KR102494720B1 - Low alloy duplex stainless steel with improved impact toughness of weld zone - Google Patents

Abstract

Disclosed herein is a low-alloy duplex stainless steel with improved weld impact toughness.
According to one embodiment of the disclosed low-alloy duplex stainless steel with improved impact toughness, in weight percent, C: 0.05% or less, Si: 1.0% or less, Mn: 1.0 to 2.0%, Cr: 18 to 24%, Ni: 1 to 4%, Mo: 0.5 to 1.0%, Cu: 1 to 3%, N: 0.15 to 0.25%, Zr: 0.1% or less and the remainder including Fe and unavoidable impurities, parallel to the rolling direction, When a surface perpendicular to the steel surface is observed with a transmission electron microscope, the area ratio of rod-shaped M ₂ X precipitates having an aspect ratio of 5 or more within an observation area of 200 μm ² may be 1.5% or less.

Description

Low alloy duplex stainless steel with improved impact toughness of weld zone}

The present invention relates to a low-alloy duplex stainless steel with improved weld impact toughness.

Duplex stainless steel has two phases, an austenite phase and a ferrite phase, and is used in petrochemicals, desalination facilities, pumps, and chemical tank materials due to its high strength and high corrosion resistance. In addition, since duplex stainless steel generally has a lower Ni content than austenitic stainless steel, it is attracting attention as a material with low cost and relatively little fluctuation in raw material prices.

Recently, compared to S32205 (representative component: 22Cr-5Ni-3Mo-0.16N) standardized in ASTM A240, the existing duplex stainless steel, low-alloy duplex stainless steel with lower Ni and Mo content is being developed to further reduce costs. . Low-alloy duplex stainless steel has a similar level of corrosion resistance to STS304 but higher yield strength, so it is used for sluice gates, desalination equipment members, and construction materials. 1.5Ni-5Mn-0.22N).

In the low-alloy duplex stainless steel, the ratio of the austenite phase and the ferrite phase should be maintained at 40 to 60%, respectively, while reducing the Ni content, which is an austenite phase stabilizing element. Accordingly, it is necessary to actively utilize Mn or N components that can replace Ni and stabilize the austenite phase, and as the addition of nitrogen in molten steel has become easier with the development of steelmaking technology, low-alloy duplexes with a high nitrogen component system of 0.15% by weight or more Stainless steel is being developed.

However, the high nitrogen component system has a problem in that it acts as a cause of deteriorating impact toughness by depositing excessive nitrides in a welding heat affected zone (HAZ) during a cooling process after welding. In particular, since precipitation of Cr nitride is further promoted in the heat-affected zone under high heat input welding conditions in which heat input is increased to improve welding productivity, it is necessary to suppress precipitation of Cr nitride to improve impact toughness of the heat-affected zone.

Korean Patent Publication No. 10-2016-0077370 (published on July 4, 2016)

In order to solve the above problems, the present invention is to provide a low-alloy duplex stainless steel with improved weld impact toughness that can improve welding productivity by suppressing the decrease in impact toughness and corrosion resistance of the weld under high heat input welding conditions.

As a means for achieving the above object, the low-alloy duplex stainless steel with improved weld impact toughness according to an embodiment of the present invention contains, by weight, C: 0.05% or less, Si: 1.0% or less, and Mn: 1.0 to 2.0%. , Cr: 18 to 24%, Ni: 1 to 4%, Mo: 0.5 to 1.0%, Cu: 1 to 3%, N: 0.15 to 0.25%, Zr: 0.1% or less, and the rest is Fe and unavoidable impurities Including, when the surface parallel to the rolling direction and perpendicular to the steel surface is observed with a transmission electron microscope, the area ratio of rod-shaped M ₂ X precipitates with an aspect ratio of 5 or more within an observation area of 200 μm ² is 1.5% or less can

In addition, the low-alloy duplex stainless steel with improved impact toughness of each welded part according to the present invention may have f _Cr represented by the following formula (1) of 0.001 or less.

(1) f _Cr = [%Cr]/%Cr

In the above formula (1), [%Cr] means the Cr weight% contained in the precipitate when the residue is extracted from the precipitate in the steel material, and %Cr means the average Cr weight% of the base material.

