JPH03260012A - Production of ultrahigh tensile strength steel excellent in stress corrosion cracking resistance and having high toughness - Google Patents

Abstract

PURPOSE:To obtain an ultrahigh tensile strength steel excellent in stress corrosion cracking resistance and toughness at low temp. by subjecting a slab of a steel having a composition in which respective contents of C, Si, Mn, Ni, Mo, Cr, V, etc., are specified to hot rolling and to quench-and-temper treatment under respectively prescribed conditions. CONSTITUTION:A steel having a composition consisting of, by weight, 0.03-0.1% C, 0.02-0.5% Si, 0.4-1.5% Mn, 11-13% Ni, 0.8-2.5% Mo, 0.2-1% Cr, 0.02-0.15% V, 0.01-0.08% Al, and the balance Fe is refined. A slab of this steel is heated to 1000-1250 deg.C and hot-rolled from <=1000 deg.C at >=50% cumulative reduction of area, and rolling is finished at >=Ar3 and water quenching is started in the above state and stopped at <=150 deg.C. Subsequently, after this hardening, the steel is further reheated at a temp. between (the Ac3 point + 20 deg.C) and (the Ac3 point + 120 deg.C) and then hardened. Then, tempering treatment is carried out at a temp. not higher than the Ac1 point.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention has high strength despite the low carbon content (L), and has high toughness and stress corrosion cracking resistance in stress corrosion environments such as seawater or salt water. Excellent yield stress of 125kg f/-
The present invention relates to a method for manufacturing the above-mentioned high-toughness ultra-high tensile strength steel.

(Conventional technology) In recent years, the demand for energy has been increasing year by year, and in order to ensure a stable supply of energy, there has been increasing interest in ocean development such as undersea resource development and undersea crustal geological surveys. There is increasing activity in the construction of offshore oil production bases and offshore oil production bases.

It is desired that the materials used in these devices have extremely high structural strength and toughness, and also have stress corrosion cracking resistance even under usage environmental conditions such as seawater.

In order to meet the demand for such safe, reliable, high-strength, and high-toughness materials, Ni-containing low alloy steels have been developed and their quality improved. For example, Tokuko Sho 83-4973
As in Publication No. 9, f (12-16%)=lOC+N
Ni is a component that becomes i + Cr + 1/2 Mo and provides a good hardened structure in a wide cooling rate range.
-Cr-Mo-based high-strength, high-toughness steel, or C, Sl, and Mn to ensure toughness at an ultra-high strength level, as in Japanese Patent Publication No. 63-49738.

Ni-Cr that lowers the content of P and S and causes precipitation strengthening of Ti and Al in the martensite matrix
ultra-high tensile strength steel, and 1ONi-8Co developed in the United States.
There are ultra-high tensile strength steels, etc.

The former Ni-Cr-Mo steel and Nj-Cr steel are both effective in increasing strength and toughness.

The latter 1 (lNf-8Co steel has a yield stress of 130 kg
A high strength of f/- can be obtained, but the low temperature toughness is not sufficiently high, and since it contains 8% Co, it is very expensive.

Furthermore, no studies have been conducted on any of these steels that take into account stress corrosion in seawater in environments where they come into contact with saltwater, such as in marine structures.

(Problem to be Solved by the Invention) Regarding stress corrosion cracking in high-strength steel, the theory of linear fracture mechanics mode has been adopted, and the theory of how cracks or defects congenitally existing in the material react to the corrosive environment A method of quantifying the fracture behavior using the KM (stress intensity factor) at the crack tip has been used, and has achieved practical results.

In other words, in the stress corrosion cracking test, a pre-cracked test piece is used under the usage environment conditions, and a severe condition is created at the notch tip to cause delayed fracture. By conducting a constant load test at a certain value level, the limit KIS that does not cause destruction below a certain value can be determined.
Stress corrosion cracking resistance is evaluated by determining the CC value.

However, regarding the steel obtained by the above-mentioned manufacturing method,
No consideration has been taken in manufacturing to withstand such evaluation.
It cannot be said that it is sufficiently safe for use.

(Means for Solving the Problem) The present inventors aimed to develop a Ni-containing low alloy steel that has stress corrosion cracking resistance in seawater or salt water, has ultra-high strength, and has high toughness. As a result of various studies on steel and its manufacturing methods, we found that the stress corrosion cracking resistance of ultra-high strength steel is significantly affected by the amount of carbon in the steel, and that reducing the amount of carbon is effective. When carbon-Nl-containing low alloy steel is normally rolled and quenched and tempered, the desired strength cannot be obtained and the limit KIscc' is low;
Also, when controlled rolling is performed and direct quenching/tempering is performed, the target strength and limit K15CC value are similarly low.
It became clear that it was not practical.

