Austenitic stainless steel with improved corrosion resistance and machinability and its manufacturing method

Abstract

Provided are austenitic stainless steel with improved corrosion resistance and machinability, and a method for producing the same. [Solution] The austenitic stainless steel with improved corrosion resistance and machinability according to the present invention contains, by weight, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16-22%, Ni: 9-15%, Mo: more than 0 and 3%, N: 0.15-0.25%, B: 0.004-0.06%, with the remainder being Fe and unavoidable impurities, and is characterized by having BN precipitate phases distributed at 10 particles / 100x100μm2 or more, MnS precipitate phases distributed at 10 particles / 100x100μm2 or less, and the length of the major axis of the MnS precipitate phases being 1μm or more.

Description

[Technical field]

[0001] The present invention relates to an austenitic stainless steel having improved corrosion resistance and machinability and a manufacturing method thereof, and more particularly to an austenitic stainless steel having improved corrosion resistance and machinability that is usable in corrosive environments such as salt water where machinability is required, and a manufacturing method thereof. [Background technology]

[0002] Austenitic stainless steels used for machine parts such as frames, chambers, dies, etc. are manufactured into their final shapes through cutting processes such as milling. In order to reduce the cutting load during cutting, increase the cutting speed, and improve the tool life, the machinability of stainless steel materials is required.

[0003] As stainless steels with excellent machinability, those that have been widely known have improved machinability by adding Mn and S to the steel and utilizing MnS compounds, which are non-metallic inclusions. However, MnS compounds are easily dissolved in corrosive environments such as salt water, or act as the starting point for pitting corrosion, which deteriorates the corrosion resistance of stainless steel. Therefore, stainless steels that utilize MnS compounds have a problem in that their use is limited in applications that require corrosion resistance due to exposure to corrosive environments. As a result, there is a demand for the development of stainless steels that can ensure both machinability and corrosion resistance. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide an austenitic stainless steel having improved corrosion resistance and machinability, and a method for producing the same. [Means for solving the problem]

[0005] The austenitic stainless steel of the present invention, which has improved corrosion resistance and machinability, contains, by weight, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16 to 22%, Ni: 9 to 15%, Mo: more than 0 and 3%, N: 0.15 to 0.25%, B: 0.004 to 0.06%, and the remainder being Fe and unavoidable impurities, and has a BN precipitate phase of 10 / 100x100μm. 2 Distributed as above.

[0006] In addition, the MnS precipitate phase is 10 pieces / 100x100μm 2 It can be distributed as follows:

[0007] In addition, there are 10 MnS precipitates per 100x100μm with a major axis length of 1μm or more. 2 It can be distributed as follows:

[0008] In addition, Cu:0 more than 1% may be further included.

[0009] The pitting potential may be 300 mV or more.

[0010] The method for producing an austenitic stainless steel having improved corrosion resistance and machinability according to the present invention includes the steps of heating a steel slab containing, by weight percent, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16-22%, Ni: 9-15%, Mo: more than 0 and 3%, N: 0.15-0.25%, B: 0.004-0.06%, and the remainder being Fe and unavoidable impurities, at 1,150-1,250°C for 1 hour and 30 minutes or more, hot rolling the heated steel slab, and maintaining the rolled material at 1,100-1,250°C for 30 seconds or more. Effect of the Invention

[0011] According to the present invention, the austenitic stainless steel having improved corrosion resistance and machinability can ensure both corrosion resistance and machinability at the same time. [Brief description of the drawings]

[0012] [Figure 1a] 6 is a photograph showing the appearance of Example 7 and Comparative Example 2 after hot rolling. [Figure 1b] 6 is a photograph showing the appearance of Example 7 and Comparative Example 2 after hot rolling. [Figure 2a] 3 is a photograph of the cross section of the stainless steel of Example 7 and Comparative Example 1 observed by SEM. [Figure 2b] 3 is a photograph of the cross section of the stainless steel of Example 7 and Comparative Example 1 observed by SEM. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The austenitic stainless steel of the present invention, which has improved corrosion resistance and machinability, contains, by weight, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16 to 22%, Ni: 9 to 15%, Mo: more than 0 and 3%, N: 0.15 to 0.25%, B: 0.004 to 0.06%, and the remainder being Fe and unavoidable impurities, and has a BN precipitate phase of 10 / 100x100μm. 2 Distributed as above.

