JP5418734B2 - Method for producing austenitic stainless steel - Google Patents

Description

  The present invention relates to a method for producing austenitic stainless steel exhibiting corrosion resistance against concentrated nitric acid. More specifically, the present invention relates to a method for producing a high Si-containing austenitic stainless steel that can be used in a high-temperature, high-concentration nitric acid environment.

The main components of the nitric acid production plant are exposed to a high temperature and high concentration nitric acid environment. In general, stainless steel is used as a corrosion resistant material for plants. Stainless steel forms a stable passive film in nitric acid and exhibits excellent corrosion resistance. However, high-temperature, high-concentration nitric acid is extremely oxidative, and general stainless steel causes transpassive corrosion. In the passive state corrosion, full-surface corrosion accompanied by elution of Cr ₂ O ₃ forming a passive film and intergranular corrosion occur near the grain boundary where sensitization (sensitivity of intergranular corrosion increases).

  High Si austenitic stainless steels such as 17Cr-14Ni-4Si (Patent Document 1) and 11Cr-17Ni-6Si (Patent Document 2) are known as materials having corrosion resistance even in such an environment. These high-Si stainless steels exhibit excellent nitric acid corrosion resistance by re-oxidizing Si eluted by corrosion in the passive state region to form a silicate film. One or more of Nb, Ta, Ti, and Zr are added to these high Si stainless steels. These additive elements have an effect of fixing C in steel and suppressing sensitization. In particular, it is effective in suppressing sensitization of the weld heat affected zone, and has a remarkable effect in improving the intergranular corrosion resistance in high-concentration nitric acid.

  The slab heating temperature in the hot working is advantageously as high as possible from the viewpoint of productivity. However, high Si stainless steel has a problem that cracking of the slab occurs during hot working when heated to a temperature higher than a predetermined temperature during hot working. This is because Si has a lower solid solubility in the austenite phase, and the higher the amount of Si, the more embrittled phases such as intermetallic compounds and δ ferrite are generated at high temperatures, resulting in deterioration of high temperature ductility. Therefore, in order to stably produce high-Si stainless steel on an industrial scale, it is necessary to appropriately manage the heating temperature in hot working.

  Patent Document 3 discloses an ingot of high Si stainless steel containing 5 to 8% Si (in this specification, “%” means “% by mass” unless otherwise specified) unless otherwise specified). A method of hot rolling or hot forging in a temperature range of 900 ° C. or higher after soaking in a temperature range satisfying 1100 ° C. and T (° C.) <1470−35 × Si-5 × Ni (%) is disclosed. ing. As the Si content increases, a low-melting intermetallic compound is formed in the cast solidified structure, and when the soaking temperature rises, the intermetallic compound partially melts, so that cracking occurs during hot working. The soaking temperature is specified to prevent this cracking.

  Non-Patent Document 1 discloses the relationship between intermetallic compounds and hot workability in high-Si stainless steel (6.5Si-17Cr-22Ni-0.01Pd). Ni-rich intermetallic compound crystallizes, and if it exists in a large amount, the hot workability becomes low, and (b) when the ingot crystallized with the intermetallic compound is soaked at 1000 to 1150 ° C, at 1150 ° C It is reported that the intermetallic compound partially dissolves and causes cracking, but no melting of the intermetallic compound is observed at 1100 ° C., and cracking during hot working does not occur when dissolved.

  That is, the high-Si stainless steel of Non-Patent Document 1 has broken down due to partial melting of the low-melting point Ni—Si intermetallic compound at 1100 ° C. and crack propagation along the grain boundary. It is presumed that the heating temperature in the hot working is defined as 1100 ° C. or lower.

  In Patent Document 4, a slab of high-Si stainless steel containing 4 to 10% of Si and having S and O regulated to 30 ppm or less is hot-rolled by soaking treatment at 1100 ° C. or more and 1250 ° C. or less for 2 hours or more, A method is disclosed in which hot rolling is terminated at 950 ° C. or higher and solution heat treatment is performed at 1000 ° C. or higher and 1200 ° C. or lower. In Patent Document 4, factors affecting the high-temperature ductility of high-Si austenitic stainless steel are (1) impurity elements of S and O, and (2) intermetallic compounds that precipitate during cooling of the slab, and It is disclosed that hot workability is improved by eliminating intermetallic compounds by reducing S and O and soaking slabs. Although the component of this intermetallic compound is not specified, it is presumed that it is a low melting point Ni—Si intermetallic compound as in Non-Patent Document 1.

