JPH0120210B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for producing a high-strength, high-toughness steel material having a yield point of 90 Kgf/mm ² or more, a tensile strength of 97 Kgf/mm ² or more, and an impact fracture transition temperature of -60°C or less, and in particular a plate having the above characteristics. The present invention relates to a method that can stably produce high-strength steel plates having a thickness of 40 to 150 mm. In recent years, various steel structures have become larger and larger, and the high-tensile steel plates used are becoming increasingly stronger. HT80 steel plates (with a tensile strength of 80Kgf/mm2 or ^higher as a guide) are now being used. However, although HT60 steel plates and the aforementioned HT80 steel plates, which have a tensile strength of 60 Kgf/mm ² or higher, are currently used in this type of structure, they have a yield point of 90 Kgf/mm ² or higher. , Tensile strength: 97Kgf/ ^mm2
At present, it has not been possible to use HT100 grade steel sheets, which have such high strength. This is because, for thick steel plates with a plate thickness of 40 mm or more, the yield point is:
It is extremely difficult to obtain a high strength of 90 Kgf/mm ² or more, tensile strength: 97 Kgf/mm ² or more, and at the same time, it has excellent low-temperature toughness with an impact fracture transition temperature of -60°C or less. It is extremely difficult to make steel materials satisfy these requirements, and furthermore, it is difficult to give the above-mentioned high-strength steel enough weldability to cause no problems when constructing actual structures. This was because the problem could not be resolved. By the way, the HT100 steel material that has been prototyped so far,
It is mainly a steel plate with a thickness of 20 to 40 mm, and contains 0.15% or more of C or 0.06% of V to ensure strength.
Therefore, not only is the toughness of the base material inferior (impact fracture surface transition temperature: -40°C or higher), but also it cannot be said that sufficient consideration has been given to weldability. Moreover, in order to increase the plate thickness to about 100 mm, it is necessary to increase the hardenability more than before, which inevitably necessitates an increase in the amount of alloying elements added. Simply increasing the amount of elements could not avoid the problem of further deterioration of base metal toughness and weldability. From the above-mentioned viewpoints, the present inventors have developed a material with a toughness value (impact fracture transition temperature: -60°C or less) and weldability [weld cracking susceptibility index (PcM) of 0.31% or less] that can be used as a welded structure member. :However, PcM is
Formula, PcM=C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5 × B (%) ), and hereinafter, % representing the component proportion is expressed as weight %]. In particular, as a result of research focusing on the influence of trace elements on conventional high-strength steel, we have obtained the findings shown in (a) to (e) below. That is, (a) Conventional high-strength steel usually contains about 0.008 to 0.015% N as an impurity;
If the content is reduced to a low value, especially 0.0040% or less, the hardenability of the steel will further improve, and a mixed structure of martensite and bainite can be easily obtained without adding large amounts of alloying components. (b) When the N content in steel is reduced to 0.0040% or less, sufficient precipitation strengthening can be obtained with the addition of a small amount of V, and therefore it is possible to limit the amount of V added to 0.06% or less. (c) In addition, after the P content is lowered to 0.008% or less and after being heated and held at a specified temperature following quenching treatment, When tempering is performed using water cooling,
(d) In addition, conventional high-strength steel has low deoxidation and tempering softening resistance. It contains about 0.3% Si for the purpose of imparting the same properties, but if this Si content is reduced to 0.15% or less, a small amount of bainite will be generated during quenching, which will significantly improve low-temperature toughness. (e) In this way, the amount of N in the steel is reduced and further Si,
The content of P and S is kept low, and a trace amount of V
By adding B and B, and then subjecting it to quenching and tempering, which involves heating to a predetermined temperature and cooling with water, even if the wall thickness exceeds 40 mm, it will have excellent strength and toughness, and 100Kgf/mm class ² high tensile strength steel with good weldability can be stably obtained. This invention was made based on the above findings, and includes C: 0.07 to 0.15%, Si: 0.15% or less, Mn: 0.40 to 1.00%, Cr: 0.40 to 1.20%, Ni: 2.0 to 4.2%, Mo : 0.40 to 0.80%, V: 0.01 to 0.06%, B: 0.0004 to 0.0015%, Cu: 0.50% or less, Sol.Al: 0.010 to 0.100%, P: 0.008% or less, S: 0.003% or less, N: 0.0040 % or less, Fe and other unavoidable impurities: remaining, and the weld cracking susceptibility index (PcM) is
It has a component composition range of 0.31% or less, plate thickness: 40
mm~150mm steel plate is heated to a temperature range of [Ac ₃ transformation point ~1000℃], quenched, then heated to 560~630℃, and then water-cooled to achieve excellent toughness and high It is characterized by the stable production of high-strength steel materials, including those with wall thicknesses of 40 mm to 150 mm, that have strength and good weldability. Next, the reason why the chemical composition and heat treatment conditions of the steel are numerically limited as described above in the method of the present invention will be explained. A Chemical composition of steel a) C The C component has the effect of ensuring the hardenability and strength of steel, but if its content is less than 0.07%, the desired effect cannot be obtained, and on the other hand If the C content exceeds 0.15%, the weldability, especially the cold crackability, will be significantly deteriorated, so the C content was set at 0.07 to 0.15%. b) Si Si is usually used to deoxidize steel and ensure strength.
