CN1317414C - Ferritic steel sheet with improved formability, high-temperature strength, high-temperature oxidation resistance, and low-temperature toughness - Google Patents

Abstract

The invention provides an inexpensive steel sheet having both "formability" and "high-temperature strength, high-temperature oxidation resistance, and low-temperature toughness". In mass%, C: 0.02% or less, Si: 0.7 to 1.1%, Mn: 0.8% or less, Ni: 0.5% or less, Cr: 8.0 to less than 11.0%, N: 0.02% or less, Nb: 0.10 to 0.50%, Ti: 0.07 to 0.25%, Cu: 0.02-0.5%, B: 0.0005 to 0.02%, V: 0 (not added) to 0.20%, one or 2 of Ca and Mg: total of 0 (not added) to 0.01%, 1 or more elements among Y and REM: 0 to 0.20% in total, and the balance of Fe and inevitable impurities, and satisfies the requirements of 3Cr +40Si ≥ 61, Cr +10Si ≤ 21, 420C-11.5Si +7Mn +23Ni-11.5Cr-12Mo +9Cu-49Ti-25(Nb + V) -52Al +470N +189 ≤ 70.

Description

Ferritic steel sheet with improved formability, high-temperature strength, high-temperature oxidation resistance, and low-temperature toughness

technical field

The present invention relates to a steel plate suitable for an exhaust passage member of a motor vehicle engine that can be used in a high temperature environment of 800 to 900°C, and relates to improved formability, high temperature strength, and high temperature resistance at the same time, such as deep drawability, stretch formability, etc. Ferritic steel sheet with oxidation resistance and low temperature toughness.

technical background

Compared with austenitic stainless steel, ferritic stainless steel has a smaller coefficient of thermal expansion and excellent thermal fatigue characteristics and high-temperature oxidation characteristics, so it is used in heat-resistant applications where thermal deformation is a problem. As a typical application, for example, exhaust gas Vehicle engine exhaust path components such as manifolds (hereinafter referred to as "exhaust pipes"), front pipes, catalyst carrier outer cylinders, intermediate pipes, mufflers, and tail pipes.

In recent motor vehicle engines, aiming at improving exhaust gas purification efficiency and output, the temperature of exhaust gas tends to rise, and high heat resistance is required for components close to the engine, such as exhaust pipes, front pipes, and catalyst carrier outer cylinders. Properties (high temperature strength, high temperature oxidation resistance). In addition, in recent years, the shape of exhaust path members tends to become more complicated. In particular, exhaust pipes and catalyst carrier outer cylinders are complicated to be formed by various methods such as mechanical stamping, servo stamping, spinning, and hydroforming. shape. For this reason, it is not enough for the materials used for them to have only good tensile elongation and bendability, but also require excellent formability represented by deep drawability and stretch formability, and also require in-plane processing properties. Anisotropy is small. In addition, since it is necessary to consider the prevention of ductile fracture and brittle fracture in secondary processing and tertiary processing, low-temperature toughness must also be excellent. In addition, when the shape is complex, thermal deformation tends to concentrate in one place and cause thermal fatigue damage when the engine is started and stopped. At the same time, abnormal oxidation is likely to occur due to local material temperature rise, so it is not possible to improve formability and Low temperature toughness at the expense of heat resistance.

SUH409L and SUS430J1L are well-known ferritic stainless steels with high heat resistance. SUH409L is excellent in workability and low-temperature toughness, and is often used for exhaust path members. However, when considering its heat resistance level, it is not ideal for use in applications where the temperature of the material exceeds 800°C. In addition, there is no sufficient deep drawability available for complex shaped members. SUS430J1L has excellent heat resistance that can be used at 900°C. However, it is hard and poor in formability.

Therefore, the following heat-resistant ferritic steels have been developed.

Patent Document 1 below discloses a ferritic heat-resistant stainless steel having a Cr level of 17.0 to 25.0%. In this steel, the high-temperature strength is improved by the combined addition of Mo and Cu, and scale peeling is suppressed by the addition of Mn. In addition, the reduction of the impact value caused by Mo is overcome to some extent by the composite addition of Cu and Ni. However, they do not have formability sufficient to cope with complex-shaped exhaust path members. Due to the high Cr level, it is disadvantageous in cost.

Patent Document 2 discloses a ferritic stainless steel that exhibits heat resistance equal to or higher than that of an 18% Cr system in a 13% Cr system and has improved high-temperature salt damage corrosion resistance. It improves high-temperature strength by ensuring solid solution of Nb, improves high-temperature oxidation characteristics by adding a large amount of Mn and Si, and improves high-temperature salt damage corrosion resistance by this Si. However, no special consideration is given to formability and low-temperature toughness, and the above-mentioned strict requirements in recent years cannot be fully satisfied.

