JP4507680B2 - Method for producing ferritic stainless steel sheet having excellent ductility and ridging resistance - Google Patents

Description

  The present invention relates to a method for producing a ferritic stainless steel sheet having excellent ductility and ridging resistance. Here, ridging refers to undulations extending in the rolling direction that appear on the surface of a product after processing such as press molding, drawing and bending.

Examples of conventional ferritic stainless steel sheet manufacturing techniques aimed at improving ridging resistance include the following.
1) A steel material containing Cr: 13 to 20% and with the upper limit of the amount of C, Si and Mn being hot-rolled, wound at 500 to 750 ° C., and annealed, the coiling temperature is 0.5. ℃ / s or more, more than 0.1 ℃ / s or more, raise the temperature to below the austenite formation temperature, hold for 30 minutes or less, cool, and cold-roll annealing at least once with intermediate annealing What performs a process (patent document 1).
2) Cr: 13 to 20%, Ti: 0.1 to 0.5%, Nb: 0.1 to 0.8%, any one or two of C + N, Si, In the process of hot rolling a steel material with the upper limit of Mn content, winding it at 750 ° C. or lower, and annealing it, heating it to 0.1 ° C./s or higher up to the coiling temperature and raising the temperature to 780 to 950 ° C. And cooling after holding for 30 minutes or less, followed by intermediate annealing, rolling at an intermediate cold rolling reduction ratio of 50% or more and a final cold rolling reduction ratio of 30% or more and less than 50%, and then performing final annealing to <111> / / ND. A fiber structure is developed (Patent Document 2).

Note that annealing (batch annealing) of hot rolled coils of ferritic stainless steel SUS430 is usually performed at a temperature of 780 to 850 ° C. (Non-patent Document 1).
JP 52-66816 A JP-A-53-48018 Japan Iron and Steel Association, Iron and Steel Handbook 3rd edition III (1) Rolling foundation and steel plate, page 700 (issued by Maruzen on May 15, 1980)

  According to the above prior art, ridging resistance and ductility can be improved, but there is a problem that cold rolling and subsequent annealing (cold rolled sheet annealing) are performed twice or more, that is, an additional process is required. .

  The present invention provides a method for producing a ferritic stainless steel sheet having excellent ductility and ridging resistance, which can solve the above-mentioned problems and can produce a ferritic stainless steel sheet having excellent ridging resistance and ductility without additional steps. The purpose is to provide.

In the present invention, a ferritic stainless steel slab is hot-rolled to form a hot-rolled sheet, then the hot-rolled sheet is annealed, then pickled, then cold-rolled to form a cold-rolled sheet, In a method for producing a ferritic stainless steel sheet having a step of annealing a cold-rolled sheet, the hot-rolled sheet annealing is performed under conditions of soaking at 970 ° C. for 6 to 12 hours and then cooling. This is a method for producing a ferritic stainless steel sheet having excellent ridging properties.

  In the present invention, it is preferable that the cooling time from the end of the soaking to 650 ° C. is 20 hours or more.

  According to the present invention, since hot-rolled sheet annealing is performed under conditions of soaking for 6 to 12 hours at 950 ° C. or higher, excellent ductility and ridging resistance can be achieved by one cold-rolled-cold-rolled sheet annealing. There is an effect that it is obtained.

  A steel piece (slab) of ferritic stainless steel as a starting material used in the present invention is not particularly limited in its steel type, but can be preferably used for SUS430 defined in JIS, for example. These exhibit a two-phase structure of ferrite (α) + austenite (γ) in a normal hot rolling temperature range. Further, either an ingot slab or a continuously cast slab may be used.

  Hot rolling may be performed by a normal method. As main operation conditions, for example, slab heating temperature: 1100 to 1300 ° C., finish rolling temperature: 900 to 1100 ° C., rolling reduction: 40 to 95%, finished plate thickness: 2.0 to 8.0 mm, winding temperature: 400-700 degreeC is mentioned.

In the present invention, the hot-rolled sheet annealing (batch annealing) is performed under conditions of cooling after annealing at 970 ° C. or more for 6 to 12 hours. For example, as shown in the heat pattern diagram in FIG. As a result, dissolution of C and N in the γ phase proceeds, carbonitrides as precipitates decrease, α grains recrystallize, and the hot-rolled structure is improved. -Ductility improves even if cold-rolled sheet annealing is performed once.

  In addition, when the γ phase transformed and precipitated along the α phase grain boundary at high temperature by the soaking process is retransformed into the α phase by cooling, the crystal orientation of the retransformed α phase is different from the original α phase. Thus, since the orientation becomes random, the ridging resistance is improved even if the subsequent cold-rolling and cold-rolling sheet annealing is performed once.

  When the soaking temperature holding temperature is less than 950 ° C., or when the soaking temperature holding time is less than 6 hours, the effects of improving the ductility and ridging resistance cannot be obtained. Further, if the soaking time holding time exceeds 12 hours, the effect of improving ductility is saturated, which is economically disadvantageous. For this reason, in this invention, the holding temperature of the soaking in hot-rolled sheet annealing was 950 degreeC or more, and holding time was limited to 6 to 12 hours. In addition, it is preferable that the said holding temperature shall be 1050 degrees C or less from a viewpoint of saturation of a ductility improvement effect.

