TWI667357B - Ferritic stainless steel and automotive exhaust path components - Google Patents

Abstract

The present invention provides a ferritic stainless steel, which contains 0.03 mass% or less of C, 0.1 to 0.8 mass% of Si, 1.0 mass% or less of Mn, 0.04 mass% or less of P, 0.01 mass% or less of S, and 0.5 mass% or less. Ni, 12.0 ~ 15.0% by mass of Cr, 0.03% by mass or less of N, 0.1 ~ 0.5% by mass of Nb, 0.8 ~ 1.5% by mass of Cu, 0.1% by mass or less of Al, and the remainder consists of Fe and unavoidable impurities And γmax represented by the following formula (1) is 55 or less. γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 189 ... (1), where C, Si, Mn, Ni, Cr, N, Cu, and Al refer to the mass% of the element .

Description

Ferritic stainless steel and automotive exhaust path components

The invention relates to a ferritic stainless steel and a ferritic stainless steel for automobile exhaust path components.

Compared with austenitic stainless steel, ferritic stainless steel has a small thermal expansion coefficient, excellent thermal fatigue characteristics and high-temperature oxidation characteristics, and is therefore used in heat-resistant applications where thermal deformation becomes a problem. Typical applications include automotive exhaust path components such as exhaust manifolds, front pipes, catalyst carrier outer cylinders, center pipes, mufflers, and tail pipes.

Recently, in order to improve the efficiency and power of exhaust gas purification, automobile engines have a tendency to increase the temperature of exhaust gas. Exhaust manifolds, front pipes, catalyst carrier outer cylinders, and other components close to the engine are particularly demanding high heat resistance (high temperature strength, High temperature oxidation resistance). In addition, in recent years, the shape of the exhaust path member has tended to be complicated. In particular, the exhaust manifold and the outer shell of the catalyst carrier are formed into a complicated shape by various methods such as mechanical compression molding, servo press molding, spinning processing, and hydraulic molding. When the shape becomes complicated, the thermal deformation accompanying the start and stop of the engine is concentrated in one place, which is prone to thermal fatigue failure, and at the same time, the local material temperature rises, and abnormal oxidation is also prone to occur. Therefore, in order to improve moldability, heat resistance cannot be sacrificed.

As ferritic stainless steels having high heat resistance, SUH409L and SUS430J1L are known. SUH409L has good processability and is used in a large number of exhaust path components. However, considering the heat resistance level, it is preferably not used in applications where the material temperature exceeds 800 ° C. On the other hand, SUS430J1L has excellent heat resistance that can also be used at 900 ° C. However, since SUS430J1L is hard, it may be difficult to apply it in terms of processability.

Therefore, a ferritic stainless steel has been developed as described below.

Patent Document 1 proposes a technique based on the composition of a SUS429-based steel, improving workability by not adding Nb, and suppressing a decrease in thermal fatigue characteristics by adding Cu. However, if it is kept within the Cu precipitation temperature range for a long time, the precipitates of Cu aggregate and become coarse, and the effect of improving the high-temperature strength becomes small. Therefore, the thermal fatigue characteristics of the ferritic stainless steel may be reduced.

Patent Document 2 proposes a technique based on the composition of a SUS429-based steel, adding Nb and Cu to improve thermal fatigue characteristics, and increasing γmax to retain martensite in the slab to improve slab toughness. However, since the γmax of this ferritic stainless steel is high, a martensite phase is generated when heated to a high temperature such as welding, and thermal fatigue characteristics may be reduced. Patent literature

Prior Art Documents Patent Document 1: Japanese Patent Application Laid-Open No. 2012-188748 Patent Document 2: Japanese Patent Application Laid-Open No. 2012-007195

As described above, ferritic stainless steels used for applications such as automotive exhaust path components can be processed into complex shapes by various forming methods, and excellent processability is required that can contribute to the expansion of the degree of freedom in designing the components. In addition, ferritic stainless steels used for applications such as automotive exhaust path components are required to have excellent thermal fatigue characteristics and oxidation characteristics even at high temperatures, and therefore it is not desirable to reduce heat resistance. However, it is clear from the above-mentioned patent documents that the present situation is that a ferritic stainless steel which has both improved excellent workability and excellent heat resistance has not been obtained.