  In the techniques disclosed in Patent Documents 3 and 4, the hot workability is improved by setting the heating temperature to be equal to or lower than the melting temperature of the Ni—Si intermetallic compound. However, in a high Si content austenitic stainless steel used in a high temperature and high concentration nitric acid environment, since it contains a large amount of Si, the solid solubility of C tends to be lowered and sensitized. Intergranular corrosion resistance was poor.

  The high-Si stainless steels disclosed in Patent Documents 1 and 2 contain Nb, Ta, Ti, and Zr, thereby suppressing sensitization and greatly improving nitric acid corrosion resistance. There was a new problem that surface defects called wrinkles were likely to occur.

  The cause of this is not clear, and there was a tendency to reduce it by lowering the slab heating temperature.However, depending on the chemical composition of the steel, it may be difficult to obtain a sufficient reduction effect. It was forced to carry out grinding and the like for removing the wrinkles after hot rolling, which caused a significant cost increase.

Japanese Patent No. 3237132 specification Japanese Patent No. 1119398 JP-A-6-93389 JP-A-5-51633

NKK Technical Report, No.154, 1996, p.14-19

  The object of the present invention is to reliably produce high-Si austenitic stainless steel with corrosion resistance suitable for use in a high-temperature, high-concentration nitric acid environment without generating lashes during the hot rolling process. It is to be.

  The present inventors have developed a high Si-containing austenitic stainless steel suitable for use in a high-temperature, high-concentration nitric acid environment (hereinafter austenitic stainless steel). As a result of investigating conditions for reliably producing (sometimes simply referred to as stainless steel), the following items (i) to (iii) were clarified.

  (I) In a stainless steel containing a large amount of Si, a Ni—Si intermetallic compound is generated. The melting point is estimated to be in the range of 1100 to 1150 ° C. as disclosed in Non-Patent Document 1, and a large slab crack that makes hot rolling difficult due to the formation of this intermetallic compound occurs.

  (Ii) When a stainless steel containing a large amount of Si contains Nb, Ta, Ti, Zr, etc., Ni—Si—X (X = Nb, Ti, Zr) is generated as an intermetallic compound. Its melting point is in the range of approximately 1150 to 1200 ° C., and for example, Ni—Si—Nb is about 1160 ° C. from the results of the phase diagram calculation. This Ni-Si-X ternary system (X = Nb, Ta, Ti, Zr) intermetallic compounds are finely dispersed because Nb, Ta, Ti, Zr, etc. are elements that hardly segregate in steel. Exist. Since the Ni—Si—X intermetallic compound is finely dispersed in such a high melting point, it does not cause large slab cracking that makes rolling difficult.

  (Iii) However, when cracks originating from the Ni—Si—X intermetallic compound occur in the vicinity of the slab surface, the cracks propagate to the surface, and the inside of the cracks are oxidized, resulting in frequent occurrence of scabs. . This beard is extremely quantitative in quantity because the intermetallic compound of Ni—Si—X is finely dispersed, and becomes a large amount of beard.

  From the above results, it has been found that the occurrence of whipping in the process of hot rolling is caused by the propagation of cracks to the surface starting from the Ni—Si—X ternary intermetallic compound. Since it is essential to contain Si and X elements from the viewpoint of preventing corrosion in a concentrated nitric acid environment, a means for suppressing the propagation of cracks in the vicinity of the surface was examined.

  In general, cracks tend to propagate when the ductility decreases at high temperatures, so the relationship between components in steel and ductility was studied. As a result, the following knowledge was obtained.

  (A) By defining the heating temperature at the time of hot rolling in relation to the Si, Cr, and Ni contents in the steel composition, defects on the product surface can be prevented.

  (B) By defining the temperature range and cooling method of finish annealing after rolling, sensitization can be suppressed while ensuring elongation and proof stress.

Based on the above findings, the present invention provides C: 0.04% or less, Cr: 7-20%, Ni: 10-22%, Si: 2.5-7%, Mn: 10% or less, sol. Al: 0.03% or less, P: 0.03% or less, S: 0.03% or less, N: 0.035% or less, a total of one or more of Nb, Ti, Ta, and Zr: containing 0.05 to 0.7 percent, a slab of stainless steel having a chemical composition the balance being Fe and impurities, the heating temperature during the hot rolling and _{T h,} (1) _formula: T h = 1135 -90Si-2.9Cr + 40Ni-ΔT ΔT in is method of manufacturing an austenitic stainless steel which is characterized in that the heated hot rolling the heating temperature _{T h} at 60 ° C. or higher.