[0014] This specification does not describe all elements of the present embodiment, and content that is common in the technical field to which the present invention belongs or that overlaps between embodiments will be omitted. Furthermore, when any part is said to "comprise" a certain component, this does not mean to exclude other components, but means that the part may further include other components, unless specifically stated to the contrary. The singular expression includes the plural expression unless the context clearly dictates otherwise.

[0015] The present invention will be described in detail below.

[0016] The following examples are presented in order to fully convey the concept of the present invention to those skilled in the art to which the present invention pertains. The present invention is not limited to the examples presented herein and may be embodied in other forms.

[0017] In the present invention, the formation of MnS, which deteriorates corrosion resistance, is eliminated to prevent the formation of MnS precipitates, and a BN compound is introduced to replace MnS and improve machinability.

[0018] However, if B is added in excess of the appropriate level, a problem occurs in that fracture occurs during hot rolling to produce a plate material. Therefore, the inventors of the present invention have discovered, through various experiments, the optimized contents of B, N and other elements for the formation of BN at a level effective for improving machinability while suppressing fracture during hot rolling.

[0019] The austenitic stainless steel of the present invention with improved corrosion resistance and machinability contains, by weight, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16-22%, Ni: 9-15%, Mo: more than 0 and 3%, N: 0.15-0.25%, B: 0.004-0.06%, and the remainder being Fe and unavoidable impurities.

[0020] In addition, Cu:0 more than 1% may be further included.

[0021] The reasons for limiting the numerical values ​​of the element contents of the alloy components of the present invention will be described below. In the following, the unit is weight percent unless otherwise specified.

[0022] The C content is greater than 0 and less than 0.05%. C is an austenite forming element and acts as an inevitable impurity. If the C content exceeds 0.05%, it may impair the corrosion resistance of the welded part, so the C content is limited to 0.05%.

[0023] The Si content is greater than 0 and 2%. Silicon is an element that can be added as a deoxidizer to improve corrosion resistance. However, if the silicon content exceeds 2%, it can impair toughness, so the silicon content is limited to 2% or less.

[0024] The Mn content is greater than 0 and 2%. Mn is an element that stabilizes austenite. However, if the Mn content exceeds 2%, it may impair corrosion resistance, so the Mn content is limited to 2% or less.

[0025] The S content is 0.01% or less. The S content is limited to 0.01% or less in order to prevent the formation of MnS, which is the compound sought to eliminate in the present invention.

[0026] The Cr content is 16-22%. Cr is an element that improves the corrosion resistance of austenitic stainless steel. If the Cr content is less than 16%, it is difficult to achieve the above-mentioned effects, and if the Cr content exceeds 22%, it may cause an increase in raw material costs and a decrease in toughness. Therefore, in the present invention, the Cr content is limited to 16-22%.

[0027] The Ni content is 9-15%. Ni is an element that stabilizes austenite. If the Ni content is less than 9%, it is difficult to exhibit the above-mentioned effects, and if the Ni content exceeds 15%, it will increase the raw material cost. Therefore, in the present invention, the Ni content is limited to 9 to 15%.

[0028] The Mo content is greater than 0 and 3%. Mo is an element that improves corrosion resistance. However, if the Mo content exceeds 3%, it increases the raw material cost. Therefore, in the present invention, the Mo content is limited to 3%.

[0029] The B content is 0.004 to 0.06%. B is an element added to ensure BN. If the B content is less than 0.004%, sufficient BN, which is the target of the present invention, cannot be formed, and if the B content exceeds 0.06%, fracture occurs during hot rolling. Therefore, in the present invention, the B content is limited to 0.004 to 0.06%.

[0030] The N content is 0.15-0.25%. N is an element added to ensure BN. If the B content is less than 0.15%, sufficient BN cannot be formed, and if the B content exceeds 0.25%, the toughness of the steel is impaired. Therefore, in the present invention, the N content is limited to 0.15 to 0.25%.

[0031] The Cu content is greater than 0 and 1%. Cu is an element that improves corrosion resistance and is added as necessary in the present invention. However, if the amount of Cu exceeds 1%, hot workability may deteriorate, so in the present invention, the amount of Cu is limited to 1%.

[0032] The balance outside the alloy composition is Fe. The austenitic stainless steel with improved corrosion resistance and machinability of the present invention may contain other impurities that may be contained in the industrial production process of steel. Since such impurities are known to anyone with ordinary skill in the art to which the present invention pertains, the present invention does not particularly limit the type and content of the impurities.

