JPH08199237A - Production of hot rolled ferritic stainless steel strip excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE: To produce a hot rolled ferritic stainless steel strip excellent in toughness at low temp. as well as in heat resistance at a low cost by subjecting a hot rolled plate, having a specific composition consisting of C, Si, Mn, Cr, Nb, Ti, Cu, N, Al, O, and Fe and also having specific plate thickness, to water cooling and then to coiling at specific temp. CONSTITUTION: A ferritic stainless steel, having a composition consisting of, by mass, <=0.03% C, 0.80-1.20% Si, 0.60-1.50% Mn, >13.5-15.5% Cr, 0.20-0.80% Nb, <=0.1% Ti, 0.02-<0.30% Cu, <=0.03% N, <=0.05% Al, <=0.012% O, and the balance Fe with inevitable impurities at manufacture, is hot-rolled at >=800 deg.C to 4.5-9.0mm final plate thickness. Then, the resulting hot rolled plate is immediately water-cooled to avoid the precipitation of Laves phase and coiled. At this time, coiling temp. T is regulated so that it satisfies the inequality, t×T<=3600 [where (t) is the thickness of the hot rolled plate].

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a low-cost ferritic stainless steel excellent in low-temperature toughness and suitable for use in exhaust gas pipeline members such as various internal combustion engines and gas turbines.
It relates to a method for manufacturing hot-rolled sheets from 4.5 mm to 9.0 mm.

[0002]

2. Description of the Related Art In recent years, as interest in global environmental issues has increased, there has been a demand for a thermal power generation system and an engine with good combustion efficiency, as well as an automobile engine that can meet exhaust gas regulations. If measures are taken to satisfy these requirements, the temperature of the combustion gas will rise and the temperature of peripheral members such as the exhaust gas purification system will rise. As a result, these components are
Greater heat resistance is required.

Austenitic stainless steel has higher high temperature strength than ferritic stainless steel. However, since thermal expansion is large, thermal strain is large, and cracks due to thermal fatigue easily occur when subjected to repeated heating and cooling. Austenitic stainless steel is C
Since it contains a large amount of r and Ni, the manufacturing cost also increases. For these reasons, ferritic stainless steel is often used for automobile exhaust gas materials and heat-resistant members for various plants.

In Japanese Patent Publication No. 62-206, in order to avoid slab cracking of Nb-containing ferritic stainless steel, Nb
It is disclosed that the cooling condition of the slab needs to be strict so as to suppress the formation of the system carbide. In addition,
In 6-17516, mainly in order to avoid 475 ° C. embrittlement, the cooling rate and the coiling temperature after hot rolling are strictly specified. The present inventors have developed a low Cr ferritic stainless steel having excellent heat resistance as disclosed in JP-A-5-125491.

This steel is an ultra-low carbon nitrogen ferritic stainless steel having a basic composition of 11% or more and less than 17% Cr, and Si, Mn, and Nb added together, and has a normal hot-rolled steel strip thickness. Is about 3.0 mm to 4.5 mm, and Cr
Since the amount is less than 17%, N containing 18% or more of Cr
It is not necessary to pay attention to the following as to the deterioration of the toughness of the slab and hot rolled sheet peculiar to b-containing ferritic stainless steel. That is, in the steel disclosed in Japanese Patent Laid-Open No. 5-125491, since the upper limit of the amount of carbon is strictly defined, the generation of carbide is small and it is not necessary to define the cooling conditions for the slab. Further, since elements such as Cr, Mo, and Al that promote embrittlement at 475 ° C. are minimized, there is no fear of cracking due to coil expansion after hot rolling.

