JP5328478B2 - Stainless steel plate with excellent surface properties - Google Patents

Description

  The present invention relates to a stainless steel thick plate having excellent surface properties.

  Since austenitic stainless steel contains a large amount of expensive Ni, Cr, Mo and the like, yield improvement is an important issue from the viewpoint of reducing manufacturing costs. However, it contains a large amount of Ni, Cr, Mo, N, etc. in terms of corrosion resistance and strength, so it is inferior in hot workability, causing intergranular cracking during hot working, resulting in surface defects and lowering yield. There is a problem of doing.

  Generally, in order to improve the hot workability of austenitic stainless steel, harmful S and O are reduced as in Patent Document 1 and Patent Document 2, for example, and Ca, Mg, or Ce is added to be harmful. A method of fixing S and O is known. However, when Ca, Mg or Ce is added, there is a problem that water-soluble sulfides are generated in the steel and the corrosion resistance is deteriorated.

  Further, as disclosed in Patent Document 3, a method for improving the hot workability of stainless steel by appropriately controlling the B, S concentration and the amount of δ-Fe in the steel is disclosed. However, when B is added, the problem is that the corrosion resistance decreases due to sensitization by the formation of borides.

  Further, as means for improving the hot workability, as in Patent Document 2 and Patent Document 3, control of the amount of δ-Fe in the cast structure can be mentioned. It is known that the presence of δ-Fe reduces the segregation of S and O to the grain boundaries and improves hot workability. However, the influence of each component on the amount of δ-Fe and hot workability is not always clear, and it is not said that it is sufficiently applied to improve hot workability from the aspect of component design. .

Japanese Patent Laid-Open No. 1-168846 Japanese Patent Publication No. 5-7457 JP 7-41912 A Japanese Patent Publication No. 5-7457

T.A.Siewert, C.N.McCowan, and D.L.Olso: Welding Journal, 67 (1988), 292s. D.J.Kotecki and T.A.Siewert: Welding Journal, 71 (1992), 171s

  Although the conventional technology improves hot workability by adding trace elements, corrosion resistance deterioration due to inclusions and precipitates occurs, so it is often not applicable to stainless steel thick plates that have strict requirements for corrosion resistance. It is.

  The present invention has been made in view of such a conventional problem, and in a stainless steel thick plate, surface properties are not generated during hot rolling by controlling the components and the amount of δ-Fe. An austenitic stainless steel excellent in the above is provided.

(1) By mass%, C: 0.005 to 0.07%, Si ≦ 0.8%, Mn: 0.5 to 4.0%, P ≦ 0.04%, S ≦ 0.008%, Cr: 16 to 19%, Ni ≦ 13%, Al ≦ 0.05%, N ≦ 0.08%, balance Fe and inevitable impurities, and the calculated value of δ-Fe defined by the following formula (1) is Austenite with excellent surface properties, characterized by satisfying 0.5 or more and 7.0 or less and having a measured δ-Fe value of 0.1% to 0.7% by volume on the surface of the plate after thick plate rolling Stainless steel plate.
δ-Fe calculated value = 2.9 × ([Cr] +0.3 [Si] + [Mo] +0.7 [Nb]) − 2.6 × ([Ni] +0.3 [Mn] +0.25 [ Cu] +35 [C] +20 [N])-18 (1)
(2) The surface according to the above (1), further comprising one or more of Mo ≦ 3.0%, Cu ≦ 3.5%, and Nb ≦ 0.5% by mass%. Austenitic stainless steel plate with excellent properties.
(3) The austenitic stainless steel thick plate having excellent surface properties according to the above (1) or (2), wherein RA value ≧ 78 defined by the following formula (2) is satisfied.
RA value = 106−3.4 × (calculated value of δ−Fe) −2036 × [S] −81 × [N] (2)
Unlike the three prior arts described above, the present invention achieves improvement in the surface quality of the thick plate by controlling the amount of δ-Fe by component design and slab heating during hot rolling. In the present invention, “after thick plate rolling” means a state in which cooling after thick plate rolling is completed. When heat treatment such as solution treatment is further performed after thick plate rolling, the state before the heat treatment is meant. In the above formulas (1) and (2), [element] means the content (% by mass) of each element.

  The present invention adjusts the components in stainless steel and the amount of δ-Fe after thick plate rolling (measured value of δ-Fe), and further controls the amount of δ-Fe during rolling after slab heating. A crack in rolling is suppressed, and an austenitic stainless steel thick plate having excellent surface properties is obtained.

