JPH08176727A - High tensile steel and production thereof - Google Patents

Abstract

PURPOSE: To obtain a high tensile steel having excellent weld cracking sensitivity and large heat input weldability by specifying the chemical compsn and PCM value of the steel and a relation among Ceq, contents of Nb and V and the thickness of sheet. CONSTITUTION: The compsn. of the steel is composed, by weight %, of 0.06 to 0.1% C, 0.01 to 0.4% Si, 0.5 to 1.6% Mn, <=0.3% Mo, 0.005 to 0.05% Nb, <=0.1% V, 0.005 to 0.1% Al, 0.0005 to 0.008% N, <0.005% Ti, <0.0003% B, and the balance Fe and inevitable impurities. The PCM value defined by PCM= C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B is specified to <=0.2. Further, the relation 625Nb+250V+210Ceq>=40+t is satisfied by using the Cqe value defined by Cqe=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14, the contents of Nb and V and the thickness t (mm) of the steel sheet. The range of the optimum amt. of the alloy elements to be added according to the thickness of sheet exists in this steel.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a 600 N / mm ² class high strength steel excellent in weld cracking susceptibility and large penetration weldability, which is used for steel structures such as bridges, tanks, iron pipes, warehouses and buildings, and its production. Regarding the method.

[0002]

^2. Description of the Related Art Conventionally, there have been many demands for improving the performance of 600 N / mm ² class high strength steel, and many studies have been made so far. Among these, as a technique characterized by lowering C and adding Ti-B for the purpose of improving susceptibility to welding cracks, JP-A-49-37814 and Japanese Patent Publication No. 4-13406.
The gazette is known. 600N / mm with improved weld crack susceptibility by the technologies represented by these
^Although class ² high-strength steel can be obtained, the tensile strength required for 600 N / mm ² class high-strength steel is achieved by utilizing B, so the base material properties are unstable due to changes in chemical composition and manufacturing conditions. Is a concern, and the hardness of the weld heat affected zone increases significantly. This increase in the hardness of the weld heat affected zone is not preferable because it causes deterioration of the toughness of the bond portion, which is most feared particularly in the high heat input welded joint.

Japanese Unexamined Patent Publication (Kokai) No. 2-8322 discloses a combination of low carbon content, utilization of precipitation hardening elements, and direct quenching method to improve SS resistance.
This is a technique aimed at improving the C property and the weld cracking susceptibility.
Steel plates produced by this technology have been thoroughly reduced in carbon content to ensure SSC resistance. Therefore, when joints are made by general large penetration welding, the performance of the weld metal is not secured and sufficient joint strength is obtained. It's hard to be beaten. Further, since the strength required for high-strength steel of 600 N / mm ² class has been achieved by lowering C, it is necessary to add a large amount of hardenability improving element in addition to a large amount of precipitation hardening element. As a result, the hardness of the coarse-grained zone of the heat-affected zone is increased during high heat input welding, and there is concern about joint toughness.

Japanese Examined Patent Publication No. 61-12970 is intended to provide a 600 N / mm ² class high-strength steel excellent in susceptibility to welding cracks by combining reduction of C, addition of V and direct quenching. According to the description of this publication, 600N / mm ² class high-strength steel having excellent weld crack susceptibility can be obtained at least up to a plate thickness of 30 mm, but there is no description relating to weld joint performance. It can be said that the large heat input weldability is unsolved.

Japanese Unexamined Patent Publication No. 55-69241 is a proposal relating to a 600 N / mm ² class high-strength steel excellent in high heat input weldability. The characteristics of this technique are low Si content and high Al content, and the weld crack susceptibility is not always solved.

As other conventional techniques, Watanabe et al., "Development of steel NK-HIWEL for large heat input welding", Nippon Steel Pipe Technical Report N
o. 97 (1983) and Japanese Patent Laid-Open No. 174820/1985, but these are mainly 500 N.
/ Mm ² It is a technology for ² class steel, 600N / mm
Grade ² high-strength steel is out of the scope.

