CN1303241C - Stainless steel for use under circumstance where organic acid and saline are present - Google Patents

Abstract

The present invention provides a stainless steel which is suitable for use in the food manufacturing plant, particularly a soy sauce manufacturing plant. A stainless steel to be used in the environment which contains organic acid and common salt comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, Ni; 40.0 wt % or less, 16.0 wt <=Cr<=26.0 wt %, 2.0 wt %<=Mo<=8.0 wt %, 0.05 wt %<=Al<=0.100 wt %, 0.10 wt %<=N<=0.30 wt %, Mg: 0.005 wt % or less, Ca; 0.0010 wt % or less and balance consisting of Fe and inevitable impurities, and satisfying equation (1), Cr+3.3Mo+20N<=38 (1) wherein, Cr, Mo and N show the content of each ingredients by weight %.

Description

Stainless steel used in environments containing organic acids and salt

technical field

The invention relates to stainless steel, which has good crevice corrosion resistance and stress corrosion crack resistance, and is suitable for food manufacturing equipment, especially food equipment that generates organic acids such as amino acids, citric acid, and acetic acid during the manufacturing process and contains high concentrations of table salt , especially soy sauce manufacturing equipment.

Background technique

Traditionally, in food manufacturing equipment, stainless steel, steel coated with inorganic or organic materials, or FRP was used depending on the operating conditions such as the ingredients of the food used and the temperature. In recent years, the use of stainless steel has been increasing from the perspective of easy maintenance and reduced maintenance costs of equipment, and especially from the perspective of cleanability. Generally, general-purpose stainless steel such as SUS304 and SUS316 is used in food manufacturing equipment such as soft drinks and beer or milk. It does not cause major problems such as leakage due to corrosion. In addition, even in foods containing salt, if it is used near normal temperature , and there is no need to worry too much about pitting corrosion and crevice corrosion, or local corrosion problems such as stress corrosion cracking, which are enough to withstand use. However, in the manufacture of condiments such as soy sauce containing a lot of salt, even at room temperature, if SUS304 and SUS316 are used, there are many cases where significant localized corrosion occurs and the corrosion resistance is insufficient. If SUS329 series stainless steel with higher corrosion resistance than the above-mentioned stainless steel is used, the possibility of localized corrosion is small, but even so, when the temperature rises from normal temperature, there are concerns about crevice corrosion and stress corrosion cracking of the welded part. So its use is limited. Therefore, the current situation is that in soy sauce manufacturing equipment in such a special environment, it is necessary to use steel coated with inorganic or organic materials, or to use FRP, and to use nickel alloys and titanium, which are more expensive than stainless steel, instead of stainless steel.

Contents of the invention

The present invention has been studied in view of the above facts, and its purpose is to provide stainless steel suitable for food equipment, especially soy sauce production equipment or vinegar (sour wine) equipment that generates organic acids during fermentation and contains high concentrations of salt.

The inventors of the present invention conducted various studies on stainless steel suitable for use in food production equipment, especially in food production equipment for fermentation processes including condiments such as soy sauce containing a large amount of salt. As a result, it became clear that during the fermentation process, amino acids, When organic acids such as citric acid and lactic acid are used, these acids accelerate the corrosion rate of stainless steel, especially the acceleration of crevice corrosion and stress corrosion cracking.

The mechanism by which organic acids accelerate the corrosion of stainless steel is considered to be that during the fermentation process, the amino acids produced act as reducing agents to age the passive coating on the surface that imparts corrosion resistance to stainless steel. On the other hand, citric acid, lactic acid, etc. Acting as a chelating agent on the surface of stainless steel, it promotes the dissolution of the so-called oxide-based inclusions in the steel that are not covered by the surface passive film of CaO and MgO, forming the starting point of crevice corrosion and stress corrosion cracking, making the corrosion resistance change. Difference. Therefore, it was found that in order to improve the corrosion resistance of the passive film on the surface of stainless steel and the corrosion resistance of the base metal in an environment containing a high concentration of common salt in the presence of organic acids, it was first necessary to satisfy the conditions in the following formula (1).

Cr+3.3Mo+20N≥38(1)

(In the formula, Cr, Mo, and N represent the content (weight %) of each component)

At the same time, it was found that reducing the CaO and MgO contained in stainless steel inclusions and making the composition mainly composed of SiO ₂ and Al ₂ O ₃ can improve the corrosion resistance in a high-concentration salt environment containing organic acids. That is, the experimental results show that by satisfying the conditions of the following formula (2):

Si+Al-100(Ca+Mg)≥0 (2)

Also, when the weight ratio of CaO+MgO in the inclusions is 20% or less, crevice corrosion and stress corrosion cracking, which cause main corrosion in stainless steel, can be suppressed. The present invention has thus been accomplished.

The gist of the first invention of the present invention is to contain C: 0.05% by weight or less, Si≤1.00% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, Ni: 40.0% by weight or less, 16.0% by weight≤Cr≤26.0% by weight, 2.0% by weight≤Mo≤8.0% by weight, 0.005% by weight≤Al≤0.100% by weight, 0.10% by weight≤N≤0.30% by weight, Mg: 0.0005% by weight % or less, Ca: 0.0010% by weight or less, the rest is Fe and unavoidable impurities, and satisfies the conditions of the following formula (1), stainless steel that can be used in environments containing organic acids and salts;

Cr+3.3Mo+20N≥38 (1)

(In the formula, Cr, Mo, and N represent the content (weight %) of each component)

The gist of the second invention of the present invention is to contain C: 0.05% by weight or less, Si≤1.00% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, 15.0% by weight %≤Ni≤40.0 wt%, 16.0 wt%≤Cr≤26.0 wt%, 2.0 wt%≤Mo≤8.0 wt%, 0.005 wt%≤Al≤0.100 wt%, 0.10 wt%≤N≤0.30 wt%, the rest Austenitic stainless steel that is Fe and unavoidable impurities, and satisfies the conditions of the following formula (1), and can be used in environments containing organic acids and salts;