In addition, as a method for improving workability, there is a method for reducing Cr and Si as a general method for the purpose of achieving low alloying. However, in this method, since γmax is increased, a martensite phase is easily generated when used at a high temperature, and thermal fatigue characteristics are reduced. In addition, when Cr and Si are reduced, the high-temperature oxidation characteristics are also reduced.

In addition, as a general method for improving the workability, there is a method of reducing the heating temperature of the slab to increase the strain during hot rolling. However, in this case, it is known that the surface quality is reduced. Moreover, its cause and countermeasures have not yet been determined.

An object of the present invention is to provide a ferritic stainless steel which is excellent in workability and heat resistance, and also has a good surface quality, and a ferritic stainless steel for automobile exhaust path components.

In ferritic stainless steels, when Cr and Si are reduced in order to improve workability, γmax rises, and a martensite phase tends to be generated, so thermal fatigue characteristics are reduced. Therefore, in the present invention, the relationship between γmax and the generation of martensite phase and thermal fatigue characteristics were investigated. As a result, it was found that when γmax is 55 or less, no martensite phase is generated and there is no effect on thermal fatigue characteristics .

In addition, if the heating temperature of the slab is lowered during hot rolling in order to improve workability, the surface quality is lowered. Therefore, in the present invention, various studies have been made focusing on the state of generation of scale when the heating temperature of the slab is lowered. As a result, it was found that when the slab is heated, the oxide scale of the Fe main body is not uniformly generated but is locally generated, which is one of the reasons for the decrease in surface quality. When the oxide scale of the Fe main body is locally generated, it is considered that a surface defect occurs due to the thin portion of the oxide scale of the Fe main body coming into contact with the roll of the hot rolling mill. Therefore, the present inventors conducted intensive studies, and as a result, found that when the slab heating temperature during hot rolling is lowered, Si and Cr have a large influence on the generation of scale. Furthermore, it has been found that, by specifying the addition amounts of Si and Cr, even if the heating temperature of the slab is reduced, scales of the Fe main body are uniformly generated, and the surface quality during hot rolling can be improved.

That is, the present invention relates to a ferritic stainless steel containing 0.03% by mass or less of C, 0.1 to 0.8% by mass of Si, 1.0% by mass or less of Mn, 0.04% by mass or less of P, 0.01% by mass or less of S, and 0.5 Mass Ni or less, 12.0 to 15.0 mass% Cr, 0.03 mass% or less N, 0.1 to 0.5 mass% Nb, 0.8 to 1.5 mass% Cu, or 0.1 mass% Al or less. Avoided impurity structure, and γmax represented by the following formula (1) is 55 or less.

γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 189 ... (1)

In the formula, C, Si, Mn, Ni, Cr, N, Cu, and Al mean the mass% of the element.

In addition, the present invention also relates to a ferritic stainless steel for automobile exhaust path components, which contains 0.03% by mass or less of C, 0.1 to 0.8% by mass of Si, 1.0% by mass or less of Mn, 0.04% by mass or less of P, 0.01 S mass% or less, Ni 0.5 mass% or less, 12.0 to 15.0 mass% Cr, 0.03 mass% or less N, 0.1 to 0.5 mass% Nb, 0.8 to 1.5 mass% Cu, or 0.1 mass% Al The remainder is composed of Fe and unavoidable impurities, and γmax represented by the following formula (1) is 55 or less.

γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 189 ... (1)

In the formula, C, Si, Mn, Ni, Cr, N, Cu, and Al mean the mass% of the element.

According to the present invention, it is possible to provide a ferritic stainless steel excellent in workability and heat resistance and having a good surface quality, and a ferritic stainless steel for automobile exhaust path components.