Patent Document 3 discloses a ferritic heat-resistant stainless steel containing Nb having a Cr level of 11.0 to 15.5%, in which high-temperature oxidation resistance and scale adhesion are improved. These characteristics are remarkably improved by strictly limiting Mn/Si in the range of 0.7 to 1.5. In addition, a method of improving low-temperature toughness and workability by adding Cu is taught. For example, data showing no cracking in the adhesive bending test is shown for workability. However, in view of the fact that the requirements for the shape of the exhaust path member are becoming stricter, these materials are required to have excellent formability that can sufficiently cope with various forming methods (above). In this regard, there is no steel in Patent Document 3 that focuses on draw formability such as deep drawability, and it cannot be said that it can sufficiently satisfy recent strict requirements. In addition, the Cr level is 11.0% or more which is required to contain stainless steel, and in exhaust path members that do not require "stainless steel", it is more desirable to reduce the cost by lowering Cr.

Patent Document 4 discloses a ferritic stainless steel for an exhaust pipe containing 11 to 14% of Cr. In this steel, Si is actively added to Nb-containing steel to increase the high-temperature strength. Its heat resistance is considered to be the same as that of the steel of Patent Document 3. However, no consideration has been given to improving the formability and low-temperature toughness beyond existing ones, and this steel cannot be said to sufficiently satisfy the recent stringent requirements. In addition, the level of Cr is desired to be further reduced.

Patent Document 5 discloses a ferritic heat-resistant steel for engine exhaust passage members having a Cr level of 8.0 to 10.0%. Compared with SUH409L, this steel has improved heat resistance, and at the same time realized cost reduction by lowering Cr. In addition, it is pointed out that Cu is effective for improving low-temperature toughness and workability. For example, in terms of processability, it has ductility comparable to SUH409L in a tensile test at room temperature. However, the in-plane anisotropy and deep drawability to improve ductility have not been realized, and the problem of imparting formability that can sufficiently cope with various forming methods (mentioned above) has not yet been solved. In addition, a method of stably imparting excellent low-temperature toughness is unknown. Therefore, the steel of Patent Document 5 cannot be said to fully satisfy the recent strict requirements for exhaust path members.

In Patent Documents 6 and 7, it is disclosed that the Cr level for improving the corrosion resistance to condensed water required for low-temperature components such as mufflers, or for improving the high-temperature strength required for high-temperature components such as exhaust pipes is 10 to less than 15% ferritic steel. However, there is no specific disclosure about the high-temperature oxidation resistance, and the workability is evaluated only by the proof stress. Patent Document 6 and Patent Document 7 fail to realize that stable and reproducible improvement of high-temperature oxidation resistance and formability at the same time is unknown, and the method is unknown. Therefore, the steels introduced in Patent Documents 6 and 7 are difficult to say are perfect in terms of formability when processing of various exhaust path members having complex shapes is considered.

[Patent Document 1] Japanese Unexamined Patent Publication No. 3-274245 (1 line in the upper right column on page 3 - 9 lines in the upper right column on page 4)

[Patent Document 2] JP-A-5-125491 (paragraphs 0012-0016)

[Patent Document 3] JP-A No. 7-11394 (paragraphs 0014-0021, 0028-0028, Table 6, Figure 1)

[Patent Document 4] JP-A-7-145453 (paragraphs 0011-0021)

[Patent Document 5] JP-A-10-147848 (paragraphs 0003-0005, 0014)

[Patent Document 6] JP-A-10-204590 (paragraphs 0026-0036, 0072)

[Patent Document 7] Japanese Unexamined Patent Publication No. 10-204591 (paragraphs 0028-0037, 0074)

Contents of the invention

As described above, steel sheets for automobile exhaust passage members are required to have excellent "formability" that can be processed into complex shapes by various forming methods and contribute to the expansion of design freedom of the members. However, for high temperature strength and high temperature oxidation resistance, it is desired to maintain the same level as SUS430J1L at 800 to 900°C, and also to have excellent low temperature toughness. However, as indicated by the above-mentioned documents, the present situation is that a steel sheet that has been improved to a high level simultaneously with excellent formability, high-temperature strength, high-temperature oxidation resistance, and low-temperature toughness has not yet appeared.

The object of the present invention is to provide a motor vehicle exhaust path member having excellent "formability" applicable to complicated shapes, excellent "high-temperature strength" and "high-temperature oxidation resistance" that can be used at 900°C, and energy It is a new ferritic heat-resistant steel that has excellent "low-temperature toughness" with a transition temperature of -50°C or lower, and lowered the Cr level to less than 11% by mass to achieve cost reduction.

The inventors investigated the unresolved cause of the simultaneous improvement of excellent formability and excellent high-temperature strength, high-temperature oxidation resistance, and low-temperature toughness, and considered that among the above-mentioned characteristics, especially "formability " and "high temperature oxidation resistance" means have not been found is the main reason, therefore, the results of detailed studies have shown that in the case of adjusting the balance of austenite as in the formula (3) described later, as in the formula (1) described later As represented by the formula (2), there is a range in which "formability" and "high-temperature oxidation resistance" are compatible in terms of the Si and Cr contents.

In addition, in order to evaluate the workability on exhaust path members of complex shapes, "deep drawability" in formability cannot be underestimated. In heat-resistant ferritic steels to which Nb has been added, it has been known that in addition to Nb, combined addition of Ti is effective in improving deep drawability. Furthermore, partial recrystallization of hot-rolled sheets has been obtained. The understanding of deep drawability (average plastic deformation ratio r _AV ) and its in-plane anisotropy (plastic anisotropy Δr) can be improved.