It is a figure which shows the result of having investigated the relationship between (delta) -Fe calculation value and the occurrence of shave. It is a figure which shows the result of having investigated the relationship between the amount of (delta) -Fe (measured value of (delta) -Fe) on the surface of a thick plate, and the occurrence of hege. It is a figure which shows the result of having investigated the relationship between RA value and the occurrence of shaving. It is a figure which shows the result of having investigated the relationship between RA value and the cross-sectional shrinkage rate in 1100 degreeC.

  In the present invention, in order to prevent the occurrence of hot working cracks, the amount of δ-Fe during slab and hot rolling is regulated. Specifically, the following δ-Fe calculated value is used as the δ-Fe amount of the slab, and the δ-Fe measured value on the plate surface after thick plate rolling is used as the δ-Fe amount during hot rolling. . In addition, the amount of δ-Fe is displayed in volume% for both the calculated value and the measured value.

In the present invention, the calculated value of δ-Fe uses the following formula (1) in which terms of Nb and Cu are added to the formula described in non-patent document 1 as described in non-patent document 2. This is because the δ-Fe amount of the slab can be accurately predicted with respect to a wider component range than that conventionally used, for example, the δ-Fe amount prediction equation of Patent Document 4.
δ-Fe calculated value = 2.9 × ([Cr] +0.3 [Si] + [Mo] +0.7 [Nb]) − 2.6 × ([Ni] +0.3 [Mn] +0.25 [ Cu] +35 [C] +20 [N])-18 (1)

FIG. 1 shows the relationship between the calculated value of δ-Fe calculated from the steel component according to the above equation (1) and the occurrence of lashes after thick plate rolling. From the result of visual observation of the state of occurrence of whipping on the thick plate, the evaluation was divided into ○, Δ, and ×. ○ indicates that there is no problem as a product with no maintenance at all, Δ indicates that it can be remedied by grinding (entire maintenance), and × indicates that a failure that cannot be used at all has occurred. The calculated value of δ-Fe represents the amount of δ-Fe in the slab surface layer. As is apparent from FIG. 1, when the calculated value of δ-Fe is less than 0.5%, the hot workability is poor. This is because the amount of δ-Fe that dissolves [S] is small. The [delta]-Fe calculated value of 7.0% in the ultra scab generation is increased. This is because cracks occur at the phase boundary due to the difference between the δ phase having a small deformation resistance and the γ phase having a large deformation resistance. In addition, what is shown in FIG. 1 is the component range prescribed | regulated to Claim 1 of this invention, Comprising: It confirmed that it exists in the range of 0.1-0.7% about the following (delta) -Fe measured value. ing.

  FIG. 2 shows the relationship between the measured value of δ-Fe on the surface of the thick plate after rolling and the occurrence of shaving after the rolling of the thick plate. The measured value of δ-Fe was measured with a ferrite scope after the plate surface after thick plate rolling was polished with No. 1000 Emily paper. It was found that when the measured value of δ-Fe is in the range of 0.1 to 0.7%, the occurrence rate of hege is small. In addition, what is shown in FIG. 2 is the component range prescribed | regulated to Claim 1 of this invention, Comprising: About the said (delta) -Fe calculated value, it confirms that it exists in the range of 0.5 to 7.0%. ing.

That, (1) δ-Fe Calculated formula with a range of 0.5~7.0%, δ-Fe measurement of the plate surface after Plate Mill of from 0.1 to 0.7% If it was in the range, it was found that the occurrence of shaving after thick plate rolling can be prevented.

  A heat treatment was applied to the sample for high temperature tensile test by a laboratory experiment, and a high temperature tensile test at 1100 ° C. was performed. Further, the amount of δ-Fe (volume%) on the surface of the test piece was measured to obtain the amount of residual δ-Fe. When the heat treatment conditions were changed and the relationship between the amount of residual δ-Fe and the cross-sectional shrinkage rate of the high-temperature tensile test was observed, the high-temperature ductility was highest when the amount of residual δ-Fe was 0.5%, and 0.1 to 0. It was found that the range of 7% was good. As described above, when the amount of residual δ-Fe is large, it is considered that cracking occurs due to the deformation resistance values of the δ and γ phases. On the other hand, when the amount of residual δ-Fe is too small, it is considered that the solid solution of [S] due to δ-Fe disappears and segregation of [S] is promoted to deteriorate the hot workability. The disappearance of δ-Fe means transformation into a fine γ phase of the crystal structure, and it is presumed that generation of microvoids due to transformation shrinkage also adversely affects hot workability. From the above test results, it is clear that the δ-Fe measured value on the surface of the plate after thick plate rolling is 0.1 to 0.7% as in the present invention.

