JP7178791B2 - Steel plates for ferritic stainless steel pipes - Google Patents

Description

The present invention relates to a steel plate for ferritic stainless steel pipes.

Since ferritic stainless steel has a small thermal expansion coefficient, it has been used for automotive exhaust system parts that undergo repeated heating and cooling. In recent years, with the aim of reducing the weight of the car body, the thickness of the material steel of the parts has been reduced. Furthermore, due to the downsizing of the vehicle body itself, the mounting space for parts has become smaller along with the vehicle body, and the machining shape of exhaust system parts including steel pipes has been restricted. However, in bending, the smaller the bending radius R of the steel pipe to be processed and the thinner the steel pipe, the more likely defects such as cracks will occur.

In order to prevent cracks, it is important to suppress variations in plate thickness by performing bending while applying an appropriate pushing force (pressure) to the steel pipe from the bending machine. However, when the ratio (t/D) of the steel pipe outer diameter D to the plate thickness t becomes small, the steel pipe is likely to break due to wrinkles. Therefore, in order to prevent wrinkles from occurring, the pressure applied from the bending machine to the steel pipe is restricted, and it becomes difficult to sufficiently suppress variations in plate thickness. Therefore, the smaller the t/D of the steel pipe to be worked, the better the bendability of the steel pipe.

In addition, when the ratio (R/D) of the outer diameter D to the bending radius R of the steel pipe to be worked decreases, the absolute value of the strain applied to the steel pipe due to bending increases. That is, the thickness reduction rate increases on the outer diameter side of bending, and cracks are likely to occur. Therefore, it is required that the steel pipe to be processed has characteristics capable of suppressing an increase in the wall thickness reduction rate as the R/D of the steel pipe to be processed becomes smaller.

Exhaust manifolds, which are one of the exhaust system parts of automobiles, are switching from pressed parts to steel pipes for the purpose of improving fuel efficiency and performance. Exhaust manifolds have a shape that is particularly difficult to bend, even among exhaust pipe parts that use steel pipes.

Yuji Hashimoto, Proceedings of the Spring Meeting of the Japan Society for Technology of Plasticity, P291-292 (2004)

As mentioned above, in recent years, for the purpose of reducing the weight and size of automobiles, strict conditions such as large diameter, thin wall, and small R bending are required as processing conditions for exhaust system parts to be mounted. Steel sheets for steel pipes are required to have properties that can withstand such working conditions. Specifically, there is a demand for a material capable of suppressing the rate of thickness reduction even in large-diameter, thin-walled, small-R bending such that t/D≦0.04 and R/D≦1.2.
However, even if either t/D ≤ 0.04 or R/D ≤ 1.2 is satisfied, if the steel pipe is bent to 90° as in Non-Patent Document 1, cracking occurs or cracks occur. It is known that even without it, the thickness reduction rate exceeds 40%, which is just before cracking.
Automobile exhaust pipe parts are subjected to loads due to thermal fatigue, etc., so parts that crack even with a small load cannot be mounted. Considering these points, it is preferable to use a steel plate for steel pipes that can suppress the wall thickness reduction rate after bending to 30% or less. The actual situation is that a steel plate for steel pipes that can achieve this with diameter, thinness, and small radius bending has not yet been realized.

The present invention has been devised in view of the current situation, and is suitable for steel pipes that are bent with a large diameter, thin wall, and small radius. An object of the present invention is to provide a steel plate for a ferritic stainless steel pipe.

In order to solve the above-mentioned problems, the present inventors have found the work hardening index (n value), the average value (average r value) of the plastic strain value (r value), and the anisotropy of the r value of the steel plate for ferritic stainless steel pipes. As a result of various examinations, we have obtained the following new knowledge.

In the case of a steel pipe using a steel plate with a high average r-value, thinning is less likely to occur during bending. Therefore, the higher the average r-value, the smaller the thickness reduction rate of the steel pipe during bending, which can prevent work cracking. In particular, the r-value (r ₀ ) in the rolling direction and the r-value (r ₉₀ ) in the 90-degree direction of rolling promote the flow in the circumferential direction due to deformation during bending of the steel pipe. It is known that the fluidity greatly contributes to suppression of thinning.
However, as a result of the present inventors performing small R bending on thin and large-diameter steel pipes using various anisotropic steel plates, the r value (r ₀ ) in the rolling direction and the r value in the 90-degree direction of rolling It was found that when (r ₉₀ ) is extremely large (in-plane anisotropy Δr is large) compared to (r ₄₅ ) in the 45° rolling direction, the thickness reduction rate increases. In other words, it has been found that simply increasing the average r value is not sufficient to enhance the suppression of thinning, and it is effective to appropriately control the in-plane anisotropy Δr as well. In addition, when bending a steel pipe with a large diameter, thin wall, and small R such that t/D ≤ 0.04 and R/D ≤ 1.2, it is a reduction that can stably prevent the occurrence of cracks. As a result of investigating the anisotropy of material steel sheets with a wall thickness of less than 30%, we obtained new knowledge that the anisotropy is represented by the following formula (1).