However, the addition of Ti leads to a decrease in low-temperature toughness. In order to improve this low-temperature toughness, it has been proved that the combined addition of Cu and B is more effective than the addition of Cu alone.

However, when the amount of Cu added was increased, abnormal oxidation was rapidly induced. Furthermore, an appropriate range of Cu that can simultaneously improve "low temperature toughness" and "high temperature oxidation resistance" was found.

The present invention was accomplished based on these findings.

That is, the above object is based on mass %, C: 0.02% or less, Si: 0.7 to 1.1%, Mn: 0.8% or less, Ni: 0.5% or less, Cr: 8.0 to less than 11.0%, N: 0.02% or less, Nb: 0.10 to 0.50%, Ti: 0.07 to 0.25%, Cu: 0.02 to 0.5%, B: 0.0005 to 0.02%, V: 0 (not added) to 0.20%, preferably 0.01 to 0.20%, one of Ca and Mg Or two kinds: 0 (not added) to 0.01% in total, preferably 0.0003 to 0.01%, one or more elements among Y and REM: 0 (not added) to 0.20% in total, preferably 0.01 to 0.20%, and the rest is composed of Fe and The unavoidable impurity composition is achieved by having a ferritic steel sheet that satisfies all of the chemical compositions of the following formulas (1) to (3) and simultaneously improves formability, high-temperature oxidation resistance, high-temperature strength, and low-temperature toughness.

3Cr+40Si≥61...(1)

Cr+10Si≤21...(2)

420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-25(Nb+V)-52Al+470N+189≤70...(3).

In addition, among the above-mentioned steel sheets, there is provided a steel sheet in which Mo: 0.50% or less and Al: 0.10% or less are added.

Here, the value of the content in mass % of each element is substituted for the elements in the formulas (1) to (3). However, in the formula (3), zero is substituted at the place where no element is included.

In addition, in the present invention, among the above steel sheets, there is provided a copper sheet having a metal structure obtained by cold rolling and annealing a partially recrystallized hot rolled sheet.

Here, the partially recrystallized hot-rolled sheet refers to a hot-rolled sheet in which recrystallized grains account for 10 to 90% by volume, and the remainder is composed of non-recrystallized structures. The amount of recrystallized grains can be determined by optical microscope observation of the section of the hot-rolled sheet. A hot-rolled sheet is a hot-rolled steel sheet and means that it is no longer cold-rolled, regardless of whether heat treatment is performed after hot-rolling. The metal structure obtained by cold rolling and annealing is finally a completely recrystallized structure.

In addition, in the present invention, among the above steel sheets, there is provided a steel sheet having a metal structure obtained by cold rolling and annealing a completely recrystallized hot rolled sheet.

here. The completely recrystallized hot-rolled sheet refers to a hot-rolled sheet in which recrystallized grains exist in an amount exceeding 90% by volume.

In addition, in the present invention, there is provided the above-mentioned steel sheet, especially a steel sheet processed into an exhaust passage member of a motor vehicle engine and used.

Owing to the present invention, simultaneous improvement of "formability" and "high temperature strength, high temperature oxidation resistance, and low temperature toughness" is realized in ferritic heat-resistant steel sheets. In particular, the "formability" is a property that is optimized in terms of deep drawability and isotropy, which are necessary for various forming methods. A steel plate with new performance unintentionally given to a hot steel plate. In addition, even in terms of "high temperature strength, high temperature oxidation resistance, and low temperature toughness", performance equivalent to or better than that of existing materials used for exhaust path members is ensured. High compatibility of "formability" and "high-temperature strength, high-temperature oxidation resistance, and low-temperature toughness" is difficult in conventional steel sheets, but in the present invention, this compatibility is achieved at a Cr level of 11% or less. Therefore, the present invention can use ferritic heat-resistant steel for the complex-shaped exhaust path member, expand the design freedom of the member, and make a great contribution to cost reduction.

Brief description of the drawings

Fig. 1 shows that for ferritic steel with 10Cr-0.9Si-0.3Nb-0.1V-0.1Cu as the basic composition, the influence of Ti content and the difference between partial recrystallization or complete recrystallization after hot rolling are relatively Graph of the effect of the r-value (r _D ) at 45° to the direction of calendering.

Figure 2 shows the oxidation increment and energy of the Cu content in the atmosphere after heating at 900°C for 200 hours for a ferritic steel with 10Cr-0.9Si-0.3Nb-0.1Ti-0.1V-0.001B as the basic composition Graph of the effect of migration temperature.

Figure 3 shows the effect of Cu content and Si content on high temperature oxidation resistance and formability for ferritic steel with 8-14Cr-0.5-1.0Si-0.3Nb-0.1Ti-0.1V-0.1Cu as the basic composition. A graph of the impact.

Fig. 4 shows that for 8～14Cr-0.5～1.0Si-0.3Nb-0.1Ti-0.1V-0.1Cu as the basic composition and satisfying above-mentioned (1) formula and (2) formula ferritic steel, use AM= Graph of the relationship between AM value defined by 420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-25(Nb+V)-52Al+470N+189 and elongation under room temperature tensile test.

