JP6311633B2 - Stainless steel and manufacturing method thereof - Google Patents

Description

  The present invention relates to a ferrite-marten excellent in low-temperature toughness and press formability, which is a material for a structure to be press-molded, and is particularly suitable as a material for a structure that requires moderate toughness even in cold regions. The present invention relates to a site duplex stainless steel and a manufacturing method thereof.

  Ferritic stainless steel is excellent in corrosion resistance and inexpensive, and is therefore applied to various applications that require corrosion resistance. In recent years, ferritic stainless steel having a plate thickness exceeding 5 mm has been manufactured on a commercial basis due to progress in manufacturing technology, and the application range of ferritic stainless steel has further expanded. Among them, by adopting ferritic stainless steel, there is a movement to make maintenance such as repainting unnecessary and to suppress life cycle cost.

  When ferritic stainless steel is used as a structural material, it is required that the ferritic stainless steel has excellent low-temperature toughness in consideration of use in cold regions. Refinement of crystal grains is effective for improving low-temperature toughness, and a martensite phase is used as a technique for this. By controlling the manufacturing conditions to generate an appropriate martensite phase, the crystal grains can be refined and the toughness can be improved. However, since the martensite phase is a very strong structure, if the martensite phase is simply generated, the strength becomes too high and the springback after press molding becomes noticeable, or the press molding itself. In other words, excessive pressure is required, which makes molding difficult.

  Thus, it is difficult to obtain a ferritic stainless steel having excellent low temperature toughness and press formability, which is required for a structure material used in a cold region.

  For example, as a structure material, Patent Document 1 discloses stainless steel for rail wagons. This forms a martensite phase in the weld heat affected zone to improve the corrosion resistance of the weld zone, and further regulates the FFV value to suppress the occurrence of surface defects. However, this stainless steel has insufficient low-temperature toughness.

  Further, as a stainless steel plate having excellent toughness, Patent Document 2 discloses a thick-walled martensitic stainless steel having excellent toughness with suppressed generation of δ ferrite. However, this stainless steel is too strong and is difficult to press. Moreover, this stainless steel may also lack low temperature toughness.

  Further, as a stainless steel with less spring back after processing, Patent Document 3 discloses a stainless steel in which the shape crystallinity during bending is improved by controlling the average crystal grain size and texture. However, this stainless steel has low low temperature toughness and is not suitable for use in cold regions.

JP 2012-12702 A JP-A 61-136661 Japanese Patent Laid-Open No. 2001-32050

  As described above, with the techniques disclosed in these prior documents, it is difficult to obtain stainless steel having excellent low temperature toughness and press formability that can be preferably applied in cold districts.

  The present invention has been made in view of such circumstances, and is suitable as a material for a structure used in a cold region, and has excellent low temperature toughness and press formability, and a ferrite-martensite duplex stainless steel and its production It aims to provide a method.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive research on the influence of the steel structure, the component composition, etc. on the low temperature toughness and press formability. The findings obtained as a result are as follows.

  It is known that refinement of crystal grains is effective for improving low temperature toughness. By adjusting the component composition and manufacturing conditions, a ferrite-martensite two-phase structure can be obtained, whereby crystal grains can be refined and low-temperature toughness can be improved. However, an increase in the martensite phase fraction increases the yield stress and increases the springback when the press molding is performed, and thus reduces the press formability. In addition, the refinement of crystal grains similarly increases the yield stress, thus reducing the press formability. From the above, it is difficult to achieve both low temperature toughness and press formability at a high level as in the conventional case even in a ferrite-martensite two-phase structure.

  Therefore, in the present invention, by focusing on the structure control during hot rolling, by setting the heating temperature and winding temperature of hot rolling to an appropriate range according to the component, a metal in which martensite phase and ferrite phase are mixed A microstructure was formed and both moderate low temperature toughness and press formability were achieved.

  In addition, it has been found that the formation of coarse precipitates such as TiN, which becomes the starting point of fracture, can be suppressed by adjusting the components to an appropriate range, and the low temperature toughness can be improved.

  The present invention is constituted by the above findings. Specifically, the present invention is as follows.

