JP4239247B2 - Method for producing Ti-containing ferritic stainless steel sheet with excellent workability - Google Patents

Description

【００２６】
【表２】

【００２７】
【表３】

【００２８】
以上の条件によって製造した鋼板について加工性を評価した。評価の方法として、引張り試験の結果から算出される平均ｒ値および伸びを採用した。ここで、ｒ値 (ｒ) および伸び (Ｅl ）は、圧延方向に対して０度、45度、90度の方向からJIS 13号Ｂ形状の引張試験片を採取し、各方向の測定値から、それぞれ、ｒ＝（ｒ₀ ＋２×ｒ₄₅＋ｒ₉₀）／４およびＥl ＝（Ｅl₀＋２×Ｅl₄₅ ＋Ｅl₉₀ ) ／４の式により算出した値である。得られたｒ値および伸びを表３に合わせて示す。表３から明らかなように、本発明に従った成分および工程の組合せで製造した場合には、ｒ値および伸びは良好な値を示し、優れた加工性を有する鋼板が製造可能であることがわかる。
【００２９】
【発明の効果】
以上説明したように、本発明によれば、従来技術では得られなかった優れた加工性を有するTi含有フェライト系ステンレス鋼板を製造することが可能となる。そしてこの鋼板は、厳しい加工性とともに、耐食性、溶接性が求められる自動車排気系のパイプおよびチューブ等に好適に使用でき、産業上優れた効果をもたらすものである。
【図面の簡単な説明】
【図１】 18wt％Cr−0.3 wt％Ti鋼における粗圧延後の再結晶率に及ぼす、粗圧延の累積圧下率および終了温度の影響を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a manufacturing technology for a Ti-containing ferritic stainless steel sheet having excellent workability, and in particular, ferrite used for automobile exhaust pipes and mufflers that require corrosion resistance and weldability in addition to excellent workability. The present invention relates to a method for manufacturing a stainless steel plate.
[0002]
[Prior art]
Since ferritic stainless steel sheets containing Ti have both corrosion resistance and weldability and are relatively inexpensive, they have recently been used for automobile exhaust system members and the like. This Ti-containing ferritic stainless steel sheet is generally subjected to hot rolling consisting of rough rolling and finishing rolling on a continuously cast slab, wound into a coil, and softened and homogenized by annealing (batch type or continuous type). After that, it is manufactured by performing cold rolling and finish annealing.
In hot rolling of Ti-containing ferritic stainless steel, generally, the rolling method in SUS430, which is a general-purpose steel type, has been followed from the viewpoint of operability. Since SUS430 has a higher content of C and N in a solid solution state than a Ti-containing ferritic stainless steel sheet, it has a high high-temperature strength and a high rolling load. Therefore, in the hot rolling of SUS430, in order to reduce the rolling load, the slab heating temperature is set to a high temperature and the rolling is performed at a high temperature, and the rolling is performed by increasing the number of passes to reduce the rolling reduction per pass. It has been considered important to do. Therefore, the conditions for reducing the rolling load have been adopted also in the Ti-containing ferritic stainless steel.
[0003]
When rolling under such conditions, the most serious problem with Ti-containing ferritic stainless steels is that the strip structure at the center of the plate thickness is not sufficiently divided because the rolling reduction per pass is small, and cooling is not possible. It was that the workability (for example, r value and elongation) of the steel sheet (cold-rolled annealed sheet) after being rolled and finish-annealed was not sufficiently obtained.
By the way, there have been some proposals so far regarding methods for improving the workability of a Ti-containing ferritic stainless steel sheet. For example, Japanese Patent Application Laid-Open No. 57-137427 discloses cold rolling by lowering the slab heating temperature, increasing the finish rolling start temperature, lowering the finish rolling end temperature, and further defining the lower limit of the finish rolling reduction. A method for improving the workability of an annealed plate is disclosed.
[0004]
[Problems to be solved by the invention]
This method is intended to improve workability by recrystallization during finish rolling, but in normal finish rolling by tandem rolling of 5 to 7 stand, the time between passes is short, so There was a problem of not recrystallizing.
Japanese Patent Application Laid-Open No. 9-194937 describes a method in which rolling at a reduction rate of 80% or more is performed in a temperature range of 1000 ° C. or more and 1100 ° C. or less, and the product is allowed to cool for 30 seconds or more and then finish rolling. However, this method has a problem that, when the slab heating temperature is high, the initial crystal grains become coarse, and the crystal grains cannot be sufficiently refined even under the subsequent strong pressure.
Therefore, the present invention is intended to solve the above-mentioned problems of the prior art, advantageously control the recrystallization behavior by hot rolling conditions, and has a Ti-containing excellent workability unprecedented It aims at proposing the manufacturing technology of a ferritic stainless steel plate.
[0005]
[Means for Solving the Problems]
The inventors examined in detail the hot rolling conditions of Ti-containing ferritic stainless steel in order to achieve the above-mentioned object. As a result, the inventors have obtained the knowledge that the problem can be solved by controlling rough rolling and finish rolling within an appropriate range, and have completed the present invention. The summary composition is as follows.
[0006]
(1) C: 0.010 wt% or less, N: 0.010 wt% or less, and C + N: 0.015 wt% or less, Cr: 6 wt% or more, 25 wt% or less, Ti: 6 × (C + N) wt% or more, A slab containing 0.5 wt% or less, with the balance being Fe and unavoidable impurities is heated to 1160 ° C. or less, and the cumulative rolling reduction is 85% or more and the final pass end temperature is 950 ° C. or more. Rolling is performed, and the sheet bar obtained by rough rolling is held at 900 ° C. or higher for 20 seconds or longer , and then finish rolling is performed so that the cumulative rolling reduction is 85% or higher and the final pass end temperature is 800 ° C. or lower and 650 ° C. or higher. Made of a Ti-containing ferritic stainless steel sheet excellent in workability, characterized in that it is wound on a coil at 680 ° C. or lower and 350 ° C. or higher, and then subjected to annealing, cold rolling and finish annealing. Method.
