JPH01319631A - Production of extra thick steel plate for pressure vessel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to Cr-Mo steel for chemical reaction vessels such as oil refining, and is directed to the production of extra-thick steel plates for pressure vessels with excellent toughness in the surface layer of the steel plate. It is about the method.

[Prior art] Cr-Mo steel used for chemical reaction vessels, etc. is conventionally rolled, then cooled and then heated again to a predetermined temperature (usually about 93,000).
It has been used after being hardened or normalized and then tempered. Due to the high temperature and high pressure requirements for blunt use, there is a demand for higher strength steel materials, and efforts are being made to develop composition systems with the addition of new alloying elements.

However, quenching at high temperatures, which is essential for effective utilization of alloying elements, causes coarsening of austenite crystal grains, resulting in a decrease in low-temperature toughness.

On the other hand, there is a method known from JP-A-58-107415 or JP-A-61-87818, that is, a production method that utilizes the high temperature of the as-rolled product and quenches it from a temperature equal to or higher than the A ra temperature. In the (direct quenching method), since the alloying elements are sufficiently solid-dissolved, high strength can be obtained, and at the same time, it is possible to improve low-temperature toughness by refining the γ grains by rolling.

However, chemical reaction vessels often use relatively thick steel plates with a thickness of 75 mm or more, and in this case, because the temperature and other conditions differ between the surface layer and the inside during rolling, it is difficult to maintain uniform toughness in the thickness direction. There are drawbacks that are difficult to obtain.

[Problems to be Solved by the Invention] Chemical reaction vessels are used at high temperatures and require tensile strength and creep rupture strength at high temperatures. In addition, chemical reaction vessels are required to undergo pressure resistance tests during periodic inspections, and the materials used are required to have excellent low-temperature toughness in each part of the plate thickness.

The method of the present invention is an extremely thick C manufactured by rolling direct quenching method.
The present invention provides a method for improving the deterioration in toughness in the surface layer of a steel sheet, which is a drawback of r-Mo steel.

[Means for Solving the Problems] As a result of various studies on the influence of process conditions on the material quality in the surface layer of a Cr-Mo steel direct quenching mill containing Nb, the inventor found that It has been found that the toughness of the surface layer can be improved by controlling the surface temperature.

The present invention was made based on the above-mentioned knowledge, and in terms of weight %, 0003-017%, Si0.02-0.5
%, Mn: 0, 3-1, 0%, Cr: 1.5
-50%, Mo: 0.5-1.5%, V: 0.
03-0.5%, Nb: 0 old-0.1%, Al: 0.
005~0.05%, NO1 old% or less, P: 0.0
2% or less, S: 0.02% or less as a basic component, and if necessary, B: 0.0002-0.005% alone or Tl: 0.01-0.05% and silk together. , steel consisting of residual Fe and unavoidable impurities is heated to 1100~
After heating to 1280°C, hot rolling is started before the surface temperature drops below 950°C, rolling is finished at 800 to 1050°C, and immediately quenching and tempering is performed from a temperature of 800°C or higher. This invention relates to a method for manufacturing extra-thick steel plates for pressure vessels.

[For production] The present invention will be explained in more detail below.

C is an effective element for increasing the strength at room temperature and high temperature, and at least 0.03% is required to meet the strength level required for steel for chemical reaction vessels.

As C poison increases, the toughness of the steel material decreases and weldability also deteriorates, so the upper limit is set at 0.17%.

Si is added in an amount of 0.02% or more to deoxidize and increase strength, but if the amount added is too large, the toughness decreases, so the first limit is set to 0.02%.
5%.

Mn is an effective element for fixing S and increasing strength, but if added in a large amount, it will cause significant segregation within the material and increase the anisotropy of toughness. do.

P is an element that not only micro-segregates in steel and causes significant directional differences in toughness, but also segregates at grain boundaries during tempering and post-weld heat treatment, reducing toughness. Therefore, it is desirable to reduce it, so the upper limit should be set. is 0.02%.

S forms nonmetallic inclusions MnS in steel, increases the directional difference in toughness, and lowers the upper shelf energy in the Charpy test, so the upper limit is set to 0.02%.

Cr not only increases hardenability, but also precipitates carbonitrides during tempering/smoothing and post-weld heat treatment, improving high-temperature strength.

Further, Cr is added in an amount of 1.5% or more in order to stabilize carbonitrides and improve the hydrogen corrosion resistance of the steel. However, 5.0
Since addition of more than % is unnecessary for steel for chemical reaction vessels,
The upper limit is set to 50%.

Mo is added to increase high temperature strength, especially creep rupture strength. However, if it is added less than 0.5%, the effect is not noticeable, and if it exceeds 1.5%, the effect is saturated, so the amount added (
)5 to 15%.

