JP2015086443A - Ferrite-martensite two-phase stainless steel excellent in low temperature toughness and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferrite-martensite two-phase stainless steel that has corrosion resistance and workability each required for wagon body material, and that is excellent in low temperature toughness, and to provided a method of producing the ferrite-martensite two-phase stainless steel.SOLUTION: The ferrite-martensite two-phase stainless steel excellent in low temperature toughness is provided which has a specific component composition, satisfies following inequality (I) to (III) and has a steel structure consisting of 2 phases of a ferrite phase and a martensite phase, the content of the martensite phase being 5% to 95%of, by vol.%. 10.5≤Cr+1.5×Si≤13.5...(I), 2.0≤30×(C+N)+Ni+0.5×Mn≤6.0...(II), Mn/Ni≤1.0...(III), where Cr and Si in the inequality (I), C, N, Ni and Mn in the inequality (II) and Mn and Ni in the inequality (III) mean contents (mass%) of respective elements.

Description

  The present invention relates to a ferrite-martensite duplex stainless steel excellent in low-temperature toughness suitable as a body use material for a freight car that carries coal, oils, and the like in a cold region, and a method for producing the same.

  The amount of freight transport by rail has been increasing year by year as the global rail laying distance has increased. Freight cars such as rail wagons and containers are used for this rail freight transportation, and in recent years, ferritic stainless steel has been used as the material.

  However, there is a problem that ferritic stainless steel is not suitable for use in cold regions such as inland parts of the Eurasian continent where the temperature is -30 ° C or lower in winter because of low temperature toughness. In particular, excellent low-temperature toughness is required for materials used in freight car bodies that carry liquids such as oils.

  As a stainless steel for rail wagons, for example, a stainless steel in which a martensite phase is formed in the weld heat affected zone to improve the corrosion resistance of the welded portion, and the occurrence of surface defects is regulated by defining the FFV value is disclosed in Patent Literature 1 is disclosed. However, this stainless steel has insufficient low-temperature toughness.

  As a stainless steel plate having excellent toughness, for example, Patent Document 2 discloses a high-strength, high-toughness stainless steel plate having excellent bendability. In this high-strength, high-toughness stainless steel plate, the length in the rolling direction of the MnS-based inclusion particles is 3 μm or less, and the ratio between the rolling direction length of the MnS-based inclusion particles and the length in the direction perpendicular thereto is 3.0 or less. The bendability is improved. However, in the invention described in Patent Document 2, the corrosion resistance required as a material for body use of a freight car, particularly the corrosion resistance of the welded portion is insufficient, and the toughness at low temperatures may not be sufficient.

  Patent Document 3 discloses a thick-walled martensitic stainless steel with excellent toughness that suppresses the formation of δ ferrite. However, the strength of this stainless steel is too high, and it is difficult to press it for application to rail wagons and containers for rail freight. In addition, the stainless steel described in Patent Document 3 may also lack low temperature toughness.

JP 2012-12702 A Japanese Patent Laid-Open No. 11-302791 JP-A 61-136661

  As described above, the stainless steels disclosed in these patent documents are not suitable as materials for cargoes that carry liquids such as oils in cold regions because of low temperature toughness. In addition, the stainless steel disclosed in the above patent document may not have the corrosion resistance and workability required for the material for body use of freight cars.

  The present invention has been made in view of such circumstances, and has ferritic-martensitic duplex stainless steel having corrosion resistance and workability required for freight car body materials and excellent low-temperature toughness, and a method for producing the same. The purpose is to provide.

  In order to solve the above-mentioned problems, the present inventors have intensively studied the influence of the structure and components on the low temperature toughness.

  As a method for evaluating the influence of the structure on the low temperature toughness, a method using the Hall-Petch law showing a correlation between the crystal grain size and the low temperature toughness is known. According to this law, the ductile brittle transition temperature decreases in proportion to the -1/2 power of the crystal grain size. That is, the finer the crystal grain size, the lower the low temperature toughness. Based on this finding, the present inventors have studied the components and the production method in order to make the crystal grain size of stainless steel finer. FIG. 1 shows the correlation between the martensite phase fraction (content of martensite phase expressed in volume%) of stainless steel and the average crystal grain size in the component range of the present invention. It has been found that the average grain size decreases with a martensite phase fraction of 5% to 95%. As a result, the low temperature toughness can be improved through minimizing the average crystal grain size. The method for measuring the average crystal grain size is as described in the examples.

