KR101587699B1 - Martensitic stainless steel and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a martensitic stainless steel having excellent corrosion resistance and processability by optimizing the size and distribution of chromium carbide distributed in a microstructure, and a method for producing the same. More particularly, the martensitic stainless steel according to one embodiment of the present invention comprises Wherein at least 80/100 占 퐉 ² or more of chromium carbide is distributed in the structure of martensitic stainless steel containing 0.60 to 0.70% of C and 0.5 to 2% of Cu.

Description

TECHNICAL FIELD [0001] The present invention relates to a martensitic stainless steel and a method of manufacturing the same.

More particularly, the present invention relates to a martensitic stainless steel having excellent corrosion resistance and processability by optimizing the size and distribution of chromium carbide distributed in the microstructure, and a process for producing the same. will be.

Generally, molten steel is a type of steel which is used not only for various knives, scissors, razor blades, but also for medical instruments such as knives, etc., and is a steel type requiring high hardness while maintaining cutting and abrasion resistance. In addition, since the molten steel is easily contacted with moisture or stored in a humid atmosphere, corrosion resistance is required, so high-martensitic stainless steels having high hardness are generally used.

The high carbon martensitic stainless steel is a steel containing 0.45 to 0.70% by weight of carbon, at most 1.0% of manganese, at most 1.0% of silicon and 12.0 to 15.0% of chromium by weight, And about 12.0 ~ 15.0% chromium-based component system are widely used as a material for a hardness requiring high hardness.

High hardness materials for water require very hardness, so they are very brittle. Therefore, it is necessary to soften the material for the water to a predetermined level so as to facilitate the processing. For this purpose, a BAF (Batch Annealing Furnace) process which can easily perform the heat treatment of the brittle material is included.

During the calcination process, the material reacts with chromium in the ferrite matrix to disperse fine particles of chromium carbide in the ferrite matrix, which lowers the solid carbon content in the matrix and facilitates the application of the stainless steel manufacturing process such as rolling and pickling. In addition, the distribution of fine chromium carbide uniformly distributed in the structure of the ferrite matrix enables rapid reutilization of chromium and carbon to a high temperature austenite phase in a strengthening heat treatment process performed by the logistics manufacturer, and the hardness of the martensite structure And an important factor for improving the corrosion resistance.

In order to secure a martensite structure of high hardness with excellent corrosion resistance, it is essential to form a fine chromium carbide in the annealed ferrite structure. However, in the case of the conventional 420-type martensitic stainless steel, chromium carbides locally precipitate in the grain boundaries due to grain-size preferential precipitation and growth of chromium carbide when 0.5% or more of high carbon is added, and the stockability ratio to the austenite phase Resulting in lowering of hardness and corrosion resistance.

Therefore, in order to secure a material having high hardness and corrosion resistance of a high carbon martensitic stainless steel for water, it is required to develop a new steel material capable of uniformly distributing fine chromium carbide in a microstructure and to establish an annealing pattern.

In order to uniformly distribute fine chromium carbide in the ferrite base structure of the present application, a high corrosion resistant martensitic stainless steel having Mo and W added thereto and a manufacturing method thereof (Patent Document 1) have been filed. In addition to Patent Document 1, Continuous research was carried out to uniformly distribute the carbide in the ferrite matrix.

Patent No. 10-1268736 (Feb. 201, 201)

The present invention provides a martensitic stainless steel having a proper amount of Cu and a heat treatment pattern of a material so as to uniformly distribute fine chromium carbide in a ferrite matrix and improve hardness and corrosion resistance, and a method for manufacturing the same.

Martensitic stainless steel according to one embodiment of the present invention in weight%, C: 0.60 ~ 0.70% , Cu: 80 gae in the martensitic stainless steel tissue containing 0.5 ~ ^2% / 100㎛ 2 or more of chromium carbide Is distributed.

Wherein the martensitic stainless steel further contains 0.03 to 0.06% of N, 0.1 to 0.6% of Si, 0.5 to 1.0% of Mn, and 12 to 14% of Cr in weight percent and Fe and unavoidable impurities in the balance .

The martensitic stainless steel has an elongation of 18% or more.