Average r value -1.1 - 1.1 x (Δr - 0.3) ⁴ ≥ 0 (1)

Based on this new knowledge, it is possible to sufficiently reduce the wall thickness reduction rate and stably prevent the generation of cracks in large-diameter, thin-walled, small-R steel pipes such as t/D ≤ 0.04 and R/D ≤ 1.2. It is possible to bend the

The present invention has been completed based on the above findings, and the gist of the invention is as follows.

(1) C: 0.020% or less, N: 0.020% or less, Si: 0.03 to 1.20%, Mn: 0.02 to 1.61 %, P: 0.020% or less, in mass % 050% or less, S: 0.0050% or less, Ni: 0.010-0.153%, Cr: 10.0-30.0%, Mo: 0.005-2.50%, Cu: 0.005 ~2.00%, Nb: 0.001-0.800%, Ti: 0.001-0.300%, B: 0-0.005%, Al: 0-0.30%, Sn: 0- 0.12 %, V: 0-1.00%, Co: 0-0.30%, W: 0-0.05 %, Sb: 0-0.50%, REM: 0-0.20% The balance is Fe and impurities, elongation at break ≥ 28%, work hardening index (n value) ≥ 0.18, average plastic strain ratio (average r value) ≥ 1.10, rolling direction plastic strain ratio r ₀ A steel plate for ferritic stainless steel pipes, wherein ≧0.80 and 0.40≧ average r value−1.1−1.1×(Δr−0.3) ⁴ ≧ 0.09 .
(2) In mass%, B: 0.0001 to 0.005%, Al: 0.01 to 0.30%, Sn: 0.001 to 0.50%, V: 0.03 to 1.00 %, Co: 0.01 to 0.30%, W: 0.005 to 3.00%, Sb: 0.005 to 0.50%, REM: 0.001 to 0.20% one or two The steel plate for ferritic stainless steel pipes according to (1) above, characterized in that it contains at least one species.

According to the present invention, there is provided a steel plate for ferritic stainless steel pipes, which is suitable for large-diameter, thin-walled, and small-radius steel pipes that are bent, reduces the reduction in thickness during bending, and prevents work cracks. can be done. In particular, according to the steel pipe made of the steel plate for ferritic stainless steel pipe according to the present invention, even with a large diameter and thin shape such as t/D ≤ 0.04, it is possible to sufficiently suppress the reduction in wall thickness, and the occurrence of work cracks. It is possible to perform small-R bending that can prevent this.
Further, according to the present invention, by controlling the chemical composition of the steel sheet within a suitable range, it is possible to prevent not only work cracks but also wrinkles during bending.

4 is a graph showing the relationship between the Δr value, the average r value, and the maximum wall thickness reduction rate of Examples 1 to 16 and Comparative Examples 1 to 8. FIG. It is a schematic side view of a rotary draw bending machine used in the rotary draw bending test of the present example.

The chemical composition of the steel plate for ferritic stainless steel pipes (hereinafter also simply referred to as steel plate) of the present embodiment will be described. In the following description, "%" of the content of each element means % by mass unless otherwise specified.

When the content of C and N is large, it degrades formability and corrosion resistance. Therefore, the upper limits of the contents of both C and N are set to 0.020% or less. Preferably, it is 0.015% or less. On the other hand, excessively reducing the C and N contents leads to an increase in manufacturing costs, so the lower limits of both C and N are preferably 0.001% or more. More preferably, it is 0.002% or more.

Si is known as a deoxidizing element, and is contained in an amount of 0.03% or more in order to improve high-temperature oxidation resistance. Preferably, it is 0.20% or more. On the other hand, if the Si content exceeds 1.20%, the bending workability is remarkably deteriorated, leading to an increase in the thickness reduction rate and the occurrence of wrinkles. It is preferably 1.10% or less, more preferably 1.00% or less.