Cr+3.3Mo+20N≥38(1)

(In the formula, Cr, Mo, and N represent the content (weight %) of each component)

The gist of the third invention of the present invention is the stainless steel according to the first invention or the second invention, wherein the organic acid contains amino acid and one or two or two selected from citric acid, acetic acid and lactic acid. More than one acid;

The gist of the fourth invention of the present invention is to include C: 0.05% by weight or less, Si≤1.00% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, Ni: 40.0% by weight or less, 16.0% by weight≤Cr≤26.0% by weight, 2.0% by weight≤Mo≤8.0% by weight, 0.005% by weight≤Al≤0.100% by weight, 0.10% by weight≤N≤0.30% by weight, Mg: 0.0005% by weight % or less, Ca: 0.0010% by weight or less, the rest is Fe and unavoidable impurities, and stainless steel for food equipment that satisfies the following formula (1);

Cr+3.3Mo+20N≥38(1)

(In the formula, Cr, Mo, and N represent the content (weight %) of each component)

The gist of the fifth invention of the present invention is to contain C: 0.05% by weight or less, Si≤1.00% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, 15.0% by weight %≤Ni≤40.0 wt%, 16.0 wt%≤Cr≤26.0 wt%, 2.0 wt%≤Mo≤8.0 wt%, 0.005 wt%≤Al≤0.100 wt%, 0.10 wt%≤N≤0.30 wt%, the rest Austenitic stainless steel for food equipment that is Fe and unavoidable impurities and satisfies the following formula (1):

Cr+3.3Mo+20N≥38(1)

(In the formula, Cr, Mo, and N represent the content (weight %) of each component)

The gist of the sixth invention of the present invention is the austenitic stainless steel described in the above-mentioned first invention to the fifth invention, wherein the above-mentioned stainless steel satisfies the condition of the following formula (2), and the oxide system in the steel The weight ratio of CaO+MgO in inclusions is 20% by weight or less;

Si+Al-100(Ca+Mg)≥0 (2)

(In the formula, Si, Al, Ca, and Mg represent the content (% by weight) of each component

The gist of the seventh invention of the present invention is the stainless steel described in the above-mentioned inventions 1 to 6, wherein the above-mentioned stainless steel can be used in soy sauce manufacturing equipment or vinegar manufacturing equipment;

The gist of the eighth invention of the present invention is the stainless steel described in the above-mentioned first invention to the seventh invention, wherein it further contains %, 0.01 wt% ≤ Co ≤ 1.0 wt% 1 or 2 or more substances;

The gist of the ninth invention of the present invention is the stainless steel according to the above-mentioned first invention to the eighth invention, wherein 0.001% by weight≤B≤0.010% by weight is contained.

In the stainless steel according to the first or second invention, it is preferable to contain one selected from the group consisting of 0.01% by weight≤Cu≤1.0% by weight, 0.01% by weight≤W≤1.0% by weight, and 0.01% by weight≤Co≤1.0% by weight Or two or more kinds of substances, it is also preferable to contain 0.001% by weight≤B≤0.010% by weight.

A brief description of the drawings

Fig. 1 shows the AES analysis result curve of the sample surface immersed in the test solution for 1 week.

Figure 2 shows the curve of the natural immersion potential versus time when the sample is immersed in the test solution.

Figure 3 shows the relationship between Cr+3.3Mo+20N and the ratio of CaO+MgO in the inclusions and the curve of whether there is corrosion in the corrosion test.

Fig. 4 shows a relationship curve of the relationship Si+Al-100 (Ca+Mg) and the ratio of CaO+MgO in inclusions.

The best solution for implementing the present invention

As described above, the stainless steel of the present invention is defined by (i) the chemical composition and its appropriate content range, (ii) the relationship between Cr, Mo, and N that particularly contributes to the improvement of corrosion resistance, (iii) the inclusions in the steel The composition of the material and the appropriate content ranges of Al, Si, Ca, and Mg constituting them will be described below for the experimental results that form the basis of the present invention.

Experiment 1

The present inventors first studied how the environment of a soy sauce production facility having a fermentation process in which organic acids such as amino acids and lactic acid are produced differs from the environment in which such organic acids do not exist. The test uses the 2mm thick SUS316L sold on the market as the test material, cuts two pieces into 80mm×25mm×2mm and 60mm×20mm×2mm samples to overlap, and performs 4-point resistance welding to make a corrosion test with welding gaps. Sample. General soy sauce contains many kinds of organic acids, but in order to simplify the system, the following four test solutions were prepared, adding glutamic acid and aspartic acid, which are typical organic acids produced during fermentation, Lactic acid, citric acid, and acetic acid that are not amino acids.

1-①: 17% saline solution

1-②: 17% saline + 1% glutamic acid

1-③: 17% saline + 1% glutamic acid + 1% lactic acid

1-④: 17% saline + 1% glutamic acid + 1% aspartic acid + 1% lactic acid + 0.2% citric acid + 0.15% acetic acid

These four test solutions were stored at 35°C, and the above-mentioned samples were respectively immersed in each test solution for one month. Cut with a cutting machine after dipping, so that the cutting line passes through the center of the nuclei of the solder joints formed by spot resistance welding. Observe the cross-section with an optical microscope, and evaluate the depth of crevice corrosion and the length of stress corrosion cracks. Show the evaluation results in Table 1. It was confirmed that only crevice corrosion occurred in the solution (1-①) containing only common salt. In contrast, in the solution (1-②) containing glutamic acid as an amino acid, in addition to crevice corrosion, Stress corrosion cracking occurs. On the other hand, in the solution (1-③) containing both lactic acid and glutamic acid, which are not amino acids but have a chelating structure, the depth and length of crevice corrosion and stress corrosion cracking increased. In addition, in the solution (1-④) containing various organic acids mixed with common salt, the corrosion also increased significantly. From the above results, it can be seen that the environment inside the soy sauce manufacturing equipment containing high concentration of salt, which produces organic acids such as amino acids and lactic acid during the fermentation process, is significantly more corrosive than the environment with the same high concentration of salt but no organic acids .

