JPH062904B2 - High strength low alloy steel Extra thick steel manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is a production for increasing the high temperature strength (especially the creep strength) of a Mo type, Cr-Mo type low alloy steel extra thick steel material used for a high temperature pressure vessel. It is related to the method.

(Prior art and problems) Mo-based and Cr-Mo-based low alloy heat-resistant steels are widely used in high temperature and high pressure reaction vessels such as chemical industry, petrochemistry, and oil refining due to their excellent high temperature strength and hydrogen corrosion resistance. Has been done. By the way, recent high-temperature reaction vessels are becoming larger, higher in temperature, and higher in pressure to improve efficiency, and as a result, the thickness of the apparatus tends to be further increased. In the case of manufacturing with a monoblock, the extremely thickening leads to a decrease in the cooling rate at the center of the plate thickness, resulting in a decrease in strength and toughness. Further, an increase in wall thickness requires a longer time for stress relief annealing, which also leads to a decrease in strength.

Under these circumstances, the high temperature strength is increased to prevent an extreme increase in wall thickness compared to the conventional component systems, and the high toughness and tempering resistance to prevent brittle fracture due to the pressure test during regular inspections. New consideration is required from the aspect of strength and toughness such as chemical conversion.

Further, compared with the conventional operating temperature, the movement toward higher temperatures for increasing the reaction efficiency requires a steel having higher hydrogen corrosion resistance and higher creep strength than the conventional steels. As a steel that can cope with such a high temperature, for example, 3Cr-1Mo steel is said to endure up to 538 ° C. in terms of hydrogen attack, but it has a drawback of low high temperature strength.

That is, as a conventionally known Cr-Mo low alloy steel,
JP-A-50-130621 or JP-A-55-
There is steel known from Japanese Patent No. 41961 and the like,
None of these can guarantee sufficient strength at high temperatures, and the above-mentioned problems cannot be solved only by the steel material components.

(Means and Actions for Solving Problems) The inventors of the present invention repeated various experiments in order to further increase the strength of the conventional low alloy heat resistant steel as described above, and as a result, an appropriate amount of V, Nb, Ti was obtained. It has been found that the strength and toughness of a steel obtained by hot working in a specific temperature range, followed by off-line quenching and tempering are improved by using steel to which strengthening elements such as A patent application was filed as 207763 (Japanese Patent Laid-Open No. 61-87818). While continuing research, the present invention has been completed by finding that it is possible to manufacture a low alloy, extremely thick steel material having high strength and high toughness equivalent to or higher than that of the prior invention, at low cost.

That is, in the present invention, C0.05 to 0.20% by weight, Si
0.01 to 0.80% or less, Mn 0.2 to 1.5%, C
r0.2-5.0%, Mo0.4-1.5%, V0.0
1 to 0.35%, and one or two of Nb and Ti in total of 0.
01-0.12%, Sol. A steel ingot or slab containing 0.01 to 0.1% Al and the balance Fe and unavoidable impurities, or B0.0003 to 0.0
The steel ingot or slab containing 02% and N 0.005% or less and the balance Fe and unavoidable impurities is 1100 to 12
After heating to 80 ° C., hot working is performed so that the reduction ratio (thickness before processing / finished thickness) at 800 to 1050 ° C. becomes 1.2 or more,
Then, the method for producing a high-strength low-alloy steel extra-thick steel material is characterized in that it is immediately quenched directly from a temperature of 800 ° C. or higher and then tempered.

Hereinafter, the present invention will be described in detail.

First, in the present invention, the extremely thick steel material refers to a material having a thickness range of more than 100 mm. This is because the thickness has increased from the conventional thickness of 100 mm or less due to the large size of the equipment or the increase in pressure in the applications such as the chemical industry and petroleum refining mentioned above. It is intended for.

Next, the reasons for limiting each component of the steel targeted by the method of the present invention will be described.

