JPS60121228A - Manufacture of tempered high tension steel plate - Google Patents

Abstract

PURPOSE:To improve the strength, toughness and weldability by tempering a steel having a composition contg. reduced amounts of N, V, Si, P and S at a reduced temp. and by water-cooling the tempered steel. CONSTITUTION:The composition of a steel is composed of, by weight, 0.07- 0.15% C, <0.15% Si, 0.4-1.2% Mn, 0.4-1.2% Cr, 0.4-0.8% Mo, 0.01-0.06% V, <0.002% B, 0.01-0.1% sol.Al, <0.004% N, <0.01% P, <0.003% S and the balance Fe, and the weld crack sensitivity index (PCM) represented by the equation is adjusted to <=0.28%. The resulting steel is heated to the Ac3 transformation point -1,000 deg.C, quenched, tempered at 560-630 deg.C, and water-cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to a method for producing high-strength steel plates, particularly heat-treated high-tensile 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 itself can be made lighter, and the transport and assembly work associated with it have been improved.
Furthermore, it is intended to take advantage of advantages such as improved weldability due to the thinner walls of each structural part +4.

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 copper plates used are also becoming stronger.

Conventionally, this type of high-strength steel plate is 11T60.
IIT80 class steel is used, but the yield point is 9.
llT1 has a high strength of 0 kgf/1.
00 class steel has not yet been put into practical use.

This is because: ■ It is difficult to stably secure the specified strengths of yield strength of 90 kgf/m or more and tensile strength of 97 kgf/nA or more; ■It is extremely difficult to simultaneously satisfy 4J weldability, which does not pose a problem in implementation; This is because it is difficult to feel satisfied.

The conventionally developed IIT100 class steel types contain more than 0.15% of C or more than 0.06% of ■ to ensure strength, so the toughness of the base material is not very good. Tehana (, brittle fracture transition temperature (ν″r
S) was -40°C or lower''2, and the weldability could not be said to be sufficient.

On the other hand, similar steel types with good base material toughness contain 50% Ni.
% or more, and therefore has the disadvantage of being significantly inferior in economic efficiency.

(Related invention) The inventors have already proposed a IIT100 class steel containing 2 to 3% Ni and having good strength, toughness and weldability (Japanese Patent Application No. 58-1'77701). Compared to IIT80 steel, it also contains a considerably large amount of Ni, and has the disadvantage of high production cost.

(Summary of the Invention) In order to improve the drawbacks of the above-mentioned prior art and prior invention, the inventors of the present invention have conducted detailed research focusing on trace elements, particularly impurities in steel, and have found the following results: I gained this knowledge.

(i) Even in llT100 class high strength steel,
By reducing N, which is normally contained in large amounts as an impurity in high-strength steel sheets, the hardenability of base A can be increased. Therefore, it is possible to simply reduce the N content without adding large amounts of alloying components. High strength can be achieved simply by
In addition, such a low N content also improves the toughness of the welded joint.

(11) By reducing the N content, ■ the amount added can be reduced to 0.06
Even if you limit it to below %, such li! Since sufficient precipitation strengthening can be obtained with V in &fi, deterioration in base material toughness and weldability due to the addition of a large amount of (2) can be suppressed.

(iii) On the other hand, by suppressing the Si content to 0.15% or less, a mixed structure of martenzite and fine heinite can be easily obtained, and the base material toughness can be significantly improved. At the same time, by reducing the Si content, even in a welded joint where the cooling rate is relatively high, the formation of island martensite that adversely affects toughness can be suppressed, so the toughness of the welded joint is improved.

(iv) 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 even if tempering is performed at a lower temperature than conventionally, sufficient low-temperature toughness can be achieved. In addition, the S content can be further reduced to 0.0.
By controlling the amount to 0.3% or less, sufficient low-temperature toughness can be imparted regardless of the rolling direction.

Therefore, based on the above knowledge, we continued our research and found that by combining the reduction of N content, ■ reduction of content, reduction of Si content, and further reduction of P and S content, we found that By combining the steel with the composition obtained in this way with a lower tempering temperature and water cooling after tempering, it is possible to ensure satisfactory strength and toughness as well as rough weldability. It has become possible to significantly reduce the amount of Ni that was added in large amounts to 111100 class steel, and it has become possible to provide mechanical properties of II T] and 00 class steel with economical efficiency equivalent to 11T80. After discovering what happened, he completed this invention.

Here, this invention has the following properties in weight%: c: o, o7 to 0.15%, Si: 0.15% or less, Mn: 0.40 to 1.20%, Cr: 0.4
0-1.20%, Mo: 0.40-0.80%,
V: 0.01-0.06%, B: 0.0020
% or less, sol, Al1: 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, W: 1.00% or less, Ti
: 0.010% or less and Ni: 1.00% or less, the balance is Fe and unavoidable impurities, and the PcM (weld crack susceptibility index) expressed by the formula: is 0.28
% or less is heated to a temperature of 2,1000°C or less and quenched to a temperature of 560°C or more, 63
This is a method for producing a tempered high tensile strength 11i1 plate with excellent strength, toughness and weldability, which is characterized by tempering at a temperature of 0'C or lower and then water cooling.

