. In order to ensure the quality of the edge of a knife, the smaller the size of the remaining carbide, the better. However, hot rolled annealed materials produced through general casting, hot rolling, and batch annealing (BAF) processes contain primary chromium carbide (M ₇ C ₃ ) and shows a structure composed of chromium carbide (M ₂₃ C ₆ ), which is a secondary carbide that precipitates first along the grain boundaries during top annealing after hot rolling, and ferrite. In particular, primary chromium carbide generated at high temperature in the center of the material does not decompose during hot rolling and top annealing and remains coarsely larger than 3㎛. Additionally, as the carbon content of primary chromium carbide increases, the size becomes coarser and the number of primary chromium carbides increases.

Coarse carbides larger than 3㎛ remain as coarse carbides larger than 3㎛ because they are difficult to segment even when cold rolling is performed by applying a certain level of reduction. Coarse carbides remaining in such cold rolled materials are difficult to decompose into the matrix during strengthening heat treatment, which is mainly performed by continuous heat treatment, and remain. In addition, it is concentrated in the C and Cr segregation area in the center of the thickness, and when it remains, linear defects remain on the surface after being polished with a knife, which causes problems that prevent it from being used as a product and increases manufacturing costs.

US Patent Document No. 6273973 B1 proposes a method of heat treating a cast ingot at a high temperature for a long time to remove coarse M ₇ C ₃ carbides, such as removing primary carbides generated in high carbon martensitic steel. To this end, methods of heat treating materials in an equilibrium temperature range where primary carbides do not occur are mainly proposed. In addition, in order to remove these coarse primary carbides, a method of manufacturing hot-rolled steel sheets by heat-treating at a high temperature for a long time after manufacturing the ingot, going through a repetitive forging process to manufacture hot-rolled slabs, and then reheating at a high temperature and hot pressing is used. I'm doing it.

However, this method must be maintained for a long time at a high temperature where primary carbides are dissolved, so that all of the generated primary carbides are dissolved, and the actual yield decreases due to shrinkage due to solidification during ingot casting, so it costs a lot of money to produce the material and reduces productivity. It has a falling limit.

US Patent No. 6273973 B1 (December 2, 1999)

The purpose of the present invention to solve the above-described problem is to provide a martensitic stainless steel and a method of manufacturing the same so that the number of primary carbides inside the hot-rolled annealed material is 22ea/mm ² or less without a forging process.

The problem to be solved by the present invention is not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

As a means to achieve the above-described object, the martensitic stainless steel cast according to an example of the present invention has, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn : 0.4 to 0.9%, Cr: 13.6 to 15.0%, Ni: 0.01 to 0.3%, the remainder includes Fe and inevitable impurities, and the area fraction of primary carbide having an area of 2000㎛ ² or more in the central segregation zone is 2.5%. It may be a martensitic stainless steel cast steel.

The martensitic stainless steel cast according to an example of the present invention may be a martensitic stainless steel cast further containing one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%.

The martensitic stainless steel cast according to an example of the present invention may be a martensitic stainless steel cast with a thickness of 250 to 320 mm.

The martensitic stainless steel according to an example of the present invention has, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 13.6%. 15.0%, Ni: 0.01 to 0.3%, the remainder contains Fe and inevitable impurities, and the number of primary carbides with a diameter of 3 ㎛ or more is 22 (ea/mm ² ) or less. It may be a martensitic stainless steel.

The martensitic stainless steel according to an example of the present invention may be a martensitic stainless steel that further contains one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%.

The martensitic stainless steel according to an example of the present invention may be a martensitic stainless steel with a thickness of 4 to 8 mm.

The method for manufacturing a martensitic stainless steel cast according to an example of the present invention is, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 15.0%, Ni: 0.01 to 0.3%, the remainder being Fe and the step of reducing the slab containing inevitable impurities at a reduction rate of 2 to 4% during continuous casting, and 2000㎛ ² at the center segregation area. It may be a method of manufacturing a martensitic stainless steel cast piece in which the area fraction of primary carbides having the above area is 2.5% or less.

The method of manufacturing a martensitic stainless steel cast according to an example of the present invention may be a method of manufacturing a martensitic stainless steel cast, further comprising one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%. there is.

The method of manufacturing a martensitic stainless steel cast according to an example of the present invention may be a method of manufacturing a martensitic stainless steel cast having a thickness of 250 to 320 mm.

In the method of manufacturing a martensitic stainless steel cast steel according to an example of the present invention, the rolling step may be a method of manufacturing a martensitic stainless steel cast steel, which is a step of rolling down with an in-line roller.

The martensitic stainless steel manufacturing method according to an example of the present invention is, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: Reducing a slab containing 13.6 to 15.0% Ni, 0.01 to 0.3% Ni, and the remainder Fe and inevitable impurities at a reduction rate of 2 to 4% during casting to produce a cast piece; First reheating the cast steel; A step of first hot rolling the first reheated cast steel; Secondary reheating of the primary hot rolled cast steel; Secondary hot rolling the secondary reheated cast steel; winding step; and a top annealing step; wherein the first reheating step is performed at 1200 to 1300° C. for 1 to 4 hours, the first hot rolling step is performed at a reduction ratio of 50 to 60%, and the second hot rolling step is performed at a reduction rate of 50 to 60%. The reheating step may be a martensitic stainless steel manufacturing method performed at 1200 to 1300° C. for 1 to 4 hours.

