JPS6356290B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a hot-rolled sheet of ferritic stainless steel, and more specifically to an improvement in an annealing method for hot-rolled steel strip. Generally, ferritic stainless steel such as SUS430, which is typified by 17% Cr steel, has a (α+γ) two-phase mixed structure in the hot rolling temperature range, and the γ phase transforms during the cooling process after hot rolling. This γ
When the cooling rate is low, the phase transforms to pearlite (α+ carbide), when the cooling rate is extremely high, it transforms to martensite, and at intermediate cooling rates, it transforms to a bainitic intermediate stage structure. In other words, as the cooling rate increases, γ
The phase transforms into a highly hard structure. After hot rolling, the hot rolled steel strip wound into a coil is normally left to cool in the atmosphere and brought to room temperature, but the cooling rate during this period is sufficiently slow for most hot rolled steel strips. , the γ phase formed in the hot rolling temperature range transforms into soft pearlite. However, since both edges of the coil and the inner and outer winding parts (the beginning and end of winding) are in direct contact with the atmosphere, the cooling rate is faster than that at the center of the coil, causing the intermediate stage structure to deteriorate. Transform into a site. Therefore, both edges of the coil and the inner and outer windings have higher hardness than the center of the coil. A hot-rolled steel strip in which both edges of the coil and the inner and outer windings have high hardness is not suitable for cold rolling without annealing. In other words, when cold rolling a steel strip that includes a highly hard coil edge and inner and outer circumferences, not only does the rolling load increase, resulting in deterioration of cold rolling workability, but also the unevenness of hardness results in poor rolling after cold rolling. This increases plate thickness variation and, furthermore, because the coil edge portions on which tension acts during cold rolling are hardened, cracks are likely to occur, and cold rolling often has to be stopped midway. For this reason, hot-rolled steel strips are usually subjected to bell-shaped furnace annealing at a temperature in the range of 700 to 900°C for several hours to more than ten hours before being introduced into a cold rolling process.
In other words, the hard structure containing bainite and martensite is reheated to become a soft structure consisting of α+ carbides, and at the same time uniformity within the coil is achieved to create a hot rolled steel strip suitable for cold rolling. It is. However, annealing in a bell furnace involves reheating the once cooled steel strip to a high temperature of 700 to 900°C, which not only requires a large amount of energy, but also takes time to raise and lower the temperature. The actual annealing time requires a long time of one to several days. As described above, annealing hot-rolled ferritic stainless steel strips in a bell-type furnace is sufficiently effective and industrially stable, but it has the problem of requiring a large amount of energy and time. It is clear that there would be a great advantage if the annealing in the bell furnace could be omitted, and the inventors have considered omitting the annealing in the bell furnace. The point that the present inventors focused on is that the hardness of ferritic stainless steel hot rolled steel strip is
During cooling after winding, only the coil edges and the inner and outer windings cool at a high rate, while the center of the coil transforms into a pearlite phase due to extremely slow cooling.
The point is that it is sufficiently soft to allow cooling rolling. In other words, by reheating the coil immediately after winding, the cooling of the coil edge and the inner and outer turns can be delayed so that the transformation to pearlite, which is a soft transformation product, is completed. If only the inner and outer turns are softened, a hot-rolled steel strip that is sufficiently cold-rollable should be obtained. From the above points of view, the present researchers conducted many experiments and researched a method for producing hot rolled steel strip that is soft and can be sufficiently cold rolled without annealing in a bell furnace. I found out. That is, according to the present invention, in the method for manufacturing a hot rolled ferritic stainless steel strip, the hot rolled steel strip is wound into a coil at a temperature of 550 to 950°C after normal hot rolling. Within 20 minutes after finishing
Provided is a method for producing a hot-rolled steel strip in which the entire steel strip including the coil edge and inner and outer winding portions is softened, the method comprising holding the steel strip at a temperature of 650 to 850° C. for 10 to 30 minutes and then allowing it to cool. It will be well understood by those skilled in the art that the method of the present invention can be applied to ferritic stainless steel in general, which undergoes γ phase transformation during cooling from hot rolling temperature. However, in order to apply this method and obtain sufficient effects, the desirable materials are:
C: 0.005-0.15%, Si: 1.0% or less, Mn: 2.0%
The following steel contains 14.0 to 20.0% Cr, 0.10% or less N, and the balance is Fe and unavoidable impurities. The method of the present invention can be applied even if the composition deviates somewhat from this composition, or even if a metal element, so-called a minor additive component, is added, as long as the γ phase transformation is not essentially affected. Next, the method of the present invention will be explained in detail based on experiments. Figure 1 shows C: 0.068%, Si: 0.53%, Mn:
0.26%, Cr: 16.51%, N: 0.013% hot-rolled SUS430 steel plate was heated at 1000℃, kept at constant temperature at 600-900℃, and then rapidly cooled, showing the change in hardness with constant temperature holding time. It is something. As the constant temperature holding time increases, the amount of transformation into soft pearlite increases, resulting in a decrease in hardness. Especially from 650
When kept at a constant temperature of 850°C, most of the γ phase transforms into pearlite and exhibits low hardness after a holding time of 10 to 30 minutes. The above experimental results show that the steel strip after hot rolling is 650~850
This indicates that most of the γ phase transforms into pearlite if held at ℃ for 10 to 30 minutes, and even in the coil edge and inner and outer winding parts, where the cooling rate is relatively high, it is possible to reheat immediately after winding and transform the upper part into pearlite. This shows that it is possible to obtain a sufficiently softened steel strip if the conditions listed above are satisfied. Reheat the coil immediately after winding to heat the coil edges and inner and outer windings to 650℃.