Table 1 test solution contains ingredients Maximum crevice corrosion depth Maximum stress corrosion crack length 1-① 17% table salt 14.5μm - 1-② 17% table salt + 1% glutamic acid 15.5μm 26.5μm 1-③ 17% Salt + 1% Glutamic Acid + 1% Lactic Acid 39.0μm 45.0μm 1-④ 17% Salt + 1% Glutamic Acid + 1% Aspartic Acid + 1% Lactic Acid + 0.2% Citric Acid + 0.15% Acetic Acid 70.0μm 71.0μm

Experiment 2

In order to ascertain the mechanism of the increase in corrosion caused by such organic acids, the present inventors conducted surface analysis on SUS316L immersed in high-concentration salt water containing organic acids for a long time, and performed electrochemical measurements in the solution. . Specifically, three test solutions were prepared:

2-①: 17% saline solution

2-②: 17% saline + 1% glutamic acid

2-③: 17% saline + 1% lactic acid

Store these solutions at 35°C, immerse the SUS316L flat plate sample wet-ground with No. 400 emery paper in the above three test solutions for one week, and use an Auger electron spectrometer (hereinafter referred to as AES) to analyze its surface. Passive membrane structure was analyzed. The surface of the sample after similar immersion was observed with a scanning electron microscope (hereinafter referred to as SEM). Further, with saturated calomel as a reference electrode, the natural immersion potential of each sample during the 1-week immersion period was measured. Before measuring the natural immersion potential, air was passed through various test solutions for 24 hours in advance to make the dissolved oxygen reach a saturated state.

First, the AES analysis results of the surface of the sample after immersion in each solution for 1 week are shown in FIG. 1 . Figure 1 shows the numerical values of [Cr]/[Cr]+[Fe], which is an indicator of the strength of the passivation film formed by analyzing the constituent elements of the surface passivation film in the depth direction with the Ar acceleration voltage adjusted to 1kV. [Cr] and [Fe] here represent their respective atomic %, and the higher the index, the stronger the passive film, that is, the better the corrosion resistance. As clearly shown in Figure 1, no difference in the structure of the surface passive film was found in 17% saline (2-①), or in the solution of 17% saline with 1% lactic acid added (2-③), but in 17% saline In the solution (2-②) to which 1% glutamic acid was added to saline, the value of [Cr]/[Cr]+[Fe] in the outermost layer decreased compared to 2-① and 2-③. This shows that glutamic acid played a role in the aging of the passive membrane. At the same time, the measurement results of natural immersion potential in each solution are shown in Fig. 2. It can be seen that in solutions 2-① and 2-③, the natural immersion potential changes little from the beginning of the measurement, but in the solution containing glutamic acid In solution 2-②, immediately after the start of the measurement, the spontaneous immersion potential decreased rapidly. From the analysis of the above results, it can be considered that, even among organic acids, glutamic acid, which is an amino acid, functions as a reducing agent, and finally destabilizes the passive film on the surface.

On the other hand, when the surface of the sample after immersion in each solution for one week was observed by SEM, there was no change in the solutions 2-① and 2-② compared with before immersion. Only in the solution 2-③ containing lactic acid, minute pores were formed on the surface. This part is where the inclusions originally existed. The result of detailed observation shows that although the inclusions are also inclusions, the inclusions mainly composed of Al ₂ O ₃ or SiO ₂ still exist after immersion, but there is a selective Sexually dissolves and falls off. As the mechanism of this phenomenon, it can be considered that because lactic acid has a chelating structure, it preferentially reacts with Ca and Mg with strong affinity, resulting in the selective dissolution of CaO and MgO-based inclusions, forming crevice corrosion and stress corrosion. The beginning of the crack. In view of the above reasons, it can be concluded that even among organic acids, lactic acid and citric acid with a chelate structure have the effect of increasing corrosion. In addition, if there are inclusions with CaO and MgO as the main body, the corrosion resistance will be improved. worse.

Experiment 3

The above explains the particularity in the environment of soy sauce manufacturing equipment containing high concentration of salt where organic acids exist, and organic acids cause aging of the passive film, or selectively dissolve inclusions mainly composed of CaO or MgO to increase The mechanism of great corrosion, and then in order to find a stainless steel composition that exhibits good corrosion in this environment and may be applied, the present inventors conducted the following experiments.

Use an induction furnace (atmospheric melting furnace) in the following composition range, and make the weight ratio of CaO+MgO in the oxide-based inclusions in the steel reach various ratios, melt stainless steel, and obtain steel ingots, C: 0.008～0.035 % by weight, Si: 0.02 to 0.24% by weight, Mn: 0.13 to 0.92% by weight, P: 0.017 to 0.034% by weight, S: 0.001 to 0.003% by weight, Ni: 6.44 to 34.83% by weight, Cr: 16.51 to 25.12% by weight , Mo: 2.06-7.47% by weight, Cu: 0.01-0.86% by weight, W: 0.01-0.73% by weight, Co: 0.01-0.75% by weight, Al: 0.006-0.092% by weight, N: 0.02-0.30% by weight, Ca : 0.0001 to 0.0052% by weight, Mg: 0.0001 to 0.0018% by weight, and B: 0.0001 to 0.0036% by weight. Then, under the condition of 1250°C, the steel ingot was heat treated for 8 hours, forged, cold-rolled, and at 1150°C, it was heated for 30 minutes and then water-cooled for solution treatment to produce a cold-rolled plate with a thickness of 2mm. Next, a sample was taken from a 2 mm cold-rolled sheet in the same manner as in Experiment 1, and a sample with a weld gap was produced by spot resistance welding. In the corrosion test, fermented condiment soy sauce containing about 17% of salt was used as a test solution, and the test solution was kept at 35° C., and the above-mentioned sample was immersed in the test solution for 5 months. After immersion, cut so that the cutting line passes through the center of the core of the solder joint, observe its cross-section with an optical microscope, and evaluate the occurrence of crevice corrosion or stress corrosion cracking. Whatever corrosion occurred was rated as x, and a material that did not corrode at all was rated as ○.

