JPH02141528A - Manufacture of tempered-type high tensile steel plate excellent in toughness - Google Patents

Abstract

PURPOSE:To stably manufacture an extra thick high tensile steel plate having superior toughness at low temp. by controlling compositional limit values and also carrying out treatments under respectively prescribed process conditions. CONSTITUTION:A steel having a composition consisting of, by weight, 0.10-0.20% C, <=0.30% Si, 0.40-1.20% Mn, <=0.5% Cu, >3.5-4.5% Ni, 0.10-1.20% Cr, 0.05-0.80% Mo, 0.005-0.1% V, 0.005-0.03% Nb, 0.015-0.10% solAl, 0.0003-0.0030% B, <=0.010% P, <=0.005% S, <=0.004% N, and the balance essentially iron is prepared. This steel is heated up to >=1000 deg.C to undergo hot rolling, and the resulting hot rolled plate is reheated up to a temp. in the region between te Ac3 point and 1050 deg.C to undergo first quenching, further heated up to a temp. in the region between the Ac3 point and 950 deg.C and also equal to or lower than the first quenching temp. to undergo second quenching, tempered at a temp. of the Ac1 point or below, and water-cooled.

Description

Detailed Description of the Invention <Industrial Application Field> This invention is directed to the production of high-strength steel sheets having yield strength of 90 kgf/- or more, tensile strength of 97 kgf/- or more, and impact transition temperature of 60° C. or less. Regarding the method, in particular,
The present invention relates to a method for manufacturing a heat-treated high-strength steel plate that can stably provide the above-mentioned performance even when the target is an extremely thick material with a plate thickness of 100 x m or more.

<Prior art and its problems> In recent years, the tendency for welded structures to become larger is becoming more and more remarkable, and the structural steel plates used in these structures are becoming increasingly high tensile strength and extremely thick. For example, high tensile strength steel plates with a thickness of 18 (Jan) and 80 kgf/- are used for penstock pipes in pumped storage power plants, and high tensile strength steel plates with a thickness of 80 kgf/- and 100 to 150 mm are used as rank materials for jack-up oil drilling rigs. It has come to be used.

By the way, HT60 and HT80 class steels have traditionally been used as high-strength steel plates for this type of use, but for welded structures in recent years that have become significantly larger as mentioned above, yield strength: 90 kgf/ It has become desirable to use an extremely thick high tensile strength steel plate of the HT100 class having a high strength of mm1 or more and a thickness of 100 mm or more.

However, it is industrially extremely difficult to stably secure the high strength of the HT100 class, which is yield strength: 90 kgf/mnt or more and tensile strength: 97 kgf/- or more, in an extremely thick steel plate over 100 mm thick, and even if Even if high strength can be imparted, it is even more difficult to ensure sufficient toughness at the same time, and therefore, at present, a stable and large-scale supply system for the above-mentioned extra-thick high-strength steel sheets has not been established.

In other words, 100kgf/m of plate thickness exceeding 1001mm
When manufacturing M1 class high tensile strength extra-thick steel plates, it is necessary to design components with extremely high hardenability in order to ensure strength at the center of the plate thickness. There is a large difference in the cooling rate between the central part and the central part, and if the strength of the central part is to be ensured, the strength of the surface part becomes too high, resulting in significantly worse low-temperature toughness than that of the central part.

Therefore, stably obtaining a steel plate with an excellent strength-toughness balance throughout the thickness direction is the biggest challenge in manufacturing "1001qrf/mJ class high tensile strength extra-thick steel plate exceeding 100mm thickness". However, this problem has not yet been fully overcome.

Means for Solving the Problem> Therefore, from the above-mentioned viewpoints, the present inventors have developed a material that has yield strength: 90 kgf/- or more, tensile strength: 97 kgf/- or more, and impact transition temperature knee 60°C or less. However, as a result of intensive research aimed at providing a means of manufacturing extra-thick high-strength steel plates that can stably provide the above-mentioned performance even when the plate thickness exceeds 100 mm, we have obtained the following knowledge. It's arrived. That is, (a) in order to obtain a 100 kgf/- class high tensile strength steel that is practical in terms of cost while ensuring the desired weldability, it is inappropriate to add a large amount of reinforcing elements, and the steel structure due to quenching is inappropriate. martensite organization is essential-U
There isn't.