  [1] By mass%, C: 0.005 to 0.030%, N: 0.005 to 0.020%, Si: 0.05 to 0.50%, Mn: 0.05 to 3.0% , P: 0.040% or less, S: 0.02% or less, Cr: 9.0 to 16.0%, Ni: 0.1 to 5.0%, Nb: 0.05 to 0.5%, Ti: Contains 0.10% or less, the balance is composed of two components of Fe and inevitable impurities, and a ferrite phase and a martensite phase, and the volume ratio of the martensite phase is 5 to 60%. A ferritic-martensitic duplex stainless steel having a steel structure and a yield stress of 420 MPa or less.

  [2] The component composition further includes, by mass%, Al: 0.20% or less, V: 0.20% or less, Cu: 1.0% or less, Mo: 2.0% or less, W: 1. The ferrite-martensite duplex stainless steel according to [1], containing one or more of 0% or less and Co: 0.5% or less.

  [3] The component composition further includes, in mass%, one of Ca: 0.01% or less, B: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less. The ferrite-martensite duplex stainless steel according to [1] or [2], comprising two or more kinds.

[4] A method for producing a ferrite-martensite duplex stainless steel excellent in low temperature toughness according to any one of [1] to [3], wherein the heating temperature T _H (° C.) is the following formula (1): Hot rolling is performed under conditions where the coiling temperature T _C (° C.) satisfies the following formula (2), and after the hot rolling, annealing is performed under conditions where the annealing temperature is 680 to 780 ° C. and the annealing time is 1 hour or more. A method for producing a ferrite-martensite duplex stainless steel characterized by the above.
2600C + 1700N-20Si + 20Mn-40Cr + 50Ni + 1460 ≦ T _H ≦ 2600C + 1700N-20Si + 20Mn-40Cr + 50Ni + 1660 (1) Formula T _C ≦ 875-400C-500N-110Mn + 10Cr-125Ni (2) Formulas (1) and (2) The element symbol means the content (% by mass) of each element.

  According to the present invention, a ferrite-martensite duplex stainless steel having excellent low temperature toughness and press formability, which is suitable as a material for a structure used in a cold region and press formed, can be obtained.

It is a figure which shows the influence of the yield stress which acts on the amount of springback.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

  First, the ferrite-martensite duplex stainless steel of the present invention (hereinafter sometimes referred to as “stainless steel of the present invention”) will be described in the order of component composition, steel structure, and characteristics.

<Ingredient composition>
The stainless steel of the present invention is, as an essential component, in mass%, C: 0.005 to 0.030%, N: 0.005 to 0.020%, Si: 0.05 to 0.50%, Mn: 0.05 to 3.0%, P: 0.040% or less, S: 0.02% or less, Cr: 9.0 to 16.0%, Ni: 0.1 to 5.0%, Nb: 0 0.05% to 0.5%, Ti: 0.10% or less, and as optional components, by mass, Al: 0.20% or less, V: 0.20% or less, Cu: 1.0% or less , Mo: 2.0% or less, W: 1.0% or less, and Co: 0.5% or less, one or two or more by mass%, Ca: 0.01% or less, B: 0.01 % Or less, Mg: 0.01% or less, and REM: 0.05% or less. The balance other than these essential components and optional components is Fe and inevitable impurities. Hereinafter, each component will be described. In the following description, “%” representing the content of a component means “mass%”.

C: 0.005-0.030%
C is an austenite stabilizing element. Since the martensite phase fraction increases as the C content increases, C is an element useful for adjusting the martensite phase fraction. These effects can be obtained by setting the C content to 0.005% or more. However, since C is an element that increases the yield stress, an excessive content of C is not preferable from the viewpoint of press formability. In the present invention, the C content is suitably 0.030% or less. Therefore, the C content is in the range of 0.005 to 0.030%. Preferably, it is 0.008 to 0.020% of range.

N: 0.005-0.020%
N is an austenite stabilizing element. Since the martensite phase fraction increases as the N content increases, N is an element useful for adjusting the martensite phase fraction. These effects can be obtained by making the N content 0.005% or more. However, when N is contained excessively, the production of coarse TiN is promoted, and this TiN lowers the low temperature toughness. Therefore, the N content is suitably 0.020% or less. Therefore, the N content is in the range of 0.005 to 0.020%. Preferably, it is 0.008 to 0.015% of range.