Meanwhile, a method for producing a martensitic stainless steel according to an embodiment of the present invention comprises casting a stainless steel molten steel containing 0.60 to 0.70% of C and 0.5 to 2% of Cu in weight percent, It characterized in that a heat treatment to be 80 / 100㎛ ² or more chromium carbides are distributed in stainless steel tissue.

The stainless steel may further contain 0.03 to 0.06% of N, 0.1 to 0.6% of Si, 0.5 to 1.0% of Mn, and 12 to 14% of Cr, with the balance being Fe and unavoidable impurities.

Wherein the heat treatment includes a first cracking process for uniformly distributing Cu precipitates in the structure of the martensitic stainless steel, a second cracking process for uniformly distributing the chromium carbide in the martensitic stainless steel texture, And a third cracking process to spheroidize the particles.

The first cracking process is performed at 500 to 600 ° C, the second cracking process is performed at 800 to 900 ° C, and the third cracking process is performed at 600 to 750 ° C.

The first cracking process is continued for 5 to 15 hours, the second cracking process is continued for 15 to 25 hours, and the third cracking process is continued for 5 to 15 hours.

A temperature rising step of raising the temperature at a rate of 40 to 200 ° C / h from the first cracking step until reaching the second cracking step; and a step of raising the temperature to 10 ° C / h or more And the air cooling process is further performed after the third cracking process.

According to the embodiment of the present invention, it is possible to produce a material having high hardness, high corrosion resistance and workability by uniformly distributing fine chromium carbide in the matrix structure of martensite.

FIG. 1 is a graph showing a heat treatment process according to a method for producing a martensitic stainless steel according to an embodiment of the present invention,
FIG. 2 is a photograph showing the microstructure of the Cu precipitate according to the first cracking condition,
Fig. 3 is a photograph showing the microstructure of the silk blast furnace according to the Cu content,
4 is a graph showing a change in the number of carbides according to the amount of Cu added.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know.

FIG. 1 is a graph showing a heat treatment process according to a method for producing martensitic stainless steel according to an embodiment of the present invention, FIG. 2 is a photograph showing the microstructure of a Cu precipitate according to a first cracking condition, FIG. FIG. 4 is a graph showing the change in the number of carbides according to the amount of Cu added. FIG.

The martensitic stainless steel according to one embodiment of the present invention may contain 0.60 to 0.70% of carbon (C), 0.03 to 0.06% of nitrogen (N), 0.1 to 0.6% of silicon (Si) (Fe), and unavoidable impurities, in an amount of 0.1 to 1.0% Mn, 12 to 14% of chromium (Cr) and 0.5 to 2% of copper (Cu)

First, the reason for limiting the function and the content of the trapping elements constituting the martensitic stainless steel of the present invention will be described.

When the content of carbon (C) is low, the hardness of the martensitic stainless steel after the tempering treatment is lowered, and machinability and abrasion resistance can not be ensured. On the other hand, if the content is excessively large, the chromium carbide is excessively formed, and the corrosion resistance of the material itself is lowered, and there is a risk of formation of coarse carbides in the annealed structure due to carbon segregation, so the upper limit is limited to 0.70% .

Nitrogen (N) is an element added to improve both corrosion resistance and hardness. It does not cause local fine segregation even when added in place of carbon (C), so that coarse precipitates are not formed on the product. Add 0.03% or more to achieve this effect. However, when added excessively, the upper limit is limited to 0.06% because there is a possibility of pore due to nitrogen during casting.

Since silicon (Si) is an essential element added for deoxidation, 0.1% or more is added. However, since excessive Si addition decreases the acidity and increases the brittleness of the material, its upper limit is limited to 0.6%.

Manganese (Mn) is an element added for deoxidation, so add 0.5% or more. However, if it is added excessively, the surface quality of the steel is inhibited and the upper limit is limited to 1.0% because the retained austenite of the final heat treatment material is inhibited from securing high hardness properties.

Chromium (Cr) is a basic element for ensuring corrosion resistance, so add 12% or more. However, if it is added excessively, the manufacturing cost is increased, the fine segregation of the chromium component in the tissue is increased, and the corrosion resistance and hardness of the reinforcing heat treatment material may be lowered by locally causing chromium carbide coarsening, .

