JP2562771B2 - Method for producing ultra-high strength steel with excellent resistance to stress corrosion cracking - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a high strength despite a low carbon content and a yield strength of 1080M which is excellent in stress corrosion cracking resistance in a stress corrosion environment such as seawater or salt water.
The present invention relates to a method for producing an ultrahigh tensile strength / high toughness steel of Pa or more.

[0002]

2. Description of the Related Art In recent years, interest in ocean development in the deep sea has rapidly increased due to global geophysical quests such as exploration and excavation of energy resources and occurrence of earthquakes. The development of research vessels is becoming active. When used in the deep sea, very high pressure is applied to various containers, so that the steel materials used are desired to have high toughness in terms of structurally high strength. In order to meet the demand for such safe and reliable high strength and high toughness materials, Ni-containing low alloy steels have been developed and their quality has been improved. For example, C + 1 / 8Mo in Ni-Cr-Mo-V system as disclosed in JP-A-56-9358.
High strength and high toughness steel with + V> 0.26 and Cr <0.8Mo, or stable high strength and high toughness at a wide range of cooling rates during quenching treatment, as in JP-A-57-188655. Many production methods have been proposed, such as the obtained Ni-Cr-Mo-V ultrahigh-strength steel and a method for producing a Ni-containing steel material treated with extremely low P and extremely low S in order to secure high toughness. . However, although these are effective in achieving ultra-high tensile strength and high toughness, there is a concern that they are inferior in reliability in the environment targeted by the present application. That is, the container used in the deep sea is
Since it is exposed to seawater, the steel material is required to have sufficiently good seawater corrosion resistance, that is, high seawater stress corrosion cracking resistance.

As an ultra-high-strength steel material having high reliability in deep sea, for example, as in Japanese Patent Publication No. Sho 64-11105, Ni-containing steel is used to reduce N and O, and Al (%) × N.
(%) × 10 ⁴ <1.5, a Ni—Cr—Mo—V based high toughness ultra-high tensile steel has been proposed, and a great effect is observed. However, the weld heat affected zone has lower stress corrosion cracking resistance in seawater than that in the base metal compared to that in the base metal, and therefore, the
Ingenuity and ingenuity are required to improve reliability. Further, as in Japanese Patent Publication No. 1-51526, a method for producing an ultra-high-strength steel having excellent stress corrosion cracking resistance by directly quenching and tempering a Ni-Mo-Nb steel containing 5-8% Ni. Is proposed. However, the strength of the steel material is lower than that of the object of the present invention, and in the production of thick high-strength steel, in general, in view of material uniformity and anisotropy in the plate thickness direction, the direct quenching and tempering method is used. Strict control is required to manufacture the steel sheet, and there is a concern that the stability of the material in the width direction and the longitudinal direction in the steel sheet may be reduced.

[0004]

As described above, in the conventional super high tensile strength and high toughness steel materials, stress corrosion cracking resistance,
In particular, in the heat-affected zone of welding, the resistance to stress corrosion cracking in seawater is lower than that in air, and the material uniformity in the thickness direction of thick-walled materials and the stability of material in steel sheets are It is a disadvantageous manufacturing method, and further improvement of steel materials and manufacturing methods is desired.

[0005]

DISCLOSURE OF THE INVENTION The inventors of the present invention are based on the fact that they have a resistance to stress corrosion cracking in seawater or salt water, in particular, a stress corrosion cracking resistance of a heat-affected zone of a weld, and have an ultrahigh tensile strength. In order to stably produce a Ni-containing low alloy steel having high toughness with a high temperature, various examinations have been made on steel components and their production method, in particular, hot rolling-reheating quenching and tempering treatment. As a result, Ni with reduced C, Si and Mn has been obtained. After adding Mo, V and Nb to the contained steel and sufficiently solidifying these elements in the hot rolling step,
By controlling the heating rate and heating temperature range in the reheating and quenching process, solid-solution Mo, V and Nb are precipitated during heating and non-diffusion type reverse transformation consisting of acicular austenite group with high dislocation density It has been found that γ grains are formed, a unique strengthening mechanism works in the Ni-containing steel, high strength can be achieved, and the target steel can be manufactured.

