KR101092244B1 - Ferritic steel sheet concurrently improved in formability high-temperature strength high-temperature oxidation resistance and low-temperature toughness - Google Patents

Abstract

The present invention provides an inexpensive steel sheet having both "formability" and "high temperature strength, high temperature oxidative resistance and low temperature toughness". Ferritic steel sheet of the present invention is C 0.02% by mass or less, Si 0.7 to 1.1% by mass, Mn 0.8% by mass or less, Ni 0.5% by mass or less, Cr 8.0 to less than 11.0% by mass, N 0.02% by mass or less, Nb 0.10 to 0.50 Mass%, Ti 0.07-0.25 mass%, Cu 0.02-0.5 mass%, B 0.0005-0.02 mass%, V 0 (no addition)-0.20 mass%, 1 or 2 types in total of Ca and Mg 0 (no addition)-0.01 It consists of the mass%, Y, and the sum of one or more elements 0 (no addition) to 0.20 mass% in Y and rare earth elements (REM), and Fe and an unavoidable impurity residual amount, satisfying the formulas (1) to (3).

Equation 1

3Cr + 40Si ≥ 61

Equation 2

Cr + 10Si ≤ 21

Equation 3

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

Ferrite, Hot Rolled, Recrystallized, Rolled, Annealed, Automobile Engine, Exhaust Gas

Description

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

1 shows the effect of Ti content on the r value (r _D ) in the 45 ° direction with respect to the rolling direction in a ferritic steel based on 10Cr-0.9Si-0.3Nb-0.1V-0.1Cu, and The graph shows the effect of the difference between partial recrystallization and complete recrystallization after hot rolling.

FIG. 2 shows the Cu content on the oxidative increase and energy transition temperature after heating for 200 hours at 900 ° C. in a ferritic steel based on 10Cr-0.9Si-0.3Nb-0.1Ti-0.1V-0.001B. It is a graph showing the influence of.

3 shows the effects of Cr content and Si content on high temperature oxidation resistance and formability in ferritic steels based on 8 to 14 Cr-0.5 to 1.0 Si-0.3Nb-0.1Ti-0.1V-0.1Cu. It is a graph.

4 is an AM value (AM = 420C-) in a ferritic steel having 8 to 14 Cr-0.5 to 1.0 Si-0.3Nb-0.1Ti-0.1V-0.1Cu as a basic composition and satisfying Equations 1 and 2 11.5 Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-25 (Nb + V) -52Al + 470N + 189) and a graph showing the relationship of elongation in the 45 ° direction with respect to the rolling direction in the room temperature tensile test to be.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a steel sheet suitable for exhaust gas passage members of an automobile engine that can be used in a high temperature atmosphere at 800 to 900 ° C., and has high moldability, high temperature resistance, high temperature oxidation resistance and low temperature toughness such as deep drawability and tensile moldability. The present invention relates to an improved ferritic steel sheet.

Ferritic stainless steels are used in heat-resistant applications where thermal stress is a problem because they have a lower coefficient of thermal expansion and superior thermal fatigue characteristics and high temperature oxidation characteristics compared to austenitic. Typical examples thereof include automotive engine exhaust gas passage members such as an exhaust manifold (hereinafter referred to as "Igmani"), a front pipe, a catalyst carrier outer cylinder, a center pipe, a muffler and an exhaust pipe.

Recent automobile engines tend to raise the exhaust gas temperature for the purpose of improving the exhaust gas purification efficiency and output, and particularly high heat resistance (high temperature strength) for members close to the engine such as igmanis, front pipes, and catalyst carrier outer cylinders. And high temperature oxidative resistance). Also, in recent years, the shape of the exhaust gas passage member tends to be complicated. In particular, the igmani and catalyst carrier outer cylinders are molded into complex shapes by various methods such as mechanical press molding, servo press molding, spinning processing, hydrofoam and the like. Therefore, the material used here is not enough simply to have good tensile elongation and bendability, but is also required to be excellent in moldability, which is represented by deep drawing and tensile moldability, and to have low workability in-plane anisotropy. In addition, since it is necessary to consider the prevention of ductile cracking and brittle cracking in secondary processing and tertiary processing, it must be excellent in low-temperature toughness. In addition, when the shape is complicated, thermal stress due to engine start-up and stoppage is concentrated in one place, and thermal fatigue breakage is likely to occur, and the material temperature rises locally, and abnormal oxidation is also likely to occur, thereby improving moldability and low temperature toughness. Heat resistance cannot be sacrificed in planning.

As ferritic stainless steel having high heat resistance, SUH409L and SUS430J1L are known. SUH409L has good workability and low temperature toughness and is widely used for exhaust gas passage members. However, considering its heat resistance level, application in applications where the material temperature exceeds 800 ° C. is undesirable. Moreover, it does not have sufficient deep-axle property applicable to the member of a complicated shape. SUS430J1L has excellent heat resistance that can be used even at 900 ℃. However, it is hard and falls in terms of formability.

Accordingly, the following heat resistant ferritic steels have been developed.