Detailed ways

In Fig. 1, the effect of Ti content and the difference between partial recrystallization and complete recrystallization of hot-rolled sheet are shown for ferritic steel whose basic composition is 10Cr-0.9Si-0.3Nb-0.1V-0.1Cu. Effect of the r-value (r _D ) at a 45° orientation relative to the direction of calendering. As a partially recrystallized hot-rolled sheet, a hot-rolled sheet having a structure in which recrystallized grains account for 10 to 90% by volume was prepared by heating a hot-rolled sheet with a thickness of 4.0 mm at 700 to 1000°C for 1 minute. As the hot-rolled sheet, a hot-rolled sheet having a thickness of 4.0 mm was heated at about 1050° C. for 1 minute. After these hot-rolled sheets were cold-rolled to 2.0 mm, annealed at 1050°C and completely recrystallized, tensile test pieces were cut out of them. As can be seen from FIG. 1 , when Ti is contained in an amount of 0.07% by mass or more, the r _D value increases rapidly. In addition, the r _D value in the entire Ti content range is further improved by partial recrystallization after hot rolling.

These reasons are not necessarily clear-cut, but the following considerations can be made. That is, Ti, which is stronger than Nb in forming carbonitrides, fixes C and N, reduces solid solution C and solid solution N, makes the matrix highly purified, and promotes the improvement of workability at the time of recrystallization during final annealing. The development of (111) surface collection tissue. When the Ti content is 0.07% by mass or more, the effect is considered to be significant. In addition, in the case of partially recrystallizing the hot-rolled sheet, it is considered that fine Nb-Ti-based precipitates are uniformly generated, and the precipitates suppress the development of the (100) planar structure that hinders the improvement of workability during annealing, and at the same time promote (111) Face the development of collective organization.

In Fig. 2, for a ferritic steel with 10Cr-0.9Si-0.3Nb-0.1Ti-0.1V-0.001B as the basic composition, the Cu content is shown in relation to the energy transfer temperature and after heating at 900°C for 200 hours in the atmosphere. The effect of oxidation increment. As a sample, a partially recrystallized hot-rolled sheet with a thickness of 4.0 mm was cold-rolled to 2.0 mm, then finished annealed at 1050° C. and completely recrystallized. Here, the energy transfer temperature is determined by a pendulum impact test. Make the impact direction parallel to the rolling direction, use the No. 5 test piece (width 2mm) according to JIS Z 2202, and conduct the test at a temperature of -100 to 25°C according to JIS Z 2242, and obtain it from the relationship between the test temperature and the absorbed energy Energy transfer temperature. According to JIS Z 2281, the oxidation gain is obtained by measuring the weight increase of a test piece heated continuously at 900°C for 200 hours in the atmosphere. As is known from FIG. 2 , in ferritic steel containing an appropriate amount of B, addition of Cu in a small amount of about 0.02% by mass is effective in improving low-temperature toughness. However, when it exceeds 0.5% by mass, it is newly found that the oxidation resistance at 900° C. deteriorates rapidly.

The reasons for these are not clear from the current point of view, but regarding low-temperature toughness, it is considered that the generation of twins, which is one of the main causes of low-temperature brittleness, is suppressed, and regarding abnormal oxidation, it is considered that Cu promotes Destabilization of the phase equilibrium of the matrix caused by oxidation of Cr and Si.

Fig. 3 shows the effect of Cr content and Si content on high-temperature oxidation resistance and formability for ferritic steels with 8-14Cr-0.5-1.0Si-0.3Nb-0.1Ti-0.1V-0.1Cu as the basic composition . The sample was produced by the same process as that in Fig. 2 . Here, as an index of formability, 0.2% proof stress in a room temperature tensile test at a direction of 45° with respect to the rolling direction is used. It is judged that the steel plate with a pressure exceeding 300 MPa basically does not have formability for various forming methods as an exhaust path member. As is known from FIG. 3 , when the content of Cr and Si decreases, abnormal oxidation occurs during heating at 900° C. for 100 hours in the air. On the other hand, when the content of Cr and Si increases, formability deteriorates. However, in the combination of the contents of Cr and Si, it can be seen that there is a region where both the high-temperature oxidation resistance at 900°C and the formability can be satisfied. Previously, the existence of such regions was not known, so although various ferritic heat-resistant steels were developed, as a result, steels with poor high-temperature oxidation resistance or poor formability, stable and good reproducibility, and satisfying the above two properties at the same time could not be specified.

The range where high-temperature oxidation resistance and formability can be satisfied at the same time is the range of the curve marked with ○ in the figure, and is specified by the following formulas (1) and (2).

3Cr+40Si≥61.....(1)

Cr+10Si≤21....(2)

Fig. 4 shows that for the ferritic steel with 8 ~ 14Cr-0.5 ~ 1.0Si-0.3Nb-0.1Ti-0.1V-0.1Cu as the basic composition, and satisfying the above formula (1) and formula (2), use AM= The AM value defined by 420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-25(Nb+V)-52Al+470N+189 and the room temperature tensile test at 45° relative to the rolling direction elongation relationship. The AM value is a value indicating the balance between the ferrite phase and the austenite phase. As can be seen from FIG. 4 , high ductility is obtained only in the range where the AM value is 70 or less, and the ductility drops sharply when the AM value exceeds 70. Therefore, only when the formulas (1) and (2) are satisfied and the following formula (3) is satisfied, formability and high-temperature oxidation resistance can be simultaneously improved.

