JP2016065298A - Method for manufacturing high strength austenitic stainless thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an austenitic stainless thick steel plate capable of securing excellent corrosion resistance, high strength or the like in corrosive environment such as submergence environment containing salt content close to coast.SOLUTION: There is provided a method for manufacturing a high strength austenitic stainless thick steel plate having a heating process for heating a steel raw material consisting of a specific composition component to 1100 to 1250°C, a hot rolling process for hot rolling the steel raw material heated in the heating process with a condition of a rolling finish temperature of 800 to 1000°C and cumulative draft of 30% or more, and a cooling process for cooling a hot rolled sheet obtained in the hot rolling process with the condition of a cooling stop temperature of 500°C or less and cooling rate of 3°C/s or more.SELECTED DRAWING: None

Description

  The present invention has a high pitting corrosion resistance that can be applied to steel structure members, particularly members that require high corrosion resistance (pitting corrosion resistance) and high strength and hardness (high strength, etc.) such as rails such as sluices and dams. The present invention relates to a method for producing a strength austenitic stainless steel plate.

  Stainless steels are martensitic stainless steel represented by SUS410 (13Cr), ferritic stainless steel represented by SUS430 (18Cr), and austenitic stainless steel represented by SUS304 (18Cr-8Ni). And SUS630 (17Cr-4Ni-4Cu-Nb), which are roughly classified into precipitation hardening type stainless steels.

  Among these various stainless steels, austenitic stainless steel is excellent in corrosion resistance in a general corrosive environment, particularly pitting corrosion resistance. From the viewpoint of corrosion resistance, particularly pitting corrosion resistance, SUS304 (18Cr-8Ni) and SUS316 (18Cr-10Ni-2Mo) are often used among austenitic stainless steels.

  Since SUS316 has Mo added compared to SUS304, SUS316 has a feature of excellent pitting corrosion resistance. Parts for uses such as sluices assumed in the present invention are required to have excellent pitting corrosion resistance and high strength. At present, a material obtained by subjecting SUS304N2 (18Cr-8Ni-0.20N-0.1Nb) to a solution treatment at a temperature of about 1150 ° C. is often used after performing thick plate rolling. However, since SUS304N2 has a high N content, it has poor hot workability, surface cracks and the like, resulting in poor manufacturability and a very low yield during production. SUS304N2 is a material with excellent pitting corrosion resistance, but if this material is used in an environment with a high salt concentration such as close to the coast, there is a risk of pitting corrosion.

  As described above, austenitic stainless steel such as SUS304N2 is subjected to a solution treatment for the purpose of homogenizing the structure and improving the corrosion resistance by solid solution of carbide. However, there is a problem that a material formed by such a solution treatment tends to have low strength. As a countermeasure against this, a technique that applies thermomechanical processing (TMCP) has been proposed.

  In Patent Document 1, as a main component, by mass%, C: 0.03% or less, Cr: 16-20%, Ni: 7-15%, N: 0.08% or less, Mo: 3.0% Austenitic stainless steel thickness which heat-treats the steel raw material containing the following on the conditions of 1150 degreeC-1300 degreeC, and rolls the heat-treated steel raw material on the conditions whose cumulative reduction in the range of 900-950 degreeC is 20-45%. A method for manufacturing a plate is disclosed.

  In Patent Document 2, as a main component, C: 0.05% or less, N: 0.15 to 0.30%, Ni: 7 to 11%, Cr: 18 to 20%, Nb: 0 as a main component. A manufacturing method of a high-strength austenitic stainless steel rolled material in which a steel material containing 0.01 to 0.25% and Mo: 1.0% or less is heat-treated is disclosed.

  In Patent Document 3, as a main component, C: 0.040% or less, Cr: 16-20%, Ni: 7-15%, N: 0.25% or less, Ti: 0.003% as main components. A steel material containing 0.020% is heated to 1150 to 1300 ° C., and the heated steel material is rolled under a condition that the cumulative reduction ratio in the temperature range of 900 to 700 ° C. is 5 to 30%. A method for producing a high-strength austenitic stainless steel sheet that cools a hot-rolled sheet is disclosed.