In addition, the low-alloy duplex stainless steel with improved impact toughness of each weld according to the present invention may have an austenite phase formation rate (V _α→γ ) of 0.24 or more, expressed by the following formula (2), during solidification of the molten region by welding. there is.

(2) V _α→γ = [%γ _1100℃ ]/(T ₁ - 1100℃)

In Equation (2), [%γ _{1100 °C} ] means the area fraction of the austenite phase at 1100 °C, and T ₁ means the temperature at which the formation of the austenite phase begins.

In addition, the low-alloy duplex stainless steel with improved impact toughness of each welded part according to the present invention may have a PCIV of 120 or more represented by the following formula (3).

(3) PCIV = -30.3 + 3.87*[%γ] + 247*V _α→γ - 68326*f _Cr

In the above formula (3), [%γ] means the area fraction of the austenite phase in the weld heat-affected zone. V _α→γ is the rate of formation of the austenite phase during solidification of the molten region by welding and is derived from the following formula, V _α→γ = [%γ _1100℃ ]/(T ₁ - 1100℃), [%γ _{1100 ℃} ] means the area fraction of the austenite phase at 1100 ℃, T ₁ means the temperature at which the formation of the austenite phase begins. f _Cr means the value obtained by dividing the Cr weight % contained in the precipitate by the average Cr weight % of the base metal when the precipitate in the steel material is extracted.

Further, in the low-alloy duplex stainless steel with improved impact toughness of each welded part according to the present invention, the area fraction of the austenite phase in the weld heat-affected zone may be 45 to 54%.

In addition, the low-alloy duplex stainless steel with improved impact toughness of each welded part according to the present invention may have a room temperature Charpy impact value of 120 J/cm ² or more in the weld heat-affected zone.

In addition, the low-alloy duplex stainless steel having improved impact toughness of each welded part according to the present invention may have a critical pitting temperature of 25° C. or higher.

According to the present invention, it is possible to improve welding productivity by suppressing the decrease in impact toughness of the heat-affected zone under high heat input welding conditions, and to reduce the content of expensive alloy elements such as Ni and Mo, so that the cost is relatively low-alloy duplex. Stainless steel can be provided. Specifically, the distribution of M ₂ X precipitates, the ratio (f _Cr ) of dividing the weight % of Cr contained in the precipitates by the average weight % of the parent material when the precipitates of steel materials are extracted from the residue (f Cr ), the austenite phase during solidification of the region melted by welding It is possible to provide a low-alloy duplex stainless steel with improved weld impact toughness by controlling the formation rate (V _α→γ ) and the area fraction of the austenite phase in the heat-affected zone of the weld.

According to the present invention, it is possible to provide a low-alloy duplex stainless steel that can simultaneously secure improved impact toughness and sufficient corrosion resistance of a welded part through an appropriate alloy design.

The low-alloy duplex stainless steel according to the present invention has excellent weld impact toughness, sufficient corrosion resistance, and low raw material cost compared to STS304 or STS316L, so it can be applied as a material for various industrial facilities such as desalination facilities, food and beverage facilities, and sluice gates. .

1 is a graph showing a Charpy impact value predicted by PCIV compared to a room temperature Charpy impact value actually measured in a weld heat-affected zone.

Preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified in many different forms, and the technical spirit of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Terms used in this application are only used to describe specific examples. Therefore, for example, expressions in the singular number include plural expressions unless the context clearly requires them to be singular. In addition, the terms "include" or "have" used in this application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, but other features It should be noted that it is not intended to be used to preliminarily exclude the presence of any steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Accordingly, certain terms should not be interpreted in an overly idealistic or formal sense unless clearly defined herein. For example, in this specification, a singular expression includes a plurality of expressions unless there is a clear exception from the context.

In addition, "about", "substantially", etc. in this specification are used at or in the sense of or close to the value when manufacturing and material tolerances inherent in the stated meaning are presented, and are accurate to aid in understanding the present invention. or absolute numbers are used to prevent unfair use by unscrupulous infringers of the stated disclosure.