  The martensite phase fraction can be controlled by adjusting the Cr equivalent (Cr + 1.5 × Si) and Ni equivalent (30 × (C + N) + Ni + 0.5 × Mn) and the annealing temperature. By adjusting these parameters, a ferrite-martensite duplex stainless steel having a fine average crystal grain size and excellent low-temperature toughness can be obtained.

  Furthermore, the present inventors have focused on the toughening of the martensite phase and studied the contents of Ni and Mn, which are austenite stabilizing elements, and the toughness of the martensite phase. Various stainless steels in the component range of the present invention were annealed at 900 ° C. for 10 hours, and water-cooled after annealing to produce stainless steel having a 100% martensite structure. Using this stainless steel, a sub-size Charpy test piece was prepared, and the absorbed energy at −50 ° C. was measured. The results are shown in FIG. It was confirmed that the lower the Mn / Ni, the better the low temperature toughness of the martensite phase. That is, as an element for forming a martensite phase, it is possible to obtain a ferrite-martensite duplex stainless steel having further excellent low-temperature toughness by increasing the Ni ratio and suppressing the Mn ratio.

  The present invention has been completed based on the above findings. That is, this invention makes the following structure a summary.

  (1) By mass%, C: 0.005 to 0.030%, N: 0.005 to 0.030%, Si: 0.05 to 1.00%, Mn: 0.05% or more and 1.00 %: P: 0.04% or less, S: 0.02% or less, Al: 0.01 to 0.15%, Cr: 10.0 to 13.0%, Ni: 0.3 to 5.0 %, Ti: 4 × (C content (%) + N content (%))% or more and 0.4% or less, V: 0.005 to 0.10%, the balance being Fe and inevitable It consists of impurities, satisfies the following inequalities (I) to (III), has a steel structure consisting of two phases of a ferrite phase and a martensite phase, and the content of the martensite phase is 5% to 95% by volume%. A ferritic-martensitic duplex stainless steel characterized by

10.5 ≦ Cr + 1.5 × Si ≦ 13.5 (I)
2.0 ≦ 30 × (C + N) + Ni + 0.5 × Mn ≦ 6.0 (II)
Mn / Ni ≦ 1.0 (III)
Here, Cr and Si in the inequality (I), C, N, Ni and Mn in the inequality (II), and Mn and Ni in the inequality (III) are the contents (mass of each element). %).

  (2) By mass%, Cu: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, and Co: 0.5% or less, containing one or more kinds The ferritic-martensitic duplex stainless steel as described in (1).

  (3) By mass%, Ca: 0.01% or less, B: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less, containing one or more. The ferrite-martensite duplex stainless steel as described in (1) or (2).

  (4) A method for producing a ferrite-martensite duplex stainless steel according to any one of (1) to (3), wherein the steel slab is heated to a temperature of 1100 to 1300 ° C, and then a temperature of over 900 ° C. At a temperature of 700 to 850 [deg.] C. and annealing for 1 hour or more is performed at a low temperature. A method for producing ferrite-martensite duplex stainless steel having excellent toughness.

  ADVANTAGE OF THE INVENTION According to this invention, it has the corrosion resistance and workability which are required for the body use material of a freight car which carries coal, oils, etc. in a cold region, and is excellent in low-temperature toughness, and its manufacture. A method is obtained. According to the present invention, the effect of improving the low temperature toughness of the welded portion can be obtained as a result of excellent low temperature toughness as a material.

  Moreover, according to the present invention, it is possible to produce the above-mentioned ferrite-martensite duplex stainless steel having excellent properties at low cost and high efficiency.

It is a figure which shows the influence of the martensite phase fraction which acts on an average crystal grain diameter. It is a figure which shows the influence of Mn and Ni which give to the absorbed energy of -50 degreeC of a martensitic phase. It is a figure which shows the example of a measurement of element distribution of the hot-rolled steel plate by EPMA.

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

  First, the component composition of the ferrite-martensite duplex stainless steel of the present invention (sometimes referred to as “stainless steel” in the present specification) will be described. In the following description of each component,% indicating the content of each element is mass% unless otherwise specified.

