JPH0536492B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ferritic chromium stainless steel that is excellent in toughness and intergranular corrosion resistance of welded parts and corrosion resistance of a base metal, particularly pitting corrosion resistance and acid resistance. Ferritic chrome stainless steel has too much
Since it does not contain Ni and has improved corrosion resistance with Cr and Mo, the cost of raw materials during production is low, but it has been considered to have lower toughness and ductility and inferior corrosion resistance than austenitic steel. However, with recent advances in steelmaking technology, technology has been established to significantly reduce C and N, which are impurities in steel that cause property deterioration, and technology for adding alloying elements or stabilizing elements has also been developed. Since this has been established, efforts have been made to increase the production of high-purity ferritic chromium stainless steel, focusing on its low raw material cost, and on the other hand, studies have been conducted on measures to further improve corrosion resistance. Typical high-purity ferritic chromium stainless steels include 18%Cr-2%Mo, 26%Cr-1%
Mo, 29%Cr-4%Mo, 29%Cr-4%Mo-2%
Examples include Ni, 30% Cr-2% Mo, etc. The greatest feature of these high-purity ferritic chromium stainless steels is that they exhibit excellent stress corrosion cracking resistance (also known as SCC resistance) in solutions containing chlorides. Applications are being expanded, mainly in neutral environments containing Cl ^- ions, where austenitic stainless steel cannot be used. By the way, methods of improving the corrosion resistance of ferritic chromium stainless steel that are being implemented today include increasing the Cr content,
Mo addition, Ni addition, extremely low (C+N), and even
This includes adding stabilizing elements such as Ti and Nb. However, both the increase in Cr and the addition of Mo act in the direction of deteriorating the toughness of ferritic stainless steel, especially the toughness of welded parts, and rapid deterioration of toughness is observed below room temperature.
Such deterioration in toughness has hindered the wide range of applications for high-purity ferritic chromium stainless steel. In addition, reducing (C+N) is not sufficient to prevent deterioration of the toughness of welds, much less Ti, Nb
When trying to reduce the effect of (C+N) by adding stabilizing elements such as charcoal,
As a result of the generation of a large amount of nitrides, deterioration of the toughness of the weld is inevitable. Thus, the purpose of the present invention is to eliminate the problems of deterioration in toughness and corrosion resistance, which have been considered drawbacks of conventional high-purity ferritic chromium stainless steels, and to create a high-quality stainless steel that dramatically improves both of these properties. Our goal is to provide high purity ferritic chrome stainless steel. Here, the present inventors investigated various conventional methods for improving toughness, and found that they could be improved by ultra-low C and N, as well as low Si and Mn, as well as ultra-low S and Al addition. As a comprehensive effect, the present invention was completed by unexpectedly discovering that the low-temperature toughness of ferritic chromium stainless steel, including welded parts, is significantly improved. Furthermore, by extremely lowering the S content in the steel to a level that sets it apart from conventional methods, the present inventors unexpectedly prevented the decline in hot deformability and deterioration in corrosion resistance that accompany lower Mn. , rather, ultra-low and low S content in steel.
The corrosion resistance of the base material is improved due to the synergistic effect with Mn conversion.
In particular, the present invention was completed by discovering that pitting corrosion resistance, rust resistance, and acid resistance are significantly improved. In addition, the present inventors have also achieved ultra-low C,
The present invention was completed by discovering that the toughness of the weld and the intergranular corrosion resistance of the weld are significantly improved by adding N and Al, and if desired, adding Ti and Nb as stabilizing elements. That's what I did. Here, the present invention includes, in weight%, Cr: 15.00-50.00%, Mo: 5.00% or less, Ni: 0.01-6.00%, Al: 0.015-1.00%, and further, if desired, Ti: 1.00% or less [However, , Ti≧5x(C+N)%] and Nb: 1.00% or less [However, Nb≧8x(C+N)%] or two.