(b) However, when hardening an extra-thick steel plate, there will inevitably be a large difference in the cooling rate between the center of the plate and the surface, so it is necessary to ensure sufficient strength at the center. The surface part is hardened too much and the low-temperature toughness is extremely deteriorated.In this case, if modifications are made to obtain a fine-grained martensitic structure by minimizing the γ grain size during hardening, Sufficient low-temperature toughness is maintained even on the surface of the steel material, which tends to be excessively hardened (C)
In order to achieve the desired fine γ grains during quenching, the growth of γ grains during heating of the steel material is suppressed by adding Nb, and at the same time, quenching treatment is performed in advance to form the heated surface structure (structure of the γ-hardened part). ) is extremely effective, and by utilizing the synergistic effect of Nb addition and double quenching, it is possible to stably secure an extremely fine martensitic structure through fine γ grains. To become.

(dl However, in this case, in order to keep the γ grains fine before the second quenching, the second quenching needs to be performed at a relatively low temperature in the r range, and there is a concern that the quenching effect may be insufficient. This problem can be solved very effectively by adding B, which is an element that improves hardenability.

(Q) Also, if the steel has a specific composition that has been reduced by a specific amount or more through hot rolling, the same effect can be obtained by using direct quenching following hot rolling instead of the first quenching treatment. be secured.

The present invention requires fine-grained austenite (T) to achieve excellent low-temperature toughness in r100kgf/- edge high tensile strength steel.
This was done based on the above findings obtained through research from the viewpoint that "the key point of development is to obtain a martensitic structure transformed from grains", and rc:o, 10-0.20% ( (Hereinafter, percentages representing component proportions are expressed as weight percentages.)

Si: 0.30% or less, Mn: 0.40~
1.20%.

Cu: 0.5% or less, Ni: more than 3.5%
4.5%.

Cr s 0.10-1.20%, Mo: 0.
05-0.80%.

V: 0.005-0.1%, Nb: 0.0
05-0.03%.

Sol, AI! : 0.015-0.10%.

B: 0.0003-0.0030%, P: 0.
010% or less.

S: o, oos% or less, N: 0.004
% or less, with the remainder substantially consisting of Fe, after heating it to 1000°C or higher and hot rolling it as shown in Figure 1,
Ac, reheated and quenched to a temperature range of point to 1050°C,
After that, quenching is performed again by heating in the temperature range of Ac3 point to 950℃ and below the first quenching temperature, or as shown in Fig. 2, the above-mentioned "hot rolling-quenching treatment" is performed. Instead, perform "a process in which a cumulative reduction of 30% or more is applied in a temperature range of 900°C or higher, finish rolling is completed at 800°C or higher, and then water-cooled as it is", followed by tempering at a temperature below the Act point and water-cooling. As a result, the plate thickness is 1
Yield strength: 90kg even if the thickness exceeds 0.00mm
f/- or more, tensile strength: 91 kgf/- or more, and impact transition temperature knee of 60° C. or less, which can be mass-produced on an industrial scale.

Next, the reason why the manufacturing conditions of the high-strength steel plate are limited as described above in the present invention will be explained together with the effects that supported the manufacturing conditions.

A) Composition of steel material a) C C is an element necessary to ensure the strength of the steel plate, but if its content is less than 0.10%, it will not work as a 100 kgf/mJ class high tensile strength steel. It is not possible to secure the necessary strength (this tendency is especially noticeable in the case of extra-thick steel plates with a thickness of 100 mm or more), and on the other hand, if the content exceeds 0.20%, the susceptibility to welding cold cracking increases, etc. Since this would cause problems, the C content was set at 0.10 to 0.20%.

b) Si Si is usually an element added to deoxidize steel and ensure strength, but by reducing Si, the toughness of welded joints and weld heat affected zones can be improved. this is,
This is because the reduction in Si makes it possible to suppress the formation of island-shaped martensite that adversely affects toughness even in welded joints where the cooling rate is relatively fast. In order to sufficiently secure the above effects, it is necessary to keep the Si content at 0.30% or less, so the Si content was limited to 0.30% or less, but preferably 0.15% or less. By doing so, even better effects can be obtained.

c) Mn In addition to acting as a deoxidizing agent for steel, the Mn component also has the effect of ensuring hardenability, but if its content is less than 0.40%, the desired effect due to the above effect cannot be obtained, On the other hand, 1.2
Since Mn content exceeding 0% causes deterioration of weldability and base metal toughness, the Mn content is set at 0.40 to 1.20%.