The martensitic stainless steel manufacturing method according to an example of the present invention may be a martensitic stainless steel manufacturing method further comprising one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%.

The martensitic stainless steel manufacturing method according to an example of the present invention may be a martensitic stainless steel manufacturing method in which the coiling step is performed at a temperature of 700°C or higher.

In the method of manufacturing martensitic stainless steel according to an example of the present invention, the upper annealing step is performed by charging the steel into a hot-rolled annealing furnace at a temperature of 600°C or higher, maintaining it at 800 to 900°C for 3 to 10 hours, and then heating the steel to a temperature of 700 to 790°C. It may be a method of manufacturing martensitic stainless steel, which involves hot-rolling annealing by maintaining it at ℃ for 5 to 15 hours.

The martensitic stainless steel manufacturing method according to an example of the present invention may be a martensitic stainless steel manufacturing method in which the cast steel has a thickness of 250 to 320 mm and a total reduction ratio of hot rolling is 96.8% or more.

The martensitic stainless steel manufacturing method according to an example of the present invention is a martensitic stainless steel in which the number of primary carbides with a diameter of 3 ㎛ or more is 22 (ea/mm ² ) or less. It could be a way.

According to one embodiment of the present invention, by omitting the forging process and controlling the number of primary carbides (M ₇ C ₃ ) inside the hot rolled annealed steel sheet to 22ea/mm ² or less, marten is economical and has excellent quality of primary carbides. Site can provide stainless steel plates and methods for manufacturing the same.

Figure 1 is a diagram showing the primary carbide of a hot-rolled annealed material obtained under the condition of applying hot rolling twice.
Figure 2 is a diagram showing the primary carbide of a hot-rolled annealed material obtained under the condition of applying hot rolling once.
Figure 3 is a diagram showing linear defects of the final product commercialized as a comparative example of the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression, unless the context clearly requires it to be singular. In addition, terms such as "comprise" or "comprise" used in the present application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to indicate other features. It should be noted that it is not used to preliminarily rule out the existence of any elements, steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used in this specification should be viewed as having the same meaning as generally understood by those skilled in the art to which the present invention pertains. Therefore, unless clearly defined in this specification, specific terms should not be interpreted in an overly idealistic or formal sense.

In addition, in this specification, "about", "substantially", etc. are used in the meaning of or close to that value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used accurately to aid understanding of the present invention. It is used to prevent unscrupulous infringers from unfairly exploiting disclosures where absolute figures are mentioned.

The martensitic stainless steel cast according to an example of the present invention has, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 13.6%. 15.0%, Ni: 0.01 to 0.3%, the remainder may include Fe and inevitable impurities.

In addition, the martensitic stainless steel cast according to an example of the present invention may further include one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2% by weight.

Additionally, the martensitic stainless steel according to an example of the present invention may be equal to the weight percent of the cast steel.

The reason for limiting the composition range of each alloy element is described below.

The content of C may be 0.4 to 0.55% by weight.

C is an essential element that improves the hardness of steel, and must be added appropriately to ensure hardness after quenching and tempering heat treatment. Considering this, C may be added in an amount of 0.4% or more to satisfy the purpose of the present invention. However, if the content is excessive, the toughness of the steel sheet may decrease. Considering this, the upper limit of C content can be limited to 0.55%. Preferably, the C content may be 0.43% or more and 0.53% or less.

The content of N may be 0.01% to 0.1% by weight.

N, like C, is an effective element in improving the hardness of steel. Considering this, N may be added in an amount of 0.01% or more. However, if the N content is excessive, chromium nitride, a low-temperature precipitated phase, is generated and causes the γ phase to remain, which may result in insufficient strength after strengthening heat treatment. Therefore, if the N content is excessive, fatigue resistance may be poor. Considering this, the upper limit of N content may be limited to 0.1%.

The content of Si may be 0.2 to 0.6% by weight.

Si is added to deoxidize steel. In addition, Si is an effective element in securing strength through solid solution strengthening. Considering this, Si may be added in an amount of 0.2% or more. However, if the Si content is excessive, scale may form on the surface of the steel during hot rolling, deteriorating surface quality. Considering this, the upper limit of Si content may be limited to 0.6%.

The content of Mn may be 0.4 to 0.9% by weight.

Mn is a very effective element in improving hardenability and producing a solid solution strengthening effect by forming a substitutional solid solution within the matrix structure. Additionally, if the Mn content is low, it may not sufficiently combine with S (sulfur) introduced as an impurity in the steel, causing playing cracks. Considering this, Mn may be added in an amount of 0.4% or more. However, if the Mn content is excessive, the toughness of the steel may become inferior. Considering this, the upper limit of Mn content may be limited to 0.9%.