[0007]
(2) In the production method described in (1) above, the slab may contain, in addition to the above components,
Ni: 1.0 wt% or less,
Mo: A method for producing a Ti-containing ferritic stainless steel sheet excellent in workability, characterized by containing one or two of not more than 3.0 wt%.
[0008]
(3) The Ti-containing ferritic stainless steel excellent in workability, characterized in that in the production method described in (1) or (2) above, rolling is performed with a rolling reduction of at least one pass of rough rolling being 35% or more. A method of manufacturing a steel sheet.
[0009]
(4) In the manufacturing method according to any one of the above (1) to (3), a sheet bar obtained by rough rolling is not less than 900 ° C. between the end of rough rolling and the start of finish rolling. A method for producing a Ti-containing ferritic stainless steel sheet excellent in workability, characterized by holding in a temperature range for 20 seconds or more.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The inventors first reexamined the prior art, and in the conventional hot rolling method, the degree of strong rolling in the rough rolling is insufficient, the strip structure is not divided, and the rough rolling It was clarified that the conditions necessary for the subsequent recrystallization were not satisfied, and the softening due to the recrystallization before the finish rolling was not sufficient. And as a result of these, the load at the time of finish rolling becomes high, and from the upper limit of the mill power, the strong reduction in the finish rolling is limited, and the strip structure remains until after rolling, and the workability of the cold-rolled annealed sheet is reduced. I also found it worsening.
[0011]
In view of these circumstances, the inventors comprehensively studied rolling conditions in rough rolling and finishing rolling. As a result of detailed experiments, it was concluded that in order to improve the workability, it is necessary to pay attention to the following points.
(1) First, by reducing the slab heating temperature as much as possible, the coarsening of crystal grains during heating is suppressed and the initial grain size is reduced. (2) In rough rolling, crystal grains should be made finer by performing high-temperature and low-pressure. (3) After rough rolling, hold a sufficient time above the recrystallization temperature before the start of finish rolling, so that a recrystallized structure is once formed. (4) Accumulate sufficient strain by further rolling down in subsequent finish rolling and winding in a low temperature range. (5) Promote recrystallization during annealing of hot-rolled sheet by this accumulated strain.
[0012]
Next, manufacturing conditions for realizing the above items will be specifically described.
・ Heating temperature Slab heating temperature shall be 1160 ℃ or less. This is because when the temperature exceeds 1160 ° C., the crystal grains become coarse and the refinement of the structure after rough rolling is hindered, and as a result, the workability of the cold-rolled and annealed steel sheet is not improved. If the hot rolling is possible, the slab heating temperature is low, and the improvement of the processability of the slab becomes large. Therefore, the slab heating temperature is preferably 1120 ° C. or less.
[0013]
-In rough rolling and rough rolling, strong reduction is effective for sufficiently refining crystal grains. When the cumulative reduction ratio from the slab to the end of rough rolling is 85% or more, there is an effect of improving workability. Furthermore, when at least one of the rough rolling reduction passes has a reduction rate of 35% or more, a higher effect can be obtained.
Also, in order to keep the sheet bar above the recrystallization temperature after rough rolling and promote recrystallization before rolling, the recrystallization temperature of the Ti-containing ferritic stainless steel according to the present invention is 900 ° C or higher. Therefore, the final end temperature of the rough rolling must be 950 ° C. or higher.
In order to further promote recrystallization, it is desirable to hold the sheet bar obtained by rough rolling at 900 ° C. or higher for 20 seconds or longer before finish rolling under the rough rolling finish temperature.
When the slab heating temperature is low, it is necessary to reduce the rough rolling pass or increase the rough rolling speed in order to increase the rough rolling final pass end temperature. Capability is improved, and production with such a high rolling load that has been impossible in the past is possible.
[0014]
Fig. 1 shows the production of ferritic stainless steel based on 0.006 wt% C-0.008 wt% N-18 wt% Cr-0.3 wt% Ti in the laboratory, heated to 1120 ° C, and then rolled roughly. The thickness of the sheet bar at the end was changed to change the cumulative rolling reduction, and the rolling end temperature was changed in the range of 850 ° C to 1000 ° C and held at that temperature for 20 seconds. The recrystallization rate was investigated.
FIG. 1 shows that a recrystallized structure of 50% or more can be obtained by setting the rolling end temperature to 950 ° C. or higher and the cumulative rolling reduction to 85% or higher.
[0015]
-Finish rolling After such rough rolling, it is also important to control the rolling conditions and the coiling temperature in the finish rolling within an appropriate range in order to exhibit the excellent workability of the cold-rolled annealed sheet.