  In the present invention, from the viewpoint of making the crystal orientation of the α grains after hot-rolled sheet annealing more random, it is preferable that the cooling time from the end of the soaking is 650 ° C. is 20 hours or more. Such cooling conditions can be satisfied by gradual cooling with a heat insulating cover. After the temperature falls below 650 ° C., it may be allowed to cool.

  Pickling may be performed by a usual method (such as nitric acid electrolytic pickling). In addition, you may perform normal mechanical scale removal (shot blasting etc.) after hot-rolled sheet annealing and before pickling. Further, normal picking (coil surface grinding) may be performed after pickling and before cold rolling.

  Cold rolling and cold rolled sheet annealing may be performed by a normal method (continuous annealing). The main operating conditions for cold-rolled sheet annealing include an ultimate holding temperature of 800 to 950 ° C. and a holding time of 15 to 120 seconds.

  A continuous cast slab equivalent to SUS430 having the chemical composition shown in Table 1 is heated to 1200 ° C., hot-rolled at a finish rolling temperature of 950 ° C., a reduction ratio of 90%, and a finished sheet thickness of 4 mm, and 750 ° C. And rolled into a coil, and this was subjected to hot-rolled sheet annealing in the batch annealing equipment under the conditions shown in Table 2.

  Next, after shot blasting-sulfuric acid electrolytic pickling-coil surface grinding, it is cold-rolled to a finished sheet thickness of 0.8 mm, and then at a final holding temperature of 880 ° C. and a holding time of 30 seconds in a continuous annealing facility Under conditions, cold-rolled sheet annealing was performed.

  After the soaking, the coil was covered with a heat insulating cover heated by a burner for cooling to 650 ° C. Regarding the cooling time, a difference of 21 hours and 18 hours was realized by controlling the burner output.

  The steel sheets after cold-rolled sheet annealing are 3 (L (direction parallel to the rolling direction), C (direction perpendicular to the rolling direction), D (direction of 45 ° to the rolling direction) in accordance with JIS Z 2241. Tensile tests were performed in the direction and the elongation was measured. In addition, using the same L direction tensile test piece as above, the surface after the 15% tensile test was visually observed, and ridging was evaluated by ◯ indicating no ridging, △ indicating partial occurrence, and x indicating overall occurrence. And the average value of the value which measured the wave | undulation of the test piece surface in the direction perpendicular | vertical to a tension direction with the contact-type surface roughness meter was evaluated as surface wave | undulation height.

  These results are shown in Table 2. Compared with the conventional example, the inventive example is remarkably improved in both elongation and ridging score, and it can be seen that the workability is improved by one cold rolling-cold rolling sheet annealing. 2 (a) and 2 (b) show examples No. 1 of the present invention after hot-rolled sheet annealing. 1 and conventional example No. 1 3 and FIGS. 3 (a) and 3 (b), Example No. of the present invention after cold-rolled sheet annealing. 1 and conventional example No. 1 3 shows an optical microscope observation structure of No. 3 (the left-right direction of the paper corresponds to the L direction). From FIG. 2, it can be seen that in the example of the present invention, the grains grow uniformly. Thereby, the ridging resistance after cold-rolled sheet annealing improved. Further, FIG. 3 shows that in the example of the present invention, there are few fine precipitates that are considered to be fine carbonitrides. This improved the ductility.

  In addition, for the cooling time up to 650 ° C. after the soaking, the invention example No. No. 1 is 21 hours, Example No. 2 is 18 hours, and as a result, the ridging evaluation is No. 2. No. 1 21 hours are marked with a circle, no ridging has occurred, and no. The 18 hours of 2 are indicated by Δ, indicating that ridging occurred partially.

  The present invention can be used in a manufacturing process of a ferritic stainless steel sheet.

It is a heat pattern figure which shows one example of the hot-rolled sheet annealing conditions of this invention compared with a prior art example. The steel sheet structure after hot-rolled sheet annealing was obtained as Example No. of the present invention. 1 and conventional example No. 1 FIG. 3 is a copy diagram of an optical microscope structure photograph shown in comparison with FIG. The steel sheet structure after the cold-rolled sheet annealing is referred to as Example No. of the present invention. 1 and conventional example No. 1 FIG. 3 is a copy diagram of an optical microscope structure photograph shown in comparison with FIG.

Claims (2)

A steel piece of ferritic stainless steel is hot-rolled to form a hot-rolled sheet, and then the hot-rolled sheet is annealed, then pickled, and then cold-rolled to form a cold-rolled sheet. In the method for producing a ferritic stainless steel sheet having a step of annealing, the hot-rolled sheet annealing is performed under conditions of soaking at 970 ° C. for 6 to 12 hours and then cooling, and having excellent ductility and ridging resistance A method for producing a ferritic stainless steel sheet.   2. The method for producing a ferritic stainless steel sheet having excellent ductility and ridging resistance according to claim 1, wherein the cooling time from the end of the soaking is 650 [deg.] C. is 20 hours or more.

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