[0006]

A plant plate having a thickness of 4.5 mm or more may be required for a plant member such as an exhaust gas duct of a combined cycle thermal power generation system. However, the hot-rolled steel strip disclosed in JP-A-5-125491 above has a bendability and low-temperature toughness of the hot-rolled steel strip less than 4.5 mm when the sheet thickness becomes 4.5 mm or more. Since it is low, if bending or tension is applied to the front surface of the annealing line during annealing after hot rolling, base material cracking may occur. In order to prevent the hot-rolled sheet from breaking and to manufacture it safely, it is a guideline that the Charpy impact value at room temperature (20 ° C) is 50 (J / cm ² ) or more.

An object of the present invention is to provide a method for manufacturing a hot-rolled steel strip to improve the bendability and low-temperature toughness deterioration of the hot-rolled steel strip peculiar to a plate thickness of 4.5 mm or more as described above. An object of the present invention is to provide a low-cost ferritic stainless steel that can be applied to members that require high properties.

[0008]

According to the present invention, the mass% is
As C: 0.03% or less, Si: 0.80% to 1.20%, M
n: 0.60% to 1.50%, Cr: over 13.5% to 15.5%, N
b: 0.20% to 0.80%, Ti: 0.1% or less, Cu: 0.02%
Above 0.30%, N: 0.03% or less, Al: 0.05% or less,
O: 0.012% or less, the balance of Fe and ferritic stainless steel containing unavoidable impurities in production,
After hot rolling at 800 ° C or higher so that the finished plate thickness of the hot rolled plate is 4.5 mm or more and 9.0 mm or less, it is immediately water-cooled, and the plate thickness t after hot rolling and the coiling temperature T during hot rolling are , T × T ≦ 3600 (1) A method for producing a ferritic stainless hot-rolled steel strip having excellent low-temperature toughness, which comprises winding at a temperature satisfying the relationship (1).

[0009]

The present inventors have developed a low-cost 14Cr-Si-Mn.
-Nb ferritic stainless steel plate thickness 4.5mm or more 9.0m
As a result of extensive research from the aspect of hot rolling conditions to improve the low temperature toughness of hot rolled steel strips of m or less, in order to improve the low temperature toughness of hot rolled steel strips, avoid the 475 ° C brittle zone. Instead, it was clarified that it is effective to cool the hot-rolled steel strip under cooling conditions that avoid the precipitation zone of the Laves phase containing Si and Nb. As a result of further detailed investigation, it was clarified that in order to obtain the minimum toughness necessary for annealing the hot-rolled steel strip, the cooling condition must be strictly specified by the plate thickness.

FIG. 1 shows 14Cr-1Mn-1Si-0.5N.
The influence of aging temperature and time on the Charpy impact value at 20 ° C. was examined by using hot-rolled sheets of b-0.2Cu steel having thicknesses of 3 mm and 6 mm. The hot-rolled sheet was hot-rolled in a laboratory and then water-cooled in a water tank. Board thickness 3m
It can be seen that the m-material has almost no problem in toughness even if the aging time changes, although the impact value of the 700- ° C aging material slightly decreases. Therefore, it can be seen that even if air cooling is performed after hot rolling, it has sufficient toughness. On the other hand, the 6 mm thick material is
The impact value of the 475 ° C. aged material does not decrease, but the impact value sharply decreases by aging at 700 ° C.

FIG. 2 shows 14Cr-1Mn-1Si-0.5N.
b-0.2Cu steel, hot rolling finish temperature 850 ℃,
Finished plate thickness is 6mm, winding temperature is 500 ℃ and 70
It shows the Charpy impact characteristics of a plate hot-rolled under the condition of 0 ° C. Also, the plate thickness 3mm wound at 700 ℃
The impact characteristics of the hot rolled sheet are also shown. From the results in Figure 2,
When the plate thickness is 6 mm, the toughness of the hot-rolled sheet is low when wound at 700 ° C and then slowly cooled. However, when it is passed through the temperature range around 700 ° C in a short time by rapid cooling and wound at a lower temperature, It was found that the same toughness as a 3 mm thick hot-rolled sheet was obtained.

