KR100327618B1 - W-containing duplex stainless steel casting alloy having high corrosion resistance, good structure stability - Google Patents

Abstract

The present invention relates to a super corrosion resistance-high strength or higher stainless steel for casting containing tungsten (W), more specifically local corrosion resistance, embrittlement resistance and flowability to '475 ℃ brittle' is improved and ultra corrosion resistance and high strength It relates to a duplex stainless steel for castings containing tungsten.

It is an object of the present invention to provide an ideal stainless steel for casting containing tungsten having local corrosion resistance, embrittlement resistance to '475 ° C. brittleness' and flowability and having super corrosion resistance and high strength.

The ideal stainless steel for casting according to the present invention improves the flow rate of the molten alloy and desulfurization and deoxidation during casting of the alloy by optimizing the Si content of the Fe-25Cr-7Ni-1.5Mo-3W alloy and increasing the Mn content tolerance. By enhancing the effect, it is possible to manufacture complex shapes of castings.

Description

WT-containing duplex stainless steel casting alloy having high corrosion resistance, good structure stability}

The present invention relates to a super corrosion resistance-high strength or higher stainless steel for casting containing tungsten (W), more specifically local corrosion resistance, embrittlement resistance and flowability to '475 ℃ brittle' is improved and super corrosion resistance and high strength It relates to a duplex stainless steel for castings containing tungsten.

Typical abnormal stainless steels are based on the composition of Fe- (20-27)% Cr- (4-7)% Ni- (1.5-4.5)% Mo- (1.0-0.35) N, such as Si, Mn, Cu, Al, etc. Is added, the content of C is limited to 0.03% by weight or less in order to suppress the production of chromium carbide. Among the alloying elements of the abnormal stainless steel, Mo greatly increases the local corrosion resistance of the abnormal stainless steel, but promotes the formation of a sigma (σ) phase that greatly lowers the impact strength and local corrosion resistance of the alloy.

In general, local corrosion is a phenomenon in which the passivation film on the metal surface is destroyed and its parts are intensively corroded. Crevice corrosion and pitting corrosion are the most common forms of local corrosion. In particular, in the above stainless steel, the fitting resistance is greatly influenced by the composition of the alloy, and thus the PREw value is widely used as a measure to evaluate the fitting corrosion resistance according to the alloy composition (H. Okamoto, Proc. Conf.Application of stainless steels 92, 1992).

PREw = wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) + 16 wt% N (1)

In Formula (1), the contribution of W in the fitting resistance is defined as 1/2 of Mo. However, even in the case of abnormal stainless steels having the same PREw value, the fitting resistance was different when the Mo: W ratio was changed. In particular, J.S Kim and H.S. Kwon's study reported that when the Mo: W weight ratio was adjusted to 1: 2, the formation of sigma phase was significantly suppressed and showed excellent local corrosion resistance (Corrosion, JS Kim and HS Kwon, 'Effects of W on the Corrosion'). and Kinetics of Sigma Phase Formation of 25 Cr Duplex Stainless Steels' (1999).

Abnormal stainless steels, when used in a relatively low temperature range of 250 to 550 ° C for a long time, generally embrittlement phenomenon known as '475 ° C brittle' is observed. The high temperature brittleness due to the precipitation of sigma phase can be controlled to some extent through the heat treatment process of the product, whereas the 475 ° C brittleness appears during the actual use of the final product, so it is difficult to find a method for controlling it.

The cause of brittleness at 475 ° C. occurring in the above stainless steel is known to be due to spinodal decomposition of the ferrite phase into the α- phase, which is a Cr-rich phase, and the α phase, which is a Fe-rich phase. The α 'phase reduces the mobility of dislocations in the matrix and induces the formation of microcracks, thereby reducing the toughness of the material, and Cr-depleted regions are formed around the α' phase, which is a Cr-rich phase, to reduce the corrosion resistance of the abnormal stainless steel. Known.