The content of Cr may be 13.6 to 15.0% by weight.

Cr is an element that is effective in improving corrosion resistance and improving hardness and wear resistance by forming chromium carbide. Considering this, Cr can be added at more than 13.6%. However, if the Cr content is excessive, hardenability may increase more than necessary and manufacturing costs may increase. Considering this, the upper limit of Cr content may be limited to 15.0%.

The content of Ni may be 0.01% to 0.3% by weight.

Ni is an essential element added to martensitic stainless steel to transform the metal structure into an austenite structure in the hot working area. In addition, Ni is an element that improves corrosion resistance and hardenability when added in a small amount. Considering this, Ni may be added in an amount of 0.01% or more. However, if the Ni content is excessive, workability may deteriorate, excessive austenite may remain after strengthening heat treatment, making it difficult to secure the hardness of the product, and manufacturing costs may increase. Considering this, the upper limit of Ni content may be limited to 0.3%.

The content of Mo may be 0.01 to 0.8% by weight.

Mo is an element effective in improving corrosion resistance and hardenability, so it can be selectively added in the present invention. In addition, Mo, along with V, is an element that plays a role in suppressing the miniaturization and growth of carbides. Considering this, Mo may be added in an amount of 0.01% or more. However, if the Mo content is excessive, manufacturing costs may increase. Considering this, the upper limit of Mo content can be limited to 0.8%.

The content of V may be 0.05% to 0.2% by weight.

V is an element that forms carbides and is effective in suppressing the coarsening of chromium carbides, so it can be added selectively in the present invention. Additionally, V is an element effective in preventing grain coarsening and improving wear resistance during heat treatment. However, if the content of V is excessive, carbides may be formed more than necessary, which may lower the toughness of the steel and increase the manufacturing cost. Considering this, the upper limit of V content may be limited to 0.2%.

Meanwhile, Mo and V may be added in combination when manufacturing steel.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all of them are not specifically mentioned in this specification.

Hereinafter, a martensitic stainless steel cast according to an embodiment of the present invention having the above-described alloy composition will be described.

The martensitic stainless steel cast of the present invention may be a martensitic stainless steel cast in which the area fraction of primary carbides having an area of 2000㎛ ² or more in the central segregation portion is 2.5% or less. Here, the central segregation area means -15mm to 15mm from the center of the cast steel.

Additionally, the martensitic stainless steel cast of the present invention may be a martensitic stainless steel cast with a thickness of 250 to 320 mm.

In the martensitic stainless steel cast of the present invention, the area fraction of primary carbides having an area of 2000㎛ ² or more in the center segregation portion of the cast can be controlled to 2.5% or less, and the thickness can be controlled to 250 to 320 mm. Using this, the number of primary carbides with a diameter of 3㎛ or more can be controlled to 22 or less while manufacturing 8mm hot-rolled annealed stainless steel. Through this, it is possible to produce high-quality martensitic stainless steel sheets with no surface linear defects after knife processing.

Hereinafter, a martensitic stainless steel according to an embodiment of the present invention having the above-described alloy composition will be described.

The martensitic stainless steel of the present invention may be a martensitic stainless steel in which the number of primary carbides having a diameter of 3 μm or more is 22 (ea/mm ² ) or less.

Additionally, the martensitic stainless steel of the present invention may have a thickness of 4 to 8 mm. Preferably, the martensitic stainless steel may have a thickness of 5 mm.

The martensitic stainless steel of the present invention can produce high-quality products by controlling the number of coarse primary carbides with a diameter of 3 ㎛ or more to 22 or less, preventing surface linear defects in the final product.

Hereinafter, a method for manufacturing a martensitic stainless steel cast steel according to an embodiment of the present invention having the above-described alloy composition will be described.

The martensitic stainless steel cast of the present invention has, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 15.0%, Ni: 0.01 to 0.3%, the remainder may include the step of reducing the slab containing Fe and inevitable impurities at a reduction rate of 2 to 4% during continuous casting.

The slab having the above-described alloy composition is supplied to a tundish using a molten steel transfer device for a continuous casting process. The molten steel from the tundish is supplied to the water-cooled mold and solidification begins. At this time, the cast piece with only the surface solidified exits the casting mold and undergoes secondary cooling with cooling water to solidify. At this time, unlike the surface area in contact with the continuous mold, solidification of the center of the cast steel progresses slowly, causing C, Cr, etc. to segregate in the center, causing coarse primary carbides to precipitate.

The method for manufacturing a martensitic stainless steel cast of the present invention may include the step of reducing the cast at a reduction rate of 2 to 4% before the cast is completely solidified. The reduction of the present invention uses an in-line roller to reduce the cast steel in a state before it is completely solidified by 2 to 4%. By reducing the cast before it is completely solidified, the shrinkage cavity in the center of the cast can be removed, and by weaving the segregated molten steel to minimize segregation, the area where primary carbides are generated can be reduced.