Therefore, when manufacturing the material of this component system, it is not necessary to consider the cooling condition after hot rolling for a thin plate, but if the plate thickness becomes thick, the toughness of the hot rolled plate may depend on the cooling condition. It became clear that it became extremely low. In addition,
The reason why the impact value differs depending on the plate thickness under the same hot rolling conditions is not necessarily clear, but in the Charpy test, the plastic deformation amount of the thin plate material is larger than that of the thick plate material because the plastic deformation amount near the surface layer of the plate is larger than that near the center. It is considered that this is because the ratio of the deformation amount becomes high. As a result, it is assumed that the thick plate material is more significantly affected by the embrittlement factor that affects the impact value.

FIG. 3 shows 14Cr-1Mn-1Si-0.5N.
The relationship between the coiling temperature and the hot-rolled finished sheet thickness of a b-0.2Cu steel, which was rapidly water-cooled to various temperatures after completion of hot rolling and wound, and then air-cooled, is shown. From the results shown in FIG. 3, it can be seen that there is a region showing a low toughness with a Charpy impact value of 50 J / cm ² or less when winding is performed at a high temperature after finish hot rolling. Furthermore, it was revealed that this low toughness region strongly depends not only on the winding temperature but also on the plate thickness.

This cause is not due to 475 ° C. brittleness, as is clear from the result of aging at 475 ° C. in FIG. Therefore, it is considered that the toughness decreasing mechanism of this component system is different from that generally known for high Cr ferritic stainless steels of 18% or more.

Using the aging material and the hot-rolled sheet obtained in FIGS. 1 to 3, electrolytic extraction (10% acetylacetone + 1% tetramethylammonium chloride + methyl alcohol) was carried out to measure the amount of residue, and the extracted residue was X As a result of identification by the line, most were Laves phase. In addition, it was revealed that the impact value was inferior as the amount of residue increased. Further, chemical analysis confirmed that the Laves phase contained a large amount of Nb and Si.

From the above results, 14Cr-1Mn-1Si
Although it is not necessary to be concerned about the brittleness at 475 ° C, the component system of the present invention, which has a basic component of -0.5 Nb, requires careful attention to the formation of the Laves phase. It was clarified that the temperature range for rapid cooling differs depending on the thickness of the rolled sheet. In addition, the amount of decrease in toughness is Laves
It became clear that a good Charpy impact value can be obtained by reducing the generation after hot rolling, which depends on the amount of phases generated.

The action of each component in the steel of the present invention and the reasons for limiting the content thereof will be individually outlined below.

C and N: Generally, they are important elements for enhancing the high temperature strength, but on the other hand, when the content is large,
It causes deterioration of oxidation resistance, workability, and toughness. Further, C and N form a compound with Nb to reduce the amount of effective Nb in the ferrite phase. Also, this carbonitride
During continuous casting, coarse precipitation occurs at grain boundaries, which may significantly reduce the toughness of the slab. For these reasons, each content is set to 0.03% or less.

Si: An element essential for improving the high temperature oxidation resistance. Even a steel having a relatively small amount of Cr, such as the steel of the present invention, is very effective in imparting excellent high temperature oxidation resistance. However, if added in excess, it becomes hard,
0.80% to 1.20% as it causes deterioration of workability and toughness
Range.

Mn: An important element of the steel of the present invention. By adding Si like the steel of the present invention, in addition to suppressing the increase in oxidation, addition of Mn significantly improves the adhesion of the surface oxide. However, if added excessively, abnormal oxidation is rather induced by precipitation of an austenite phase. Therefore, the range is set to 0.60% to 1.50%.

Cr: A very effective element for imparting high temperature oxidation resistance, and it is necessary to add 11% or more, preferably more than 13.5%, in order to maintain high temperature oxidation resistance. . On the other hand, excessive addition causes embrittlement of the steel, hardens the workability, and raises the raw material cost. Therefore, the range of Cr exceeds 13.5% to 1
5.5%.