420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-25(Nb+V)-52Al+470N+189≤70...(3)

Next, matters specific to the present invention will be described.

Generally speaking, C and N are effective for improving high temperature strength such as creep strength and creep rupture strength. However, in ferritic steel, when the C and N contents are high, the low-temperature toughness deteriorates. At this time, in order to stabilize the carbonitrides, it is necessary to increase the addition amount of Nb and Ti, and the cost of steel materials increases. On the other hand, the drastic reduction of C and N places a heavy burden on steelmaking, which in turn leads to an increase in cost. As a result of various investigations, in the present invention, the content of C and N is allowed to be 0.02% by mass. Furthermore, if the addition amount of Ti and Nb is appropriate, especially good formability and heat resistance can be obtained in the amount of C+N in the range of 0.01 to 0.02% by mass. Therefore, it is preferable to make the total content of C and N into 0.01-0.02 mass %.

Both Si and Cr are very effective in improving the high-temperature oxidation resistance, but on the contrary, they harden the steel. In order to simultaneously obtain excellent formability and high-temperature oxidation resistance, the content of Si and Cr must be controlled within the range satisfying the above formula (1) and (2) (above Fig. 3 ). In addition, from the viewpoint of securing corrosion resistance and low-temperature toughness, lower and upper limits for Si and Cr are added to these relational expressions. That is, when the content of Si and Cr is too small, the minimum necessary SUH409L level of corrosion resistance cannot be maintained, and conversely, when the content is too large, the low temperature toughness of the steel level cannot be maintained. Therefore, the Si content is specified at 0.7 to 1.1% by mass. A more preferable range of the Si content is 0.8 to 1.0% by mass. On the other hand, the Cr content is specified to be 8.0 to less than 11.0% by mass. The more preferable range of Cr content is 9.0 to less than 11.0 mass %, and the most preferable range is 9.0 to less than 10.0 mass %.

When Mn is added in excess, the steel material is hardened, resulting in a decrease in low-temperature toughness and formability. In addition, especially with the composition system of the present invention, there is a possibility that an austenite phase will be formed during heating and use, thereby adversely affecting the high-temperature oxidation resistance. Therefore, the upper limit of the Mn content is set to 0.8% by mass. Furthermore, in the component system of the present invention, especially when excellent flake adhesion at 900° C. is required, it is preferable to contain Mn in the range of 0.2 to 0.8% by mass.

Ni is effective in improving low-temperature toughness, but excessive addition will harden the steel and result in poor formability. In addition, in the composition system of the present invention, like Mn, when heated and used, there is a danger that an austenite phase will be formed and the high-temperature oxidation resistance will be deteriorated. Therefore, the upper limit of the Ni content is limited to 0.5 mass %.

Nb is very effective in improving high temperature strength. In the present invention, due to the addition of Ti, there is almost no Nb fixed in C and N, and it is considered that all of the added Nb effectively contributes to the improvement of the high-temperature strength. The effect becomes remarkable at 0.10% by mass or more. On the other hand, excessive addition of Nb deteriorates formability and low-temperature toughness. Therefore, the Nb content is set at 0.10 to 0.50% by mass. In order to obtain higher formability and high-temperature strength, it is preferably in the range of 0.10 to 0.40% by mass.

Ti is known to fix C and N and generally improves intergranular corrosion resistance, but in the present invention, it is very important to improve formability (especially deep drawability). The effect of improving the formability is remarkable at a Ti content of 0.07% by mass or more (the above-mentioned FIG. 1 ). However, excessive addition of Ti deteriorates the toughness and adversely affects the surface properties of the product. Therefore, the Ti content is specified to be 0.07 to 0.25% by mass. In order to obtain a high level of high temperature strength, it is preferable to add Ti so as to satisfy Ti≧6(C+N). In addition, in order to obtain a product having a surface property equal to or greater than that of SUH409L, the Ti content is preferably within a range of 0.20% by mass or less.

Mo is effective for improving the high-temperature strength, but a large amount of Mo causes embrittlement of steel materials. In addition, Mo is a very expensive element. Even without adding Mo, sufficient heat resistance can be ensured by optimizing the content of elements of other components, but by adding Mo, the degree of freedom in component design can be increased. When Mo is contained, it is preferable to carry out in the range of 0.50 mass % or less. In addition, when heat resistance is more important than cost, Mo may be added in excess of 0.50% by mass, but Mo should not be added in excess of 3.0% by mass, which extremely lowers low-temperature toughness.

Cu improves low-temperature toughness, but it is important to contain 0.02% by mass or more of Cu in the combination with B described later in order to remarkably improve the low-temperature toughness required in the exhaust passage member. However, when Cu exceeds 0.5% by mass, the high-temperature oxidation resistance rapidly deteriorates (the above-mentioned FIG. 2 ). Therefore, in the present invention, the Cu content is regulated to be 0.02 to 0.5% by mass.

