JP3550726B2 - Method for producing high strength steel with excellent low temperature toughness - Google Patents

Description

【００３４】
【表２】

【００３５】
実施例２
表３に示す組成の各鋼を溶製してスラブとした後、1150℃に加熱した後、表４に示す各条件で圧延および焼入れの後、570 ℃において焼もどし処理を行った（発明法、比較法）。この他に、鋼 2,3,6,8〜13については従来のＱＱ’Ｔ処理（890 ℃で１時間加熱後水冷と700 ℃で20分間加熱後水冷の後、570 ℃で焼もどしの処理）によっても製造した。
これらの鋼板の降伏強さ、引張強さおよびシャルピー衝撃試験における破面遷移温度、−60℃および−110 ℃での吸収エネルギーおよび再現ＨＡＺ靭性について調べた結果を表４に示す。ここで、再現ＨＡＺの条件は、ピーク温度が1400℃および 800℃の単サイクル熱履歴とし、800 ℃から 500℃までの冷却時問を30秒とした。
【００３６】
【表３】

【００３７】
【表４】

【００３８】
【発明の効果】
以上説明したように本発明の製造方法によれば、従来のＱＱ’Ｔ処理によって得られるよりも優れた特性、すなわち、低温靱性に優れるとともに良好なＨＡＺ特性と高い強度を、他を犠牲にすることなく実現した低温用鋼を提供できるほか、こうした鋼材を高い生産性の下で経済的に製造することができる。[0001]
[Industrial applications]
The present invention relates to a method for producing a high-tensile steel having excellent low-temperature toughness.
[0002]
[Prior art]
Recent developments in energy resources have been conducted in harsh environments that were previously neglected. This tendency is remarkable in fields such as oil and natural gas, and development is being vigorously carried out, for example, in extremely cold regions such as the Arctic Circle.
The development of energy resources in such an environment also demands better quality than ever before for low-temperature steel materials such as pipelines used to transport the extracted resources, such as strength. Demands for low-temperature toughness are becoming more severe.
On the other hand, the use of liquefied gases such as LPG, LEG, and LNG as energy sources is also steadily increasing. As steel materials used for transporting and storing these liquefied gases, excellent materials, especially at low temperatures, are economical. It is desired to supply a low-temperature steel material having the above characteristics.
[0003]
By the way, as a low-temperature steel material provided for each of the above-mentioned uses, 2.5 to 9% Ni steel has been conventionally developed and widely used. This known low-temperature steel material is obtained by cooling the steel material after hot rolling and then reheating the steel material to a temperature higher than the transformation point of Ac ₃ and normalizing or quenching from that temperature (hereinafter, this heat treatment is referred to as “Q”). Abbreviated), then quenched after reheating to the Ac ₁ transformation point to the Ac ₃ transformation point (hereinafter, this heat treatment is abbreviated as “Q ′”), and tempered at a temperature lower than the Ac ₁ transformation point (hereinafter, referred to as This heat treatment is abbreviated as “T”), or a so-called QQ′T treatment in which Q ′ is omitted, or a so-called QT treatment in which Q ′ is omitted.
The above-mentioned heat treatment method for obtaining excellent low-temperature toughness of 2.5 to 9% Ni steel is not only economical, but also inferior in productivity because heating and quenching operations need to be performed several times. There is a problem that the surface cleanliness of the steel sheet is impaired by the large amount of scale generated.
Further, when a precipitation hardening element such as Nb or V is added as the above steel composition, carbonitride precipitates are coarsened during two heating treatments of austenite / ferrite region heating and tempering heating, and low temperature toughness is reduced. There was also a problem of deterioration.
[0004]
On the other hand, methods for producing Ni-containing steel without the complicated heat treatment as described above have been proposed in Japanese Patent Publication No. Sho 61-17885 and Japanese Patent Application Laid-Open No. Sho 58-100624.