The method for manufacturing a martensitic stainless steel cast of the present invention is to reduce the cast at a reduction rate of 2 to 4% before it is completely solidified, thereby reducing the area fraction of primary carbide having an area of 2000㎛ ² or more based on the center segregation of the cast. This may be less than 2.5%. Here, the central segregation area means -15mm to +15mm from the center of the cast steel.

If the cast steel is reduced with a reduction ratio of less than 2% before the cast steel is completely solidified, the slab is not damaged, but the area fraction of primary carbide with an area of 2000㎛ ² or more in the center of the cast steel exceeds 2.5%. If the reduction rate exceeds 4%, the slab may crack or be damaged. Therefore, in the present invention, the reduction rate before the cast steel is completely solidified is set to 2 to 4%.

The method for manufacturing a martensitic stainless steel cast of the present invention may be a manufacturing method for manufacturing a cast with a thickness of 250 to 320 mm. When the thickness is less than 250 mm, even if the cast steel is reduced at a reduction rate of 2 to 4% before it is completely solidified, the area fraction of primary carbide with an area of 2000㎛ ² or more in the central segregation area of the cast steel exceeds 2.5%. In addition, in order to ensure that the total reduction rate of hot rolling in the subsequent steps is 96.8% or more, the thickness of the cast steel is preferably 250 to 320 mm.

Through this, hot-rolled annealed stainless steel of 4 to 8 mm can be obtained from the cast steel manufactured by the martensitic stainless steel cast manufacturing method of the present invention.

Hereinafter, a method for manufacturing martensitic stainless steel according to an embodiment of the present invention having the above-described alloy composition will be described.

The method for producing martensitic stainless steel of the present invention is, in weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 15.0%. , Ni: 0.01 to 0.3%, the remainder being Fe, and manufacturing a cast piece by reducing a slab containing unavoidable impurities at a reduction rate of 2 to 4% during casting; First reheating the cast steel; A step of first hot rolling the first reheated cast steel; Secondary reheating of the primary hot rolled cast steel; Secondary hot rolling the secondary reheated cast steel; winding step; and a top annealing step. It may be a method of manufacturing martensitic stainless steel.

The process of manufacturing a hot rolled steel sheet even though the cast steel is reduced at a reduction rate of 2 to 4% before it is completely solidified, and the area fraction of primary carbide with an area of 2000㎛ ² or more based on the center segregation of the cast steel is controlled to 2.5% or less. Coarse carbides may remain.

In the method of manufacturing martensitic stainless steel of the present invention, after the step of manufacturing a cast steel, the first reheating step is performed at 1200 to 1300 ° C. for 1 to 4 hours, and the first hot rolling step is performed at a reduction rate of 50. to 60%, and the secondary reheating step is performed at 1200 to 1300°C for 1 to 4 hours, and includes the steps of secondary hot rolling, coiling, and top annealing of the secondary reheated cast steel, It may be a method of manufacturing martensitic stainless steel.

The method for manufacturing martensitic stainless steel of the present invention includes two reheating and hot rolling steps to ensure thermal decomposition and mechanical pulverization of primary carbides. After primary reheating and hot rolling, secondary reheating and hot rolling can be performed without top annealing.

In general, cast steel manufactured by ingot-forging or continuous casting is reheated at a high temperature and then goes through hot rolling, blowing, and top annealing steps to obtain an annealed hot rolled steel sheet. However, continuous casting involves one reheating and hot rolling step, and the thermal decomposition and mechanical grinding processes of the primary carbide are insufficient. Therefore, the quality of the primary carbide is inferior compared to the case including the ingot-forging step. However, when an ingot-forging step is included, the production time becomes longer and the actual yield decreases due to the formation of shrinkage cavities during ingot casting, which limits productivity and cost.

According to the method for manufacturing martensitic stainless steel of the present invention, the cast steel is first reheated and hot rolled, and the first hot rolled material is first hot rolled without winding and top annealing, followed by second reheating and hot rolling. By continuous casting, the quality of the primary carbide is superior to that of ingot-forging, while ensuring productivity and cost.

According to the method for manufacturing martensitic stainless steel of the present invention, the thickness of the cast steel can be set to 250 to 320 mm in order to ensure the thickness of the stainless steel is 4 to 8 mm.

The martensitic stainless steel cast of the present invention may have a thickness of 250 to 320 mm and undergo primary hot rolling and secondary hot rolling at a reduction ratio of 50 to 60%, and the total hot rolling reduction ratio may be 96.8% or more. Through this, the thickness of the hot rolled annealed stainless steel plate can be secured to 4 to 8 mm.

When the step of reheating and hot rolling the cast steel is performed only once, even if the area fraction of primary carbides with an area of 2000㎛ ² or more in the center of the cast steel is less than 2.5%, the number of primary carbides with a diameter of 3㎛ or more in the hot-rolled annealed material may exceed 22. Hot-rolled annealed materials with coarse carbide clusters generate defects in the surface linearity after knife processing. If you proceed with primary reheating and hot rolling and immediately proceed with secondary reheating and hot rolling without going through top annealing, the number of primary carbides with a diameter of 3㎛ or more in the hot rolled annealed material is reduced to 22 or less, and the primary carbides are refined to 1. The quality of secondary carbide can be excellent. These hot rolled annealed materials may not generate surface linear defects after knife processing.