The present invention is constructed on the basis of such knowledge, and the gist thereof is C: 0.04 to 0.09%, S
i: 0.01 to 0.10%, Mn: 0.05 to 0.65
%, Ni: 8.0 to 11.0%, Mo: 0.5 to 1.5
%, Cr: 0.2 to 1.5%, V: 0.02 to 0.20
%, Al: 0.01 to 0.08%, with the balance being iron and inevitable impurities, or Cu:
0.2-1.5%, Nb: 0.005-0.10%, T
i: Strength improving element group consisting of 0.005 to 0.03%,
Or Ca with an inclusion morphology control effect: 0.0005-
0.005%, REM: 0.0005-0.0100%
1 or 2 or more, and the balance is iron and unavoidable impurities, the steel slab is heated between 1000 and 1250 ° C., and hot-rolled to finish at an Ar ′ point temperature or higher, and then air-cooled. Then, after further reheating to a temperature range of Ac ₃ points −40 ° C. to Ac ₃ points + 40 ° C. at a heating rate of 120 ° C./min or less, quenching treatment is performed, and then Ac ₁
A method for producing an ultra-high-strength steel excellent in stress corrosion cracking resistance, characterized by performing tempering treatment at a temperature below the point.

[0007]

The present invention will be described in detail below. First,
The reasons for limiting the steel components of the present invention will be described. C: C is an effective element that improves hardenability and easily increases strength. On the other hand, it is an element that most affects the improvement of the stress corrosion cracking resistance of the weld heat affected zone of the ultra-high strength steel of the present invention. If it exceeds 0.09%, the stress corrosion cracking resistance of the heat-affected zone of the weld markedly deteriorates. Further, if it is less than 0.04%, the strength becomes insufficient. Therefore, the C content is 0.04 to 0.
It is set to 09%. Si: Si is effective for improving strength.
Further, it is an unavoidable element in steel making, and 0.01% will be contained in the steel, but if it exceeds 0.10%, in the case of high Ni content as in the present steel, temper embrittlement becomes large, Low temperature toughness decreases. Therefore, the Si content is 0.01 to 0.
10%.

Mn: Mn is necessary for improving hardenability and hot workability, but if it is less than 0.05%, its effect is not obtained. On the other hand, in the case of the Ni-containing steel of the present invention, the addition of Mn increases the susceptibility to temper embrittlement and lowers the stress corrosion cracking resistance of the base metal and the weld heat affected zone. There is. Figure 1 shows 0.06% C-9.9%
Ni-1.0% Mo-0.1% V composition and Mn addition amount of 0.
Using a steel slab changed to 15 to 1.05%, after hot rolling-air cooling, the toughness of a steel sheet reheat-quenched at 770 ° C and tempered at 540 ° C and a stress corrosion cracking test in artificial seawater (KIscc test) ) The results are shown. It is understood that the low temperature toughness and the stress corrosion cracking resistance are improved as the Mn is reduced.
Therefore, the Mn content is set to 0.05 to 0.65%.

Ni: Ni increases stacking fault energy,
It is effective in increasing cross-slip, making stress relaxation easier, increasing impact absorption energy, and improving low temperature toughness. Furthermore N
i is most effective in coexistence with Mo, V, etc. included in the present invention. That is, after hot rolling, Ac ₃ points −40 ° C.
When reheated to a temperature range of Ac ₃ point + 40 ° C., diffusion-type reverse transformation γ composed of massive austenite formed by melting of carbides and non-diffusion-type reverse transformation γ composed of acicular austenite groups not accompanied by dissolution of carbides Although mixed grains are formed with the grains, the non-diffusion type reverse transformation γ grains have a higher dislocation density than the diffusion type reverse transformation γ grains and act extremely effectively for increasing the strength. That is, Ni has the effect of delaying the dissolution of carbides such as Mo and V, and can hold acicular austenite stably up to a high temperature. Therefore, it is necessary to add 8.0% or more to secure the strength of the non-diffusion type reverse transformation γ grains by stabilizing at high temperature. Further, if added in excess of 11.0%, austenite precipitates during tempering and reduces strength / toughness. Therefore, the Ni content is set to 8.0 to 11.0%.

Mo: Mo is an element effective in suppressing precipitation hardening by tempering and temper embrittlement, and at the same time, is an important element of the present invention like Ni. That is, during the reheating and quenching treatment, fine carbides mainly composed of Mo precipitated in the heating process remain as undissolved carbides up to a high temperature, and thus it is necessary to maintain the acicular austenite group having a high dislocation density at a high temperature and secure the strength. . However, if it is less than 0.5%, the Mo carbide is dissolved during the reheating quenching treatment, the non-diffusion type reverse transformation γ grains are rapidly eroded by the diffusion type reverse transformation γ grains, and the target strength cannot be obtained. On the other hand, if it exceeds 1.5%, the strength improving effect is saturated, and rather coarse alloy carbides increase and the toughness decreases. Therefore, the content of Mo is set to 0.5 to 1.5%.