The technique disclosed in Japanese Patent Publication No. 61-17885 discloses rolling of a high-pressure reduction at a temperature not lower than the Ar ₃ transformation point, and subsequently maintaining the temperature in the temperature range of the Ar ₃ transformation point to the Ar ₁ transformation point for a certain period of time. This method involves quenching and then tempering at a temperature lower than the Ac ₁ transformation point. However, due to insufficient hardenability, good welding heat affected zone (hereinafter simply referred to as “HAZ”) characteristics are obtained. There was a problem that it could not be obtained.
The technique disclosed in Japanese Patent Application Laid-Open No. 58-100624 is a method in which a two-phase zone rolling is performed, followed by rapid cooling and a tempering treatment. There was a problem of shortage.
[0005]
[Problems to be solved by the invention]
The present invention advantageously solves the above-described problems, and has excellent low-temperature toughness by overcoming the problems of steel materials manufactured by the conventional method without resorting to complicated heat treatment of repeating heating and quenching. The main purpose is to propose a method for producing high strength steel.
Another object of the present invention is to provide a low-temperature steel material having excellent surface cleaning, high strength, and excellent HAZ characteristics.
Still another object of the present invention is to establish a technique for economically producing the above steel with high productivity.
[0006]
[Means for Solving the Problems]
Under the recognition of such a problem, the inventors have studied in detail the steel composition, rolling conditions, and subsequent heat treatment conditions. As a result, it was found that by appropriately combining these conditions, the above problem could be solved if the steel structure was made to be a high-strength fine tempered martensite or a mixed structure of tempered martensite and lower bainite. The invention has been completed.
According to this method, a steel excellent in low-temperature toughness, strength, HAZ characteristics, and surface cleanliness can be obtained without performing so-called QQ'T treatment, which is conventionally desirable for improving the properties of steel materials. It can be manufactured.
[0007]
That is, the present invention is a method of achieving the above-mentioned new knowledge by controlling the composition of steel, the conditions of rolling and subsequent heat treatment, and the gist configuration thereof is as follows.
(1) C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt%, B : 0.0003 to 0.002 wt%, P: 0.010 wt% or less, S: 0.005 wt% or less, and the balance is a steel substantially composed of Fe. After heating, the rolling reduction at 1000 ° C or less is 30 to 70. %, And the rolling end temperature is (Ar ₃ transformation point +300) ° C. to (Ar ₃ transformation point +100) ° C., and then quenched from a temperature of (Ar ₃ transformation point +50) ° C. or higher, A method for producing a high-strength steel excellent in low-temperature toughness, characterized in that tempering is performed at a temperature equal to or lower than the Ac ₁ transformation point.
[0008]
(2) C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt%, B : 0.0003~0.002wt%, P: 0.010wt% or less, S: include: 0.005 wt%, or one Mo: 0.8 wt% or less, Cu: 0.5 wt% or less, V: 0.2 wt% or less, Ti: 0.1 wt % Or less selected from the group consisting of at least one selected from the group consisting of iron and a balance of substantially Fe. Rolling is performed at a rolling end temperature of (Ar ₃ transformation point +300) ° C. to (Ar ₃ transformation point +100) ° C., followed by quenching from a temperature of (Ar ₃ transformation point +50) ° C. or higher, and then an Ac ₁ transformation point A method for producing a high-strength steel excellent in low-temperature toughness, characterized by performing tempering at the following temperature.
[0009]
(3) C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt%, B : 0.0003-0.002 wt%, P: 0.010 wt% or less, S: 0.005 wt% or less, and any one or two selected from La: 0.03 wt% or less, Ce: 0.03 wt% or less After heating a steel substantially composed of Fe, the rolling reduction at 1000 ° C. or less is 30 to 70%, and the rolling end temperature is (Ar ₃ transformation point + 300) ° C. to (Ar ₃ Rolling at (transformation point +100) ℃, quenching from (Ar ₃ transformation point +50) ℃ or higher, and then tempering at temperature below Ac ₁ transformation point. Excellent method for producing high tensile steel.