d) Cu Cu is an effective element for increasing strength without impairing toughness, and this effect can be confirmed even when added in a trace amount, but adding more than 0.5% is not enough to justify the cost increase. The Cu content is 0.5 because not only does it not have the effect of increasing strength, but it also has a negative effect on high-temperature ductility.
% or less.

e) The NtNi component has the effect of ensuring the hardenability of steel and improving the low-temperature toughness, but if its content is 3.5% or less, it is necessary to ensure the necessary strength for a 100kgf/-edge high tensile strength steel plate exceeding 100fl thickness. On the other hand, adding more than 4.5% not only saturates the effect but also increases costs, so the Ni content is set in a range of more than 3.5% and less than 4.5%. Ta.

f) Cr The Cr component has the effect of ensuring the hardenability and strength of steel, but if its content is less than 0.1%, the desired effect due to the above effect cannot be obtained. C
The r content was determined to be 0.1 to 1.2%.

g) M.

Mo is an effective element for increasing the hardenability of steel and increasing temper softening resistance to ensure desired strength.
If the content is less than 0.05%, sufficient effects cannot be obtained, while if the content exceeds 0.80%, weldability will be significantly deteriorated.

The content was determined to be 0.05 to 0.80%.

h) V ■ is an element added to steel to ensure strength,
If the content is less than 0.005%, it is difficult to secure the desired strength. On the other hand, if the content exceeds 0.10%, the toughness and weldability of the base material will be significantly deteriorated. 0.005 to 0.10%.

i) Nb The Nb component contains austenite (γ) as fine precipitates.
By existing in the Nb region, the pinning effect suppresses the growth of austenite grains and makes the austenite grains finer. However, if the Nb content is less than 0.005%, the desired effect due to the above action cannot be obtained. On the other hand, 0.0
Since Nb content exceeding 3% impairs weldability, the Nb content is set at 0.005 to 0.03%.

j) Sol, Aj! The beta component has the effect of deoxidizing the steel as well as refining the austenite grains and improving toughness, but if its content is less than 0.015%, the desired effect due to the above effect cannot be obtained; If the content exceeds 0.10%, the toughness will be impaired due to an increase in deoxidation products such as alumina, so sol.

The hel content was determined to be 0.015 to 0.10%.

k) B B is an element that significantly improves the hardenability of steel when added in small amounts, and is a very effective component for improving the strength and toughness of steel, but if its content is less than 0.0003% However, on the other hand, with 0.0
Since the effect is saturated even if the S content exceeds 0.03%, the S content was determined to be 0.0003 to 0.003%.

1) PP P is an impurity element that promotes temper brittleness of steel and deteriorates toughness. In particular, high tensile strength steel that can handle 100 kgf/- is susceptible to this effect. However, the P content was limited to 0.01% or less because the above-mentioned adverse effects can be suppressed to an acceptable level by suppressing the P content to 0.01% or less.

m) S S is an impurity element that normally exists in the form of MnS in steel and is elongated by rolling to produce anisotropy in toughness. In high-strength steel, elongated inclusions in particular cause significant toughness deterioration, but the S content is reduced to 0.005
By suppressing the S content to 0.005% or less, the adverse effects can be suppressed to an acceptable level.

n) N Reducing N to 0.004% or less 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 set to 0.004% or less and the so7. A
By adjusting the β content to 0.015 to 0.10%, the amount of solid solution B can be made 0.0003% or more,
A significant improvement in hardenability is achieved. Also, the amount of N is 0.0
When it is reduced to 0.04% or less, coarsening of AfN is suppressed and toughness is also improved. Furthermore, since the generation of VN is suppressed by reducing the N content, (2) becomes a uniform solid solution at the normal austenitizing temperature, and therefore the effect of reducing the amount of V added can also be secured. For this reason, the N content is 0.00
It was limited to 4% or less.

B) Rolling/Heat Treatment Conditions Pressure - 2 Rolling/Shino Input a) Rolling Heating Temperature During rolling, it is desirable to heat at a high temperature in order to dissolve V carbonitrides, BN, etc.
Since sufficient solid solution of the above-mentioned precipitates is not achieved at a temperature lower than 1000°C, the rolling heating temperature was set at 1000°C or higher.

b) First quenching temperature The first quenching is carried out for the following purposes. That is, (a) By making the previous structure a quenched structure, the grain size during heating during the second quenching is made fine.

