JP3319222B2 - Manufacturing method of high chromium ferritic steel with excellent creep characteristics of welded joint - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high chromium ferrite heat-resistant steel used in thermal power plants, chemical plants, etc., and particularly to imparting good toughness and improving the creep rupture characteristics of welded joints. And a method for producing a high chromium ferritic steel.

[0002]

2. Description of the Related Art A high chromium heat-resistant steel sheet containing 9 to 12% Cr is usually subjected to normalizing treatment at a high temperature of 1000 ° C. or more after hot rolling, and further to tempering.
In this case, it is known that the higher the normalizing temperature, the higher the creep strength after tempering.

For this reason, high-temperature heating in rolling is regarded as normalizing treatment, and direct normalizing is performed without normalizing after hot rolling. Such a method is disclosed in JP-A-2-182826 and the like. In this method, in order to ensure toughness, a high pressure of 50% or more is required in a temperature range of 1125 ° C. or more where recrystallization is promoted during hot working. However, the temperature tends to decrease on the slab surface, and it is practically difficult to satisfy the above conditions.

When these heat-resistant steels are welded, a softened portion is formed in the heat-affected zone of the weld. Therefore, in a creep rupture test of a welded joint, rupture occurs at the softened portion, and the creep rupture strength of the joint is reduced by the base metal. In some cases, it may decrease, and improvement is required.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a high-temperature welded joint having excellent creep characteristics without impairing the toughness of the base material by directly tempering after the hot rolling without the normalizing step. An object of the present invention is to provide a method for producing chromium ferrite steel.

[0006]

Means for Solving the Problems The inventors of the present invention have studied the mechanical properties of heat-resistant steels in a case where a high chromium ferritic heat-resistant steel is rolled and then a normalizing process is omitted and tempering is adopted. As a result of various studies, it has been found that by defining the combination of the chemical composition and the rolling conditions in a specific range, not only the base material but also the welded joint exhibit excellent creep rupture strength without impairing the toughness of the base material. .

[0007] The present invention has been made based on such findings, and in terms of mass% , C: 0.05 to 0.15.
%, Si: 0.01 to 0.5%, Mn: 0.1 to 1.0
%, Cr: 8 to 13%, Mo: 0.7 to 1.3%, V:
0.01-0.3%, Nb: 0.01-0.15%, A
l: 0.003 to 0.05%, N: 0.005 to 0.1
% High chromium ferritic steel is heated to a temperature of 1200 ° C or higher, and the cumulative rolling reduction is performed in a temperature range of 1125 ° C or higher.
Hot rolling of 10% or more and less than 50% is performed, and
Hot rolling at temperatures below 25 ° C, 800 ° C or more
Finish rolling at a temperature of, then tempering at a temperature below A _c1 point
Method for producing a high chromium ferritic steel having excellent creep properties of a welded joint, characterized in that to is to provide.

[0008]

The reason why the method of the present invention improves the creep rupture characteristics of a high chromium ferritic steel welded joint will be described based on experimental results. As described above, a softened portion is generated in the weld heat affected zone of steel of this type of system,
The creep rupture strength of the welded joint may decrease,
The main cause is Nb, V due to the welding heat cycle during welding.
Therefore, it is necessary to precipitate these precipitates in the steel as finely as possible before the welding, because it is considered to be due to the coarsening of carbonitrides. for that purpose,
First, at the time of slab heating, it is effective to heat to a temperature at which the total amount of Nb and V forms a solid solution, to perform a direct tempering treatment after cooling, and to finely disperse these precipitates in advance. Therefore, the slab heating temperature needs to be 1200 ° C. or higher. In this case, there is a concern that the austenite grains become coarse and the toughness is reduced. Therefore, the hot rolling condition is 1
Cumulative rolling reduction of 10% or more and not more than 50% in the temperature range of 125 ° C or more
With various changes in the full range,
Hot rolling in the temperature range and rolling at a temperature of 800 ° C or higher
After completion of tempering, tempering at a temperature not higher than A _c1 point
Ri, industrially sufficiently feasible, and it was confirmed that the toughness level required as high chromium heat resistant steel can be ensured.

0.10C-0.34Si-0.44Mn
−8.52Cr−0.99Mo−0.20V−0.08
Nb: 0.013Al-0.048N, using a 100 mm thick steel slab composed of the balance Fe and unavoidable impurities, after heating at 1250 ° C, the cumulative draft in the temperature range of 1125 ° C or more is 10% or more and less than 50%. Hot-rolling with various changes in the range of
A 25 mm thick steel plate was obtained by hot rolling . Note that the rolling end temperature is set to 800 ° C. or higher where the anisotropy of the microstructure becomes remarkable, and as a result, the toughness does not decrease. Thereafter, tempering treatment was performed at 800 ° C. for 1 hour, and a 2 mm V notch Charpy impact characteristic and a creep rupture characteristic were examined.

[0010] Fig. 1 shows the effect of 1 on the absorbed energy at 0 ° C.
The effect of the cumulative draft in a temperature range of 125 ° C. or more is shown. As is clear from the figure, the absorbed energy value after tempering is extremely good at about 200 J at a rolling reduction of 10% or more.
Normalizing-Temperature shows almost the same level as that of the conventional steel.

