KR20230077482A - Two phase stainless steel with improved strength and manufacturing method therefor - Google Patents

Abstract

A duplex phase stainless steel with improved strength, in which the martensite fraction within materials is appropriately controlled by optimizing alloy components and annealing and pickling processes, and a manufacturing method thereof are disclosed. The duplex phase stainless steel with improved strength according to an embodiment of the present invention includes 0.02 to 0.05% by weight of C, more than 0 up to 0.5% by weight of Si, more than 0 up to 0.5% by weight of Mn, more than 0 up to 0.5% by weight of Ni, 11.5 to 13.0% by weight of Cr, more than 0 up to 0.5% by weight of Mo, 0.01 to 0.02% by weight of N, and the remainder Fe and inevitable impurities, and the volume fraction of martensite, as a microstructure, maybe 40 to 60%.

Description

Ideal stainless steel with improved strength and its manufacturing method {TWO PHASE STAINLESS STEEL WITH IMPROVED STRENGTH AND MANUFACTURING METHOD THEREFOR}

The present invention relates to stainless steel with improved strength and a method for manufacturing the same, and more particularly, to stainless steel with improved strength by optimizing alloy components and an annealing and pickling process, and a method for manufacturing the same.

In general, ferritic stainless steel has excellent corrosion resistance with a small amount of expensive alloy elements such as Ni added. Therefore, ferritic stainless steel is a material having high price competitiveness compared to austenitic stainless steel.

Ferritic stainless steel is widely used as a building member requiring corrosion resistance. In particular, ferritic stainless steel sheets used for stone substrates require high strength with a hardness of HBR 83 or higher.

The strength of ferritic stainless steels can be improved by forming martensite in the matrix. However, if martensite is excessively formed in the ferritic stainless steel matrix, problems such as plate breakage may occur in the continuous annealing process. Therefore, in order to manufacture a ferritic-martensitic or higher stainless steel having improved strength through martensite formation, it is necessary to optimize alloy components and manufacturing processes.

An object of the present invention for solving the above problems is to provide an ideal stainless steel with improved strength and a method for manufacturing the same by appropriately controlling the martensite fraction in the material by optimizing the alloy components and the annealing and pickling process.

In the ideal stainless steel having improved strength according to an embodiment of the present invention, in weight%, C: 0.02% or more and 0.05% or less, Si: 0% or more and 0.5% or less, Mn: 0% or more and 0.5% or less, Ni: 0 % more than 0.5%, Cr: more than 11.5% and less than 13.0%, Mo: more than 0% and less than 0.5%, N: more than 0.01% and less than 0.02%, including the remainder Fe and unavoidable impurities, as a microstructure, martensite volume fraction It may be 40 to 60%.

In addition, the stainless steel with improved strength according to an embodiment of the present invention may have a Rockwell hardness of HRB 83 or more.

In addition, in the ideal stainless steel having improved strength according to an embodiment of the present invention, the number of repetitions of 90°, bending, and testing at room temperature may be five or more.

In addition, in the manufacturing method of stainless steel with improved strength according to an embodiment of the present invention, in weight%, C: 0.02% or more and 0.05% or less, Si: more than 0% and 0.5% or less, Mn: more than 0% and 0.5% or less , Ni: more than 0% and less than 0.5%, Cr: more than 11.5% and less than 13.0%, Mo: more than 0% and less than 0.5%, N: more than 0.01% and less than 0.02%, preparing a slab containing the remaining Fe and unavoidable impurities ; Reheating the slab to 1100 to 1300 ° C; preparing a hot-rolled material by hot-rolling the reheated slab; And pickling after hot rolling annealing of the hot rolled material at 850 to 1000 ° C., and the strength number (HN) represented by Equation (1) below may satisfy 100 or more and 107 or less.

Equation (1): 804.8*C + 155.4*N + 14.46*Si - 7.35*Cr + 0.283*T -106

In Equation (1), C, N, Si, and Cr mean the content (wt%) of each component element, and T means the heat treatment temperature (° C.) during annealing.

According to one embodiment of the present invention, by optimizing the alloy components and annealing heat treatment conditions to appropriately control the martensite fraction in the material, it is possible to provide an ideal stainless steel with improved strength and a manufacturing method thereof.

1 is a graph showing changes in hardness (HRB) according to Equation (1).

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention may be embodied in other forms without being limited to only the embodiments presented herein. In the drawings, in order to clarify the present invention, illustration of parts irrelevant to the description may be omitted, and the size of components may be slightly exaggerated to aid understanding.