(El) When producing extremely thick materials that exceed the Loom thickness, coarse precipitation of BN and Nb (CN) occurs during slow cooling after rolling, but by aiming at solid solution of these precipitates, hardenability is improved by B. The aim is to improve the effect and the γ-grain refinement effect due to Nb.

To achieve this, it is necessary to heat the product to at least the Ac or intended γ range, but if it is heated to a temperature range exceeding 1050°C, the growth of γ grains will become significant, and during the second quenching process, which is the next step, The first quenching temperature was set to Ac, ~1050°C, since fine γ grains could not be ensured.

C) Second quenching temperature The second quenching is aimed at obtaining a fine-grained martensitic transformed structure by quenching from fine particles.

And the structure before quenching during the second quenching is fine particles! 1
In order to maintain the iIl texture, it is necessary to quench from the low-temperature γ region where the pinning effect of Nb can be utilized. However, if the quenching temperature is too low (A C3
If the temperature is below the point), solid solution Bit cannot be ensured, and the hardenability improving effect of B cannot be utilized. On the other hand, when heated to a high temperature exceeding 950° C., BN and AfN decompose, resulting in an increase in free N, which combines with solid solution B during quenching and loses the hardenability improvement effect of B. For this reason, the second quenching temperature should be Ac3~
The temperature was set at 950°C.

d) Tempering treatment conditions Tempering treatment removes the strain introduced by quenching,
It is also carried out to improve the strength-toughness balance by finely precipitating carbides. This tempering is generally carried out at a temperature below the Ac point, and since a sufficient effect can be obtained at this temperature, the tempering temperature is also set at a temperature below the Ac1 point in the present invention. Ta.

In addition, the main condition of the present invention is to heat and maintain the material at a tempering temperature and then cool it with water.

This is because if the steel plate to be manufactured (100 kgf/- high tensile strength steel plate) exceeds 100 kgf thickness, cooling after tempering must be water-cooled, otherwise the cooling rate will be extremely slow, and the susceptibility to tempering brittleness will increase. This is because it leads to deterioration of toughness.

In the case of rolling and tempering, the reasons for limiting the rolling heating temperature, second quenching temperature, and tempering treatment conditions in this case are the same as in the case of "rolling-three-day quenching and tempering" described above. However, the reason why the rolling and direct quenching conditions, excluding the rolling heating temperature, were limited as described above is as follows.

a) The purpose of the present invention is to be able to stably manufacture high-toughness, extra-thick, high-strength steel plates with a rolling reduction exceeding 1001 mm thickness. If such a relatively thin piece of steel is used as a starting material, it may not be possible to achieve fine grain structure at the center of the sheet thickness due to an insufficient forging ratio. In the present invention, since the ultimate aim is to obtain a fine-grained martensitic structure, it is necessary to make the preliminary structure as fine as possible as described above. For this purpose, it is necessary to apply a reduction of 30% or more in a temperature range of 900°C or higher, so if direct quenching is applied, the temperature will be 900°C.
It was determined that a cumulative pressure of 30% or more should be applied in the above temperature range.

b) Rolling Finish Temperature Direct quenching is an alternative to the first quenching in the case of three-day quenching described above. Therefore, it is necessary to bring BN and Nb (CN) into a solid solution state immediately before direct quenching, but for this purpose it is necessary to finish rolling at 800° C. or higher. Because of this, the finishing rolling temperature was set at 800°C or higher.

C) Direct quenching Direct quenching replaces the first quenching in the case of three-day quenching, so the steel plate after finish rolling is immediately water-cooled.

Next, the present invention will be explained in more detail with reference to Examples.

(Example) First, slabs having the component compositions shown in Table 1 were obtained according to a conventional method, and then treated under the conditions shown in Table 2.
A 150m thick steel plate was manufactured.

Next, test pieces were cut out from each of the obtained steel plates and evaluated for mechanical properties and weldability, and the results are also shown in Table 2.

The weldability was evaluated by a y-groove restraint cracking test. The y-groove restraint cracking test was performed by preheating diagonal y-groove restraint cracking test pieces (plate thickness 50) taken from each steel plate to 125°C. After that, heat input: 17 KJ/Cm and manual welding current: 1
70 A, voltage: 25 V, speed = 15 am/m1n), and the test was carried out under conditions to check for the presence or absence of surface cracks, root cracks, and cross-sectional cracks.