V and N, like Ti, are carbonitride-forming elements, and are effective in improving the intergranular corrosion resistance and the toughness of the welded heat-affected zone. In addition, like Nb, in a solid solution state, they can improve high-temperature strength. The effect is particularly remarkable in the coexistence state with Nb. In addition, V is also considered to be effective in improving high-temperature oxidation resistance. However, when it exceeds 0.02 mass %, workability and low-temperature toughness will fall. Therefore, when V is added, it must be performed in the range of 0.20% by mass or less. In addition, in order to sufficiently obtain the above-mentioned effect of V, it is preferable to add in the range of 0.01 to 0.20% by mass.

Al is very effective in improving high-temperature oxidation resistance, but in the present invention, the composition is designed so that high-temperature oxidation resistance can be ensured even if Al is not contained. Excessive addition of Al deteriorates formability, weldability, and low-temperature toughness, and in the present invention, since Ti and Si are added, deoxidation by Al is not particularly required. When Al is contained, it must be within the range of 0.1% by mass or less. When Al is contained and formability, weldability, and low-temperature toughness are particularly important, it is preferable to limit the Al content to 0.07% by mass or less.

B suppresses low-temperature embrittlement and secondary work embrittlement in a ferrite system in which Nb and Ti coexist, and it is clear that the effect becomes remarkable when it is added in combination with Cu. In order to sufficiently improve the low-temperature toughness, it is necessary to add 0.0005% by mass or more of B. On the other hand, when B is added excessively and exceeds 0.02% by mass, borides are generated, the formability deteriorates, and the low-temperature toughness conversely deteriorates. In the present invention, B is contained in the range of 0.0005 to 0.02 mass % together with Cu: 0.02 to 0.5 mass %.

Ca and Mg have a strong bond with S, reduce the amount of MnS produced and improve corrosion resistance. In addition, the Ca and Mg elements themselves effectively contribute to the improvement of the high-temperature oxidation resistance, and therefore, when the corrosion resistance and the oxidation resistance are emphasized, these elements may be added as necessary. However, adding a large amount increases intermediates and deteriorates low-temperature toughness and formability. Therefore, when adding one or both of Ca and Mg, the total content must be within the range of 0.01% by mass or less. In order to obtain a remarkable effect by adding Ca and Mg, the total content of Ca and Mg is preferably 0.003 to 0.01% by mass.

REM (rare earth elements) such as Y, La, and Ce stabilize the Cr oxide film formed on the surface of the steel sheet, and improve the high-temperature oxidation resistance of the steel sheet by improving the adhesion between the steel substrate and the oxide film. Therefore, these elements can be added as needed when high-temperature oxidation resistance is important. However, when a large amount is added, not only the formability and low-temperature toughness will be deteriorated, but also intermediates starting from abnormal oxidation will easily be formed, conversely leading to deterioration of high-temperature oxidation resistance. Therefore, when adding one or more elements among Y and REM, the total content must be within the range of 0.20% by mass or less. In order to obtain a remarkable effect by adding Y and REM, the total content of one or more elements among Y and REM is preferably 0.01 to 0.20% by mass.

As other elements, one or two or more of Zr, Hf, Ta, W, Re, and Co effective for improving high-temperature strength may be contained. However, since the addition of large amounts leads to embrittlement of the steel, when these elements are added, the total content must be within the range of 3.0% by mass or less, and the total content is preferably 0.5% by mass or less.

P, S, O, Zn, Sn, Pb, etc., which are general impurity elements, should be reduced as much as possible from the viewpoint of ensuring formability and low-temperature toughness. Specifically, as the most relaxed restrictions, P: 0.04 mass % or less, S: 0.03 mass % or less, O: 0.02 mass % or less, Zn: 0.10 mass % or less, Sn: 0.10 mass % or less, Pb: 0.10% by mass or less. In an actual manufacturing site, it is preferable to set stricter restrictions according to the required quality.

The above formulas (1) to (3) define the necessary composition ranges (above) for simultaneously improving formability and high-temperature oxidation resistance. Here, the lower limit of the value on the left side of the formula (3) (AM value) is not particularly specified. However, steel with a low AM value usually contains a variety of ferrite-forming elements called Si, Cr, Mo, Ti, Nb, V, and Al. When these elements are contained in large amounts, the formability or low-temperature toughness decreases. From the results of various studies, it can be said that it is preferable to adjust the components so that the AM value is 40 or more.

By satisfying the above chemical composition, formability, high-temperature oxidation resistance, high-temperature strength, and low-temperature toughness can be simultaneously improved.

On top of this, in order to further improve formability, it is very effective to perform cold rolling and annealing after partial recrystallization treatment of hot-rolled sheets, that is, after making recrystallized grains account for 10 to 90% by volume, the remaining The so-called complete recrystallization process of annealing a hot-rolled sheet partially composed of a non-recrystallized structure after cold rolling can greatly improve the r value (above-mentioned Fig. 1 ), which is an index of deep drawability. The steel sheet having the metal structure obtained in this way has formability sufficient to cope with the latest exhaust path members that have strict shape requirements.