  In a preferred embodiment, the method according to the present invention comprises heat-treating the austenitic stainless steel subjected to the hot rolling in a temperature range of 1100 to 1160 ° C., and then cooling at a cooling rate of 100 ° C./min or more. In addition.

  According to the present invention, a high Si-containing austenitic stainless steel suitable for use in a high-temperature, high-concentration nitric acid environment can be reliably produced without generating lashes during hot rolling.

FIG. 1 is a graph showing the results of a torsion test of the test steel 1. FIG. 2 is a graph showing the relationship between ΔT of the test steel 1 and the rate of occurrence of whipping. FIG. 3 is a graph showing the relationship between the heat treatment temperature of the test steel 1 after rolling, 0.2% proof stress, and elongation.

  Hereinafter, the manufacturing method of the austenitic stainless steel according to the present invention will be described in more detail with reference to the accompanying drawings. As described above, “%” related to the chemical composition of steel is “% by mass”. Moreover, the balance of the chemical composition of steel is Fe and impurities.

[Chemical composition of steel]
[C: 0.04% or less]
Although C is an element that increases the strength of steel, it is an element that deteriorates corrosion resistance, for example, forms Cr carbide at the grain boundary in the heat-affected zone of the weld and causes sensitization. Therefore, the C content is set to 0.04% or less. The C content is preferably 0.03% or less, and more preferably 0.02% or less.

[Cr: 7-20%]
Cr is a basic element for ensuring the corrosion resistance of stainless steel, and its content is 7% or more and 20% or less. If the Cr content is less than 7%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content is excessive, a co-existence of Si and Nb results in a two-phase structure in which a large amount of ferrite is precipitated, resulting in deterioration of workability and impact resistance, so the upper limit of Cr content is 20%. To do. The lower limit of the Cr content is preferably 10%, and more preferably 11%. On the other hand, the upper limit of the Cr content is preferably 19%, and more preferably 18%.

[Ni: 10-22%]
Ni is a stabilizing element of the austenite phase and also has an effect of increasing the zero ductility temperature. Ni is contained in an amount of 10% to 22%. If the Ni content is less than 10%, the desired corrosion resistance and toughness cannot be obtained. If the Ni content exceeds 22%, the cost increases significantly. The lower limit of the Ni content is preferably 12%, and more preferably 13%. Further, the upper limit of the Ni content is preferably 20%, and more preferably 16%.

[Si: 2.5-7%]
Si is contained in an amount of 2.5% or more and 7% or less in order to enhance the corrosion resistance in concentrated nitric acid. In order to form a silicate film that ensures corrosion resistance in nitric acid, Si is contained in an amount of 2.5% or more. On the other hand, when Si is contained excessively, the zero ductility temperature is lowered. Further, not only the cost is increased, but also the weldability is lowered, so the upper limit of the Si content is set to 7%. The lower limit of the Si content is preferably 3.0%, and more preferably 3.5%. The upper limit of the Si content is preferably 6%, and more preferably 5%.

[Mn: 10% or less]
Mn is a stabilizing element of the austenite phase and is also contained as a deoxidizer, so it is contained in an amount of 10% or less. If the Mn content exceeds 10%, the corrosion resistance decreases, hot cracking during welding, and further the workability decreases. The upper limit of the Mn content is preferably 6%, and more preferably 4%. Moreover, in order to acquire the said effect of Mn reliably, it is preferable that Mn content is 0.5% or more, and it is further more preferable that it is 1.0% or more.

[Sol.Al: 0.03% or less]
Al is contained in the steel as a deoxidizing agent. However, since excessive inclusion of Al generates harmful inclusions, the content of sol.Al is set to 0.03% or less.

[P: 0.03% or less, S: 0.03% or less]
P and S are both elements harmful to corrosion resistance and weldability, and the lower the content of each, the better. Therefore, the P content is 0.03% or less, and the S content is 0.03% or less.

[N: 0.035% or less]
N has a high affinity with Nb, Ti, Ta, and Zr, and inhibits the fixation of C by these elements. Therefore, the N content is preferably as low as possible. Therefore, the N content is 0.035% or less.