Therefore, we focused on the behavior of carbides, and after controlled rolling and direct quenching, we reheated to various austenitizing temperatures, and then quenched and
As a result of a detailed study of the tempering process, it was found that in a specific reheating and quenching temperature range, the strength significantly increases, and it is possible to produce a steel material with high toughness and a sufficiently high limit KIscc value.

The present invention was constructed based on such knowledge. C; 0.03-O, 10%, si.

0.020-0.50%, Mn; 0.4-1.5%, N
i; 11.0-13.0%, Mo; 0, 8-2
, 5%, Cr; 0.2-1.0%, V; (+, (12
A steel billet containing ~0.15%, Al; 0.01~0.08%, and the balance consisting of iron and inevitable impurities, further 1: Cu:o, 2~1.5%, N b;0.0
05-0.05%, Ti; 0.005-0.03
9o, Co; strength improving element group of 0.5 to 2.096,
Or Ca for inclusion shape control effect: 0.0005 to 0.0
1000 to 1 steel billet having one or more of 0.05%
Heating to 250°C, hot rolling from 1000°C or lower at a cumulative reduction rate of 50% or more with respect to the finished plate thickness, Ar3
Finish rolling at the point or higher and start water cooling as it is for 15 minutes.
A quenching process is performed that stops at a temperature of 0°C or lower, and then the temperature is further increased from A c s point +20°C to A c a point +120°C.
Yield stress 12 characterized by being reheated during the process, quenching, and then tempering at a temperature below the A C1 point.
This is a method for producing high-toughness ultra-high tensile strength steel with excellent stress corrosion cracking resistance of 5 kg f/- or more.

The present invention will be explained in detail below.

First, the reasons for limiting the steel components of the present invention will be described.

C: C is an effective element for improving hardenability and easily increasing strength. On the other hand, it is an element that also influences the improvement of stress corrosion cracking resistance of ultra-high strength steel, which is the objective of the present invention.

Figure 1 shows 11%N I-1,0~1.5%Mo system with C
A steel billet produced by changing 0.03-0.13% of
Heating at 50°C - Rolling start 960°C (cumulative reduction rate 60%, finished plate thickness 40mm) - Water cooling (water cooling stop 100°C or less) - Reheating 750-775°C Quenching - 500-520°C
The results of an investigation on the influence of the amount of C on tempered steel below the limit KIscc value are shown.

It can be seen that the lower the C content, the better the limit 5scc value becomes. In other words, when C exceeds 0.10%, the KI
It lowers the SCC value and reduces stress corrosion cracking resistance. Further, if C is less than 0.03%, strength cannot be obtained. Therefore, the C content was set to 0.03 to 0.1090.

Si; Si is effective in improving strength. In addition, it is an unavoidable element in steelmaking, and 0.02% is contained in steel, but if it exceeds 0.50%, tempering brittleness increases in Ni-containing steel, and low-temperature toughness decreases. . Therefore, the Sj content was set to 0.02 to 0.50%.

Mn: Mn improves hardenability and is effective in ensuring strength and toughness, but if Ml is high, temper brittleness increases as in 81, so it needs to be 1,596 or less. or,
If it is less than 0.4%, strength and toughness will decrease. Therefore, the Mn content was set to 4 to 1.5%.

Nj: Ni increases stacking fault energy, increases cross-over, facilitates stress relaxation, increases impact absorption energy, and is effective in improving low-temperature toughness.

Furthermore, Ni is most effective when coexisting with Mo included in the present invention, but the A c s point +20 during reheating and quenching treatment.
When heated to a temperature between ℃ and A c a point + 120℃, newly generated massive austenite grains are suppressed, and many acicular γ grains are generated from martensite laths in prior γ grains, and these γ grains (hereinafter referred to as martensitic reverse transformed γ grains) have an extremely high dislocation density and have a significant effect on strength improvement. If it is less than 11.0%, this effect will be small, and if it is added in excess of 13%, no further effect will be observed. Therefore, the content of N1 is reduced to 1
The content was set at 1.0 to 13.0%.