Patent Document 1 discloses a ferritic heat resistant stainless steel having a Cr content of 17.0 to 25.0%. Such steel improves high temperature strength by the combined addition of Mo and Cu and suppresses scale peeling by adding Mn. Moreover, the reduction of the impact value by Mo is overcome to some extent by the composite addition of Cu and Ni. However, it does not have moldability which can fully cope with a complicated exhaust gas passage member. The high Cr content is also disadvantageous in terms of cost.

Patent Document 2 discloses a ferritic stainless steel which exhibits heat resistance equal to or higher than that of 18% Cr based on 13% Cr based and improves high temperature salt corrosion. This improves high temperature strength by securing solid solution Nb, improves high temperature oxidation characteristics by adding Mn and Si several times, and improves high temperature salt corrosion corrosion resistance by such Si. However, no special consideration is given to the improvement of moldability and low temperature toughness, and it is not possible to sufficiently cope with the recent stringent requirements described above.

Patent document 3 discloses steel which aimed at the improvement of high temperature oxidation resistance and scale adhesiveness in Nb containing ferritic heat resistant stainless steel with Cr content of 11.0-15.5%. These properties are remarkably improved by strictly restricting Mn / Si in the range of 0.7 to 1.5. In addition, it is taught that addition of Cu can improve low-temperature toughness and workability. For example, regarding workability, data has been described that no crack occurs in the close bending test. However, in consideration of the phenomenon in which the demand for the shape of the exhaust gas passage member is becoming more severe, the material is required to have excellent moldability that can sufficiently cope with various molding methods (see above). In this respect, the steel of Patent Document 3, which does not pay attention to the tensile moldability such as the deep axiality, can be said to be sufficient to cope with the existing strict requirements. In addition, the Cr content also contains 11.0% or more required for stainless steel, and in the exhaust gas passage member which does not necessarily need to be "stainless", further cost reduction by reduction of Cr content is desired.

Patent document 4 discloses ferritic stainless steel for igmani containing Cr: 11 to 14%. Such steels increased the high temperature strength by actively adding Si to the Nb-containing steel. Its heat resistance is considered to be equivalent to the steel of Patent Document 3. However, it is not considered to improve the formability and low temperature toughness more than conventionally, and it cannot be said that such steel can sufficiently cope with existing strict requirements. In addition, further reduction is also desired for the Cr content.

Patent Document 5 discloses a ferritic heat resistant steel for an engine exhaust gas passage member having a Cr content of 8.0 to 10.0%. These steels realize cost reduction by reducing Cr content while improving heat resistance than SUH409L. In addition, it is taught that Cu is effective in improving both low-temperature toughness and workability. For example, the workability has a ductility comparable to that of SUH409L in a tensile test at room temperature. However, the improvement of ductile in-plane anisotropy and deep axiality is not intended, and the problem which gives moldability which can fully respond to various shaping | molding methods (refer the above description) is not solved. Moreover, the method of stably providing excellent low-temperature toughness is not known. Therefore, it cannot be said that the steel of patent document 5 can fully cope with the existing strict requirements for an exhaust gas passage member.

Patent Documents 6 and 7 disclose ferritic steels having a Cr content of less than 10 to 15%, which have improved corrosion resistance against condensation water required for low temperature members such as mufflers or high temperature strength required for high temperature members such as igmanis. . However, there is no specific disclosure regarding high temperature oxidation resistance, and workability is only evaluated by a proof strength. Patent Document 6 and Patent Document 7 do not intend to improve the high temperature oxidative resistance and moldability stably and reproducibly satisfactorily, and the method thereof is not known. Therefore, the steels disclosed in Patent Documents 6 and 7 cannot be said to satisfy all the requirements in terms of formability in consideration of processing on various exhaust gas passage members having a complicated shape.

Patent Document 1. Japanese Patent Application Laid-Open No. 3-274245 (See the first row from the top right of the third page to the ninth row from the top right of the fourth page)

Patent Document 2. Japanese Patent Application Laid-Open No. 5-125491 (see paragraphs 0012 to 0016)

Patent Document 3. Japanese Patent Application Laid-Open No. 7-11394 (see paragraphs 0014 to 0021, 0028 to 0029, Table 6 and FIG. 1)

Patent Document 4. Japanese Patent Application Laid-Open No. 7-145453 (see paragraphs 0011 to 0021)

Patent Document 5. Japanese Patent Application Laid-Open No. 10-147848 (see paragraphs 0003 to 0005 and 0014)

Patent Document 6. Japanese Patent Application Laid-Open No. 10-204590 (see paragraphs 0026 to 0036 and 0072)

Patent Document 7. Japanese Patent Application Laid-Open No. 10-204591 (see paragraphs 0028 to 0037 and 0074)

As described above, the steel sheet for automobile exhaust gas passage members is required to have excellent "molding" which can be processed into various shapes by various molding methods and which can contribute to the expansion of design freedom of the members. However, regarding high temperature strength and high temperature oxidative resistance, it is desired to maintain the level equivalent to SUS430J1L at 800-900 degreeC, and also excellent low-temperature toughness. However, as can be seen from the above-mentioned patent document, it is the present situation that the steel plate which improved the outstanding moldability, the outstanding high temperature strength, the high temperature oxidative resistance, and the low temperature toughness simultaneously to a high level has not appeared yet.