The method of this invention is characterized by limiting the Si content to 0.15% or less, which promotes the formation of bainite and improves the mixing of martensite and bainite. It improves the toughness of steel by obtaining a structure. And the Si content is 0.15
If the Si content exceeds 0.15%, the effect of promoting bainite formation decreases, so the Si content was set at 0.15% or less. Note that there is no need to set a lower limit for the Si content; the lower the Si content, the better the effect. However, in consideration of ease of steel manufacturing, it is preferable to reduce the Si content to about 0.01%. c) Mn The Mn component has the effect of ensuring the hardenability of steel, but if its content is less than 0.40%, the desired effect cannot be obtained;
If the Mn content is exceeded, the toughness and weldability will deteriorate.
It was set at 0.40-1.00%. d) Cr The Cr component should be contained at 0.40% or more to ensure hardenability and strength, but if it is contained in excess of 1.20%, toughness will deteriorate, so the Cr content should be set at 0.40 to 1.20%. Established. e) Ni The Ni component has the effect of ensuring hardenability and improving low-temperature toughness, so it should be contained at 2.0% or more, but in consideration of economic efficiency, the upper limit was set at 4.2%. In addition, Ni is an element that can increase hardenability without reducing toughness, and
2.7 to produce 150mm thick HT100 steel plate
It is desirable to contain it in an amount of % or more. f) Mo Mo component increases the hardenability of steel,
It has the effect of increasing the tempering resistance and increasing its strength, but its content is 0.40
If the Mo content is less than 0.80%, the desired effect cannot be obtained, and if it exceeds 0.80%, the weldability will be significantly deteriorated.
It was set at 0.40-0.80%. g) V The V component has the effect of increasing strength through precipitation strengthening, and is contained at 0.01% or more to ensure the desired strength.
If the V content exceeds 0.06%, the toughness and weldability will deteriorate.
It was set at 0.01-0.06%. In addition, in the method of the present invention, the N content is kept low at 0.004% or less, and by performing low-temperature tempering, sufficient strength can be ensured with a small amount of V addition, so the V content is reduced to 0.01%. It is most desirable to adjust the content to ~0.04% in order to ensure toughness. h) B The B component can greatly improve the hardenability of steel by adding a small amount, so it should be included at 0.0004% or more, but if it is included in excess of 0.0015%, the toughness will deteriorate, so B
The content was set at 0.0004% to 0.0015%. In addition,
In the method of the present invention, the N content is reduced to 0.0040%.
The B content is kept low as below.
A range of 0.0004 to 0.0010% is desirable. i) Cu The Cu component has the effect of improving the strength, toughness, and corrosion resistance of steel, but if it is contained in excess of 0.50%, it may deteriorate hot workability, toughness, or hot cracking resistance during welding. Therefore, the Cu content was set at 0.50% or less. In addition,
Although the Cu content is small and its addition effect is recognized, in order to obtain a noticeable effect, 0.05
% or more of Cu is desirable. j) Sol.Al The Sol.Al component has the effect of deoxidizing the steel and increasing the hardenability improvement effect of the B component, but if its content is less than 0.010%, the desired effect is not obtained. On the other hand, if the Sol.Al content exceeds 0.100%, the toughness will deteriorate, so the Sol.Al content was set at 0.010 to 0.100%. k) P and S It is desirable to reduce these impurities as much as possible in order to improve the toughness of steel. Yield strength of steel by quenching and tempering:
In order to satisfy 90Kgf/mm ² or more, tensile strength: 97Kgf/mm ² or more, it is preferable to temper at a low temperature, but at this time, if the P content is higher than 0.008%, tempering will cause brittleness. Toughness will be significantly reduced. Therefore, the P content
It was set at 0.008% or less. Furthermore, if the S content increases beyond 0.003%, coarse MnS will be produced, which will be elongated during rolling and cause deterioration of toughness, so the S content was set at 0.003% or less. l) N Setting N to less than 0.0040% is an extremely effective means for increasing the hardenability of steel and improving the strength and toughness of the base metal. That is, the N content is 0.0040
% or less, and by adjusting the Sol.Al content to 0.01 to 0.10%, the amount of solid solution B can be reduced to 0.0003ppm.
As a result, the hardenability is improved. Furthermore, since the amount of N is low, coarsening of AlN is suppressed and toughness is improved. Furthermore,
Since the generation of VN is suppressed by lowering the N content, V is uniformly dissolved in solid solution at the normal austenitizing temperature, which also has the effect of limiting the amount of V added. For this reason, the N content was reduced to 0.0040%.