[0007]

The present invention has been made in view of the above circumstances, and an object thereof is to provide a 600 N / mm ² class high-strength steel excellent in weld cracking susceptibility and high heat input weldability. .

[0008]

In order to improve the weld crack susceptibility, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
It is effective to reduce the Pcm value defined by i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B. From the comparison of the coefficients, it is easily known that the reduction of C is most effective, but extremely low C makes it difficult to secure the joint strength.
Furthermore, in order to secure the strength of the base metal, it is necessary to add a large amount of alloying elements, which not only leads to an increase in cost, but there is a concern about joint toughness associated with hardening of the coarse-grained zone of the heat-affected zone of the welding heat affected zone.

As a means of securing the strength of the base material instead of adding a large amount of alloying elements, B addition is considered, but there is a concern that the joint toughness may deteriorate due to a marked increase in hardness of the weld heat affected zone.

Therefore, Ti from the viewpoint of effectively utilizing B
Addition is not essential, but it is preferable not to add Ti in order to stably obtain good base material toughness. So 60
For the purpose of improving the weld crack susceptibility of 0 N / mm ² class high-strength steel and ensuring high heat input weldability, optimization of the C content and the amount of additional alloying elements was investigated and the present invention was obtained based on the following findings.

(1) In order to obtain a sound weld metal, obtain sufficient joint strength, and avoid adding a large amount of alloying elements, it is necessary to contain 0.06% or more of C in the base metal.

(2) However, the addition of C exceeding 0.1% increases the weld crack susceptibility and deteriorates the joint toughness.

(3) By adopting the direct quenching method, precipitation hardening of Nb can be utilized to secure the strength of the base material.

(4) The addition of Nb makes it possible to stably secure the high heat input welded joint strength.

(5) There is a range of optimum addition amounts of alloying elements depending on the steel plate thickness.

That is, in the present invention, C: 0.0% by weight.
6 to 0.1%, Si: 0.01 to 0.4%, Mn: 0.
5 to 1.6%, Mo: 0.3% or less, Nb: 0.005
~ 0.05%, V: 0.1% or less, Al: 0.005-
0.1%, N: 0.0005 to 0.008%, Ti <
0.005%, including B <0.0003%, Pcm = C
+ Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 +
Pcm value defined by Cr / 20 + Mo / 15 + V / 10 + 5B is 0.2 or less, and Ceq = C + Mn / 6
+ Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V /
Using the Ceq value defined by 14, the Nb and V contents, and the steel plate thickness t (mm), 625Nb + 250V + 21
It is a high-strength steel satisfying the relationship of 0 Ceq ≧ 40 + t and having the balance of weld cracking susceptibility and large heat input weldability consisting of iron and unavoidable impurities.

Further, in% by weight, C: 0.06 to 0.1
%, Si: 0.01 to 0.4%, Mn: 0.5 to 1.6
%, Mo: 0.3% or less, Nb: 0.005 to 0.05
%, V: 0.1% or less, Al: 0.005-0.1%,
N: 0.0005 to 0.008%, Ti <0.005
%, B <0.0003%, and further Cu: 0.5%
Hereinafter, one or more of Ni: 1.5% or less and Cr: 0.5% or less is included, and Pcm = C + Si / 30 + Mn /
20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 1
Pcm value defined by 5 + V / 10 + 5B is 0.2 or less, and Ceq = C + Mn / 6 + Si / 24 + Ni /
Ce defined by 40 + Cr / 5 + Mo / 4 + V / 14
A high-strength steel excellent in weld crack susceptibility and large heat input weldability, which satisfies the relation of 625Nb + 250V + 210Ceq ≧ 40 + t by using the q value, Nb, V content, and steel plate thickness t (mm), and the balance is iron and inevitable impurities. Is.