C: 0.005-0.030%, N: 0.005-0.030%
C and N are austenite stabilizing elements. When the contents of C and N increase, the martensite phase fraction in the stainless steel of the present invention tends to increase. Thus, C and N are useful elements for adjusting the martensite phase fraction. The effect is obtained by setting both the C content and the N content to 0.005% or more. However, since C and N are also elements that reduce the toughness of the martensite phase, it is appropriate that both the C content and the N content be 0.030% or less. Accordingly, the C and N contents are both in the range of 0.005 to 0.030%. More preferably, both are in the range of 0.008 to 0.020%.

Si: 0.05-1.00%
Si is an element used as a deoxidizer, and in order to obtain the effect, the Si content needs to be 0.05% or more. Si is a ferrite stabilizing element, and the martensite phase fraction tends to decrease as the Si content increases. Therefore, Si is an element useful for adjusting the martensite phase fraction. On the other hand, if the content exceeds 1.00%, the ferrite phase becomes brittle and the toughness decreases. For this reason, content of Si shall be 0.05 to 1.00% of range. More preferably, it is 0.11 to 0.40%.

Mn: 0.05% or more and less than 1.00% Mn is an important element in the present invention. Mn is an austenite stabilizing element, and the martensite phase fraction increases as the Mn content increases. The effect obtained by containing Mn can be obtained by making the Mn content 0.05% or more. However, since Mn is an element that lowers the toughness of the martensite phase, it is preferable that the Mn content is small from the viewpoint of low temperature toughness. This is considered because cross slip is suppressed when the Mn content increases. This reduction in the toughness of the martensite phase becomes significant when the Mn content is 1.00% or more, and it becomes difficult to control. Therefore, the Mn content is in the range of 0.05 to less than 1.00%. More preferably, it is 0.11 to 0.40% of range.

P: 0.04% or less P is preferably smaller in terms of hot workability. In the present invention, the allowable upper limit of the P content is 0.04%. A more preferable upper limit value is 0.035%.

S: 0.02% or less S is preferably smaller in terms of hot workability and corrosion resistance. In the present invention, the allowable upper limit of the S content is 0.02%. A more preferred upper limit is 0.005%.

Al: 0.01 to 0.15%
Al is generally an element useful for deoxidation, and the effect can be obtained by making the Al content 0.01% or more. On the other hand, when the content exceeds 0.15%, a large Al-based inclusion is generated and causes surface defects. Therefore, the Al content is in the range of 0.01 to 0.15%. More preferably, it is 0.03 to 0.14% of range.

Cr: 10.0-13.0%
Since Cr forms a passive film, it is an essential element for ensuring corrosion resistance. In order to acquire the effect, it is necessary to contain 10.0% or more of Cr. Cr is a ferrite stabilizing element, and is a useful element for adjusting the martensite phase fraction. However, if the Cr content exceeds 13.0%, not only the production cost of stainless steel increases, but it becomes difficult to obtain a sufficient martensite phase fraction. Therefore, the Cr content is in the range of 10.0 to 13.0%. More preferably, it is 10.5 to 12.5%.

Ni: 0.3-5.0%
Ni is an important element in the present invention. Ni, like Mn, is an austenite stabilizing element and is an element useful for adjusting the martensite phase fraction. The effect is acquired by making Ni content 0.3% or more. Furthermore, in the present invention, Ni is utilized as an element for improving the toughness of the martensite phase. As shown in FIG. 2, low temperature toughness is improved by reducing the Mn content and increasing the Ni content. This is thought to be because cross-sliding becomes easier due to the addition of Ni. However, if the Ni content exceeds 5.0%, it becomes difficult to control the martensite phase fraction, and the toughness decreases. Therefore, the Ni content is in the range of 0.3 to 5.0%. More preferably, it is 1.0 to 3.0% of range. More preferably, it is 1.2 to 2.7% of range.

Ti: 4 × (C content (%) + N content (%))% or more and 0.4% or less Ti is C, N in steel is Ti carbide, nitride or carbonitride (hereinafter, Three types of carbide, nitride, and carbonitride are collectively named and referred to as carbonitride), and have the effect of suppressing the formation of Cr carbonitride and the like. As a result, Ti combines with C and N to form carbonitrides, so that the amount of C and N dissolved in the martensite phase decreases, and a decrease in the toughness of the martensite phase is suppressed. Further, by suppressing the precipitation of Cr carbonitride, the corrosion resistance, particularly the corrosion resistance of the welded portion is improved. Ti is an element having these two effects. These effects can be obtained by setting the content to 4 × (C content (%) + N content (%))% or more. On the other hand, even if the Ti content exceeds 0.4%, the effect is not only saturated, but a large amount of Ti carbonitrides and the like are precipitated in the steel, leading to deterioration of toughness. Therefore, the Ti content is 4 × (C content (%) + N content (%))% or more and 0.4% or less. More preferably, it is a range that satisfies 0.15 to 0.25% and 4 × (C content (%) + N content (%)) or more.