The partial recrystallization treatment of the hot-rolled sheet can be carried out directly in the hot-rolling process or by heating after hot-rolling to before cold-rolling.

In order to carry out the partial recrystallization treatment in the hot rolling step, for example, a method of hot rolling in the temperature range of 950 to 1250° C., coiling, and air cooling can be employed. Appropriate conditions can also be selected according to equipment specifications and hot rolling bead schedule. In addition, when the partial recrystallization treatment is performed by heating after hot rolling, for example, a method of heating the steel sheet cooled after hot rolling in a temperature range of 850 to 1000° C. can be employed. This heating may be performed at any stage before cold rolling.

After cold-rolling such a partially recrystallized hot-rolled sheet, annealing is performed to completely recrystallize it. The cold rolling rate can be implemented in the range of 30-90%, for example. In the case of being used for an automobile exhaust passage member, the final plate thickness is adjusted to, for example, about 0.4 to 1.2 mm. The annealing temperature is preferably in the range of, for example, 950 to 1150°C. The obtained ferritic steel sheet has excellent formability and low-temperature toughness, which are maintained after working on welded steel pipes.

When emphasizing the beauty of the surface appearance of the processed product, it is preferable to use a completely recrystallized hot-rolled sheet. A completely recrystallized hot-rolled sheet can be obtained by heat treatment at, for example, a temperature range of 950 to 1100° C. after hot rolling.

[Example 1]

Ferritic steels having the chemical compositions shown in Table 1 and Table 2 were melted in a high-frequency vacuum melting furnace, and cast into 30 kg steel ingots. After hot calcining them, hot rolling was performed to obtain hot rolled sheets with a sheet thickness of 4.0 mm. The hot rolling conditions are as follows: hot rolling temperature: 700-1250°C. Average rolling reduction per pass: about 30%, water cooling after hot rolling, and then heating at 900 to 1000°C for 1 minute. The metal structure of the cross-section of the hot-rolled sheet was observed with an optical microscope. In any sample, recrystallized grains accounted for 10 to 90% by volume, and the rest was non-recrystallized structure, which confirmed that the partial recrystallization treatment was completed. These partially recrystallized hot-rolled sheets were cold-rolled to a thickness of 2 mm, and then annealed at 1050° C. for 1 minute to completely recrystallize, thereby obtaining cold-rolled annealed sheets. In addition, Nos. 1 to 21 in Table 1 are ferritic steels satisfying the predetermined chemical composition in the present invention, and Nos. 22 to 31 in Table 2 are comparative steels other than these. Among them, No. 22 is steel equivalent to SUH409L, and No. 23 is steel equivalent to SUS430J1L.

[Table 1] (mass%)

Tr.: below the analysis limit

[Table 2] (mass%)

Tr.: below the analysis limit, *: out of the scope specified in the present invention

Cut test pieces from each cold-rolled annealed plate for tensile test, pendulum impact test, high-temperature tensile test, and high-temperature oxidation test.

The 0.2% proof stress, elongation at break, and plastic deformation ratio were obtained from the tensile test, and formability was evaluated. In the direction parallel to the rolling direction, in the direction of 45° relative to the rolling direction, and in the direction of 90° relative to the rolling direction, the No. 13B test piece specified in JIS Z 2201 is cut out from each test steel plate as a tensile test. piece. The 0.2% proof stress and the elongation at break were determined by performing a test specified in JIS Z 2241 with a test piece oriented at 45° to the rolling direction. The plastic deformation ratio was determined by a tensile test based on JIS Z 2254 using the test piece in the above three directions. That is, the ratio of plastic deformation in each direction was calculated from the ratio of transverse deformation and sheet thickness deformation when a 15% uniaxial tensile pre-strain was applied, and the average plastic deformation ratio r _AV and in-plane anisotropy Δr were obtained from the following formula .

r _AV ＝(r _L +2r _D +r _T )/4

Δr＝(r _L -2r _D +r _T )/2

in:

r _L : Plastic deformation ratio in the direction parallel to the rolling direction

r _D : Plastic deformation ratio in the direction of 45° relative to the rolling direction

r _T : Plastic deformation ratio in the direction of 90° relative to the rolling direction

The pendulum impact test was carried out in the direction illustrated in FIG. 2, and the energy transfer temperature was obtained as an index of low-temperature toughness.

The high-temperature tensile test uses the above-mentioned tensile test piece in the 45° direction, according to the method of JIS G 0657, and obtains the 0.2% proof stress at 900°C as the high-temperature strength index.

High-temperature oxidation test, according to JIS Z 2281, calculates the oxidation increment after heating at 900°C in the atmosphere for 200 hours, as an index of high-temperature oxidation resistance.

These results are shown in Table 3.

