JP2006241590A - Austenitic stainless steel hot rolled steel having satisfactory corrosion resistance, proof stress and low temperature toughness and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an austenitic stainless steel hot rolled steel having more satisfactory seawater resistance and higher strength than those of the conventional steel while securing low temperature toughness required for a structural member, i.e., whose three characteristics of corrosion resistance, proof stress and low temperature toughness are all satisfactory. <P>SOLUTION: The austenitic stainless steel hot rolled steel satisfies the relations of PI=Cr+3.3(Mo+0.5W)+16N is 35 to 40, and δcal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+32C+20N)-18 is -6 to +2, and has the characteristics that 0.2% proof stress at room temperature is ≥550MPa, Charpy impact value by a V notch test piece at -40°C is ≥100 J/cm<SP>2</SP>, and a pitting potential (Vc'100) measured in a deaerated 10% NaCl aqueous solution heated at 50°C is ≥500 mV (Vs saturated Ag/AgCl). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a structural stainless steel product with excellent corrosion resistance that is used in marine / gulf environments and chloride environments. For example, it is used as a material for outer shells, bulkheads, aggregates, hydrofoils, etc. for hull structures. In doing so, the present invention relates to a hot-rolled steel material of high-strength austenitic stainless steel having seawater resistance and low-temperature toughness, and a method for producing the same.

  Conventionally, coated steel plates with heavy anticorrosion have been used for hull structures. In the past, demand for high-speed ships equipped with hydrofoil has increased. In this application, high-speed seawater flow is in contact, and therefore, materials with excellent seawater resistance that do not require painting are required. Further, a high-strength material is desired in order to reduce the hull weight.

  Austenitic stainless steel is promising as a material with excellent seawater resistance, but in a normal production method, it is softened because it is subjected to solution annealing after hot rolling, and the yield strength is at most 400 MPa.

  In order to increase the strength, it can be realized by omitting solution annealing and hot working under a specific temperature condition, and there are many past findings (Patent Documents 1 to 3). Among them, Patent Document 2 discloses a method for producing a high-strength austenitic stainless steel while maintaining low-temperature toughness, but the steel does not consider seawater resistance. In Patent Literature 3, 0.3% or more of N-added steel and 0.5 to 3.0% Mo-added steel are heat-treated under specific conditions, so that the yield strength is 500 MPa or more and excellent in seawater resistance. Although a manufacturing technique for austenitic stainless steel is disclosed, there is no provision for toughness.

  Elements that enhance seawater resistance include Cr, Mo, and N, and the corrosion resistance order of the steel types is arranged by a mathematical formula such as PI = Cr + 3.3 (Mo + 0.5W) + 16N as a pitting corrosion index. When the PI value of the component shown in the example of Patent Document 3 is calculated, the minimum value is about 32. However, as a stainless steel satisfying a higher PI value (35 or more), the austenite series contains 23% or more of Ni. SUS836L, 890L, and SUS329J4L containing 5.5 to 7.5% Ni in a two-phase system.

  Two-phase SUS329J4L has a high yield strength because it contains a ferrite phase. In recent years, a duplex stainless steel called super duplex with increased amounts of Mo and W has been developed, and its application has started as a high strength and high corrosion resistance material. On the other hand, as a high-strength steel material of austenitic high corrosion-resistant stainless steel, those having a PI value exceeding 35 have not been put into practical use yet.

JP 60-208459 A Japanese Patent Laid-Open No. 2-97649 Japanese Patent Laid-Open No. 4-6214

  Stainless steel causes crevice corrosion when it has a crevice shape, and more severe corrosion than a flat plate portion. Therefore, in order to use it for general purpose and maintenance-free as a hull structure, development of a high corrosion resistance steel material higher than the steel material shown in Patent Document 3 has been required.

  On the other hand, there is a growing demand for reliable stainless steel materials for reef groundings and collisions between ships. The reliability of the steel material is both the characteristics of the base material and the weldability. As the reliability of the base material, high toughness is required in preparation for a collision accident. Of Cr, Mo, and N that improve corrosion resistance, the addition of Mo and Cr will greatly reduce the hot workability due to the effect of delta ferrite in the slab steel slab, as well as high Cr and Mo steel. In some cases, the toughness is greatly deteriorated by the influence of an intermetallic compound generally called σ phase, and it is necessary to contain a large amount of Ni in order to eliminate the influence of both. However, considering the recent rise in Ni and Mo raw material prices, development of resource-saving, low-cost, high-corrosion resistant stainless steel is desired. In addition, it cannot be adopted from the point of low temperature toughness about the duplex stainless steel.