Cr: Cr improves the hardenability and is effective for securing the strength. At least 0.2% is necessary.
If it exceeds 1.5%, the increase in strength is saturated and the toughness decreases.
Therefore, the content of Cr is set to 0.2 to 1.5%. V: V is effective for securing strength by forming carbonitrides during precipitation treatment and precipitation hardening. Further, as in the case of Mo, during the reheating and quenching treatment, V is finely precipitated during heating, so that the non-diffusion type reverse transformation γ composed of acicular austenite group is formed.
It is effective in increasing the grain stability and ensuring strength. 0.02%
If it is less than 0.2%, the target strength cannot be obtained, and if it exceeds 0.20%, the toughness decreases. Therefore, the content of V is 0.02
0.20%. Al: Al is an element necessary for deoxidation, and at the same time,
It combines with N in the steel to form a nitride of AlN, which is effective in refining the structure. However, if it is less than 0.01%, its effect is small, and if it exceeds 0.08%, alumina-based inclusions increase to impair the toughness. Therefore, the content of Al is set to 0.01 to 0.08%.

In the present invention, in addition to the above components, (Cu, Nb,
One or more of Ti) and (Ca, REM) are added. The Cu, Nb, and Ti components have an equal effect of improving the strength of the steel, and the Nb and Ti components are also effective for making the austenite grains finer. The lower limit is Cu: 0.2
%, Nb: 0.005%, Ti: 0.005%. However, Cu: 1.5%, Nb:
If the content exceeds 0.10%, Ti; 0.03%, the low temperature toughness is lowered and the stress corrosion cracking susceptibility is increased.

Ca, REM: Ca and REM have a spheroidizing effect of non-metallic inclusions and are effective in improving toughness and anisotropy, which requires 0.0005%, but Ca is less than 0.
If 005% and REM exceed 0.0100%, the inclusions increase and the toughness decreases. Therefore, its content is C
a: 0.0005 to 0.005%, REM: 0.000
5 to 0.0100%. In addition to the above-mentioned components, P, S, N, O and the like as unavoidable impurities are harmful elements that lower the toughness and the stress corrosion cracking resistance, which are the characteristics of the present invention, so their amounts should be small. Preferably P: 0.0
05% or less, S: 0.003% or less, N: 0.0050
%, O: 0.0030%.

Next, a manufacturing method which is another skeleton of the present invention will be described. That is, even with the above steel composition, the manufacturing method must be appropriate in order to obtain the desired strength, toughness, and stress corrosion cracking resistance. For this reason,
The reason for limiting the heating, rolling, and reheating quenching / tempering conditions of the steel piece will be described. First, a steel slab having the above steel composition is heated to 1000 to 1250 ° C. In this heating, in addition to refining the heated austenite grains, in order to utilize the above-mentioned non-diffusion type reverse transformation γ and strengthening by fine precipitation in the reheating quenching-tempering treatment after hot rolling, 100
It is necessary to heat the steel slab to 0 ° C. or higher and sufficiently dissolve Mo, Cr, V, Nb, etc. At this time, at a low temperature of less than 1000 ° C., this solid solution action becomes insufficient, the undissolved alloy carbide (M ₆ C) becomes coarse, and on the contrary, sufficient precipitation hardening cannot be expected at the time of tempering, and the cause of decrease in toughness. Will also be. On the other hand, at temperatures above 1250 ° C, Mo,
Although alloyed carbides such as Cr and V are sufficiently solid-solved, in the Ni-containing steel of the present invention, oxides increase on the surface of the steel slab and finally surface flaws after rolling occur. Further, the heated austenite grains become coarse, and it is difficult for the austenite grains to become finer in the subsequent rolling, which causes a decrease in toughness. Therefore, considering these, the heating temperature of the billet is 100
It shall be 0-1250 ° C.

Next, hot rolling is carried out to a temperature above the Ar ′ transformation point of the thus heated steel slab and air cooling is carried out. The steel of the present invention has a low Ar ′ point temperature of about 400 ° C. and satisfies this condition only by being processed in a normal rolling process. Since the steel of the present invention has a sufficiently high hardenability, it becomes a martensite single-phase structure containing a sufficiently large amount of dislocations only by air cooling. Since the non-diffusion type reverse transformation γ grains that contribute to strengthening are the same as the γ grain size after hot rolling, if it is necessary to secure lower temperature toughness, the γ grain size by rolling-recrystallization is reduced. A method of appropriately lowering the rolling finishing temperature for the purpose of granulation is preferable, but not particularly limited.