(2) itself forms a carbonitride, which increases the strength, and also forms a solid solution in the Cr carbonitride, which has the effect of further stabilizing the Cr carbonitride. For this purpose, it is necessary to add 0.03% or more, and if it exceeds 0.5%, the effect is saturated and the effect corresponding to the amount added cannot be obtained, so it is set at 0.03 to 0.5%.

Nb is an element that forms stable carbonitrides during the thermal history of heating and cooling, and has the effect of significantly improving the high temperature strength of steel. Further, it prevents deformation-induced precipitation due to rolling, movement of grain boundaries, and coarsening of recrystallized grains. For this reason, 0.01% or more is added, but if it exceeds 0.01%, the effect commensurate with the amount added cannot be obtained, so it is economically suppressed to 0.1% or less.

8β is an essential element for deoxidizing steel, and for this purpose, the lower limit is set to 0.005%. However, if the amount of Aρ added is high, it will harm the creep rupture strength, so the first limit of addition should be reduced to 0.
．． 05%.

Like C, N increases the strength of steel, but in normal melting methods, addition of more than 0.03% causes formation of old pores in the steel ingot. If the pores are not compressed even by rolling, the ductility and toughness will decrease, so the addition amount should be 0.03% or less.

In addition, when B is added and made to act, the effect of B is impaired if Nz is too large, so the N content should be 0.01% or less.

In the present invention, the above component composition may be used as the basic component, or B may be added alone or in combination with Ti to improve strength and toughness.

B is an element that increases hardenability by 1 when added in a small amount, and is added when further hardenability is required.

Is the hardening improvement effect recognized from 0.0002%?
．． There is no point in increasing it beyond 0.005%. Therefore, the amount added is set to 0.0002 to 0.005%.

Ti combines with N, and B has no effect on improving hardenability.
:Has the effect of preventing becoming. Therefore, it can be added together with B. However, the effect is not sufficient with 0 and old% not t&. When adding B, the amount of nitrogen in the steel is reduced to 0.
．． The maximum amount added is 0.005% as it is regulated below the old percentage.
The following is fine.

Steel with the above chemical composition is obtained by melting in a converter or electric furnace, then subjecting it to ladle smelting or vacuum degassing treatment as necessary, and is usually formed into an ingot using a mold or one-way solidification mold. Afterwards, it is made into slabs by blooming.

Slabs may be manufactured directly from molten steel by continuous casting.

Next, hot rolling conditions will be described.

Any soaking and pressing methods may be used for the blooming. Immediately, the slab may be soaked after cooling, or the slab may be charged as a hot slab into a soaking furnace.

After soaking at 1000 to 1280°C, a slab is formed by rolling or forging. The thickness of the slab is preferably about 13 to 2.5 times the thickness of the product board.

It is essential that the slab be heated to a temperature at which some or all of the Nb and V contained in the steel are dissolved. Therefore, it is heated at a temperature of 1100°C or higher. However, when the temperature exceeds 1280°C, the austenite grains become coarse and cannot be made finer even by rolling.
The temperature shall be below 0℃.

The heated slab is transported to a rolling mill by a crane, table roller, etc., and hot rolled to a predetermined thickness. During this transport time, the surface temperature of the slab decreases.

Figure 1 shows 0,15%C-0,18%5j-0,5]%M
n -0,008%P-[1,0[18%May-31%Cr-1,,08%Mo-0,22%V-0,04,
3%Nb-[],]f]07%Al-0.008%N, rolled to a thickness of 1.20 mm by varying the surface temperature of rolling start [1, +1, and quenched from a temperature of 800°C or higher. , shows the change in the Charpy fracture surface transition temperature of the surface layer (inside 8 areas from the rolled surface) when tempered at 710°C for 10 hours.

If the slab surface temperature at the start of rolling is less than 950°C, vTr
While s increases and toughness decreases, at temperatures above 950°C, excellent vTrs of -506C or less can be stably obtained. Therefore, in order to improve the toughness of the surface layer, it is necessary to control the surface temperature at the start of rolling to 950° C. or higher.

In order to prevent the surface temperature from decreasing, it is also effective to keep the slab warm after it has been extracted from the heating furnace by insulating it or covering it with heat-generating material.

Such regulation of the surface temperature at the start of rolling is particularly important when directly quenching an extra-thick steel plate exceeding 75 mm in thickness to obtain excellent surface toughness. The rolling end temperature is 800 to 105 from the viewpoint of securing the quenching start temperature described below.
The temperature shall be 0°C.

However, if it is less than 800°C, it is impossible to ensure a quenching start temperature of 800°C or higher, making it difficult to secure the material quality. - direction, 10
If the temperature exceeds 50°C, the austenite grains will not be sufficiently refined by rolling, the structure will become coarse, and it will be difficult to ensure the material quality, especially the toughness, which is not preferable.