  On the other hand, the addition of N as disclosed in Patent Document 3 is effective in terms of ensuring the strength, but excessive addition of N tends to cause bubbles in the weld, and conversely the joint strength of the weld. And may reduce reliability.

  The object of the present invention is to have better seawater resistance and higher strength than conventional steel while ensuring low temperature toughness required for structural members of high-speed ships. That is, it is to realize an austenitic stainless steel hot-rolled steel material having excellent corrosion resistance, proof stress, and low temperature toughness.

  From the viewpoint of weldability, the inventors of the present invention have the strength and toughness of a thick steel plate obtained by casting, hot working, and heat treatment for an austenitic component system having an N amount of 0.35% or less and a PI value of 35 or more. The corrosion resistance was investigated. In particular, the toughness is not limited only by the Ni content, but it has been found that the content of the intermetallic compounds having a high Cr and Mo content in the steel material controls the toughness. The formation of such a metal structure starts from solidification of steel and can occur in any process in hot rolling. Therefore, investigation was started from the influence of chemical composition on the solidification structure, and the influence on rough rolling, homogenization heat treatment, hot working, and heat treatment conditions of cast steel was also investigated. As a result, overcoming the problems of the prior art, limiting the content of constituent elements and solidification structure to obtain austenitic stainless steel with excellent corrosion resistance, toughness, high strength and hot workability, limiting the metal structure of the steel And found an effective manufacturing method for manufacturing the steel material.

The gist of the present invention is as follows.
That is, in mass% C: 0.001 to 0.03%
Si: 0.1 to 1.5%
Mn: 0.1 to 3.0%
P: 0.005 to 0.05%
S: 0.0001 to 0.003%
Ni: 15.0-21.0%
Cr: 22.0-28.0%
Mo: 1.5-3.5%
N: 0.15-0.35%
O: 0.0005 to 0.007% is contained,
The balance consists of Fe and inevitable impurities,
0.2% proof stress at room temperature is 550 MPa or more,
Charpy impact value by V notch test piece at −40 ° C. is 100 J / cm ² or more,
This is an austenitic stainless hot-rolled steel material having a pitting corrosion potential (Vc′100) measured in a degassed 10% NaCl aqueous solution at 50 ° C. of 500 mV (vs saturated Ag / AgCl) or more.

The steel material which has the said characteristic can be obtained by prescription | regulation of the following component or metal structure.
The PI value represented by the formula (1) satisfies the relationship of 35 to 40 and the δcal value represented by the formula (2) satisfies -6 to +2, and the content of the intermetallic compound contained in the steel material is 0.5. % Or less PI = Cr + 3.3 (Mo + 0.5W) + 16N (1)
δcal = 2.9 (Cr + 0.3Si + Mo + 0.5W)
-2.6 (Ni + 0.3Mn + 0.25Cu + 32C + 20N) -18
.... (2)

In addition to this, the following alloy elements can be contained.
1) 1% or 2 types of W: 0.3 to 3.0%, Al: 0.005 to 0.1% in mass% 2) Cu: 0.3 to 2.0% in mass%, Sn: 0.1% or less of 1 type or 2 types or more 3) By mass%, Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, REM: 0.005-0.10% One or more,
(3) PV value represented by the formula is 0 or less PV = S + O-0.8Ca-0.3Mg-0.3REM-30 (3)
4) B in mass%: 0.0003 to 0.0060%
5) By mass: Ti: 0.003-0.03%, Nb: 0.02-0.20%, Zr: 0.003-0.03%, V: 0.05-0.5%, Ta : 0.01-0.1% of 1 type or 2 types or more

  As a production method, a homogenized heat treatment at 1200 to 1300 ° C. for 1 hour or more is added to a slab or steel slab, and reheated at 1100 ° C. to 1300 ° C. In the rolling process, 850 ° C. or more is maintained, and It is effective that rolling is performed at a reduction rate of 10% or more at 50% or more, 1050 to 850 ° C., and the average cooling rate of 800 ° C. to 500 ° C. is 150 ° C./min or more after rolling, and no solution heat treatment is performed .

