Austenitic stainless steel having improved corrosion resistance and machinability and method for manufacturing the same

Abstract

An austenitic stainless steel with improved corrosion resistance and machinability is disclosed. The austenitic stainless steel with improved corrosion resistance and machinability according to the invention comprises, by weight percent: C: 0.05% or less (excluding 0), Si: 0% (excluding) to 2%, Mn: 0% (excluding) to 2%, S: 0.01% or less, Cr: 16% to 22%, Ni: 9% to 15%, Mo: 0% (excluding) to 3%, N: 0.15% to 0.25%, B: 0.004% to 0.06%, and the balance being Fe and unavoidable impurities.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an austenitic stainless steel having improved corrosion resistance and machinability and a method of manufacturing the same, and more particularly, to an austenitic stainless steel having improved corrosion resistance and machinability for use in a corrosive environment such as salt water and an environment requiring machinability and a method of manufacturing the same. BACKGROUND

[0002] An austenitic stainless steel for mechanical parts such as frames, chambers, molds, etc. is manufactured into a final shape through a machining process such as milling. Machinability of the stainless steel is required to reduce machining load, increase machining speed, and improve tool life.

[0003] A steel type to which Mn and S are added and which uses MnS compounds as non-metallic inclusions is widely known as a stainless steel having excellent machinability. However, the MnS compounds are easily eluted in a corrosive environment such as salt water or act as a starting point of pitting, which deteriorates the corrosion resistance of the stainless steel. Therefore, the stainless steel using the MnS compounds is limited in applications requiring corrosion resistance due to exposure to a corrosive environment. Thus, there is a need to develop a stainless steel that is both machinable and corrosion resistant. SUMMARY

[0004] TECHNICAL PROBLEM

[0005] One aspect of the present disclosure provides an austenitic stainless steel having improved corrosion resistance and machinability and a method of manufacturing the same.

[0006] TECHNICAL SOLUTION

[0007] According to one embodiment of the present disclosure, an austenitic stainless steel having improved corrosion resistance and machinability includes, in weight percent (wt.%): 0.05% or less but not including 0 of C, 2% or less but not including 0 of Si, 2% or less but not including 0 of Mn, 0.01% or less of S, 16% to 22% of Cr, 9% to 15% of Ni, 3% or less but not including 0 of Mo, 0.15% to 0.25% of N, 0.004% to 0.06% of B, and the remainder of Fe and unavoidable impurities, per 100 x 100 μm 2 10 or more BN precipitates can be distributed.

[0008] According to one embodiment of the present disclosure, 10 or more MnS precipitates can be distributed per 100 x 100 μm 2 10 or more MnS precipitates can be distributed per 100 x 100 μm

[0009] According to one embodiment of the present disclosure, 10 or more MnS precipitates can be distributed per 100 x 100 μm 2The MnS precipitates can be distributed with 10 or less of the MnS precipitates having a long axis length of 1 μm or more.

[0010] According to one embodiment of the present disclosure, the austenitic stainless steel having improved corrosion resistance and machinability can further include 1% or less but not including 0 of Cu in terms of weight percentage (wt%).

[0011] According to one embodiment of the present disclosure, the pitting potential can be 300 mV or more.

[0012] According to one embodiment of the present disclosure, the method of manufacturing the austenitic stainless steel having improved corrosion resistance and machinability can include: heating a stainless steel at 1,150°C to 1,250°C for 1 hour and 30 minutes or more, the stainless steel including, in terms of weight percentage (wt%): 0.05% or less but not including 0 of C, 2% or less but not including 0 of Si, 2% or less but not including 0 of Mn, 0.01% or less of S, 16% to 22% of Cr, 9% to 15% of Ni, 3% or less but not including 0 of Mo, 0.15% to 0.25% of N, 0.004% to 0.06% of B, and a remainder of Fe and unavoidable impurities; hot-rolling the heated stainless steel; and holding the hot-rolled steel at 1,100°C to 1,250°C for 30 seconds or more.