The present invention provides excellent "formability" that can be easily applied to a complicated automobile exhaust gas passage member, excellent "high temperature strength" and "high temperature oxidative resistance" and energy transition temperature that can withstand use at 900 ° C. It is an object to provide a novel ferritic heat-resistant steel which simultaneously combines excellent "low temperature toughness" of -50 DEG C or lower and reduces Cr content to less than 11 mass%.

The inventors of the present invention have investigated the cause that the simultaneous improvement of the excellent moldability, the high temperature strength, the high temperature oxidative resistance and the low temperature toughness is not solved. Among the above characteristics, the "formability" and the "high temperature oxidative resistance" are stably and reproducibly stable. I thought that there was a big reason for the fact that the means of reconciliation were not known. Therefore, as a result of detailed examination, when the austenite balance is adjusted as in Equation 3, as described in Equation 1 and Equation 2, "formability" and "high temperature oxidative resistance" It has been found that there is a region compatible with.

In addition, in order to evaluate the workability with respect to a complicated exhaust gas passage member, "deep axiality" cannot be overlooked also in moldability. In heat-resistant ferritic steels containing Nb, it was found that the addition of Ti in combination with Nb was effective in improving the deep axial properties. In addition, it was found that by partially recrystallizing the hot rolled sheet, the deep axial property (average plastic strain ratio r _average ) and its in-plane anisotropy (plastic anisotropy Δr) are improved.

However, addition of Ti causes the fall of low-temperature toughness. In order to improve such low temperature toughness, the combined addition of Cu and B proved to be more effective than the addition of Cu alone.

However, increasing the amount of Cu added suddenly causes abnormal oxidation. And the appropriate range of Cu which can improve "low temperature toughness" and "high temperature oxidative resistance" was found.

The present invention has been completed based on these findings.

That is, the said objective is C 0.02 mass% or less, Si 0.7-1.1 mass%, Mn 0.8 mass% or less, Ni 0.5 mass% or less, Cr 8.0-1 less than 11.0 mass%, N 0.02 mass% or less, Nb 0.10-0.50 mass% , Ti 0.07-0.25 mass%, Cu 0.02-0.5 mass%, B 0.0005-0.02 mass%, V 0 (no addition)-0.20 mass%, Preferably 0.01-0.20 mass%, 1 type or 2 types in Ca and Mg Total 0 (no addition) to 0.01 mass%, preferably 0.0003 to 0.01 mass%, Y and one or more elements total 0 (no addition) to 0.20 mass%, preferably 0.01 to 0.20 mass%, in Y and rare earth elements (REM), And a ferritic steel sheet which is made of Fe and an unavoidable impurity residual amount, and which has a chemical composition satisfying all of the formulas (1) to (3), wherein moldability and high temperature oxidizing resistance, high temperature strength, and low temperature toughness are simultaneously improved.

3Cr + 40Si ≥ 61

Cr + 10Si ≤ 21

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

Moreover, the thing which added the prescription | regulation of 0.50 mass% or less of Mo and 0.10 mass% or less in the said steel plate is provided.

Here, a value indicating the content of each element in mass% is substituted at the positions of the elements of the equations (1) to (3). However, 0 is substituted in the position of the element which is not contained in Formula (3).

The present invention also provides that having a metal structure obtained by cold rolling and annealing a hot rolled plate partially recrystallized from the steel sheet.

Here, the partially recrystallized hot rolled plate refers to a hot rolled plate in which recrystallized particles occupy 10 to 90% by volume and the remaining portion is made of unrecrystallized structure. The amount of recrystallized particles can be specified by optical microscope observation of the hot rolled plate cross section. The hot rolled plate means that the steel sheet after hot rolling is not cold rolled, and it does not matter whether or not the heat treatment is performed after the hot rolling. The metal structure obtained by cold rolling and annealing is finally completely recrystallized.

The present invention also provides that having a metal structure obtained by cold rolling and annealing a hot rolled plate completely recrystallized in the steel sheet.

Here, the fully recrystallized hot rolled plate means a hot rolled plate in which the abundance of recrystallized particles exceeds 90% by volume.

In addition, the present invention provides that the steel sheet is processed and used in particular as an exhaust gas passage member of an automobile engine.

Best Mode for Carrying Out the Invention

1 shows the effect of Ti content on the r value (r _D ) in the 45 ° direction with respect to the rolling direction in a ferritic steel based on 10Cr-0.9Si-0.3Nb-0.1V-0.1Cu, and The effect of the difference between partial recrystallization and complete recrystallization after hot rolling is shown. As the partially recrystallized hot rolled sheet, a hot rolled sheet having a plate thickness of 4.0 mm was heated at 700 to 1000 ° C. for 1 minute to prepare a having a structure in which 10 to 90 vol% is occupied by recrystallized particles, and fully recrystallized hot As the rolled plate, a hot rolled plate having a plate thickness of 4.0 mm was heated at about 1050 ° C. for 1 minute. These hot rolled plates were cold rolled to 2.0 mm, then annealed at 1050 ° C. to complete recrystallization to cut tensile test pieces therefrom. As can be seen from FIG. 1, when Ti is contained 0.07% by mass or more, the r _D value rises rapidly. In addition, the r _D value is further improved in the entire Ti content range by partial recrystallization after hot rolling.