Mo; Mo is also an important element in the present invention like Nl. That is, A c a point +20 of reheating and quenching treatment
When heated between ℃ and Ac3 point + 120℃, the M□ carbide that precipitates during the heating process remains as a fine precipitate up to high temperature, which is necessary to maintain the formation of martensitic reverse transformed γ grains. be.

It is also an effective element for suppressing precipitation strengthening ε tempering brittleness due to tempering. However, if it is less than 0.89o, the effect of forming martensitic reverse-transformed γ grains is small up to high temperatures, and the target strength cannot be obtained, and if it exceeds 2.5%, no strength improvement effect is observed, and on the contrary, , coarse M6C precipitates increase and reduce toughness. Therefore, the Mo content was set to 0°8 to 2.5%.

Cr: Cr improves hardenability and is effective in ensuring strength, and is required at least 0.2%. Moreover, if it exceeds 1.0%, the increase in strength will be saturated and the toughness will decrease. therefore,
The Cr content was set to 0.2 to 1.0%.

v; V is necessary to generate carbonitrides and ensure strength during tempering. If it is less than 0.02%, the target strength cannot be obtained, and if it exceeds 0.15%, the toughness will decrease.

Ap and All form nitrides in the high temperature range during heating and heat treatment of the steel billet, and are effective in refining austenite grains.

However, if it is less than 0.01%, the effect is small;
．． If it exceeds 0.8%, alumina-based inclusions will increase and the toughness will be impaired. Therefore, the Aj) content should be adjusted from 0.01 to 0.01.
It was set at 08%.

In the present invention, in addition to the above basic components (Cu, Nb).

One or more of TI, Co) and Ca are added.

Cu, V, Nb, Ti, and Co components have an equal effect of improving the strength of steel, and in order to ensure the desired effect, the lower limit content of Cu; 0.2%, N
b; 0.005%, Ti; 0.005%, Co; 0.0
It needs to be 5%. However, each Co; 1.5
%, Nb; 0.05%, Ti; o, oa%, Co; 2
．． If the content exceeds 0%, low-temperature toughness decreases and stress corrosion cracking susceptibility increases, so it is limited as described above.

Ca; Ca is extremely effective in spheroidizing nonmetallic inclusions, and has the effect of improving low-temperature toughness and reducing toughness anisotropy. It requires 0.0005%, but 0.005
%, the toughness decreases due to an increase in inclusions. Therefore, the content was set to 0.0005 to 0.05%.

In addition to the above-mentioned components, unavoidable impurities such as P and SN are harmful elements that reduce the toughness and stress corrosion cracking resistance, which are the characteristics of the present invention, so the smaller the amount, the better. Preferably P, 0.005% or less, S, O, O39
Adjust to 6 or less, N: 0.0080% or less.

Next, the manufacturing method, which is another gist of the present invention, will be described.

That is, even if the steel composition is as described above, the manufacturing method must be appropriate in order to obtain the desired strength, toughness, and stress corrosion cracking resistance. For this reason, the reason why the heating temperature, rolling, cooling, reheating quenching and tempering conditions of the steel billet were limited will be explained.

First, a steel piece having the above steel composition was heated to 100 to 1250°C.
Heat to. In this heating, the heated austenite grains are refined, martensitic type reverse transformed γ grains are formed by maintaining the fine Mo carbide at high temperature in the reheating and quenching process, and Mo, Cr, V, Nb, etc. are added during the tempering process. In order to utilize the strengthening caused by the precipitation of fine carbonitrides, it is necessary to sufficiently dissolve carbides such as Mo, Cr, V, and Nb that are present in the steel slab.

At this time, at a low temperature of less than 1000° C., this solid solution action is not sufficient, and the presence of undissolved precipitates such as M6C cannot expect sufficient precipitation hardening during tempering and also causes a decrease in toughness.

On the other hand, at temperatures exceeding 1250°C, Mo and Cr.

Although carbonitrides such as V and Nb are sufficiently dissolved in solid solution, in the N1-containing steel of the present invention, oxides increase on the surface of the steel piece,
Eventually, surface flaws occur after rolling.

Moreover, the heated austenite grains become coarse, making it difficult to refine the austenite grains during subsequent rolling, which also causes a decrease in toughness. Therefore, in consideration of these problems, the heating temperature of the steel slab was set at 1000 to 1250'C.

Next, the steel billet heated in this way is hot rolled at a cumulative reduction rate of 50% or more relative to the finished plate thickness from 1,000°C or below, and the rolling is completed at a temperature above the A ra point, and then water cooling is performed as it is. A quenching process is performed that starts and stops at a temperature of 150°C or less.