The energy and time required to hold it at ~850°C for 10-30 minutes is extremely low. This is because the hot-rolled steel strip immediately after coiling originally has a temperature of 500°C or higher, and furthermore, the coil edges and inner and outer winding parts, which have a high cooling rate, are heated from the outside when heated. It is easy to take in heat, and it takes until reheating after winding.
Even if the temperature drops below 650℃, it can be used within a short period of time.
This is because it can be reheated to 650°C or higher. That is, in the method for producing a hot-rolled ferritic stainless steel strip according to the present invention, a hot-rolled steel strip that has been subjected to normal hot rolling and coiled at 550 to 950°C is preferably heated within 20 minutes after the completion of coiling. is charged into a heating furnace within 10 minutes, held at 650-850°C for 10-30 minutes, and then taken out from the heating furnace and allowed to cool. By performing the reheating described above, the cooling of the coil edge and the inner and outer windings is delayed, and during this time, most of the γ phase generated during hot rolling transforms into pearlite, making it soft and cold rolling sufficient. A possible hot rolled steel strip is obtained. Note that the heating furnace used for reheating does not require a large amount of heat or time, so a simple one such as a tunnel furnace is sufficient. It may be installed on the connected conveyor. Next, in the present invention, the reason why the preferred range of ingredients of the material is limited will be described. C is an austenite-forming element and is an element that increases the hardness of the hot rolled steel strip, but if it is less than 0.005%, the hardness of the hot rolled steel strip is sufficiently low even without the reheating of the present invention; If it exceeds this, it will be impossible to sufficiently soften it even if the reheating of the present invention is performed, so the content is limited to 0.005 to 0.15%. Si is a ferrite-forming element that reduces the amount of γ phase, but excessive addition impairs ductility and toughness, so it was limited to 1% or less. Mn improves toughness, but is an austenite-forming element that increases the amount of γ phase and if added in excess, sufficient softening cannot be obtained even with the reheating of the present invention. % is allowed. Cr is an important element in stainless steel that provides corrosion resistance and oxidation resistance, but it is not directly related to the purpose of the present invention and is used more widely than the SUS430 standard.
It can be included in the range of 14.0-20%. 14.0
If it is less than 20.0%, both properties will deteriorate significantly, and if it exceeds 20.0%, workability will deteriorate. N has the same effect as C, but if it exceeds 0.10%, sufficient softening cannot be obtained even if the reheating of the present invention is performed, so the content is set to 0.10% or less. P, S, Ni, O, etc. are present in the amounts normally contained in this type of steel and pose no problem. Next, the reason for the limitations on heating conditions will be described.
The coiling temperature after hot rolling was set at 550 to 950℃.
This is because if the temperature is lower than 550°C, a large amount of heating and time will be required to complete the transformation of the γ phase into pearlite. Although it is preferable that the winding temperature be high, if it exceeds 950°C, undesirable results such as coarsening of ferrite grains will occur. The reheating temperature was limited to 650 to 850℃.