C is necessary for maintaining the strength, but if it exceeds 0.20%, the weldability and toughness are impaired, so the upper limit is made 0.20%, and if the lower limit is less than this, the strength when high temper parameters are adopted during post-welding heat is adopted. Since it is difficult to hold the content, it was set to 0.05%. Here, the temper parameter (T.P.) is T.P.
P. = T (20 + logt). However, T: temperature (K) and t = time (hour).

Si is added as a deoxidizing agent in an amount of 0.01% or more, and is an element effective in improving strength. But 0.
If it exceeds 8%, the weldability and toughness will be adversely affected, so 0.0
It was set to 1 to 0.80%.

Mn is a component necessary not only for deoxidation but also for strength retention. However, if it exceeds 1.5%, it is not preferable from the viewpoint of toughness, so the upper limit was made 1.5%, and the lower limit was made 0.2% from the viewpoint of ensuring the strength of the extra thick material.

Cr is required to be 0.2% or more from the viewpoint of oxidation resistance, hydrogen corrosion resistance and strength, but if added in excess of 5%, problems occur with respect to weldability, so it was set to 0.2-5%.

Mo is an element that significantly enhances high temperature strength, but 0.4%
If it is less than 1.0%, the effect is extremely reduced, and if it exceeds 1.5%, the effect is hardly increased and the weldability is adversely affected. Therefore, the upper limit is 1.5% and the lower limit is 0.4%.

V is 0.01% because it significantly increases the resistance to temper softening.
The above is necessary and is an element that has a remarkable effect on the improvement of high temperature strength like Mo, but if added in excess of 0.35%, there is a detrimental adverse effect on weldability, so 0.01-0.3.
It was set to 5%.

Next, Nb and Ti are elements that make the crystal grains finer and improve the strength, but if the amount is alone or less than 0.01% in total, there is no effect, and if it exceeds 0.12%, the croup strength is rather increased. Lower, so the upper limit is 0.12% and the lower limit is 0.01%
I decided.

Sol · Al is an element effective in improving toughness, but 0.01
%, The effect is weak, and if it exceeds 0.10%, the hot workability is adversely affected, so the upper limit is 0.10% and the lower limit is 0.1%.
It was set to 01%.

The above are the basic components of the steel according to the present invention, but if the plate thickness becomes extremely thick, a component system considering hardenability is required.

B is an extremely thick material and is an element effective for preventing the precipitation of ferrite and securing a bainite structure when the cooling rate during quenching is extremely slow, but if less than 0.0003%,
No matter how much the amount of Al is increased, no matter how much the amount of N described below is lowered, the hardenability is not effective. Further, if it exceeds 0.0020%, the workability and weldability are adversely affected due to uneven bending, so the upper limit is set to 0.
0020% and the lower limit was made 0.0003%.

N is an extremely small amount of B as described above, and it is effective to suppress the amount thereof with addition of Al in order to secure hardenability.
When the content is 0.005% or less, the effect of the trace amount B appears for the first time, so the content is determined to be 0.005% or less.

The above is the steel to which the manufacturing method according to the present invention is applied. The manufacturing method for increasing the high temperature strength and similarly securing the toughness using this steel will be described below.

First, ingots or slabs are melted by ordinary steelmaking means,
The ingot is made by continuous casting or ordinary ingot, and then heating prior to hot rolling is performed in order to obtain solid solution of NbC and TiC in austenite and to expect strengthening by subsequent precipitation.
It is necessary to heat at 100 ° C. or higher. But,
When the heating temperature exceeds 1280 ° C, coarsening of crystal grains starts and the toughness of the final product is adversely affected. Therefore, the upper limit of the heating temperature is 1280 ° C and the lower limit is 1100 ° C.

Next, hot working refers to casting, ring rolling, roll rolling, etc. in this case.

In this hot working, the reason for carrying out hot working with a reduction ratio (pre-working thickness / partition thickness) of 1.2 or more in the temperature range of 800 to 1050 ° C is that the thickness of the extra-thick steel plate is sufficient up to the center By performing various processing, the grain size is refined to the central portion and the toughness is improved.