Si: 0.05 to 0.50%
Si is an element used as a deoxidizer. In order to acquire the effect, it is necessary to make Si content 0.05% or more. Si is a ferrite phase stabilizing element, and the martensite phase fraction decreases as the Si content increases, so Si is a useful element for adjusting the martensite phase fraction. On the other hand, if the Si content exceeds 0.50%, the ferrite phase becomes brittle and the toughness decreases. Therefore, the Si content is in the range of 0.05 to 0.50%. Preferably, it is 0.11 to 0.40%.

Mn: 0.05 to 3.0%
Mn is an austenite stabilizing element, and since the martensite phase fraction increases as the Mn content increases, Mn is an element useful for adjusting the martensite phase fraction. The effect of containing Mn is obtained when the Mn content is 0.05% or more. However, if the Mn content exceeds 3.0%, not only the effect is saturated, but the yield stress increases and the press formability decreases. Therefore, the Mn content is in the range of 0.05 to 3.0%. Preferably, it is 0.11 to 2.5% of range.

P: 0.040% or less P is preferably smaller from the viewpoint of low temperature toughness, and the upper limit value of the content is set to 0.040%. Preferably, the P content is 0.035% or less.

S: 0.02% or less S is preferably smaller in terms of hot workability and corrosion resistance, and the allowable upper limit of the content is 0.02%. Preferably, the S content is 0.005% or less.

Cr: 9.0 to 16.0%
Cr is an essential element for forming a passive film on the surface of stainless steel and ensuring corrosion resistance. In order to obtain the effect, it is appropriate to set the Cr content to 9.0% or more. Further, Cr is a ferrite phase stabilizing element, and is a useful element for adjusting the martensite phase fraction because the martensite phase fraction decreases as the Cr content increases. However, when the Cr content exceeds 16.0%, not only the cost increases, but it becomes difficult to obtain a sufficient martensite phase fraction. Therefore, the Cr content is in the range of 9.0 to 16.0%. More preferably, it is 10.5 to 13.5%.

Ni: 0.1 to 5.0%
Ni, like Mn, is an austenite stabilizing element and is a useful element for adjusting the martensite phase fraction because the martensite phase fraction increases as the Ni content increases. These effects are obtained when the Ni content is 0.1% or more. However, if the Ni content exceeds 5.0%, it becomes difficult to control the martensite phase fraction, and it becomes difficult to obtain an appropriate low temperature toughness. Therefore, the Ni content is in the range of 0.1 to 5.0%. Preferably, it is 0.4 to 3.0% of range. More preferably, it is 0.6 to 2.2% of range.

Nb: 0.05 to 0.5%
Nb generates C, N, and carbides, nitrides, and carbonitrides in steel and suppresses formation of Cr carbonitrides and the like. Thereby, the corrosion resistance, particularly the corrosion resistance of the welded portion can be improved. The effect is obtained when the Nb content is 0.05% or more. On the other hand, when the Nb content exceeds 0.5%, the hot workability decreases, the hot rolling load increases, the recrystallization temperature of the hot-rolled steel sheet rises, and the appropriate martensite phase fraction is increased. It becomes difficult to perform annealing at a temperature at a rate. Therefore, the Nb content is set to 0.05 to 0.5%. Preferably, it is 0.10 to 0.3%.

Ti: 0.10% or less Ti, like Nb, precipitates and fixes C and N in steel as Ti carbide, nitride or carbonitride, and suppresses the formation of Cr carbonitride and the like. Have. However, when coarse TiN is generated, the TiN serves as a starting point for fracture, thereby lowering the low temperature toughness. It is one of the features of the present invention to reduce this coarse TiN production and to reduce the fracture starting point. When the Ti content exceeds 0.10%, the toughness drop due to coarse TiN becomes remarkable. Therefore, the Ti content is set to 0.10% or less. More preferably, it is 0.04% or less, More preferably, it is 0.02% or less. In addition, in this invention, since Ti is so preferable that there is little, a minimum is 0%.

  Next, optional components will be described.