  In the present invention, austenitic stainless steel having excellent seawater resistance, proof stress and low temperature toughness was obtained by limiting the components and performing a specific heat treatment.

  The present invention realizes austenitic stainless steel suitable for a hull structure that satisfies all the seawater resistance, proof stress and low-temperature toughness required for structural members of high-speed ships, and has a significant industrial contribution. .

  Below, the reason for limitation of Claim 1 of this invention is demonstrated first.

  First, characteristics useful as ship structural materials are defined.

  As for corrosion resistance, it is necessary to endure in seawater without heavy anti-corrosion coating.

  That is, the normal pitting potential is measured in 30 ° C-3.5% NaCl, but considering the seawater resistance in the tropics, the water temperature is often 50 ° C. In many cases, the salinity is concentrated from 3.5% NaCl. Eventually, the pitting corrosion potential (Vc′100) is measured in a degassed 50 ° C.-10% NaCl aqueous solution, and if this potential is 500 mV or more, it is practical. It turns out that there is no problem. In addition, saturated Ag / AgCl was used for the reference electrode.

Concerning impact resistance, conversely, it becomes a problem in cold regions, and at -40 ° C., Charpy impact value of 100 J / cm ² or more, which is generally considered not to cause problems in ships, is satisfied.

  As for strength, the higher the yield strength, the better for weight reduction. However, in the present invention, the steel material that has a 0.2% yield strength at room temperature of 550 MPa or more after satisfying the above corrosion resistance and impact resistance. Can be provided.

  Next, the reason why the components are limited in the present invention will be described.

  C limits the content to 0.03% or less in order to ensure the corrosion resistance of the stainless steel. If the content exceeds 0.03%, Cr carbide is produced and the corrosion resistance and toughness deteriorate. However, since extremely reducing increases the cost of scouring, the lower limit was made 0.001%. Preferably, it is 0.01 to 0.03%.

  Si is added in an amount of 0.1% or more for deoxidation. However, if added over 1.5%, the toughness deteriorates. Therefore, the upper limit is limited to 1.5%. A preferable range is 0.2 to 1.0%.

  Mn is added in an amount of 0.1% or more for deoxidation. However, if it exceeds 3.0%, corrosion resistance and toughness deteriorate. Therefore, the upper limit is limited to 3.0%. A preferable range is 0.2 to 1.5%.

P is limited to 0.05% or less in order to deteriorate hot workability and toughness. However, since the excessive reduction leads to high scouring costs, the lower limit was made 0.005%. Preferably it is 0.01 to 0.03%.
S degrades hot workability, toughness, and corrosion resistance, so is limited to 0.003% or less. However, since extremely reducing increases the cost of scouring, the lower limit was made 0.0001%. Preferably, it is 0.0005 to 0.001%.

  Ni is contained in an amount of 15.0% or more in order to stabilize the austenite structure and improve corrosion resistance to various acids and further toughness. On the other hand, it is an expensive alloy and is limited to a content of 21.0% or less from the viewpoint of cost.

  Cr is contained at 22.0% or more in order to ensure basic corrosion resistance. On the other hand, if the content exceeds 28.0%, intermetallic compounds are liable to precipitate and inhibit toughness. Therefore, the Cr content is set to 22.0% or more and 28.0% or less.

  Mo is a very effective element that additionally increases the corrosion resistance of stainless steel, and the steel of the present invention contains 1.5% or more. On the other hand, since it is an extremely expensive element and promotes the precipitation of intermetallic compounds together with Cr, the upper limit is defined as 3.5% or less. A desirable content is 2.0 to 3.0%.

  N is an effective element that improves the strength and corrosion resistance by dissolving in the austenite phase. For this reason, it is made to contain 0.15% or more. For the base metal, it is possible to make a solid solution up to 0.4% with the steel of the present invention, but in order to increase the sensitivity of bubble generation when welding is performed, the upper limit content was set to 0.35%. . Desirably, it is 0.30% or less.