Thus, increasing the '475 ° C brittle' resistance in abnormal stainless steels has a direct impact on extending the life of abnormal stainless steel products used in actual industrial structures and production equipment.

The present inventors conducted a study to compensate for the problem of brittleness at 475 ° C., which is a disadvantage of the conventional abnormal stainless steel, and the alloy containing 1.5 wt% Mo and 3 wt% W having a weight fraction of Mo: W of 1: 2 was the most. It shows excellent local corrosion resistance and embrittlement resistance, and also increases the content of Si from 0.7 to 1.1% by weight and the maximum allowable content of Mn to 1.5% by weight to deoxidize and desulfurize the molten metal by Si and Mn during casting. The present invention was completed by finding that it is possible to obtain a W-containing abnormal stainless steel which can be obtained, and the castability of the molten metal can be improved to manufacture a complex shape casting.

Accordingly, an object of the present invention is to provide an ideal stainless steel for casting containing tungsten having local corrosion resistance, embrittlement resistance to '475 ° C brittleness', and flowability, and having super corrosion resistance and high strength.

1 is a graph showing a change in the fitting corrosion resistance to the content of tungsten.

Figure 2 is a graph showing the impact toughness change of the alloy with the aging treatment time at 475 ℃ for 3Mo, 2Mo-2W, 1.5Mo-3W, 6W alloy.

Figure 3 is a graph showing the average flow length for the alloys with varying Si content of Fe-25Cr-7Ni-1.5Mo-3W super corrosion resistance stainless steel or more.

Figure 4 is a graph showing the yield strength and tensile strength of the alloy and the comparative alloy according to the present invention.

The ideal stainless steel for casting according to the present invention optimizes the content ratio of Mo and W to 1: 2, Cr content of 24.0 to 27.0 wt%, Ni content of 6.5 to 7.5 wt%, and Mo content of 1.0 to 2.0 Weight%, W content 2.0 to 4.0 weight%, N content 0.2 to 0.35 weight%, C content up to 0.03 weight%, P content up to 0.02 weight%, S content up to 0.004 weight% and the balance consists of Fe In the above stainless steel, casting corrosion resistance-high strength for tungsten containing tungsten, which improved the corrosion resistance, embrittlement resistance, and flowability of the alloy by limiting the Mn content up to 1.5 wt% and the Si content from 0.7 to 1.1 wt%. It is characterized by the above stainless steel.

The ideal stainless steel according to the present invention has a PREw value of 38.9, but the fitting corrosion resistance and '475 ° C brittleness' of the ideal stainless steel were changed by changing the weight ratio of Mo: W to 3Mo, 2Mo-2W, 1.5Mo-3W and 6W, respectively. As a result of evaluating the resistance, the 1.5Mo-3W alloy having a Mo: W weight ratio of 1: 2 showed the highest local corrosion resistance and embrittlement resistance to '475 ° C brittleness'.

In general, as a method of evaluating the castability of the molten metal, a method of comparing the fluidity of the molten metal is widely used. The flow rate of the molten metal indicates how well the molten metal flows into the mold. The flow rate of the molten metal is measured by using the flow rate of the molten metal. Factors affecting the flowability of the molten metal include the composition of the molten alloy, the content of impurities such as sulfur and phosphorus in the molten metal, the contents of inclusions, and the injection temperature of the molten metal.

In the abnormal stainless steel for casting, the deoxidation effect of the molten metal can be expected and the flowability of the molten metal can be improved by maintaining the Si content above an appropriate level. However, since Si is known to promote the formation of sigma (σ) phase which greatly reduces local corrosion resistance and impact strength in abnormal stainless steels, it is generally limited to 1.5 wt% or less. According to the present invention, Fe-25Cr In the alloy of -7Ni-1.5Mo-3W composition, it can be seen that the flowability of the molten metal is excellent between 0.7 wt% and 1.1 wt%.