That is, in the finish rolling, the lower the end temperature, the more the strip structure is divided, the more strain is accumulated, and the recrystallization is promoted by the subsequent annealing. Such an effect is reduced when the finish rolling finish temperature exceeds 800 ° C. On the other hand, when the rolling end temperature is lower than 650 ° C., the rolled material is hardened and the rolling load becomes remarkably large. Therefore, when the rolling reduction is increased, the surface of the rolled material is wrinkled due to contact with the rolling roll. . Further, in finish rolling, by making the cumulative reduction ratio as high as possible, the strip structure is divided, distortion is accumulated, and recrystallization is promoted by annealing in the next step. If the cumulative rolling reduction is less than 85%, such an effect cannot be expected.
Therefore, in finish rolling, it is necessary that the cumulative rolling reduction is 85% or more and the final pass end temperature is 800 ° C. or less and 650 ° C. or more. In the present invention, since recrystallization is performed after rough rolling, rolling with a cumulative reduction of 85% or more is sufficiently possible even when the finish rolling finish temperature is 800 ° C. or lower.
[0016]
-Winding Further, the winding temperature after finish rolling needs to be 680 ° C or lower in order to sufficiently accumulate the strain imparted by rolling without releasing it. However, if the winding temperature is lower than 350 ° C., the rolled material is markedly hardened, making it difficult to wind, and if it is forcibly wound, creases will be generated on the surface. Therefore, coil winding after finish rolling must be performed in a temperature range of 680 ° C or lower and 350 ° C or higher. A preferable winding temperature for sufficiently accumulating rolling strain is 600 ° C. or lower.
[0017]
The conditions for annealing, cold rolling, and finish annealing of the hot-rolled sheet, which are performed following the above process, may be carried out in accordance with a conventional method and need not be particularly defined, but the following conditions are particularly recommended.
The annealing of the hot-rolled sheet is held at 800 ° C. or higher for 1 minute or longer, the cold rolling is at a rolling reduction of 65% or higher, and the final annealing is held at 850 ° C. or higher for 30 seconds or longer.
Further, after hot rolling annealing and after finish annealing, descaling by pickling is performed if necessary. For descaling, electrolytic pickling in nitrate is suitable.
[0018]
Below, the reason which limited the component composition is demonstrated.
C: 0.010 wt% or less C is an element having an adverse effect on workability. If it exceeds 0.010 wt%, the effect appears remarkably, so it is limited to 0.010 wt% or less. In order to obtain better processability, the C content is desirably limited to 0.005 wt% or less.
[0019]
N: 0.010 wt% or less and C + N: 0.015 wt% or less N is an element that has an adverse effect on workability, as with C. If it exceeds 0.010 wt%, the effect becomes significant. % Or less. In order to obtain better workability, it is desirable to limit to 0.007 wt% or less.
From the viewpoint of improving workability, the total amount of C and N (C + N) is limited to 0.015 wt% or less.
[0020]
Ti: 6 × (C + N) wt% or more, 0.5 wt% or less
Ti is an element that fixes C and N in steel and improves workability and weldability. These effects are exhibited by containing Ti 6 × (wt% C + wt% N) or more. However, even if added over 0.5 wt%, not only the effect is saturated, but the solid solution Ti raises the recrystallization temperature of the steel and hinders the softening of the steel after the end of the rough rolling. Therefore, Ti is added in the range of 6 × (C + N) wt% or more and 0.5 wt% or less. In order to further improve the workability by recrystallization after rough rolling, the Ti content is desirably 0.3 wt% or less.
[0021]
Cr: 6wt% or more, 25wt% or less
Cr is an element that improves corrosion resistance. For this effect, a content of less than 6 wt% is insufficient. On the other hand, when the content exceeds 25 wt%, embrittlement occurs and becomes a practical obstacle. Therefore, Cr content is limited to the range of 6-25 wt%.
[0022]
Ni: 1.0 wt% or less, Mo: 3.0 wt% or less
Ni and Mo are both elements useful for improving the corrosion resistance, and one or two of these are added as necessary. However, adding Ni over 1.0 wt% and Mo over 3.0 wt% not only saturates the effect, but also impairs manufacturability and economy, so each of these values is added as the upper limit.
[0023]
Ingredients other than those described above are unavoidably included in the material. Of these, Si, Mn, and Al are elements necessary for deoxidation in the steelmaking process, and are usually contained in steel in the ranges of 1.0 wt% or less, 1.0 wt% or less, and 0.1 wt% or less, respectively.
[0024]
【Example】
Ferritic stainless steel with the chemical composition shown in Table 1 is made into a 200 mm thick slab by continuous casting, and after heating this slab, the pass schedule and rolling reduction shown in Table 2) are adopted, and various final results are obtained. Rough rolling was performed at the end temperature of the pass. Subsequently, using a finish rolling mill consisting of 7 stages, rolling was performed while changing the final pass end temperature and the cumulative rolling reduction, and after water cooling, it was wound around a coil at the temperature shown in Table 1. These rolling conditions are summarized in Table 3. Here, all the inventive examples were held at 900 ° C. or higher for 30 seconds during the period from rough rolling to finish rolling. The hot-rolled sheet thus obtained was annealed and then pickled, cold-rolled, finish-annealed and pickled to obtain a cold-rolled steel sheet having a thickness of 0.6 mm.
[0025]
[Table 1]