  O is an important element that constitutes an oxide that is representative of non-metallic inclusions. Excessive inclusion inhibits toughness, and on the other hand, a coarse clustered oxide causes surface defects. For this reason, the upper limit of the content was set to 0.007%. In addition, since the excessive reduction increases the cost of scouring, the lower limit was made 0.0005%. Preferably it is 0.001 to 0.004%.

  The reason for limitation according to claim 2 of the present invention will be described.

  The PI value: pitting index represented by the above formula (1) is an index of corrosion resistance of stainless steel to the chloride environment, and by setting it to 35 or more, necessary characteristics could be obtained. SUS836L and the like exist in stainless steel exceeding 40, but the Ni content is 24% or more, and becomes very expensive. In the present invention, since the austenitic stainless steel having corrosion resistance commensurate with the cost is targeted, the upper limit of the PI value is set to 40. In the present invention not containing W, W in formula (1) is set to zero.

  Δcal represented by the above formula (2) is an index representing the amount of delta ferrite appearing in the solidified structure of austenitic stainless steel, and is generally 0 to 7% in order to reduce the susceptibility to solidification cracking or make the structure finer. It is controlled to the extent. However, in a steel having a high Cr content such as the steel of the present invention, the delta ferrite in the solidified structure is changed to an intermetallic compound during the hot manufacturing process, and remains in the steel material as a product to inhibit toughness. Therefore, the upper limit of δcal is limited to +2 so that delta ferrite is reduced. If this value is exceeded, it will be difficult to obtain high toughness even if the device is devised in the hot manufacturing process. On the other hand, a small (minus) side of δcal means that the amount of delta ferrite is substantially 0%, and not only the above effect is saturated, but also an excessively high amount of Ni is contained. 6 was the lower limit. A preferred range is -3 to +1. In the present invention that does not contain W or Cu, W or Cu in formula (2) is set to zero.

The content ratio of the intermetallic compound contained in the steel material is an important factor governing the toughness of the austenitic stainless steel material in the present invention. An intermetallic compound is a compound having Cr, Mo, or W as a main component, called σ phase or χ phase. The content of this compound can be measured by observing an optical microscope of about 400 times with alkaline electrolytic corrosion of the microstructure. The present inventors have found that the Charpy absorbed energy of the steel material is less than 100 J / cm ² when the content ratio as an average value of the cross section observation of the steel material exceeds 0.5%, and the upper limit is set to 0.5%. It was.

  The reason for limitation according to claim 3 of the present invention will be described.

  W, like Mo, is an element that additionally improves the corrosion resistance of stainless steel, and 0.3 to 3.0% can be contained in the steel of the present invention for this purpose.

  Al is an important element for deoxidation of steel, and is contained in an amount of 0.005% or more in order to reduce oxygen in the steel. On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and inhibits the toughness of stainless steel. The degree depends on the N content, but when Al exceeds 0.1%, the toughness deteriorates remarkably, so the upper limit of the content is set to 0.1%.

  The reason for limitation according to claim 4 of the present invention will be described.

  Cu is an element that additionally increases the corrosion resistance of stainless steel to acids, and can be contained for this purpose. The effect is preferably 0.3% or more, but even if the content exceeds 2.0%, the effect corresponding to the cost is saturated, so the upper limit was made 2.0%.

  Sn also improves the corrosion resistance of the steel, but if added excessively, hot working cracks occur, so the upper limit was made 0.1%. The lower limit of Sn is preferably 0.005%.

  The reason for limitation according to claim 5 of the present invention will be described.

  Ca, Mg, and REM are all elements that improve the hot workability of steel, and one or more of them are added for that purpose. In any case, excessive addition conversely decreases hot workability, so the upper and lower limits of the content were determined as follows. It is 0.0005 to 0.0050% for Ca and Mg, and 0.005 to 0.10% for REM. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

Furthermore, the PV value defined by the following formula (3) is 0 or less. This formula clarifies the necessary amount for adding Ca, Mg, and REM by the amount of S, and by making it 0 or less, hot workability can be further improved by appropriate addition. .
PV = S + O-0.8Ca-0.3Mg-0.3REM-30 (3)

  The reason for limitation according to claim 6 of the present invention will be described.

  B can increase the grain boundary strength and improve hot workability by adding 0.0003% or more. However, excessive addition causes the hot workability to be impaired by excessive precipitation boride, so the upper limit is made 0.0060%.