[0013] Advantages

[0014] The present disclosure provides an austenitic stainless steel having improved corrosion resistance and machinability and a method of manufacturing the same. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1a and 1b are photographs showing the appearance of Example 7 and Comparative Example 2, respectively, after hot-rolling; and

[0016] Figure 2a and 2b are photographs of cross sections of the stainless steels of Example 7 and Comparative Example 1, respectively, observed by a scanning electron microscope (SEM).

[0017] BEST MODE FOR CARRYING OUT THE INVENTION

[0018] The austenitic stainless steel having improved corrosion resistance and machinability according to one embodiment of the present disclosure contains, in weight percent (wt.%): 0.05% or less but not including 0 of C, 2% or less but not including 0 of Si, 2% or less but not including 0 of Mn, 0.01% or less of S, 16% to 22% of Cr, 9% to 15% of Ni, 3% or less but not including 0 of Mo, 0.15% to 0.25% of N, 0.004% to 0.06% of B, and the remainder of Fe and unavoidable impurities, and each 100 x 100 μm 2 The 10 or more BN precipitates are distributed. DETAILED DESCRIPTION

[0019] The present specification does not describe all elements according to the embodiments of the present disclosure, and omits the description or overlapping parts which are well known in the art to which the present disclosure pertains.

[0020] Throughout the specification, unless otherwise specified, the term "comprising" one element does not exclude other elements, but can also include additional elements.

[0021] As used herein, the singular form "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0022] Hereinafter, the embodiments of the present disclosure will be described in detail.

[0023] The following embodiments of the present disclosure provide a full disclosure of the present disclosure to a person of ordinary skill in the art to which the present disclosure pertains. The present disclosure is not limited to the embodiments shown herein, but can be presented in other forms.

[0024] According to the present disclosure, the formation of MnS deteriorating corrosion resistance is excluded to prevent the formation of MnS precipitates. In addition, BN compounds are introduced instead of MnS to improve machinability.

[0025] However, the addition of B beyond an appropriate level causes cracking during hot rolling for producing a plate. Accordingly, the present inventors have found an optimized content of B, N, and other elements, enabling the formation of BN at an effective level for improving machinability, while suppressing cracking during hot rolling.

[0026] According to one embodiment of the present disclosure, an austenitic stainless steel having improved corrosion resistance and machinability contains, in weight percent (% by weight), 0.05% or less but not including 0 of C, 2% or less but not including 0 of Si, 2% or less but not including 0 of Mn, 0.01% or less of S, 16% to 22% of Cr, 9% to 15% of Ni, 3% or less but not including 0 of Mo, 0.15% to 0.25% of N, 0.004% to 0.06% of B, and the remainder of Fe and unavoidable impurities.

[0027] Further, the austenitic stainless steel having improved corrosion resistance and machinability can also contain, in weight percent (% by weight), 1% or less but not including 0 of Cu.

[0028] Hereinafter, reasons for numerical limits on alloy element contents in embodiments of the present disclosure will be described. Units are % by weight unless otherwise specified.

[0029] Carbon (C) has a content of 0.05% or less but not including 0.

[0030] Carbon (C) is an austenite-forming element and acts as an unavoidable impurity. When the content of C exceeds 0.05%, the corrosion resistance of a welded portion can be impaired, and thus the content of C is controlled to 0.05%.

[0031] Silicon (Si) has a content of 2% or less but not including 0.

[0032] Si is added as a deoxidizer and is an element for improving corrosion resistance. However, when the content of Si exceeds 2%, the toughness can be deteriorated, and thus the content of Si is controlled to 2% or less in the present disclosure.

[0033] Manganese (Mn) has a content of 2% or less but not including 0.

[0034] Mn is an austenite phase stabilizing element. However, when the content of Mn exceeds 2%, the corrosion resistance can be deteriorated, and thus the content of Mn is controlled to 2% or less in the present disclosure.

[0035] Sulfur (S) has a content of 0.01% or less.