At temperatures below 850°C and below 850°C, the rate of transformation of the γ phase to pearlite is slow, and reheating in the range of 650 to 850°C completes the pearlite transformation within a short time and softens the steel strip. This is because it can be achieved. The reason for limiting the start of reheating after winding to within 20 minutes after winding is that if a coil is left to cool in the atmosphere for more than 20 minutes after winding, the temperature of the coil edge and inner and outer windings will be below 500°C, making it impossible to reheat the coil. Heating requires a large amount of heat and a large amount of time. As a result, the reheating furnace that can accommodate the hot rolling pitch becomes so large that it loses its economic significance. Figure 2 shows a plate thickness of 3.6mm.
The figure shows the temperature change at the coil edge and outer winding part when the coil is hot-rolled to a width of 1000 mm and rolled at 730°C with an outer diameter of 1500 mm.The figure shows the temperature change at the coil edge and outer winding part when the coil is left to cool in the atmosphere after winding. After winding, the temperature drops to 500°C after being left to cool in the air for 20 minutes. I repeat,
The main purpose of the present invention is to soften the coil edge and the inner and outer windings by heating for a short time.However, the temperature of the coil decreases significantly before reheating, and reheating requires a large amount of heat and takes a long time. If time is required, the industrial meaning is lost. For this reason, we limited the temperature of the coil at the start of reheating to 500°C or higher using a normal coil within 20 minutes after winding. The reason why we limited the reheating time to 10 to 30 minutes is because, as shown in Figure 1, when heating at 650 to 850℃,
This is because heating for less than 30 minutes does not result in a sufficient reduction in hardness, and heating for more than 30 minutes does not result in any further significant reduction in hardness. In addition, from the viewpoint of the capacity of the reheating furnace, the shorter the heating time, the more preferable. Next, the effects of the present invention will be explained with reference to Examples and Comparative Examples of the present invention. Figure 3 shows C: 0.063%, Si: 0.55
%, Mn: 0.26%, Cr: 16.42%, N: 0.023%. (Rolling diameter 1500 mm) Comparative example 1 (Fig. 3-a) without reheating and Example of the present invention (Fig. 3-b and c) in which the hot rolled steel strip was reheated after winding. It is a figure showing hardness. The experimental conditions are as follows. Comparative Example 1: After being wound at 650°C, it was allowed to cool in the atmosphere. Example 1 of the present invention: After being wound at 700°C, it was charged into a heating furnace 8 minutes later, and reheated so that the coil edge was maintained at 720°C for 20 minutes, and then allowed to cool in the atmosphere. Example 2 of the present invention: After being wound at 710°C, it was charged into a heating furnace 7 minutes later, and reheated so that the coil edge was maintained at 680°C for 20 minutes, and then allowed to cool in the atmosphere. In Comparative Example 1, the hardness of the central part in the width direction of the coil is approximately Hv205, but the hardness of the coil edges and the inner and outer winding parts is Hv240-300, which is not suitable for cold rolling as it is. be.
However, in Examples 1 and 2, significant softening was obtained at the coil edges and the inner and outer turns, and the entire coil became Hv200 or less, making it possible to cold-roll the coil as it is without annealing in a bell furnace. Table 1 shows the hardness and tensile properties of the hot rolled steel strips of Examples 1 and 2 of the present invention and Comparative Example 1 shown in FIG.
The hardness and tensile properties of an annealed steel strip annealed for ×10 hours are shown. In the hot-rolled steel strip of Comparative Example 1, the elongation was less than 20% at any position and the ductility was low, but in Examples 1 and 2 of the present invention, a significant decrease in strength and improvement in ductility were observed, and the coil edge and inner and outer The rolled part also shows an elongation of over 20%. The annealed steel strip obtained by annealing the hot-rolled steel strip of Comparative Example 1 in a bell-shaped furnace has a Vickers hardness of Hv162 to 163, and an elongation of nearly 30%. It is impossible to expect a rolled steel strip to have exactly the same hardness and ductility as an annealed steel strip that has been heated for a long time.
However, the softening obtained by the present invention is sufficient for cold rolling. Table 2 shows Examples 1 and 2 of the present invention and Comparative Example 1.