  The reason for limitation according to claim 7 of the present invention will be described.

  Ti is an element that forms oxides, nitrides, and sulfides in a very small amount to refine the crystal grains of the steel, and is an element that can be actively used in the steel of the present invention. In order to reduce the intermetallic compound content in steel, it is effective to limit the upper limit of δcal and perform homogenization heat treatment on the steel slab. Among these, in the latter method, heat treatment is performed at a high temperature of about 1250 ° C. for several hours, but the inclusion of an appropriate amount of Ti effectively suppresses the growth of crystal grains during the heat treatment at such a high temperature. . For this purpose, a content of 0.003% or more is necessary. On the other hand, Ti is an element having a very high ability to form nitrides. In the steel of the present invention containing N, if Ti is contained in an amount exceeding 0.03%, coarse TiN will inhibit the toughness of the steel. For this reason, the content was determined to be 0.003 to 0.03%. The preferable content rate when it is contained is 0.005 to 0.02%.

  Nb forms carbide and fixes C, thereby suppressing the formation of Cr carbide and improving corrosion resistance and toughness. Also, nitrides are formed to suppress crystal grain growth, and the steel material is refined to increase the strength. For the purpose of improving the corrosion resistance and increasing the strength, 0.02% or more can be added. However, if added over 0.2%, a large amount of Nb carbonitride precipitates during hot working and hinders hot recrystallization, leaving a coarse structure in the steel product. The upper limit was set to 0.2%. A preferable content range in the case of adding is 0.05% to 0.15%.

  V is an element that forms carbonitrides similarly to Nb, and can be added to ensure corrosion resistance and toughness. For this purpose, it is contained in an amount of 0.05% or more, but if it exceeds 0.5%, coarse V-based carbonitrides are produced, and conversely, toughness deteriorates. Therefore, the upper limit is limited to 0.5%. Preferably, it is 0.1 to 0.3% of range.

  Zr and Ta can also be added to suppress adverse effects on the corrosion resistance of C and S, but if added excessively, adverse effects such as a reduction in toughness occur, so the content is Zr: 0.003 to 0 0.03%, Ta: limited to 0.01-0.1%.

  The reason for limitation according to claim 8 of the present invention will be described.

  In the present invention, in order to increase the toughness of the steel material, the amount of intermetallic compounds contained in the steel material is limited to 0.5% or less, but the solution heat treatment after the final hot rolling is omitted in order to obtain high yield strength. Must. Therefore, it is necessary to reduce the amount of intermetallic compounds in the slab and not to generate as much as possible in the hot rolling process.

  First, the technique for reducing the intermetallic compound in the slab needs to combine the control of δcal and the homogenization heat treatment for the steel slab described in this claim. In the steel material to which the present invention is applied, the temperature at which the intermetallic compound is formed when there is no solidification segregation is about 1000 ° C. or less. However, a steel slab with component segregation due to solidification requires a production process in which segregation is diffused and homogenized in order to reduce the content of intermetallic compounds. The temperature and time of this homogenization heat treatment vary somewhat depending on the solidification rate and cross-sectional area of the slab, the hot workability when it is made into a steel slab, the chemical composition such as δcal, etc., but the diffusion of Cr, Mo, Ni, etc. Since the rate is limited, the necessary temperature is 1200 ° C. or higher. On the other hand, when the temperature exceeds 1300 ° C., oxide scale is abnormally generated. The longer the time, the better, but a minimum of 1 hour is required. This object can also be achieved by taking soaking of 1200 ° C. × 1 h or more in the heating of the steel slab for product rolling. From the above, it was defined that a homogenization heat treatment at 1200 to 1300 ° C. for 1 hour or longer was applied. Considering the effect and economy, the desirable range of soaking time is 2 to 20 hours.

  About rolling conditions, it is reheated at 1100 ° C. to 1300 ° C., and a rough rolling stage in which the total reduction amount is 50% or more at 1050 ° C. or higher, and subsequently, the final reduction amount is 10% or more at 1050 to 850 ° C. It consists of a rolling stage. The rough rolling stage is a stage for mainly breaking the solidified structure and obtaining a uniform recrystallized structure, and the finish rolling stage is a stage for introducing work strain by rolling and increasing the strength after rolling. Furthermore, re-precipitation of intermetallic compounds is suppressed by performing all rolling at 850 degreeC or more. And by controlling cooling from 800 ° C. to 500 ° C. at an average cooling rate of 150 ° C./min or more after rolling, re-precipitation of intermetallic compounds is suppressed and work strain introduced by finish rolling is recovered. Suppress.