Al: 0.20% or less Al is an element generally useful for deoxidation. In order to obtain the effect, the Al content is preferably 0.001% or more. On the other hand, if the content exceeds 0.20%, a large Al-based inclusion is generated and causes surface defects. Therefore, the Al content is 0.20% or less. More preferably, it is 0.03 to 0.14% of range.

V: 0.20% or less V, like Ti and Nb, is an element that generates a nitride and suppresses a decrease in corrosion resistance due to sensitization of the weld. In order to acquire the effect, it is preferable to make V content 0.005% or more. However, if the V content exceeds 0.20%, the corrosion resistance of the oxide film formed on the weld called a temper collar decreases. Therefore, the V content is 0.20% or less. More preferably, it is 0.010 to 0.10%.

Cu: 1.0% or less Cu is an element that improves corrosion resistance, and is an element that particularly reduces crevice corrosion. For this reason, it is preferable to contain Cu when high corrosion resistance is required. In order to sufficiently exhibit the effect of improving the corrosion resistance, it is effective to make the Cu content 0.03% or more. However, when the Cu content exceeds 1.0%, the hot workability decreases. Therefore, when Cu is contained, the upper limit of the content is 1.0%. A more preferable Cu content is 0.3 to 0.5%.

Mo: 2.0% or less Mo is an element that improves corrosion resistance, and is particularly preferably contained when high corrosion resistance is required. In order to sufficiently exhibit corrosion resistance, it is effective to contain 0.03% or more. However, if the content exceeds 2.0%, the workability in the cold state is deteriorated, and the rough surface in the hot rolling occurs, so that the surface quality is extremely lowered. Therefore, when Mo is contained, the upper limit is made 2.0%. A more preferable range is 0.1 to 1.3%.

W: 1.0% or less W is an element that improves corrosion resistance, and it is preferable to contain W when particularly high corrosion resistance is required. In order to obtain the effect, the W content is preferably 0.01% or more. However, excessive W content increases strength and decreases manufacturability. Therefore, the W content is set to 1.0% or less.

Co: 0.5% or less Co is an element that improves toughness, and it is preferable to contain Co when higher toughness is required. In order to obtain the effect, the Co content is preferably 0.01% or more. However, excessive Co content reduces manufacturability. Therefore, the Co content is set to 0.5% or less.

Ca: 0.01% or less Ca is an element that suppresses nozzle clogging due to precipitation of Ti-based inclusions that are likely to occur during continuous casting. In order to obtain the effect, the Ca content is preferably 0.0001% or more. However, excessive Ca content generates CaS, which is a water-soluble inclusion, and reduces corrosion resistance. Therefore, the Ca content is set to 0.01% or less.

B: 0.01% or less B is an element that improves the secondary work brittleness. In order to obtain the effect, it is effective to make the B content 0.0001% or more. However, excessive addition of B causes a decrease in ductility due to solid solution strengthening. Therefore, the B content is set to 0.01% or less.

Mg: 0.01% or less Mg is an element that improves the equiaxed crystal ratio of the slab and contributes to the improvement of workability. In order to acquire the effect, it is preferable to make Mg content 0.001% or more. However, excessive addition of Mg deteriorates the surface properties of the steel. Therefore, the Mg content is set to 0.01% or less.

REM: 0.05% or less REM is an element that improves oxidation resistance and suppresses the formation of oxide scale. Among REMs, La and Ce are particularly effective. In order to obtain the effect, it is effective to make the REM content 0.001% or more. However, excessive content of REM reduces productivity such as pickling properties and increases costs. Therefore, the content of REM is set to 0.05% or less.

  In the present invention, in addition to the above-described essential components and optional components, other elements may be contained based on conventional knowledge. The balance other than the elements specified above is Fe and inevitable impurities. Specific examples of the inevitable impurities include Zn: 0.03% or less and Sn: 0.3% or less.

<Steel structure>
Next, the steel structure will be described. The steel structure of the stainless steel of the present invention consists of two phases, a ferrite phase and a martensite phase. And the volume ratio of a martensite phase is 5 to 60%.