2 is a table showing the hardness and tensile properties of a 0.4 mm thick cold-rolled plate product produced by two cold rolling processes. The outline of the process is as follows. Examples 1 and 2 of the present invention Hot rolled steel strip 7 to 8 minutes → Reheating (680 to 720℃×
20 minutes) → Cold rolling (3.6mm → 1.0mm) → Intermediate annealing →
Cold rolled (1.0mm → 0.4mm) → Finish annealing comparison example 1 Hot rolled steel strip → Bell-shaped furnace annealing (800℃ x 10 hours)
→ Cold rolling (3.6mm → 1.0mm) → Intermediate annealing → Cold rolling (1.0mm → 0.4mm) → Finish annealing Intermediate annealing is 850℃ x 1 minute, finish annealing is 810℃ x 1 minute. And so. As is clear from the above steps, in Examples 1 and 2 of the present invention, the annealing step in the bell-shaped furnace is omitted and the steps are shortened. There is no difference in mechanical properties between the examples of the present invention and the comparative examples, and it is clear that the method of the present invention can produce products comparable to those of the conventional method. In addition, in the cold rolling in Examples 1 and 2 of the present invention, the cold rolling could be carried out in the same manner as when cold rolling an annealed steel strip annealed in a bell-shaped furnace, without causing any particular operational problems. Figure 4 shows C: 0.112%, Si: 0.38%, Mn:
In hot rolling of SUS430 steel with chemical components of 0.68%, Cr: 16.42%, and N: 0.031%, normal hot rolling was performed to obtain a plate thickness of 3.6 mm and width of 1000 mm, and after coiling (rolling diameter 1500mm) Comparative Example 2 without reheating (Figure 4-a) and Example 3 of the present invention with reheating after winding
(FIG. 4-b) is a diagram showing the hardness at each position of the hot rolled steel strip. The experimental conditions are as follows. Comparative Example 2: After being wound at 670°C, it was allowed to cool in the atmosphere. Example 3 of the present invention: After being wound at 710°C, it was charged into a heating furnace 7 minutes later, and reheated so that the coil edge was maintained at 720°C for 20 minutes, and then allowed to cool in the atmosphere. The hot rolled steel strip of Comparative Example 2 has a higher content of austenite-forming elements than the chemical composition shown in Figure 3, and the coil edges and inner and outer turns are
High hardness of Hv300-360. However, in Example 3 of the present invention, by reheating, the hardness of the coil edge and the inner and outer windings decreased, and the entire steel strip was softened to approximately Hv220 or less, making it a hot rolled steel that can be cold rolled. Obi is obtained.

【table】

[Table] The hot-rolled steel strip obtained by the production method of the present invention is not only used as a material for producing cold-rolled sheets, but also exhibits good ductility with an elongation of 20% or more. Of course, it is also possible to use the ferritic stainless steel as it is as a hot-rolled steel sheet.

[Brief explanation of the drawing]

Figure 1 shows the change in hardness with constant temperature holding time, where (▲) indicates 600℃, (〇) indicates 650℃,
(△) mark is 750℃, (●) mark is 850℃, (▽) mark is 900℃
Each isothermal temperature in °C is shown. FIG. 2 is a diagram showing temperature changes at the coil edge portion and outer winding portion of the coil which was left to cool in the atmosphere after winding. Figures 3 and 4 are diagrams showing the hardness distribution of hot rolled steel strips, where Figure 3-a is Comparative Example 1, Figure 3-b is Invention Example 1, and Figure 3-c is Example 2 of the present invention, Fig. 4-a shows Comparative Example 2, and Fig. 4-b shows Example 3 of the present invention, where the (〇) mark indicates the inner winding part of the steel strip, and the (△) mark indicates the inner winding part of the steel strip. The center winding part of the steel strip and the symbol (●) indicate the outer winding part of the steel strip, respectively.

Claims (1)

[Claims] 1. In a method for producing a hot rolled ferritic stainless steel strip, after finishing normal hot rolling,
A hot-rolled steel strip wound into a coil at a temperature of 550 to 950℃ is heated to a temperature of 650 to 850℃ within 20 minutes after winding is completed.
A method for producing a hot-rolled steel strip in which the entire steel strip including the coil edge and inner and outer winding portions is softened, the method comprising holding the strip for 10 to 30 minutes and then allowing it to cool. 2. A method for producing a hot rolled steel strip according to claim 1, wherein the composition of the ferritic stainless steel is C: 0.005 to 0.15%, Si: 1.0% or less,
A method characterized in that it contains Mn: 20% or less, Cr: 14.0 to 20.0%, N: 0.10% or less, and the balance consists of Fe and inevitable impurities.