In the above stainless steels, Mn is an austenite stabilizing element, and has an effect of deoxidation and desulfurization of the molten alloy and improves the solubility of nitrogen in the alloy. However, since MnS is formed to lower the corrosion resistance of the alloy, it is generally limited to 1.5% by weight or less.

Since the alloy according to the present invention has a high fitting resistance, it can be used for a long time even in a more severe local corrosion atmosphere without corrosion damage, and has excellent resistance to brittleness at 475 ° C., so that the casting product has a long life and has excellent fluidity. It enables the production of shaped castings.

The superior fitting corrosion resistance and the 475 ° C. brittleness suppression effect of the alloy having a weight ratio of Mo: W of 1: 2 are shown in the drawings. In addition, the flow rate measurement results and tensile test results of the alloys with varying Si content based on Fe-25Cr-7Ni-1.5Mo-3W or more stainless steel are shown in the drawings.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

Figure 1 shows the content of Mo and W while maintaining a constant value (38.9) of PREw (= wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) + 16 wt% N), a measure of the fitting corrosion resistance, 3 Mo, The change in fitting corrosion resistance for Fe-25Cr-7Ni-xMo-yW-0.25N alloys changed to 2Mo-2W, 1.5Mo-3W, 6W is shown as a function of W content. The 1.5 Mo-3W alloy with the Mo: W ratio of 1: 2 by weight (about 1: 1 by atomic fraction) exhibited the best fitting corrosion resistance.

2 is a result showing the impact toughness change of the alloy with the aging treatment time at 475 ℃ for 3Mo, 2Mo-2W, 1.5Mo-3W, 6W alloy. Compared to 3Mo alloy, 2Mo-2W and 1.5Mo-3W alloys, in which Mo was partially replaced by W, showed excellent embrittlement resistance when aged at 475 ° C.

3 is a view comparing average flow lengths for alloys of varying Si content of Fe-25Cr-7Ni-1.5Mo-3W super corrosion resistance stainless steel or higher. The Si content of 0.7 wt% to 1.1 wt% showed excellent flowability of each alloy melt.

4 is a view comparing the yield strength and tensile strength of the alloys 1, 2, 3 of the present invention and the comparative alloys 1, 2, 3 (Table 2.). The yield strength of the alloy showing excellent fluidity is about 500 MPa and the tensile strength is about 790 MPa.

Hereinafter, the present invention will be described in more detail with reference to raw material composition steps and examples, but these are only exemplary embodiments of the present invention and should not be interpreted in a limited sense.

<Raw Material Composition Step 1>

In order to investigate the effect of Mo and W on the local corrosion resistance and '475 ° C brittleness' of the above stainless steels, the content of Mo and W was made to have the same PREw (= 38.9) value in the basic composition of Fe-25Cr-7Ni-0.25N. An alloy of Table 1 was prepared, changed to 3Mo, 2Mo-2W, 1.5Mo-3W, 6W.

Table 1.Chemical composition of Fe-25Cr-7Ni-xMo-yW-0.25N or higher stainless steel (wt%)

Alloy Cr Ni Mo W N Fe 3Mo Comparative Alloy 24.6 6.6 3.12 - 0.25 bal. 2Mo-2W Comparative Alloy 25.01 6.81 2.15 2.10 0.25 bal. 1.5Mo-3W Alloy 24.82 6.79 1.60 3.25 0.25 bal. 6 W Comparative Alloy 24.34 6.54 - 6.30 0.25 bal.

<Raw Material Composition Step 2>

In order to investigate the change in the fitting corrosion resistance according to the W content of the alloy prepared in step 1 of the raw material composition, anodizing polarization test was performed on a degassing 80 ° C. and 4M NaCl solution to determine the fitting potential. The measured and compared result is shown in FIG.

The 1.5Mo-3W alloy with the Mo: W ratio adjusted to 1: 2 by weight (about 1: 1 by atomic fraction) exhibited the best fitting corrosion resistance and increased or decreased W content. In both cases, the fitting resistance tends to decrease.