[One or more of Nb, Ti, Ta, Zr: 0.05 to 0.7%]
Nb, Ti, Ta, and Zr all have an effect of fixing C and suppressing the decrease in intergranular corrosion resistance due to sensitization, and are particularly effective elements for improving the corrosion resistance of the weld heat affected zone. If the total content of these elements is less than 0.05%, the effect of improving the intergranular corrosion resistance cannot be obtained, and further hot working cracks due to the formation of a low melting point Ni—Si intermetallic compound become large. On the other hand, if the total content of these elements exceeds 0.7%, the workability decreases. Therefore, the content of these elements is 0.05% or more and 0.7% or less in total of one or more kinds.

[Production conditions]
Method of manufacturing austenitic stainless steel according to the present invention, a slab of stainless steel having the above chemical composition, the heating temperature during the hot rolling and _{T h,} (1) _formula: T h = 1135-90Si-2. and 9Cr + 40Ni-ΔT in [Delta] T is the hot rolling step of performing the inter-heated hot rolling heating temperature _{T h} at 60 ° C. or higher, after heat treatment preferably still a temperature range of 1100 to 1160 ° C., 100 ° C. / min It consists of a heat treatment step (annealing step) for cooling at the above cooling rate.

"Hot rolling process"
In order to clarify the optimum heating temperature range for hot rolling, the relationship between chemical composition and high temperature deformability was investigated by high temperature torsion test. Thereby, the zero ductility in hot rolling can be investigated.

In the high-temperature torsion test, one side of a test piece having a parallel part diameter of 8 mm and a length of 30 mm is fixed and held at a predetermined temperature, with a rotational speed of 300 rpm (strain speed of 4.2 sec ⁻¹ ) and an axial force of 0 kgf in one direction. The number of rotations until twisting was applied and breaking was taken as the number of twists.

  As an example, FIG. 1 shows the results of a high temperature return test using a test piece of high Si stainless steel having the chemical composition shown in Table 1 as test steel 1 as the relationship between the heating temperature and the number of twists.

  In FIG. 1, the number of twists reached a maximum near 1100 ° C., and the number of twists decreased significantly at a temperature higher than that. That is, it can be seen that the temperature at which the ductility of the high Si stainless steel shown as the test steel 1 in Table 1 becomes zero (hereinafter referred to as “zero ductility temperature”) is approximately 1275 ° C.

Using the same method, the high-temperature torsion test was conducted on high-Si stainless steels having various chemical compositions containing one or more of Nb, Ta, Ti, and Zr, and the zero ductility temperature was examined. As a result, it was found that the zero ductility temperature (T ₀ ) can be expressed by the following regression equation (2) as the relationship with the Si, Cr, Ni concentration.

T ₀ = 1135-90Si-2.9Cr + 40Ni (2)
The heating temperature (T _h ) in the hot working is made 30 ° C. lower than the zero ductility temperature (T ₀ ), that is, the heating temperature (T _h ) is expressed by the following formula (1):
T _h = 1135-90Si-2.9Cr + 40Ni-ΔT (1)
By setting the temperature at ΔT at 30 ° C. or higher, cracks originating from Ni—Si—X ternary (X = Nb, Ta, Ti, Zr) intermetallic compounds are less likely to occur. Decrease. If the amount of lashes is reduced, it is possible to proceed to the next process with simple surface care, which is excellent in economic efficiency.

  The forged slab was heated at a predetermined temperature and then hot rolled to a thickness of 4 mm. Thereafter, the scale was removed by pickling, and then the rate of occurrence of baldness was investigated by the following method.

  The steel plate surface was divided into 100 mm unit meshes, and the ratio of the number of meshes with scabs out of the total number of meshes investigated was defined as the scab generation rate (%). If the rate of occurrence of lashes is 5% or less, it is possible to proceed to the next step with a simple rework.

From the components of the test steel 1 shown in Table 1, the zero ductility temperature T ₀ = 1275 ° C. is obtained by the equation (2). FIG. 2 shows the relationship between ΔT of the test steel 1 (Table 1) and the rate of occurrence of lashes.

As shown in the graph of FIG. 2, by setting the heating temperature T _h of hot rolling so as to satisfy the [Delta] T ≧ 30 ° C., scab defect occurrence rate of 5% or less. On the other hand, as ΔT was smaller than 30 ° C. and closer to the zero ductility temperature, the rate of occurrence of lashes rapidly increased.

That is, in order to minimize the scab defect is, [Delta] T is 30 ° C. or more, preferably such that 60 ° C. or higher may be set the heating temperature T _h of the hot rolling. The holding time to this heating temperature is not particularly limited. In the present invention, the heating temperature is set in order to prevent the occurrence of baldness after rolling, so that the surface temperature of the slab may be a predetermined temperature. However, in order not to interfere with hot rolling, it is preferable to heat the slab to the center until the temperature is substantially uniform. The heating time required for this depends on the size of the slab, but in general, the heating time is preferably 60 minutes or more.