Contains seeds, the remainder consists of Fe and impurities, and by refining, impurities such as C, N,
Al-containing ferritic chromium stainless steel with excellent corrosion resistance and toughness, characterized by reducing S, Si, Mn, and O to the following ranges: C: 0.0060% or less, N: 0.0150% or less, S: Si: less than 0.10%, Mn: less than 0.15%, O: less than 0.015%. As is clear from the above, the steel of the present invention is characterized by extremely low levels of C, N, S, Si, Mn, and O impurities; This reduction was made possible for the first time by a newly developed special vacuum refining method described below. Conventional high-purity ferritic chromium stainless steel has been produced by AOD method, VOD method, VIM method, electron beam melting method, etc., but these conventional techniques contain C, N, and S as impurities. ,Si,
It was impossible to simultaneously reduce Mn and O to the above-mentioned ranges (hereinafter referred to as the "invention steel level"). i.e. for example AOD
Although it was possible to reduce S in steel to 0.0020% or less using the method, it was not possible to reduce C and N in steel to the level of the steel of the present invention, and furthermore, it was not possible to reduce S in steel to the level of the steel of the present invention. It was also not possible to reduce the content to the level of the invention steel due to deoxidizing restrictions. Also,
Although it was possible to reduce C, N, Mn, and O in steel to the level of the steel of the present invention using vacuum melting methods such as the VOD method, VIM method, and electron beam melting method, the S content of the steel could be reduced to the level of the steel of the present invention. It was impossible to reduce it to the level of steel. In this way, even in the conventional technology, only some of the above-mentioned impurities C, N, S, Si, Mn, and O were reduced to an extremely low level, but all of them were reduced at the same time. It was impossible to do so, and the effect at that time was unknown. In other words, like conventional high-purity ferritic chromium stainless steel, C, N, S, Si, Mn,
If any one of the O elements is high, the original properties of the reduction of each element that should be exhibited when these elements are simultaneously reduced to the level of the steel of the present invention are lower than the level of the steel of the present invention. It can be said that this is hidden by the presence of some impurity elements that have a high Since the relationship between each of these impurities was not clarified, the reduction effect as an impurity saturates, for example, at about 50 ppm for S, and at about 100 ppm and 200 ppm for C and N, respectively. It was thought that. Next, the reason why the composition range of each component in the ferritic chromium stainless steel of the present invention steel is limited as described above will be explained. Hereinafter, unless otherwise specified, "%" in this specification means "% by weight." Chromium (Cr): Cr is an important element that determines the basic corrosion resistance of the steel of the present invention. Corrosion resistance improves as the content increases, but if the content exceeds 50.00%, forging is impossible;
If the content is less than
It was set as 15.00-50.00%. Molybdenum (Mo): Mo is an additive element that has the effect of significantly increasing the corrosion resistance of the steel of the present invention. It has great effects on improving rust resistance, acid resistance, pitting corrosion resistance, and crevice corrosion resistance. If the content exceeds 5.00%, σ
The deterioration of mechanical properties due to the precipitation of intermetallic compounds such as (sigma) phase and χ (chi) phase becomes significant.
The upper limit was set at 5.00%. Nickel (Ni): Ni improves acid resistance and improves the passivation ability of the steel of the invention. If the content exceeds 6.00%, it becomes difficult to obtain a single ferrite phase, and furthermore, the precipitation of the σ phase becomes significant, so the upper limit was set at 6.00%. The above effects are not observed at less than 0.01%. Aluminum (Al): Al has the effect of improving the toughness of the weld heat affected zone and reducing toughness deterioration due to N in steel. Furthermore, it has the effect of reducing O in steel, reduces nonmetallic inclusions that have a notch effect, and further reduces solid solute N.
Improves the toughness of the base material by stabilizing the On the other hand, if it is added in excess of 1.00%, the base material will become noticeably brittle and cracking will occur during hot heating, so the upper limit was set at 1.00%. In the steel of the present invention, Al is an element that is actively added, and the residual Al content is distinguished from the unavoidable impurity level Al content that remains in normal Al deoxidation, and the lower limit is 0.015%. .
Preferably it is 0.015%-0.4%. Titanium, Niobium (Ti, Nb): Ti and Nb are added, if desired, in order to prevent deterioration of corrosion resistance and toughness in the welded part due to C and N contamination caused by external factors during welding. When adding Ti and/or Nb, Ti and Nb have the effect of suppressing the coarsening of crystal grains in the base metal and the weld heat affected zone, and in order to fully exhibit this effect, it is necessary to add Ti and Nb. ≧
Only an amount of 5x(C+N)% and Nb≧8x(C+N)% is required. However, when both Ti and Nb are present in large amounts, Laves phase precipitation becomes noticeable and the deterioration of toughness becomes noticeable, so the upper limit for each was set at 1.00%. Next, the reason for suppressing impurities as an important feature of the steel of the present invention and the effects obtained thereby will be further explained. Carbon, nitrogen (C, N): C and N are component elements that have a large effect on the toughness of high-purity ferritic chromium stainless steel, as well as the intergranular corrosion resistance, rust resistance, and acid resistance of welded parts. According to their knowledge, the effect of reducing these elements does not reach saturation, and the lower the content of C and N in steel, the more desirable it is.