  In Patent Document 4, the formula F1 = S + ((P + Sn) / 2) + ((As + Zn + Pb + Sb) / 5), the formula F2 = Nb + Ta + Zr + Hf + 2Ti + (V / 10) is 0.05 ≦ F2 ≦ 1.7−9 × F1. Satisfied, in Patent Document 5, the formula P1 = S + ((P + Sn) / 2) + ((As + Zn + Pb + Sb) / 5), the formula P2 = Nb + 2 (V + Ti) is 0.2 ≦ P2 ≦ 1.7−10 × P1 An austenitic stainless steel characterized by satisfaction is disclosed. The element symbols in the formulas described in Patent Documents 4 and 5 mean the content of each element.

  Patent Document 6 discloses a high-strength metastable austenitic stainless steel containing, by mass%, Si: 0.03-3.0%, Al: 0.01% or less, and O: 0.001-0.005%. Is disclosed.

  Patent Documents 7 to 9 disclose manufacturing methods for increasing the strength of austenitic stainless steel by hot rolling.

JP-A-8-73936 JP-A-9-49015 JP-A-8-104920 Special table 2009-44802 gazette Special table 2009-44796 JP 2001-262281 A JP-A-60-197817 JP 60-26619 A JP-A-61-272317

  In patent document 1, in order to obtain high strength, the thermomechanical processing is applied to the austenitic stainless steel thick plate. In Patent Document 1, the C content is as low as 0.03% or less and the N content is as low as 0.08% or less, and it cannot be said that the increase in strength due to solute C or solute N is sufficient. Moreover, in patent document 1, since rolling completion temperature is 900 degreeC or more, although a certain amount of dislocation reinforcement effect is acquired, the effect is still inadequate. Moreover, since the N content is low, the invention described in Patent Document 1 has insufficient pitting corrosion resistance.

  In Patent Document 2, an austenitic stainless steel having an N content of 0.15 to 0.30% is further heat-treated. For this reason, in Patent Document 2, an increase in strength can be expected due to solid solution strengthening by solid solution N and dislocation strength by thermomechanical treatment. However, since the Nb content is 0.01 to 0.25%, there is a problem that the pitting corrosion resistance is inferior as described later. Moreover, in invention of patent document 2, since there is little Mo content which is an element which improves pitting corrosion resistance, pitting corrosion resistance is inadequate.

  In Patent Document 3, in order to increase the strength, an austenitic stainless steel sheet having an N content of 0.25% or less is subjected to a thermomechanical treatment for terminating rolling in a temperature range of 900 to 700 ° C. However, since the cumulative rolling reduction in that temperature range is as low as 5 to 30%, there is a problem that a sufficient dislocation strengthening effect cannot be obtained. Moreover, in patent document 3, since it cools after rolling, precipitates, such as Cr carbide | carbonized_material, produce | generate and lead to deterioration of pitting corrosion resistance. Moreover, since the invention of patent document 3 does not contain Mo, pitting corrosion resistance is inadequate.

  Patent Documents 4 and 5 require a certain amount or more of Nb, V, Ti, etc. that can fix C in order to improve corrosion resistance. However, as will be described later, there is a problem that NbC becomes a starting point of pitting corrosion and deteriorates pitting corrosion resistance.

  The technique described in Patent Document 6 relates to a wire used for a spring or the like. In patent document 6, in order to raise the intensity | strength after cold rolling, it is set as the metastable austenitic stainless steel. In Patent Document 6, the strength after cold rolling is as high as 1800 MPa in TS (tensile strength). Considering that many properties, including tensile strength, affect other properties, and the overall properties are determined by harmony of these properties, TS with a TS of 1800 MPa and the TS assumed in the present invention are 700 to 700. It can be said that the kind of steel is different from steel of about 900 MPa.