If a large amount of Mn is contained, the work hardening index (n value) of the steel sheet cannot be sufficiently ensured, and formability is deteriorated. Preferably, it is 1.50% or less. On the other hand, considering the refining cost, about 0.02% of Mn is inevitably mixed, so the lower limit is set to 0.02% or less. Preferably it is 0.10% or more.

Since P is an element harmful to corrosion resistance and toughness, its content should be as small as possible. Therefore, the upper limit is limited to 0.050% or less. Preferably, it is limited to 0.020% or less. On the other hand, excessive reduction of P leads to an increase in manufacturing cost, so it is preferably 0.001% or more. More preferably, it is 0.010% or more.

Since S is an element that deteriorates corrosion resistance, the smaller the content, the better. Therefore, the upper limit is limited to 0.0050% or less. On the other hand, excessive reduction of S leads to an increase in manufacturing cost, so it is preferably 0.0001% or more. More preferably, it is 0.0005 to 0.0050%.

Ni is an element that improves corrosion resistance when contained. However, the lower r-value and hardening due to the formation of the austenite phase lead to deterioration of formability, so the upper limit of the content is made 2.000% or less. Preferably, it is 0.500% or less. In addition, about 0.010% of Ni is at a level that is unavoidably mixed, so the lower limit is made 0.010% or more. Preferably, it is 0.100% or more.

Cr is an element that ensures corrosion resistance, which is a basic characteristic of stainless steel. The lower limit of the Cr content is set at 10.0% or more in consideration of the corrosion resistance required for automobile exhaust system parts. Preferably, it is 10.5% or more. On the other hand, if a large amount of Cr is contained, the moldability is lowered, so the upper limit was made 30.0% or less. From the viewpoint of workability and manufacturing cost, the content is preferably 25.0% or less, more preferably 22.0% or less. Even more preferable is 20.0% or less.

Like Cr, Mo is also an element that improves corrosion resistance. However, if it is contained excessively, there is a possibility that the formability is lowered. Therefore, considering formability and cost, the upper limit of Mo is set to 2.50% or less. It is preferably 2.20% or less, more preferably 2.00% or less. On the other hand, considering the refining cost, about 0.005% is the level of unavoidable contamination, so the lower limit of the Mo amount is set to 0.005% or more. Preferably it is 0.05% or more.

Cu improves corrosion resistance and high-temperature strength, and is effective in refining crystal grains through phase transformation. Since these effects are exhibited from 0.005%, the lower limit of the Cu content is made 0.005% or more. Preferably, it is 0.05% or more. On the other hand, an excessive Cu content makes the steel hard and lowers toughness and formability. Therefore, the upper limit of the Cu content is made 2.00% or less. It is preferably 1.80% or less, more preferably 1.50% or less.

Nb is an element that combines with C and N to form precipitates and improve formability. Also, it improves corrosion resistance, especially the corrosion resistance of welds. However, since an excessive Nb content lowers the moldability, the upper limit was made 0.800% or less. It is preferably 0.700% or less, more preferably 0.500% or less. On the other hand, considering the refining cost, about 0.001% is a level that is unavoidably mixed, so the lower limit of the Nb content is made 0.001% or more. Preferably, it is 0.050% or more.

Ti, like Nb, is an element that forms precipitates by combining with C and N to improve formability, and also improves corrosion resistance, particularly corrosion resistance of welds. However, an excessive Ti content causes deterioration in formability and defects due to inclusions, so the upper limit is made 0.300% or less. Preferably, it is 0.200% or less. On the other hand, considering the refining cost, about 0.001% is a level that is unavoidably mixed, so the lower limit of the Ti content is set to 0.001% or more. Preferably, it is 0.050% or more.

The steel plate for ferritic stainless steel pipes according to the present embodiment is composed of Fe and impurities other than the above-described elements (remainder). In addition, the "impurities" in the present embodiment are components that are mixed due to various factors in the manufacturing process including raw materials such as ores and scraps when steel is manufactured industrially, and are unavoidably mixed. Including ingredients.
Moreover, in this embodiment, the following elements can be contained as necessary. The lower limit for containing these elements is 0%.

B is an element that improves secondary workability, and suppresses secondary work cracking in various processes after expanding the steel pipe. In order to obtain the effect, the B content is desirably 0.0001% or more. More preferably, it is 0.0003% or more. Moreover, when B is contained excessively, the moldability is lowered, so the upper limit is preferably 0.005% or less.