The allowable C and N concentrations in the steel of the present invention are Cr
It decreases significantly as the concentration increases. C 0.0060
If N exceeds 0.015%, the weld toughness deteriorates significantly and intergranular corrosion resistance deteriorates, so the upper limit for C is set at 0.006%.
% and N were set at 0.0150%. Preferably C: 0.003% or less and N: 0.0060% or less. Sulfur (S): S significantly deteriorates corrosion resistance and hot workability, and also tends to lower the impact value in the ductile fracture region at room temperature. S is 0.0020
% or less, preferably 0.0010% or less. As will be described later, in the steel of the present invention, for the purpose of improving toughness, etc.
The amount of Mn is limited to 0.15% or less, but if the amount of S is high, the sulfide in the steel will not be MnS but will instead be in the form of (Fe, Mn)S, reducing the corrosion resistance and hot deformability of the base metal. do. Therefore, S needs to be limited to the above range. Silicon (Si): Si reduces the elongation of the base material through solid solution strengthening,
Shift the brittle fracture surface transition temperature to the high temperature side. In conventional melting methods, approximately 0.20% of Si is normally required as a deoxidizing agent, but in the steel of the present invention, the addition of Si as a deoxidizing agent is unnecessary due to the adoption of advanced vacuum refining technology. However, a very small amount of about 0.15% may be mixed in as an unavoidable impurity during melting, but the allowable upper limit is less than 0.10% Si. Manganese (Mn): Mn has a remarkable property of shifting the brittle fracture surface transition temperature to the high temperature side, and the lower the Mn content, the more desirable it is. The allowable upper limit was set at 0.15%, which is the level of unavoidable impurities. Preferably it is 0.10% or less. Oxygen (o): Oxygen exists as oxide-based nonmetallic inclusions in steel and acts as a crack initiation point at the notch, so the presence of oxygen increases the brittle fracture transition temperature. Furthermore, it tends to reduce the impact absorption energy in the ductile fracture region above the brittle fracture transition temperature. Therefore, the upper limit of oxygen is
It was set as 0.015%. When oxygen is contained in an amount exceeding 0.015%, not only the toughness but also the corrosion resistance of the base material tends to deteriorate. Preferably oxygen is 0.010
% or less. In addition, in the present invention, 0.7% or less of Cu and 0.5% or less of V may be contained as other unavoidable impurities. These unavoidable impurities have no adverse effect on the purpose of the present invention. Next, the present invention will be described in more detail with reference to Examples, but prior to that, a method for producing the steel of the present invention will be described. As already mentioned, the present invention simultaneously reduces each of the above-mentioned impurities to an extremely low level, and obtains excellent properties through the synergistic effect of the reduction effects of each impurity. Although the method for achieving the simultaneous reduction of such impurities is not limited to a specific method, the new refining method developed and put into practical use by the present inventors described below allows simultaneous reduction of such impurities. can be easily achieved. In other words, a 2.5-ton vacuum smelting furnace equipped with a high-frequency induction heating coil that has oxygen top-blowing capacity, gas bottom-blowing capacity, and powder top-blowing capacity is used. For denitrification, oxygen powder is used as an oxygen supply source, respectively.
It is supplied onto the molten metal surface by upward blowing using a special porous lance at the speed of sound with a gas carrier. This smelting furnace is already in use in actual production, and is configured to allow casting under reduced pressure (vacuum). In this way, by top-blowing the above-mentioned powder at the sonic level under vacuum, the stirring ability of molten steel can be dramatically improved. In a preferred embodiment of the invention, in 26% Cr steel, C is, for example, less than 0.0010% (less than 10 ppm), N is, for example, less than 0.0040% (less than 40 ppm), and S is, for example, less than 0.0040% (less than 40 ppm).