Throughout the specification, when a certain component is said to "include", it means that it may further include other components without excluding other components unless otherwise stated.

Expressions in the singular number include plural expressions unless the context clearly dictates otherwise.

Hereinafter, the reason for limiting the numerical value of the alloy component content in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, units are % by weight.

In the ideal stainless steel having improved strength according to an embodiment of the present invention, in weight%, C: 0.02% or more and 0.05% or less, Si: 0% or more and 0.5% or less, Mn: 0% or more and 0.5% or less, Ni: 0 % more than 0.5% or less, Cr: 11.5% or more and 13.0% or less, Mo: more than 0% and 0.5% or less, N: 0.01% or more and 0.02% or less, the remaining Fe and unavoidable impurities may be included.

The content of C (carbon) may be 0.02% or more and 0.05% or less.

When the content of C is low, the hardness of the steel is lowered after heat treatment, making it difficult to secure machinability and wear resistance. Considering this, C may be added by 0.02% or more. However, when the content of C is excessive, chromium carbide may be excessively formed and corrosion resistance may deteriorate. In addition, when the content of C is excessive, coarse carbides may be formed in the annealed structure due to carbon segregation. Considering this, the upper limit of the C content may be limited to 0.05%. Preferably, the content of C may be 0.03% or more and 0.045% or less.

The content of Si (silicon) may be greater than 0% and 0.5% or less.

Si is a ferrite phase forming element and can be added for deoxidation. However, when the content of Si is excessive, there is a concern that steel-manufacturing Si inclusions may increase and surface defects may occur. Considering this, the upper limit of the Si content may be limited to 0.5%. Preferably, the Si content may be 0.1% or more and 0.5% or less, more preferably 0.1% or more and 0.4% or less.

The content of Mn (manganese) may be greater than 0% and less than or equal to 0.5%.

Mn is an austenite phase stabilizing element, and may be added to secure a certain level of austenite phase fraction at a hot rolling reheating temperature. However, when the Mn content is excessive, pitting resistance may be deteriorated by forming precipitates such as MnS. Considering this, the upper limit of the Mn content may be limited to 0.5%. Preferably, the content of Mn may be 0.1% or more and 0.3% or less.

The content of Ni (nickel) may be more than 0% and 0.5% or less.

Ni is an austenite phase stabilizing element, which is effective in inhibiting pitting evolution and is also effective in improving toughness of a hot-rolled steel sheet. However, when the content of Ni is excessive, there is a concern that material hardening and toughness decrease due to solid solution strengthening. In addition, when the content of Ni is excessive, raw material costs may increase. Considering this, the upper limit of the Ni content may be limited to 0.5%. Preferably, the content of Ni may be 0.02% or more and 0.3% or less.

The content of Cr (chromium) may be 11.5% or more and 13.0% or less.

Cr is an essential element for securing corrosion resistance of stainless steel in a condensate atmosphere. Considering this, Cr may be added by 11.5% or more. However, when the content of Cr is excessive, the elongation rate may decrease and the raw material cost may increase. Considering this, the upper limit of the Cr content may be limited to 13.0%. Preferably, the Cr content may be 11.8% or more and 12.8% or less.

The content of Mo (molybdenum) may be greater than 0% and less than or equal to 0.5%.

Mo is an element effective in improving the corrosion resistance of stainless steel. However, if the Mo content is excessive, it may cause an increase in raw material cost. In addition, when the Mo content is excessive, deterioration in hot workability may occur due to a decrease in grain boundary mobility. Considering this, the upper limit of the Mo content may be limited to 0.5%. Preferably, the upper limit of the Mo content may be limited to 0.1%.

The content of N (nitrogen) may be 0.01% or more and 0.02% or less.

N is an element effective in simultaneously improving corrosion resistance and hardness. In addition, N is an element having the advantage of not forming coarse precipitates because it does not cause local fine segregation even when added instead of C. Considering this, N may be added in an amount of 0.01% or more. However, when the content of N is excessive, it may be difficult to control the composition system as the solubility limit in molten steel is exceeded during casting. In addition, when the N content is excessive, pinhole defects may occur on the steel surface. Considering this, the upper limit of the N content may be limited to 0.02%.

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

Stainless steel having improved strength according to an embodiment of the present invention has a microstructure and may have a martensite volume fraction of 40 to 60%.

In order to improve the strength of ferritic stainless steel, it is necessary to form martensite in the material. However, since excessive formation of martensite results in poor workability, it is essential to secure an appropriate level of martensite fraction. According to the present invention, by optimizing the alloy components and the annealing heat treatment process, the martensite volume fraction can be controlled to about 40 to 60%.