The method for manufacturing martensitic stainless steel of the present invention includes a winding step after secondary hot rolling; And it may include a top annealing step. The winding step can be performed at a temperature of 700°C or higher. The upper annealing step may be a step of hot rolling annealing by charging the steel into a hot rolling annealing furnace at a temperature of 600°C or higher, maintaining it at 800 to 900°C for 3 to 10 hours, and then maintaining it at 700 to 790°C for 5 to 15 hours. .

The method for producing martensitic stainless steel of the present invention may be a martensitic stainless steel production method for producing martensitic stainless steel in which the number of primary carbides with a diameter of 3 ㎛ or more is 22 (ea/mm ² ) or less. there is. In addition, the martensitic stainless steel of the present invention can produce high-quality products by preventing surface linear defects in the final product.

With the method for manufacturing the martensitic stainless steel cast of the present invention, the martensitic stainless steel cast of the present invention can be manufactured. In addition, the martensitic stainless steel of the present invention can be manufactured using the martensitic stainless steel cast of the present invention. In addition, the martensitic stainless steel of the present invention can be manufactured by the martensitic stainless steel manufacturing method of the present invention.

The martensitic stainless steel of the present invention can be used as a high-hardness, high-corrosion-resistant ceramic material. Additionally, the martensitic stainless steel of the present invention can be used for automobile parts. In addition, it can be applied to other uses that can be utilized due to the physical properties of the stainless steel obtained by the present invention.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

{Example}

The slab having an alloy composition according to Table 1 below was reduced by 0 to 6% using an in-line roller before the cast steel was completely solidified.

In Comparative Examples 1, 3, 5, 8, 10, 11, 12, 14, and 15, the cast steel was reheated at about 1250°C for about 3 hours, hot rolled to a final thickness of 8 mm, and reheated and hot rolled once.

In the case of Invention Examples 1 to 4 and Comparative Examples 2, 4, 6, 9, and 13, the cast steel was first reheated at about 1250 ° C. for about 3 hours, first hot rolled at a reduction rate of about 55%, and then first rolled. The steel sheet was reloaded into the reheating furnace and subjected to secondary reheating and secondary hot rolling at approximately 1250°C for approximately 3 hours, followed by hot rolling to a final thickness of 8mm, and reheating and hot rolling a total of two times.

The manufactured hot rolled steel sheet was wound at approximately 700°C. For top annealing, the wound coil was charged into a hot rolling annealing furnace at about 600°C and kept at 850°C for about 10 hours. Then, in the second stage, it was kept at about 750°C for about 10 hours to perform top annealing.

A cast specimen was manufactured from the center of the manufactured cast steel in the width direction, and the fraction of coarse carbides with an area of 2000㎛ ² or more in the central segregation area of the cast steel (-15 mm to +15 mm from the center) was analyzed through optical microscope image analysis. , To analyze primary carbides, a test piece was produced by cutting a hot-rolled specimen, and analyzed through image analysis with an optical microscope. The number of primary carbides with a diameter of 3㎛ or more (ea/mm ² ) was obtained, and the crack length was obtained. The presence or absence of slab damage was confirmed based on cases exceeding 5 mm, and the presence or absence of surface linear defects after knife processing was visually confirmed and shown in Table 2.

Table 1 shows the composition of the alloy composition.

Alloy composition C Si Mn Cr Ni Mo V N 0.51 0.4 0.45 14.6 0.15 0.55 0.11 0.03

Table 2 shows the thickness of the cast steel, the reduction ratio of the cast steel before it is completely solidified during casting, the presence or absence of slab damage, the area fraction of primary carbides, the number of reheating and hot rolling, the number of primary carbides, and the presence or absence of surface linear defects.

Additionally, Table 2 is based on a thickness of 8mm of hot rolled annealed material.

Cast plate thickness (mm) Slab reduction rate (%) before the cast is completely solidified during casting Is there any damage to the slab? Cast center segregation section
^≥2000㎛2 at (±15mm)
Primary carbide area fraction (%) Number of reheating and hot rolling Hot-rolled annealed material ≥3㎛
Number of primary carbides (ea/mm ² ) Presence of surface linear defects after knife processing Invention Example 1 250 2 Nothing 2.3 2 22 Nothing Invention Example 2 250 4 Nothing 1.6 2 15 Nothing Invention Example 3 300 2 Nothing 2.5 2 19 Nothing Invention Example 4 300 4 Nothing 2.0 2 12 Nothing Comparative Example 1 200 0 Nothing 6.6 One 71 have Comparative Example 2 200 0 Nothing 6.3 2 56 have Comparative Example 3 200 2 Nothing 5.3 One 53 have Comparative Example 4 200 2 Nothing 5.2 2 41 have Comparative Example 5 200 4 Nothing 2.6 One 36 have Comparative Example 6 200 4 Nothing 2.6 2 28 have Comparative Example 7 200 6 have - - - - Comparative Example 8 250 0 Nothing 5.7 One 67 have Comparative Example 9 250 0 Nothing 5.3 2 49 have Comparative Example 10 250 2 Nothing 2.5 One 51 have Comparative Example 11 250 4 Nothing 1.7 One 29 have Comparative Example 12 300 0 Nothing 5.7 One 63 have Comparative Example 13 300 0 Nothing 6.1 2 27 have Comparative Example 14 300 2 Nothing 2.4 One 46 have Comparative Example 15 300 4 Nothing 1.8 One 27 have Comparative Example 16 300 6 have - - -