Next, the steel sheet after hot rolling and air cooling has Ac ₃ points-
Reheat to a temperature range of 40 ° C to Ac ₃ points + 40 ° C, and perform quenching treatment. In the heat treatment step of reheating with a martensite structure as a pre-structure, the diffusion-type reverse transformation γ-grains composed of general agglomerated austenite in the old austenite grain boundaries when heated to the α-γ two-phase coexistence temperature range Needle-like austenite groups are formed from the martensite inside and coexist with carbides and ferrite. Since acicular austenite is produced by non-diffusion (martensite type) reverse transformation, it has a large amount of dislocations and contributes to strengthening. Further Ac
_When heated to ₃ points −40 ° C. to Ac ₃ points + 40 ° C., the acicular austenite group increases in area and forms non-diffusion type reverse transformation γ grains, which are stably maintained up to high temperature and diffusion type reverse transformation γ grains. And a martensite structure in which more dislocations have been introduced from this temperature range, and ultrahigh-strength steel can be achieved.

Further, when heated to a temperature of Ac ₃ point + 40 ° C. or higher, a non-diffusion type reverse transformation γ which contributes to strengthening after quenching.
The grains change to general diffusion type reverse transformation γ grains, and the strength of the steel sheet decreases. Therefore, the reheating and quenching temperature is Ac ₃ points -40.
In order to stabilize the non-diffusion type reverse transformation γ grains within the range of 0 ° C to Ac ₃ points + 40 ° C, it is preferable to set the range of Ac ₃ points ± 20 ° C.

The heating rate during reheating is 120 ° C./min.
The following heating rate is also one of the features of the present invention. FIG. 2 shows 0.06% C-9.9% Ni-1.0% Mo.
-Showing the test result of the yield strength of the steel plate which was obtained by heating-rolling-air-cooling a steel piece having a 0.1% V composition to 1150 ° C, varying the heating rate up to a reheating quenching temperature of 790 ° C, and then tempering at 540 ° C. . It can be seen that the strength increases as the heating rate decreases. Generally, it has been reported that the non-diffusion type reverse transformation γ is generated when rapidly heated, but in the steel of this component containing a large amount of Ni, the non-diffusion type reverse transformation γ is formed even without rapid heating. Moreover, contrary to the conventional wisdom, a new finding has been obtained that a heating rate of 120 ° C./min or less is advantageous for increasing the strength of steel. As a result of a detailed examination of this cause, M that precipitates during the slow heating process
The stability of the non-diffusion type reverse transformation γ once generated is increased by the carbon / nitrides such as o, Cr, V, and Nb, and the area ratio of the non-diffusion type reverse transformation γ grains that contribute to strengthening is increased. It was found to be due to that.

Next, the reheat-quenched steel sheet is then tempered at a temperature not higher than the Ac ₁ point. When the temperature exceeds the Ac ₁ point, the strength and toughness deteriorate due to the formation of unstable austenite. Therefore, the tempering temperature is limited to the Ac ₁ point or less in order to sufficiently strengthen the precipitation by fine precipitation of Mo, Cr, V, etc. and obtain high strength and high toughness. The steel obtained by such a manufacturing process has ultra-high tensile strength and high toughness in spite of low carbon, and the stress corrosion cracking resistance of ultra-high-strength steel,
In particular, the characteristics of the weld heat affected zone are remarkably improved.

[0020]

EXAMPLES Steel pieces obtained by smelting steel having the composition shown in Table 1 were manufactured into steel plates with a thickness of 20 to 80 mm based on the respective manufacturing conditions of the present invention method and comparative method shown in Table 2. . For these, the mechanical properties of the base metal and the KIscc value (critical fracture toughness value for stress corrosion cracking resistance) of the base metal portion and the weld heat affected zone were investigated. The welding was performed by TIG welding with a heat input of 25 kJ / cm. Mechanical properties of the base materials obtained by the steels having the chemical compositions shown in Table 1 and the production conditions shown in Table 2 and AST in artificial seawater containing 3.5% NaCl
Table 3 shows the KIscc test results of the base metal portion and the weld heat affected zone using the test piece shown in ME 399. The thick underlined parts in the table show the parts outside the scope of the invention and those with insufficient properties.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