  Patent Documents 7 and 8 can achieve high strength by thermomechanical processing. However, the inventions described in Patent Document 7 and Patent Document 8 do not consider pitting corrosion resistance.

  Even in Patent Document 9, as in Patent Documents 7 and 8, pitting corrosion resistance is not considered. Further, in Patent Document 9, since it is studied to improve the strength by rolling in the recrystallized region, the technique described in Patent Document 9 aims at the dislocation strengthening effect by rolling in the non-recrystallized region. is not.

  An object of this invention is to provide the manufacturing method of the austenitic stainless steel plate which can ensure the outstanding pitting corrosion resistance, high intensity | strength, etc. in corrosive environments, such as a submerged environment containing the salt near the coast.

    The inventors of the present invention have made extensive studies to solve the above problems and have obtained the following knowledge.

  1. The N content for obtaining the solid solution strengthening effect by the solid solution N was determined to such an extent that the manufacturability and the yield were not adversely affected, and an appropriate range of the components capable of obtaining the target high strength was found.

  2. Conventionally, in order to suppress sensitization caused by the formation of Cr carbide, it is often performed to add Nb to fix C as Nb carbide to improve corrosion resistance. However, it has been found that Nb carbide deteriorates the corrosion resistance due to the starting point of pitting corrosion. Therefore, in order to reduce the Nb content as much as possible and further improve the pitting corrosion resistance, the inventors have found an appropriate range of the N content considering the balance with manufacturability.

  3. In order to achieve high strength, it is desirable to lower the rolling end temperature, but it has been found that pitting corrosion resistance deteriorates when the temperature is lowered to less than 800 ° C. The pitting corrosion resistance degradation mechanism is unknown in detail, but it is thought that the starting point of pitting corrosion increases probably by accumulating more than a certain amount of strain.

  4). As described above, since there is a limit to lowering the rolling end temperature, in order to achieve further higher strength, the target strength is achieved by increasing the cumulative rolling reduction to 30% or more and utilizing the dislocation strengthening effect. It was found that can be obtained.

  As a result of further studies based on the above knowledge, the present inventors have found that the above problems can be solved by combining an appropriate alloy component and rolling under an appropriate thermomechanical condition, thereby completing the present invention. More specifically, the present invention provides the following.

  [1] By mass%, C: 0.010 to 0.030%, Si: 0.20 to 1.00%, Mn: 0.20 to 1.30%, P: 0.045% or less, S: 0.020% or less, Ni: 10.50 to 14.50%, Cr: 16.50 to 18.50%, Mo: 2.00 to 3.00%, N: 0.12 to 0.22%, A steel material containing Al: 0.01 to 0.05%, O: 0.010% or less and Nb: 0.005% or less, with the balance being Fe and unavoidable impurities, 1100 to 1250 ° C. A hot rolling process in which the steel material heated in the heating process is hot rolled at a rolling end temperature of 800 to 1000 ° C. and a cumulative rolling reduction of 30% or more, and the hot The hot-rolled sheet obtained in the rolling process is subjected to a cooling stop temperature of 500 ° C. or lower and a cooling rate of 3 ° C./s or higher. High strength austenitic method of manufacturing a stainless steel plate and having a cooling step for retirement, the.

  According to the present invention, it is possible to obtain an austenitic stainless steel plate having excellent pitting corrosion resistance, high strength, and the like in a corrosive environment such as a normal atmospheric environment or a submerged environment containing some salt, such as near the coast. It is very useful in industry.

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

Component composition of steel material The steel material used as a raw material in the production method of the present invention is mass%, C: 0.010 to 0.030%, Si: 0.20 to 1.00%, Mn: 0.20. 1.30%, P: 0.045% or less, S: 0.020% or less, Ni: 10.50 to 14.50%, Cr: 16.50 to 18.50%, Mo: 2.00 to 3 0.000%, N: 0.12 to 0.22%, Al: 0.01 to 0.05%, O: 0.010% or less, and Nb: 0.005% or less, with the balance being It consists of Fe and inevitable impurities. Hereinafter, the component composition of the steel material will be described. In the following description, “%” representing the content means “% by mass”. In addition, the component composition of a steel raw material turns into a component composition of the thick steel plate which is a product.