Volume ratio of martensite phase: 5-60%
When the steel structure of the stainless steel of the present invention includes a martensite phase, the ferrite phase crystal grains are refined, and the low temperature toughness is improved. If the volume ratio of the martensite phase is less than 5%, a sufficient crystal grain refining effect cannot be obtained, and it is difficult to set the low temperature toughness to an appropriate value. When the volume fraction of the martensite phase exceeds 60%, the yield stress becomes too high, and the spring back during press forming becomes significant. Therefore, the volume ratio of the martensite phase is set to 5 to 60%. More preferably, it is 10 to 30%.

  In the present invention, the ferrite phase becomes fine due to the presence of the martensite phase. The term “fine” means that the average crystal grain size as a whole metal structure including the ferrite phase and the martensite phase measured by a cutting method is 20 μm or less. The average crystal grain size was measured as follows. A 400 μm long line segment is made vertically and horizontally in the plate thickness direction for a 400 μm square L cross-sectional structure where crystal grain boundaries are exposed by aqua regia etching. A value obtained by dividing the length by the number of intersections between the line segment and the crystal grain boundary was defined as the average crystal grain size.

Volume ratio of ferrite phase: 40 to 95%
Since the stainless steel of the present invention has an appropriate volume fraction of the ferrite phase, it suppresses springback due to press working and obtains good press formability. The effect is obtained when the volume fraction of the ferrite phase is 40% or more. However, normally, when the ferrite phase is annealed for a long time as in the present invention, the crystal grains become excessively coarse. Therefore, when a ferrite phase is introduced in order to obtain good press formability, there is a problem that the low temperature toughness is lowered by normal annealing. In the present invention, by appropriately controlling the volume fraction of the ferrite phase and optimizing the annealing conditions, the ferrite phase is prevented from becoming coarse and the excellent low-temperature toughness is maintained while maintaining excellent press formability. However, if the volume fraction of the ferrite phase exceeds 95%, it becomes difficult to suppress the coarsening of crystal grains even under the annealing conditions of the present invention. Therefore, the volume fraction of the ferrite phase is set to 40 to 95%.

Volume ratio of other phases: 5% or less The present invention is a ferrite-martensite duplex stainless steel, and the steel structure is a two-phase structure of a ferrite phase and a martensite phase. That is, these two phases are the main phases of the steel structure, and it is not necessary to include phases other than these two phases. However, even if the steel structure contains a small amount of phases other than these two phases, the effect of the present invention is not impaired. Specifically, the total of other phases may be 5% or less by volume ratio. Examples of other phases include residual austenite phase and various inclusions such as carbides and nitrides.

<Characteristic>
Yield stress: 420 MPa or less The influence of the yield stress is significant in the springback by press molding. The greater the yield stress, the greater the springback after press molding, making it more difficult to stabilize the shape.

  Deviation of angle from 90 ° (spring back amount Δθ) and yield stress after bending 90 ° using a punch with R = 5mm for a specimen with 5mm thickness having various yield stresses FIG. 1 shows the relationship.

  In addition, the plot of FIG. 1 is the result which used the test numbers B, E, A, N, J, S, C, and L of Table 2 as a test material in order from the left (in order from a small yield stress). Correspond.

  As shown in FIG. 1, it is understood that the yield stress is 420 MPa or less and the springback amount Δθ is 5 ° or less, so that the press formability is good. Therefore, the yield stress is set to 420 MPa or less. Preferably it is 400 MPa or less.

  In the present invention, for the stainless steel having the component composition of the present invention, by controlling the heating temperature of the hot rolling to a two-phase region of δ + γ, the structure before hot rolling is made relatively fine and the elemental The tissue has a concentration distribution. Furthermore, by setting the coiling temperature of the subsequent hot rolling to the α single phase region, it promotes appropriate coarsening of the ferrite phase, and appropriately controls the volume ratio of the martensite phase-ferrite phase and its crystal grain size. Yes. According to the structure control during hot rolling, the yield stress is set to 420 MPa or less in the present invention.

<Manufacturing method>
Then, the manufacturing method of the stainless steel of this invention is demonstrated.

  First, molten steel adjusted to the above component composition is melted in a commonly used melting furnace such as a converter or an electric furnace, and then vacuum degassing (RH method), VOD (Vacuum Oxygen Decarburization) method, AOD (Argon Oxygen Decarburization) etc. are used for refining by a known refining method, and then a steel slab (steel material) is obtained by a continuous casting method or an ingot-bundling method. The casting method is preferably continuous casting from the viewpoint of productivity and quality. The slab thickness is preferably 200 mm or more.