Copper (Cu) is the most important alloy element in the stainless steel of the present invention. In order to secure a uniform distribution of fine chromium carbide, it is necessary to preferentially precipitate the Cu precipitate over the chromium carbide, A Cu content of 0.5% or more is required. However, when Cu is added excessively, there is a possibility that the composition, workability and corrosion resistance may be lowered, so the upper limit is limited to 2.0%.

The martensitic stainless steel according to an embodiment of the present invention having the above composition can be produced by a continuous casting or casting of steel ingot and then a hot rolled steel sheet which can be processed by hot rolling. The produced hot-rolled steel sheet has a thickness that can be used for rolling, and softening work is carried out through heat treatment to ensure good workability before proceeding with the process such as precision rolling.

As shown in Fig. 1, the unslit heat treatment is performed by a first cracking process in which spherical Cu precipitates are uniformly distributed in a martensitic stainless steel structure in a preferential manner, a first cracking process in which a martensitic stainless steel (hereinafter referred to as "hot- A second cracking process for uniformly distributing the chromium carbide in the texture of the chromium carbide, and a third cracking process for spheroidizing the fine particles of the chromium carbide. After the first cracking process, the temperature of the hot-rolled steel sheet is raised to a temperature until reaching the second cracking process, and the temperature of the hot-rolled steel sheet is lowered until the third cracking process after the second cracking process. Cooling process, and an air cooling process for cooling the hot-rolled steel sheet after the third cracking process.

The first cracking process is a process of uniformly distributing Cu precipitates in the structure of hot-rolled steel sheet, in which the hot-rolled steel sheet is uniformly heated for 5 to 15 hours in a constant temperature atmosphere of 500 to 600 ° C. In this process, fine Cu precipitates are uniformly distributed in the structure as shown in Fig. 2 (a), and exist in a size of several tens nm. This Cu precipitate generally acts as a deposition point of chromium carbide, which is known to precipitate first in the grain boundary, and then induces precipitation of uniform chromium carbide in the second cracking process. When the cracking temperature is less than 500 ° C., the Cu precipitates are not formed as shown in FIG. 2 (b), and when the temperature exceeds 600 ° C., the chromium carbide precipitates simultaneously with the Cu precipitates, It is preferentially deposited on these grain boundaries to prevent uniform distribution of fine carbides.

Further, when the cracking time is less than 5 hours in the first cracking process, Cu precipitation does not occur and it is impossible to secure a uniform distribution of chromium carbide. When the cracking time exceeds 15 hours, The number of Cu precipitates decreases and local Cu precipitates are distributed. As a result, it becomes difficult to obtain uniform distribution of chromium carbide. The first cracking process is preferably performed at a temperature of 500 to 600 ° C for 5 to 15 hours.

The heating process is a process of raising the hot-rolled steel sheet at a rate of 40 to 200 ° C / h until the second cracking process after the first cracking process. When the heating rate is less than 40 ° C / h during the heating process, the time period over which the chromium carbide becomes coarse, for example, 700 to 750 ° C, increases and the chromium carbide becomes large in size, The density of the carbide can be reduced. On the other hand, if the heating rate is more than 200 ° C / h, the durability time of the chromium carbide is decreased to secure the minute chromium carbide, but the chromium carbide distribution imbalance There are disadvantages that result. Therefore, the temperature raising rate in the temperature raising process is preferably 40 占 폚 / h or more and 200 占 폚 / h or less.

The second cracking process is a process of uniformly distributing chromium carbide in the structure of the hot-rolled steel sheet after the heating process, and is a process of uniformly heating the hot-rolled steel sheet in a constant temperature atmosphere of 800 to 900 ° C for 15 to 25 hours. In this process, chromium carbide is uniformly distributed in the tissue. If the crack temperature is less than 800 ° C., the coagulated portion can be formed due to the chromium carbide locally precipitated at the grain boundary during the cracking treatment. If the crack temperature is higher than 900 ° C., coarse chromium carbide is formed in the vicinity of grain boundaries, And coarse chromium carbide cause a local material imbalance of the material, making it difficult to secure ductility and causing material quality deterioration during the final heat treatment.