In addition, "high heat input welding" in the present specification may be defined as welding having a high welding heat input, and generally refers to a welding technique having a higher heat input than conventional welding techniques. "High heat input welding condition" means a welding condition having a higher heat input than normal welding, and may mean, for example, a welding condition with a welding heat input of about 3.5 MJ/m or more.

In addition, in this specification, the meaning of the description that improved impact toughness can be secured even under high heat input welding conditions is that the heat input is high compared to general welding conditions, even under high heat input welding conditions where the possibility of lowering the impact toughness of the weld is relatively high. It is to emphasize the effect that improved impact toughness can be secured, and it is necessary to note that improved impact toughness can be secured even under general welding conditions.

In addition, in this specification, "welding heat-affected zone" means a region of a steel material whose metal structure or properties are changed by welding heat, from the point where the temperature rises due to welding heat to the point where the temperature does not rise due to welding heat and is at room temperature. covers the area of In this specification, the term "welded portion" refers to a region of a portion including a weld metal and a weld heat-affected zone.

In order to improve the weld heat-affected zone impact toughness of low-alloy duplex stainless steel, it is important to reduce Cr nitride precipitated in the weld heat-affected zone. Duplex stainless steel begins to solidify into a ferrite single phase during solidification of the molten region by welding and transforms into an austenite phase, and finally each phase has an area fraction of 40 to 60%. Since the ferrite phase has a body-centered cubic crystal structure, the solubility of nitrogen, an interstitial element, is lower than that of the austenite phase. Therefore, nitrogen becomes supersaturated in the ferrite single-phase section in the initial stage of solidification, and the supersaturated nitrogen reacts with Cr to form and precipitate Cr nitride at a temperature in the range of 500 to 600 ° C. Cr nitride precipitated in this way has a rod shape with an aspect ratio of 5 or more, and is precipitated in a cluster form to reduce impact toughness.

Therefore, if the amount of precipitation of Cr nitrides in the form of clusters in the microstructure of the steel is controlled to be small, it is possible to prevent a decrease in impact toughness. Focusing on this, the researchers of the present invention observed a surface parallel to the rolling direction and perpendicular to the steel surface with a transmission electron microscope, and the area ratio of rod-shaped M ₂ X precipitates with an aspect ratio of 5 or more was 1.5% within an observation area of 200 μm ² It was confirmed that good impact toughness can be secured even under high heat input welding conditions when controlled so as to be as follows. Here, the M ₂ X precipitate refers to a compound in which a metal atom (M) and an interstitial element (X) are stoichiometrically bonded at a ratio of 2:1, and for example, Cr nitride (Cr ₂ N) is there is.

In addition, various types of precipitates may exist in the heat-affected zone of welding depending on the alloy components of the duplex stainless steel, but most of the precipitates are Cr-containing, unless alloying elements such as Ti, Nb, and V are easily formed. It exists as a precipitate. According to an example of the present invention focusing on this, when the precipitates of the following low-alloy duplex stainless steel are extracted, the ratio (f _Cr ) of dividing the Cr weight % contained in the precipitate by the average Cr weight % of the base material is substituted even when the value is 0.001 or less. It is possible to secure good impact toughness even under heat welding conditions.

(1) f _Cr = [%Cr]/%Cr

In the above formula (1), [%Cr] means the Cr weight% contained in the precipitate when the residue is extracted from the precipitate in the steel material, and %Cr means the average Cr weight% of the base material.

As a method of suppressing the generation of Cr nitride that lowers the impact toughness of the weld heat-affected zone, a method of reducing the content of nitrogen forming Cr nitride by reducing supersaturated nitrogen in the ferrite phase formed by solidification of the molten region by welding is there is. The inventors of the present invention paid attention to the fact that the austenite phase has a face-centered cubic crystal structure, so the solid solubility for nitrogen is greater than that of the ferrite phase, and the formation rate of the austenite phase (V _α→γ ) during solidification of the molten region by welding It was confirmed that the supersaturated nitrogen in the ferrite phase can be reduced by increasing .

However, since it is difficult to measure the austenite phase formation rate (V _α→γ ) experimentally, V _α→γ was derived through thermodynamic calculation. Thermo-Calc software was used for thermodynamic calculation, and the average slope between the temperature at which the austenite phase starts to form (T ₁ ) and the austenite phase area fraction at 0.0% at 1100 ° C is as follows: It was derived by calculation.