<Raw Material Composition Step 3>

In order to investigate the change in impact toughness of the alloy according to the aging treatment time at 475 ° C., the alloy prepared in Example 1 was subjected to aging at 475 ° C. for 1 hour, 10 hours, 100 hours, and 300 hours, respectively, and then the impact toughness was measured. The results are shown in FIG.

Compared to 3Mo alloys, 2Mo-2W and 1.5Mo-3W alloys, in which Mo is partially replaced by W, show excellent embrittlement resistance at which the impact toughness decreases slowly during 475 ° C aging.

<Example 1>

To investigate the castability of molten alloys in the 1.5Mo-3W alloy with excellent fitting resistance and embrittlement resistance, the alloys of Table 2 with Fe-25Cr-7Ni-1.5Mo-3W as the basic composition and varying the Si content were subjected to atmospheric induction melting. The flow rates were measured for each of them. The flow rates were compared by measuring the length of the flow measurement sand until the alloy melted.

The fluidity measurement sandpaper consists of eight channels through which the melt flows, and averaged the eight measured values to obtain average fluidity length.

As shown in FIG. 3, the average flow length of each alloy measured as described above showed the largest value between 0.7 wt% and 1.1 wt% of silicon. In the case of the comparative alloys, the average flow length of 100 mm or less was shown, while in the case of the alloy, the castability was about 140 mm or more.

Table 2.Chemical composition of Fe-25Cr-7Ni-1.5Mo-3W-xSi-0.25N or higher stainless steel (wt%)

Alloy Cr Ni Mo W Mn Si Fe A01 Comparative Alloy 1 24.76 6.96 1.48 2.92 0.997 0.471 bal. N01 Alloy 1 25.35 6.75 1.49 2.86 0.726 0.794 bal. N02 Alloy 2 25.37 6.86 1.49 2.90 0.640 0.852 bal. N03 Alloy 3 25.66 7.02 1.55 3.01 0.755 0.942 bal. A02 Comparative Alloy 2 24.65 6.73 1.51 2.95 0.903 1.17 bal. A03 Comparative Alloy 3 24.61 6.65 1.45 3.04 0.911 1.20 bal.

<Example 2>

In order to determine the strength of the castings for the alloys according to the present invention showing excellent fluidity, a room temperature tensile test of each alloy prepared in Example 1 was conducted.

Each alloy was cut in the casting state and subjected to solution treatment at 1050 ° C. for 2 hours, and then processed into tensile test pieces having a gauge length of 25 mm and a diameter of 6.25 mm according to ASTM A370. Tensile test was carried out at room temperature, INSTRON 4206 was used.

The tensile test results are shown in FIG. 4, and the yield strength of the alloy showing excellent fluidity is about 500 MPa, and the tensile strength is about 790 MPa. This shows that the high strength is maintained in the alloy of the present invention having a Si content of 0.7% by weight to 1.1% by weight showing excellent flowability.

The ideal stainless steel for casting according to the present invention improves the flow rate of the molten alloy and desulfurization and deoxidation during casting of the alloy by optimizing the Si content of the Fe-25Cr-7Ni-1.5Mo-3W alloy and increasing the Mn content tolerance. By enhancing the effect, it is possible to manufacture complex shapes of castings.

Claims (1)

By optimizing the content ratio of Mo and W to 1: 2, the Cr content is 24.0 to 27.0 wt%, the Ni content is 6.5 to 7.5 wt%, the Mo content is 1.0 to 2.0 wt% and the W content is 2.0 to 4.0 wt% %, N content of 0.2 to 0.35% by weight, C content of up to 0.03% by weight, P content of up to 0.02% by weight, S content of up to 0.004% by weight, and the balance of Fe stainless steel. Super corrosion-resistant high strength stainless steel for castings containing tungsten with 1.5% by weight and Si content between 0.7 and 1.1% by weight.