  Here, in order to improve the low temperature toughness, it is important to suppress the Ti content to 0.10% or less as described above. Specifically, when scrap is not used or when scrap is used, the Ti content of the scrap is analyzed to control the total amount of Ti in the scrap, and this is immediately after the steel grade containing Ti is melted. It is necessary to employ a melting method that strictly restricts the mixing of Ti, such as not melting the invention steel.

Next, the steel slab is heated to a heating temperature defined by the formula (1) and hot-rolled to obtain a hot-rolled steel sheet. In the present invention, the heating condition is one of important conditions. The formula (1) is represented by 2600C + 1700N-20Si + 20Mn-40Cr + 50Ni + 1460 ≦ T _H ≦ 2600C + 1700N-20Si + 20Mn-40Cr + 50Ni + 1660, T _H means the heating temperature (° C.), and the element symbol in the formula (1) is the content of each element (Mass%) is meant.

  Equation (1) is a condition for the heating temperature to be a two-phase region of δ phase and γ phase. The left side of equation (1) indicates the boundary temperature of γ / δ + γ, and the right side indicates the boundary temperature of δ + γ / δ. Therefore, when the formula (1) is satisfied, the heating temperature of the hot rolling is in the range of δ + γ. In this temperature range, the presence of both the δ phase and the γ phase suppresses the coarsening of the crystal grains before hot rolling as compared with the case of heating in the temperature range of the γ single phase. Further, concentration of the ferrite phase stabilizing element into the δ phase and concentration of the austenite stabilizing element into the γ phase occur, respectively, and the concentration distribution of the element proceeds. Thus, by making the structure before hot rolling relatively fine and having a concentration distribution of elements, the martensite phase and the ferrite phase are mixed by subsequent hot rolling and annealing. You can get an organization. As a result, a stainless steel having both good low temperature toughness and press formability can be obtained. When the heating temperature exceeds the right side of equation (1) and becomes a δ single-phase temperature range, concentration distribution at the heating temperature does not occur and a substantially single-phase structure is formed, so two steps of δ-γ transformation and γ-α transformation occur. Due to the transformation, fine crystal grains are formed in a wide region after hot rolling annealing, and the yield stress increases. When the heating temperature is lower than the left side of the formula (1), a relatively coarse structure with little concentration distribution after hot rolling is formed, but this forms a coarse structure with a small martensite fraction by annealing, thereby reducing low temperature toughness. Reduce. Therefore, formula (1) was determined.

  In the hot rough rolling of the steel slab heated to the above temperature, it is preferable to carry out rolling with a rolling reduction of 30% or more in a temperature range exceeding 900 ° C. for one pass or more. By this strong rolling, the crystal structure of the steel sheet is refined and the toughness is improved. After hot rough rolling, finish rolling is performed according to a conventional method. The finishing temperature of finish rolling is preferably 750 ° C. or higher. More preferably, it is 800 degreeC or more. This is because hot rolling is terminated at a temperature range of α + γ with a γ single phase or a large γ fraction, and an excessively coarse ferrite phase is formed in the cooling process after the hot rolling is completed. This is to prevent it.

After the hot rolling, the coil is wound around the coil at a winding temperature defined by the formula (2). Adjustment of the coiling temperature is also important in the production method of the present invention. (2) is represented by _{T C ≦ 875-400C-500N-110Mn} + 10Cr-125Ni. (2) _{T C} denotes a coiling temperature (℃) In the equation. Moreover, the element symbol in Formula (2) means content (mass%) of each element.

  Equation (2) is a condition for the coiling temperature to be in the α single phase temperature range. By winding in this temperature range, most of the microstructure after hot rolling becomes a ferrite phase, but a part of the region where the austenite stabilizing element is concentrated remains as a martensite phase. In such a structure in which the majority is a ferrite phase and partly contains martensite, there are few factors that suppress the coarsening of the ferrite phase in the subsequent annealing temperature rising process, and a moderately coarse ferrite phase is formed. As a result, press formability is improved. On the other hand, when the coiling temperature exceeds the right side of equation (2) and falls within the temperature range of α + γ, the martensite phase after hot rolling increases, suppressing the coarsening of the ferrite phase and increasing the yield stress. Reduces press formability. Therefore, formula (2) was determined. The lower limit of the coiling temperature is not particularly limited, but if the coiling temperature falls below 350 ° C., the strength of the steel strip may increase due to the temperature decrease, and the coiling temperature may be difficult. It is preferable that it is 350 degreeC or more.