Nb: It is an important element of the steel of the present invention because it effectively acts to maintain high temperature strength. In order to maintain high temperature strength, it is necessary to add at least 0.20% or more. On the other hand, if Nb is excessively added, the toughness of the slab is significantly reduced. The upper limit of Nb is set to 0.80% so as to maintain sufficient high temperature strength and not significantly affect the deterioration of the slab toughness.

Cu: An element effective in improving both low temperature toughness and workability, and its effect becomes remarkable when 0.02% is added. However, if Cu is added excessively, workability is impaired. Therefore, the Cu content range is
The mass% should be 0.02% or more and less than 0.30%.

Al: Decarburization is carried out by oxygen blowing in the steelmaking process. At this time, oxygen remaining in the steel deteriorates weldability. Therefore, in general, Al is an essential element as a deoxidizer. Since the steel of the present invention contains Si, deoxidation with Al is not always necessary. However, if Al is excessively mixed in the steelmaking process, a large amount of Al-based oxide is generated during welding, which conversely reduces weldability. Therefore, in order not to adversely affect the weldability, Al
The range of the content of is 0.05% or less by mass%.

Ti: It is known that it is an element which improves the r value of the steel sheet and is effective for deep drawability. However, when Ti is added, TiN is likely to be generated, resulting in a decrease in yield due to the occurrence of bald defects in the steel sheet and a decrease in weldability.
Therefore, the content range of Ti is set to 0.10% or less by mass%. Furthermore, when severe processing is performed, TiN
In order to prevent the formation of a., It is preferably 0.05% or less.

O: As described above, since the weldability is adversely affected, it is desirable that the O content be as low as possible, but the lower the O content, the higher the manufacturing cost. On the other hand, in the steel of the present invention, the Al content is set within a range having sufficient weldability in consideration of the case where deoxidation is performed by adding Al. Therefore, the range of O content is 0.012% or less by mass%.

If the hot-rolling finishing temperature is too low, the work strain introduced during hot-rolling will not be recovered, and a large amount of Laves phase may be produced using this as a production nucleus. Therefore, the hot rolling finishing temperature is set to 800 ° C or higher. Although the cooling rate after hot rolling is not specified, it is a factor that affects the amount of Laves phase that adversely affects toughness.
In other words, if the cooling rate after hot rolling is too slow, the time for heating to the Laves phase formation temperature range becomes long, and there is a possibility that a large amount of Laves phase will grow. In order to suppress the precipitation amount of these, it is preferable that the cooling rate after hot rolling is 30 ° C./second or more.

The winding temperature is one of the most important factors in the present invention. After finishing hot rolling at 800 ℃ or higher, quenching and winding at the lowest temperature possible
Although it is desirable to suppress the generation of the s-phase, as shown in FIG. 3, the allowable range (upper limit temperature) of the winding temperature changes depending on the plate thickness. That is, in FIG. 3, the upper limit temperature T (° C.) at the start of water cooling at which the impact value at 20 ° C. becomes 50 J / cm ² is proportional to the reciprocal of the hot-rolled sheet thickness t (mm), and t × T ≦ 3600.・ ・ The relationship of (1) should be satisfied. If the cooling rate is controlled according to the equation (1), efficient operation becomes possible. The amount of Laves phase produced is preferably 0.5 mass% or less.

The method of cooling the steel strip after hot rolling specified in the present invention can be achieved by defining the hot-rolled finished sheet thickness and the coiling temperature. Even if the entire coil is rapidly cooled later in a water tank or the like, there is no problem. Further, in the present invention, only the hot-rolled steel strip having a plate thickness of 4.5 mm or more and 9.0 mm or less is specified, but the hot-rolled steel strip having a plate thickness of less than 4.5 mm can also be hot-rolled by applying the method of the present invention. It is possible to improve the low temperature toughness of the plate.