(2) V _α→γ = [%γ _1100℃ ]/(T ₁ - 1100℃)

In Equation (2), [%γ _{1100 °C} ] means the area fraction of the austenite phase at 1100 °C, and T ₁ means the temperature at which the formation of the austenite phase begins. The reason why the austenite formation rate was derived based on 1100 ° C is that the area fraction of the austenite phase increases linearly up to 1100 ° C in thermodynamic calculations, and the phase fraction changes with time at temperatures exceeding 1100 ° C experimentally This is because it is judged that the austenite phase is at a stable temperature because is insignificant.

In addition, the austenite phase has better impact toughness than the ferrite phase due to its crystallographic characteristics, and since the ductile transition region does not appear as the temperature decreases, the impact toughness tends to improve as the area fraction of the austenite phase increases. In the low-alloy duplex stainless steel with improved weld impact toughness according to one embodiment of the present invention, the area fraction of the austenite phase in the weld heat-affected zone may be 45 to 54%.

The inventors of the present invention have developed the above-mentioned f _Cr , V _α→γ , and the area fraction of the austenite phase in the heat-affected zone of the weld, which can improve the impact toughness of the weld zone in order to secure more improved impact toughness of the weld zone under high heat input welding conditions. In combination, the parameter PCIV that can evaluate the room temperature impact toughness of the weld heat-affected zone was derived.

(3) PCIV = -30.3 + 3.87*[%γ] + 247*V _α→γ - 68326*f _Cr

In the above formula (3), [%γ] means the area fraction of the austenite phase in the weld heat-affected zone. V _α→γ is the rate of formation of the austenite phase during solidification of the region melted by welding and is derived from the above-described equation (2). f _Cr is a value obtained by dividing the Cr weight % contained in the precipitate by the average Cr weight % of the base material when residue is extracted from the precipitate in the steel material, and is derived from the above-described equation (1).

PCIV is a parameter for securing the desired impact toughness by more precisely estimating the room temperature Charpy impact value of the weld heat-affected zone. Referring to FIG. 1, it can be seen that the room temperature Charpy impact value actually measured in the weld heat-affected zone and the Charpy impact value predicted by PCIV match in direct proportion. In the present invention, PCIV is controlled to be 120 or more to secure sufficient impact toughness.

The distribution of M ₂ X precipitates, f _Cr , V _α→γ , the area fraction of austenite phase in the weld heat-affected zone, and PCIV, which are described as means for improving the impact toughness of the weld zone in the present invention, are independent means and depend on each other. It should be noted that no In addition, in order to improve the weld impact toughness, the above-mentioned means may be applied individually or in combination. For example, it is possible to improve the impact toughness by controlling the distribution of M ₂ X precipitates and controlling the f _Cr value, and for another example, V _α→γ and the austenite area in the weld heat affected zone. The impact toughness can be improved by controlling the fraction.

Low-alloy duplex stainless steel with improved weld impact toughness according to an example of the present invention, by weight, C: 0.05% or less, Si: 1.0% or less, Mn: 1.0 to 2.0%, Cr: 18 to 24%, Ni : 1 to 4%, Mo: 0.5 to 1.0%, Cu: 1 to 3%, N: 0.15 to 0.25%, Zr: 0.1% or less, and the rest may include Fe and unavoidable impurities.

Hereinafter, the content range of the alloy components of the low-alloy duplex stainless steel with improved weld impact toughness according to the present invention will be described.

The content of C may be 0.05% by weight or less.

C is an effective element for increasing material strength by solid solution strengthening, but if the C content is excessive, it is easily combined with carbide-forming elements such as Cr, which are effective for corrosion resistance at the ferrite-austenite phase boundary, and reduces the corrosion resistance by lowering the Cr content around the grain boundary. let it In order to ensure sufficient corrosion resistance, the content of C is limited to 0.05% by weight or less in the present invention. In terms of improving corrosion resistance, the content of C in the present invention may be preferably 0.03% by weight or less.

The content of Si may be 1.0% by weight or less.