  The reason for condition limitation will be described in more detail.

  In order to enable rolling with a total reduction amount of 50% or more at 1050 ° C. or more, and to lower the deformation resistance and facilitate rolling, the steel ingot needs to be heated to 1100 ° C. or more. However, when heated above 1300 ° C, the grain boundary portion melts and cracks occur during rolling, so the heating temperature is limited to 1100 ° C to 1300 ° C.

  In the rough rolling stage, in order to break the solidified structure and obtain a uniform recrystallized structure, the total reduction must be 50% or more at 1050 ° C. or higher. When the rolling temperature is less than 1050 ° C. or the total reduction amount is less than 50%, a uniform recrystallized structure cannot be obtained.

  In the finish rolling stage, in order to obtain a target yield strength of 550 MPa, finish rolling is required in which the total rolling reduction at 1050 ° C. to 850 ° C. is 10% or more in the component range limited in the present invention. Moreover, if it rolls above 1050 degreeC, it will recrystallize, a pressurization strain will not accumulate | store, and sufficient strength will not be acquired, but if it rolls below 850 degreeC, precipitation of an intermetallic compound will be accelerated | stimulated and toughness will be drastically improved. Will be reduced. Therefore, it is necessary to perform rolling while maintaining 850 ° C. or higher in all rolling processes.

  Finally, high strength can be maintained by omitting the solution treatment.

  Examples are described below. Table 1 shows the chemical composition of the test steel. In addition, the content of inevitable impurity elements other than the components described in the table is the same as that of ordinary stainless steel. Moreover, the part in which content is not described about the component shown in Table 1 shows that it is an impurity level. REM in the table means lanthanoid rare earth elements, and the content indicates the total of these elements.

  These steels were melted in a laboratory 50 kg vacuum induction furnace and cast into a flat steel ingot having a thickness of about 100 mm.

  A steel plate having a thickness of 12 to 22 mm was produced by the above-mentioned test steel by partial rolling, homogenization heat treatment, and product rolling. The partial rolling was soaked at 1180 ° C. for 2 hours and then rolled to 65 mm. The steel pieces were subjected to homogenization heat treatment under the conditions shown in Tables 2 and 3. Some steel pieces omitted the homogenization heat treatment. This steel slab was ground to 60 mm, used as a material for product rolling, and further hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel material. Note that spray cooling was performed from a state where the steel material temperature immediately after rolling was 800 ° C or higher to 500 ° C or lower. Some steel plates were subjected to solution heat treatment under conditions of water cooling after soaking at 1100 ° C. for 20 minutes.

  With respect to the thick steel plate obtained under the above production conditions, a No. 4 tensile test piece and a JIS No. 4 V-notch Charpy test piece were cut out from the direction perpendicular to the rolling direction, and 0.2% offset proof stress and impact value at −40 ° C. were measured. After the surface was polished by # 600, the pitting potential (Vc′100) was measured in a degassed 10% NaCl aqueous solution at 50 ° C. Moreover, the test piece for microstructure observation was cut out, and after mirror finishing, the intermetallic compound was revealed by 10% KOH electrolytic etching, and the content rate was measured with an optical microscope. For the measurement of the content rate, point counting was performed in each 10 visual fields 400 times in 1/4, 1/2, and 3/4 thick parts, and all average values were calculated to obtain the intermetallic compound content of the steel material. The obtained results are shown in Tables 2-4.

  The hot workability was evaluated relative to the occurrence of ear cracks during product rolling. In the steel materials (Steel Nos. F to N) according to Invention Example 5 or 6, it was confirmed that no cracks occurred and good hot workability was exhibited except for the case where the reheating temperature was excessively high. . On the other hand, it was confirmed that in other steel examples of the invention, an ear crack of about 5 to 10 mm occurred on one side, and the yield was slightly reduced. The length of the ear crack is shown in Tables 2-4.