If the cracking time in the second cracking process is less than 15 hours, it is advantageous for formation of fine chromium carbide, but chromium carbide is not uniformly distributed, can be distributed and distributed, and if it exceeds 25 hours, The coalescence of the carbides and the local chromium carbide coarsening are progressed, and the process efficiency is decreased due to the increase of the heat treatment time and the manufacturing cost is increased. Therefore, it is preferable that the second cracking process is performed at a temperature of 800 to 900 DEG C for 15 to 25 hours.

The cooling process is a process of cooling the hot-rolled steel sheet to 600 to 750 ° C until the third cracking process after the second cracking process, and it is preferable to cool the hot-rolled steel sheet at a rate of 10 ° C / h or more. If the cooling rate is less than 10 ° C / h, the time period over which the size of the chromium carbide microstructure passes through the coarse temperature range is increased, thereby causing the chromium carbide microstructure in the microstructure to coarser, It is difficult to secure corrosion resistance and hardness.

The third cracking process is a process of cooling down the fine particles of the chromium carbide in the hot-rolled steel sheet and cooling the hot-rolled coil at a constant temperature of 600 to 750 ° C for 5 to 15 hours . The minimum temperature condition for spheroidizing the chromium carbide is 600 ° C. If the temperature exceeds 750 ° C., the spheroidized chromium carbide is excessively grown to decrease the number of chromium carbide and decrease the ductility.

In addition, if the holding time of the third cracking process is less than 5 hours, the progress of spheroidization of the chromium carbide is insufficient, and if it exceeds 15 hours, the spheroidized carbide is excessively grown to form coarse microstructure. Therefore, it is preferable that the third cracking process is performed for 5 to 15 hours under a constant temperature atmosphere of 600 to 750 ° C.

After the third cracking process, the hot-rolled steel sheet is air-cooled in the air to complete the annealing process.

Hereinafter, the present invention will be described by comparing Examples and Comparative Examples.

First, hot rolled steel sheets according to Examples and Comparative Examples were prepared which contain the same composition as Table 1 and contain the remaining iron (Fe) and unavoidable impurities (wt%).

division C Si Mn Cr Cu N Comparative Example 1 0.683 0.408 0.693 13.21 0 0.03 Comparative Example 2 0.695 0.391 0.701 13.16 2.52 0.04 Example 1 0.687 0.402 0.730 13.24 0.51 0.04 Example 2 0.651 0.4 0.688 13.3 1.02 0.06 Example 3 0.700 0.380 0.638 12.76 1.46 0.03 Example 4 0.692 0.424 0.722 13.55 1.98 0.05

The hot-rolled steel sheets were prepared from martensitic stainless steels having compositions of the comparative examples and the examples, and hot-rolled to obtain a steel sheet having a thickness of 3 mm. Then, the surface and edge quality of the hot-rolled steel sheet according to each of the comparative examples and the examples were confirmed.

The hot-rolled steel sheet thus prepared was subjected to heat treatment using the following annealing conditions and then air-cooled to observe the microstructure and evaluate the elongation. The results are shown in Table 2.

[Condition for heat treatment]

- First cracking process: 500 ° C for 10 hours

-Heating process: heating rate: 100 ° C / hr

- Second cracking process: 20 hours at 850 ° C

- Cooling process: Temperature lowering rate 10 ° C / hr

- Third cracking process: 7 hours at 650 ° C

division
Cu content
(Wt%) Hot-rolled steel sheet quality material Surface portion Edge portion Hardness (Hv) Elongation (%) Carbide aspect Comparative Example 1 0 Good Good 198 21.1 Uniformity Comparative Example 2 2.52 Cracking Cracking 231 17.5 Uniformity Example 1 0.51 Good Good 203 20.8 Uniformity Example 2 1.02 Good Good 215 19.3 Uniformity Example 3 1.46 Good Good 218 19.2 Uniformity Example 4 1.98 Good Good 224 19.2 Uniformity

(Comparative Example 1, Examples 1 to 4) When the Cu content in the hot-rolled steel sheet was 0 to 2% (Comparative Example 1, Examples 1 to 4), the quality of the surface and edge portions of the material were satisfactorily observed after hot rolling. And an elongation of 18% or more.

On the other hand, when the Cu content was 2.5% or more (Comparative Example 2), it was confirmed that a large amount of cracks were generated on the surface and the edge portion of the material after hot rolling, and this was attributed to the disadvantage of hot workability due to the addition of a large amount of Cu do. In addition, the elongation rate was estimated to be less than 18% even after the application was made, which is considered to be due to the surface crack introduced during hot rolling. Based on the above results, it can be seen that the Cu content should be limited to 2% or less in order to ensure good processability.