In Invention Examples 1 to 4, the thickness of the cast steel was set to 250 to 320 mm, and the slab was reduced at a reduction ratio of 2 to 4% before the cast steel was completely solidified during casting.

It can be confirmed that the slabs of Invention Examples 1 to 4 were not damaged, and that the area fraction of carbide with an area of 2000㎛ ² or more in the central segregation area of the cast slab was 2.5% or less. Through this, it can be confirmed that by reducing the slab at a reduction ratio of 2 to 4% before the cast slab is completely solidified during casting, the shrinkage cavity in the center of the cast slab is removed and segregation is minimized, thereby reducing the area where coarse primary carbides occur. there is.

In addition, in Inventive Examples 1 to 4, the slab was reduced at a reduction ratio of 2 to 4% before the cast slab was completely solidified, and the hot rolled annealed material subjected to reheating and hot rolling twice had a number of primary carbides with a diameter of 3 ㎛ or more. It can be confirmed that there are 22 or less. Through this, when performing secondary reheating and hot rolling without forging, through continuous casting, and without top annealing immediately after primary reheating and hot rolling, the number of primary carbides with a diameter of 3㎛ or more in the hot rolled annealed material is reduced to 22 or less. It can be seen that the primary carbide is refined and the quality of the primary carbide is excellent. In addition, it can be confirmed that this hot-rolled annealed material does not generate surface linear defects after knife processing.

In Comparative Examples 1 and 2, the slab was not reduced before the cast slab was completely solidified during casting. Although the slabs of Comparative Example 1 and Comparative Example 2 were not damaged, it can be seen that the area fraction of coarse primary carbide in the central segregation area was excessive at 6.6% and 6.3%.

In addition, in Comparative Example 1, the first hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 71, confirming that the quality of the primary carbides was inferior. It can be confirmed that surface linear defects occur in the product processed in Comparative Example 1. In Comparative Example 2, secondary hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot-rolled annealed material was 56, which confirmed that the quality of primary carbides was less inferior than that of Comparative Example 1. However, it can be confirmed that surface linear defects occur in Comparative Examples 1 and 2 after product processing. Through this, it can be seen that the quality of the primary carbide is excellent when the reheating and hot rolling steps are performed a total of two or more times in the manufacturing method of martensitic stainless steel.

In Comparative Examples 3 and 4, the slab was reduced at a reduction rate of 2% before the cast slab was completely solidified during casting. Although the area fraction is smaller than Comparative Example 1 and Comparative Example 2, it can be seen that the area fraction of coarse primary carbide in the central segregation area of the cast steel is still excessive at 5.3% and 5.2%. Through this, it can be confirmed that when the thickness of the cast steel is 200 mm for making a hot-rolled annealed material with a thickness of 8 mm, the effect of reducing the area fraction of primary carbide in the center segregation zone of the cast steel, which is the object of the present invention, cannot be obtained.

In addition, in Comparative Example 3, the first hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 53, confirming that the quality of the primary carbides was inferior. It can be confirmed that surface linear defects occur in the product processed in Comparative Example 3. In Comparative Example 4, secondary hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot-rolled annealed material was 41, which confirmed that the quality of primary carbides was less inferior than that of Comparative Example 3. However, it can be confirmed that surface linear defects occur in Comparative Examples 3 and 4 after product processing. Through this, it can be seen that the quality of the primary carbide is excellent when the reheating and hot rolling steps are performed a total of two or more times in the manufacturing method of martensitic stainless steel.

In Comparative Examples 5 and 6, the slab was reduced at a reduction rate of 4% before the cast slab was completely solidified during casting. Although the area fraction of coarse primary carbide is smaller than that of Comparative Example 3 and Comparative Example 4, it can be seen that the area fraction of coarse primary carbide in the central segregation area of the cast steel is still excessive at 2.6% and 2.6%. Through this, it can be confirmed that when the thickness of the cast steel is 200 mm for making an 8 mm hot-rolled annealed material, the effect of reducing the area fraction of primary carbide in the center of the cast steel, which is the object of the present invention, cannot be obtained.