It was manufactured by the above manufacturing method,
The ferritic stainless hot rolled steel strip of the present invention is excellent in low temperature toughness of the hot rolled sheet. Moreover, it can be manufactured at a lower cost than ferritic stainless steel containing 18% or more of Cr.

[0031]

EXAMPLES The low temperature toughness of the hot rolled steel strip of the present invention will be described with reference to examples.

Table 1 shows the chemical composition values of the test materials.

[0033]

[Table 1]

Steels A to C in the table are included in the scope of the present invention. All of the steels were melted in a vacuum melting furnace, and hot rolled into hot-rolled steel strips having various thicknesses, and various cooling conditions after hot rolling were changed. Table 2 shows the hot rolling method. Using the obtained hot rolled sheet, the low temperature toughness and bendability of the hot rolled sheet were examined. Table 2 also shows the test results.

[0035]

[Table 2]

First, the low temperature toughness was evaluated by a Charpy impact test value at 20 ° C. That is, a V notch test piece with a plate thickness of 3.0 mm to 9.0 mm conforming to "JIS Z 2202" was prepared,
The metal material test method (Charpy impact test) specified in "JIS Z 2242" was performed to determine the impact value.

The bendability of the hot rolled sheet was evaluated by a bending test. That is, a metal material bending test piece conforming to "JIS Z 2204 No. 1" was prepared, and a bending angle by a pressing bending method specified in "JIS Z 2248" was measured.

As can be seen from Table 2, the materials hot-rolled by the method according to the present invention all have a Charpy impact value at 20 ° C. of 50 J / cm ² or more. Also, the bendability of the hot-rolled sheet was good, and no cracks were formed at 180 ° bending.

On the other hand, in Comparative Examples 9, 10 and 13, since the winding temperature is not regulated by the plate thickness and the winding is performed at a temperature higher than the range of the present invention, the Charpy impact value is low. ing. In particular, Comparative Examples 9 and 13 are also inferior in bendability of the hot rolled sheet.

Further, in Comparative Example 11, the plate thickness is out of the scope of the claims of the present invention, so the impact value and bendability are inferior even when hot rolling according to the method of the present invention is performed. Further, Comparative Example 12
Although the winding temperature is included in the method of the present invention, since the hot rolling finishing temperature is low, the amount of Laves phase produced falls outside the scope of the claims of the present invention and the impact value is poor.

[0041]

EFFECTS OF THE INVENTION According to the present invention, a low-cost ferritic stainless steel hot rolled steel strip having a plate thickness of 4.5 mm or more and 9.0 mm or less and excellent in low temperature toughness, which is suitable for members in which heat resistance is important Can be provided.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between a Charpy impact test value at 20 ° C. and aging time.

FIG. 2 is a graph showing the effect of winding temperature on Charpy impact properties.

FIG. 3 is a graph showing a relationship between a winding temperature after hot rolling and a hot-rolled finished sheet thickness, which affects a Charpy impact value at 20 ° C.

Claims (1)

[Claims] 1. As a mass%, C: 0.03% or less, Si:
0.80% to 1.20%, Mn: 0.60% to 1.50%, Cr: 13.5%
Over 15.5%, Nb: 0.20% ~ 0.80%, Ti: 0.1%
Below (including no additives), Cu: 0.02% or more and less than 0.30%,
N: 0.03% or less, Al: 0.05% or less (including no additions), O: 0.012% or less, and the balance of Fe and the unavoidable impurities in the production of ferritic stainless steel. Is hot rolled at 800 ℃ or more so as to be 4.5 mm or more and 9.0 mm or less, and immediately water-cooled to obtain the sheet thickness t after hot rolling and the coiling temperature T during hot rolling.
However, the method for producing a ferritic stainless steel hot rolled steel strip having excellent low temperature toughness of a hot rolled steel sheet, characterized in that the hot rolled steel sheet is wound at a temperature satisfying the relationship of t × T ≦ 3600 (1).