Fig. 3 shows the results of corrosion tests by dividing the weight ratio of CaO+MgO in the oxide-based inclusions in steel into 20% or less and 20% or more. The abscissa of Fig. 3 represents the value of Cr+3.3Mo+20N (wherein, Cr, Mo, N represents the content (weight %) of each element), and this value is in the alloy composition, selects and has great influence on corrosion resistance Cr, Mo, and N, in consideration of the degree of influence, each element affects the total amount in an approximately equal amount. It can be confirmed from FIG. 3 that when the weight ratio of CaO+MgO in the oxide-based inclusions is 20% or more, corrosion does not occur until the value of Cr+3.3Mo+20N exceeds 44. On the contrary, if the weight ratio of CaO+MgO is 20% or less, and the value of Cr+3.3Mo+20N is 38 or more, corrosion does not occur. It is obvious that the larger the value of Cr+3.3Mo+20N, the better the corrosion resistance. Corresponding high-priced elements must be added to the alloy, which will inevitably lead to an increase in cost. However, if the composition of oxide-based inclusions is controlled so that the weight ratio of CaO+MgO is 20% or less, the lower limit of Cr+3.3Mo+20N necessary for corrosion resistance can be lowered. But at the same time, it also shows that even if this control is carried out, the index value of Cr+3.3Mo+20N is at least 38 or above, otherwise the material may corrode in soy sauce manufacturing equipment containing high concentrations of salt and organic acids.

Next, the inventors of the present invention further conducted repeated studies on methods for stably controlling the weight ratio of CaO+MgO in oxide-based inclusions in steel to 20% or less. The results showed that considering Ca, Mg, if the contents of Si and Al of the deoxidizing material components are limited within a certain range, the above-mentioned ratio can be achieved. That is to say, it is found that as shown in Figure 4, if the contents of Si and Al are respectively limited to 0.01 to 0.25% by weight and 0.005 to 0.100% by weight, and the content relationship between Ca and Mg satisfies Si+Al-100(Ca +Mg)≥0, it is possible to stably make the weight ratio of CaO+MgO in the inclusions 20% or less. As described above, it can be concluded that by controlling the range of Cr, Mo, N, and Si, Al components and the composition of inclusions, it is possible to provide good resistance in soy sauce manufacturing equipment containing high concentrations of salt and organic acids. Corrosive austenitic stainless steel.

The reason for limiting each component will be described below.

C: 0.05% by weight or less C is an element that induces sensitization and lowers corrosion resistance especially during welding, so its content is desirably low, but extremely reducing its content will lead to a decrease in strength and increase the manufacturing cost. Since the content of C is allowable up to 0.05% by weight, this value is taken as the upper limit.

Si: 1.00% by weight or less

Si is an element effective for deoxidation, especially in order to reduce the ratio of CaO+MgO in oxide-based inclusions in steel, and to constitute the main body of oxide-based inclusions together with aluminum, Si is an ideal element. However, if it is added in excess, it will lead to a decrease in ductility and an increase in strength while its effect is saturated, and it will also increase the precipitation of intermetallic compounds equal to σ phase and x, which will reduce corrosion resistance, so the amount must be 1.0 % or less, preferably 0.70% or less, 0.50% or less, 0.25% or less, 0.20% or less, more preferably 0.10% or less.

Mn: 1.00% by weight or less

Mn is an element that suppresses the precipitation of intermetallic compounds such as σ phase and x, and in order to suppress the reduction of corrosion resistance, it is necessary to reduce its amount as much as possible. Therefore, its amount must be 1.00% by weight or less, preferably 0.30% by weight or less. Below, more preferably 0.20% by weight or below is suitable.

P: 0.040% by weight or less P is an element unavoidably mixed as an impurity, and its use is desirably small from the viewpoints of easy segregation in grain boundaries, corrosion resistance, and hot workability. However, if the content of P is extremely reduced, the manufacturing cost will increase. The allowable value of the P content can be up to 0.040% by weight, so this value is taken as the upper limit. However, it is preferably 0.030% by weight or less.

S: 0.003% by weight or less Like P, S is also an element unavoidably mixed as an impurity, and its use is desirably small from the viewpoints of easy segregation in grain boundaries, corrosion resistance, and hot workability. In particular, when its content exceeds 0.003% by weight, its harmfulness is evident, so its content is limited to 0.003% by weight or less. However, it is preferably 0.002% by weight or less.

Ni: 40.0% by weight or less

Ni is an element effective in suppressing the precipitation of intermetallic compounds equal to the σ phase and x, and is also an essential element in forming the structure into austenite. It is an element effective in improving stress corrosion cracking resistance. However, if its content exceeds 40.0% by weight, it leads to poor hot workability and increased heat deformation resistance. Therefore, the content of Ni is limited to 40.0% by weight or less. The Ni content is preferably 18.0 to 30% by weight, more preferably 24.0 to 26% by weight.

