JPH0247526B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a manufacturing method for increasing the high temperature strength (especially creep strength) of Cr-Mo based low alloy steel extra thick steel used in high temperature pressure vessels. (Prior art and problems) Cr-Mo based low alloy heat-resistant steel is used in the chemical industry, petrochemical industry, etc. due to its excellent high temperature strength and hydrogen corrosion resistance.
Widely used in high-temperature, high-pressure reaction vessels such as oil refining. Nowadays, high-temperature reaction vessels are becoming larger, higher in temperature, and higher in pressure in order to improve efficiency, and along with this, there is a tendency for the thickness of the apparatus to become thicker and thicker. When manufacturing as a monoblock, increasing the thickness leads to a decrease in the cooling rate at the center of the plate thickness, resulting in a decrease in strength and toughness. In addition, an increase in wall thickness requires a longer stress relief annealing time, which also leads to a decrease in strength. Due to these circumstances, we have increased high-temperature strength to avoid an extreme increase in wall thickness compared to conventional component systems, and developed high-toughness and tempering-resistant brittleness to prevent brittle fractures caused by pressure tests during periodic inspections. New considerations will need to be made in terms of strength and toughness, such as oxidation resistance. Additionally, the trend toward higher temperatures than conventional operating temperatures in order to increase reaction efficiency requires steels with higher hydrogen attack resistance and higher creep strength than conventional steels. For example, 3Cr-1Mo steel is said to be able to withstand hydrogen attack up to 538℃ as a steel that can withstand such high temperatures.
It has the disadvantage of low high temperature strength. In other words, conventionally known Cr-Mo based low alloy steels include steels known from JP-A-50-130621 and JP-A-55-41961, but both of these steels are However, sufficient strength cannot be guaranteed, and the above-mentioned problems have not been solved by using steel components alone. (Means and effects for solving the problem) The present inventors have attempted to further increase the strength of the heat-resistant low-alloy steel as described above by adding an appropriate amount of V.
We tried adding Nb, Ti, etc., but learned that the strength level and toughness of these steels vary greatly depending on the manufacturing history, so we decided to establish manufacturing conditions that would ensure balanced strength and toughness. It was a success. That is, the present invention has C0.10 to 0.20% and Si0.80% by weight.
% or less, Mn0.2~1.5%, Cr2.1~5.0%, Mo0.4~
Contains 1.5%, V 0.35% or less, 0.01 to 0.12% in total of one or both of Nb and Ti, Sol, Al 0.01 to 0.1%,
Alternatively, a steel ingot or slab to which 0.0003 to 0.002% of B is added and N is limited to 0.005% or less is heated to 1100 to 1280℃, and then hot worked in a temperature range of 800℃ or higher to continue forming austenite. This is a method for manufacturing extra-thick steel materials for high-temperature and high-pressure vessels, which is characterized by holding the steel material at a temperature between 880 and 1050° C. for oxidation, and then quenching or normalizing it. The present invention will be explained in detail below. First, in the present invention, the extremely thick steel refers to a steel with a thickness of over 100 mm. This is due to the fact that in applications such as the chemical industry and oil refining mentioned above, the thickness has increased compared to the conventional thickness of 100 mm or less due to larger equipment or higher pressure. It is intended for those of. Next, the reason for limiting each component of the steel to be subjected to the method of the present invention as described above will be described. C is necessary to maintain strength, but if it exceeds 0.20% it will impair weldability and toughness, so the upper limit is set at 0.20%, and the lower limit is that it is difficult to maintain strength when a high tempering parameter is used during post-weld heating. Therefore, it was set at 0.10%. Here, the temper parameter (T.
P.) is determined by TP=T(20+1ogt). However, T: temperature (K) and t: time (hour). Although Si is added as a deoxidizing agent, it is also an element that is effective in improving strength. However, if it is too large, weldability and toughness will be adversely affected, so it is set at 0.80% or less. Mn is a necessary component not only for deoxidizing but also for maintaining strength. However, if it exceeds 1.5%, it is undesirable from the viewpoint of toughness, so the upper limit was set at 1.5%, and the lower limit was set at 0.2% from the viewpoint of guaranteeing the strength of extremely thick materials. Cr is necessary from the viewpoints of oxidation resistance, hydrogen corrosion resistance, and strength, but if it is added in excess of 5%, problems will occur with weldability, so the upper limit was set at 5%. The lower limit is determined from the perspective of hydrogen erosion resistance.
Considering the content of Mo, V, etc., it was set at 2.1%. Mo is an element that significantly increases high-temperature strength,
If it is less than 0.4%, the effect is extremely reduced, and if it exceeds 1.5%, there is almost no increase in the effect and it has a negative effect on weldability, so the upper limit was set to 1.5% and the lower limit was set to 0.4%. V significantly increases tempering softening resistance,
Like Mo, it is an element that has a remarkable effect on improving high-temperature strength, but adding more than 0.35% has a decisive negative effect on weldability, so the upper limit was set at 0.35%. Next, Nb and Ti are elements that make crystal grains finer and improve strength, but their amount alone or in total is 0.01
If it is less than 0.12%, it will not be effective, and if it exceeds 0.12%, the creep strength will decrease, so the upper limit should be set to 0.12%,
The lower limit was set at 0.01%. Sol and Al are effective elements for improving toughness, but
If it is less than 0.01%, the effect is weak, and if it exceeds 0.10%, it will adversely affect hot workability, so the upper limit was set to 0.10% and the lower limit was set to 0.01%. The above are the basic components of the steel according to the present invention, but when the plate thickness becomes extremely thick, a component system that takes hardenability into consideration is required. B is an effective element for preventing the precipitation of ferrite and ensuring a bainite structure when the cooling rate during quenching becomes extremely slow in extremely thick materials.
If it is less than 0.0003%, there is no effect on hardenability no matter how large the amount of Al is or how much the amount of N (described later) is decreased.
In addition, if it exceeds 0.0020%, it will adversely affect workability and weldability due to segregation, so the upper limit should be set at 0.0020% and the lower limit should be set at 0.0020%.
It was set as 0.0003%. It is effective to keep the amount of N at a low level along with the addition of Al in order to ensure hardenability with the extremely small amount of B mentioned above, but the effect of the very small amount of B appears for the first time when it is reduced to 0.005% or less. Because it comes,
We decided to keep it below 0.005%. The above is the steel to which the manufacturing method of the present invention is applied. A manufacturing method using this steel to increase high-temperature strength and ensure toughness at the same time will be described below. First, a steel ingot or slab is melted using normal steelmaking methods and made into an ingot using continuous casting or ordinary ingot making.
After that, heating prior to hot rolling must be heated to 1100°C or higher in order to dissolve NbC and TiC in austenite and expect strengthening through subsequent precipitation. However, if heated above 1280℃, the crystal grains will begin to coarsen, which will have a negative effect on the toughness of the final product, so the upper limit of the heating temperature must be set.
The temperature was set at 1280℃, and the lower limit was set at 1100℃. Next, hot processing in this case refers to forging, ring rolling, roll rolling, etc., and the lower limit of the temperature range for such processing was set at 800°C. The reason for this is that when the temperature is lowered below this temperature, the amount of NbC and TiC precipitated increases, and these precipitates do not dissolve into solid solution again during the subsequent austenitization process, resulting in precipitates that do not contribute to the strength of the steel in the final stage. This is because the amount increases. Also, the temperature is immediately held between 880 and 1050°C, but the reason for holding it immediately is the same as the reason for limiting the processing temperature range.
This is to suppress the precipitation of NbC and TiC, and the reason for keeping it within this temperature range is to homogenize the steel material, and the reason why the lower limit was set at 880℃ is that below this it would take too long to homogenize. The reason why the upper limit was set at 1050°C is that exceeding 1050°C adversely affects toughness. Note that the holding time is not particularly determined, but from the viewpoint of soaking, it should be 20 minutes or more. Next, quenching or normalizing is an operation in which the steel material is cooled from a temperature in the austenite range, and in this case, it is performed to transform the steel material into martensite, bainite, etc., and obtain homogeneous and excellent strength characteristics. The effects of the present invention will be described in more detail with reference to Examples below. (Example) Table 1 shows the chemical composition of the test steel. The test steel was melted and ingot-formed in a high-frequency furnace, and then forged to 60t×
The material has a shape of 100w x 200l. Table 2 also shows hot working and cooling conditions, cooling rate,
The actual steel plate thickness corresponding to that cooling rate, T (20
+logt) and various properties, namely room temperature, high temperature tensile properties, creep rupture properties, and impact value vEo at 0°C. Note that the room temperature tensile test was conducted using JIS No. 4 high temperature tensile test, and the creep rupture test was conducted using a JIS standard test piece.

