JPH0125371B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of the Invention) The present invention relates to a method for manufacturing high-strength steel plates, particularly heat-treated high-strength thick steel plates with excellent strength, toughness, and weldability. (Prior Art) As a recent trend, as welded structures become larger, the amount of high-strength steel plates used has increased in order to reduce the weight of the structures. By using high-strength steel plates, the structure takes advantage of the advantages of lighter weight, improved transportation and assembly workability, and improved welding performance due to the thinner walls of each structural member. This is what I am trying to do. For example, in recent years, welded structures such as pumped storage power plants, pressure vessels, bridges, and offshore structures are becoming larger in size, and the high tensile strength steel plates used are also becoming more tensile. Traditionally, this type of high-strength steel plate is
Although HT60 and HT80 class steel types are used,
High strength with a yield point of 90Kgf/mm2 or ^higher
HT100 class steel has not yet reached the stage of practical use. This has a yield strength of 90Kgf/ ^mm2 or more and a tensile strength of
It is difficult to stably secure the specified strength of 97Kgf/mm ² or more, and even if the specified strength is satisfied, it is difficult to simultaneously provide excellent low-temperature toughness.
Furthermore, this is because it is extremely difficult to impart weldability that does not pose a problem in terms of implementation. The conventionally developed HT100 class steel types contain more than 0.15% C to ensure strength.
Alternatively, since it contains more than 0.06% of V, the toughness of the base material is not very good, the brittle fracture surface transition temperature (vTs) is -40℃ or higher, and the weldability is not sufficient. Nakatsuta. On the other hand, similar steel types with good base metal toughness are
It contains 5% or more of Ni, which has the disadvantage of significantly lower economic efficiency. (Related Invention) The inventors have already proposed HT100 class steel containing 2 to 3% Ni and having good strength, toughness and weldability (Patent Application 177701/1982), but still there is no improvement in HT80 steel. In comparison, it contains a considerably large amount of Ni and has the disadvantage of high manufacturing cost. (Summary of the Invention) In order to improve the drawbacks seen in the above-mentioned prior art and prior inventions, the inventors of the present invention have conducted detailed research focusing on trace elements, particularly impurities in steel, and have found the following: This invention was completed based on the following findings: (i) Even in HT100 class high-strength steel, by reducing N, which is normally contained in large amounts as an impurity in high-strength steel sheets, the base material can be improved. Hardenability can be increased, and therefore high strength can be achieved simply by reducing the N content without adding large amounts of alloying components; The toughness of welded joints is also improved. (ii) Normally, reducing the amount of N in steel has the disadvantage that austenite crystal grains become coarser and toughness deteriorates. However, even in low-N steel, by adding a small amount of Nb, austenite grains can be refined and toughness can be improved. Nb is often used as a precipitation-strengthening element or a recrystallization suppressing element in conventional controlled rolled steel, but it is rarely used in tempered steel.
This invention differs greatly from the prior art in that Nb is used as a grain refining element rather than as a precipitation strengthening element. (iii) By reducing N, the amount of V added can be reduced to 0.06%.
Even if it is limited to the following, it is possible to obtain sufficient precipitation strengthening with such a small amount of V. Therefore, the deterioration of base material toughness and weldability due to the addition of a large amount of V can be suppressed, and it can be used as a weldable thick steel plate. It can provide sufficient toughness. (iv) On the other hand, by suppressing the Si content to 0.15% or less, a mixed structure of martensite and fine bainite can be easily obtained, and the toughness of the base material can be significantly improved. At the same time, such low
Even in welded joints where the cooling rate is relatively fast, the formation of island-like martensite that adversely affects toughness can be suppressed by siliconization, so the toughness of the welded joints is improved. (v) Tempering brittleness can be suppressed by lowering the P content, which is an impurity element, to 0.010% or less and water cooling after tempering, and sufficient low-temperature toughness can be imparted even if tempering is performed at a lower temperature than conventionally. In addition, by further reducing the S content to 0.003% or less,
Sufficient low-temperature toughness can be imparted regardless of the rolling direction. (vi) In this way, by combining not only the reduction of N content and the addition of Nb, but also the reduction of V content, Si content, and P and S content, Furthermore, by combining the steel with the composition thus obtained with lowering the tempering temperature and water cooling after tempering, it is possible to ensure satisfactory strength and toughness as well as excellent weldability. Here, this invention provides: In weight%, C: 0.07-0.15%, Si: 0.15% or less, Mn: 0.40-1.20%, Ni: 1.00-3.50%, Cr: 0.40-1.20%, Mo: 0.40-0.80 %, V: 0.01-0.06%, Nb: 0.005-0.030%, B: 0.0020% or less, sol.Al: 0.01-0.10%, N: 0.0040% or less, P: 0.010% or less, S: 0.003% or less, and Cu: 0.50% or less, Ca: 0.005 as required
% or less and W: 1.00% or less, the balance is Fe and unavoidable impurities, and the formula: P _CM (%) = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) /
20+Ni(%)/60+Cr(%)/20+Mo(%)/15+V(
%)/10+5×B(%) P _CM (welding crack susceptibility index) is 0.28%
After heating the steel to above 1050℃, then hot rolling, followed by reheating to the temperature range from Ac ₃ transformation point to 1050℃, quenching, then tempering below Ac ₁ transformation point. Manufacture of tempered high-strength thick steel plates with excellent weldability, characterized by water cooling, yield point of 90 Kgf/mm ² or higher, tensile strength of 97 Kgf/mm ² or higher, and impact fracture transition temperature of -60°C or lower. It's a method. Note that the thick steel plate herein is generally not particularly limited, but preferably refers to one having a thickness of 5 mm or more. (Aspects of the Invention) Next, the reason why the steel composition and heat treatment conditions are limited as described above in this invention will be explained.