[0036] In order to prevent the formation of MnS to be excluded in the present disclosure, the content of S is controlled to 0.01% or less.

[0037] Chromium (Cr) has a content of 16% to 22%.

[0038] Cr is an element for improving corrosion resistance of the austenitic stainless steel. When the Cr content is less than 16%, the above effect can not be obtained. A Cr content exceeding 22% can increase raw material costs and reduce toughness. Therefore, the Cr content is controlled to 16 to 22% in the present disclosure.

[0039] The content of nickel (Ni) is 9 to 15%.

[0040] Ni is an austenite phase stabilizing element. When the Ni content is less than 9%, the above effect can not be obtained. A Ni content exceeding 15% results in an increase in raw material costs. Therefore, the Ni content is controlled to 9 to 15% in the present disclosure.

[0041] The content of molybdenum (Mo) is 3% or less (excluding 0).

[0042] Mo is an element for improving corrosion resistance. However, a Mo content exceeding 3% results in an increase in raw material costs, and therefore the Mo content is controlled to 3% in the present disclosure.

[0043] The content of boron (B) is 0.004 to 0.06%.

[0044] B is added to secure BN. When the content of B is less than 0.004%, it can not be possible to form sufficient BN targeted in the present disclosure, and when the content of B exceeds 0.06%, a breakage occurs during hot rolling. Therefore, the content of B is controlled to 0.004 to 0.06% in the present disclosure.

[0045] The content of nitrogen (N) is 0.15 to 0.25%.

[0046] N is added to secure BN. When the content of N is less than 0.15%, it can not be possible to form sufficient BN, and when the content of N exceeds 0.25%, toughness is deteriorated. Therefore, the content of N is controlled to 0.15 to 0.25% in the present disclosure.

[0047] The content of copper (Cu) is 1% or less but excluding 0.

[0048] Cu is an element for improving corrosion resistance, and is added as necessary in the present disclosure. However, when the content of Cu exceeds 1%, hot workability can be deteriorated, and therefore the Cu content is controlled to 1% in the present disclosure.

[0049] The remaining component of the alloy composition of the present disclosure is iron (Fe). The austenitic stainless steel having improved corrosion resistance and machinability according to the present disclosure can contain other impurities that can be contained in a typical industrial production process of steel. Since the types and amounts of these impurities are known to those skilled in the art to which the present disclosure pertains, they are not specifically limited in the present disclosure.

[0050] In any portion of the austenitic stainless steel according to the present disclosure, 10 or less MnS precipitates having a long axis length of 1 μm or more are distributed per 100 x 100 μm 2 In this case, the MnS precipitates can contain a total of 50 atomic % or more of Mn and S.

[0051] According to the present disclosure, since the formation of MnS that causes deterioration of corrosion resistance is suppressed, corrosion resistance can be ensured, and the pitting potential of the austenitic stainless steel of the present disclosure can be 300 mV or more.

[0052] In any portion of the austenitic stainless steel according to the present disclosure, 10 or less MnS precipitates having a long axis length of 1 μm or more are distributed per 100 x 100 μm 2 In this case, the MnS precipitates can contain a total of 50 atomic % or more of Mn and S. According to the present disclosure, MnS is replaced with BN, thereby ensuring machinability while suppressing deterioration of corrosion resistance.

[0053] Next, a method of manufacturing the austenitic stainless steel having improved corrosion resistance and machinability according to one embodiment of the present disclosure will be described.

[0054] The austenitic stainless steel having improved corrosion resistance and machinability according to one embodiment of the present disclosure can be manufactured in various methods, and the manufacturing method is not particularly limited. However, as one embodiment, the austenitic stainless steel having improved corrosion resistance and machinability according to one embodiment of the present disclosure can be manufactured as described below.