The ferritic stainless steel of the present invention contains C, Si, Mn, P, S, Ni, Cr, N, Nb, Cu, and Al, and the remaining portion is composed of Fe and unavoidable impurities. In addition, the ferrite system does not The rust steel may further contain, as an optional component, one or more selected from Ti, Mo, V, Zr, W, Co, and B.

Here, in this specification, it is shown that the content of an element having no lower limit is set to an unavoidable level of impurities.

The reasons for limiting each element will be described below.

C and N are generally considered to be effective elements for improving high-temperature strength such as creep strength. However, if C and N are excessively contained, a martensite phase is likely to occur, and thermal fatigue characteristics, oxidation characteristics, and workability are degraded. In the steel composition in which Nb is added as a fixed element in which C and N are carbonitrides, it is necessary to add Nb in an amount in accordance with the C and N concentrations, so the cost of the ferritic stainless steel increases. On the other hand, if C and N are to be drastically reduced, the burden on steelmaking becomes excessive and costs increase. For these reasons, in the present invention, both C and N are specified to be 0.03 mass% or less. Moreover, considering oxidation characteristics and processability, it is desirable that both C and N are 0.015 mass% or less.

Both Si and Cr have a great influence on high-temperature oxidation characteristics and processability. The more Si and Cr are added, the better the high-temperature oxidation characteristics are, but the workability is reduced. On the other hand, when the high-temperature oxidation characteristics become good, and when the slab heating temperature during hot rolling is lowered, the scale of the Fe main body is not uniformly generated but locally generated, so the surface quality is lowered. In order to impart surface quality, the addition ranges of Si and Cr also need to be strictly defined. Therefore, in order to have both workability, high-temperature oxidation resistance, and surface quality during hot rolling, Si is set to 0.1 to 0.8% by mass, preferably 0.2 to 0.6% by mass. For the same reason, Cr is specified to be 12.0 to 15.0 mass%.

Mn is an alloy element that improves the high-temperature oxidation characteristics of ferritic stainless steels, particularly film peelability. However, if Mn is added in excess, workability is deteriorated. In addition, Mn is an austenite phase stabilizing element. Therefore, when Mn is excessively added to a steel type with a small amount of Cr added, a martensite phase is likely to occur, resulting in deterioration of thermal fatigue characteristics and workability. Therefore, Mn is specified to be 1.0% by mass or less, and preferably 0.8% by mass or less.

Since P and S adversely affect the high temperature oxidation resistance and the toughness of the hot-rolled sheet, it is preferable to reduce P and S as much as possible. Therefore, P is set to 0.04 mass% or less, and S is set to 0.01 mass% or less.

Ni is an element effective for improving low-temperature toughness. However, since Ni is an austenite phase stabilizing element, when Ni is excessively added to a steel type with a small amount of Cr, a martensite phase is generated like Mn, and thermal fatigue characteristics and workability are reduced. In addition, since the price of Ni is high, excessive addition of Ni should also be avoided. Therefore, the Ni content is specified to be 0.5% by mass or less. The lower limit of the Ni content is not particularly limited, but it is preferably more than 0% by mass, and more preferably 0.01% by mass or more.

Nb fixes C and N in the form of carbonitrides, and the remaining solid solution Nb to which carbonitrides are fixed exhibits the effect of improving the high-temperature strength. However, if an excessive amount of Nb is added, the workability is reduced. Therefore, the Nb content is specified as 0.1 to 0.5% by mass, preferably 0.2 to 0.4% by mass.

Cu is an element that increases high-temperature strength. In order to obtain the necessary high-temperature strength, the Cu content needs to be 0.8% by mass or more. However, as the Cu content increases, processability and high-temperature oxidation resistance characteristics decrease. Therefore, the Cu content is specified to be 0.8 to 1.5% by mass, preferably 0.9 to 1.3% by mass.

Al is added as a deacidifier during steel making, and it also exhibits the effect of improving high-temperature oxidation resistance. However, excessive addition of Al will reduce surface properties and adversely affect workability. Therefore, it is desirable that the content of Al is as small as possible, and is set to 0.1% by mass or less, and preferably 0.05% by mass or less.