The reason for this is not necessarily clear, but is considered as follows. In other words, Ti, which has higher carbonitride-producing ability than Nb, fixes C and N to decrease solid solution C and solid solution N, and the matrix is highly purified, which is advantageous for the improvement of workability during recrystallization during final annealing. The development of aggregates is promoted. When the Ti content is 0.07% by mass or more, the effect is considered to be realized. Further, in the case of partial recrystallization of the hot rolled plate, fine Nb-Ti-based precipitates are uniformly produced, and their annealing inhibits the development of the (100) surface texture in which the precipitates suppress the improvement in workability and (111) It is thought to promote the development of cotton tissue.

FIG. 2 shows the Cu content on the oxidation increase after heating for 200 hours at 900 ° C. in an energy transition temperature and in an atmosphere of ferritic steel based on 10Cr-0.9Si-0.3Nb-0.1Ti-0.1V-0.001B. The effect of this is illustrated. Samples are those obtained by cold rolling a hot rolled plate having a thickness of 4.0 mm, partially recrystallized to 2.0 mm, followed by final annealing at 1050 ° C. for complete recrystallization. Here, the energy transition temperature is obtained by Charpy impact test. No.5 test piece (width 2mm) was taken in accordance with JIS Z2202 so that the impact direction was parallel to the rolling direction, and the test was carried out at a temperature of -100 to 25 ° C in accordance with JIS Z2242, and the relationship between test temperature and absorbed energy. Obtain the energy transition temperature from Oxidation increase is obtained by measuring the weight increase of the test piece when heating for 200 hours continuously at 900 ° C in the air according to JIS Z2281. As can be seen in FIG. 2, in ferritic steels containing an appropriate amount of B, Cu effectively acts to improve low temperature toughness by adding a trace amount of about 0.02 mass%. However, when exceeding 0.5 mass%, it turned out newly that the oxidation resistance in 900 degreeC deteriorates rapidly.

The reason for this is not clear at the present time. However, in terms of low-temperature toughness, it is thought that occurrence of twin crystals, which is one of low-temperature brittleness factors, is suppressed, and in terms of abnormal oxidation, phase balance of the matrix due to oxidation of Cr or Si is suppressed. It is believed that destabilization of is promoted by Cu.

FIG. 3 shows the effects of Cr content and Si content on high temperature oxidation resistance and formability in ferritic steels based on 8 to 14 Cr-0.5 to 1.0 Si-0.3Nb-0.1Ti-0.1V-0.1Cu. . The sample is produced by the same process as that of FIG. Here, as the index of the molding to the rolling direction at room temperature tensile test of 45 ^o direction using the 0.2% proof stress. If it exceeds 300 MPa, it is judged that it does not have moldability which can respond to various shaping | molding methods basically for the exhaust gas passage member. As can be seen in Figure 3, when the content of Cr and Si is reduced, abnormal oxidation occurs by heating for 100 hours at 900 ℃ in the air. On the other hand, when the contents of Cr and Si are increased, moldability deteriorates. However, it has become apparent that in the combination of the contents of Cr and Si there exists a region that can satisfy both high temperature oxidative resistance and moldability at 900 ° C. In the past, the existence of such an area was unclear, and although various ferritic heat-resistant steels were developed, it was found that the high-temperature oxidation resistance and the moldability were deteriorated as a result, and both characteristics were stably reproduced with good reproducibility. At the same time, no satisfactory steel was specified.

The region where the high temperature oxidation resistance and the moldability can be satisfied at the same time is a range in which a? Mark in the figure exists and is specified by the following equations (1) and (2).

Equation 1

3Cr + 40Si ≥ 61

Equation 2

Cr + 10Si ≤ 21

4 is a base composition of 8 to 14 Cr-0.5 to 1.0 Si-0.3Nb-0.1Ti-0.1V-0.1Cu, and is defined by the following general formula in a ferritic steel that satisfies Equations 1 and 2 above. The relationship between elongation at room temperature tensile test in 45 ° direction with respect to AM value and rolling direction is shown.

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

AM value shows the balance of a ferrite phase and an austenite phase. As can be seen from FIG. 4, high ductility is obtained only when the AM value is in the range of 70 or less, and when it exceeds 70, the ductility drops rapidly. Therefore, the moldability and the high temperature oxidation resistance are improved at the same time only when the following equations (1) and (2) are satisfied.

Equation 3

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

EMBODIMENT OF THE INVENTION Hereinafter, the matter which specifies this invention is demonstrated.