  There is no particular upper limit for ΔT. In a normal hot rolling facility, hot rolling can be performed if the hot rolling end temperature is 700 ° C. or higher. Preferably, this end temperature is set to 950 ° C. or higher.

  Hot rolling can be performed in one or more stages. In the case of multi-stage rolling, if necessary, it can be heated between rolling stands. The heating temperature at this time is not particularly required to be a temperature at which ΔT is 30 ° C. or higher, but is preferably a temperature at which ΔT is 30 ° C. or higher. Thereby, since the crystal grain size of the surface is refined during the subsequent hot rolling, the propagation of cracks hardly occurs, and the occurrence of lashes is further suppressed. After hot rolling, pickling is generally performed by a conventional method to remove oxide scale on the surface of the rolled material.

[Heat treatment process]
Since the stainless steel plate obtained by hot rolling can adjust mechanical characteristics (elongation, yield strength) by performing a heat treatment for annealing, it is preferable to perform the heat treatment after the hot rolling. When the heat treatment temperature is increased, the proof stress is reduced although the elongation is increased. When the cooling rate after the heat treatment is slow, chromium carbide precipitates, and the corrosion resistance deteriorates. Therefore, it is necessary to set the heat treatment temperature and the subsequent cooling rate so as to achieve both elongation and yield strength and prevent sensitization.

  FIG. 3 shows the relationship between the heat treatment temperature of the test steel 1 and the 0.2% proof stress and elongation. The circle plot in the graph of FIG. 3 shows 0.2% yield strength (MPa), and the square plot shows elongation (%).

  As shown in FIG. 3, by performing heat treatment at 1100 ° C. or more and 1160 ° C. or less, good elongation and sufficient yield strength, specifically, elongation: 50-53%, 0.2% yield strength: 325-290 MPa The stainless steel shown can be obtained.

  Further, when the cooling rate after the heat treatment is low, sensitization occurs and the intergranular corrosion sensitivity increases. By setting the cooling rate to 100 ° C./min or more, sensitization does not occur, and the stainless steel exhibits good nitric acid corrosion resistance.

  Thus, according to the present invention, a high Si content austenitic stainless steel suitable for use in a high-temperature, high-concentration nitric acid environment can be reliably produced without generating lashes in the hot rolling process. can do.

  A slab obtained by forging a 10 kg ingot obtained by melting the component test steels 1 to 5 shown in Table 1 above in a high-frequency electric furnace was heated at a predetermined temperature shown in Table 2 for 120 minutes and then thickened with a two-stage rolling mill. Hot rolled to 4 mm. In the state where the obtained stainless steel plate was pickled and the scale was removed, the rate of occurrence of lashes on the surface of the steel plate was investigated by the method described above. The results are summarized in Table 2.

As shown in Table 2, when the heating temperature at the time of hot rolling was lower by 30 ° C. or more than the zero ductility temperature T ₀ (° C.) calculated from the components, the rate of occurrence of whipping was 5% or less. .

On the other hand, when the heating temperature is higher than a temperature that is 30 ° C. lower than the zero ductility temperature T ₀ (° C.) calculated from the components, the rate of occurrence of whipping exceeds 5%, and the hot rolling process Thus, a high Si content austenitic stainless steel suitable for use in a high-temperature, high-concentration nitric acid environment could not be reliably produced without generating baldness.

Claims (1)

In mass%, C: 0.04% or less, Cr: 7-20%, Ni: 10-22%, Si: 2.5-7%, Mn: 10% or less, sol. Al: 0.03% or less, P: 0.03% or less, S: 0.03% or less, N: 0.035% or less, a total of one or more of Nb, Ti, Ta, and Zr: containing 0.05 to 0.7 percent, a slab of stainless steel having a chemical composition the balance being Fe and impurities, the heating temperature during the hot rolling and _{T h,} is ΔT in the following (1) 60 hot rolling step ° C. by heating to at a heating temperature T _h more performing hot rolling, and the austenitic stainless steel obtained by the hot rolling and heat treatment in the temperature range of from 1,100 to 1160 ° C., then 100 ° C. The manufacturing method of an austenitic stainless steel further including the heat processing process which cools with the cooling rate of / min or more.
T _h = 1135-90Si-2.9Cr + 40Ni-ΔT (1)

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