C: 0.010 to 0.030%
C is a component that improves the strength by the solid solution strengthening effect. If the C content is less than 0.010%, sufficient strength as general structural steel cannot be obtained. For this reason, C content shall be 0.010% or more. On the other hand, C precipitates as Cr carbide in the heat history during rolling and inhibits corrosion resistance. For this reason, C content shall be 0.030% or less.

Si: 0.20 to 1.00%
Si is an effective component for securing strength and deoxidation. In order to obtain this effect, the Si content is set to 0.20% or more. However, if the Si content exceeds 1.00%, Si remains as non-metallic inclusions, resulting in deterioration of pitting corrosion resistance and hot workability. Therefore, the Si content is in the range of 0.20 to 1.00%. The range is preferably 0.30 to 0.70%, and more preferably 0.40 to 0.60%.

Mn: 0.20 to 1.30%
Mn is also an effective component for securing strength and deoxidation. In order to obtain this effect, the Mn content is set to 0.20% or more. However, if the Mn content exceeds 1.30%, non-metallic inclusions such as MnS remain, the pitting corrosion resistance is remarkably deteriorated, and the hot workability is also deteriorated. Therefore, the Mn content is in the range of 0.20 to 1.30%. Preferably, it is 0.50 to 1.20% of range. More preferably, it is set as 0.70 to 1.10% of range.

P: 0.045% or less P is an impurity element that segregates at the grain boundary and degrades the corrosion resistance. Therefore, the P content is 0.045% or less. Preferably, it is 0.030% or less. However, extremely reducing the P content leads to a significant increase in refining costs. Therefore, the P content is preferably 0.010% or more. Therefore, a preferable P content is in the range of 0.010 to 0.045%.

S: 0.020% or less S is an impurity element similar to P. The inclusion of S causes non-metallic inclusions such as MnS to precipitate and deteriorates the pitting resistance. Therefore, the S content is 0.020% or less. Preferably, it is 0.015% or less. However, extremely reducing the S content leads to a significant increase in refining costs. Therefore, the S content is preferably 0.001% or more. Therefore, preferable S content is 0.001 to 0.015% of range.

Ni: 10.50-14.50%
Ni is a main element of austenitic stainless steel and is an element that improves pitting corrosion resistance. The inclusion of Ni increases the adhesion of an oxide film (passive film) formed by Cr, which will be described later, and the pitting corrosion resistance is improved by the combined addition of Cr and Ni. However, since Ni is an expensive element, the Ni content is in the range of 10.50 to 14.50% in consideration of the balance between pitting corrosion resistance and price. Preferably, it is 10.50 to 12.50% of range.

Cr: 16.50-18.50%
Cr is an element that forms a highly protective oxide film (passive film) on the surface of the metal and improves pitting corrosion resistance. However, since Cr is an expensive element, it is necessary to consider the balance between pitting corrosion resistance and price. Therefore, the Cr content is in the range of 16.50 to 18.50%. Preferably, it is in the range of 17.00 to 18.50%.

Mo: 2.00 to 3.00%
Mo is an element that improves pitting corrosion resistance and crevice corrosion resistance. In order to obtain this effect, the Mo content is set to 2.00% or more. Although the corrosion resistance is improved by the combined effect of Cr and Ni, since Mo is an expensive element, the Mo content is 3.00% or less. In consideration of the balance between the addition amount of Cr and Ni and the price, the Mo amount is set to a range of 2.00 to 3.00%.

N: 0.12-0.22%
N is an effective component which improves the strength by solid solution strengthening as in C. N is an effective component for improving pitting corrosion resistance. In order to obtain these effects, the N content is set to 0.12% or more. On the other hand, if the N content exceeds 0.22%, the hot workability is deteriorated and the productivity is lowered. For this reason, N content was limited to 0.12-0.22%. Preferably, it is 0.14% to 0.18% of range.