Si acts as a stabilizing element in the ferrite phase and is added for deoxidation purposes. However, since excessive addition of Si deteriorates mechanical properties related to impact toughness, the content of Si is limited to 1.0% by weight or less in the present invention.

The content of Mn may be 1.0 to 2.0% by weight.

Mn is contained to control the fluidity of the molten metal and is added as an austenite phase stabilizing element to replace expensive Ni. In addition, an appropriate amount of Mn can improve hot workability, so Mn is added in an amount of 1.0% by weight or more in the present invention. However, if the Mn content is excessive, corrosion resistance may be deteriorated by forming MnS by combining with S in the steel, and hot workability may also deteriorate.

The content of Cr may be 18 to 24% by weight.

Cr, along with Mo, is a ferrite stabilizing element that not only plays a major role in securing the ferrite phase of duplex stainless steel, but is also an essential element for securing corrosion resistance. Corrosion resistance is improved when the Cr content is increased, but there is a disadvantage in that the expensive Ni content must be added in proportion to maintain the austenite phase fraction. In consideration of this, in order to secure an appropriate level of corrosion resistance while maintaining a desirable phase fraction of duplex stainless steel, the content of Cr is limited to 18 to 24% by weight in the present invention. In view of the above, the content of Cr may be preferably 19 to 23% by weight, more preferably 20 to 22% by weight.

The content of Ni may be 1 to 4% by weight.

Ni, together with Mn and N, plays a major role in securing the austenite phase fraction of duplex stainless steel as an austenite phase stabilizing element. In order to reduce cost, the content of Mn and N, which are other austenite phase forming elements, can be increased by replacing expensive Ni. However, an excessive reduction in Ni content may result in excessive addition of Mn and N to secure the austenite phase fraction, resulting in a decrease in corrosion resistance and hot workability, or low addition of Cr and Mo, which are ferrite phase stabilization elements, resulting in poor corrosion resistance. It is difficult. Considering this, the content of Ni in the present invention is limited to 1 to 4% by weight. For cost reduction, the content of Ni may be preferably 1 to 3% by weight.

The content of Mo may be 0.5 to 1.0% by weight.

Mo, like Cr, is a ferrite phase stabilizing element and at the same time a strong corrosion resistance improving element, is added in an amount of 0.5% by weight or more in the present invention. However, Mo is a very expensive element, and if the content is excessive, it easily forms a sigma phase during heat treatment to reduce impact toughness and corrosion resistance. In order to secure the phase fraction of the ferrite phase and secure corrosion resistance, the content of Mo may be preferably 0.6% by weight or more. In order to suppress sigma phase formation and reduce cost, the content of Mo may be preferably 0.7% by weight or less.

The content of Cu may be 1 to 3% by weight.

Cu is an austenitic phase stabilizing element, which suppresses the phase transformation to martensite phase when steel is cold-formed and improves corrosion resistance in a sulfuric acid atmosphere, so Cu is added in an amount of 1% by weight or more in the present invention. However, since Cu reduces pitting resistance in a chlorine atmosphere and may deteriorate hot workability when excessively added, the Cu content is managed to 3% by weight or less in the present invention.

The content of N may be 0.15 to 0.25% by weight.

In duplex stainless steel, N is one of the elements that greatly contributes to the stabilization of the austenite phase along with Ni. If the N content is too small, a small amount of Cr and Mo must be added to secure the austenite phase fraction, resulting in weld strength and phase stability. It is difficult. In addition, since an increase in N content can incidentally improve corrosion resistance and strength of steel, N is added in an amount of 0.15% by weight or more in the present invention. However, if the content of N is excessive, hot workability may be deteriorated to reduce the real yield, so in the present invention, N is added in an amount of 0.25% by weight or less. In terms of improving the real rate, N may be preferably managed to 0.20% by weight or less.

In addition, Zr may be optionally added to the low-alloy duplex stainless steel having improved weld impact toughness according to the present invention.

The content of Zr may be 0.1% by weight or less.