Al is an element added mainly as a deoxidizing element, and has the effect of suppressing the peeling of oxide scale. In order to suppress peeling of oxide scale, the content is preferably 0.01% or more. From the viewpoint of further improving the effect of suppressing the peeling of oxide scale, it is more desirable to make it 0.03% or more. Moreover, when Al is contained excessively, it affects the decrease in elongation and weld penetration. In this embodiment, the upper limit of the Al content is desirably 0.30% or less in order to prevent deterioration of workability due to a decrease in elongation.

V is an element that suppresses crevice corrosion. In order to obtain this effect, it is desirable to contain 0.03% or more. Moreover, it is more desirable to contain 0.05% or more from the viewpoint of further improving the effect. Moreover, when V is contained excessively, it hardens and deteriorates formability. Therefore, it is desirable to set the upper limit of the V content to 1.00% or less.

Sn is an element that contributes to the improvement of corrosion resistance and high-temperature strength. Since this is manifested by containing 0.001% or more, it is desirable to set the lower limit of the Sn content to 0.001% or more. Preferably it is 0.005% or more. However, if the Sn content exceeds 0.50%, slab cracking may occur during steel sheet production. Therefore, it is desirable to set the upper limit of the Sn content to 0.50% or less.

Since W contributes to the improvement of corrosion resistance and high-temperature strength, it is desirable to contain 0.005% or more as necessary. However, if the content of W exceeds 3.00%, it leads to deterioration of toughness during steel sheet production, so the upper limit is preferably 3.00% or less.

Co is an element that contributes to the improvement of high-temperature strength. Since this effect is manifested when the Co content is 0.01% or more, it is desirable to set the lower limit of the Co content to 0.01% or more. However, when the Co content exceeds 0.30%, there is a possibility of causing deterioration in toughness. Therefore, it is desirable to set the upper limit to 0.30% or less.

Sb is an element that contributes to improvement of high-temperature strength. This effect is manifested by the segregation of Sb at grain boundaries, and this phenomenon occurs when the amount of Sb is 0.005% or more. Therefore, it is desirable that the lower limit of the Sb content is 0.005% or more. However, if the Sb content exceeds 0.50%, weld cracking occurs due to excessive segregation, so the upper limit is preferably 0.50% or less.

REM (rare earth elements) is a generic term for two elements, scandium (Sc) and yttrium (Y), and fifteen elements (lanthanides), from lanthanum (La) to lutetium (Lu), according to a general definition.
REM contributes to the improvement of oxidation resistance, and may be contained in an amount of 0.001% or more, if necessary. However, if the content of REM exceeds 0.20%, the sulfide of REM may lower the corrosion resistance, so the upper limit is preferably 0.20% or less. When REM is contained, it may be a single element in the group of elements described above or a mixture of two or more elements.

The steel plate for ferritic stainless steel pipes of the present embodiment may contain elements other than those described above as long as the effects of the present invention are not impaired.

Next, the elongation, work hardening index (n value), and plastic strain ratio (r value) of the steel plate for ferritic stainless steel pipes according to this embodiment will be described.

In the present embodiment, it is important to control the plastic strain ratio (r value) of the steel plate as the raw material in order to reduce the thickness reduction rate when bending the steel pipe. Specifically, the average r value and the rolling direction r value (r ₀ ) are controlled within appropriate ranges.

An average r value of 1.10 or more is necessary to reduce the rate of thickness reduction and prevent cracking during work, so this was made the lower limit. In order to stably reduce the thickness reduction rate and further suppress work cracks, it is preferable to set the average r value to 1.15 or more. Note that the upper limit of the average r-value is not particularly set, and may be determined appropriately in consideration of the average r-value of ferritic stainless steel sheets that can be manufactured with general manufacturing equipment, devices, etc. For example, 3.00 The following may be set as the upper limit.
Note that the average r value can be obtained by the following formula (3).

Average r value=(r ₀ +2r ₄₅ +r ₉₀ )/4 (3)
However, in the formula (3), _r0 indicates the r value in the rolling direction, _r90 indicates the r value in the direction perpendicular to the rolling direction, and r45 indicates the r value in the ₄₅ ° rolling direction.

