JP4494237B2 - Austenitic stainless steel material excellent in corrosion resistance, toughness and hot workability, and method for producing the same - Google Patents

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. Therefore, the present invention provides an inexpensive stainless steel material having excellent strength and toughness.

  Conventionally, coated steel plates with heavy anticorrosion have been used for hull structures. In the past, the demand for high-speed ships equipped with hydrofoil has increased, and because high-speed seawater flows in this application, materials with excellent seawater resistance that do not require painting are required, and austenitic stainless steel with excellent seawater resistance Steel high-strength materials have been studied (Patent Documents 1-3).

  In these documents, a technology for producing austenitic stainless steel with high strength and seawater resistance of 500 MPa or more by controlled rolling, 0.3% or more of N addition, and 0.5 to 3.0% of Mo addition, , Nb and other elements are added, and the manufacturing technology of austenitic stainless steel with less weld softening is disclosed.

  Cr, Mo, and N are important as elements that enhance seawater resistance, and the corrosion resistance order of 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 1 is calculated, the minimum value is about 32. However, as a stainless steel satisfying a higher PI value (35 or more), the austenitic system contains 23% or more of Ni. SUS836L, 890L, and SUS329J4L containing 5.5 to 7.5% Ni in the two-phase system.

  Since the two-phase SUS329J4L contains a ferrite phase, the yield strength is high. 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. However, as a high-strength steel material of austenitic high corrosion-resistant stainless steel, those having a PI value exceeding 35 have not yet been put into practical use.

Japanese Examined Patent Publication No. 7-13252 Japanese Patent No. 2783895 Japanese Patent No. 2784896

  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 the hull structure for general purpose and maintenance-free purposes, development of a high corrosion resistance steel material higher than the lower limit of the steel material shown in Patent Document 1 has been required.

  On the other hand, there is an increasing demand for reliable seawater-resistant stainless steel materials for grounding on rock reefs and collision accidents between ships. The reliability of the steel material is both the weldability and the characteristics of the base material.

  Although the effects of N and Nb on the welded part as disclosed in Patent Documents 2 and 3 are certainly effective in ensuring strength, excessive addition of N tends to cause bubbles in the welded part, conversely welding. The joint strength and reliability of the part may be reduced.

  On the other hand, as the reliability of the base material, high toughness is required in preparation for a collision accident. Generally, high toughness can be secured with stainless steel by adding a large amount of Ni. In the austenitic stainless steel containing a large amount of Cr and Mo having a PI value of 35 or more, a larger amount of Ni needs to be added. 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. These austenitic stainless steels are very expensive due to their high Cr, Mo and Ni contents and high hot production costs.

  From the viewpoint of weldability, the inventors of the present invention have the strength of thick steel plates obtained by casting, hot working, and heat treatment for austenite-based component systems with N content of 0.35% or less and PI value of 35 or more. The toughness and corrosion resistance were investigated. In particular, the toughness is not limited only by the Ni content, but it has been found that the content of non-metallic inclusions contained in the steel material and the intermetallic compounds having a high Cr and Mo composition ratio dominate the toughness. In particular, in the austenitic stainless steel having a high Cr and Mo content targeted by the present invention, it is important to control the content of intermetallic compounds contained in the steel material. Since the formation of the metal structure begins with the solidification of the steel, we investigated the influence of the chemical composition on the solidification structure, and further investigated the effects of rough rolling, homogenization heat treatment, hot working, and heat treatment conditions on the cast steel. As a result, overcoming the problems of the prior art, limiting the content of component elements and solidification structure to obtain an austenitic stainless steel material excellent in corrosion resistance, toughness and hot workability, limiting the metal structure of the steel material, They also found an effective manufacturing method for manufacturing the steel material.