In addition, the stainless steel having improved strength according to an embodiment of the present invention may have a Rockwell hardness of HRB 83 or more by controlling the martensite fraction described above.

In addition, in the ideal stainless steel having improved strength according to an embodiment of the present invention, the number of times of 90° repeated bending test at room temperature may be 5 or more. Preferably, the number of repetitions "bending" testing may be 10 or more and 30 or less.

Next, a method for manufacturing stainless steel having improved strength according to another aspect of the present invention will be described.

According to an embodiment of the present invention, the manufacturing method of stainless steel with improved strength, in weight%, C: 0.02% or more and 0.05% or less, Si: more than 0% and 0.5% or less, Mn: more than 0% and 0.5% or less, Ni : More than 0% and 0.5% or less, Cr: 11.5% or more and 13.0% or less, Mo: more than 0% and 0.5% or less, N: 0.01% or more and 0.02% or less, the remaining Fe and unavoidable impurities Preparing a slab containing; Reheating the slab to 1100 to 1300 ° C; preparing a hot-rolled material by hot-rolling the reheated slab; And pickling after hot rolling annealing of the hot rolled material at 850 to 1000 ° C., and the strength number (HN) represented by Equation (1) below may satisfy 100 or more and 107 or less.

Equation (1): 804.8*C + 155.4*N + 14.46*Si - 7.35*Cr + 0.283*T -106

In Equation (1), C, N, Si, and Cr mean the content (wt%) of each component element, and T means the heat treatment temperature (° C.) during annealing.

The reasons for limiting the range of components of each alloy composition are as described above, and each manufacturing step will be described in detail below.

First, after manufacturing a slab satisfying the alloy composition, a series of reheating, hot rolling, hot rolling annealing, and pickling may be performed.

First, the prepared slab may be reheated to 1100 to 1300 ° C.

When the reheating temperature is low, it may be difficult to re-decompose the coarse precipitates generated during slab manufacturing. Considering this, the reheating temperature may be 1100°C or higher. However, if the reheating temperature is too high, the internal crystal grains may become too coarse. Considering this, the upper limit of the reheating temperature may be limited to 1300°C.

Next, a hot-rolled material may be manufactured by hot-rolling the reheated slab. After that, the hot-rolled material may be pickled after hot rolling annealing at 850 to 1000 ° C.

When the hot rolling annealing temperature is low, the precipitated martensite may not be sufficient to exhibit desired strength. In consideration of this, the hot rolling annealing temperature may be 850° C. or higher. However, when the hot rolling annealing temperature is too high, production may be limited due to problems such as plate breakage during production due to excessive precipitation of martensite. In consideration of this, the upper limit of the hot rolling annealing temperature may be limited to 1000 °C.

In addition, in the method for manufacturing stainless steel with improved strength according to an embodiment of the present invention, the strength number (HN) represented by Equation (1) below may satisfy 100 or more and 107 or less.

Equation (1): 804.8*C + 155.4*N + 14.46*Si - 7.35*Cr + 0.283*T -106

In Equation (1), C, N, Si, and Cr mean the content (wt%) of each component element, and T means the heat treatment temperature (° C.) during annealing.

By performing an annealing and pickling process that satisfies Equation (1), an appropriate amount of martensite phase transformation may be induced, thereby obtaining an ideal stainless steel having improved strength.

When the strength number (HN) is less than 100, martensite formation may not be sufficiently performed, resulting in inferior strength. However, when the strength number (HN) exceeds 107, excessive formation of martensite may cause problems such as plate breakage during annealing and pickling.

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

{Example}

For the various alloy composition ranges shown in Table 1 below, slabs were prepared, reheated at 1200 ° C., and hot rolled to prepare hot rolled materials, followed by hot rolling annealing and pickling.

division alloy component C Si Mn Ni Cr Mo N steel grade A 0.044 0.331 0.23 0.07 11.888 0.01 0.0138 steel grade B 0.033 0.361 0.218 0.07 11.974 0.01 0.0106 steel grade C 0.031 0.045 0.12 0.02 12.7 0.01 0.02

Table 2 below shows the hot rolling annealing temperature, formula (1) value, martensite volume fraction, Rockwell hardness, and repeated bending test results.

The value of formula (1) was shown by calculating the following formula (1).

Equation (1): 804.8*C + 155.4*N + 14.46*Si - 7.35*Cr + 0.283*T -106

In the above formula (1), C, N, Si, and Cr mean the content (wt%) of each component element, and T means the heat treatment temperature (° C.) during annealing.