【table】

【table】

[Table] (Note) ○: Comparative example Water cooling corresponds to quenching, and air cooling corresponds to normalizing.
All retention times during austenitization are 1
It's time.
In Table 2, Nos. 1, 2, 5, 6, 8, and 9 are comparative examples, and Nos. 3, 4, and 7 are examples of the present invention. No. 1 is the normal process condition, i.e. heating at 1250℃,
This refers to the process of cooling the product to 250°C or less after finishing processing at 1100° to 950°C, but since it is cooled after rolling and then reheated to form austenite, sufficient strength cannot be obtained. Further, in Nos. 2 and 5, the heating conditions do not meet the requirements of the present invention, and the high temperature strength of the former does not improve much compared to the conventional process, and the toughness of the latter deteriorates significantly. Further, No. 6 is a material whose processing temperature is below the lower limit at the end of processing, and No. 9 is a material which exceeds the upper limit of the austenitization retention temperature, and both have problems in terms of strength or toughness. Further, No. 8 is below the lower limit of the austenitizing temperature and has low creep rupture strength. On the other hand, Nos. 3, 4, and 7 that meet the requirements of the present invention
The steel produced under these conditions has significantly improved creep rupture strength and comparable toughness compared to the comparative example. (Effects of the Invention) As described above, according to the manufacturing method of the present invention, it is possible to provide a steel material that has higher creep rupture strength and higher temperature strength than conventional manufacturing methods, and is well balanced with toughness. Therefore, the size of high-temperature and high-pressure equipment has increased,
It can handle high temperatures and is useful for reducing the weight of equipment, making it an extremely important contribution to industry.

Claims (1)

[Claims] 1% by weight: C 0.10-0.20%, Si 0.80% or less, Mn 0.2-1.5%, Cr 2.1-5.0%, Mo 0.4-1.5%, V 0.35% or less, one of Nb, Ti Or a steel ingot or slab containing 0.01-0.12% in total of the two types, Sol, Al 0.01-0.1%, or further adding B 0.0003-0.002% and limiting N to 0.005% or less at 1100-1280℃ After heating, hot processing is performed in a temperature range of 800℃ or higher,
A method for producing an extra-thick steel material for high-temperature and high-pressure vessels, which comprises immediately holding the temperature at a temperature between 880 and 1050°C for austenitization, and then quenching or normalizing it.