C and N are generally effective for improving high temperature strength such as clip strength and clip breaking strength. However, in ferritic steels, high C and N contents deteriorate low temperature toughness. In this case, in order to stabilize as carbonitride, it is necessary to increase the amount of Nb and Ti added and the cost of the steel material is increased. On the other hand, in order to drastically reduce C and N, the burden on steelmaking becomes excessive, and conversely, the cost increases. As a result of various studies, in the present invention, up to 0.02% by mass of both C and N are allowed. In addition, by optimizing the addition amount of Ti and Nb, particularly good moldability and heat resistance are obtained because the amount of C + N is 0.01 to 0.02% by mass. Therefore, it is preferable to make the total content of C and N into 0.01 to 0.02 mass%.

Both Si and Cr are very effective at improving the high temperature oxidation characteristics, while hardening steel. In order to achieve both excellent moldability and high temperature oxidation resistance, it is necessary to adjust the content of Si and Cr in a range satisfying both Equations 1 and 2 (see FIG. 3). In addition to these relational expressions, the lower and upper limits of Si and Cr are regulated from the viewpoint of securing corrosion resistance and low temperature toughness. In other words, if the content of Si and Cr is too small, at least the required SUH409L level of corrosion resistance cannot be maintained, while if too high, the low temperature toughness of the same steel level cannot be maintained. Therefore, Si content is prescribed | regulated to 0.7-1.1 mass%. The more preferable range of Si content is 0.8-1.0 mass%. In addition, Cr content is prescribed | regulated to 8.0-1 less than 11.0 mass%. The more preferable range of Cr content is 9.0 to less than 11.0 mass%, and still more preferable range is 9.0 to 10.0 mass%.

Excessive addition of Mn causes the steel material to harden, leading to lowering of low temperature toughness and formability. Moreover, especially in the component system of this invention, an austenite phase is produced | generated at the time of heat use, and there exists a possibility that it may adversely affect high temperature oxidative resistance. Therefore, the upper limit of Mn content is prescribed | regulated as 0.8 mass%. In addition, in the component system of the present invention, it is preferable to contain Mn in the range of 0.2 to 0.8% by mass, particularly when excellent scale adhesion is required at 900 ° C.

Ni is effective for improving low temperature toughness, but excessive addition hardens steel materials, resulting in deterioration of formability. In addition, in the component system of the present invention, when heated and used in the same manner as Mn, austenite phase may be generated to deteriorate high temperature oxidative resistance. Therefore, the upper limit of Ni content is limited to 0.5 mass%.

Nb is very effective for improving the high temperature strength. In the present invention, since Ti is added, there is almost no Nb fixed to C and N, and it can be considered that substantially all of the added Nb effectively serves to improve the high temperature strength. The effect becomes remarkable at 0.10 mass% or more. On the other hand, addition of excess Nb deteriorates moldability and low temperature toughness. Therefore, Nb content is prescribed | regulated as 0.10 to 0.50 mass%. In order to obtain higher moldability and high temperature strength, it is preferable to be in the range of 0.10 to 0.40% by mass.

Ti is known to fix C and N and generally improve intergranular corrosion resistance, but is very important in the present invention for improving moldability (especially deep tack). The improvement of formability is remarkable at Ti content of 0.07 mass% or more (see FIG. 1). However, excessive addition of Ti deteriorates toughness and also adversely affects the surface properties of the product. Therefore, Ti content is prescribed | regulated as 0.07-0.25 mass%. To obtain high levels of high temperature strength, it is preferred to add Ti to satisfy Ti ≧ 6 (C + N). Moreover, in order to obtain the product of the surface property equivalent to or more than SUH409L, it is preferable to contain Ti in the range of 0.20 mass% or less.

Mo is effective for raising the high temperature strength, but a large amount of content causes embrittlement of the steel material. Mo is an extremely expensive element. Although sufficient heat resistance can be ensured by optimizing the content of other component elements without adding Mo, the degree of freedom in component design is increased by adding Mo. When it contains Mo, it is preferable to carry out in 0.50 mass% or less. In the case where the heat resistance is more important than the cost, Mo may be added in excess of 0.5% by mass, but should not be added in excess of 3.0% by mass, where the low temperature toughness is extremely lowered.

Cu improves low temperature toughness, but in order to significantly improve the low temperature toughness required for the exhaust gas passage member, it is important to contain Cu in an amount of 0.02% by mass or more in combination with the following B. However, when Cu exceeds 0.5 mass%, high temperature oxidative resistance deteriorates rapidly (refer FIG. 2). Therefore, in this invention, Cu content is prescribed | regulated as 0.02-0.5 mass%.

V is a carbonitride generating element similar to Nb and Ti, and is effective for improving intergranular corrosion resistance and toughness of weld heat affected zones. Moreover, it contributes to the improvement of high temperature strength in solid solution state similarly to Nb. The effect is particularly noticeable in the coexistence state with Nb. In addition, V is thought to be effective in improving the high temperature oxidative resistance. However, when it exceeds 0.20 mass%, the workability and low-temperature toughness will fall. Therefore, when adding V, it is necessary to carry out in 0.20 mass% or less. Moreover, in order to fully acquire the effect of V, it is preferable to add in 0.01-0.20 mass%.