Cr: 16.0% by weight≦Cr≦26.0% by weight Cr is an element effective in improving crevice corrosion resistance, and in order to obtain this effect, it is necessary to contain 16.0% by weight or more of Cr. However, if the content exceeds 26.0% by weight, it will promote the formation of intermetallic compounds equal to σ phase and x, which will reduce the crevice corrosion resistance instead, so its usage is limited to 16.0% to 26.0% by weight. The Cr content is preferably 20.0% by weight or more, more preferably 22.0% by weight or more.

Mo: 2.0 wt% ≤ Mo ≤ 8.0 wt%

Mo is also an effective element for improving crevice corrosion resistance. In order to obtain this effect, its content must be 2.0% by weight or more. However, if the content exceeds 8.0% by weight, the precipitation of intermetallic compounds will increase, and the corrosion resistance will decrease on the contrary. , so the content of Mo is limited to 2.0% by weight to 8.0% by weight. The Mo content is preferably 3.0% by weight or more, more preferably 5.0% by weight or more.

Al: 0.005 wt% ≤ Al ≤ 0.100 wt%

Al is a strong deoxidizer, as shown in Experiment 3, especially in order to reduce the ratio of CaO+MgO in the oxide-based inclusions in steel, so that it can form the main body of oxide-based inclusions together with Si, it is necessary to actively pay attention to the addition of Al . However, if the content exceeds 0.10% by weight, the effect will be saturated and the precipitation of intermetallic compounds will be promoted, so the content is limited to 0.10% by weight or less.

N: 0.10 wt% ≤ N ≤ 0.30 wt%

N, like Cr and Mo, is an effective element that can improve crevice corrosion resistance and at the same time suppress the precipitation of intermetallic compounds. In order to obtain this effect, the content must be 0.10% by weight or more. However, if the content exceeds 0.30% by weight, the heat deformation resistance will be extremely increased and hot workability will be hindered, so the content of N is limited to 0.10% by weight to 0.30% by weight. The N content is preferably 0.15% by weight or more.

Mg: 0.0005% by weight or less

Mg is an unavoidable element contained in oxide-based inclusions in general steel. As is evident from the results of Experiment 3, it is necessary to make the content 0.0005% by weight or less from the viewpoint of corrosion resistance. That is, if it exceeds 0.0005% by weight, inclusions soluble in organic acids having a chelate structure are likely to be formed, resulting in lowered corrosion resistance.

Ca: 0.0010% by weight or less

Like Mg, Ca is an unavoidable element contained in oxide-based inclusions in steel. It is clear from the results of Experiment 3 that the content must be 0.0010% by weight or less from the viewpoint of corrosion resistance. That is, if it exceeds 0.0010% by weight, inclusions soluble in organic acids having a chelate structure are likely to be formed, resulting in lowered corrosion resistance.

Cu: 0.01 to 1.0% by weight

W: 0.01 to 1.0% by weight

Co: 0.01 to 1.0% by weight

In the present invention, in addition to the above components, one or two of 0.10% by weight≤Cu≤1.0% by weight, 0.01% by weight≤W≤1.0% by weight, and 0.10% by weight≤Co≤1.0% by weight may also be contained. More than 2 elements. These elements are generally effective for improving corrosion resistance. However, in order to obtain this effect, the content must be 0.01% by weight or more. On the other hand, if the content exceeds 1.0% by weight, hot workability will be hindered, so the content of each element is set to 0.01% by weight to 1.0% by weight.

B: 0.001% by weight ≤ B ≤ 0.010% by weight

In the present invention, in addition to the above-mentioned components, 0.001% by weight≦B≦0.010% by weight may be contained. B is an element very effective in improving hot workability, but when the content thereof is less than 0.001% by weight, the effect is very poor, and if it exceeds 0.010% by weight, hot workability is degraded on the contrary. Therefore, the content of B is limited to 0.001% by weight to 0.010% by weight.

Cr+3.3Mo+20N≥38

In the present invention, the reason why the relation of Cr, Mo, N is limited to the following relational formula is because the result of experiment 3 shows that if Cr+3.3Mo+20N is lower than 38, even if Si, Al, Ca, Mg In order to control the weight ratio of CaO+MgO in the oxide-based inclusions in steel, which is the main constituent element of the present invention, to an optimum level, it does not have sufficient corrosion resistance in soy sauce manufacturing equipment containing high concentrations of salt and organic acids sex. Preferably Cr+3.3Mo+20N is 40 or above, and more ideal if it is 44 or above.

Cr+3.3Mo+20N≥38

(In the formula, Cr,, Mo, N represent the content of each element (weight%)) The weight ratio of CaO+MgO in the oxide-based inclusions in the steel is 20% or lessSi+Al-100(Ca+Mg )≥0

In the present invention, the reason why the weight ratio of CaO+MgO in the oxide-based inclusions in steel is limited to 20% or less, and the relationship among Si, Al, Ca, and Mg is limited to the following relational expression is that Experiment 3 The results showed that if this relationship is not satisfied, there is insufficient corrosion resistance in soy sauce manufacturing equipment containing high concentrations of salt and organic acids.

Si+Al-100(Ca+Mg)≥0

(In the formula, Si, Al, Ca, and Mg represent the content (% by weight) of each element component)

In the present invention, it is not necessary to make all the oxide-based inclusions in the steel be single substances or composite oxides of SiO ₂ , Al ₂ O ₃ , CaO, MgO, no matter what kind of inclusions, as long as the ratio of CaO+MgO is within 20% or less is fine. Of course, sometimes other oxides may be formed alone or form composite oxides together with the above-mentioned oxides. MnO, FeO, TiO ₂ etc. may be considered as other oxides.

Examples and Comparative Examples

Next, the present invention will be described on the basis of the examples shown below. Steels having various compositions shown in Experiment 3 above are also recorded here.