[0010]
(4) C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt% , B: 0.0003~0.002wt%, P: 0.010wt% or less, S: 0.005 wt% include the following, or one Mo: 0.8 wt% or less, Cu: 0.5 wt% or less, V: 0.2 wt% or less, Ti: It contains one or more selected from 0.1 wt% or less, and further contains one or two selected from La: 0.03 wt% or less and Ce: 0.03 wt% or less. After heating a steel substantially composed of Fe, the rolling reduction at 1000 ° C or less is 30 to 70%, and the rolling end temperature is (Ar ₃ transformation point + 300) ° C. to (Ar ₃ transformation Excellent in low-temperature toughness, characterized by rolling at (point +100) ° C, then quenching from (Ar ₃ transformation point +50) ° C or higher, and then tempering at temperature below Ac ₁ transformation point. Method of manufacturing high-strength steel. In each of the above inventions, the combination of the additional elements added to the basic components is
(1) For Mo type,
Mo- (one or more of Cu, V, Ti)
Mo- (one or more of Cu, V, Ti)-(one or two of La, Ce)
(2) For Cu-based materials,
Cu- (one or two of V and Ti)
Cu- (one or two of V and Ti)-(one or two of La and Ce)
(3) For the V type,
V-Ti
V-Ti- (one or two of La and Ce)
(4) For Ti series,
Ti
Ti- (one or two of La and Ce)
Is preferred.
[0011]
[Action]
Next, in the present invention, the reason why the steel composition is limited according to the above-mentioned summary configuration will be described.
C: 0.20 wt% or less;
C is an element effective for improving hardenability and easily increasing strength, but when too much, significantly impairs toughness and increases weld cracking susceptibility. Therefore, its content needs to be 0.20 wt% or less. The preferred content is 0.10 wt% or less.
[0012]
Si: 0.03 to 0.8 wt%;
Si is an element that effectively promotes deoxidation and works effectively to increase the strength. The effect can be obtained by adding 0.03 wt% or more, but if it is too much, the low-temperature toughness and weldability are significantly impaired, so the content must be 0.03-0.8 wt%, and the preferred content is 0.10-0.8 wt%. 0.20 wt%.
[0013]
Mn: 0.4 to 4.5 wt%;
Mn is an extremely useful element that replaces expensive Ni because it has the effect of improving hardenability and increasing both the strength and low-temperature toughness of steel. The effect can be obtained by adding 0.4 wt% or more. However, adding more than 4.5 wt% significantly increases the susceptibility to weld cracking and increases the temper brittleness, so it is necessary to add it in the range of 0.4 to 4.5 wt%. is there. The preferred content is 0.4 to 1.0 wt%.
[0014]
Al: 0.07 wt% or less;
Al has a deoxidizing effect in the steelmaking process, and is also useful because it forms nitrides during hot rolling and heat treatment to make the structure finer, but too much increases the amount of Al ₂ O ₃ inclusions and increases weldability. Harms. Therefore, the upper limit of the content needs to be 0.07 wt% or less.
[0015]
Ni: 3.5 to 6.0 wt%;
Ni is an extremely useful element for improving low-temperature toughness and strength. The effect can be obtained by adding 3.5 wt% or more, but if it is too much, it is an expensive element and the effect is not improved so much, so it is necessary to add it in the range of 3.5 to 6.0 wt%. In addition, the preferable content of Ni is 4.0 to 6.0 wt%.
[0016]
Nb: 0.005 to 0.2 wt%;
Nb forms a solid solution in steel by high-temperature heating, and finely precipitates as carbonitride in the subsequent rolling process. For this reason, the austenite grains have a very fine structure as a result of the crystal growth being remarkably suppressed during recrystallization. It also contributes to an increase in strength due to precipitation hardening. These effects are exhibited by adding 0.005 wt% or more under the production conditions of the present invention. However, if the added amount of Nb is too large, the upper limit is 0.2 wt% in order to reduce the weld toughness. The preferred content of Nb is 0.005 to 0.015 wt%.