V: 0.005-0.10%
V is an element that forms a nitride like Ti and suppresses a decrease in the toughness of the martensite phase. The effect is acquired by making V content 0.005% or more. However, if the content of V exceeds 0.10%, V is concentrated in the temper collar portion of the welded portion and the corrosion resistance is lowered. Therefore, the V content is 0.005 to 0.10%. More preferably, it is 0.01 to 0.06%.

  The stainless steel of the present invention contains the above components, with the balance being Fe and inevitable impurities. Inevitable impurities refer to components that are contained even if not intentionally added (unavoidable impurities), and components that do not impair the effects of the present invention even if intentionally added. Specific examples of the inevitable impurities include Zn: 0.03% or less and Sn: 0.3% or less. Depending on the type of element, it is impossible to distinguish between inevitable impurities and components that do not detrimentally affect the effects of the present invention. For example, a part of the Sn content may be mixed into the steel as an inevitable impurity, and the others may be intentionally added to the steel. In this case, the sum of the amount contained as an unavoidable impurity and the amount contained by intentional addition may be 0.3% or less.

  In addition to the above components, the stainless steel of the present invention further includes Cu: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, and Co: 0.5% by mass%. You may contain 1 type, or 2 or more types among the following.

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, when applying the stainless steel of this invention to the use for which high corrosion resistance is requested | required, it is preferable that stainless steel contains Cu. However, if the Cu content exceeds 1.0%, the hot workability decreases, the austenite phase at high temperatures increases, the phase balance of ferrite-martensite is disrupted, and excellent low temperature toughness is obtained. It becomes difficult. Therefore, when the stainless steel of the present invention contains Cu, the upper limit is made 1.0%. Moreover, in order to fully exhibit the corrosion-resistance improvement effect, it is preferable that content of Cu is 0.3% or more. A more preferable range of the Cu content is 0.3 to 0.5%.

Mo: 1.0% or less Mo is an element that improves corrosion resistance. For this reason, when applying the stainless steel of this invention to the use for which high corrosion resistance is requested | required, it is preferable that stainless steel contains Mo. However, if the Mo content exceeds 1.0%, workability in cold rolling is deteriorated, and surface roughness in hot rolling occurs, resulting in extremely low surface quality. Therefore, when Mo is contained in the stainless steel of the present invention, the upper limit of the content is preferably set to 1.0%. Further, in order to sufficiently exhibit the effect of improving the corrosion resistance, it is effective to contain 0.03% or more of Mo. A more preferable range of the Mo content is 0.1 to 0.8%.

W: 1.0% or less W is an element that improves corrosion resistance. For this reason, when the stainless steel of the present invention is applied to applications requiring high corrosion resistance, the stainless steel preferably contains W. The effect is obtained by making the W content 0.01% or more. However, when the content of W becomes excessive, the strength increases and the manufacturability decreases. Therefore, the content of W is set to 1.0% or less.

Co: 0.5% or less Co is an element that improves toughness. For this reason, when the stainless steel of the present invention is applied to an application that requires particularly high toughness, the stainless steel preferably contains Co. The effect can be obtained by setting the Co content to 0.01% or more. However, if the Co content is excessive, productivity is reduced. Therefore, the content of Co is set to 0.5% or less.

  In addition to the above components, the stainless steel of the present invention may further include, in mass%, Ca: 0.01% or less, B: 0.01% or less, Mg: 0.01% or less, and REM: 0.05%. You may contain 1 type, or 2 or more types among the following.

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. The effect is acquired by making Ca content 0.0001% or more. However, when Ca is contained excessively, CaS that is a water-soluble inclusion is generated, and the corrosion resistance is lowered. Therefore, the Ca content is preferably 0.01% or less.