Further, a steel having the above chemical composition is added to 10
After heating to a temperature range of 0 to 1250 ° C., hot rolling to a predetermined plate thickness, then directly quenching from the Ar3 transformation point or higher, and tempering at a temperature of the Ac1 transformation point or lower to obtain weld cracking susceptibility and large penetration. It is a method for producing high-strength steel, which is characterized by having excellent heat weldability. Then, in producing the steel in this manner, after heating to a temperature range of 1000 to 1250 ° C. and hot rolling at a cumulative reduction of 20% or more at 1050 ° C. or less, continuous quenching is performed from the Ar3 transformation point or more, A method for producing high-strength steel, which is characterized by having excellent weld crack susceptibility and high heat input weldability by performing tempering treatment at a temperature not higher than the Acl transformation point.

[0019]

The reasons for limiting the constituent features of the present invention will be described below.

<C> As described above, 0.06% or more is added in order to secure joint strength and avoid adding a large amount of alloying elements. C
If the amount is less than 0.06%, it is necessary to add 0.2% or more of Mo, which leads to high cost, deterioration of the toughness of the large heat input joint, and the strength of the large heat input welded joint cannot be secured. If the C content exceeds 0.1%, the weld crack susceptibility deteriorates, and the large heat input joint toughness may deteriorate.

<Si> Si is effective in ensuring the strength of the base metal and the high heat input welding strength, so 0.01% or more is added. However, the addition of more than 0.4% deteriorates the weld crack susceptibility and the toughness of large welded joints.

<Mn> Mn works effectively in securing the strength of the base metal and the high heat input welding strength, so it is added in an amount of 0.5% or more.
However, addition of more than 1.6% deteriorates the weld crack susceptibility.

<Mo> Mo has a tendency to deteriorate weld crack susceptibility and toughness of large-entry welded joints, so the upper limit is made 0.3%. However, since Mo works effectively in securing the base material strength and the high heat input welding strength, 0.0
It is preferable to add 1% or more and 0.2% or less.

<Nb> Nb works effectively in securing the strength of the base metal and the high heat input welding strength, so 0.005% or more is added. However, the addition of more than 0.05% deteriorates the toughness of the weld metal.

<V> When V exceeds 0.1%, the weld crack susceptibility is deteriorated and the base metal toughness is impaired.
% Or less. However, 0.01% or more is added if necessary because it works effectively in securing the base material strength and the high heat input welding strength.

<Al> Al is added for deoxidizing steel,
Usually, 0.005% or more is contained. Further, it is added in an amount of 0.01% or more in order to secure the toughness of the base material by refining the microstructure. However, addition of Al in excess of 0.1% impairs base material toughness.

<Ti, B> Ti has the effect of improving the toughness of the base material and the welded joint through grain refinement of the microstructure. In addition, B-added steel is often positively added in order to secure B that works effectively for hardenability. However, in the present invention, it is not necessary to add Ti because the strength of the base material is secured without adding B and the toughness of the welded joint is achieved by reducing the hardness in the coarse grained zone of the heat affected zone. Rather, there is concern about instability in the base material performance due to the addition of Ti, so that as an impurity element,
Although it is regulated to less than 005%, it is desirable to be less than 3.4 times the N content described later.

B must be regulated to less than 0.0003% as an impurity element in order to reduce the hardness of the heat affected zone.

<N> N reacts with Al, Nb, etc. to form a precipitate, thereby making the microstructure finer and improving the toughness of the base metal, and reacting with Nb, V, etc. during tempering to cause precipitation hardening. Add to secure strength.

Addition of less than 0.0005% does not form a precipitate necessary for refining the microstructure and ensuring strength,
Addition of more than 0.008% rather impairs the toughness of the base metal and the welded joint.

<P, S> P and S are both impurity elements, and are 0.0 to obtain a sound base metal and weld joint.
It is desired to be regulated to 15% or less, preferably 0.01% or less.

<Cu, Ni, Cr> Cu, Cr has the effect of improving the strength of the base material and the weld joint. Ni has a function of further improving toughness. Although these alloying elements are not essential in the present invention, they may be added within the range of the formula of limiting chemical components. In particular, by replacing a part of Mn with these elements, improvement of toughness and reduction of segregation can be expected.