[table 3]

*: The object of the present invention was not achieved, *1: 900°C, *2: 900°C×200 hours

As shown in Table 3, the steel plates No. 1 to 21 as examples of the present invention all have an intermediate level of softness (0.2% proof stress) of SUH409L (No. 22) and SUS430J1L (No. 23) , showing the same ductility (elongation) as SUH409L. In terms of deep drawability, it exhibits an average plastic deformation ratio r _AV and an in-plane anisotropy Δr value superior to SUH409L and SUS403J1L. Low temperature toughness (energy migration temperature) has good properties comparable to SUH409L. In terms of heat resistance (high temperature strength, high temperature oxidation resistance) at 900°C, it is significantly better than SUH409H, and has the same performance as SUS 430J1L. That is, the steel sheet of the example of the present invention is excellent in "formability", and also sufficiently maintains "high temperature strength, high temperature oxidation resistance, and low temperature toughness".

On the other hand, steel No. 22 corresponding to SUH409L as a comparative example was inferior in deep drawability and heat resistance, and steel No. 23 corresponding to SUS430J1L was hard and had insufficient formability. No. 24 and No. 25 are steel grades that have a proven track record of use as exhaust path members for motor vehicle engines, but No. No. 24 has poor formability and low-temperature toughness because the content of Si and Cr is outside the range of the present invention without adding Ti. No. 25 has high C and Nb content and the content of Si and Cr is outside the range of the present invention. Poor formability, low temperature toughness and high temperature oxidation resistance. No. 26 is poor in formability and high-temperature oxidation resistance because the phase stability becomes stable on the austenite side. Since Nos. 27 to 31 contained elements detrimental to low-temperature toughness and exceeded the range specified by the present invention, low-temperature toughness deteriorated.

[Example 2]

Part of the steels (No. 1-10, No. 22-26) in Table 1 and Table 2 were hot-rolled, then heated at 950-1100° C. for 1 minute for heat treatment, and completely recrystallized hot-rolled sheets were produced. After each hot-rolled sheet was cold-rolled to a sheet thickness of 2.0 mm, it was annealed at 1050° C. for 1 minute for complete recrystallization to obtain a cold-rolled annealed sheet.

For each cold-rolled and annealed sheet, the 0.2% proof stress, elongation at break, plastic deformation ratio, and in-plane anisotropy were determined in the same manner as in Example 1. In addition, in order to evaluate the surface appearance after processing, the samples cut out from each cold-rolled annealed sheet were subjected to 20% plastic deformation in the rolling direction, and then measured with a stylus type roughness meter. Surface roughness in the direction (ten-point average roughness R _z according to JIS B 0660, reference length 10 mm). For comparison, the surface roughness was similarly measured for samples derived from partially recrystallized hot-rolled sheets (samples shown in Table 3).

The results are shown in Table 4

[Table 4]

*: The object of the present invention is not achieved

*1: Measured value of a sample (Table 3) using a partially recrystallized hot-rolled sheet

When comparing the data of the "inventive examples" of Table 4 and Table 3, samples from fully recrystallized hot-rolled sheet (Table 4) compared to samples from partially recrystallized hot-rolled sheet (Table 3) , the average plastic deformation ratio is equal or slightly lower, and the in-plane anisotropy tends to become slightly larger. This is considered to be because, when a completely recrystallized hot-rolled sheet is used, the r value in the 45° direction with respect to the rolling direction slightly decreases. On the contrary, the data in Table 4 show that the surface roughness after processing is significantly reduced by using a completely recrystallized hot-rolled sheet. That is, due to the complete recrystallization treatment of the hot-rolled steel sheet, it is possible to provide a steel sheet in which the beauty of the surface appearance of the processed product is suitable for the desired application.

In addition, the samples of the comparative examples were basically poor in formability.

Claims (8)

1. A ferritic steel plate, in mass %, C: 0.02% or less, Si: 0.7～1.1%, Mn: 0.8% or less, Ni: 0.5% or less, Cr: 8.0 to less than 11.0%, N: 0.02% or less, Nb: 0.10-0.50%, Ti: 0.07～0.25%, Cu: 0.02～0.5%, B: 0.0005～0.02%, V: 0～0.20%, One or both of Ca and Mg: 0 to 0.01% in total, One or more elements among Y and other REMs: 0 to 0.20% in total, The remainder is composed of Fe and unavoidable impurities, has a chemical composition that satisfies the following formulas (1) to (3) and simultaneously improves formability, high temperature oxidation resistance, high temperature strength, and low temperature toughness, 3Cr+40Si≥61...(1) Cr+10Si≤21...(2) 420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-25(Nb+V)-52A1+470N+189≤70...(3). 2. The steel sheet according to claim 1, wherein the V content is 0.01 to 0.20%. 3. The steel sheet according to claim 1, wherein the total content of one or both of Ca and Mg is 0.0003 to 0.01%. 4. The steel sheet according to claim 1, wherein the total content of Y and one or more elements among other REMs is 0.01 to 0.20%. 5. The steel sheet according to claim 1, wherein Mo: 0.50% or less and Al: 0.10% or less are added. 6. The steel sheet according to any one of claims 1 to 5, characterized in that it has a metal structure obtained by cold rolling and annealing a partially recrystallized hot rolled sheet. 7. The steel sheet according to any one of claims 1 to 5, which has a metal structure obtained by cold rolling and annealing a fully recrystallized hot rolled steel sheet. 8. The steel sheet according to any one of claims 1 to 5, characterized in that it is processed as an exhaust path member for a motor vehicle engine.

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