  The thickness of the hot-rolled steel sheet obtained by hot rolling is not particularly limited, but is preferably 2.0 to 8.0 mm.

  The hot-rolled steel sheet is annealed under appropriate conditions. In order to make the structure of the hot-rolled steel sheet have an appropriate crystal grain size, the annealing conditions are important in the present invention. The specific conditions are an annealing temperature of 680 to 780 ° C. and an annealing time of 1 hour or more.

  Annealing conditions will be described. In a relatively fine structure in which the ferrite phase is mostly formed after hot rolling, the ferrite phase is moderately coarsened in the annealing temperature rising process. Thereafter, when the temperature rises to the α + γ region, a new γ phase is generated from the region where the austenite stabilizing element is concentrated. This γ phase transforms into a martensite phase after annealing, and forms a fine structure together with the martensite phase present after hot rolling. In this way, a structure in which a relatively coarse ferrite phase and a fine martensite phase are mixed is formed. If the annealing temperature is less than 680 ° C., in addition to insufficient recrystallization, the formation of martensite phase is also insufficient, resulting in a structure with low low temperature toughness. On the other hand, if the annealing temperature exceeds 780 ° C., the yield stress increases with the increase of the martensite phase fraction, the springback becomes remarkable, and it becomes difficult to stabilize the shape during press molding. Therefore, the annealing temperature was set to 680 ° C to 780 ° C. Preferably, it is 700 to 740 ° C. In addition, when the annealing time is less than 1 hour, transformation to the γ phase becomes insufficient, so the annealing time was set to 1 hour or more. The upper limit of the annealing time is not particularly limited, but when annealing is performed for an extremely long time, the crystal grains become coarser. Therefore, the annealing time is preferably 100 hours or less.

  After annealing, descaling such as shot blasting and pickling is preferably performed. Descaling conditions may be determined as appropriate.

  After descaling such as pickling, skin pass rolling may be performed. Further, cold rolling, annealing, pickling and the like may be further performed in accordance with a conventional method to form a cold rolled sheet.

  For welding of the stainless steel according to the present invention, all the usual welding methods such as arc welding including TIG and MIG, seam welding, resistance welding such as spot welding, and laser welding can be applied.

  Steel having the component composition shown in Table 1 was vacuum-melted and cast into a steel slab having a thickness of 250 mm. The produced steel slab was heated at the temperature shown in Table 2, 9-pass hot rolling was performed, and the steel slab was wound at the winding temperature shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 5 mm. The obtained hot-rolled steel sheet was annealed under the conditions shown in Table 2, and then the scale was removed by shot blasting and pickling. In the rough rolling of hot rolling, rolling with a rolling reduction of 30% or more was performed for one pass or more in a temperature range exceeding 900 ° C. Moreover, the finishing temperature of the finish rolling of the hot rolling was set to 750 ° C. or higher.

  An L cross section (vertical cross section parallel to the rolling direction) having a shape of 20 mm × 10 mm was taken from the hot-rolled steel sheet from which the scale had been removed, and the element distribution of Ni and Cr was measured using EPMA. The crystal grains in which Ni was concentrated and Cr was reduced as compared with other regions were determined as the martensite phase, and the martensite phase fraction was determined by image processing. The results are shown in Table 2. No. F, No. Since I was an unrecrystallized structure, the martensite phase fraction could not be measured. Further, it was confirmed by Vickers hardness (HV 0.01 is 200 or less) that the phase other than the martensite phase is a ferrite phase.