[0055] For example, a manufacturing method of an austenitic stainless steel having improved corrosion resistance and machinability according to one embodiment of the present disclosure includes: heating a stainless steel at 1,150 to 1,250 °C for 1 hour and 30 minutes or more, the stainless steel including, in weight percent (wt%): 0.05 or less but not including 0 of C, 2% or less but not including 0 of Si, 2% or less but not including 0 of Mn, 0.01% or less of S, 16 to 22% of Cr, 9 to 15% of Ni, 3% or less but not including 0 of Mo, 0.15 to 0.25% of N, 0.004 to 0.06% of B, and a remainder of Fe and unavoidable impurities; hot-rolling the heated stainless steel; and holding the hot-rolled steel at 1,100 to 1,250 °C for 30 seconds or more.

[0056] In this case, the heating is a process for forming as much BN as possible, and can be performed at 1,150 to 1,250 °C for 1 hour and 30 minutes or more.

[0057] Further, the hot-rolling can be performed to a thickness of 8 mm, but the thickness can vary according to the use without being limited thereto.

[0058] Further, the holding process after the hot-rolling is for forming BN again, and can be performed at 1,100 to 1,250 °C for 30 seconds or more.

[0059] Hereinafter, the present disclosure will be described in more detail by examples. However, it is noted that the following examples are intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by matters described in the claims and capable of being reasonably inferred therefrom.

[0060] Embodiments

[0061] An alloy satisfying the alloy composition of Table 1 was melt-cast, and an austenitic stainless steel casting was heated at 1,200 °C for 1 hour and 30 minutes. Thereafter, the heated steel casting was hot-rolled to a thickness of 8 mm. Subsequently, the hot-rolled steel was held at a temperature of 1,150 °C for 30 seconds or more to form BN precipitates, thereby obtaining a hot-rolled steel sample.

[0062] [Table 1]

[0063]

[0064] For the hot-rolled steel samples of Examples 1 to 10 and Comparative Examples 1 to 6, whether or not a fracture occurred after the hot-rolling was observed, and in Table 2 below, the case where a fracture occurred was marked as O, and the case where a fracture did not occur was marked as X.

[0065] [Table 2]

[0066]

[0067] Referring to Table 2, in Examples 1 to 10, which satisfy the alloy composition of this disclosure, no fracture occurred during hot rolling. However, in Comparative Example 2, the B content was satisfactory, but the N content did not reach the lower limit proposed in this disclosure, resulting in fracture during hot rolling. Figure 1a and Figure 1b Photographs showing the appearance of Example 7 and Comparative Example 2 after hot rolling. (Refer to...) Figure 1a It can be determined that the steel plate in Embodiment 7 according to this disclosure does not appear to be broken. Instead, referring to... Figure 1b It can be determined that Comparative Example 2 has a satisfactory B content, but the N content does not reach the lower limit proposed in this disclosure, and therefore fractures during hot rolling.

[0068] In Comparative Example 3, although the B content was satisfactory, the N content did not reach the lower limit proposed in this disclosure, and therefore fracture occurred during hot rolling.

[0069] Subsequently, BN precipitates and MnS precipitates were observed in the hot-rolled steel samples of Comparative Examples 1 and 4 to 6, and Examples 1 to 10, which did not fracture during hot rolling, and their corrosion resistance and machinability were evaluated, as shown in Table 3 below.

[0070] BN and MnS precipitates were mirror-polished on any cut surface of the steel plate, and then observed at 100 × 100 μm using a scanning electron microscope (SEM) connected to an energy-dispersive spectroscopy (EDS) instrument. 2 The amount of MnS precipitates of 1 μm or larger and per 100 × 100 μm 2 The amount of BN precipitates is shown.

[0071] Corrosion resistance was assessed by pitting potential. Pitting potential was measured by immersing a hot-rolled steel sample in an aqueous solution containing 3.5 wt% NaCl, connecting electrodes, applying a voltage, and measuring the voltage at the point where the current reaches 0.1 mA as the voltage gradually increases from its natural potential.

[0072] Machinability was evaluated by measuring the cutting load torque under conditions of a 2 mm depth of cut, a 5 mm thickness of cut, and an end mill speed of 2,000 rpm when cutting with an end mill. However, since the cutting environment may vary, the torque of Comparative Example 1 was used as a reference (100%).