  As is clear from the results of Table 1 and Tables 2 to 4, the steel composition and the intermetallic compound content that are within the scope of the present invention, and the steel material that satisfies the manufacturing conditions, the corrosion resistance, proof stress, and Charpy impact value all satisfy the specified conditions. .

  As can be seen from the above examples, the steel material of the present invention is clearly a high-strength austenitic stainless steel material having good corrosion resistance and toughness.

  The present invention realizes austenitic stainless steel suitable for a hull structure that satisfies all the seawater resistance, proof stress and low-temperature toughness required for structural members of high-speed ships, and has a significant industrial contribution. .

Claims (8)

C: 0.001 to 0.03% in mass%
Si: 0.1 to 1.5%
Mn: 0.1 to 3.0%
P: 0.005 to 0.05%
S: 0.0001 to 0.003%
Ni: 15.0-21.0%
Cr: 22.0-28.0%
Mo: 1.5-3.5%
N: 0.15-0.35%
O: 0.0005 to 0.007%
Containing
The balance consists of Fe and inevitable impurities,
0.2% proof stress at room temperature is 550 MPa or more,
Charpy impact value by V notch test piece at −40 ° C. is 100 J / cm ² or more,
Austenitic stainless steel with good corrosion resistance, proof stress, and low temperature toughness, characterized by a pitting corrosion potential (Vc′100) measured in a degassed 10% NaCl aqueous solution at 50 ° C. of 500 mV (vs saturated Ag / AgCl) or higher. Hot rolled steel. In the austenitic stainless hot-rolled steel material according to claim 1, the PI value represented by the formula (1) further satisfies the relationship of 35 to 40, the δcal value represented by the formula (2) satisfies the relationship of -6 to +2, and An austenitic stainless hot-rolled steel material having good corrosion resistance, proof stress, and low-temperature toughness, wherein the content of intermetallic compounds contained in the steel material is 0.5% or less.
PI = Cr + 3.3 (Mo + 0.5W) + 16N (1)
δcal = 2.9 (Cr + 0.3Si + Mo + 0.5W)
-2.6 (Ni + 0.3Mn + 0.25Cu + 32C + 20N) -18 (2) The austenitic stainless hot-rolled steel material according to claim 1 or 2, further comprising one or two of W: 0.3 to 3.0% and Al: 0.005 to 0.1% in mass%. An austenitic stainless hot-rolled steel with good corrosion resistance, proof stress, and low-temperature toughness.   The austenitic stainless hot-rolled steel material according to claims 1 to 3, further comprising one or more of Cu: 0.3 to 2.0% and Sn: 0.1% or less in mass%. Austenitic stainless hot rolled steel with good corrosion resistance, proof stress and low temperature toughness. The austenitic stainless hot-rolled steel material according to any one of claims 1 to 4, wherein Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, REM: 0.005-0. Containing one or more of 10%,
An austenitic stainless hot-rolled steel material having good corrosion resistance, proof stress, and low-temperature toughness, wherein the PV value represented by the formula (3) is 0 or less.
PV = S + O-0.8Ca-0.3Mg-0.3REM-30 (3)   6. The austenitic stainless hot-rolled steel material according to claim 1, further comprising B: 0.0003 to 0.0060% by mass%. Austenitic stainless steel having good corrosion resistance, proof stress, and low temperature toughness. Hot rolled steel.   In the austenitic stainless hot-rolled steel material according to any one of claims 1 to 6, Ti: 0.003-0.03%, Nb: 0.02-0.20%, Zr: 0.003-0. Austenitic series with good corrosion resistance, proof stress, and low temperature toughness characterized by containing one or more of 03%, V: 0.05 to 0.5%, Ta: 0.01 to 0.1% Stainless hot rolled steel.   A homogenization heat treatment of 1200 to 1300 ° C for 1 hour or longer is added to the slab or steel slab of the austenitic stainless hot-rolled steel material according to claims 1 to 7, and reheated at 1100 to 1300 ° C, and 850 ° C in the rolling step The above is maintained, and rolling is performed at a reduction rate of 50% or more at 1050 ° C. or more and 10% or more at 1050 to 850 ° C., and after cooling, an average cooling rate of 800 ° C. to 500 ° C. is 150 ° C./min or more. A method for producing an austenitic stainless hot-rolled steel material having good corrosion resistance, yield strength and low-temperature toughness, characterized by not performing heat treatment.