  Three Charpy test pieces in the C direction (direction perpendicular to the rolling direction) were produced from the hot-rolled steel sheet from which the scale had been removed, and a Charpy test was performed at -50 ° C. The Charpy test piece was a sub-size test piece of 5 mm (thickness) × 55 mm (width) × 10 mm (length). Each test material was tested three times to determine the average absorbed energy. Table 2 shows the obtained absorbed energy. In the examples of the present invention, absorption energy of 20 J or more is obtained, and it can be seen that the low temperature toughness is good. No. which is a comparative example. B, No. In K, since the heating temperature of the hot rolling was lower than the left side of the formula (1), the martensite phase fraction was less than 5%, the crystal grains were coarsened, and the absorbed energy at −50 ° C. was less than 20J. No. In O, since the heating temperature of the hot rolling exceeded the right side of the formula (1), the refinement of crystal grains progressed, and the yield stress became 420 MPa or more. No. Since F has a low annealing temperature, No. Since I has a short annealing time, recrystallization of the hot-rolled steel sheet structure was insufficient, and the absorbed energy at −50 ° C. was less than 20 J. No. In X, the Ti content was out of the scope of the present invention, and coarse TiN serving as a fracture starting point was generated, so the absorbed energy at −50 ° C. was less than 20 J.

  A JIS No. 5 tensile test piece was produced in the L direction (in the direction parallel to the rolling direction) from the hot-rolled steel sheet from which the scale had been removed, a tensile test was performed, and the yield stress was measured. When the yield point could not be observed, the 0.2% proof stress was taken as the yield stress. The results are shown in Table 2. In all the examples of the present invention, the yield stress is 420 MPa or less, which indicates that the press formability is good. No. which is a comparative example. C, No. In L, since the winding temperature deviates from the equation (2), the yield stress exceeded 420 MPa. No. F and No. No. H shows the annealing temperature deviating from the present invention. Since the annealing time of I was outside the present invention, the yield stress was over 420 MPa. No. W had a high martensite phase fraction and a yield stress of over 420 MPa because the Mn content deviated from the scope of the present invention even when the hot rolling and annealing conditions were within the scope of the present invention.

  From the above results, according to the present invention, it was confirmed that a ferrite-martensite duplex stainless steel excellent in low temperature toughness and press formability can be obtained.

  According to the present invention, a ferrite-martensite 2 excellent in low-temperature toughness and press-formability, which is a structure to be press-molded and is suitable as a material used in applications requiring moderate toughness even in cold regions. Phase stainless steel and its manufacturing method are obtained.

Claims (4)

In mass%, C: 0.005 to 0.030%, N: 0.005 to 0.020%, Si: 0.05 to 0.50%, Mn: 0.05 to 3.0%, P: 0.040% or less, S: 0.02% or less, Cr: 9.0 to 16.0%, Ni: 0.1 to 5.0%, Nb: 0.09 to 0.5%, Ti: 0 A composition comprising 10% or less, the balance being Fe and inevitable impurities,
A steel structure consisting of two phases of a ferrite phase and a martensite phase, wherein the volume fraction of the martensite phase is 5 to 60%,
Features and be away stainless steel that yield stress is less than or equal to 420MPa. Further, the composition of the component is, by mass, Al: 0.20% or less, V: 0.20% or less, Cu: 1.0% or less, Mo: 2.0% or less, W: 1.0% or less. and Co: stainless steel according to claim 1, characterized in that it contains 0.5% of one or more of the following. The component composition is, in mass%, Ca: 0.01% or less, B: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less, or one or more of them. characterized in that it contains the claim 1 or claim 2 stainless steel according to. A method of manufacturing a stainless steel according to any one of claims 1 to 3,
Hot rolling is performed under conditions where the heating temperature T _H (° C.) satisfies the following formula (1) and the winding temperature T _C (° C.) satisfies the following formula (2):
After the hot rolling, the annealing temperature is 680 to 780 ° C., the manufacturing method of the characteristics and to Luz stainless steel that annealing time performs annealing for 1 hour or more.
2600C + 1700N-20Si + 20Mn-40Cr + 50Ni + 1460 ≦ T _H ≦ 2600C + 1700N-20Si + 20Mn-40Cr + 50Ni + 1660 (1) Formula T _C ≦ 875-400C-500N-110Mn + 10Cr-125Ni (2) Formulas (1) and (2) The element symbol means the content (% by mass) of each element.

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