Zr is an alloying element capable of improving the strength of steel, and suppresses precipitation of Cr nitride without forming other precipitates. For this effect in the present invention, Zr may be optionally added. However, if the content of added Zr exceeds 0.1% by weight, the effect is saturated and only increases the alloy cost, which is not preferable.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

According to an example, the low-alloy duplex stainless steel having improved weld impact toughness as described above may have a Charpy impact value of 120 J/cm ² or more at room temperature in a weld heat-affected zone.

In addition, the low-alloy duplex stainless steel with improved weld impact toughness according to the present invention can secure sufficient corrosion resistance through an appropriate alloy design. According to one example, the critical pitting temperature may be 25°C or higher.

The low-alloy duplex stainless steel according to the present invention has excellent weld impact toughness, sufficient corrosion resistance, and low raw material cost compared to STS304 or STS316L, so it can be applied as a material for various industrial facilities such as desalination facilities, food and beverage facilities, and sluice gates. .

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

{Example}

After melting the steel having the chemical composition shown in Table 1 in a vacuum induction melting furnace, hot rolling was performed, and then solution heat treatment was performed in the range of 1100 to 1200 ° C. to prepare a hot-rolled sheet material having a thickness of 15 mm.

Alloy composition (% by weight) C Si Mn Cr Ni Mo Cu N Zr Invention example 1 0.022 0.51 1.48 21.0 2.51 0.71 1.51 0.17 - Invention example 2 0.020 0.49 1.53 21.0 2.57 0.75 1.00 0.16 - Invention example 3 0.018 0.50 1.50 20.9 1.96 0.63 2.98 0.17 - Invention example 4 0.030 0.76 1.78 20.5 2.00 0.60 2.00 0.19 0.01 Invention Example 5 0.035 0.50 1.56 19.5 2.05 0.6 1.98 0.17 - Example 6 0.021 0.48 1.69 20.1 2.13 0.71 2.34 0.18 - Example 7 0.020 0.56 1.71 22.4 2.51 0.68 2.05 0.17 0.06 Comparative Example 1 0.029 0.58 1.82 21.4 2.12 0.59 0.59 0.18 - Comparative Example 2 0.019 0.50 4.90 21.0 1.97 0.81 0.98 0.17 - Comparative Example 3 0.018 0.51 1.44 21.1 1.54 0.60 1.00 0.15 - Comparative Example 4 0.032 0.62 2.56 20.5 2.00 0.58 1.52 0.18 - Comparative Example 7 0.032 0.66 0.85 21.3 2.13 0.62 1.52 0.17 - Comparative Example 8 0.025 0.58 0.92 21.5 2.20 0.64 2.21 0.18 - Comparative Example 9 0.026 0.53 1.57 20.9 2.24 0.12 2.00 0.18 - Comparative Example 10 0.033 0.65 1.84 22.1 2.05 0.32 1.98 0.19 - Comparative Example 11 0.036 0.64 1.89 21.8 0.90 0.63 1.54 0.17 - Comparative Example 12 0.028 0.71 1.81 21.5 2.00 0.61 2.33 0.28 -

A welding heat cycle simulation test was performed to evaluate the welding characteristics. The welding heat cycle test was performed in the following way. The specimen was heated to 1350 ° C at a heating rate of 130 ° C / s and maintained for 5 seconds, then cooled to 500 ° C at a slow cooling rate of 10 ° C / s, and rapidly cooled from 500 ° C or less to room temperature. Here, the cooling rate of 10°C/s is an experiment assuming a high heat input welding condition in which the welding heat input is greater than the normal condition.

Table 2 below shows the formation rate of austenite phase (V _α→γ ), area fraction of austenite phase in the weld heat-affected zone ([%γ]), When residues are extracted from precipitates in steel materials, the Cr weight % contained in the precipitates divided by the average Cr weight % of the parent material (f _Cr ), the precipitate area ratio (%), the critical pitting temperature (℃), and the Charpy impact value at room temperature in the heat-affected zone of welding (J) is shown.

The formation rate of the austenite phase (V _α→γ ) during solidification of the molten region by welding was calculated using Thermo-Calc, a thermodynamic calculation software. As described above, the temperature at which the austenite phase starts to form (T ₁ ) The average slope between the area fractions of the austenite phase at 1100 ° C. from the area fraction of the austenite phase at that time (0.0%) was calculated and derived as follows.