B: 0.01% or less B is an element that improves secondary work brittleness, and in order to obtain the effect, the B content is made 0.0001% or more. However, when B is contained excessively, ductility is lowered 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. The effect is acquired by making Mg content 0.0001% or more. However, when Mg is contained excessively, the surface properties of steel deteriorate. 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. From the viewpoint of suppressing oxide scale, it is particularly effective to use La and Ce among REM. The effect can be obtained by setting the REM content to 0.0001% or more. However, when REM is contained excessively, productivity such as pickling properties is reduced and manufacturing cost is increased. Therefore, the content of REM is set to 0.05% or less.

  In the present invention, in addition to the optional elements as described above, other elements may be further contained based on conventional knowledge. Even in that case, it is important to consider the phase balance between the ferrite phase and the austenite phase at the annealing temperature. Note that Nb is preferably not contained in the present invention because Nb is combined with C and N to greatly collapse the phase balance. Even when the stainless steel of the present invention contains Nb, its content is preferably 0.05% or less.

  Next, the steel structure of the ferrite-martensite duplex stainless steel of the present invention will be described. In addition,% which represents content of each phase in steel structure shall be volume%.

The content of martensite phase is 5% to 95% by volume
In the stainless steel of the present invention, the crystal grains are refined by including the martensite phase, and the low temperature toughness is improved. As shown in FIG. 1, when the content of the martensite phase is less than 5% or more than 95% by volume, the average crystal grain size exceeds 10.0 μm, and improvement in toughness due to refinement of crystal grains cannot be expected. Therefore, the content of the martensite phase is 5% to 95% by volume. More preferably, it is 15% to 90%, and most preferably 30% to 80%. If the content of the martensite phase is 30% to 80%, the average crystal grain size becomes very small as shown in FIG.

  The present invention is an invention in which low temperature toughness is improved by refining crystal grains. In the present invention, a reverse transformation to an austenite phase by annealing is used as a method for refining crystal grains. This is because the structure of the ferrite phase and martensite phase after hot rolling is annealed under appropriate temperature conditions, so that part of the martensite phase is transformed into the austenite phase and the grains are refined. It is a technique to do. The structure transformed into the austenite phase by annealing transforms again into the martensite phase in the cooling process after annealing, and generates finer crystal grains. What is important here is the annealing temperature and the austenite phase fraction at that temperature (content of austenite phase expressed in volume%). If the austenite phase fraction at the annealing temperature is too small, the amount of reverse transformation is small and the effect of crystal grain refinement is insufficient. If the austenite phase fraction at the annealing temperature is too large, the austenite phase grows after reverse transformation and fine crystal grains cannot be obtained. Therefore, an appropriate austenite phase fraction at the annealing temperature is required for crystal grain refinement by reverse transformation. The moderate austenite phase fraction is 5% to 95% because the austenite phase fraction at the annealing temperature is considered to be the martensite phase fraction after cooling.

10.5 ≦ Cr + 1.5 × Si ≦ 13.5 (I)
2.0 ≦ 30 × (C + N) + Ni + 0.5 × Mn ≦ 6.0 (II)
The austenite phase fraction at a predetermined annealing temperature can be adjusted by so-called Cr equivalent and Ni equivalent. Since the austenite phase at the annealing temperature transforms to the martensite phase in the cooling process after annealing, the martensite phase fraction of the stainless steel can be adjusted by adjusting the austenite phase fraction at the annealing temperature. In the present invention, formula (I) and formula (II) using Ni equivalent are defined as Cr equivalents, and the respective ranges are defined. Here, when the formula (I) using the Cr equivalent is less than 10.5, the Cr equivalent is too small, and therefore it is difficult to adjust the Ni equivalent to bring the austenite phase fraction at a predetermined annealing temperature into an appropriate range. Become. On the other hand, if the formula (I) exceeds 13.5%, the Cr equivalent is too much, and even if the Ni equivalent is increased, it becomes difficult to obtain an appropriate austenite phase fraction at a predetermined annealing temperature. Therefore, the formula (I) is set to 10.5 or more and 13.5 or less. More preferably, it is 11.0 or more and 12.5 or less. Similarly, in the formula (II) using Ni equivalent, if the Ni equivalent is less than 2.0, the Ni equivalent is too small, so it is difficult to obtain an austenite phase at a predetermined annealing temperature, and if it exceeds 6.0, an appropriate austenite phase fraction is obtained. It becomes difficult to obtain. Therefore, the formula (II) is set to 2.0 or more and 6.0 or less. More preferably, it is 2.5 or more and 5.0 or less.