That is, the gist of the present invention is as follows.
In mass%, C: 0.03% or less, Si: 0.1-1.5%, Mn: 0.1-3.0%, P: 0.05% or less, S: 0.003% or less, Ni: 15.0 to 21.0%, Cr: 22.0 to 28.0%, Mo: 1.5 to 3.5%, Cu: 2.0% or less, N: 0.15 to 0.35 %, Al: 0.005 to 0.1% or less, satisfying the relationship of PI: 35 to 40, δcal: −6 to +4, and further contained in steel materials called σ phase and χ phase The content of the intermetallic compound is 0.5% or less, and Ti: 0.003 to 0.03%, Nb: 0.02 to 0.20%, V: 0.05 to 0.5% as necessary. , W: one or more of 0.3 to 3.0%, and / or B: 0.0003 to 0.0060%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 An austenitic stainless steel material excellent in corrosion resistance, toughness and hot workability, characterized by containing one or more of ˜0.0050% and REM: 0.005 to 0.10%. Furthermore, in order to reduce the content rate of the intermetallic compound in steel materials with respect to the slab or the steel slab after rough hot working, the said steel materials characterized by adding the homogenization heat processing for 1 hour or more at 1200-1300 degreeC Manufacturing method.
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 + 35C + 20N) -18 (2)
Here, the element name is expressed in mass%.

  The present invention realizes an austenitic stainless steel material having good corrosion resistance, toughness, and hot workability suitable as a structural material for ship hulls having seawater resistance, and greatly contributes industrially.

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

  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.

  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. Preferably, it is 0.03% or less.

  S degrades hot workability, toughness, and corrosion resistance, so is limited to 0.003% or less. Preferably, it is 0.001% or less.

  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%. . Preferably, it is 0.30% or less.

  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%.

  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%. Preferably it is 0.004% or less.

  The PI value: pitting corrosion index represented by the above formula (1) is an index of the corrosion resistance of stainless steel to the chloride environment, and is required to be at least 35 or more in order to obtain the corrosion resistance commensurate with the purpose. 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 the formula (1) is set to zero.

  Δcal shown in the above equation (2) is an index representing the amount of delta ferrite appearing in the solidified structure of austenitic stainless steel, and is generally about 0 to 7% in order to reduce the solidification cracking susceptibility or make the structure finer. Are controlled by 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 +4 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 +3. In the present invention that does not contain W or Cu, W or Cu in formula (2) is set to zero.

  The content 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 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%.

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

  Cu is an element that additionally enhances the corrosion resistance of stainless steel to acids, and for this purpose, it can be contained in an amount of 0.1% or more. Even if the content exceeds 2.0%, the effect corresponding to the cost is saturated, so the upper limit was made 2.0%.

  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.

  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.

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

  B, 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. B is 0.0003 to 0.0060%, Ca and Mg are 0.0005 to 0.0050%, and REM is 0.005 to 0.10%. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

  The reason for limitation according to claim 4 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 as the method, the amount of delta ferrite in the solidified structure called δcal is predicted. It is a homogenization heat treatment for the steel slab described in the chemical composition formula and 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, a 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.

  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.

  The above test steel was subjected to rough rolling, homogenization heat treatment, and product rolling under various conditions. The rough rolling was performed at 1180 ° C. for 2 hours and then rolled to 65 mm. The steel slab was subjected to a homogenization heat treatment at 1220 to 1280 ° C. Some steel pieces omitted the homogenization heat treatment. This steel slab was ground to 60 mm to obtain a material for product rolling. Product rolling was soaked at 1220 ° C. for 1 h to 2 h, and then rolled at a finishing temperature of 850 to 950 ° C. to obtain a 12 mm thick steel plate. Note that spray cooling was performed from a state where the steel material temperature immediately after rolling was 800 ° C or higher to 300 ° C or lower. The final solution heat treatment was carried out under conditions of water cooling after soaking at 1100 ° C. for 20 minutes. In some steel plates, solution heat treatment was omitted.