The structure subjected to water cooling treatment after rolling needs to be sufficiently refined into a martensitic structure. This is because the martensitic reverse-transformed γ grains formed during the next reheating and quenching process inherit the austenite grains formed during hot rolling.
This is because it is necessary to sufficiently refine the austenite grains by rolling.

Rolling start temperature is 1000℃ or higher and 50% of finished plate thickness
At the following cumulative reduction ratios, the grain refining effect is small and may cause a decrease in toughness. Therefore, it is necessary to start rolling at 1000° C. or lower and at a cumulative reduction rate of 50% or higher. A preferable upper limit of the cumulative reduction rate is 85%.

In addition, the water cooling start temperature after rolling is determined by the A r
It is necessary to measure the temperature from three or more points. This makes the structure sufficiently martensitic, and in the subsequent reheating process, acicular austenite is generated from the martensite laths within the austenite grains, resulting in martensitic reverse transformation γ
This is also to create a pre-texture for grain formation. Therefore, the finishing temperature of hot rolling needs to be finished at a temperature higher than the A r 3 point.

However, the A r 3 point of the target steel of the present invention is low, and rolling in this temperature range is difficult due to large deformation resistance. Preferably it is 700°C or higher.

In addition, the reason why the water cooling stop temperature after rolling is specified to be 150°C or less is because if water cooling is stopped at a temperature exceeding 150°C, martensitic transformation may not be completed in the case of the steel subject to the present invention. This is because strength and toughness may be significantly affected.

Next, the steel that has been water-cooled after hot rolling has an A c a point +20
It is reheated and quenched to an appropriate temperature in the temperature range from °C to A c a point +120 °C. Figure 2 shows the concept of the formation process of γ grains as the quenching temperature increases.

In the process of heating the martensite in the previous structure, massive diffuse-type transformed γ grains are generated mainly at the prior austenite grain boundaries, and acicular austenite grows from the martensite laths due to reverse transformation inside the grains. As the temperature rises, the area of each increases, and at point A, the whole becomes austenite.

Furthermore, in the temperature range from the A c point +20°C to the A c s point +120°C, the martensitic reverse-transformed γ grains in which acicular austenite is combined have a high dislocation density and are therefore harder than the diffusion-type transformed γ grains.

Furthermore, when the temperature exceeds A c point a + 120°C, M.

With the solid solution of carbide and coarsening of agglomeration, the harder martensitic reverse-transformed γ grains are eroded by the diffusion-type transformed γ grains and become fully diffused transformed γ grains. Therefore, (as in /9 < A
If the reheating temperature exceeds the a point +120°C, the strength will decrease, and the stress corrosion cracking resistance will also decrease due to the coarsening of grain boundary carbides.

Further, at temperatures below the A c s point + 20° C. as in (a), some ferrite structure may be mixed, resulting in a decrease in strength.

On the other hand, as shown in 0) < A c from 3 points + 20°C
c In the temperature range of point a + 120℃, diffusion-type transformed γ grains occur at the prior austenite grain boundaries, martensitic reverse-transformed γ grains with high dislocation density occur inside the grains, and furthermore, due to the presence of fine carbides mainly composed of Mo, Strength, toughness and stress corrosion cracking susceptibility are significantly improved.

Figure 3 shows the strengthening phenomenon caused by reheating as shown in Table 1 for inventive steel B (11% Ni-1.0% Mo system) and comparison steel K (11% NiO, 35% Mo system). This figure shows the strength after hot rolling and water cooling followed by heating and quenching at varying temperatures and tempering.

In the case of inventive steel B, the reheating and quenching temperature range of the inventive method (
It can be seen that a phenomenon of increased strength appears from the A point +20°C to the A ca point +120°C).

In addition, FIG. 4 shows the limit KIsc in this reheating process.
The change in c value was calculated based on the present invention steel B (11%Ni-1,0%Mo
System, heating at 1150℃ - rolling start at 950℃・60% reduction・
The results of an investigation regarding water cooling, reheating and quenching, and tempering at 500°C are shown below.

Limit KISC in the reheating and quenching temperature range of the method of the present invention
It can be seen that the C value has improved.

The reheated and quenched steel is then tempered at a temperature below Ac. At temperatures exceeding the A C1 point, toughness decreases due to the formation of unstable austenite. Therefore, Mo, Cr.