<Pcm> Pcm is an index showing the susceptibility to welding cracks, and is regulated to 0.2 or less in order to eliminate the need for preheating during welding in a normal environment.

<Calculation formula: 625Nb + 250V + 210
Ceq> Calculation formula: 625Nb + 250V + 210Ceq
Is an index representing the strength of the base material at a plate thickness of 1/2 t,
In addition to the carbon equivalent formula (Ceq) generally known by those skilled in the art, the contribution of Nb and V, which are the key points of the present invention, is taken into consideration and further about 15
It is a mathematical formula arranged in consideration of the plate thickness effect in the plate thickness range of -60 mm.

The plate thickness effect means that when the steel plate is forcibly cooled from the Ar3 transformation point or higher by direct quenching after hot rolling,
This means that the cooling rate inevitably changes depending on the plate thickness, and therefore the base metal strength changes.

In order to obtain a steel sheet that conforms to JIS G3106 SM570Q, which is classified as 600 N / mm ² class high strength steel, the calculation formula is: 625 Nb + 250 V + 210 Ce
It is necessary that q exceeds a value obtained by adding 40 to the plate thickness (mm).

<Heating temperature before hot rolling> In order to homogenize the alloying elements and form a solid solution of Nb, it is necessary to set the heating temperature to 1000 ° C. or higher. However, the heating temperature is 1250 ° C
If it exceeds, the toughness of the base material cannot be ensured due to the coarsening of the microstructure, so the upper limit is 1250 ° C., preferably 120.
Set to 0 ° C.

<Rolling Conditions> The step of hot rolling the uniformly heated steel of the present invention to a predetermined plate thickness may be performed under ordinary conditions. From the viewpoint of more stably securing and improving the toughness of the base material, it is desirable to apply a cumulative reduction of 20% or more in a temperature range of 1050 ° C. or less. Cumulative reduction rate of 20
When the content is at least%, fine graining by recrystallization can be achieved, and the toughness of the base material can be secured and improved more stably.
For the same reason, it is desirable to secure a rolling reduction of 5% or more, more preferably 10% or more, for each pass of rolling.

The rolling finish temperature must be above the Ar3 transformation point. If the rolling is completed at a temperature below the Ar3 transformation point, the subsequent direct quenching will result in incomplete quenching, and good base material properties cannot be secured. From the viewpoint of ensuring the acoustic anisotropy of the base material, the rolling finish temperature is preferably higher than 900 ° C. and higher.

<Direct Quenching> After completion of hot rolling, it is necessary to quench the steel sheet having a temperature higher than the Ar3 transformation point by forced cooling. For forced cooling, it is necessary to uniformly apply a cooling medium such as water to the steel sheet so as to achieve a cooling rate of approximately 3 ° C./sec or more near the transformation temperature of the sheet thickness 1 / 2t.

<Temperature Temperature> In tempering, when tempering is carried out at a temperature exceeding the Acl transformation point, the strength is remarkably reduced.
Since the strength as 0 N / mm ² class high-strength steel cannot be ensured, it is performed below the Ac1 transformation point. However, in order to secure the strength of the welded joint and to secure the strength of the base metal due to the precipitation effect of Nb, it is preferable to carry out at 570 ° C, preferably 600 ° C or higher.

[0042]

EXAMPLES Table 1 shows the chemical composition of the steel used in the examples of the present invention. Steel with the chemical composition shown in Table 1 was melted to form a steel ingot, hot-rolled to a specified plate thickness under the manufacturing conditions shown in Table 2, then directly quenched and then tempered, and then the sample steel Got The finishing temperature of hot rolling is 900 to 10 in all cases.
The temperature was set to 00 ° C and the tempering temperature was set to 580 to 680 ° C. In addition, a part of the test steel was reheat-quenched and tempered and used as a comparative example.