Ti is an element that fixes solid solutions C and N in steel in the form of carbonitrides to improve ductility and processability. In addition, Ti suppresses the intergranular precipitation of Cr carbides, and the effect of improving corrosion resistance is also expected. However, if an excessive amount of Ti is added, the surface properties of the steel are deteriorated due to the generation of TiN, which adversely affects weldability and low-temperature toughness. Therefore, if necessary, Ti can be added in an amount of 0.20% by mass or less, preferably 0.1% by mass or less.

Mo, V, Zr, W, and Co are elements that improve high-temperature strength and thermal fatigue resistance by solid solution strengthening or precipitation strengthening. However, the addition of an excessive amount causes excessive hardening of the steel. Therefore, Mo, Zr, W, and Co may be added at 0.5 mass% or less, and V may be added at 0.1 mass% or less, as necessary.

B is an element that improves the secondary workability of steel and suppresses cracking during multi-stage forming. However, if B is excessively added, the manufacturability and weldability are deteriorated. Therefore, if necessary, B may be added in an amount of 0.01% by mass or less.

Equations (1) and (2) show γmax, which is an index of austenite phase generation. If γmax is too high, a martensite phase is likely to be generated, but if a martensite phase is present, the thermal fatigue characteristics are deteriorated. Therefore, in order not to generate a martensite phase, γmax is specified to be 55 or less. In addition, formula (1) represents γmax when Mo or Ti is not contained as an arbitrary component, and formula (2) represents γmax when Mo or Ti is contained as an arbitrary component.

γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 189 ... (1)

γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-12Mo-49Ti-52Al + 189 ... (2) Here, in formula (1) and formula (2), C, Si, Mn, Ni , Cr, N, Cu, Al, Mo, and Ti refer to the mass% of the element.

The manufacturing method of the ferritic stainless steel of the present invention is not particularly limited, as long as it is manufactured by sequentially performing the following steps: a step of heating a slab cast in accordance with a predetermined method at 1000 to 1250 ° C for 1 to 3 hours; Method: a process of hot rolling; a process of annealing at a temperature of 900 to 1100 ° C; a process of pickling and cold rolling according to a prescribed method; and a process of annealing at a temperature of 900 to 1100 ° C and then pickling .

Even if the ferritic stainless steel of the present invention obtained in this way is produced, even if the heating temperature of the slab is lowered, oxide scale of the Fe main body is uniformly produced, and the surface quality during hot rolling is good. In addition, the ferritic stainless steel is also excellent in workability and heat resistance. Therefore, the ferritic stainless steel of the present invention is suitable for heat resistance, especially for automobile exhaust path components. Examples

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

Various ferritic stainless steels having the steel composition of Table 1 were smelted in a vacuum melting furnace and cast into a 30 kg steel ingot. The steel ingot (slab) was heated at 1100 ° C for 2 hours, followed by hot rolling, annealing, cold rolling, and work annealing in order, thereby producing a cold rolled annealed sheet having a thickness of 1.5 mm. In addition, the ingot was forged and annealed, and a round bar annealing material was also produced. In the table, No. 1 to No. 20 are steels of the present invention, and No. 21 to No. 30 are comparative steels. Among them, No. 21 is a steel equivalent to Patent Document 1, and No. 22 is a steel equivalent to Patent Document 2. Table 1

A method for confirming the generation state of scale when the slab heating temperature is lowered will be described.

The steel ingot was cut into a thickness of 5 mm × width 25 mm × length 35 mm, and its surface was polished with a # 120 abrasive belt. The furnace was heated to 1000 ° C. for 2 hours using an electric furnace that reproduced the same amount of oxygen and water vapor as the hot-rolled heating furnace. After the internal heating, the generation state of the scale was confirmed by observing the cross section. The state of uniformly generating the scale of the Fe main body was evaluated as good (○: the same applies hereinafter), and the state of locally or not generating the scale of the Fe main body was evaluated as being poor (×: the same applies hereinafter).

A cold-rolled annealed sheet having a thickness of 1.5 mm was subjected to a high-temperature oxidation test and workability evaluation.