First, steels of the present invention and comparative steels having the compositions shown in Tables 2 and 3 were melted in an induction furnace to obtain steel ingots. Then, at 1250° C., heat treatment of steel ingot for 8 hours, forging, cold rolling, and solution treatment with water cooling after heating at 1150° C. for 30 minutes, to produce a cold-rolled sheet with a thickness of 2 mm.

Then take two pieces of samples from the 2mm cold-rolled plate, the size is 80mm×25mm×2mm and 60mm×20mm×2mm, wet grinding with No. 400 emery paper, after degreasing, it is made by point resistance welding with welding seam of samples.

In the corrosion test, fermented condiment soy sauce containing about 17% of salt was used as a test solution, and the test solution was stored at 35° C., and the sample was immersed in the test solution for 5 months. After immersion, cut along the center of the core of the solder joint formed by spot welding, observe the section of the gap with an optical microscope, and evaluate the occurrence of crevice corrosion or stress corrosion cracking. The observation results are shown in Table 3.

○ mark: Indicates a material showing good corrosion resistance without occurrence of any kind of corrosion such as crevice corrosion or stress corrosion cracking.

× mark: Indicates the material with crevice corrosion and stress corrosion cracking. Show the evaluation results in Table 3.

Table 2 No. C Si mn P S Ni Cr Mo Al N Ca Mg Cu W co B Invention steel 1 0.035 0.12 0.23 0.021 0.001 15.17 16.51 4.77 0.012 0.29 0.0003 0.0002 - - - - 2 0.011 0.11 0.13 0.022 0.001 18.45 19.37 5.83 0.038 0.12 0.0007 0.0001 - - - - 3 0.008 0.02 0.18 0.019 0.001 18.84 20.26 6.09 0.092 0.21 0.0006 0.0002 - - - 0.0033 4 0.009 0.23 0.20 0.020 0.002 19.31 20.41 6.24 0.041 0.23 0.0002 0.0002 0.71 - - - 5 0.011 0.20 0.18 0.020 0.001 24.56 20.97 6.05 0.019 0.22 0.0002 0.0001 - - - 0.0026 6 0.010 0.18 0.26 0.021 0.001 24.98 21.02 6.76 0.026 0.20 0.0003 0.0001 0.32 - - - 7 0.010 0.24 0.20 0.019 0.001 29.23 23.03 7.47 0.044 0.22 0.0009 0.0002 - - - 0.0036 8 0.006 0.09 0.21 0.018 0.001 25.85 23.13 5.34 0.052 0.19 0.0003 0.0004 - - - 0.0031 9 0.007 0.06 0.20 0.020 0.001 25.78 23.31 5.53 0.017 0.20 0.0004 0.0003 - - - 0.0022 10 0.009 0.13 0.20 0.017 0.002 24.57 20.14 6.18 0.019 0.22 0.0006 0.0002 0.86 - - 0.0041 11 0.009 0.21 0.21 0.019 0.001 25.03 22.87 5.72 0.023 0.19 0.0008 0.0005 - 0.73 - - 12 0.010 0.17 0.17 0.022 0.001 25.11 20.63 6.02 0.034 0.22 0.0001 0.0003 - - 0.75 - 13 0.011 0.18 0.38 0.034 0.003 24.89 26.84 2.06 0.063 0.30 0.0003 0.0002 - - - - 14 0.009 0.23 0.23 0.019 0.001 6.55 24.60 3.22 0.035 0.17 0.0004 0.0002 - 0.16 - 0.0017 15 0.010 0.20 0.34 0.020 0.001 6.76 25.12 3.41 0.029 0.16 0.0005 0.0001 - 0.34 - - compare steel 16 0.013 0.18 0.22 0.018 0.003 10.81 16.78 2.13 0.006 0.02 0.0008 0.0002 - - - - 17 0.010 0.17 0.19 0.019 0.002 15.04 18.13 3.98 0.008 0.15 0.0036 ^* 0.0004 - - - - 18 0.011 0.24 0.16 0.019 0.001 13.67 19.24 3.56 0.016 0.06 0.0009 0.0002 - - - - 19 0.012 0.17 0.20 0.018 0.001 25.46 20.74 5.02 0.009 0.15 0.0036 ^* 0.0004 - - - - 20 0.011 0.25 0.19 0.020 0.001 24.95 20.87 5.37 0.026 0.17 0.0052 ^* 0.0018 ^* - - - - twenty one 0.010 0.14 0.20 0.020 0.001 24.64 24.26 2.31 0.037 0.26 0.0006 0.0003 - - - - twenty two 0.008 0.21 0.18 0.017 0.001 6.44 24.72 3.16 0.037 0.17 0.0018 ^* 0.0009 ^* - - - -

The values in the table are % by weight, and the balance is iron. ^* Indicates a case outside the scope of the present invention.

table 3 No. Cr+3.3Mo+20N Si+Al-100(Ca+Mg) CaO+MgO ratio in inclusions (%) Corrosion test results in soy sauce Invention steel 1 38.05 0.082 15.1 ○ 2 41.01 0.068 12.6 ○ 3 44.56 0.032 13.5 ○ 4 45.60 0.231 <0.1 ○ 5 45.34 0.189 0.2 ○ 6 47.33 0.166 1.6 ○ 7 52.08 0.174 2.5 ○ 8 44.55 0.072 18.3 ○ 9 45.56 0.007 16.2 ○ 10 44.93 0.069 9.4 ○ 11 45.55 0.103 11.8 ○ 12 44.90 0.164 4.0 ○ 13 39.63 0.193 3.2 ○ 14 38.63 0.205 <0.1 ○ 15 39.57 0.169 6.8 ○ compare steel 16 24.21 ^* 0.086 14.9 x 17 34.26 ^* -0.002 ^* 25.3 x 18 32.19 ^* 0.146 6.0 x 19 40.31 -0.221 ^* 36.3 x 20 41.99 -0.424 ^* 95.1 x twenty one 37.09 ^* 0.087 10.2 x twenty two 38.55 -0.081 ^* 23.7 x

^* Indicates a case outside the scope of the present invention.