It was determined as follows. m) PcM (weld crack susceptibility index) Even if preheating is performed at a sufficiently high temperature during welding, the formula PcM (%) = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / PcM (welding crack susceptibility index) expressed as 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5 × B (%) is
If it exceeds 0.31%, the incidence of cold cracking becomes extremely high and sufficient weldability cannot be ensured, so PcM was set at 0.31% or less. B Heat treatment conditions a) Heating temperature before quenching The reason why the heating temperature before quenching is set to the Ac ₃ transformation point or higher is to completely austenite the steel and uniformly dissolve the alloying elements as _a solid solution.
Heating below the transformation point does not allow the alloying elements to form a uniform solid solution. On the other hand, the heating temperature
If the temperature exceeds 1000℃, the austenite crystal grains will become coarser and the toughness will _decrease .
1000℃]. b) Tempering temperature The reason for tempering at a temperature of 560℃ or higher is to remove the strain introduced by quenching and to improve strength and toughness by finely precipitating carbides. If the temperature is lower than 0.degree. C., the above effects cannot be obtained. on the other hand,
If the tempering temperature exceeds 630℃, the strength of the steel material will decrease and the yield strength will no longer satisfy the requirements of 90Kgf/mm ² or higher and tensile strength of 97Kgf/mm ² or higher. It was set as ℃. Note that one of the characteristics of the method of the present invention is that the steel material is heated and held at the tempering temperature and then water-cooled, and such treatment can significantly improve the low-temperature toughness. This is because, for example, in steel materials with a wall thickness exceeding 40 mm, the cooling rate after tempering decreases and the susceptibility to tempering brittleness increases. If you ensure cooling rate: 0.5~6.0℃/sec,
This is because the above-mentioned disadvantages will be resolved. In addition, the method of this invention has plate thickness: 40 mm ~
It is particularly effective in manufacturing 150mm high-tensile, thick-walled steel plates. This is because for steel materials with a wall thickness of less than 40 mm, it is relatively easy to ensure the strength and toughness of the base material without adding large amounts of alloying elements. Also, if the plate thickness exceeds 150 mm, the water cooling effect will not be obtained. Next, the present invention will be explained using examples and comparing with comparative examples. Examples First, steels A to F and comparison steels G to I, which are subject to the present invention and have chemical compositions as shown in Table 1, were melted. Next, this was made into a 150 mm thick slab by hot forging, and then into a 100 mm thick steel plate by hot rolling. The rolling heating temperature at this time was 1150°C. Next, the obtained 100 mm thick steel plate was reheated to 920°C as shown in Table 2, water quenched, and then heated to 580°C or 600°C for 1
It was tempered by holding it for a period of time and then cooling it in water. Separately, steels A to F targeted by the present invention were quenched at 600°C in the same manner as above.
The conventional method involves heating and holding for one hour and then cooling in the air.

[Table] Note 2) * indicates that it is outside the scope of the present invention.

[Table] Hardening and tempering treatments were also performed. A JIS No. 4 Shapey test piece and a round bar tensile test piece with a diameter of 8.5 and a parallel length of 50 mm were taken from the center of the thickness of each of these steel plates in the rolling direction, and their mechanical properties were investigated. These results are also shown in Table 2. As is clear from Table 2, methods 1 to 12 of the present invention
According to, yield point: 90Kgf/mm ² or more, tensile strength: 97
It can be seen that a steel plate can be obtained which satisfies Kgf/mm ² or more and satisfies the strength as an HT100 steel material, and also has excellent toughness with an impact fracture transition temperature of -60°C or less. On the other hand, it can be seen that the steel plates obtained by Comparative Methods 13 to 21 are all inferior in toughness to the steel plates obtained by the method of the present invention. Furthermore, y groove restrained crack test pieces (plate thickness: 50 mm) were taken from each steel plate obtained by methods 1 to 12 of the present invention, and after preheating to 125°C, MIG welding was performed at a heat input of 17 KJ/cm. When we investigated the presence or absence of surface cracks, root cracks, and cross-sectional cracks, good results were obtained, with no defects occurring in any of them. As described above, according to the present invention, it is possible to obtain a thick high-strength steel material that has both excellent toughness and high strength and also has good weldability, thereby further improving the performance of steel structures. This brings about industrially useful effects, such as making it possible to

Claims (1)

[Claims] 1. In weight percentage: C: 0.07-0.15%, Si: 0.15% or less, Mn: 0.40-1.00%, Cr: 0.40-1.20%, Ni: 2.0-4.2%, Mo: 0.40-0.80 %, V: 0.01-0.06%, B: 0.0004-0.0015%, Cu: 0.50% or less, sol.Al: 0.010-0.100%, P: 0.008% or less, S: 0.003% or less, N: 0.0040% or less, Fe and other unavoidable impurities: the remainder, and the weld cracking susceptibility index (PcM) is
It has a component composition range of 0.31% or less, plate thickness: 40
Weldability, characterized by subjecting a steel plate of mm to 150 mm to a tempering treatment in which it is heated to a temperature range of [Ac ₃ transformation point to 1000 °C], quenched, then heated to 560 to 630 °C, and then water-cooled. A method for manufacturing high-strength, high-toughness steel materials with a good tensile strength of 97 Kgf/mm ² or more.