Ferrite phase content 5 to 95% by volume
In the stainless steel of the present invention, the ferrite phase content is 5 to 95% by volume. If the content of the ferrite phase is 5% or more by volume, in addition to obtaining the effect of refining the crystal grains in the annealing process, the workability is improved, so press processing for molding the freight car body Is easy and preferable. In addition, if the content of the ferrite phase is 95% or less by volume, in addition to obtaining the effect of refining crystal grains in the annealing process, the martensite phase is increased and the strength is improved. This is preferable because the required strength can be obtained.

  As described above, the steel structure of the stainless steel of the present invention is composed of two phases of ferrite and martensite, but may contain other phases as long as the effects of the present invention are not impaired. Examples of other phases include an austenite phase and a σ phase. If the total content of the other phases is 10% or less by volume, it is considered that the effects of the present invention are not impaired.

Mn / Ni ≦ 1.0 (III)
In the present invention, the inequality (III) is an indispensable condition for making steel a desired structure. In the present invention, among the austenite stabilizing elements, Ni and Mn, by reducing the Mn content that makes the martensite phase brittle and increasing the Ni content that strengthens the martensite phase, the martensite phase itself Toughened. As a result, low temperature toughness is improved even with the same martensite phase fraction. This is presumably because, when Mn is contained in a large amount relative to Mn, cross slip occurs easily in the martensite phase, and deformation occurs before the cleavage fracture occurs to absorb the impact. Thus, a martensitic phase with a high Ni content improves low temperature toughness. The effect is obtained when Mn / Ni is 1.0 or less. More preferably, it is 0.75 or less.

  Next, a method for producing stainless steel according to the present invention will be described.

  As a method for producing the stainless steel of the present invention with high efficiency, a steel melted in the above component composition is made into a slab by continuous casting or the like, then made into a hot rolled coil, annealed, and then descaled (shot) Blasting, pickling, etc.) to make stainless steel is recommended. Specifically, this will be described in detail below.

  First, the molten steel adjusted to the component composition of the present invention 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) The steel slab (steel material) is then refined by a known refining method such as the AOD (Argon Oxygen Decarburization) method, and then by a continuous casting method or an ingot-bundling method. The casting method is preferably continuous casting from the viewpoint of productivity and quality. Further, the slab thickness is preferably set to 100 mm or more in order to secure a reduction ratio in hot rough rolling described later. A more preferable range is 200 mm or more.

  Subsequently, after heating a steel slab to the temperature of 1100-1300 degreeC, it hot-rolls to make a hot-rolled steel plate. The slab heating temperature is desirably higher in order to prevent roughing of the hot-rolled steel sheet. However, when the slab heating temperature exceeds 1300 ° C., the slab sag becomes significant, and the crystal grains become coarse and the toughness of the hot-rolled steel sheet decreases. On the other hand, when the slab heating temperature is less than 1100 ° C., the load in hot rolling becomes high, the rough surface in hot rolling becomes remarkable, recrystallization during hot rolling becomes insufficient, and the toughness of the hot-rolled steel sheet is reduced. descend.

  In the hot rough rolling process in the hot rolling, it is preferable to perform at least one pass of rolling with a rolling reduction of 30% or more in a temperature range exceeding 900 ° C. By this strong rolling, the crystal grains of the steel sheet are refined and the toughness is improved. After hot rough rolling, finish rolling is performed according to a conventional method.

  A hot-rolled steel sheet having a thickness of about 2.0 to 8.0 mm manufactured by hot rolling is annealed at a temperature of 700 to 850 ° C. Thereafter, pickling may be performed. When the annealing temperature of the hot-rolled steel sheet is less than 700 ° C., recrystallization becomes insufficient, and reverse transformation from the martensite phase to the austenite phase hardly occurs, and the amount thereof decreases, so that sufficient low temperature toughness is obtained. Absent. On the other hand, if it exceeds 850 ° C., it becomes an austenite single phase at the annealing temperature, and the crystal grains become extremely coarse and the toughness decreases. The annealing of the hot-rolled steel sheet is preferably held for 1 hour or longer by so-called box annealing.

  For the welding of stainless steel according to the present invention, all the usual welding methods such as arc welding including TIG, MIG, seam welding, resistance welding such as spot welding, laser welding, etc. can be applied, and the low temperature toughness is also applicable. Are better.