is "% by weight". C: C is necessary to ensure hardenability and strength,
In particular, in order to ensure strength of YS≧97Kgf/mm ² or more, 0.07% or more is added, but if added in excess of 0.15%, the weld cracking property and low temperature cracking property are significantly deteriorated, so in this invention, C Content 0.07~
Limited to 0.15%. Si: Si is usually added to deoxidize and ensure strength, but by reducing Si, the toughness of welded joints and heat-affected zones can be improved. This is because the reduction in Si promotes the precipitation of carbides, thereby suppressing the formation of hardened structures in the weld. For example, such effects can be sufficiently maintained by limiting Si to 0.15% or less. In addition, in order to ensure the strength of HT100 class high-strength steel, it is usually 0.25
% of Si, and therefore sufficient weldability is not necessarily obtained. Therefore, in this invention, the Si content is reduced for the purpose of improving weldability.
Limited to 0.15% or less. Mn: Mn is necessary to ensure hardenability,
Mn content should be added at 0.40% or more, but if it exceeds 1.20%, weldability and base metal toughness will deteriorate, so the Mn content should be reduced to 0.40%.
Limited to ~1.20%. Ni: Ni is effective in ensuring hardenability and improving low-temperature toughness, so it is added in an amount of 1.00% or more, but considering economic efficiency, the upper limit was set at 3.50%. Note that it is preferable from an economical point of view that the Ni content is varied within the range of 1.00 to 3.50% depending on the thickness of the steel plate. Cr: Cr is added in an amount of 0.40% or more to ensure hardenability and strength, but if it exceeds 1.20%, it deteriorates weldability and base metal toughness, so it is limited to 0.40 to 1.20%. Mo: Mo is added in an amount of 0.40% or more because it increases hardenability and temper softening resistance and is effective in securing strength, but if it exceeds 0.80%, weldability will deteriorate significantly, so its content is limited to 0.40-0.80%. V: V is used in this invention to ensure strength.
Add 0.01% or more, but if it exceeds 0.06%, the toughness and weldability of the base material will deteriorate significantly, so 0.01%
Limit to ~0.06%. In this invention, since the N content is limited to 0.0040% or less, sufficient strength can be ensured with a V addition amount of 0.06% or less. Nb: Nb is normally used as a precipitation strengthening element or as a recrystallization suppressing element in controlled rolled steel, but in this invention it is added as a grain refining element. For this purpose, 0.005% or more is added, but if it exceeds 0.030%, weldability deteriorates, so it is limited to 0.005 to 0.030%. B: B is an element that greatly improves hardenability when added in a small amount, and is effective for improving strength and toughness, but if it exceeds 0.0020%, the toughness deteriorates, so the B content is 0.0020%. Limited to the following. In addition,
In this invention, since the N content is limited to 0.0040% or less, the B content is preferably 0.0010% or less. sol.Al Al has a deoxidizing effect and also refines the austenite crystal grains and improves toughness, so in this invention, 0.01% or less is added as sol.Al.
If it exceeds 0.10%, the toughness will deteriorate, so 0.01~
Limit to 0.10%. N: In this invention, when the N content is 0.0040% or less, hardenability is enhanced, the strength and toughness of the base metal are improved, and the toughness of the welded joint is also improved. Note that N is normally fixed as AlN in steel, but since it is reheated to a high temperature in the weld bond and weld heat-affected zone, AlN becomes a solid solution in the base material.