In the examples of the present invention (1-A to 12-O in which the steel of the present invention and the method of the present invention are combined), the mechanical properties of the base material are high strength and high toughness, and the intent of the present invention is Sufficiently high stress corrosion cracking resistance for both base metal and weld heat affected zone K
Iscc value. On the other hand, even in the case of the method of the present invention, in the comparative example in which the comparative steels (P to V) deviating from the chemical composition range limited by the present invention were combined, Example 16-
In P and 17-Q, since the amounts of Mo and V are low, non-diffusion type reverse transformation γ grains are not generated, precipitation strengthening is small, and strength is insufficient. In Example 18-R, since the amount of Ni is low, non-diffusion type reverse transformation γ grains are not generated and the strength is insufficient. In Examples 19-S and 20-T, since the Mn content and the C and Mn contents are high, the toughness, the base material, and the KIscc of the weld heat affected zone are high.
The value is low. In Example 21-U, since the C content and Ni content are high, the toughness and the KIscc value of the weld heat affected zone are low. In Example 22-V, since the amount of C is high, the K of the welding heat affected zone
The Iscc value is low.

Next, even in the case of the steel of the present invention, in the comparative examples combined with the comparative methods (23 to 29) deviating from the scope of the method of the present invention, Examples 23-D and 28-J were reheat-quenched. Since the heating rate is high, the non-diffusion type reverse transformation γ grains become unstable and the diffusion type reverse transformation γ grains increase, resulting in insufficient strength. Example 24
Since -D has a low reheating and quenching temperature, a large amount of ferrite remains between the acicular γ groups, resulting in a decrease in strength and toughness. In Examples 25-B and 27-F, since the heating temperature of the billet is low, the presence of coarse undissolved precipitates of carbide and the precipitation strengthening are small,
Insufficient strength and toughness. In Examples 26-B and 29-L, since the reheating and quenching temperature is high, diffusion type reverse transformation γ grains are generated and the strength is insufficient, and further, the KIscc value of the base material is slightly lowered.

[0026]

EFFECTS OF THE INVENTION Due to the combination of the composition range and the manufacturing method of the present invention, an ultra-high tensile steel having a yield strength of 1080 MPa or more, which has good low temperature toughness and excellent stress corrosion cracking resistance of a weld heat affected zone, It has become possible to stably manufacture and supply, and it has become possible to significantly improve the reliability of containers and equipment used in the deep sea.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between the Mn content of steel and stress corrosion cracking resistance,

FIG. 2 is a table showing the relationship between the heating rate and the yield strength during reheating.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomoya Koseki, 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. (72) Ichiro Nakagawa 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama None) Inside Kawashima Steel Works Mizushima Steel Works

Claims (2)

(57) [Claims] 1. By weight%, C: 0.04 to 0.09% Si: 0.01 to 0.10% Mn: 0.05 to 0.65% Ni: 8.0 to 11.0% Mo: 0.5 to 1.5% Cr: 0.2 to 1.5% V: 0.02 to 0.20% Al: 0.01 to 0.08% with the balance being iron and unavoidable impurities the composed slab was heated to between 1000 to 1250 ° C., was subjected to hot rolling to finish up Ar' point temperature or more, air cooled, then further, Ac ₃ point or less of the heating temperature of 120 ° C. / min -
After reheating to a temperature range of 40 ° C-Ac ₃ points + 40 ° C,
A method for producing an ultra-high-strength steel excellent in stress corrosion cracking resistance, which comprises performing a quenching treatment and subsequently a tempering treatment at a temperature of Ac ₁ point or less. 2. By weight%, C: 0.04 to 0.09% Si: 0.01 to 0.10% Mn: 0.05 to 0.65% Ni: 8.0 to 11.0% Mo: 0.5 to 1.5% Cr: 0.2 to 1.5% V: 0.02 to 0.20% Al: 0.01 to 0.08%, further Cu: 0.2 to 1 0.5% Nb: 0.005 to 0.10% Ti: 0.005 to 0.03%, which has a strength improving element group or inclusion morphology control action Ca: 0.0005 to 0.005% REM: Heat that heats a steel slab containing 0.0005 to 0.0100% of one or more kinds, the balance of which is iron and unavoidable impurities, at a temperature of 1000 to 1250 ° C and finishes at a temperature of Ar 'or higher. After hot rolling, it is air-cooled, and then, at a heating temperature of 120 ° C./min or less, Ac ₃ point −40 ° C. to Ac ₃ After reheating to the temperature range of + 40 ° C, quenching is performed, then Ac ₁
A method for producing an ultra-high-strength steel excellent in stress corrosion cracking resistance, characterized by performing tempering treatment at a temperature below the point.