The lower limit temperature of hot working is 800 ° C. or higher in order to secure the subsequent cooling start temperature of 800 ° C. Further, at 1050 ° C. or higher, the effect of fine graining due to hot working is lost due to the high temperature.

If the reduction ratio (thickness before processing / partition thickness) is less than 1.2, sufficient processing cannot be performed up to the center of the plate thickness, and grain refinement is insufficient, and improvement in toughness cannot be expected. However, the reduction ratio is 800 to 10
The total reduction ratio of the number of hot workings at 50 ° C. is sufficient, and it is preferable that the total reduction ratio is as large as possible for fine grain formation.

Direct quenching immediately after the above hot working from a temperature of 800 ° C. or higher makes use of the steel components and hot working conditions described above, and provides high strength and high toughness equivalent to or higher than that of the prior invention. This is a point of the present invention for producing an ultra-thick low alloy steel. However, when the quenching start temperature is lower than 800 ° C., some solid-solution strengthening elements such as V, Nb, and Ti are precipitated and do not contribute to strengthening.

Next, tempering is performed to obtain uniform and excellent strength and toughness, and tempering of ordinary Cr-Mo steel (for example, 625
~ 700 ° C x 30 minutes or more).

The effects of the present invention will be described more specifically below with reference to examples.

(Example) Table 1 shows the chemical composition of the sample steel. The sample steel is melted and ingoted in a high-frequency furnace, and then forged by 60t x 100w x 2
It is the material of the shape of 001. Also, Table 2 shows the hot working and cooling conditions and cooling rates, the equivalent plate thickness of the actual steel plate corresponding to the cooling rate, the temper parameters calculated by T (20 + logt), various characteristics, that is, room temperature and high temperature. Tensile properties, creep rupture properties, and impact value vE ₀ at 0 ° C are shown.

The room temperature tensile test was carried out using JIS No. 4 high temperature tensile test, and the creep rupture test was carried out using JIS standard test pieces.

In Table 2, NO.1,3,6,7,8,11,1
2,14,15,16,18,19,21,22,24
Is a comparative example, NO. 2, 4, 9, 10, 13, 17, 20,
Reference numeral 23 is an example of the present invention.

NO.1 to NO.7 are related to 3Cr-1Mo steel, NO.1
Is a normal process material, that is, a material that has been once cooled to room temperature after hot working and then reheated and quenched at 950 ° C., and neither sufficient strength nor creep rupture strength was obtained. In No. 3, the heating temperature does not satisfy the requirements of the present invention, and the crystal grains become coarse and the toughness deteriorates significantly. In No. 6, the reduction ratio of hot working does not satisfy the requirements of the present invention, and the toughness value is low. Since NO.7 has a too low direct quenching temperature after hot working, it has low strength at both normal temperature and high temperature and low creep rupture strength.

On the other hand, Nos. 2, 4, and 5 were produced within the range satisfying the requirements of the present invention, and all have high creep rupture strength, high strength and high toughness, and are excellent materials.

Next, No. 8 to No. 11 relate to the 1 / 4C'r-1 / 2Mo steel. No. 8 is a normal process material, that is, a material which is once cooled to room temperature after hot working and then reheated and hardened at 930 ° C., and sufficient strength is not obtained. N
O.11 does not have sufficient strength because the direct quenching temperature after hot working is low.

No. 9 was produced within the range satisfying the requirements of the present invention, and has a high level of strength and toughness and a high creep rupture strength. NO.10 was also manufactured in a range satisfying the requirements of the present invention, and the toughness value is slightly low because the rolling reduction is close to the lower limit, but the strength and creep rupture strength are high values.

Next, NO.12 to NO.15 are related to Mn-Mo steel.
No. 12 is a normal process material, that is, a material which is once cooled to normal after hot working, reheated to 900 ° C. and hardened, and does not have sufficient strength. In NO.14, the heating temperature does not satisfy the requirements of the present invention, and the grain refinement is insufficient and the toughness value is low. No. 15 does not have sufficient strength because the direct quenching temperature after hot working does not satisfy the requirements of the present invention.