For the high-temperature oxidation test, a test piece having a size of 25 mm × 35 mm was produced, and a continuous oxidation test in a furnace was performed at 875 ° C. × 200 hours using an electric furnace in an atmospheric atmosphere, and then the weight of the test piece was measured. As a result of measurement of the oxidation increase amount, a test piece having a weight change of 5 mg / cm ² or less compared to the weight before the test was evaluated as ○, and a test piece having a weight change exceeding 5 mg / cm ² was evaluated as ×.

The workability evaluation was performed by a normal temperature tensile test. A JIS13B test piece was prepared, and the elongation at break in the rolling direction was measured. A test piece having an elongation at break of 35% or more was evaluated as ○, and a test piece having an elongation at break of less than 35% was evaluated as x.

A thermal fatigue test piece was produced from a round bar annealed material, and was subjected to a thermal fatigue test. Here, the thermal fatigue test piece was cut by using a round bar annealed material having a diameter of 10 mm, and a notch of R = 2.83 mm was provided at the center between the reference points so that the diameter reached 7 mm (the length between the reference points is 15mm). In the thermal fatigue test, a high-frequency heating device is used to perform heating and cooling at a minimum temperature of 200 ° C and a maximum temperature of 750 ° C at 3 ° C / sec, and the holding time at the minimum and maximum temperatures is set to 30 seconds, respectively. Take this as a loop. In the thermal fatigue test, the restraint rate was set to 25%, and the test was performed. The number of cycles where the maximum stress per loop was reduced by 25% compared to the value when the constant was constant was taken as the thermal fatigue life. Test pieces with a thermal fatigue life of 1,600 or more cycles were evaluated as ○, and test pieces with less than 1,600 cycles Evaluation was ×. Table 2 Evaluation test results

As shown in Table 2: The ferritic stainless steel of the example of the present invention is excellent in the generation state of scale, high-temperature oxidation characteristics, processability, and thermal fatigue characteristics.

In contrast, the ferritic stainless steel of Comparative Example 21 not containing Nb, Comparative Example 24 having Nb lower than the lower limit, and Comparative Example 28 having Cu lower than the lower limit was insufficient in high-temperature strength, and thus the thermal fatigue characteristics were reduced. Further, in the ferritic stainless steel of Comparative Example 28, the Cr content was excessive, and therefore the workability was reduced, and at the same time, the Fe-based oxide scale was unevenly generated during heating at 1000 ° C for 2 hours.

In the ferritic stainless steels of Comparative Examples 22 and 23, since γmax exceeds the upper limit value, a martensite phase is easily generated, and thermal fatigue characteristics are deteriorated. In addition, the ferritic stainless steel of Comparative Example 23 had a large C content, and thus had insufficient workability.

In the ferritic stainless steel of Comparative Example 27, since the Ni content and γmax exceeded the upper limit values, the thermal fatigue characteristics were reduced, and because the Cr content was small, the high-temperature oxidation characteristics were also insufficient.

The ferritic stainless steel of Comparative Example 25 had a large Si content, so that Fe-based oxide scale was not uniformly generated when heated at 1000 ° C. for 2 hours, and because the Si and Nb contents were large, the workability also decreased.

In the ferritic stainless steel of Comparative Example 26, since N and Al were excessive, the workability was reduced.

Since the ferritic stainless steel of Comparative Example 29 has a small Si content, high-temperature oxidation characteristics are reduced.

In the ferritic stainless steel of Comparative Example 30, since the contents of Mn and Cu are excessive, both the high-temperature oxidation characteristics and workability are reduced.

As described above, in the ferritic stainless steel of the comparative example, all of the occurrence state of scale, high-temperature oxidation characteristics, workability, and thermal fatigue characteristics are insufficient.

This application claims priority from Japanese Patent Application No. 2017-7842 filed on January 19, 2017, and the entire contents of these Japanese patent applications are incorporated herein by reference.