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 refining conditions are limited as described above in this invention will be explained. In the following, unless otherwise specified, "1%" means "wt%".

C: C is necessary to ensure hardenability and strength, especially 97
kgf/0.07% to ensure strength greater than crystal
However, if more than 0.15% is added, the mother + A
In this invention, C-containing go, o7 to 0 significantly deteriorates toughness, weld joint toughness, and low-temperature cracking resistance.
．． It was limited to 15%.

Si: Si is added to ensure proper deoxidation and strength, but in this invention, the Si content is limited to 0.15% or less. This is because the reduction in Si promotes the formation of carbides, which suppresses the formation of hardened structures such as island-like martensia I in the weld heat affected zone, thereby improving the toughness of the weld joint.

In addition, since the region where fine heinite is formed in the base metal is expanded and a mixed structure of martensite and fine heinite is obtained, the toughness is improved. Therefore, in this invention, the Si content is limited to 0.15% or less. Preferably it is 0.05 to 0.13%.

Hn: Mn is necessary to ensure hardenability, 0.40%
However, if it exceeds 1.20%, the weldability and base metal toughness will deteriorate, so the Mn content should be increased from 0.40 to 1.20%.
limited to.

Cr; Cr- is added in an amount of 0.40% or more to ensure hardenability and strength, but if it exceeds 1.20%, weldability and base metal toughness will deteriorate, so it should be added in a range of 0.40 to 1.20%. Restrict.

G1o: 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%, it significantly deteriorates weldability. Therefore, its content is limited to 0.40 to 0.80%.

■ ; ■ is 0°01 in this invention to ensure strength.
% or more, but if it exceeds 0.06%, the base material toughness and weldability will be significantly deteriorated, so 0.01 to 0.06%
%.

In this invention, since the N content is limited to 0.0040% or less, sufficient strength can be ensured with an addition amount of 0.06% or less.

In this invention, by limiting the tempering temperature to a low level, the precipitated carbides are made finer and the zeta strength is ensured, so that the addition amount is preferably 0.03 to 0.04%.

B: B is an element that greatly improves hardenability when added in a small amount, and is effective for improving strength and toughness, but Q, 002
If it exceeds 0%, the toughness will deteriorate, so the B content is limited to 0.0020% or less. In addition, in this invention, N
Since the content is limited to 0.0040% or less, it is preferable that the B content is 0,00% or less.

N: In this invention, when the N content is 0.0040% or less, the hardenability is improved by 11%, the strength and toughness of the base metal are improved, and the toughness of the weld joint is also improved. In addition, N is normally fixed as AQN in steel, but since it is reheated to a high temperature in the weld heat affected zone, AQN dissolves in the matrix and becomes free N, which deteriorates toughness. I end up.

Also, if the N content is high, 1 bar will be released as VN during reheating,
Since the amount of solid solution (■) decreases, the amount of precipitation strengthening of (■) after quenching and tempering decreases, resulting in a decrease in the strength of the base metal.

Therefore, in this invention, the N content is set to 0.004
Limited to 0% or less.

sol, AQ: AQ has a deoxidizing effect, refines austenite crystal grains, and improves toughness, so in this invention, so
l, 0.01% or more is added as AQ, but 0.10%
If it exceeds sol, AC!, the toughness will deteriorate. is 0
．． Limit to O1~0°10%.

P: P promotes tempering brittleness and deteriorates toughness, and also has /8
It is easy to segregate during solidification from steel, and abnormal structures occur in the segregated areas, so it is preferable to reduce it as much as possible.

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 expanded by rolling to produce anisotropy in toughness. In high-strength steel, elongated inclusions in particular should have an adverse effect on toughness, so in this invention the S content is limited to 0.003% or less, preferably 0.002% or less.

In addition, in a preferred embodiment of this invention, P+s is 0.010.
Will be limited to % or less.

Cu: Cu is added as necessary to improve strength and eating properties, but if it exceeds 0.50%, hot workability, toughness,
/8 This will cause deterioration of the bone joint, so in this invention, the amount of Cu added is 0.
゜Limited to 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 and temper softening resistance to increase strength, so it is added as necessary, but 1.0
If it exceeds 0%, toughness and weldability will deteriorate, so 1゜00
% or less.

Ti: T1 refines the crystal grains of the matrix 4 and suppresses the coarsening of the crystal grains in the /8 contact area, so it is added as necessary, but if it exceeds 0.010%, the matrix It is limited to 0.010% or less since it significantly deteriorates the toughness of the material.

Ni: Ni is effective in ensuring hardenability and improving low-temperature toughness, so it may be added as necessary, but in order to achieve economic efficiency comparable to II T 80, when adding Ni The amount added is limited to 1.00% or less.

PcM (weld cracking susceptibility index): Pcr4 is an index that expresses cold cracking susceptibility during welding, and is usually 8.
It is 0.28% or less for 0 kg class high tensile steel. In this invention, also in llT100, 111' 8
0.28% PCM to give weldability of 0.0
Limited to the following.