  Stainless steel having the component composition shown in Table 1 was vacuum-melted in a laboratory. The molten steel ingot was heated to 1200 ° C., and a hot rolled steel sheet having a thickness of 5 mm was obtained by hot rolling in which rolling at a reduction rate of 30% or more was performed at least one pass in a temperature range exceeding 900 ° C. The obtained hot-rolled steel sheet was annealed at 780 ° C. for 10 hours, then shot blasted and pickled to remove the scale. The annealing temperature was selected so that the austenite phase fraction of the example of the present invention was in the range of 5% to 95%.

  An L cross section (vertical cross section parallel to the rolling direction) having a shape of 20 mm × 10 mm was collected from the hot-rolled steel sheet from which the scale had been removed, and the structure was revealed with aqua regia and observed. From the observed structure, the average crystal grain size of each test material was measured by a cutting method. Each average crystal grain size is shown in Table 2.

  Furthermore, the element distribution of Ni and Cr in the L cross section was measured using EPMA. A measurement example is shown in FIG. The portion where Ni was concentrated and Cr decreased was determined to be martensite, and the martensite phase fraction was determined by image processing. The results are shown in Table 1. It was recognized that the larger the formula (2), the larger the martensite phase fraction. As shown in FIG. 1, the average crystal grain size tended to be minimized when the martensite phase fraction was around 60%.

  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 25 J or more was obtained, and it can be seen that the low temperature toughness is good. In contrast, No. of the comparative example. 27 is Mn, No. 27. 28 is Cr, No. 29 is Ni, No. No. 30 had low low temperature toughness because Ti was out of the range of the present invention. Moreover, No. of the comparative example. The component 31 was within the scope of the present invention, but the low temperature toughness was low because the formula (III) deviates from the scope of the present invention. Moreover, No. of the comparative example. 32-No. No. 35 has a low temperature toughness lower than 25J because the formula (I) or the formula (II) is out of the scope of the present invention.

  A test piece of 60 mm × 80 mm was taken from the hot-rolled steel sheet from which the scale had been removed, and the back surface and the end 5 mm were covered with water-resistant tape, and a salt spray test was performed. The salt water concentration was 5% NaCl, the test temperature was 35 ° C., and the test time was 24 h. After conducting the salt spray test, the test surface was photographed, the portion where rust was generated was converted to black, the portion where rust was not generated was converted to white, and the corrosion area ratio was measured by image processing. . Table 2 shows the obtained corrosion area ratio. Those having a corrosion area ratio of 15% or less were evaluated as having good corrosion resistance. No. which is an example of the present invention. 1-No. No. 26 had good corrosion resistance. Among the comparative examples, Mn is no. No. 27, Ni deviates from the scope of the present invention. 29 was poor in corrosion resistance.

  From the hot-rolled steel sheet from which the scale was removed, a tensile test piece of JIS No. 5 was taken in parallel with the rolling direction, a tensile test was performed, and workability was evaluated. The obtained elongation values are shown in Table 2. Those having an elongation of 15% or more were evaluated as having good processability. No. which is an example of the present invention. 1-No. No. 26 had good workability. Among the comparative examples, the formula (I) falls outside the scope of the present invention. 32, No. 32 where formula (II) departs from the scope of the present invention. No. 35 had poor workability.

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

  Steel slabs having a component composition shown in Table 3 and having a thickness of 250 mm were vacuum-melted. The produced steel slab was heated to 1200 ° C., and then a hot-rolled steel sheet having a thickness of 5 mm was obtained by 9-pass hot rolling. Table 4 shows the hot rolling conditions. After annealing the obtained hot-rolled steel sheet under the conditions shown in Table 4, the scale was removed by shot blasting and pickling.

  The element distribution of Ni and Cr in the L section (vertical section parallel to the rolling direction) was measured using EPMA. The portion where Ni was concentrated and Cr decreased was determined to be martensite, and the martensite phase fraction was determined by image processing. The results are shown in Table 4.

  Furthermore, from the hot-rolled steel sheet from which the scale had been removed, an L cross section with a shape of 20 mm × 10 mm was collected, and the structure was revealed with aqua regia and observed. From the observed structure, the average crystal grain size of each test material was measured by a cutting method. Each average grain size is shown in Table 4.