Al: 0.01 to 0.05%
Al is a component effective for deoxidation. In order to obtain this effect, the Al content is set to 0.01% or more. However, when the Al content exceeds 0.05%, Al-based inclusions remain, and the pitting corrosion resistance is remarkably deteriorated. Therefore, the Al content is in the range of 0.01 to 0.05%.

O: 0.010% or less O generates an oxide that is representative of non-metallic inclusions and deteriorates pitting corrosion resistance. For this reason, it is necessary to make O content into 0.010% or less. However, extremely reducing the O content leads to a significant increase in refining costs. Therefore, the O content is preferably 0.001% or more. Moreover, preferable O content is 0.001 to 0.008% of range.

Nb: 0.005% or less Nb may be added for the purpose of improving corrosion resistance. However, when Nb precipitates as Nb carbide, pitting corrosion resistance deteriorates. For this reason, it is necessary to reduce Nb content as much as possible. However, stainless steel is often made from scrap and inevitably contains Nb. Therefore, the Nb content is limited to 0.005% or less that can ensure sufficient pitting corrosion resistance.

  The above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities.

  Then, the manufacturing method of this invention is demonstrated. In the following description, the temperatures such as the heating temperature, the rolling end temperature, and the cooling stop temperature are the average temperatures of the thick steel plates. The average temperature is calculated from the surface temperature of the slab or thick steel plate in consideration of the parameters of the plate thickness and thermal conductivity.

  The manufacturing method of the present invention includes a heating step, a hot rolling step, and a cooling step. Hereinafter, each step will be described.

Heating process A heating process is a process of heating the steel raw material which has the said component composition to 1100-1250 degreeC.

  When the heating temperature is less than 1100 ° C., carbides and intermetallic compounds generated and precipitated in the melting process remain undissolved, and pitting corrosion resistance deteriorates. For this reason, heating temperature was 1100 degreeC or more. Moreover, when heating is performed at a heating temperature exceeding 1250 ° C., the operation load is large, and the steel plate surface flaws and patterns are generated due to the abnormally generated oxide scale. For this reason, heating temperature was made into the range of 1100-1250 degreeC. Preferably, it is the range of 1150-1200 degreeC.

  In addition, the manufacturing method of the said steel raw material is not specifically limited. For example, steel having the above-described component composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and a steel material such as a slab is formed by a conventional method using a continuous casting method or an ingot-bundling method. A method is mentioned. The melting method and casting method are not limited to the methods described above.

Hot rolling step The hot rolling step is a step of hot rolling the steel material heated in the above heating step under the conditions that the rolling end temperature is 800 to 1000 ° C and the cumulative rolling reduction is 30% or more.

  The strength improves as the rolling end temperature becomes lower. On the other hand, when the rolling end temperature is too low, pitting corrosion resistance is deteriorated. Therefore, the rolling end temperature is in the range of 800 to 1000 ° C.

  In the present invention, the dislocation strengthening effect by rolling in an unrecrystallized region of about 1000 ° C. or less is important for improving the strength. When the cumulative rolling reduction is less than 30%, a predetermined strength cannot be obtained, so in the present invention, it is set to 30% or more. However, since the hot deformation resistance is high in the low temperature region of the non-recrystallized region, increasing the cumulative rolling reduction leads to an operation load. For this reason, a preferable cumulative rolling reduction is in the range of 30 to 70%.

Cooling step The cooling step is a step of cooling the hot-rolled sheet obtained in the hot rolling step under conditions where the cooling stop temperature is 500 ° C. or lower and the cooling rate is 3 ° C./s or higher.