[0033] The austenitic stainless steel according to the present invention has a maximum diameter of 10 / 100x100μm, and the maximum diameter of the MnS precipitates is 1μm or more in any cross section. 2 In this case, the MnS precipitate phase can contain 50 at. % or more in total of Mn and S.

[0034] In the present invention, the formation of MnS, which leads to deterioration of corrosion resistance, is suppressed, so that corrosion resistance can be ensured, and the pitting potential of the austenitic stainless steel according to the present invention may be 300 mV or more.

[0035] The austenitic stainless steel according to the present invention has a BN precipitate phase of 10 / 100x100μm in any cross section. 2 At this time, the BN precipitate phase can contain 50 at.% or more in total of B and N. In the present invention, by substituting BN for MnS, it is possible to ensure machinability while suppressing deterioration of corrosion resistance.

[0036] Next, a method for manufacturing an austenitic stainless steel having improved corrosion resistance and machinability according to the present invention will be described.

[0037] The austenitic stainless steel having improved corrosion resistance and machinability according to the present invention may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an example, the austenitic stainless steel may be manufactured by the following method.

[0038] For example, the method includes the steps of heating a steel slab containing, by weight, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16 to 22%, Ni: 9 to 15%, Mo: more than 0 and 3%, N: 0.15 to 0.25%, B: 0.004 to 0.06%, and the remainder being Fe and unavoidable impurities, at 1,150 to 1,250°C for 1 hour and 30 minutes or more, hot rolling the heated steel slab, and maintaining the rolled material at 1,100 to 1,250°C for 30 seconds or more.

[0039] At this time, the heating step is a process for forming as much BN as possible, and may be performed at 1,150 to 1,250° C. for 1 hour and 30 minutes or more.

[0040] Also, hot rolling can be performed up to a thickness of 8 mm, but the thickness is not limited thereto as it can vary depending on the application.

[0041] In addition, the maintaining step after hot rolling is a process for forming BN again, and may be performed at 1,100 to 1,250° C. for 30 seconds or more.

[0042] The present invention will be described in detail below with reference to examples. However, the description of such examples is merely for illustrating the implementation of the present invention, and the present invention is not limited by the description of such examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0043] Working Example

[0044] An alloy satisfying the alloy composition in Table 1 below was melted and cast, and the austenitic stainless steel slab was heated at 1,200°C for 1 hour and 30 minutes. The heated slab was then hot rolled to a thickness of 8 mm. The hot-rolled material was then maintained at a temperature of 1,150°C for 30 seconds or more to form a BN precipitate phase, and a hot-rolled material test piece was obtained.

[0045] [Table 1]

[0046] Examples 1 to 10 and Comparative Examples 1 to 6 The hot-rolled test pieces were observed for the presence or absence of fracture after hot rolling. The results are shown in Table 2 below, with ◯ indicating that fracture occurred and X indicating that fracture did not occur.

[0047] [Table 2]

[0048] As shown in Table 2, in Examples 1 to 10 satisfying the alloy composition of the present invention, no fracture occurred during hot rolling. However, in Comparative Example 2, although the B content was satisfied, the N content did not reach the lower limit proposed by the present invention, and fracture occurred during hot rolling. (a) of FIG. 1 and (b) of FIG. 1 are photographs showing the appearances of Example 7 and Comparative Example 2 after hot rolling, respectively. Referring to (a) of FIG. 1, it can be seen that in Example 2 according to the present invention, the steel sheet has a good appearance without fracture. On the other hand, referring to (b) of FIG. 1, it can be seen that in Comparative Example 2, although the B content was satisfied, the N content did not reach the lower limit proposed by the present invention, and fracture occurred during hot rolling.

[0049] In Comparative Example 3, the B content was also satisfactory, but the N content was below the lower limit proposed in the present invention, and fracture occurred during hot rolling.

[0050] Subsequently, the hot-rolled test pieces of Comparative Examples 1 and 4 to 6 and Examples 1 to 10, which did not break during hot rolling, were observed for BN and MnS precipitates, and the corrosion resistance and machinability were evaluated. The results are shown in Table 3 below.

[0051] BN and MnS precipitates were measured by mirror polishing any cut surface of the plate material and then measuring 100x100μm 2 The number of MnS precipitates of 1 μm or more per 100 x 100 μm 2 The number of BN precipitates per sample was observed using a Scanning Electron Microscope (SEM) equipped with an Energy Dispersive Spectrometer (EDS), and the number was shown.