Carbide texture observation was performed using a sintered material having Cu content of 0 to 2.0%, and representative results thereof are shown in Fig. FIG. 3 is a photograph showing microstructure photographs of a material in which the Cu addition steel and the Cu non-added steel have been subjected to surface treatment. FIG. 3 (a) is a photograph of a microstructure for Comparative Example 1 in which Cu is not added, 3 (b) shows a microstructure photograph of Example 4 in which Cu was added at about 2%.

As a result of observation of the microstructure, it can be seen that the number of chromium carbides distributed in the structure (b) of Fig. 3 to which Cu is added compared to Fig. 3 (a) in which Cu is not added is relatively large.

In order to quantitatively analyze the change of the density of chromium carbide according to the Cu content, a 3 mm thick material was subjected to cold rolling and annealing several times and then rolled to 0.2 mm, and then the chromium carbide was evaluated for the total thickness of the material. Respectively.

As a result, the number of chromium carbide was about 60 ~ 70/100 ㎛ ² when the Cu content was 0 ~ 0.5%, while the number of chromium carbide increased about 20 ~ 100 pieces / 100 mu m < ^{2 &} gt ;.

These results are attributable to the influence of Cu precipitates formed in the first cracking process at an initial temperature of 500 to 600 ° C as described above. The Cu precipitates preferentially deposited and grown relative to chromium carbide act as starting points of the chromium carbide to precipitate fine carbides Was uniformly precipitated.

From the above results, it can be seen that the steel according to the present invention contains 0.60 to 0.70% of C, 0.03 to 0.06% of N, 0.1 to 0.6% of Si, 0.5 to 1.0% of Mn, 12 to 14% of Cr, 2% and the remainder Fe and unavoidable impurities is applied to the martensitic stainless steel under the conditions set forth in the present invention, chromium carbides of not less than 80/100 탆 ² are distributed in the structure, and the elongation It is possible to manufacture a high carbon martensitic stainless steel for a tool having an excellent hardness, corrosion resistance and workability of 18% or more.

Although the present invention has been described with reference to the accompanying drawings and the preferred embodiments described above, the present invention is not limited thereto but is limited by the following claims. Accordingly, those skilled in the art will appreciate that various modifications and changes may be made thereto without departing from the spirit of the following claims.

Claims (9)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.60 to 0.70% of C, 0.5 to 2% of Cu, 0.03 to 0.06% of N, 0.1 to 0.6% of Si, 0.5 to 1.0% of Mn, 12 to 14% unavoidable impurities martensitic stainless 80 / 100㎛ ² or more martensitic stainless steel of chromium carbides are distributed in the lecture tissue containing.
delete The method according to claim 1,
Wherein the martensitic stainless steel is an martensitic stainless steel having an elongation of 18% or more.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.60 to 0.70% of C, 0.5 to 2% of Cu, 0.03 to 0.06% of N, 0.1 to 0.6% of Si, 0.5 to 1.0% of Mn, 12 to 14% casting the molten steel, but stainless steel containing incidental impurities and subjected to heat treatment so that hot-rolled, and then a 80 / 100㎛ ² or more chromium carbide distributed in the martensitic stainless steel tissue,
Wherein the heat treatment is performed at 500 to 600 ° C for 5 to 15 hours to form a first cracking step of uniformly distributing Cu precipitates in the structure of the martensitic stainless steel and a second cracking step of continuously heating the martensite stainless steel at 800 to 900 ° C for 15 to 25 hours, A second cracking process for uniformly distributing the chromium carbide in the structure of the stainless steel, and a third cracking process for sintering the fine particles of the chromium carbide continuously at 600 to 750 ° C for 5 to 15 hours Method of manufacturing stainless steel.
delete delete delete delete The method of claim 4,
A temperature raising step of raising the temperature at a rate of 40 to 200 ° C / h from the first cracking step to the second cracking step,
A cooling step of cooling at a rate of 10 ° C / h or more from the second cracking step up to the third cracking step,
And the air cooling process further proceeds after the third cracking process.

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