[0073] [Table 3]

[0074]

[0075] Referring to Tables 2 and 3, Examples 1 to 10, which meet the requirements of this disclosure, did not form MnS precipitates, and therefore their corrosion resistance is satisfactory, with pitting potentials greater than 300 mV. Furthermore, the number of BN precipitates is greater than 11 per 100 × 100 μm. 2 The cutting load was lower than that of Comparative Example 1, so it can be determined that machinability was also ensured. Figure 2a and 2b Photographs of cross-sections of stainless steel from Example 7 and Comparative Example 1, observed by SEM. (Refer to...) Figure 2a In Example 7, it can be determined that a large number of BNs to be implemented in this disclosure have been formed. However, referring to Figure 2b Figure 2b In Comparative Example 1, it can be determined that BN was not formed because the conditions required for BN formation were not met. Some black areas appear to be oxides rather than BN.

[0076] In contrast, Comparative Example 1 exhibited satisfactory corrosion resistance, with a pitting potential of 550 mV, because no MnS was formed. However, because no B was added, no BN was formed, and the cutting load was inferior to that of the Example.

[0077] In Comparative Example 4, MnS was formed and the cutting load was low, but the N content did not reach the lower limit proposed in this disclosure. Therefore, sufficient BN was not formed, and the corrosion resistance was poor.

[0078] 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 this disclosure, so sufficient BN was not formed and the corrosion resistance was poor.

[0079] Comparative Example 6 showed satisfactory corrosion resistance, with a pitting potential of 1000 mV, because no MnS was formed. However, the cutting load was poor because the B content did not reach the lower limit set forth in this disclosure.

[0080] Although embodiments have been described for illustrative purposes, those skilled in the art will understand that various modifications, additions, and substitutions are possible without departing from the scope and spirit of this disclosure. Therefore, embodiments are not described for limiting purposes.

[0081] Industrial applicability

[0082] According to this disclosure, austenitic stainless steel with improved corrosion resistance and machinability, and a method for manufacturing the same, are provided.

Claims

1. An austenitic stainless steel with improved corrosion resistance and machinability, comprising, by weight percent (wt%): 0.05% or less but not less than 0% C, 2% or less but not less than 0% Si, 2% or less but not less than 0% Mn, 0.01% or less S, 16% to 22% Cr, 9% to 15% Ni, 3% or less but not less than 0% Mo, 0.15% to 0.25% N, 0.004% to 0.06% B, and the balance being Fe and unavoidable impurities. Among them, every 100×100 μm 2 It contains 10 or more BN precipitates, and Among them, every 100×100 μm 2 There are 10 or fewer MnS precipitates distributed.

2. The austenitic stainless steel according to claim 1, wherein the major axis length of the MnS precipitate is 1 μm or greater.

3. The austenitic stainless steel according to claim 1 further comprises 1% or less but not 0% Cu by weight percentage (wt%).

4. The austenitic stainless steel according to claim 1, wherein the pitting potential is 300 mV or greater.

5. A method for manufacturing an austenitic stainless steel with improved corrosion resistance and machinability, the method comprising: The stainless steel is heated at 1,150°C to 1,250°C for 1 hour and 30 minutes or longer, wherein the stainless steel comprises, by weight percentage (wt%): 0.05% or less but not less than 0% C, 2% or less but not less than 0% Si, 2% or less but not less than 0% Mn, 0.01% or less S, 16% to 22% Cr, 9% to 15% Ni, 3% or less but not less than 0% Mo, 0.15% to 0.25% N, 0.004% to 0.06% B, and the remainder being Fe and unavoidable impurities; Hot rolling of heated stainless steel; Hold hot-rolled steel at 1,100°C to 1,250°C for 30 seconds or longer, and The austenitic stainless steel produced therein has a thickness of 100×100 μm. 2 There are 10 or more BN precipitates distributed.

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