  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 4 shows the obtained absorbed energy. In the examples of the present invention, absorption energy of 25 J or more was obtained, and it can be seen that the low temperature toughness is good. No. which is a comparative example. D, No. In E, since the maximum rolling reduction above 900 ° C. is 30% or less, even if the maximum rolling reduction below 900 ° C. is 30% or more, the average crystal grain size is large and the absorbed energy at −50 ° C. is 25 J or less. It became. No. which is a comparative example. F, No. Since J has a low or high annealing temperature, the martensite phase fraction was less than 5% or more than 95%, and the absorbed energy at −50 ° C. was 25 J or less. No. which is a comparative example. K had an annealing time of less than 1 hour, and transformation and recrystallization due to annealing were insufficient. For this reason, it was impossible to measure the martensite phase fraction and the average crystal grain size. No. The absorbed energy of K at −50 ° C. was 25 J or less.

  A test piece of 60 mm × 80 mm was taken from the hot-rolled steel sheet from which the scale had been removed, and the back surface and the end 5 mm were covered with water-resistant tape, and a salt spray test was performed. The salt water concentration was 5% NaCl, the test temperature was 35 ° C., and the test time was 24 h. After conducting the salt spray test, the test surface was photographed, the portion where rust was generated was converted to black, the portion where rust was not generated was converted to white, and the corrosion area ratio was measured by image processing. . Table 4 shows the obtained corrosion area ratio. Those having a corrosion area ratio of 15% or less were evaluated as having good corrosion resistance. In all the inventive examples, the corrosion resistance was good. Among the comparative examples, No. 1 having a high annealing temperature. J and No. in which annealing was insufficient. The corrosion resistance of K was poor.

  From the hot-rolled steel sheet from which the scale was removed, a tensile test piece of JIS No. 5 was taken in parallel with the rolling direction, a tensile test was performed, and workability was evaluated. The obtained elongation values are shown in Table 4. Those having an elongation of 15% or more were evaluated as having good processability. In all of the inventive examples, the workability was good. Among the comparative examples, No. 1 having a high martensite phase fraction. J and No. in which annealing was insufficient. The processability of K was poor.

  According to the present invention, ferrite-martensite duplex stainless steel excellent in low temperature toughness that can be produced inexpensively and with high efficiency and is suitable as a body use material for a freight car that carries coal, oil, etc. in a cold region and its A manufacturing method is obtained. The obtained stainless steel is also excellent in the low temperature toughness of the weld zone.

Claims (4)

% By mass
C: 0.005-0.030%,
N: 0.005-0.030%,
Si: 0.05-1.00%,
Mn: 0.05% or more and less than 1.00%,
P: 0.04% or less,
S: 0.02% or less,
Al: 0.01 to 0.15%,
Cr: 10.0-13.0%,
Ni: 0.3 to 5.0%,
Ti: 4 × (C content (%) + N content (%))% or more and 0.4% or less V: 0.005 to 0.10%, with the balance being Fe and inevitable impurities ,
Satisfy the following inequalities (I) to (III),
It has a steel structure consisting of two phases, a ferrite phase and a martensite phase,
Ferritic-martensitic duplex stainless steel, characterized in that the martensite phase content is 5% to 95% by volume.
10.5 ≦ Cr + 1.5 × Si ≦ 13.5 (I)
2.0 ≦ 30 × (C + N) + Ni + 0.5 × Mn ≦ 6.0 (II)
Mn / Ni ≦ 1.0 (III)
Here, Cr and Si in the inequality (I), C, N, Ni and Mn in the inequality (II), and Mn and Ni in the inequality (III) are the contents (mass of each element). %).   It is characterized by containing one or more of Cu: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, and Co: 0.5% or less in mass%. The ferrite-martensite duplex stainless steel according to claim 1.   It is characterized by containing one or more of Ca: 0.01% or less, B: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less in mass%. The ferritic-martensitic duplex stainless steel according to claim 1 or 2.   It is a manufacturing method of the ferrite martensite duplex stainless steel in any one of Claims 1-3, Comprising: After heating a steel slab to the temperature of 1100-1300 degreeC, in a temperature range over 900 degreeC, a rolling reduction rate Ferrite-martensite two-phase, characterized by performing hot rolling including hot rough rolling in which rolling at 30% or more is performed for at least one pass and annealing at 700 to 850 ° C. for 1 hour or more Stainless steel manufacturing method.