Industrial applicability

The ferritic stainless steel according to the present invention is excellent in surface quality, high-temperature oxidation characteristics, processability, and thermal fatigue characteristics, and is suitable for exhaust manifolds, front pipes, center pipes, and outer barrels of catalytic converters. Exhaust flow path components for various internal combustion engine structures.

Claims (5)

A ferritic stainless steel composed of the following components: C: 0.03% by mass or less, Si: 0.1 to 0.8% by mass, Mn: 1.0% by mass or less, P: 0.04% by mass or less, S: 0.01% by mass or less, Ni: 0.5 Mass% or less, Cr: 12.0 ~ 15.0% by mass, N: 0.03% by mass or less, Nb: 0.1 ~ 0.5% by mass, Cu: 0.8 ~ 1.5% by mass, Al: 0.1% by mass or less, and the remainder is composed of Fe and unavoidable Impurity structure, and γmax represented by the following formula (1) is 55 or less, γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 189 ... (1) In the formula, C, Si, Mn, Ni, Cr, N, Cu, and Al refer to the mass% of the element. A ferritic stainless steel containing the following components: C: 0.03% by mass or less, Si: 0.1 to 0.8% by mass, Mn: 1.0% by mass or less, P: 0.04% by mass or less, S: 0.01% by mass or less, Ni: 0.5% by mass or less, Cr: 12.0 to 15.0% by mass, N: 0.015% by mass or less, Nb: 0.1 to 0.5% by mass, Cu: 0.8 to 1.5% by mass, Al: 0.1% by mass or less, and selected from the following components More than one type: Ti: 0.20% by mass or less, Mo: 0.5% by mass or less, V: 0.1% by mass or less, Zr: 0.5% by mass or less, W: 0.5% by mass or less, Co: 0.5% by mass or less, B: 0.01% by mass Hereinafter, the remainder is composed of Fe and unavoidable impurities, and γmax represented by the following formula (2) is 55 or less, and γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-12Mo-49Ti -52Al + 189 ... (2) In the formula, C, Si, Mn, Ni, Cr, N, Cu, Mo, Ti, and Al refer to the mass% of the element. The ferritic stainless steel according to item 1 or 2 of the scope of patent application, wherein the ferritic stainless steel is used for heat resistance. A ferritic stainless steel for automobile exhaust path components, composed of the following components: C: 0.03 mass% or less, Si: 0.1 to 0.8 mass%, Mn: 1.0 mass% or less, P: 0.04 mass% or less, and S: 0.01 mass % Or less, Ni: 0.5% by mass or less, Cr: 12.0 ~ 15.0% by mass, N: 0.03% by mass or less, Nb: 0.1 ~ 0.5% by mass, Cu: 0.8 ~ 1.5% by mass, Al: 0.1% by mass or less, the rest It is composed of Fe and unavoidable impurities, and γmax represented by the following formula (1) is 55 or less, γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 189 ... (1) Formula Here, C, Si, Mn, Ni, Cr, N, Cu, and Al refer to the mass% of the element. A ferritic stainless steel for automobile exhaust path components, comprising the following components: C: 0.03% by mass or less, Si: 0.1 to 0.8% by mass, Mn: 1.0% by mass or less, P: 0.04% by mass or less, and S: 0.01 Mass% or less, Ni: 0.5 mass% or less, Cr: 12.0 to 15.0 mass%, N: 0.015 mass% or less, Nb: 0.1 to 0.5 mass%, Cu: 0.8 to 1.5 mass%, Al: 0.1 mass% or less, and One or more selected from the following components: Ti: 0.20% by mass or less, Mo: 0.5% by mass or less, V: 0.1% by mass or less, Zr: 0.5% by mass or less, W: 0.5% by mass or less, Co: 0.5% by mass or less , B: 0.01% by mass or less, the remaining portion is composed of Fe and unavoidable impurities, and γmax represented by the following formula (2) is 55 or less, γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-12Mo-49Ti-52Al + 189 ... (2) In the formula, C, Si, Mn, Ni, Cr, N, Cu, Mo, Ti, and Al refer to the mass% of the element.