It becomes free N and deteriorates toughness. Furthermore, if the N content is high, it will precipitate as VN during reheating and the amount of solid solution V will decrease, resulting in a decrease in the amount of precipitation strengthening of V after quenching and tempering, resulting in a decrease in the strength of the base material. Therefore, in this invention, the N content
Limited to 0.0040% or less. P: P promotes temper brittleness and deteriorates toughness, and also tends to segregate during solidification from molten steel, causing abnormal structures in the segregated areas, so it is preferable to reduce it.
Therefore, in this invention, the P content is limited to 0.010% or less. S: S usually exists in the form of MnS in steel, and is elongated during rolling, resulting in anisotropy in toughness. In high-strength steel, elongated inclusions have a negative effect on toughness, so in this invention, the S content is reduced to 0.003%.
Below, it is preferably limited to 0.002% or less. In addition,
In a preferred embodiment of this invention, P+S is limited to 0.01% or less. Cu: Cu is added as necessary to improve strength and corrosion resistance, but if it exceeds 0.50%, it deteriorates hot workability, toughness, and weldability, so it is not used in this invention.
Limit the amount of Cu added to 0.50% or less. Ca: Ca is added to make inclusions such as MnS spheroidal and improve the anisotropy of toughness, but if it exceeds 0.005%, the toughness decreases, so it is limited to 0.005% or less. W: W increases hardenability, temper softening resistance, and strength, so it is added as necessary, but if it exceeds 1.00%, it deteriorates toughness and weldability, so it should be limited to 1.00% or less. . P _CM (weld cracking susceptibility index) P _CM is an index that expresses the cold cracking susceptibility during welding, and is usually 0.28% or less for 80 kg class high tensile strength steel.
In this invention, _PCM is limited to 0.28% or less in order to provide HT100 with the same weldability as HT80. Heating temperature before hot rolling: The heating temperature during hot rolling is set to 1050℃ or higher to completely dissolve NbC or Nb(C,N) before rolling, and to dissolve NbC or Nb(C,N) after rolling. )
This is to precipitate uniformly and finely and to refine the austenite crystal grains during reheating and quenching. Quenching heating temperature: The heating temperature before quenching should be set to Ac ₃ points or higher.
Since complete austenitization does not occur at temperatures below the Ac ₃ transformation point, the purpose is to completely austenitize and uniformly dissolve the alloying elements.On the other hand, the upper limit is limited to 1050℃ or less when the temperature exceeds 1050℃. This is because NbC or Nb (C, N) becomes solid solution or becomes coarse, and austenite crystal grains become coarse, resulting in deterioration of toughness. Tempering temperature: If the tempering temperature exceeds the Ac ₁ transformation point, the yield strength will drop significantly and stable strength cannot be obtained, so the tempering temperature should be below the Ac ₁ transformation point. The reason for water cooling after tempering is to improve toughness by suppressing tempering brittleness,
This is due to increased strength due to increased solid solution strengthening. Water cooling after tempering is one of the features of the present invention, and thereby the low-temperature toughness and strength can be improved. In other words, by water cooling, the cooling rate after tempering can be increased, tempering brittleness can be suppressed, and C
This is because solid solution strengthening effects such as Next, this invention will be further explained with reference to Examples, but these Examples are merely illustrative of this invention and are not intended to limit this invention. Examples Steel types A to J within the scope of the present invention and steel numbers K to Q as comparative examples having chemical compositions as shown in Table 1 were melted. Next, after hot forging this into a 120mm thick slab, steel numbers A to D and K to M are
After heating to 1150℃, hot rolling is performed to produce a steel plate with a thickness of 25mm.On the other hand, steel numbers E to J and N to Q are 1150℃.
After heating to 1050°C or 1000°C, a steel plate with a thickness of 40 mm was finished by hot rolling and cooled in air. Next, these steel plates were reheated to 900°C or 920°C, water quenched, tempered at 600°C or 615°C, and then water cooled. In addition, for the purpose of investigating the influence of quenching temperature, steel numbers A to G were heated to 800℃ or 1100℃ as comparative examples.