[0026]
[Table 2]

[0027]
[Table 3]

[0028]
The workability was evaluated for the steel sheets produced under the above conditions. As an evaluation method, an average r value and elongation calculated from the results of the tensile test were adopted. Here, the r value (r) and the elongation (El) were obtained from JIS 13 B-shaped tensile test specimens taken from 0, 45, and 90 degrees with respect to the rolling direction. These are values calculated by the formulas r = (r ₀ + 2 × r ₄₅ + r ₉₀ ) / 4 and El = (El ₀ + 2 × El ₄₅ + El ₉₀ ) / 4, respectively. The obtained r value and elongation are shown in Table 3. As is apparent from Table 3, when manufactured with a combination of components and processes according to the present invention, the r value and elongation show good values, and it is possible to manufacture a steel sheet having excellent workability. Recognize.
[0029]
【The invention's effect】
As described above, according to the present invention, it is possible to produce a Ti-containing ferritic stainless steel sheet having excellent workability that has not been obtained by the prior art. This steel sheet can be suitably used for automobile exhaust pipes and tubes that require strict workability, corrosion resistance and weldability, and has excellent industrial effects.
[Brief description of the drawings]
FIG. 1 is a graph showing the effects of the cumulative rolling reduction ratio and end temperature of rough rolling on the recrystallization rate after rough rolling in 18 wt% Cr-0.3 wt% Ti steel.

Claims (4)

C: 0.010 wt% or less, N: 0.010 wt% or less, and C + N: 0.015 wt% or less, Cr: 6 wt% or more, 25 wt% or less, Ti: 6 × (C + N) wt% or more, 0.5 wt %, With the balance being Fe and inevitable impurities slab heated to 1160 ° C or less, and rough rolling to a cumulative rolling reduction of 85% or more and final pass end temperature of 950 ° C or more The sheet bar obtained by rough rolling is held at 900 ° C. or higher for 20 seconds or longer , and then finish rolling is performed so that the cumulative rolling reduction is 85% or higher and the final pass end temperature is 800 ° C. or lower and 650 ° C. or higher. A method for producing a Ti-containing ferritic stainless steel sheet having excellent workability, wherein the coil is wound at 680 ° C. or lower and 350 ° C. or higher, and then annealed, cold-rolled and finish-annealed. The manufacturing method according to claim 1, wherein the composition of the slab further contains one or two of Ni: 1.0 wt% or less and Mo: 3.0 wt% or less in addition to the above components. The manufacturing method of the Ti containing ferritic stainless steel plate excellent in workability. 3. The method for producing a Ti-containing ferritic stainless steel sheet having excellent workability, wherein the rolling is performed at a rolling reduction of at least one pass of rough rolling at 35% or more in the production method according to claim 1 or 2. In the manufacturing method of any one of Claims 1-3, the sheet | seat bar obtained by rough rolling between the completion | finish of rough rolling and the start of finish rolling is 20 second or more in the temperature range of 900 degreeC or more. A method for producing a Ti-containing ferritic stainless steel sheet excellent in workability, characterized by holding.

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