Tensile tests and Charpy impact tests were taken in the direction perpendicular to the rolling direction from the center part of the plate thickness of all the sample steels, and the mechanical properties of the base material as 600 N / mm ² class steel were evaluated.

In addition, an oblique Y-type weld crack test was carried out in accordance with JIS Z3158 and a maximum hardness test was carried out in accordance with JIS Z3101 to evaluate the weld crack sensitivity. These test atmospheres were controlled at 20 ° C-60%, and the initial temperature of the test piece was 25 ° C.

The large heat input meltability was evaluated by making a joint by ordinary electrogas arc welding, measuring the strength thereof, and performing a Charpy impact test on the bond portion where the toughness is most concerned.

The electrogas arc welded joint was prepared by welding one side of one layer only for the sample steel having a plate thickness of 25 mm, and for the sample steel having a plate thickness larger than that, it was divided into one layer on both sides at the center of the plate thickness. Welded and manufactured. The welding heat input on the final side is specified in Table 2, but the heat input on the backing side is 7 to 8 kJ / mm in all cases.

Example No. 2 in Table 2 The steel of composition No. A used in No. 1 is a comparative steel. Calculated value of steel No. A (625N
b + 250V + 210Ceq = 82) is the thickness of the test steel plate (3
The value obtained by adding 40 to 8) (78) was exceeded, so that the tensile strength of the base metal exceeded 570 N / mm ² . Also, Pc
The m value was as low as 0.17, and welding cracks did not occur in the Y crack test. However, the high heat input welded joint strength was less than 570 N / mm ² because Nb was not added.

Example No. 2 to 4 are examples of the present invention using steel No. B steel. The test steel base materials with plate thicknesses of 25 mm and 38 mm had good mechanical properties and susceptibility to welding cracks, and a sound large heat input welded joint could be realized.

Example No. 5-11 are Mn, Mo, N
It is an Example of this invention by the steel of steel numbers C-I which changed the content of b. In all cases, the mechanical properties of the sound base metal and the high heat input welded joint were confirmed, and the weld crack susceptibility was also good. In addition, as in these examples, Mn, Mo, Nb
Even if the content is changed, the correlation between the calculated value and the base metal strength is good,
It is shown that this calculation formula is industrially useful.

Example No. Reference numerals 12 and 13 are examples of the present invention using steel No. J steel in which V is not added. No. No. 12 is the case where the heating temperature is 1000 ° C., and all Nb is not solid-solved before rolling. Even in such a case, the accuracy of the calculation formula can be maintained by setting the Nb amount substituted for the calculated value as the effective Nb amount that works for precipitation hardening.

Example No. No. 14 is a comparative example in which the reheating quenching and tempering process is applied to the steel No. J steel.
The strength of the base material as N / mm ² class high-strength steel is not secured.

Example No. No. 15 is a comparative example by steel No. K steel in which the amount of C deviates from the regulation of the present invention. Even if an appropriate amount of alloying element is found from the calculation formula, a sound weld metal cannot be obtained in the high heat input welded joint, and in this case, the joint is fractured from the weld metal part in the joint tensile test and the joint strength cannot be secured. It was

Example No. No. 16 is a comparative example of steel No. L steel having a high C content, and weld cracking occurred in the oblique Y-type weld cracking test.

Example No. 17 has a high C content, and N
It is a comparative example by the steel of steel No. M which does not contain b. No weld cracks were found in the oblique Y-type weld crack test, but the maximum hardness exceeds 290 Hv and cannot be said to be good. Further, the toughness of the high heat input welded joint is inferior to any of the examples of the present invention.

Example No. 18 to 21 and 23 are Cu,
It is an example of the present invention by the steel of steel numbers N, O, P, Q, and S when it contains Ni and Cr.

No. 22 is a comparative steel made of steel No. R steel. Contains Cr in an amount exceeding the range specified in the present invention, and has Pc
Since the m value exceeded 0.2, weld cracking occurred in the oblique Y-type weld cracking test.