[0052] The corrosion resistance was evaluated by the pitting potential, which was measured by immersing a hot-rolled test piece in an aqueous solution containing 3.5wt% NaCl, connecting electrodes, applying a voltage, and gradually increasing the voltage from the natural potential, and measuring the voltage at the point where the current reached 0.1mA.

[0053] The machinability was evaluated by measuring the cutting load torque when cutting with an end mill under the conditions of a cutting depth of 2 mm, a cutting thickness of 5 mm, and an end mill rotation speed of 2,000 rpm. However, since the cutting environment may change, the torque of Comparative Example 1 is shown as the standard (100%).

[0054] [Table 3]

[0055] Referring to Tables 2 and 3, in Examples 1 to 10 which satisfy the alloy composition of the present invention, since MnS precipitates are not formed, the pitting potential is 300 mV or more and the corrosion resistance is good, and BN precipitates are 100 x 100 μm 2It can be seen that the cutting load was also lower than in Comparative Example 1, with 11 or more per cut, and machinability was also ensured. Figures 2(a) and 2(b) are SEM photographs of the cross sections of the stainless steels of Example 7 and Comparative Example 1, respectively. Referring to Figure 2(a), it can be seen that a large amount of BN, which is to be embodied in the present invention, was formed in Example 7 according to the present invention. On the other hand, referring to Figure 2(b), it can be seen that in Comparative Example 1, the conditions for BN formation were not created, and BN was not formed. The black areas observed in some areas are determined to be oxides, not BN.

[0056] On the other hand, in Comparative Example 1, MnS was not formed, the pitting potential was 550 mV, and the corrosion resistance was good. However, since B was not added, BN was not formed, and the cutting load was inferior to that of the Examples.

[0057] In Comparative Example 4, MnS was formed and the cutting load was low, but the N content was below the lower limit proposed in the present invention, so sufficient BN was not formed and the corrosion resistance was poor.

[0058] In Comparative Example 5, MnS was formed and the cutting load was low, but the B and N contents did not reach the lower limits proposed in the present invention, so sufficient BN was not formed and the corrosion resistance was poor.

[0059] In Comparative Example 6, MnS was not formed, the pitting potential was 1000 mV, and the corrosion resistance was good; however, the B content was below the lower limit proposed in the present invention, and the cutting load was poor.

[0060] The present invention is not limited thereto, and it should be understood that a person having ordinary skill in the art can make various changes and modifications within the concept and scope of the following claims. [Industrial Applicability]

[0061] According to the present invention, it is possible to provide an austenitic stainless steel having improved corrosion resistance and machinability, and a method for producing the same.

Claims

1. The steel contains, by weight percent, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16 to 22%, Ni: 9 to 15%, Mo: more than 0 and 3%, N: 0.15 to 0.25%, B: 0.004 to 0.06%, and the remainder being Fe and unavoidable impurities, BN precipitates: 10 / 100x100μm ２ An austenitic stainless steel having improved corrosion resistance and machinability, characterized by a distribution of 0.1 to 1.0 μm or more.

2. MnS precipitates: 10 / 100x100μm ２ Distributed as follows:

2. The austenitic stainless steel having improved corrosion resistance and machinability according to claim 1, characterized in that the length of the major axis of the MnS precipitate phase is 1 μm or more.

3. 2. The austenitic stainless steel having improved corrosion resistance and machinability according to claim 1, further comprising Cu: more than 0 and 1%.

4. 2. The austenitic stainless steel having improved corrosion resistance and machinability according to claim 1, characterized in that the pitting potential is 300 mV or more.

5. heating a steel slab containing, by weight percent, C: 0.05 or less (excluding 0), Si: more than 0 and 2%, Mn: more than 0 and 2%, S: 0.01% or less, Cr: 16-22%, Ni: 9-15%, Mo: more than 0 and 3%, N: 0.15-0.25%, B: 0.004-0.06%, and the remainder being Fe and unavoidable impurities, at 1,150-1,250°C for 1 hour and 30 minutes or more; hot rolling the heated billet; and The rolled material is maintained at 1,100 to 1,250° C. for 30 seconds or more, A method for producing an austenitic stainless steel having improved corrosion resistance and machinability, characterized in that the produced austenitic stainless steel has a distribution of 10 BN precipitates / 100×100 μm 2 or more.

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