As is clear from the results shown in Table 2, the extra-thick steel plates manufactured according to the conditions specified in the present invention exhibit good weldability and have the desired strength (yield strength: 90 kgf/- or more, tensile strength It can be seen that both strength (97 kgf/- or higher) and toughness (impact transition temperature knee 60° C. or lower) are satisfied from the surface to the center.

On the other hand, even if steel with the same composition is used, if the treatment method deviates from the specifications of the present invention, the target performance will not be achieved.

For example, if the cooling after tempering is air cooling as in Test No. 5, the desired toughness cannot be ensured even if the strength is satisfied. Also,
If the second quenching is performed at a temperature higher than the first quenching temperature as in Test No. 6, a fine-grained martensitic structure cannot be obtained, so the desired toughness cannot be ensured. Furthermore, if tempering is performed at a high temperature exceeding the Ac1 point as in Test No. 7, not only the strength cannot be ensured, but also the toughness cannot be ensured. Test No. 8 is an example in which the direct quenching temperature could not be secured due to the low finish rolling temperature. Martensite could not be obtained and toughness could not be ensured.

On the other hand, test numbers 9 to 12 are examples in which the composition of the steel material is outside the range specified by the present invention, but as can be seen from the results of test numbers 9 and 11, it is necessary to improve hardenability. Even if the strength is ensured by simply using a component system with a high Ceq, the desired toughness cannot be ensured.

In addition, test number 10 was made of a steel whose composition of Si, Cu, and S exceeds the specified range in the present invention, but although the strength and toughness of the base metal were close to the target values, the weldability was poor. Therefore, it cannot be used as a practical material. Furthermore, test number 12 was made of a steel whose composition was intended to ensure strength with elements other than Ni, but the target strength could not be ensured unless a predetermined amount of Ni was added.

<Summary of Effects> As explained above, according to this invention, the plate thickness is 100 mm.
fi or more, yield strength: 90 kgf/- or more.

Tensile strength! 7kgf/- or more and shock transition temperature knee 6
Industrially, extremely useful effects are brought about, such as the ability to stably produce extremely thick high-strength steel plates that have excellent performance at temperatures below 0°C.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of manufacturing conditions of a high-tensile steel plate according to the present invention. FIG. 2 is an explanatory diagram of another high-strength steel plate manufacturing condition according to the present invention.

Claims (2)

[Claims] (1) Weight percentage: C: 0.10-0.20%, Si: 0.30% or less, M
n: 0.40 to 1.20%, Cu: 0.5% or less, Ni
: more than 3.5 to 4.5%, Cr: 0.10 to 1.20%,
Mo: 0.05-0.80%, V: 0.005-0.1
%, Nb: 0.005-0.03%, sol. Al: 0
．． 015-0.10%, B: 0.0003-0.003
0%, P: 0.010% or less, S: 0.005% or less,
After hot-rolling a steel containing N: 0.004% or less and the remainder essentially consisting of Fe at a temperature of 1000°C or higher, Ac_
Harden by reheating to a temperature range of 3 points to 1050°C, then further quenching by heating to a temperature range of Ac_3 points to 950°C and below the first quenching temperature,
A method for producing a tempered high-strength steel sheet with excellent toughness, characterized by tempering at a temperature below Ac_1 point and cooling with water. (2) Weight percentage: C: 0.10-0.20%, Si: 0.30% or less, M
n: 0.40 to 1.20%, Cu: 0.5% or less, Ni
: more than 3.5 to 4.5%, Cr: 0.10 to 1.20%,
Mo: 0.05-0.80%, V: 0.005-0.1
%, Nb: 0.005-0.03%, sol. Al: 0
．． 015-0.10%, B: 0.0003-0.003
0%, P: 0.010% or less, S: 0.005% or less,
Steel containing N: 0.004% or less and the remainder substantially consisting of Fe is heated to 1000°C or higher and hot rolled, subjected to a cumulative reduction of 30% or more in a temperature range of 900°C or higher, and finished rolled. Toughness is characterized by being finished at 800°C or higher, then water-cooled as is, then reheated to a temperature range of Ac_3 points to 950°C for quenching, and subsequently tempered at a temperature of Ac_1 points or less and water-cooled. A manufacturing method for excellent heat-treated high-tensile strength steel sheets.