  The amount of δ-Fe (measured value of δ-Fe) on the surface of the thick plate after rolling can be controlled by slab heating conditions. The disappearance rate of δ-Fe during slab heating depends on the heating temperature, and a range of 1140 ° C to 1220 ° C is preferable to sufficiently increase the disappearance rate. When the temperature exceeds 1250 ° C., the occurrence of scab increases due to reprecipitation and coarsening of δ-Fe. When the temperature is lower than 1140 ° C., the effect of disappearance of δ-Fe due to diffusion becomes small, and it is necessary to increase the heating time. The heating time is determined according to the heating temperature and the slab thickness so that the measured value of δ-Fe falls within the appropriate range. For example, in the case of a slab having a thickness of 200 mm, if it is 1220 ° C., about 5 min is required, and if it is 1140 ° C., about 20-30 min is required. For δ-Fe disappearance, the residence time at a high temperature of 1100 ° C. or higher is important from the viewpoint of diffusion rate.

FIG. 3 shows the relationship between the steel component and the value of RA calculated by the following equation (2) from the calculated δ-Fe value of the equation (1), and the occurrence of lashes after thick plate rolling. It was found that when the RA value was 78% or more, the rate of occurrence of baldness was particularly small. In addition, what is shown in FIG. 3 is the component range prescribed | regulated to Claim 1 of this invention, Comprising: About the said (delta) -Fe calculated value, it exists in 0.5 to 7.0% of range, (delta) -Fe About the measured value, it has confirmed that it exists in the range of 0.1-0.7%.
RA value = 106−3.4 × (calculated value of δ−Fe) −2036 × [S] −81 × [N] (2)

  As shown in FIG. 4, the above equation (2) expressing the RA value is obtained by regression analysis from the relationship between the hot ductility of the laboratory experimental material and the components. The vertical axis represents the cross-sectional shrinkage ratio of a high-temperature tensile test at 1100 ° C. for a test material subjected to heat treatment at 1200 ° C. × 60 min. According to the relationship between the RA value and the high temperature ductility, it is considered that, in addition to the amount of δ-Fe, segregation of S grain boundaries and increase in strength by addition of N also adversely affect hot workability.

  The reason for limiting the component composition (mass%) according to the present invention will be described together with the action of each element.

  C is a strong austenitizing element and strengthens the solid solution, so 0.005% or more is added. However, if the content increases, carbides are generated and the corrosion resistance is deteriorated, and the effect on the amount of δ-Fe is also affected. Is large, and it becomes easy to generate the beard soot.

  Si is an element that acts as a deoxidizer during the melting of stainless steel, but in the present invention, it is necessary to control it to 0.8% or less from the viewpoint of ensuring hot workability. The lower limit of Si is preferably 0.2%.

  Mn is a deoxidizer and has an effect of improving hot workability, and is an element effective for fixing S as MnS and preventing the occurrence of red hot brittleness due to the formation of FeS. However, if it is contained in a large amount, the refractory melting loss during melting is increased and the corrosion resistance deteriorates, so the content is made 4.0% or less. The lower limit of Mn is preferably 0.6%.

  P is an impurity in the steelmaking process, but if it is contained in a large amount, hot workability is impaired, so the upper limit is made 0.04% or less.

  S lowers the hot workability, makes it easier to generate cracking defects during hot rolling, and deteriorates the corrosion resistance.

  Cr is a basic element of stainless steel and contributes to improvement of corrosion resistance and oxidation resistance. In addition, since the influence on the amount of δ-Fe varies depending on the concentration level, the Cr concentration is set to 16 to 19% in order to stably achieve the control of the amount of δ-Fe in the present invention.

  Ni is an element having an effect of improving the corrosion resistance and toughness of steel, but it is also expensive, so it is set to 13% or less. The lower limit of Ni is preferably 6.0%.

  Al acts as a deoxidizer, but if a large amount of Al is contained, harmful hard oxides are generated and surface defects occur during rolling. Therefore, the upper limit of Al is set to 0.05%. The lower limit of Al is preferably 0.002%.

  N is an element having an action contributing to stabilization of austenite and the like, and at the same time, is an element effective for improving the strength. However, N ≦ 0.08% should be controlled due to surface flaws caused by hot workability. It is said. The lower limit of N is preferably 0.02%.

  Mo is not a scale that is an effective element for improving the corrosion resistance, but has an effect of strengthening solid solution. If necessary, 0.05% or more is added. However, if it exceeds 3.0%, the hot workability deteriorates rapidly, so it is controlled to 3.0% or less.