Al is very effective in improving the high temperature oxidative resistance, but in the present invention, the component is set so that high temperature oxidative resistance can be ensured even without Al. Excessive Al addition deteriorates moldability, weldability and low temperature toughness, and in the present invention, since Ti and Si are added, deoxidation by Al is not particularly necessary. When it contains Al, it is necessary to carry out in 0.1 mass% or less. In the case where Al is contained and the moldability, weldability and low temperature toughness are particularly important, the Al content is preferably regulated to 0.07% by mass or less.

B suppressed low temperature brittleness and secondary work embrittlement in ferritic steel in which Nb and Ti coexisted, and it became clear that the effect became remarkable by the complex addition with Cu. In order to fully improve low-temperature toughness, it is necessary to add B 0.0005 mass% or more. On the other hand, when B is added in excess of 0.02 mass%, boride will generate | occur | produce, moldability will deteriorate, and low-temperature toughness will also rather deteriorate. In this invention, B is contained in 0.0005-0.02 mass% with 0.02-0.5 mass% of Cu.

Ca and Mg have a strong bonding force with S and improve the corrosion resistance by reducing the amount of MnS produced. In addition, the element itself of Ca or Mg acts effectively to improve high temperature oxidative resistance. Therefore, these elements can be added if necessary in the case where emphasis is placed on corrosion resistance and high temperature oxidation resistance. However, if a large amount is added, inclusions may increase and deteriorate low-temperature toughness or moldability. Therefore, when adding one or two of Ca and Mg, the total content thereof should be 0.01 mass% or less. need. In order to obtain the remarkable effect by adding Ca and Mg, it is preferable to make the total content of Ca and Mg into 0.003-0.01 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 also improve the high temperature oxidation resistance of the steel sheet significantly by improving the adhesion between the steel matrix and the oxide film. Therefore, in the case where high temperature oxidative resistance is important, these elements can be added as necessary. However, when a large amount is added, not only the moldability and low-temperature toughness deteriorate, but also the inclusions which are the starting point for abnormal oxidation are easily produced, which leads to deterioration of high temperature oxidative resistance. Therefore, when adding 1 or more types of element in Y and REM, it is necessary to implement so that the sum total content may be 0.20 mass% or less. In order to obtain the remarkable effect by addition of Y and REM, it is preferable to make the total content of 1 or more types of elements in Y and REM into 0.01-0.20 mass%.

As other elements, one or two or more of Zr, Hf, Ta, W, Re, and Co effective for improving the high temperature strength can be contained. However, since a large amount adds embrittlement of a steel material, when adding these elements, it is necessary to carry out in the range of 3.0 mass% or less in total, and it is preferable to set it as 0.5 mass% or less in total.

It is preferable to reduce P, S, O, Zn, Sn, Pb, etc. which are general impurity elements as much as possible in order to ensure moldability and low-temperature toughness. Specifically, as the loosest regulation, it can be P 0.04% by mass or less, S 0.03% by mass or less, O 0.02% by mass or less, Zn 0.10% by mass or less, Sn 0.11% by mass or less, and Pb 0.1% by mass or less. . In actual manufacturing sites, it is desirable to set stricter regulations according to the desired quality.

Equations 1 to 3 define composition ranges necessary for simultaneously improving moldability and high temperature oxidation resistance (see above). Although the lower limit is not specifically defined in the value (AM value) of the left side of Formula (3), steel having a low AM value usually contains a large amount of ferrite generating elements such as Si, Cr, Mo, Ti, Nb, V, and Al. do. Containing a large amount of these elements leads to a deterioration of moldability or low temperature toughness. As a result of various examinations, it can be said that it is preferable to adjust components so that AM value may be 40 or more.

By satisfying the above chemical composition, moldability, high temperature oxidative resistance, high temperature strength and low temperature toughness are simultaneously improved.

In addition, in order to further improve the moldability, it is very effective to perform a partial recrystallization treatment of the hot rolled plate, and then perform cold rolling and annealing. In other words, the recrystallized particles occupy 10 to 90% by volume, and a hot rolled sheet having a balance of unrecrystallized structure is manufactured, cold rolled, and then annealed to completely recrystallize, thereby greatly reducing r value, which is an index of deep axiality. Can be improved (see FIG. 1). The steel sheet having the metal structure thus obtained has moldability that can sufficiently correspond to the existing exhaust gas passage member having a strict demand for shape.

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

In performing a partial recrystallization process in a hot rolling process, the method of hot rolling, winding, and air-cooling as it is in the temperature range of 950-1250 degreeC can be used, for example. It is advisable to select the optimum conditions according to the equipment specifications or hot rolling pass schedule. In addition, when performing partial recrystallization by heating after hot rolling, the method of heating the steel plate cooled after hot rolling in the temperature range of 850-1000 degreeC can be used, for example. The heating is preferably carried out at any stage before cold rolling.

This partially recrystallized hot rolled plate is cold rolled and then annealed to complete recrystallization. Cold rolling rate can be performed, for example in 30 to 90% of range. When providing for an automobile exhaust gas passage member use, the final plate thickness is adjusted to about 0.4 to 1.2 mm, for example. As for annealing temperature, the range of 950-1150 degreeC is preferable, for example. The obtained ferritic steel sheet has excellent formability and low temperature toughness, and its properties are retained even after being processed into welded steel pipes.