Furthermore, in order to reduce the thickness reduction rate to less than 30%, it is necessary to set _r0 to 0.80 or more, so this was made the lower limit. In order to stably reduce the thickness reduction rate and further suppress working cracks, r ₀ is preferably 0.90 or more. The upper limit of _r0 is not particularly set, but it is preferable that the upper limit of _r0 is 4.00 or less in order to prevent the anisotropy Δr from becoming extremely large. More preferably, 1.10≦r ₀ ≦3.00.

The method for controlling the average r value and the r value (r ₀ ) in the rolling direction is not particularly limited, but may be controlled within the above ranges by adjusting the conditions of the steel sheet manufacturing process. For example, the average r-value and the r-value in the rolling direction can be controlled by properly setting the manufacturing conditions such as setting the rolling reduction in cold rolling and the final annealing temperature to appropriate values.

Further, in the present embodiment, the elongation of the steel sheet is set to 28% or more because it affects the rate of reduction in thickness and the formation of wrinkles during bending after forming the steel pipe. Preferably it is 30% or more.

Further, when the steel plate has an n value of 0.18 or more, the thickness reduction rate of the steel pipe during bending can be suppressed, so the steel plate has an n value of 0.18 or more. Preferably it is 0.20 or more.

Also, in this embodiment, it is important to appropriately control the in-plane anisotropy Δr and the average r value of the steel sheet.
In general, it is known that the r value (r ₀ ) in the rolling direction and the r value (r ₉₀ ) in the 90° rolling direction greatly contribute to suppression of thinning. However, as described above, the present inventors compared the r-value (r ₀ ) in the rolling direction and the r-value (r ₉₀ ) in the 90-degree rolling direction with the r-value (r ₄₅ ) in the 45-degree rolling direction. It has been found that, conversely, when the in-plane anisotropy Δr is extremely large (the in-plane anisotropy Δr is large), the thickness reduction rate increases. That is, in order to suppress the reduction rate and prevent work cracking, it is not enough to increase the average r value, and it is important to balance r ₀ , r ₄₅ and r ₉₀ to prevent an increase in Δr. is. Specifically, by controlling the average r value and Δr so as to satisfy the following formula (1), a large diameter, thin wall, and small R such as t / D ≤ 0.04 and R / D ≤ 1.2 It is possible to stably prevent the occurrence of cracks when bending a steel pipe.
Here, the in-plane anisotropy Δr is calculated by the following formula (2).

Average r value -1.1 - 1.1 x (Δr - 0.3) ⁴ ≥ 0 (1)
Δr=(r ₀ +r ₉₀ −2r ₄₅ )/2 (2)
However, in the formulas (1) and (2), _r0 indicates the r value in the rolling direction, _r90 indicates the r value in the direction perpendicular to the rolling direction, and r45 indicates the r value in the ₄₅ ° rolling direction.

In order to satisfy the above formula (1) in the steel sheet, r ₀ , r ₄₅ and r ₉₀ and the average r value may be controlled by appropriately adjusting the conditions of the steel sheet manufacturing process. Conditional factors that affect each r value include, for example, the heating temperature and rolling reduction during rolling, and the annealing temperature. By appropriately adjusting such manufacturing conditions, a steel sheet that satisfies the formula (1) can be manufactured.
Preferred manufacturing methods and conditions for the steel sheet according to the present embodiment will be described below, but these are merely examples, and the present invention is not limited thereto.

In manufacturing the steel sheet of the present embodiment, steelmaking, hot rolling, hot-rolled sheet annealing, cold rolling, and cold-rolled sheet annealing (finish annealing) may be sequentially performed.
The steel slab after steelmaking is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness by continuous rolling. The heating temperature at this time is desirably 1150 to 1250° C. from the viewpoint of increasing the average r value, suppressing an increase in the in-plane anisotropy Δr, and satisfying the formula (1).
After hot rolling, hot-rolled sheet annealing and pickling may be performed, and the hot-rolled sheet annealing step may be omitted.
Cold rolling after hot rolling may be carried out by either a normal Sendzimir mill or a tandem mill. In the cold rolling, the conditions such as the rolling reduction, the number of rolling passes, the rolling speed, and the rolling temperature may be appropriately selected and set so as to satisfy each configuration and each condition of the steel sheet of the present embodiment. For example, since the rolling reduction is a factor that affects the r value, it is desirable to adjust the average r value and the formula (1) so that they fall within the proper ranges.
After cold rolling, cold-rolled sheet annealing (finish annealing) may be performed, but the final annealing temperature is preferably 850 to 1050° C. from the viewpoint of increasing the r value due to coarsening of grains. The finish annealing may be bright annealing in which annealing is performed in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, if necessary, or may be annealing in the atmosphere.