In addition, in Comparative Example 5, the first hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 36, confirming that the quality of the primary carbides was inferior. It can be confirmed that surface linear defects occur in the product processed in Comparative Example 5. In Comparative Example 6, secondary hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot-rolled annealed material was 28, which confirmed that the quality of primary carbides was less inferior than that of Comparative Example 5. However, in Comparative Examples 5 and 6, it can be confirmed that surface linear defects occur after product processing. Through this, it can be seen that the quality of the primary carbide is excellent when the reheating and hot rolling steps are performed a total of two or more times in the manufacturing method of martensitic stainless steel.

In Comparative Example 7, the slab was reduced at a reduction rate of 6% before the cast slab was completely solidified during casting. The slab of Comparative Example 7 was damaged. From this, it can be seen that the appropriate reduction ratio is 2 to 4% when reducing the slab before the cast slab is completely solidified during casting.

In Comparative Examples 8 and 9, the slab was not reduced before the cast slab was completely solidified during casting. Although the slabs of Comparative Examples 8 and 9 were not damaged, it could be confirmed that the area fraction of coarse primary carbide in the central segregation area of the cast steel was excessive at 5.7% and 5.3%. Comparing Comparative Examples 8 and 9 with Invention Examples 1 and 2, when the slab is reduced by 2 to 4% before the cast is completely solidified during casting, the area fraction of the coarse primary carbide in the center segregation area of the cast is It can be confirmed that it can be reduced.

In addition, in Comparative Example 8, the first hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 67, confirming that the quality of the primary carbides was inferior. It can be confirmed that surface linear defects occur in the product processed in Comparative Example 8. In Comparative Example 9, secondary hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot-rolled annealed material was 49, which confirmed that the quality of primary carbides was less inferior than that of Comparative Example 8. However, in Comparative Examples 8 and 9, it can be confirmed that surface linear defects occur after product processing. Through this, it can be seen that the quality of the primary carbide is excellent when the reheating and hot rolling steps are performed a total of two or more times in the manufacturing method of martensitic stainless steel.

In addition, in Comparative Example 9, a total of two reheating and hot rolling steps were performed as in Invention Example 1 and Invention Example 2, but as observed previously, the slab was not reduced before the cast slab was completely solidified during casting, thereby affecting the quality of the primary carbide. You can see that there is a difference. Through this, in order to obtain an 8mm hot rolled annealed material, the quality of the primary carbide is excellent when the cast steel is reduced at a reduction rate of 2 to 4% before it is completely solidified during casting with a 250mm cast steel, and the reheating and hot rolling steps are performed at least twice in total. Able to know. In addition, it can be confirmed that this hot-rolled annealed material does not have surface linear defects after final product processing.

In Comparative Examples 10 and 11, the slabs were reduced at a reduction rate of 2% and 4% before the cast slab was completely solidified during casting. It can be confirmed that the area fraction of coarse primary carbide in the center of the cast steel is 2.5% and 1.7%, so the area fraction of primary carbide can be secured below 2.5%. However, in Comparative Examples 10 and 11, the first hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 51 and 29, which confirmed that the quality of the primary carbides was inferior. In addition, it can be confirmed that surface linear defects occur in products processed this way. In addition, when Comparative Examples 10 and 11 are compared with Invention Examples 1 and 2, it can be seen that the quality of the primary carbide is excellent when the reheating and hot rolling steps are performed a total of two or more times. In addition, it can be confirmed that this hot-rolled annealed material does not have surface linear defects after final product processing.

In Comparative Examples 12 and 13, the slab was not reduced before the cast slab was completely solidified during casting. Although the slabs of Comparative Examples 12 and 13 were not damaged, it was confirmed that the area fraction of coarse primary carbide in the central segregation area of the cast steel was excessive at 5.7% and 6.1%.

In addition, in Comparative Example 12, the first hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 63, confirming that the quality of the primary carbides was poor. It can be confirmed that surface linear defects occur in the product processed in Comparative Example 12. In Comparative Example 13, secondary hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 27, which confirmed that the quality of primary carbides was less inferior than that of Comparative Example 1. However, in Comparative Examples 12 and 13, it can be confirmed that surface linear defects occur after product processing. Through this, it can be seen that the quality of the primary carbide is excellent when the reheating and hot rolling steps are performed a total of two or more times in the manufacturing method of martensitic stainless steel.

In Comparative Examples 14 and 15, the slabs were reduced at a reduction rate of 2% and 4% before the cast slab was completely solidified during casting. It can be confirmed that the area fraction of coarse primary carbide in the center of the cast steel is 2.4% and 1.8%, and the area fraction of primary carbide can be secured below 2.5%. However, in Comparative Examples 14 and 15, the first hot rolling was performed, and the number of primary carbides with a diameter of 3 ㎛ or more in the hot rolled annealed material was 46 and 27, which confirmed that the quality of the primary carbides was inferior. Additionally, in Comparative Examples 14 and 15, it was confirmed that surface linear defects occurred in the processed products. Through this, it can be seen that the quality of the primary carbide is excellent when the reheating and hot rolling steps are performed a total of two or more times in the manufacturing method of martensitic stainless steel. In addition, it can be confirmed that this hot-rolled annealed material does not have surface linear defects after final product processing.