  The No. 4 tensile test piece and the JIS No. 4 V-notch Charpy test piece were cut out from the direction perpendicular to the rolling for the thick steel plate obtained under the above production conditions, and 0.2% offset proof stress, tensile strength and impact value at −40 ° C. were measured. did. Further, a specimen for microstructural observation was cut out, and after mirror finishing, an intermetallic compound was revealed by 10% KOH electrolytic etching, and the content 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 Table 2.

The hot workability was evaluated relative to the occurrence of ear cracks during product rolling. In the steel materials (steel Nos. 3 to 15) according to Invention Example 3, it was confirmed that no ear cracks occurred and good hot workability was exhibited. On the other hand, in the steel materials of Invention Examples 1 and 2, it was confirmed that an ear crack of about 5 to 12 mm occurred on one side and the yield was slightly lowered. Table 2 shows the length of the ear cracks. That is, for steel Nos. 0 to 2, although there is a slight problem in hot workability, the Charpy impact value at −40 ° C. is 100 J for all the thick plates manufactured so that the intermetallic compound content is 0.5% or less. / Cm ² is exceeded. Steel Nos. 3 to 15 are steels containing Al, B, Ca, Mg, and REM to improve hot workability, and no ear cracks are generated. Further, in the inventive examples manufactured so that the content of the intermetallic compound is 0.5% or less, the Charpy impact value at −40 ° C. exceeds 100 J / cm ² .

  Then, in the comparative steel examples No. 21 to 27, Ti <0.03%, Nb> 0.2%, V> 0.5%, Al> 0.1%, O> 0.007%, δFe> 3% Ni> 21% (δFe <−6%), which is out of the scope of the present invention, except for the steel 27, is inferior in impact characteristics. Steel 27 has good impact characteristics, but has a high Ni content and is not suitable for the purpose of the present invention.

As is clear from the results of Tables 1 and 2, in steel materials satisfying the steel composition and intermetallic compound content that are within the scope of the present invention, the PI value, which is an index of corrosion resistance, is 35 or more, and exhibits high strength. It is clear that each Charpy impact value is 100 J / cm ² or more.

  As can be seen from the above examples, the steel material of the present invention is clearly an austenitic stainless steel material having good corrosion resistance, toughness and hot workability.

  The present invention realizes an austenitic stainless steel material having good corrosion resistance, toughness, and hot workability suitable as a structural material for ship hulls having seawater resistance, and greatly contributes industrially.

Claims (4)

In mass%, C: 0.03% or less, Si: 0.1-1.5%, Mn: 0.1-3.0%, P: 0.05% or less, S: 0.003% or less, Ni: 15.0 to 21.0%, Cr: 22.0 to 28.0%, Mo: 1.5 to 3.5%, N: 0.15 to 0.35%, Al: 0.005 0.1% or less, O: 0.007% or less, PI: 35-40, δcal: −6 to +4, the balance is composed of Fe and inevitable impurities. An austenitic stainless steel material excellent in corrosion resistance and toughness, characterized in that the content of intermetallic compounds called σ phase and χ phase 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 + 35C + 20N) -18 (2)
Here, the element name is expressed in mass%.   Further, by mass, Cu: 0.1 to 2.0%, Ti: 0.003 to 0.03%, Nb: 0.02 to 0.20%, V: 0.05 to 0.5%, W The austenitic stainless steel material having excellent corrosion resistance and toughness according to claim 1, comprising one or more of 0.3 to 3.0%.   % By mass of B: 0.0003 to 0.0060%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.005 to 0.10% The austenitic stainless steel material having excellent corrosion resistance, toughness, and hot workability according to claim 1 or 2, wherein the austenitic stainless steel material has one or more kinds. Claims 1 to 1 at 1200 to 1300 ° C. in order to reduce the content of the intermetallic compound in the steel against steel strip after processing between slab or coarse heat having a chemical composition according to any one of 3 The method for producing an austenitic stainless steel material excellent in corrosion resistance, toughness and hot workability according to any one of claims 1 to 3, wherein a homogenization heat treatment for at least hours is applied.