  Cu is an austenite stabilizing element and is an element having an action of improving the corrosion resistance. Therefore, it is desirable to add 0.2% or more. However, if contained in a large amount, the hot workability is impaired, so the content is made 3.5% or less.

  Nb is an element effective for improving corrosion resistance and has an effect of strengthening solid solution, so 0.05% or more is added as necessary. However, when it exceeds 0.5%, the hot workability deteriorates abruptly, so the content is controlled to 0.5% or less.

  It is a chemical component shown in Table 1, and the balance is No. consisting of Fe and inevitable impurities. 1 to 15 austenitic stainless steels were melted in an electric furnace and AOD process to produce a continuous cast slab having a thickness of 200 mm. These slabs were heated to a predetermined temperature of 1140 to 1220 ° C. at a temperature rising rate of 1000 ° C. or higher within a range of 1 to 3 ° C./min, and maintained at a constant temperature for 5 to 20 min. Hot rolling was performed to a thickness. In the steel components in Table 1, “-” means a case where the steel components are not intentionally added. Moreover, S and P are not added intentionally. Comparative Examples 20 and 26 did not intentionally add N. Numerical values that fall outside the scope of the present invention are underlined.

  After the rolled thick plate surface was polished with # 1000 Emily paper, δ-Fe was measured using a ferrite scope to obtain a δ-Fe measurement value. The measurement was performed at 10 locations and the average value was calculated.

  The state of occurrence of scabs on the thick plate was evaluated in four stages: the above-mentioned circles, triangles, and circles plus a circle where no defects were detected. The evaluation results are shown in “Rolling results” in Table 1. The evaluations of the lashes of the steels of the present invention were all ○ or ◎, and those having an RA value of 78 or more were で.

  Compared with this, the comparative steel 16 has a too high calculated value of δ-Fe, and therefore, galling occurred. Since the comparative steel 17 had a calculated value of δ-Fe that was too low, galling occurred. Since the comparative steel 18 was subjected to slab heating at a high temperature for a long time, the measured value of δ-Fe on the surface of the thick plate was low, and galling occurred. Since the comparative steel 19 was slab heated at a low temperature for a short time, the measured value of δ-Fe on the surface of the thick plate was too high at 1.2, and galling occurred.

  In comparative steel 20, C was too high, and the calculated value of δ-Fe was too low, so that galling occurred. Since the comparative steel 21 was too high in Si, galling occurred. Since the comparative steel 22 was too high in S, it caused baldness. Since the comparative steel 23 had too high P, scabs were generated. Since the comparative steel 24 was too high in Cr, galling occurred. In comparative steel 25, Ni was too high, and the calculated value of δ-Fe was too low. Since the comparative steel 26 was too high in Al, galling occurred. Since the comparative steel 27 was too high in N, scabs were generated. Since the comparative steel 28 was too high in Mo, scabs were generated. Since the comparative steel 29 was too high in Cu, scabs were generated. Since the comparative steel 30 was too high in Nb, scabs were generated. Since comparative steel 31 had a low Mn and an RA value that was too low, baldness was generated.

Claims (3)

In mass%, C: 0.005 to 0.07%, Si ≦ 0.8%, Mn: 0.5 to 4.0%, P ≦ 0.04%, S ≦ 0.008%, Cr: 16 19%, Ni ≦ 13%, Al ≦ 0.05%, N ≦ 0.08%, balance Fe and inevitable impurities, and the calculated value of δ-Fe defined by the following formula (1) is 0.5 An austenitic stainless steel with excellent surface properties, characterized by satisfying 7.0 or less and having a measured δ-Fe value of 0.1% to 0.7% by volume on the surface of the plate after thick plate rolling Thick plate.
δ-Fe calculated value = 2.9 × ([Cr] +0.3 [Si] + [Mo] +0.7 [Nb]) − 2.6 × ([Ni] +0.3 [Mn] +0.25 [ Cu] +35 [C] +20 [N])-18 (1)   The surface property according to claim 1, further comprising one or more of Mo ≦ 3.0%, Cu ≦ 3.5%, Nb ≦ 0.5% by mass%. Austenitic stainless steel plate. The austenitic stainless steel thick plate with excellent surface properties according to claim 1 or 2, wherein an RA value ≧ 78 defined by the following formula (2) is satisfied.
RA value = 106−3.4 × (calculated value of δ−Fe) −2036 × [S] −81 × [N] (2)

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