Next, the material is quenched quickly after rolling, in order to ensure a quenching temperature of 800° C. or higher. That is, when the quenching start temperature is lower than 800°C,
A part of the solid-dissolved strengthening elements such as Nb and V precipitate and no longer contribute to strengthening. Therefore, the quenching starting temperature is 800℃
The above shall apply.

The quenching may be performed by rapid cooling by sprinkling cooling water, and is preferably performed efficiently by using accelerated cooling or the like.

After quenching, the product is made by tempering according to the usual method. Tempering is necessary to obtain homogeneous and excellent strength and toughness.
(Hold at 25 to 750°C for 30 minutes or more).

After being shipped as a product, the extra-thick #A steel plate, which has been subjected to rolling, direct quenching and quenching, undergoes processing such as welding and bending to become a pressure vessel such as a chemical reaction vessel.

[Example] Steel having the chemical components shown in Table 1 was hot rolled and heat treated under the conditions shown in Table 2 to produce products.

A sample was cut out from the obtained steel plate and subjected to a tensile test (JI
No. 84) and a 2 mm V-notch Charpy test to investigate the tensile strength and fracture surface transition temperature (v T rs).

The results are also shown in Table 2.

- Ko. 1- However, the content of C+. and Nb in the temporary phase IA is outside the scope of the present invention, and the content of V in the temporary phase 7A is outside the scope of the present invention. In both cases, the tensile strength is high and the amount of embrittlement is large. Temporary phase 2C has a low heating temperature, poor strength and toughness, and a stagnation Δ
The amount of vTrss embrittlement is both large.

Temporary phase 3C has low rolling completion temperature and quenching temperature, so
Intensity is low, vTrs (table) value is poor, △VTrSs
The amount of embrittlement is large.

Since the temporary phase 4C has a low quenching temperature, its initial toughness level is low and the amount of ΔV T rss embrittlement is large. Since the rolling start temperature of temporary phases 5C and 6C is low, only the surface layer is rolled at a low temperature, and not only is the value of vTrs (table) inferior, but also the value of ΔvTr
s, the amount of embrittlement is also large.

Temporary phase 8C has a too high rolling end temperature, so vT in the center
rs is inferior, and the amount of ΔVTrSs embrittlement is large.

In contrast, the examples of the present invention have excellent strength.

The effect of the method of the present invention is that the toughness (v T
rs) clearly appears in the difference. vTrs or surface layer-] 5
- If the temperature is -30°C or less at the lower part and the center, the specifications as steel for chemical reaction vessels are fully satisfied. Further, if the difference between the two is less than 10°C, it can be considered that there is substantially no variation in toughness within the plate thickness.

As shown in Table 2, under the manufacturing conditions according to the method of the present invention, an extremely thick steel plate is obtained with little variation in toughness within the plate thickness.

The amount of embrittlement in Table 2 indicates the amount of change in vTrs after the step cooling heat treatment shown in FIG.

[Effect of the invention] The steel plate produced by this method not only has excellent strength, but also
It has excellent surface toughness and is extremely reliable and useful for chemical reaction vessels used at high temperatures and high pressures.
It has great industrial value.

[Brief explanation of the drawing]

Figure 1 is a chart showing the influence of the surface temperature at the start of the first rolling on the Charpy fracture surface transition temperature after rolling, direct quenching and tempering, and Figure 2 shows the heating and cooling method for measuring the amount of embrittlement. This is a diagram.

Claims (1)

[Claims] 1. In weight%: C: 0.03 to 0.17%, Si: 0.02 to 0.5%
, Mn: 0.3-1.0%, Cr: 1.5-5.0%,
Mo: 0.5-1.5%, V: 0.03-0.5%, N
b: 0.01-0.1%, Al: 0.005-0.05
%, N: 0.03% or less, P: 0.02% or less, S: 0
．． 02% or less, steel consisting of residual Fe and unavoidable impurities from 1100 to 128
After heating to 0°C, hot rolling is started before the surface temperature drops below 950°C, rolling is finished at 800 to 1050°C, and immediately quenching and tempering is performed from a temperature of 800°C or higher. A method for manufacturing extra-thick steel plates for pressure vessels. 2. In weight%, C: 0.03-0.17%, Si: 0.02-0.5%
, Mn: 0.3-1.0%, Cr: 1.5-5.0%,
Mo: 0.5-1.5%, V: 0.03-0.5%, N
b: 0.01-0.1%, Al: 0.005-0.05
%, N: 0.01% or less, P: 0.02% or less, S: 0
．． 02% or less is the basic component, and further, B: 0.0002 to 0.005
% alone or in combination with Ti: 0.01 to 0.05%, residual Fe and unavoidable impurities. manufacturing method.