The martensite volume fraction was measured using an optical microscope manufactured by LEIKA.

Rockwell hardness was measured by pressing a steel ball of 1.588 mm into the sample surface under a load of 100 kgf using an atk-f3000 Rockwell hardness tester manufactured by Akashi Co., and measured through the depth into which the tip of the indenter entered.

The results of the repeated bending test were expressed as whether or not breakage occurred when the 90 ° repeated bending test was performed 5 times under the condition of 20 R at room temperature. As a result of the repeated bending test, when fracture occurred, it was marked with 'O', and when no fracture occurred, it was marked with 'X'.

In general, if breakage does not occur when the cyclic bending test is performed 5 times, the workability is good and the product can be mass-produced, so the number of cyclic bending tests is set to 5 times.

division steel grade Hot rolling annealing temperature
(℃) Equation (1) Martensite volume fraction
(%) Rockwell Hardness
(HRB) repeated bending
Test result Example 1 A 890 101 41 84 X Example 2 A 900 104 49 90 X Example 3 B 925 101 58 85 X Example 4 B 940 106 59 90 X Example 5 C 970 104 55 87 X Comparative Example 1 A 880 98 29 80 X Comparative Example 2 A 920 109 75 97 O Comparative Example 3 B 900 94 35 78 X Comparative Example 4 C 940 95 38 80 X Comparative Example 5 C 990 110 82 94 O

Referring to Table 2, Examples 1 to 5 satisfied the alloy composition, component range, formula (1) and manufacturing process presented in the present invention. Therefore, Examples 1 to 5 satisfied the Rockwell hardness HRB 83 or more by controlling the martensite volume fraction to 40 to 60%, and satisfied the number of 90 ° 　 repetition 　 bending test 5 times or more at room temperature. That is, in Examples 1 to 5, it was confirmed that workability was secured and strength was improved.

However, in Comparative Examples 1, 3 and 4, the value of Formula (1) was less than 100. Accordingly, Comparative Examples 1, 3, and 4 had a martensite volume fraction of less than 40% and did not satisfy the Rockwell hardness of HRB 83 or higher. That is, Comparative Examples 1, 3 and 4 failed to secure high strength.

In Comparative Examples 2 and 5, the value of Formula (1) exceeded 107. Therefore, in Comparative Examples 2 and 5, the martensite volume fraction exceeded 60%, and the number of times of the 90° repeated bending test at room temperature did not satisfy 5 times or more. That is, Comparative Examples 2 and 5 failed to secure processability.

1 is a graph showing changes in hardness (HRB) according to Equation (1).

Referring to FIG. 1 , it can be seen that Equation (1) and hardness (HRB) are in a proportional relationship. In particular, it can be confirmed that the value of Equation (1) must be 100 or more in order to secure the hardness HRB 83 or more.

Claims (4)

In weight percent, C: 0.02% or more and 0.05% or less, Si: 0% or more and 0.5% or less, Mn: 0% or more and 0.5% or less, Ni: 0% or more and 0.5% or less, Cr: 11.5% or more and 13.0% or less, Mo : More than 0% and 0.5% or less, N: 0.01% or more and 0.02% or less, including the remainder Fe and unavoidable impurities,
An ideal stainless steel with improved strength, with a microstructure and a martensite volume fraction of 40 to 60%. The method of claim 1,
An ideal stainless steel with improved strength, with a Rockwell hardness of at least HRB 83. The method of claim 1,
Ideal stainless steel with improved strength, with 5 or more repeated 90° bending tests at room temperature. In weight percent, C: 0.02% or more and 0.05% or less, Si: 0% or more and 0.5% or less, Mn: 0% or more and 0.5% or less, Ni: 0% or more and 0.5% or less, Cr: 11.5% or more and 13.0% or less, Mo : Preparing a slab containing more than 0% and 0.5% or less, N: 0.01% or more and 0.02% or less, the remainder Fe and unavoidable impurities;
Reheating the slab to 1100 to 1300 ° C;
preparing a hot-rolled material by hot-rolling the reheated slab; and
Including the step of pickling after hot-rolling the hot-rolled material at 850 to 1000 ℃,
Method for producing biphasic stainless steel with improved strength that satisfies 100 or more and 107 or less in strength number (HN) represented by the following formula (1):
Equation (1): 804.8*C + 155.4*N + 14.46*Si - 7.35*Cr + 0.283*T -106
(In the above formula (1), C, N, Si, Cr mean the content (wt%) of each component element, T means the heat treatment temperature (℃) during annealing).

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