[0017]
B: 0.0003-0.002 wt%;
B is a component that enhances hardenability, contributes to an increase in the strength of the base material, and an improvement in HAZ characteristics. These effects cannot be obtained unless the amount is less than 0.0003 wt%, and when added excessively, the base material and the HAZ toughness are rather reduced. Therefore, the content of B needs to be 0.0003 to 0.002 wt%.
[0018]
P: 0.010 wt% or less;
Although P is inevitably contained in steel as an impurity, even a small amount thereof has an adverse effect on the toughness of the base metal and the welded portion. More preferably, the content is -0.005 wt% or less.
[0019]
S: 0.005 wt% or less;
S, which is inevitably contained in steel as an impurity like P, has an adverse effect on the toughness of the base metal and the welded portion as MnS-based inclusions, so it must be made 0.005 wt% or less. More preferably, the content is 0.002 wt% or less.
[0020]
Further, in this invention, in addition to the basic components described above, if necessary to supplement the properties of the steel, Mo, Cu, V, Ti, La, any one or more selected from Ce Can be added. Hereinafter, the reasons for limiting the above components will be described .
[0021]
Mo: 0.8 wt% or less;
Mo has an effect of increasing strength by solid solution strengthening and improvement of hardenability. In addition, it is an effective element to prevent tempering brittleness induced by the addition of a large amount of Mn. However, Mo is expensive and, when added in large amounts, significantly hardens the weld and adversely affects HAZ toughness. Therefore, the addition amount is 1.0 wt% or less, preferably in the range of 0.05 to 0.30 wt%.
[0022]
Cu: 0.5 wt% or less;
Cu is a solid solution strengthening element and enhances strength and is also effective in improving corrosion resistance. However, excessive addition of Cu impairs hot workability. Therefore, the content of Cu is 0.5 wt% or less, preferably 0.2 wt% or less. .
[0023]
V: 0.2 wt% or less;
V has a precipitation hardening effect and is effective for further improving the strength. However, when added in a large amount, the amount of carbonitride generated increases and the toughness of the welded portion is degraded. Therefore, the content is set to 0.2 wt% or less, preferably 0.05 to 0.15 wt%.
[0024]
Ti: 0.1 wt% or less
Ti has a precipitation hardening effect and is effective for further improving the strength. However, if added in a large amount, the amount of carbonitride generated increases and the toughness of the welded portion is degraded. Therefore, the content is made 0.1 wt% or less, preferably 0.05 wt% or less.
[0025]
La: 0.03 wt% or less;
La has the effect of improving the toughness by spheroidizing the form of the sulfide-based nonmetallic inclusions. However, when added excessively, the low-temperature toughness is rather reduced, so that 0.03 wt% or less, preferably 0.005 to 0.015 wt%. I do.
[0026]
Ce: 0.03 wt% or less;
Ce has the effect of improving the toughness by spheroidizing the form of the sulfide-based nonmetallic inclusions like La, but when added excessively, the low-temperature toughness rather decreases, so that 0.03 wt% or less, preferably 0.005 wt% or less. To 0.015 wt%.
In the present invention, other rare earth elements can be used instead of La and Ce additions.
[0027]
Next, the reason for limiting the manufacturing conditions will be described.
{Circle around (1)} Reduction at 1000 ° C or less: 30-70%;
In order to make the final steel structure a fine tempered martensite or a mixed structure of tempered martensite and lower bainite, it is necessary to control and refine the crystal grains before quenching. In order to achieve the refinement of the crystal grains before the quenching treatment, the rolling reduction at 1000 ° C. or less is important. That is, by setting the rolling reduction in this temperature range to 30% or more, strain energy is accumulated at the austenite grain boundaries, and many dislocations and deformation zones are introduced into the grains. As a result, after the quenching-tempering treatment, it is possible to increase the strength and toughness by making the crystal grains finer and introducing dislocations.
However, when the rolling reduction exceeds 70%, the strength is reduced. It is considered that the reason for this is that a large amount of ferrite precipitates at the processing strain accumulation portion as a nucleus due to excessive processing.