  In the said component composition, when a hot-rolled sheet is kept in the range (precipitation temperature range) exceeding 500 degreeC and 900 degreeC for a long time, Cr carbide will precipitate on a hot-rolled sheet and the pitting corrosion resistance of a hot-rolled sheet will deteriorate. . Therefore, it is necessary to set the cooling stop temperature to 500 ° C. or lower. The cooling method is not particularly limited, but it is desirable to perform accelerated cooling by a method such as water cooling in the above precipitation temperature range. Specifically, the precipitation can be reduced by setting the cooling rate to 3 ° C./s or more.

Thick steel plates produced by the above method are excellent in any of strength, hardness and pitting corrosion resistance, and can be preferably used for applications such as rails such as sluices and dams. In the present invention, the thickness of the thick steel plate is 10 to 60 mm.

  Note that the hardness can be increased by increasing the strength.

  Steel having the component composition shown in Table 1 was made into a slab having a thickness of 180 mm by a continuous casting method, and a steel plate having a thickness of 30 mm was produced. Table 2 shows the manufacturing conditions.

  The mechanical properties of the manufactured thick steel plate were measured. Tensile strength and proof stress were evaluated by averaging two No. 4 test pieces defined in JIS Z2241 at a position of 1 / 2t in the direction perpendicular to the rolling direction. In addition, t means the thickness of a thick steel plate.

  Tensile strength: 690 MPa or more and 850 MPa or less and proof stress: 500 MPa or more were considered good.

  The hardness of the manufactured thick steel plate was measured by a Brinell hardness test defined in JIS Z2243. The measurement conditions for hardness were indenter diameter: 10 mm, test force: 3 ton, test force holding time: 15 seconds. Moreover, the Brinell hardness test was done in the surface which ground from the thick steel plate surface to plate | board thickness depth 2mm. The hardness was good at 220 points or more in HBW.

The pitting corrosion resistance was evaluated by the ferric chloride corrosion test method-test method (A) defined in JIS G0578. The test temperature is set to 35 ° C., and after continuous immersion in a 6% ferric chloride solution for 24 hours, the adhering corrosion products are removed, and the mass is measured after washing and drying. -Evaluation was performed by calculating the corrosion rate from the weight loss obtained from the mass before drying. If the corrosion rate was 2.0 g / m ² · h or less, it was judged that the pitting corrosion resistance was excellent.

  If the tensile strength, proof stress, Brinell hardness (hardness), and pitting corrosion resistance by ferric chloride corrosion test are achieved, it has high strength, high proof stress, high hardness, and excellent pitting corrosion resistance. It can be said that the performance greatly exceeds SUS304N2.

  Table 3 shows the mechanical properties and corrosion test results of the thick steel sheets produced by manufacturing the steel of the A1 steel type of the invention example of Table 1 under the various production conditions shown in Table 2. Table 4 shows the mechanical properties and corrosion test results of the thick steel plates produced from each steel type in Table 1 under the a1 condition of the invention examples in Table 2. Example No. 1-7 (Table 3) and Example No. Nos. 13 to 15 (Table 4) had mechanical properties and pitting corrosion resistance satisfying the goals of the present invention.

Claims (1)

In mass%, C: 0.010 to 0.030%, Si: 0.20 to 1.00%, Mn: 0.20 to 1.30%, P: 0.045% or less, S: 0.020 %: Ni: 10.50 to 14.50%, Cr: 16.50 to 18.50%, Mo: 2.00 to 3.00%, N: 0.12 to 0.22%, Al: 0 A steel material containing 0.01 to 0.05%, O: 0.010% or less, and Nb: 0.005% or less, with the balance being Fe and inevitable impurities, is heated to 1100 to 1250 ° C. Heating process;
A hot rolling process in which the steel material heated in the heating process is hot-rolled under conditions where the rolling end temperature is 800 to 1000 ° C. and the cumulative reduction rate is 30% or more;
A high-strength austenite system comprising a cooling step of cooling the hot-rolled sheet obtained in the hot rolling step at a cooling stop temperature of 500 ° C. or lower and a cooling rate of 3 ° C./s or higher. Manufacturing method of stainless steel plate.