On the other hand, NO.13 is manufactured within the range that satisfies the requirements of the present invention, and not only the strength and toughness but also the creep rupture strength is at a high level.

Next, NO. 16 to NO. 18 relate to the 1/4 Cr-1 / 2 Mo steel. No. 16 is a normal process material, that is, after being hot worked, once cooled to room temperature, reheated to 930 ° C. and quenched, and has low strength and creep rupture strength and low toughness. No. 18 does not have sufficient strength because the direct quenching temperature after hot working does not satisfy the requirements of the present invention. No. 17 was produced within the range satisfying the requirements of the present invention, and has high strength and toughness as well as high creep rupture strength.

Next, NO.19 to NO.24 are related to the 2.1 / 4Cr-1Mo steel, and NO.19 and NO.22 are normal process materials, that is, 930 ° C. and 980 ° C. after being hot worked and then cooled to room temperature. ℃
It is reheated and hardened, and has low strength and croup rupture strength. No. 21 and No. 24 do not have sufficient strength because the direct quenching temperature after hot working does not satisfy the requirements of the present invention, and the creep rupture strength is also low.

NO.20 and NO.23 were produced satisfying the requirements of the present invention, and both strength and toughness are at a high level and creep rupture strength is also high.

(Effects of the Invention) As described above, according to the production method of the present invention, a steel material having higher creep rupture strength and high-temperature strength and a well-balanced toughness can be obtained as compared with the conventional production method. Therefore, not only is it suitable for increasing the size and temperature of high-temperature and high-pressure equipment and is convenient for reducing the weight of the equipment, but it is also a method of directly quenching after hot working. Since the invention does not need to be reheated to the quenching temperature, it is an invention that has a large industrial effect, such as low cost and a shortened manufacturing period.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Otoguro ▲ Yasushi ▼ Male 5-10-1, Fuchinobe, Sagamihara City, Kanagawa Prefecture, Nippon Steel Corporation 2nd Technical Research Institute (72) Inventor, Katsukuni Hashimoto Sagamihara, Kanagawa Prefecture 5-10-1 Ichibuchi Nobe, Nippon Steel Corporation Second Research Laboratory (56) References JP-A-55-76020 (JP, A) JP-A-57-210915 (JP, A) JP-A-55 -131126 (JP, A) JP-A-57-79117 (JP, A)

Claims (2)

[Claims] 1. C: 0.05 to 0.20% by weight%, Si: 0.01 to 0.80%, Mn: 0.2 to 1.5%, Cr: 0.2 to 5.0 %, Mo: 0.4 to 1.5%, V: 0.01 to 0.35%, 0.01 to 0.12 in total of one or two of Nb and Ti.
%, Sol. Al: 0.01 to 0.1% is contained, and the steel ingot or slab containing the balance Fe and unavoidable impurities is heated to 1100 to 1280 ° C., and then 800 to
Reduction ratio (thickness before processing / finished thickness) at 1050 ° C is 1.2
A method for producing a high-strength low-alloy steel extra-thick steel material, which comprises performing hot working as described above, then immediately quenching directly from a temperature of 800 ° C. or higher, and then tempering. 2. C: 0.05 to 0.20% by weight%, Si: 0.01 to 0.80%, Mn: 0.2 to 1.5%, Cr: 0.2 to 5.0 %, Mo: 0.4 to 1.5%, V: 0.01 to 0.35%, 0.01 to 0.12 in total of one or two of Nb and Ti.
%, Sol. A steel ingot or slab containing Al: 0.01 to 0.1%, B: 0.0003 to 0.002%, N: 0.005% or less, and the balance Fe and unavoidable impurities is 1100 to 1280. 800-1 after heating to ℃
A high feature characterized by performing hot working at a reduction ratio (thickness before processing / finished thickness) of 1.2 or more at 050 ° C, then immediately quenching directly from a temperature of 800 ° C or more and then tempering. Manufacturing method of high strength low alloy steel and extra thick steel.