Therefore, it is necessary to perform rolling at a rolling reduction of 1000 ° C. or less in the range of 30 to 70%. More preferably, 40 to 60% is good.
[0028]
(2) Rolling end temperature: (Ar ₃ transformation point + 300) ° C-(Ar ₃ transformation point + 100) ° C;
In order to make the final steel structure a fine tempered martensite or a fine mixed structure of tempered martensite and lower bainite, the rolling completion temperature is also an important requirement. If the rolling end temperature exceeds (Ar ₃ transformation point + 300) ° C, the recrystallized grains become coarse and adversely affect the toughness after quenching and tempering. On the other hand, if the rolling end temperature is less than (Ar ₃ transformation point + 100) ° C, This is because a large amount of ferrite is precipitated and the strength is reduced. Therefore, the rolling end temperature is required to be (Ar ₃ transformation point +300) ℃ ~ (Ar ₃ transformation point +100) ° C.. More preferably, the temperature is 900 to 700 ° C.
[0029]
( ₃ ) Quenching temperature: (Ar ₃ transformation point + 50) ° C or higher;
When quenched after the above rolling, the austenite phase is transformed into fine martensite and lower bainite. The old austenite grains have a thin and elongated structure by the rolling. If the quenching temperature is less than (Ar ₃ transformation point +50) ° C., the dislocations introduced by rolling recover and the strength decreases, so it is necessary to quench from a temperature of (Ar ₃ transformation point +50) ° C. or more. More preferably, the temperature is from 800 to 650 ° C.
[0030]
(4) Tempering temperature: below the Ac ₁ transformation point;
After quenching under the above conditions, by tempering at a temperature equal to or lower than the Ac ₁ transformation point, a high-tensile steel having a target material can be obtained for the first time. If the tempering temperature exceeds the Ac ₁ transformation point, the toughness of the base material tends to decrease, and the material is not satisfied. Since the toughness of the base material is lowered even at a tempering temperature of 400 ° C. or lower, the lower limit temperature is preferably set to 400 ° C.
[0031]
By adopting the above-mentioned chemical composition and manufacturing conditions, the final structure of the steel becomes fine tempered martensite or a mixed structure of tempered martensite and lower bainite, and the strength is mainly due to the refinement of the structure and the strengthening of precipitation of Nb and the like. , A steel excellent in low-temperature toughness and HAZ characteristics can be obtained.
The structure of the present invention is significantly different from the structure obtained by the conventional so-called QQ'T method. That is, the structure obtained by the conventional QQ'T method is such that the coarse martensite structure generated in the first quenching process (Q) is partially heated by ferrite during the second quenching (Q '), A part transforms into austenite, but a part of the austenite formed at this time becomes a martensite structure in a form surrounding a ferrite phase upon cooling. This results in a structure composed of tempered martensite surrounding the ferrite grains by the subsequent tempering (T).
[0032]
【Example】
Example 1
Steel having the composition shown in Table 1 was melted to form a slab, which was heated to 1150 ° C., and then subjected to rolling and heat treatment under the conditions shown in Table 2. Here, it was also manufactured by the conventional QQ'T treatment for comparison. The thickness of each steel plate was 25 mm. The mechanical properties (yield strength, tensile strength, fracture transition temperature in Charpy impact test, and impact absorption energy at -110 ° C) of the steel sheets produced by these methods were examined. The results are shown in Table 2.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
Example 2
Each steel having the composition shown in Table 3 was melted to form a slab, heated to 1150 ° C, rolled and quenched under the conditions shown in Table 4, and then tempered at 570 ° C (invention method). , Comparison method). In addition, for steel 2,3,6,8-13, conventional QQ'T treatment (heating at 890 ° C for 1 hour, water cooling, heating at 700 ° C for 20 minutes, water cooling, and tempering at 570 ° C) ).