By the manufacturing method described above, the steel plate for ferritic stainless steel pipes according to the present embodiment can be manufactured.

According to the steel plate for ferritic stainless steel pipes of the present embodiment described above, it can be suitably used for steel pipes that are bent with a large diameter, thin wall, and small radius, and the thickness reduction rate during bending of the steel pipe can be reduced. , processing cracks can be stably prevented.

Examples of the present invention will be described below, but the conditions in the examples are examples of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is used in the following examples. It is not limited to the conditions Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

Stainless steels (Steel Nos. 1 to 23) having chemical compositions shown in Table 1 were melted to produce slabs. After that, using the obtained slabs, Examples 1 to 16 and Comparative Examples 9 to 15 shown in Table 2 were hot-rolled at a heating temperature of 1150 to 1250 ° C., and hot-rolled to a thickness of 5.0 mm. A steel plate was manufactured. After the hot-rolled steel sheet was annealed, it was cold-rolled to a thickness of 5.0 mm to 1.2 mm and subjected to finish annealing at 850 to 1050° C. to prepare a test material.

On the other hand, in Comparative Examples 1 to 8, the following manufacturing conditions were changed from the above conditions.
In Comparative Examples 1 to 4 shown in Table 2, the thickness of the hot-rolled steel sheet was set to 4.0 mm and then cold-rolled to 1.2 mm. In Comparative Examples 5 to 8 shown in Table 2, hot rolling was performed at a heating temperature of 1100° C. to produce hot rolled steel sheets.

Next, the mechanical properties of the obtained test materials (stainless steel plates; Examples 1 to 16 and Comparative Examples 1 to 15) were evaluated by the following test methods.

A JIS No. 13B tensile test piece was taken from the test material, and a normal temperature tensile test was performed in accordance with JIS Z 2241, and the normal temperature elongation (El) and work hardening index (n value) of the steel plate, plastic strain ratio (r value). evaluated.

The work hardening index (n value) is obtained by fitting the value of the true stress-true strain curve in the range where uniform strain occurs to the following Swift formula (formula (4)) using the least squares method. n values were derived.
σ=C(ε+k) ⁿ (4)
In the equation (3), σ is true stress, ε is true strain, and C and k are constants.

For the plastic strain ratio (r value), JIS 13B tensile test pieces were taken from the test material in the rolling direction, the 45 degree rolling direction, and the 90 degree rolling direction, and a plastic strain ratio test was performed in accordance with JIS Z 2254. gone. The ratio of the strain in the width direction to the thickness strain of the test piece was measured when the elongation was 15%.
Table 2 shows the test results.

The larger the n-value of the steel sheet, the better the strain propagation property, so local deformation is suppressed. Since thinning of the steel pipe occurs locally in the bending of the steel pipe, the greater the n value of the steel plate, the more it contributes to the suppression of thinning of the steel pipe.

Next, a pipe bending test was performed to evaluate the rate of wall thickness reduction and the occurrence of wrinkles.
Specifically, steel sheets with a thickness of 1.2 mm (Examples 1 to 16, Comparative Examples 1 to 15) exhibiting the components shown in Table 1 and the mechanical properties shown in Table 2 were prepared by controlling the forming conditions and the post-tube straightening amount. Then, an electric resistance welded pipe (t/D=0.028) with an outer diameter D=φ42.7 mm was produced. This steel pipe was bent 90 degrees with a bending radius R of 45 mm using a rotary drawing and bending machine as shown in FIG.
The back booster pushing force F was set to a value at which a pushing force 0.4 times the 0.2% proof stress of the steel pipe acts on the cross section of the steel pipe. The movement speed of the pressure die was constant, and the movement amount L of the pressure die at the end of bending was set to 1.18 times the movement amount (R×π/2) of the central portion of the steel pipe.
The maximum thickness reduction rate was obtained at the position where the plate thickness t decrease on the outer side of the bending was maximum, and the presence or absence of wrinkles was evaluated visually and by touch. Those with a maximum thickness reduction rate of less than 30% were evaluated as pass (◯).