V _α→γ = [%γ _1100℃ ]/(T ₁ - 1100℃)

The area fraction of the austenite phase ([%γ]) in the weld heat-affected zone was measured 5 times for the cross section of the specimen using a ferrite scope after the welding heat cycle simulation test, and the average value was taken.

The f _Cr value was calculated by the following method after welding thermal cycle simulation test. After extracting a precipitate having a diameter of 10 μm or more by extracting the residue from the specimen, the content (wt%) of Cr contained in the precipitate was measured through ICP analysis. It was obtained by dividing the Cr weight% of the measured precipitate by the average Cr weight% of the base material.

The precipitate area ratio (%) was measured by the following method after a welding thermal cycle simulation test. The cross-section of the specimen was processed as an observation surface parallel to the rolling direction and perpendicular to the steel surface, and then the cross-section was observed with a transmission electron microscope. The area ratio of rod-shaped M ₂ X precipitates having an aspect ratio of 5 or more within an observation area of 200 μm ² was analyzed through an image analysis program.

The critical pitting temperature (℃) was determined by exposing the surface of the specimen to 1M NaCl solution in accordance with ASTM G150 standard after a welding thermal cycle simulation test, then increasing the temperature of the solution from 0 ℃ to 1 ℃ / s at a voltage of 700mV. Current density When measuring , the temperature at which a current density of 100 ㎂/cm 2 or more was maintained for 60 seconds was measured.

The Charpy impact value (J) of the welding heat-affected zone at room temperature was measured by machining the specimen so that the direction of the notch is parallel to the rolling direction, and then performing a Charpy impact test at room temperature of 25 to 30 °C.

Example V _α→γ
(%/℃) [%γ]
(%) f _Cr precipitate
area ratio
(%) threshold
official
temperature
(℃) weld heat affected zone
room temperature charpy impact value
(J) Invention example 1 0.27 46.0 0.00072 0.37 31.2 176 Invention example 2 0.26 46.7 0.00071 0.42 27.5 179 Invention Example 3 0.28 47.5 0.00091 0.81 25.2 181 Invention example 4 0.33 50.6 0.00049 0.17 28.3 197 Invention Example 5 0.30 48.4 0.00051 0.23 27.6 191 Example 6 0.32 48.6 0.00056 0.26 28.1 190 Example 7 0.30 47.5 0.00040 0.20 27.9 185 Comparative Example 1 0.20 42.4 0.00103 1.64 28.0 61 Comparative Example 2 0.24 47.0 0.00048 0.14 23.1 199 Comparative Example 3 0.20 30.2 0.00118 2.54 27.0 61 Comparative Example 4 0.24 46.1 0.00084 0.82 24.0 145 Comparative Example 7 0.22 43.2 0.00102 1.53 26.3 115 Comparative Example 8 0.23 43.2 0.00109 1.53 27.1 109 Comparative Example 9 0.26 46.5 0.00099 0.76 22.4 181 Comparative Example 10 0.27 46.3 0.00090 0.71 22.6 184 Comparative Example 11 0.21 35.1 0.00149 3.51 23.3 63 Comparative Example 12 0.28 45.8 0.00164 2.11 25.1 86

Referring to Tables 1 and 2, the alloy composition defined by the present invention, the formation rate of the austenite phase during solidification of the molten region by welding (V _α→γ ), the area fraction of the austenite phase in the heat-affected zone of welding, f _Cr value , the room temperature Charpy impact value of the welding heat-affected zone was 120J or more, and the impact toughness was improved. In addition, sufficient corrosion resistance was secured with a critical pitting temperature of 25 to 35 °C.

On the other hand, in Comparative Examples 1, 3, 7, 8, and 11, the austenite phase formation rate (V _α→γ ) during solidification of the welded molten region was lower than 0.24%/° C., resulting in nitrogen supersaturation in ferrite. As a result, Cr nitride was excessive, and the precipitate area ratio exceeded 1.5%. As a result, the impact toughness of the welded heat-affected zone was lowered, and the room temperature Charpy impact value of the welded heat-affected zone was less than 120 J.