This makes it possible to achieve extremely low levels of 0.0002% (2ppm), which was previously unimaginable. Therefore, in the present invention, as a result of sufficient stirring of molten steel under vacuum, the effect of C as a deoxidizing element can be effectively utilized, so that Si, which has conventionally been required as a deoxidizing element, It becomes possible to actively reduce the addition of elements such as Mn, and the excellent effects of reducing such elements can also be utilized.
At the same time, it became possible to actively and effectively utilize Al as an alloying element rather than as a deoxidizing agent, and it became possible to take advantage of the excellent effects of adding Al as an alloying element. . Example Ultra-high purity ferritic chromium stainless steel melted by the method described above and having the steel composition shown in Table 1 was cast into 50 kg round ingots.
After casting, each ingot was externally cut, heated to 1200°C for 1.5 hours, and then forged to 30mm x 130mm x 100mm. Next, after heating to 1200°C, hot rolling was performed at a temperature of 950°C or higher to obtain a hot rolled sheet with a thickness of 7 mm and a width of 130 mm. The hot rolled sheet thus obtained was annealed at 850°C and then air cooled.
Alternatively, after annealing at 1000°C or 1100°C, water-cooling heat treatment was performed, and then processed into test pieces. The analysis of C and S in steel was carried out using a CS-144 device manufactured by LECO, which is an ultra-high performance analyzer with a sensitivity of 0.1 ppm that utilizes a high-frequency combustion infrared absorption method. The pitting corrosion resistance, brittleness, high temperature deformability, acid resistance, and intergranular corrosion resistance of each test piece thus obtained were examined. The results of these tests are also summarized in Table 1. (1) Pitting Corrosion Resistance The pitting corrosion potential of each of the test steels was measured in a 0.01 mol NaCl aqueous solution (60°C). Measurement was performed at 20m after degassing with Ar gas at 200ml/min for 1 hour.
Evaluation was made with V'Ci = 100μA when swept to the side at V/min. The measurement data are summarized in Table 1. The changes in pitting corrosion potential with respect to S in steel are summarized in a graph based on the data for test steels 2, 3, 4, 12, 13, and 14, and are shown in FIG. This is Mn
0.10% or less, the amount of S in the 19%Cr-2%Mo steel is 0.0002%-0.0048% (2ppm-
This figure shows the effect on the pitting corrosion resistance of S in steel when the S content is changed up to 48 ppm). As is clear from the results shown in the graph of the same figure, the higher the pitting potential, the better the corrosion resistance, but the pitting potential begins to deteriorate significantly when the S concentration reaches around 0.002%. On the other hand, it can be seen that it is stable below 0.0010%. In the figure, the numbers indicate the test steel numbers in Table 1. (2) Brittleness: In the case of this example, the hot-rolled sheets obtained for each of the above-mentioned test steels were annealed at 1000°C in an Ar gas atmosphere for 20 minutes, and then water-cooled. A Charpy impact test was conducted on the test piece thus obtained, and the brittle fracture transition temperature was measured. Note that the test piece size was a JIS No. 4 full-size Shapey impact test piece. The results are summarized in Table 1. The data obtained for test steels 5, 16, and 17 are summarized in a graph and shown in Figure 2. These data show changes in the brittle fracture surface transition temperature (vTrs°C) of 26%Cr-1%Mo steel when the S content is less than 0.0010% and the Mn content is varied. As is clear from the graph shown, vTrs increases as the amount of Mn increases, and it can be seen that the lower the amount of Mn, the more desirable it is. Compared to the inventive steel level
Test steel 16, which has a high Mn content of 0.20%, which is on the same level as conventional steel, has vTrs of 35°C, which is higher than room temperature, indicating that it is a problem even when used at room temperature. Test steels 5, 6, 7, and 15 in Table 1 are used as test steels.