FIG. 2 shows the results of creep rupture test of a welded joint obtained by submerged arc welding of the present test steel.
℃ illustrates in creep rupture time of -14kgf / mm ^2. For the creep rupture test, a tensile test piece of φ6 × GL30 mm was used, and a sample was taken so that the fusion line (bond) of the welded portion was at the center of the test piece. All the results obtained show a higher creep rupture strength compared to normalized-tempered welded joints of conventional steel, which tendency is more pronounced at cumulative reductions below 50%. . The reason for this is that when the cumulative rolling reduction in the temperature range of 1125 ° C. or higher is small, in other words, when the rolling reduction is lower than 1125 ° C. , the dislocation density introduced into the steel material decreases. As a result, the precipitation of carbonitride during the tempering process is considered to be finer.

Therefore, in the present invention, the heating temperature is set to 120
Assuming a heating temperature of 0 ° C. or more, hot working of 10% or more and less than 50% is performed in a temperature range of 1125 ° C. or more. Then, continue hot in the temperature range below 1125 ° C.
Rolling is performed, rolling is completed at a temperature of 800 ° C. or more,
In temper at normal temperatures A following point _C1 is tempering temperature.

Next, the reasons for limiting each component element in the present invention will be described. C: C is an element effective for increasing the strength at normal temperature and high temperature, and requires at least 0.05% from the strength level required for high Cr heat resistant steel. However, since the weldability decreases with an increase in the amount of C, the upper limit is set to 0.15%.

Si: is added as a deoxidizing agent.
If it exceeds 5%, the toughness decreases, so the upper limit is made 0.5%. Mn: Mn is an element effective for fixing S and increasing the strength. However, when the addition amount increases, the creep rupture strength decreases, so the addition amount is set to 0.1 to 1.0%.

Cr: Cr is required to be 8% or more to improve oxidation resistance and high-temperature strength, but is set to 13% or less to avoid a decrease in weldability. Mo: Mo improves the high-temperature strength by solid solution strengthening and precipitation strengthening, but the effect is not remarkable when added less than 0.7%, and when it exceeds 1.3%, the weldability is reduced and the cost is increased, so that Mo is added. The amount is between 0.7 and 1.3%.

V: V is a precipitation strengthening element like Nb and needs to be 0.05% or more. However, since a large amount of addition lowers the weldability, the addition amount is 0.05 to 0.3%. And

Nb: Nb enhances the high-temperature strength by precipitation strengthening, but if it is less than 0.01%, its effect is not sufficient, and if it exceeds 0.15%, the effect corresponding to the added amount cannot be obtained. The amount is 0.01-0.15%.

Al: Al is added as a deoxidizing agent, and therefore, is added in an amount of 0.003% or more. However, the higher the amount of Al, the lower the creep rupture strength. Therefore, the upper limit of addition is made 0.05%.

N: N improves the strength, but is 0.00%.
If it is less than 5%, the effect is not sufficient, and in a usual smelting method, pores are generated in the steel ingot by adding more than 0.1%. .

The steel having the above chemical composition is used in a converter,
After smelting in an electric furnace, it is obtained by performing ladle refining and vacuum degassing as necessary, and after ingot making, it is made into a slab by slab rolling. The slab may be manufactured directly from molten steel by a continuous casting method.

[0021]

EXAMPLES Steel slabs having the chemical compositions A to C shown in Table 1 were subjected to hot working under various conditions shown in Table 2 and further tempered. Was subjected to post-weld heat treatment (PWHT) to investigate the Charpy impact characteristics, and the welded joint was subjected to submerged arc welding under the conditions shown in FIG. 3 and subjected to PWHT and then to a creep rupture test. In either case, the PWHT condition is
740 ° C. × 4 hours. Table 2 summarizes the obtained results.
Shown in

As is evident from Table 2, in the examples of the present invention in which the steel composition and the production conditions are within the scope of the present invention, not only the conventional steel subjected to normalizing and tempering treatment but also the cumulative rolling reduction at 1125 ° C. or more. It was confirmed that the steel exhibited an excellent balance between impact characteristics and creep rupture characteristics as compared with the comparative steels having 50% or more.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

According to the present invention, it is possible to produce a high chromium ferritic heat-resistant steel having not only good toughness but also a welded joint having excellent creep characteristics. The high chromium ferrite heat-resistant steel produced by the present invention is extremely useful as a thermal power generation or a chemical plant used at high temperature and high pressure, and has great industrial value such as high efficiency of the plant.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the cumulative rolling reduction in a temperature range of 1125 ° C. or more on the absorbed energy at 0 ° C.

FIG. 2 is a graph showing creep rupture test results of a submerged arc welded joint by creep rupture time at 600 ° C. and 14 kgf / mm ² .

FIG. 3 is a diagram showing submerged arc welding conditions for a welded joint.

────────────────────────────────────────────────── (5) References JP-A-2-182826 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38 / 00-38/60

Claims (1)

(57) [Claims] (1) C: 0.05 to 0.15% by mass %;
Si: 0.01 to 0.5%, Mn: 0.1 to 1.0%,
Cr: 8 to 13%, Mo: 0.7 to 1.3%, V: 0.
01-0.3%, Nb: 0.01-0.15%, Al:
A high chromium ferritic steel containing 0.003 to 0.05% and N: 0.005 to 0.1% is heated to a temperature of 1200 ° C. or more, and a cumulative rolling reduction 10 in a temperature range of 1125 ° C. or more.
% Or more and less than 50%.
Hot rolling is performed in a temperature range of less than 800 ° C and a temperature of 800 ° C or more.
A method for producing a high chromium ferritic steel having excellent creep properties of a welded joint , wherein rolling is completed at a temperature and then _{tempered at} a temperature not higher than A _c1 point.