Examples 1 to 3 satisfying average r value ≤ 3.0, elongation El ≥ 28%, n value ≥ 0.18, average r value -1.1-1.1 × (Δr-0.3) ⁴ ≥ 0 No. 10 had a maximum thickness reduction rate of less than 30%, and could be bent without cracks or wrinkles. In addition, in Examples 11 to 16, although wrinkles occurred, the maximum thickness reduction rate was less than 30%, cracks did not occur, and relatively good bending could be performed.
FIG. 1 shows a graph showing the relationship between the Δr value, the average r value, and the maximum thickness reduction rate of Examples 1 to 16 and Comparative Examples 1 to 8. From this graph, the examples satisfying the average r value ≤ 3.0 and the average r value -1.1 - 1.1 x (Δr - 0.3) ⁴ ≥ 0 achieved a maximum thickness reduction rate of less than 30%. It can be seen that the moldability is excellent.

Average r value ≤ 3.0 and average r value -1.1 - 1.1 × (Δr - 0.3) ⁴ ≥ 0 were not satisfied. there were. At such a maximum thickness reduction rate, defects such as cracks may occur. Furthermore, wrinkles were generated on the inner diameter side of the bend. When wrinkles occur, a load is applied to the mold, possibly shortening the life of the mold.

Comparative Examples 3 and 4, which did not satisfy r ₀ ≧0.8 and average r value −1.1−1.1×(Δr−0.3) ⁴ ≧0, had a maximum thickness reduction rate of 30% or more. . At such a maximum thickness reduction rate, defects such as cracks may occur. Furthermore, wrinkles were generated on the inner diameter side of the bend. When wrinkles occur, a load is applied to the mold, possibly shortening the life of the mold.

Comparative Examples 5 to 8, which did not satisfy the average r value -1.1 - 1.1 x (Δr - 0.3) ⁴ ≥ 0, had a maximum thickness reduction rate of 30% or more. At such a maximum thickness reduction rate, defects such as cracks may occur.

Steel No. outside the composition range of the present invention. In Comparative Examples 9 to 15 using Nos. 11 to 17, the maximum wall thickness reduction rate exceeded 30%.
Further, in Comparative Examples 13 to 17 in which Mo, Cu, Nb, Ti and Al were out of range, the elongation El of the steel sheet was less than 28%.

TECHNICAL FIELD The present invention relates to steel sheets for ferritic stainless steel pipes, which are used in applications such as automotive exhaust system parts that require severe bending, high-temperature oxidation, and thermal fatigue. It prevents cracks and wrinkles in bending with a large diameter of ≦1.2, a thin wall, and a small radius.
INDUSTRIAL APPLICABILITY As described above, the present invention provides a ferritic stainless steel welded pipe with excellent formability that does not crack or has a small wall thickness reduction rate under the severe steel pipe bending conditions required for automobile exhaust systems. It becomes possible to do things, and its industrial value is great.

Claims (2)

In % by mass,
C: 0.020% or less,
N: 0.020% or less,
Si: 0.03 to 1.20%,
Mn: 0.02-1.61 %,
P: 0.050% or less,
S: 0.0050% or less,
Ni: 0.010 to 0.153%,
Cr: 10.0 to 30.0%,
Mo: 0.005-2.50%,
Cu: 0.005 to 2.00%,
Nb: 0.001 to 0.800%,
Ti: 0.001 to 0.300%,
B: 0 to 0.005%,
Al: 0-0.30%,
Sn: 0-0.12 %,
V: 0 to 1.00%,
Co: 0-0.30%,
W: 0 to 0.05 %,
Sb: 0 to 0.50%,
REM: 0-0.20%
and the balance consists of Fe and impurities,
Elongation at break ≧28%,
Work hardening index (n value) ≥ 0.18,
Average plastic strain ratio (average r value) ≥ 1.10,
rolling direction plastic strain ratio r0≧0.80,
A steel plate for a ferritic stainless steel pipe, wherein 0.40≧ average r value−1.1−1.1×(in-plane anisotropy Δr−0.3) ⁴ ≧ 0.09 . In % by mass,
B: 0.0001 to 0.005%,
Al: 0.01 to 0.30%,
Sn: 0.001 to 0.50%,
V: 0.03 to 1.00%,
Co: 0.01 to 0.30%,
W: 0.005 to 3.00%,
Sb: 0.005 to 0.50%,
REM: The steel sheet for ferritic stainless steel pipes according to claim 1, characterized by containing one or more of 0.001 to 0.20%.

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