Table 4 shows the results of examining the yield strength, tensile strength, fracture transition temperature in the Charpy impact test, absorbed energy at −60 ° C. and −110 ° C., and reproduced HAZ toughness of these steel sheets. Here, the condition of the reproduction HAZ was a single cycle heat history with peak temperatures of 1400 ° C. and 800 ° C., and the cooling time from 800 ° C. to 500 ° C. was 30 seconds.
[0036]
[Table 3]

[0037]
[Table 4]

[0038]
【The invention's effect】
As described above, according to the manufacturing method of the present invention, characteristics superior to those obtained by the conventional QQ'T treatment, that is, excellent HAZ characteristics and high strength while having excellent low-temperature toughness, are sacrificed at the expense of others. In addition to providing a low-temperature steel that has been realized without any problems, such a steel material can be economically manufactured with high productivity.

Claims (4)

C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt%, B: 0.0003 to After heating a steel containing 0.002 wt%, P: 0.010 wt% or less, S: 0.005 wt% or less, and the balance substantially consisting of Fe, the rolling reduction at 1000 ° C or less is 30 to 70%. Rolling is performed at a rolling end temperature of (Ar ₃ transformation point +300) ° C. to (Ar ₃ transformation point +100) ° C. Then, after quenching from a temperature of (Ar ₃ transformation point +50) ° C. or higher, Ac ₁ transformation A method for producing a high-strength steel excellent in low-temperature toughness, characterized by performing tempering at a temperature not higher than the temperature. C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt%, B: 0.0003 to 0.002wt%, P: 0.010wt% or less, S: include: 0.005 wt%, or one Mo: 0.8 wt% or less, Cu: 0.5 wt% or less, V: 0.2 wt% or less, Ti: less 0.1 wt% After heating one or two or more kinds selected from the above, and the balance is substantially composed of Fe, the rolling reduction at 1000 ° C or lower is 30 to 70%, and the rolling end temperature After rolling at (Ar ₃ transformation point + 300) ℃ ~ (Ar ₃ transformation point + 100) ℃, and then quenching from a temperature of (Ar ₃ transformation point + 50) ℃ or more, then temperature below Ac ₁ transformation point A method for producing a high-strength steel excellent in low-temperature toughness, characterized in that tempering is performed in the steel. C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt%, B: 0.0003 to 0.002% by weight, P: 0.010% by weight or less, S: 0.005% by weight or less, and La: 0.03% by weight or less, Ce: 0.03% by weight or less. After heating the steel substantially composed of Fe, the reduction at a temperature of 1000 ° C. or less is 30 to 70%, and the rolling end temperature is (Ar ₃ transformation point +300) ° C. to (Ar ₃ transformation point +100). ) ° C, followed by quenching at a temperature of (Ar ₃ transformation point + 50) ° C or higher, and then tempering at a temperature of Ac ₁ transformation point or lower. Manufacturing method for tensile steel. C: 0.20 wt% or less, Si: 0.03 to 0.8 wt%, Mn: 0.4 to 4.5 wt%, Al: 0.07 wt% or less, Ni: 3.5 to 6.0 wt%, Nb: 0.005 to 0.2 wt%, B: 0.0003 to 0.002wt%, P: 0.010wt% or less, S: include: 0.005 wt%, or one Mo: 0.8 wt% or less, Cu: 0.5 wt% or less, V: 0.2 wt% or less, Ti: less 0.1 wt% It contains one or two or more selected from among them, and further contains one or two selected from La: 0.03 w% or less and Ce: 0.03 wt% or less, and the balance is after heating a steel having a composition of substantially Fe, at a reduction rate of 30 to 70 percent at 1000 ° C. or less, and the rolling finishing temperature is (Ar ₃ transformation point +300) ℃ ~ (Ar ₃ transformation point + 100) at ° C. A high-strength steel with excellent low-temperature toughness characterized by being subjected to a certain rolling, followed by quenching at a temperature of (Ar ₃ transformation point + 50) ° C or higher and then tempering at a temperature of Ac ₁ transformation point or lower. Production method.