The indexes of Cr+3.3Mo+20N and Si+Al-100 (Ca+Mg) are shown in Table 3,

At the same time, it also shows the average weight ratio (%) of CaO+MgO in oxide-based inclusions in steel, Cr+3.3Mo+20N≥38, and Si+Al-100(Ca+Mg)≥0, plus The steel of the present invention, whose weight ratio of CaO+MgO in the inclusions is 20% or less, does not corrode in such a soy sauce environment containing high concentrations of salt and organic acids. Compared with the comparative steel, it can be seen that it has excellent corrosion resistance. corrosive material.

In addition, for the steel of the present invention and the comparative example with the compositions shown in Table 4 and Table 5 with varying amounts of Mn, a cold-rolled sheet with a thickness of 2 mm was produced in the same manner as in Table 2, and from this cold-rolled sheet, according to the aforementioned Take two samples of 80×25×2mm and 60×20×2mm in the same method.

Also, the same corrosion test as in the previous example was carried out, and the results are shown in Table 5. Got the same result.

Table 4 No. C Si mn P S Ni Cr Mo Al N Ca Mg Cu W co B Invention steel 1 0.03 0.12 0.50 0.021 0.001 15.17 16.51 4.77 0.012 0.29 0.0003 0.0002 - - - - 2 0.01 0.11 0.60 0.022 0.001 18.45 19.37 5.83 0.038 0.12 0.0007 0.0001 - - - - 3 0.00 0.02 0.70 0.019 0.001 18.84 20.26 6.09 0.092 0.21 0.0006 0.0002 - - - 0.0033 4 0.00 0.23 0.60 0.020 0.002 19.31 20.41 6.24 0.041 0.23 0.0002 0.0002 0.71 - - - 5 0.01 0.20 0.52 0.020 0.001 24.56 20.97 6.05 0.019 0.22 0.0002 0.0001 - - - 0.0026 6 0.01 0.18 0.53 0.021 0.001 24.98 21.02 6.76 0.026 0.20 0.0003 0.0001 0.32 - - - 7 0.01 0.24 0.52 0.019 0.001 29.23 23.03 7.47 0.044 0.22 0.0009 0.0002 - - - 0.0036 8 0.00 0.09 0.57 0.018 0.001 25.85 23.13 5.34 0.052 0.19 0.0003 0.0004 - - - 0.0031 9 0.00 0.06 0.92 0.020 0.001 25.78 23.31 5.53 0.017 0.20 0.0004 0.0003 - - - 0.0022 10 0.00 0.13 0.60 0.017 0.002 24.57 20.14 6.18 0.019 0.22 0.0006 0.0002 0.86 - - 0.0041 11 0.00 0.21 0.53 0.019 0.001 25.03 22.87 5.72 0.023 0.19 0.0008 0.0005 - 0.73 - - 12 0.01 0.17 0.51 0.022 0.001 25.11 20.63 6.02 0.034 0.22 0.0001 0.0003 - - 0.75 - 13 0.01 0.18 0.55 0.034 0.003 24.89 26.84 2.06 0.063 0.30 0.0003 0.0002 - - - - 14 0.00 0.50 0.52 0.019 0.001 6.55 24.60 3.22 0.035 0.17 0.0004 0.0002 - 0.16 - 0.0017 15 0.01 0.70 0.54 0.020 0.001 6.76 25.12 3.41 0.029 01.6 0.0005 0.0001 - 0.34 - - compare steel 16 0.01 0.18 0.54 0.018 0.003 10.81 16.78 2.13 0.006 0.02 0.0008 0.0002 - - - - 17 0.01 0.17 0.54 0.019 0.002 15.04 18.13 3.98 0.008 0.15 0.0036 ^* 0.0004 - - - - 18 0.01 0.24 0.55 0.019 0.001 13.67 19.24 3.56 0.016 0.06 0.0009 0.0002 - - - - 19 0.01 0.17 0.57 0.018 0.001 25.46 20.74 5.02 0.009 0.15 0.0036 ^* 0.0004 - - - - 20 0.01 0.25 0.56 0.020 0.001 24.95 20.87 5.37 0.026 0.17 0.0052 ^* 0.0018 ^* - - - - twenty one 0.01 0.14 0.52 0.020 0.001 24.64 24.26 2.31 0.037 0.26 0.0006 0.0003 - - - - twenty two 0.00 0.21 0.45 0.017 0.001 6.44 24.72 3.16 0.037 0.17 0.0018 ^* 0.0009 ^* - - - -

The values in the table are % by weight, and the balance is iron. ^* Indicates a case outside the scope of the present invention.

table 5 No. Cr+3.3Mo+20N Si+Al-100(Ca+Mg) CaO+MgO ratio in inclusions (%) Corrosion test results in soy sauce Invention steel 1 38.05 0.082 15.1 ○ 2 41.01 0.068 12.6 ○ 3 44.56 0.032 13.5 ○ 4 45.60 0.231 <0.1 ○ 5 45.34 0.189 0.2 ○ 6 47.33 0.166 1.6 ○ 7 52.08 0.174 2.5 ○ 8 44.55 0.072 18.3 ○ 9 45.56 0.007 16.2 ○ 10 44.93 0.069 9.4 ○ 11 45.55 0.103 11.8 ○ 12 44.90 0.164 4.0 ○ 13 39.63 0.193 3.2 ○ 14 38.63 0.475 <0.1 ○ 15 39.57 0.669 6.8 ○ compare steel 16 24.21 ^* 0.086 14.9 x 17 34.26 ^* -0.002 ^* 25.3 x 18 32.19 ^* 0.146 6.0 x 19 40.31 -0.221 ^* 36.3 x 20 41.99 -0.424 ^* 95.1 x twenty one 37.09 ^* 0.087 10.2 x twenty two 38.55 -0.O81 ^* 23.7 x

^* Indicates a case outside the scope of the present invention.