In Comparative Example 16, the slab was reduced at a reduction rate of 6% before the cast slab was completely solidified during casting. The slab of Comparative Example 16 was damaged. From this, it can be seen that it is appropriate to reduce the slab at a reduction rate of 2 to 4% before the cast slab is completely solidified during casting.

1 and 2, in the present invention, the slab is a cast reduced by 2 to 4% during continuous casting, and the martensitic stainless steel when secondary hot rolling is performed is primary without additional reduction during casting. It can be seen that the number of primary carbides with a diameter of 3㎛ or more is smaller than that of martensitic stainless steel that has been hot rolled only. In the present invention, the cast slab obtained by reducing the slab by 2 to 4% during continuous casting may have an area fraction of carbide having an area of 2000㎛ ² or more in the center segregation portion of the cast slab of 2.5% or less. In addition, when martensitic stainless steel is manufactured by performing secondary hot rolling on this cast steel, the number of primary carbides with a diameter of 3 ㎛ or more may be 22 (ea/mm ² ) or less. Additionally, when final products are manufactured with this martensitic stainless steel, linear defects may not occur.

Referring to Figure 3, in the comparative example of the present invention, the area fraction of primary carbides having an area of 2000㎛ ² or more in the center of the cast steel is greater than 2.5% and/or the number of primary carbides having a diameter of 3㎛ or more exceeds about 22. In martensitic stainless steel, if the quality of the primary carbide is poor, it can be confirmed that linear defects are caused in the final product.

Claims (16)

In weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 15.0%, Ni: 0.01 to 0.3%, the balance being Fe and Contains inevitable impurities,
A martensitic stainless steel cast piece in which the area fraction of primary carbides having an area of 2000㎛ ² or more in the central segregation zone is 2.5% or less. In claim 1,
A martensitic stainless steel cast piece further comprising one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%. In claim 1,
Martensitic stainless steel cast with a thickness of 250 to 320 mm. In weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 15.0%, Ni: 0.01 to 0.3%, the balance being Fe and Contains inevitable impurities,
Martensitic stainless steel in which the number of primary carbides with a diameter of 3㎛ or more is 22 (ea/mm ² ) or less. In claim 4,
A martensitic stainless steel further comprising one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%. In claim 4,
Martensitic stainless steel with a thickness of 4 to 8 mm. In weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 15.0%, Ni: 0.01 to 0.3%, the balance being Fe and A step of reducing the slab containing unavoidable impurities at a reduction rate of 2 to 4% during continuous casting,
A method of manufacturing a martensitic stainless steel cast piece, wherein the area fraction of primary carbides having an area of 2000㎛ ² or more in the central segregation zone is 2.5% or less. In claim 7,
A method for producing a martensitic stainless steel cast, further comprising one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%. In claim 7,
A method of manufacturing a martensitic stainless steel cast steel having a thickness of 250 to 320 mm. In claim 7,
The reducing step is a method of manufacturing a martensitic stainless steel cast, which is a step of reducing with an in-line roller. In weight percent, C: 0.4 to 0.55%, N: 0.01 to 0.1%, Si: 0.2 to 0.6%, Mn: 0.4 to 0.9%, Cr: 13.6 to 15.0%, Ni: 0.01 to 0.3%, the balance being Fe and Reducing a slab containing unavoidable impurities at a reduction rate of 2 to 4% during casting to produce a cast steel product;
First reheating the cast steel;
A step of first hot rolling the first reheated cast steel;
Secondary reheating of the primary hot rolled cast steel;
Secondary hot rolling the secondary reheated cast steel;
winding step; and
Including a top annealing step,
The first reheating step is performed at 1200 to 1300°C for 1 to 4 hours,
The first hot rolling step is performed at a reduction ratio of 50 to 60%,
A method of manufacturing martensitic stainless steel, wherein the secondary reheating step is performed at 1200 to 1300°C for 1 to 4 hours. In claim 11,
A method of producing martensitic stainless steel, further comprising one or more of Mo: 0.01 to 0.8% and V: 0.05 to 0.2%. In claim 11,
The winding step is,
A method of manufacturing martensitic stainless steel, performed at a temperature of 700°C or higher. In claim 11,
The upper annealing step is,
After charging it into a hot-rolled annealing furnace at a temperature of over 600℃,
A method of manufacturing martensitic stainless steel, which involves hot-rolling annealing by maintaining it at 800 to 900°C for 3 to 10 hours and then maintaining it at 700 to 790°C for 5 to 15 hours. In claim 11,
The thickness of the cast steel is 250 to 320 mm,
A method of manufacturing martensitic stainless steel with a total reduction rate of 96.8% or more during hot rolling. In claim 11,
The martensitic stainless steel is,
A method of manufacturing martensitic stainless steel in which the number of primary carbides with a diameter of 3㎛ or more is 22 (ea/mm ² ) or less.