In the case where the aesthetics of the surface appearance are emphasized in the processed product, it is preferable to use a hot rolled plate that is completely recrystallized. The fully recrystallized hot rolled sheet can be obtained by performing a heat treatment after hot rolling, for example, heating at a temperature range of 950 to 1100 ° C.

Example 1

Ferritic steels of the chemical composition shown in Table 1 and Table 2 are prepared by melting in a high frequency vacuum melting furnace and cast into 30 kg of ingots. After hot forging, they are hot rolled to obtain a hot rolled plate having a sheet thickness of 4.0 mm. The hot rolling conditions are hot rolling temperature 700 to 1250 ° C. and a pressure drop rate of about 30% per pass, followed by water cooling after hot rolling, and then heating at 900 to 1000 ° C. for 1 minute. The metal structure of the hot-rolled plate cross section was observed with an optical microscope, and it was confirmed that all samples had 10 to 90% by volume of recrystallized particles, the remainder as unrecrystallized structure, and partial recrystallization treatment was achieved. These partially recrystallized hot rolled plates are cold rolled to a plate thickness of 2 mm, and then annealed at 1050 ° C. for 1 minute to completely recrystallize to obtain a cold rolled annealed plate. In addition, numbers 1-21 of Table 1 are ferritic steels which satisfy | fill the chemical composition prescribed | regulated by this invention, and numbers 22-31 of Table 2 are comparative steels other than that. Number 22 is a steel equivalent to SUH409L, and number 23 is a steel equivalent to SUS430J1L.

Test pieces are cut from each cold rolled annealing plate and provided for tensile test, Charpy impact test, high temperature tensile test and high temperature oxidation test.

The moldability was evaluated by determining the 0.2% yield strength, elongation at break and the plastic strain ratio by the tensile test. 13B specimens specified in JIS Z2201 are cut from each steel sheet along a direction parallel to the rolling direction, 45 ° direction with respect to the rolling direction, and 90 ° direction with respect to the rolling direction, to obtain a tensile test piece. The 0.2% yield strength and elongation at break are obtained by using a test piece in a 45 ° direction with respect to the rolling direction and carrying out the test specified in JIS Z2241. Plastic deformation ratio is calculated | required by the tension test based on JISZ2254 using the said test piece of the said three directions. That is, the plastic strain ratio in each direction is calculated from the ratio of the lateral distortion and the sheet thickness distortion at the time of giving a 15% uniaxial tensile preliminary distortion, and the average plastic strain ratio r _average and in-plane are calculated according to the following general formula. Find the anisotropic Δr.

r _mean = (r _L + 2r _D + r _T ) / 4

Δr = (r _L -2r _D + r _T ) / 2

Where

r _L is the plastic strain ratio in the direction parallel to the rolling direction,

r _D is the plastic strain ratio in the 45 ° direction with respect to the rolling direction,

r _T is a plastic strain ratio in the 90 ° direction with respect to the rolling direction.

The Charpy impact test is carried out by the method described in Fig. 2, and the energy transition temperature is obtained as an index of low temperature toughness.

The high temperature tensile test is performed by the method according to JIS G0657 using the said tensile test piece of the 45 degree direction, and obtains 0.2% yield strength of 900 degreeC, and makes it an index of high temperature strength.

In the high temperature oxidation test, the amount of oxidation increase after heating for 200 hours at 900 ° C. in the air is calculated in accordance with JIS Z2281 and used as an index of high temperature oxidation resistance.

These results are shown in Table 3.

As can be seen from Table 3, the steel sheets of Nos. 1 to 21 as examples of the present invention all had a moderate softness (0.2% yield strength) of SUH409L (No. 22) and SUS430J1L (No. 23), and were equivalent to SUH409L. It is soft. As for the deep axial property, the average plastic strain ratio r _average and in-plane anisotropy? R superior to SUH409L and SUS430J1L are shown. Low temperature toughness (energy transition temperature) also has good performance comparable to SUH409L. Regarding the heat resistance (high temperature strength and high temperature oxidative resistance) of 900 ° C, it is clearly superior to SUH409L, and has the same performance as SUS430J1L. That is, the steel plate of the Example of this invention is excellent in "formability" and fully maintains "high temperature strength, high temperature oxidative resistance, and low temperature toughness."

On the other hand, steel equivalent to SUH409L of the comparative example No. 22 is inferior in core axial property and heat resistance, and steel equivalent to SUS430J1L of No. 23 is hard and insufficient in formability. No. 24 and No. 25 are steel types that have been used as exhaust gas passage members in automobile engines, but No. 24 is Ti-free and has low formability and low temperature toughness from the content of Si and Cr outside the present invention. 25 is inferior in moldability, low temperature toughness and high temperature oxidation resistance from the point that C and Nb are high and the content of Si and Cr is outside the scope of the present invention. No. 26 is inferior in moldability and high temperature oxidation resistance because the stability of the phase is stable on the austenite side. Nos. 27 to 31 are inferior in low temperature toughness because they contain an element harmful to low temperature toughness beyond the scope of the present invention.