In Comparative Examples 2 and 4, the content of Mn, an austenite phase stabilizing element, exceeded 2.0% by weight, so that a sufficient austenite phase formation rate (V _{α → γ} ) and austenite phase area fraction could be secured, so the impact toughness was good. . However, there is a problem in that the corrosion resistance is lowered due to the excessive Mn content and the critical pitting temperature is less than 25 ° C.

In Comparative Examples 7 and 8, the Mn content was less than 1.0% by weight, so that the area fraction of the austenite phase was not sufficiently secured. As a result, the normal temperature Charpy impact value of the welded heat-affected zone was less than 120 J, and the impact toughness was poor.

In Comparative Examples 9 and 10, the content of Mo was added too little, less than 0.5% by weight, and the corrosion resistance was inferior.

Comparative Example 11 had an excessively small amount of Ni, less than 1% by weight, so that sufficient austenite phase formation rate and austenite phase area ratio could not be secured. As a result, the normal temperature Charpy impact value of the welded heat-affected zone was less than 120 J, and the impact toughness was poor.

In Comparative Example 12, the fCr value exceeded 0.001 because the N content exceeded 0.25% by weight and _Cr nitride was excessively precipitated. As a result, the normal temperature Charpy impact value of the welded heat-affected zone was less than 120 J, and the impact toughness was poor.

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In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those skilled in the art within the scope that does not deviate from the concept and scope of the claims described below. It will be appreciated that many changes and modifications are possible.

Claims (7)

In weight percent, C: 0.05% or less, Si: 1.0% or less, Mn: 1.0 to 2.0%, Cr: 18 to 24%, Ni: 1 to 4%, Mo: 0.5 to 1.0%, Cu: 1 to 3% , N: 0.15 to 0.25%, Zr: 0.1% or less and the remainder including Fe and unavoidable impurities,
When a plane parallel to the rolling direction and perpendicular to the steel surface is observed with a transmission electron microscope, the area ratio of rod-shaped M ₂ X precipitates having an aspect ratio of 5 or more within an observation area of 200 μm ² is 1.5% or less,
The rate of formation of the austenite phase (V _{α → γ} ) expressed by the following formula (2) during solidification of the region melted by welding is 0.26 or more and 0.33 or less,
Low-alloy duplex stainless steels with improved weld impact toughness with a critical pitting temperature above 25:
(2) V _α→γ = [%γ _1100℃ ]/(T ₁ - 1100℃)
(In the above formula (2), [%γ _{1100 ° C} ] means the area fraction of the austenite phase at 1100 ° C, and T ₁ means the temperature at which the formation of the austenite phase begins). According to claim 1,
Low-alloy duplex stainless steel with improved weld impact toughness having f _Cr of 0.001 or less, represented by the following formula (1):
(1) f _Cr = [%Cr]/%Cr
(In the above formula (1), [%Cr] means the Cr weight% contained in the precipitate when the precipitate in the steel material is extracted, and %Cr means the average Cr weight% of the base material). delete According to claim 1,
Low-alloy duplex stainless steel with improved weld impact toughness having a PCIV of 120 or more, expressed by the following formula (3):
(3) PCIV = -30.3 + 3.87*[%γ] + 247*V _α→γ - 68326*f _Cr
(In the above equation (3), [%γ] means the area fraction of the austenite phase in the weld heat-affected zone; V _α→γ is the rate of formation of the austenite phase during solidification of the molten region by welding. , V _{α → γ} = [%γ _{1100 ° C} ]/(T ₁ - 1100 ° C), [% γ _{1100 ° C} ] means the area fraction of the austenite phase at 1100 ° C, and T ₁ is the formation of the austenite phase means the temperature at which this starts; f _Cr means the value obtained by dividing the Cr weight % contained in the precipitate by the average Cr weight % of the base metal when the precipitate in the steel is extracted from the residue). According to claim 1,
A low-alloy duplex stainless steel with improved weld impact toughness in which the area fraction of the austenite phase in the weld heat-affected zone is 45 to 54%. According to claim 1,
Low-alloy duplex stainless steel with improved weld impact toughness with a Charpy impact value of 120J or more at room temperature in the weld heat-affected zone. delete

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