Water quenched after reheating to 580℃ or 615℃
A refining treatment was also performed in which the steel was tempered in water and then cooled in water. Further, as a comparative example, a specimen subjected to normal quenching and tempering, in which the specimen was tempered and then air cooled, was also cited. A JIS No. 4 2mm V-notch mechanical pea test piece and a round bar tensile test piece with a diameter of 8.5mm and a parallel length of 50mm were taken from the center of the thickness of each steel plate obtained in this way in a direction perpendicular to the rolling direction. , its mechanical properties were investigated. In addition, diagonal y-groove restraint crack test pieces (plate thickness: 25 mm or 40 mm) were taken from each steel plate, and after preheating to 125°C, hand welding was performed at a heat input of 17 KJ/cm (current:
(170A, voltage: 25V, speed: 15cm/min), and the presence or absence of surface cracks, root cracks, and cross-sectional cracks was investigated. These results are summarized in Table 2. As is clear from Table 2, as shown in the results of test numbers 1 to 16, according to the present invention, the yield point is 90 Kgf/mm ² or more, and the tensile strength is 97 Kgf/mm 2
mm ² or higher, which satisfies the required strength as HT100, and also satisfies the fracture surface transition temperature of -60°C or lower, indicating that it has excellent low-temperature toughness sufficient to be used as a welded structural member. Furthermore, it can be seen that preheating to 125°C prevents low-temperature cracking and has excellent weldability. On the other hand, the steel plates obtained in test numbers 17 to 23, which are comparative examples, have steel compositions that are outside the range of this invention, so although the strength is satisfied, the fracture surface transition temperature is -
It can be seen that the temperature is 60°C or higher and the toughness is poor. In addition, weldability cracking occurs when using steel with components in which the amount of added V exceeds the upper limit (test numbers 19 and 22), when using steel with C and P _CM exceeding the upper limit (test number 17), and when using steel with components that exceed the upper limit of Si. This occurs in steel plates when steel with P _CM exceeding the upper limit is used (Test No. 18). When the rolling heating temperature is outside the lower limit (test number 24)
-26), the toughness deteriorated even though the chemical components were within the scope of this invention. When the reheating and quenching temperature is outside the lower limit (test numbers 27 and 28), the strength decreases, and when the reheating and quenching temperature is outside the upper limit (tests nos. 29 and 30), the toughness deteriorates significantly. ing. Furthermore, when air-cooled after tempering (test number 31~
33) results in a decrease in both strength and toughness. As detailed above, according to the present invention, an HT100 class steel with excellent strength and toughness and good weldability can be obtained without using a large amount of expensive Ni.

【table】

[Table] (Note) (1) * Indicates that it is outside the scope of this invention.
Si(%) Mn(%) Cu
(%) Ni(%) Cr(%) Mo(%) V(%)
(2) ** P _CM (%) = C (%) +

Claims (1)

[Claims] 1% by weight: C: 0.07-0.15%, Si: 0.15% or less, Mn: 0.40-1.20%, Ni: 1.00-3.50%, Cr: 0.40-1.20%, Mo: 0.40-0.80 %, V: 0.01-0.06%, Nb: 0.005-0.030%, B: 0.0020% or less, sol.Al: 0.01-0.10%, N: 0.0040% or less, P: 0.010% or less, S: 0.003% or less, remainder It consists of Fe and unavoidable impurities, and has the formula: P _CM (%) = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) /
20+Ni(%)/60+Cr(%)/20+Mo(%)/15+V(
%)/10+5×B(%) P _CM (welding crack susceptibility index) is 0.28%
After heating the steel to above 1050℃, then hot rolling, followed by reheating to the temperature range from Ac ₃ transformation point to 1050℃, quenching, then tempering below Ac ₁ transformation point. A method for manufacturing an annealed high-strength thick steel plate with excellent weldability, having a yield point of 90 Kgf/mm ² or higher, a tensile strength of 97 Kgf/mm ² or higher, and an impact fracture transition temperature of -60°C or lower, which is characterized by water cooling. . 2% by weight, C: 0.07-0.15%, Si: 0.15% or less, Mn: 0.40-1.20%, Ni: 1.00-3.50%, Cr: 0.40-1.20%, Mo: 0.40-0.80%, V: 0.01- 0.06%, Nb: 0.005-0.030%, B: 0.0020% or less, sol.Al: 0.01-0.10%, N: 0.0040% or less, P: 0.010% or less, S: 0.003% or less, Cu: 0.50% or less, Ca : 0.005% or less and W: 1.00% or less, the balance consists of Fe and unavoidable impurities, and the formula: P _CM (%) = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) /
20+Ni(%)/60+Cr(%)/20+Mo(%)/15+V(
%)/10+5×B(%) P _CM (welding crack susceptibility index) is 0.28%
After heating the steel to above 1050℃, then hot rolling, followed by reheating to the temperature range from Ac ₃ transformation point to 1050℃, quenching, then tempering below Ac ₁ transformation point. A method for manufacturing an annealed high-strength thick steel plate with excellent weldability, having a yield point of 90 Kgf/mm ² or higher, a tensile strength of 97 Kgf/mm ² or higher, and an impact fracture transition temperature of -60°C or lower, which is characterized by water cooling. .