No. 24 and 25 are comparative examples of steels with steel numbers N and O containing B. In all cases, good base metal strength was obtained, but the base metal toughness was No. 1 by the steel No. O steel to which Ti was added in combination. 25 is better. Both have good weld crack susceptibility, but the toughness of the high heat input welded joint is poor.

[0058]

[Table 1]

[0059]

[Table 2]

[0060]

[Table 3]

[0061]

[Table 4]

[0062]

As described above, according to the present invention, 600 N / m, which has low weld crack susceptibility and excellent large heat input weldability.
An m ² class high-strength steel and a manufacturing method thereof can be provided.

[Brief description of drawings]

1 is an example No. 1; The figure which showed the hardness distribution of the large heat input welding joint part of this invention example corresponding to 6. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

2 is an example No. The figure which showed the hardness distribution of the large heat input welding joint part of the example of this invention corresponding to 7. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

3 is an example No. The figure which showed the hardness distribution of the large heat input welding joint part of the example of this invention corresponding to 9. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

4 is an example No. The figure which showed the hardness distribution of the large heat input welding joint part of the example of this invention corresponding to 10. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

5: Example No. The figure which showed the hardness distribution of the large heat input welding joint part of the comparative example corresponding to 24. A marked increase in hardness is recognized in the vicinity of the bond portion in the heat-affected zone as compared with the inventive examples.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Toru Kawanaka, Toru Kawanaka, 1-2, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Kazuyuki Matsui, 1-2, Marunouchi, Chiyoda-ku, Tokyo Date Main Steel Pipe Co., Ltd.

Claims (4)

[Claims] 1. C: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%, M
o: 0.3% or less, Nb: 0.005 to 0.05%,
V: 0.1% or less, Al: 0.005-0.1%, N:
0.0005-0.008%, Ti <0.005%, B
<Including 0.0003%, Pcm = C + Si / 30 + M
n / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo
Pcm value defined by / 15 + V / 10 + 5B is 0.2
Below, and Ceq = C + Mn / 6 + Si / 24 + N
Using the Ceq value defined by i / 40 + Cr / 5 + Mo / 4 + V / 14, Nb, V contents, and steel plate thickness t (mm), a weld crack that satisfies the relationship of 625Nb + 250V + 210Ceq ≧ 40 + t and the balance is iron and unavoidable impurities High strength steel with excellent sensitivity and high heat input weldability. 2. C: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%, M
o: 0.3% or less, Nb: 0.005 to 0.05%,
V: 0.1% or less, Al: 0.005-0.1%, N:
0.0005-0.008%, Ti <0.005%, B
<Including 0.0003%, further Cu: 0.5% or less,
Ni: 1.5% or less, Cr: 0.5% or less, one or two
Including more than one species, Pcm = C + Si / 30 + Mn / 20 +
Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V
Pcm value defined by / 10 + 5B is 0.2 or less, and Ceq = C + Mn / 6 + Si / 24 + Ni / 40 +
Using the Ceq value defined by Cr / 5 + Mo / 4 + V / 14, the Nb and V contents, and the steel plate thickness t (mm), the relationship of 625Nb + 250V + 210Ceq ≧ 40 + t is satisfied, and the balance of weld crack susceptibility is iron and inevitable impurities, and High tensile steel with excellent heat input weldability. 3. The steel having the chemical composition according to claim 1 or 2 is heated to a temperature range of 1000 to 1250 ° C., hot-rolled to a predetermined plate thickness, and then directly quenched from the Ar3 transformation point or higher. , A method for producing high-strength steel, which is excellent in weld crack susceptibility and high heat input weldability by tempering at a temperature not higher than the Acl transformation point. 4. When producing the steel according to claim 1 or 2, heating to a temperature range of 1000 to 1250 ° C.
After hot rolling at a cumulative reduction of 20% or more at 50 ° C or less,
A method for producing a high-strength steel having excellent weld crack sensitivity and high heat input weldability, which comprises directly quenching from an Ar3 transformation point or higher and tempering at a temperature not higher than the Ac transformation point.