In order to sufficiently strengthen precipitation of carbide-forming elements such as V and Nb and obtain strength and toughness, the tempering temperature was limited to below the AC1 point.

Steel obtained through such a manufacturing process has high strength and toughness despite its low carbon content, and also has improved stress corrosion cracking resistance.

(Example) A steel slab obtained by melting steel having the composition shown in Table 1 was produced with a thickness of 25 to 50 mm based on the manufacturing conditions of the present invention method and the comparative method shown in Table 2. Manufactured from steel plate.

The mechanical properties of the base metal and the KIscc value (critical fracture toughness value against stress corrosion cracking) were investigated for these.

Mechanical properties obtained using steel having the chemical composition shown in Table 1 and manufacturing conditions shown in Table 2, and KIsec using test pieces shown in ASTM E 399 in 3.5% artificial seawater. The test results are shown in Table 3.

Examples of the present invention (1-A - Combination of the steel of the present invention and the method of the present invention)
In 9-■), the strength, toughness and KISCC of the base material
The value is sufficiently high.

On the other hand, in Comparative Example 10-J, the amount of C was high, the rolling start temperature was also high, and the cumulative reduction rate was low, resulting in coarse grains and a decrease in toughness and KISCC value. Example 11 - M
Since the amount of o is low, martensitic reverse transformed γ grains are not formed and the strength is insufficient. In Example 12L, the reheating and quenching temperature was high and martensitic reverse-transformed γ grains were not formed, resulting in insufficient strength. In addition, the C content and Mo content were high, and the N1 content was low, resulting in a decrease in toughness and KIscc value. Example 13-M is N
Since the amount of 1 is low, the strength is insufficient. Example 1 using invention steel F
In No. 4 (comparative method), the strength and KISCC value are reduced because the reheating and quenching temperature is high. Further, in Example 15 (comparative method) using invention steel F, no reheating and quenching treatment was performed, resulting in insufficient strength and a decrease in KIscc value. In Example 16 using invention steel E, the water cooling stop temperature after rolling was high, so a complete martensitic structure could not be obtained and the strength was reduced. In Example 17 using invention steel F, the strength was insufficient due to the high reheating and quenching temperature (air cooling rather than water cooling after rolling) and high reheating quenching temperature.

/ / (Effects of the Invention) The composition range and production method of the present invention have made it possible to produce ultra-high tensile steel with a yield stress of 125) cgf/- or more, which has excellent stress corrosion cracking resistance and low-temperature toughness.

As a result, sufficient safety can be ensured under the environmental conditions in which it will be used.

[Brief explanation of drawings]

Figure 1 shows the effect of C on the limit KISCC value of the 11%Ni-1.0~1.0%Mo base material manufactured by the method of the present invention.
Fig. 2 is a schematic diagram showing the formation process of γ grains as the reheating and quenching temperature increases, and Fig. 3 shows the strengthening phenomenon caused by reheating of the steel of the present invention (11%N1-1%MO system). Figure 4 shows the effect of reheating and quenching temperature on the limit KISCC value for the invention steel B (11% Ni-1% Mo system). This is a chart showing the influence of agent

Claims (1)

[Claims] 1. C in weight%; 0.03-0.10% Si; 0.02-0.50% Mn; 0.4-1.5% Ni; 11.0-13.0 % Mo; 0.8-2.5% Cr; 0.2-1.0% V; 0.02-0.15% Al; 0.01-0.08% The remainder consists of iron and inevitable impurities 1000 pieces of steel
Heating between ~1250℃, hot rolling from below 1000℃ at a cumulative reduction rate of 50% or more with respect to the finished plate thickness, finishing the rolling at a temperature of Ar_3 or higher, and starting water cooling to 150℃. A quenching process is performed that stops at a temperature below ℃, and then further heat treatment is performed from Ac_3 point +20℃ to Ac_3
After reheating between point +120°C, quenching followed by A
A method for producing high-toughness ultra-high tensile strength steel with excellent stress corrosion cracking resistance, characterized by tempering at a temperature of c_1 point or lower. 2. Strength improving element consisting of Cu; 0.2-1.5% Nb; 0.005-0.05% Ti; 0.005-0.03% Co; 0.5-2.0% by weight. Production of the high toughness ultra high tensile strength steel with excellent stress corrosion cracking resistance according to claim 1, which contains one or more types of Ca; 0.0005 to 0.005%, which has the effect of controlling the form of inclusions. Law.