Example 2

Some of the steels (numbers 1 to 10 and numbers 22 to 26) of Tables 1 and 2 were hot rolled and then subjected to a heat treatment for 1 minute at 950 to 1100 ° C. to produce fully recrystallized hot rolled plates. . Each hot rolled plate was cold rolled to a plate thickness of 2.0 mm and then annealed at 1050 ° C. for 1 minute to completely recrystallize to obtain a cold rolled annealed plate.

0.2% yield strength, elongation at break, plastic strain ratio, and in-plane anisotropy are calculated | required about each cold rolling annealing board similarly to Example 1. In order to evaluate the surface appearance after processing, 20% plastic distortion was applied to the samples cut from each cold rolled annealing plate in the rolling direction, and then the surface of the sample was prepared using a stylus roughness meter. The surface roughness (10 point average roughness Rz and reference length 10 mm based on JIS B0660) of a perpendicular direction is measured with respect to the rolling direction of the. The surface roughness is measured similarly also about the sample (listed in Table 3) derived from the hot-rolled plate which was partially recrystallized for comparison.

The results are shown in Table 4.

In contrast to the data of the "Examples of the present invention" of Table 4 and Table 3, the sample derived from the fully recrystallized hot rolled plate (see Table 4) is the sample derived from the partially recrystallized hot rolled plate (Table 3 The average plastic strain ratio is equal to or slightly lower than that of the reference), and the in-plane anisotropy tends to be slightly larger. This is considered to be due to a slight decrease in the r-value in the 45 ° direction with respect to the rolling direction when using a fully recrystallized hot rolled plate. On the other hand, from the data in Table 4, it was found that the surface roughness after processing was significantly reduced by using a hot rolled plate that was completely recrystallized. That is, by performing a complete recrystallization treatment of the hot rolled sheet, it is possible to provide a steel sheet suitable for a use where aesthetics of the surface appearance of the processed product is required.

In addition, the thing of a comparative example is inferior to moldability fundamentally.

According to the present invention, simultaneous improvement of "formability" and "high temperature strength, high temperature oxidative resistance and low temperature toughness" is achieved in a ferritic heat resistant steel sheet. In particular, "molding" is excellent in the deep axial properties required to cope with various molding methods and isotropy thereof, and in view of this, the steel sheet of the present invention imparts new performance not intended in the conventional ferritic heat-resistant steel sheet. Moreover, also in "high temperature intensity | strength, high temperature oxidative resistance, and low temperature toughness", the performance equivalent to or more than the existing material used for the exhaust gas passage member is ensured. The high compatibility between "formability" and "high temperature strength, high temperature oxidative resistance and low temperature toughness" is difficult in a conventional ferritic steel sheet, but in the present invention, the compatibility thereof is realized at a Cr content of 11% or less. Therefore, the present invention makes it possible to apply ferritic heat resistant steel to the exhaust gas passage member having a complicated shape, and contributes to the expansion of the design freedom of the member and to the cost reduction.

Claims (10)

C 0.002 to 0.02 mass%, Si 0.7 to 1.1 mass%, Mn 0.04 to 0.8 mass%, Ni 0.02 to 0.5 mass%, Cr 8.0 to 11.0 mass%, N 0.002 to 0.02 mass%, Nb 0.10 to 0.50 mass%, Ti 0.07-0.25 mass%, Cu 0.02-0.5 mass%, B 0.0005-0.02 mass%, V 0 (no addition)-0.20 mass%, Mo 0 (no addition)-0.50 mass%, Al 0 (no addition)-0.10 mass% 0 (no addition) to 0.01% by mass in total of one or two of Ca and Mg, 0 (no addition) to 0.20% by mass of one or more elements in Y and rare earth metals (REM), and inevitable with Fe A ferritic steel sheet which has a residual amount of impurity and has a chemical composition satisfying all of the formulas (1) to (3), wherein moldability, high temperature oxidizing resistance, high temperature strength, and low temperature toughness are simultaneously improved. Equation 1 3Cr + 40Si ≥ 61 Equation 2 Cr + 10Si ≤ 21 Equation 3 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-25 (Nb + V) -52Al + 470N + 189 ≤ 70 The ferritic steel sheet according to claim 1, wherein the V content is 0.01 to 0.20 mass%. The ferritic steel sheet according to claim 1, wherein the total content of one or two of Ca and Mg is 0.0003 to 0.01 mass%. The ferritic steel sheet according to claim 1, wherein the total content of at least one element in Y and REM is 0.01 to 0.20 mass%. delete The ferritic steel sheet according to any one of claims 1 to 4, having a metal structure obtained by cold rolling and annealing a partially recrystallized hot rolled plate. The ferritic steel sheet according to any one of claims 1 to 4, having a metal structure obtained by cold rolling and annealing a completely recrystallized hot rolled plate. The ferritic steel sheet according to any one of claims 1 to 4, which is processed and used as an exhaust gas passage member of an automobile engine. The ferritic steel sheet according to claim 6, which is processed and used as an exhaust gas passage member of an automobile engine. The ferritic steel sheet according to claim 7, which is processed and used as an exhaust gas passage member of an automobile engine.

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