The toughness of the heat-affected zone of TIG tanned welds was investigated using the following method. After hot rolling, the material was annealed at 1000℃ for 20 minutes in an Ar gas atmosphere, then water cooled, further cold rolled by 60%, and finally annealed at 920℃ for 2 minutes in an Ar gas atmosphere. The sample was cooled with water after being held. Ar gas flow rate 10/min,
After TIG licking at a current of 65 to 75 A and a welding speed of 120 mm/min, the Shapey impact test piece was machined in the longitudinal direction so that the weld heat affected zone became the notch part of the Shapey impact test piece. The thus obtained test piece was subjected to a Charpy impact test, and the brittle fracture surface transition temperature was measured. The test piece size was a JIS No. 4 half-size Shapey impact test piece. The obtained results are summarized in a graph in FIG. As is clear from the graph shown, the addition of Al not only improves the low-temperature toughness of the base metal, but also improves the toughness in the weld heat affected zone. (3) High-temperature deformability: The deformability at high temperatures of high-Cr ferrite stainless steel with a content of 26% Cr or higher was evaluated by conducting a drop weight test in the manner described below. Here, the above-mentioned falling weight test means that a cylindrical test piece with a diameter of 15 mm and a height of 20 mm heated to 1200℃ is instantaneously made into a flat piece with a height of 7 mm using a hammer press. It evaluates deformability. If the deformability is good, cracks will not occur on the end face, but if the deformability is insufficient, cracks will occur. Table 1 summarizes the results. It can be seen that cracks become large when the Mn content is low and the S content in the steel is higher than the present invention steel level. (4) Acid resistance: 26% Cr test steels 5, 6, 7, 15, 16, 17,
A boiling test was conducted on test steels 8, 9, 19, and 20 containing 18 and 29% Cr in a PH1, H ₂ SO ₄ solution containing 15% NaCl in the following manner. The immersion time was 6 hours, and the evaluation was based on the average corrosion rate per 6 hours. The test piece had dimensions of 3 mm x 10 mm x 40 mm, and was polished using wet emery #600. and,
Acid resistance was also evaluated using the average value of two test pieces. The results are also summarized in Table 1, but Ni
It can be seen that the acid resistance of Sample Steel 9 containing A was significantly improved. (5) Intergranular Corrosion Resistance: In order to evaluate the intergranular corrosion resistance, a sulfuric acid-ferric sulfate test (JIS G 0572) was conducted on TIG test pieces for each of the test steels shown in Table 1. TIG tanning was performed under conditions of Ar gas flow rate of 10/min, current of 65 to 75 A, and welding speed of 120 mm/min. The results of these tests are summarized in Table 1. As is clear from these results, the intergranular corrosion resistance of the weld heat-affected zone is improved by the addition of Al, while the Al-free specimen steels 11, 12, 13, 14, and Falling off of grains was observed. Symbols G and L in the table indicate crystal grain dropout in the heat affected zone. The present invention has been described in detail above, but the high purity ferritic chromium stainless steel according to the present invention contains C, N, S, Si, Mn as impurities in the steel.
At the same time, by reducing O to a level that is in line with conventional methods, the toughness and intergranular corrosion resistance of chromium stainless steel, including welded parts, as well as the corrosion resistance of the base metal, especially pitting corrosion resistance and acid resistance, are significantly improved. This is an improvement, and its industrial value is extremely high.

【table】

【table】

[Table] Example 2 In this example, corrosion resistance and toughness tests were conducted in the same manner as in Example 1 for each sample steel having the composition shown in Table 2. The results are also summarized in Table 2.

【table】

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a graph showing the effect of S in steel on corrosion resistance; Figure 2 is a graph showing the effect of Mn in steel on toughness; and Figure 3 is a graph showing the effect of Mn in steel on toughness.
It is a graph showing the influence of addition on steel toughness.

Claims (1)

[Claims] 1% by weight, Cr: 15.00 to 50.00%, Mo: 5.00% or less, Ni: 0.01 to 6.00%, Al: 0.015 to 1.00%, the balance being Fe and impurities. C.N.
Al-containing ferritic chromium stainless steel with excellent corrosion resistance and toughness, characterized by reducing S, Si, Mn, and O to the following ranges: C: 0.0060% or less, N: 0.0150% or less, S: 0.0020% or less, Si: less than 0.1%, Mn: 0.15% or less, O: 0.015% or less. 2 Weight%: Cr: 15.00-50.00%, Mo: 5.00% or less, Ni: 0.01-6.00%, Al: 0.015-1.00%, and Ti: 1.00% or less [However, Ti≧5×
(C+N)%] and Nb: 1.00% or less [however,
Contains one or two types of Nb≧8×(C+N)%], with the balance consisting of Fe and impurities, and by refining, C, N, and
Al-containing ferritic chromium stainless steel with excellent corrosion resistance and toughness, characterized by reducing S, Si, Mn, and O to the following ranges: C: 0.0060% or less, N: 0.0150% or less, S: 0.0020% or less, Si: less than 0.1%, Mn: 0.15% or less, O: 0.015% or less.