Industrial Applicability

As described above, in the stainless steel of the present invention, the amount of each of Cr, Mo, and N relative to the total amount thereof is controlled at a predetermined ratio or more, and the amount of Si, Al, Ca, and Mg is limited within a predetermined range. The composition of oxide-based inclusions in steel enables the development of stainless steel with excellent corrosion resistance for food equipment, especially soy sauce containing high concentrations of table salt and organic acids generated during fermentation.

Claims (11)

1. Stainless steel characterized by containing C: 0.05% by weight or less, Si ≤ 1.00% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, 6.55% by weight ≤ Ni≤40.0 wt%, 16.0 wt%≤Cr≤26.0 wt%, 2.0 wt%≤Mo≤8.0 wt%, 0.005 wt%≤Al≤0.100 wt%, 0.10 wt%≤N≤0.30 wt%, Mg: 0.0005 wt% % or less, Ca: 0.0010% by weight or less, the rest is Fe and unavoidable impurities, and satisfies the following formula (1) condition, containing amino acid and one selected from citric acid, acetic acid and lactic acid or Use in environments with 2 or more organic acids and salts, Cr+3.3Mo+20N≥38(1) In the formula, Cr, Mo, N represent the content of each component, by weight %, Furthermore, the above-mentioned stainless steel satisfies the condition of the following formula (2), and the weight ratio of CaO+MgO in the oxide-based inclusions in the steel is 20% by weight or less, Si+Al-100(Ca+Mg)≥0(2) In the formula, Si, Al, Ca, and Mg represent the content of each component in % by weight. 2. Stainless steel characterized by containing C: 0.05% by weight or less, Si ≤ 1.00% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, 15.00% by weight ≤ Ni≤40.0 wt%, 16.0 wt%≤Cr≤26.0 wt%, 2.0 wt%≤Mo≤8.0 wt%, 0.005 wt%≤Al≤0.100 wt%, 0.10 wt%≤N≤0.30 wt%, the rest is Fe and unavoidable impurities, and satisfy the conditions of the following formula (1), used in an environment containing amino acids and one or two or more organic acids and salts selected from citric acid, acetic acid and lactic acid , Cr+3.3Mo+20N≥38(1) In the formula, Cr, Mo, N represent the content of each component, by weight %, Furthermore, the above-mentioned stainless steel satisfies the condition of the following formula (2), and the weight ratio of CaO+MgO in the oxide-based inclusions in the steel is 20% by weight or less, Si+Al-100(Ca+Mg)≥0(2) In the formula, Si, Al, Ca, and Mg represent the content of each component in % by weight. 3. Stainless steel for food equipment, characterized by containing C: 0.05% by weight or less, Si ≤ 1.0% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, 6.55 wt%≤Ni≤40.0 wt%, 16.0 wt%≤Cr≤26.0 wt%, 2.0 wt%≤Mo≤8.0 wt%, 0.005 wt%≤Al≤0.100 wt%, 0.10 wt%≤N≤0.30 wt%, the rest A part is Fe and unavoidable impurities, and satisfies the condition of the following (1) formula, Cr+3.3Mo+20N≥38(1) In the formula, Cr, Mo, N represent the content of each component, by weight %, Furthermore, the above-mentioned stainless steel satisfies the condition of the following formula (2), and the weight ratio of CaO+MgO in the oxide-based inclusions in the steel is 20% by weight or less, Si+Al-100(Ca+Mg)≥0(2) In the formula, Si, Al, Ca, and Mg represent the content of each component in % by weight. 4. Stainless steel for food equipment characterized by containing C: 0.05% by weight or less, Si ≤ 1.00% by weight, Mn: 1.00% by weight or less, P: 0.040% by weight or less, S: 0.003% by weight or less, 15.0 wt%≤Ni≤40.0 wt%, 16.0 wt%≤Cr≤26.0 wt%, 2.0 wt%≤Mo≤8.0 wt%, 0.005 wt%≤Al≤0.100 wt%, 0.10 wt%≤N≤0.30 wt%, the rest A part is Fe and unavoidable impurities, and satisfies the condition of the following (1) formula, Cr+3.3Mo+20N≥38(1) In the formula, Cr, Mo, N represent the content of each component, by weight %, Furthermore, the above-mentioned stainless steel satisfies the condition of the following formula (2), and the weight ratio of CaO+MgO in the oxide-based inclusions in the steel is 20% by weight or less, Si+Al-100(Ca+Mg)≥0(2) In the formula, Si, Al, Ca, and Mg represent the content of each component in % by weight. 5. The stainless steel according to claims 1 to 4, wherein the stainless steel is used in soy sauce manufacturing equipment or vinegar manufacturing equipment. 6. The stainless steel according to claims 1-4, further comprising one of 0.01wt%≤Cu≤1.0wt%, 0.01≤W≤1.0wt%, 0.01≤Co≤1.0wt% or 2 or more types. 7. The stainless steel according to claim 5, further comprising one or two of 0.01wt%≤Cu≤1.0wt%, 0.01≤W≤1.0wt%, 0.01≤Co≤1.0wt%, or 2 or more. 8. The stainless steel according to claims 1 to 4, characterized by containing 0.001% by weight≤B≤0.010% by weight. 9. The stainless steel according to claim 5, characterized by containing 0.001% by weight≤B≤0.010% by weight. 10. The stainless steel according to claim 6, characterized by containing 0.001% by weight≤B≤0.010% by weight. 11. The stainless steel according to claim 7, characterized by containing 0.001% by weight≤B≤0.010% by weight.

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