Quenching heating temperature: Setting the heating temperature before quenching to Ac3 point or higher is necessary to completely austenitize and uniformly dissolve the alloying elements in the matrix.
This is to help you. The reason why this heating temperature is limited to 100° C. or less is because if the heating temperature exceeds 100° C., the austenite crystal grains become coarse and the toughness decreases.

In addition, in this invention, since the N content is reduced, the amount of AQN produced is small and the crystal grains tend to become coarse, and the Ni content that suppresses coarse grains is also small, so the heating temperature is It is desirable to keep the temperature below 950°C.

Tempering temperature: The purpose of tempering at 560°C or higher is to remove strain introduced by quenching and finely precipitate carbides to improve strength and toughness. If the tempering temperature exceeds 630°C, the carbides become coarse and the strength decreases, so the upper limit is set at 630°C.

Water cooling after tempering is one of the features of this invention.
This improves low-temperature toughness and strength.

In other words, water cooling increases the cooling rate after tempering, suppresses temper brittleness, and increases the solid solution strengthening effect of C and the like.

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 - Perfume preparations within the scope of the present invention having chemical compositions as shown in Table 1, and Perfume preparations M to R as comparative examples, were made from steel types of /8.

Next, this was made into a 120III11 thick slab by hot forging, heated to 1100°C, hot rolled, and used for fragrance adjustment. Kaori F-L and P-R's Monobazaka 1! F
-4On+m steel plate was finished and air cooled.

Next, the steel plate is reheated to 930°C or 980°C, then water quenched to 600°C or 615'C.
After tempering, it was water-cooled.

In addition, for the purpose of investigating the influence of quenching temperature, as a comparative example, water quenching was performed after reheating at 800 ° C or 1050 ° C, or for the purpose of investigating the influence of tempering temperature,
Tempering treatment was also performed in which the material was tempered at 500°C or 650°C and then cooled with water.

Furthermore, as a conventional example, ordinary quenching and tempering was performed in which the material was tempered at 61) 0° C. and then air cooled.

From the central part of the thickness of each steel plate obtained in this way, JI
S d No. 2 Liver ■ Notch Charpy test piece and diameter 8.5m
Round bar tensile test pieces with a length of 50 IIN+ and a parallel length of 50 IIN+ were taken in a direction perpendicular to the rolling direction, and their mechanical properties were investigated.

In addition, diagonal y-groove restraint crack test pieces (thickness: 25
After preheating to 125℃, heat input: 17KJ / cm and manual welding current:
170A, voltage: 25v, speed: 1.5cm/min
), 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, test numbers 1 to 19 are +
) As shown in the results, the present invention satisfies the deterioration point of 901 gf/item or more, the tensile strength of 97 kgf/temperature or more, and satisfies the required strength as llT100, and the fracture surface It can be seen that the transition temperature also satisfies -60°C or less, and that an excellent low-temperature toughness of 1'' can be obtained, which is sufficient for use as a welded structure chariot.

In addition, preheating to 125°C prevents cold cracking.
It can be seen that it also has excellent weldability.

On the other hand, it can be seen that the steel plates obtained in test numbers 20 to 25, which are comparative examples, have satisfactory strength but are inferior in toughness.

In addition, when the quenching temperature is high (Test No. 26.27) and when the Ut return temperature is low (Test No. 31), the strength is satisfied, but the toughness is deteriorated, and when the quenching temperature is low (
28) and when the tempering temperature is high, the toughness is good, but the strength is reduced.

Furthermore, in the case of the conventional example where air cooling was performed after tempering (test number 32
~35) results in a decrease in both strength and toughness.

In addition, weld cracking occurs when steel whose additive amount exceeds the -1- limit (test number 22), when steel whose C and PCM exceed the upper limit (test number 23),
When using steel with n, B, and Ti exceeding the upper range (
Test No. 25).

As described in detail above, according to the present invention, it is possible to obtain an 11T100 class steel having excellent strength and toughness and also good weldability without using a large amount of expensive Ni.

Claims (1)

[Claims] In weight %, c: o, o7 to 0.15%, Si: 0.15% or less, Mn: 0.4C1 to 1.20%, Cr: 0.
40-1.20%, Mo: 0.4 (1-0.80%
, V: 0.01-0.06%, B: 0.002
0% or less, sol, AQ: 0.01-0.10%,
N: 0.0040% or less, p: o, oio% or less,
S: 0.003% or less, Cu: 0.50% or less, Ca
: 0.005% or less, W: 1.00% or less, Ti
: 0.010% or less and Ni: 1.00% or less, the balance is Fe and unavoidable impurities, and the Pc bar (weld crack susceptibility index) expressed by the formula: is 0.28
% or less is heated to a temperature of Ac3 transformation point or higher and 1000°C or lower and quenched, then 560°C or higher and 630°
A method for producing an annealed high-tensile steel plate with excellent strength, toughness, and weldability, characterized by tempering at a temperature of C or lower and then water cooling.