JP3215955B2 - Manufacturing method of high toughness and high strength steel sheet with excellent elongation properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has a uniform and excellent strength and low-temperature toughness, excellent elongation characteristics, and a tensile strength of 686 MPa or more which can be used for buildings, bridges, marine structures, pressure vessels, etc. it relates excellent production method for high toughness and high strength steel plate of the elongation properties.

[0002]

2. Description of the Related Art Conventionally, high-strength steels having a tensile strength of at least 686 MPa have been manufactured by adding B and performing quenching and tempering.

[0003] In order to increase the strength of this steel, a large amount of alloy must be added, and as a result, the steel is highly susceptible to weld cracking. So, to improve even a little,
For example, Japanese Patent Application Laid-Open No. 62-139815 and Japanese Patent Application Laid-Open
A process for making more effective use of the effect of improving the hardenability of B, as disclosed in Japanese Patent Publication No. 19121, is being studied.

[0004] As a manufacturing method to which a direct quenching and tempering process is applied, Japanese Patent Application Laid-Open Nos.
A method for producing a high-strength steel excellent in weldability by a method using precipitation hardening has also been proposed.

[0005]

However, with the B-added steel used here, it is impossible to avoid the increase in the hardness of the welded HAZ, and there is a limit to the improvement in weld cracking susceptibility.

[0006] JP-A-2-254120 and JP-A-2-129317 disclose a method of producing a steel sheet having excellent weldability without containing B, and the method for improving the susceptibility to weld cracking is disclosed. Has achieved certain results.

However, these methods can improve the susceptibility to weld cracking, but cannot produce a high-toughness and high-strength steel sheet having excellent elongation characteristics as described in the present invention.

Japanese Patent Application Laid-Open No. 63-266023 discloses a method for producing a high-strength steel having a low yield ratio, but does not describe a technique for improving elongation characteristics. In general, the prior art and knowledge cannot achieve the improvement in elongation properties described in the present invention.

In recent years, steels for steel structures have been required to have high elongation characteristics and high toughness in order to further enhance the safety of the structures. However, in the above prior art, the elongation characteristics and the low-temperature toughness cannot be improved together, and a new solution for improving the elongation characteristics and the low-temperature toughness of high-strength steel is awaited.

[0010] The present invention has been made in view of the above problems, and
Excellent weldability, and is intended to provide an excellent high toughness and high strength steel plate <br/> the preparation of elongation characteristics.

[0011]

The gist of the present invention is as follows: C: 0.02 to 0.11% by weight, Si:
0.05 to 0.50%, Mn: 0.3 to 2.0%, N
i: 0.3~4.0%, B: 0.000 2% or less, C
u: 0.5 to 4.0%, Al: 0.005 to 0.10
%, N: 0.0010 to 0.012%, H: 0.9 pp
m or less as an essential basic component, and the rest is quenched once or twice by quenching a steel plate consisting of iron and unavoidable impurity elements from an austenitic single phase of 1050 ° C. or less.
After being recirculated, the steel is further subjected to a quenching treatment of quenching from a two-phase region of more than Ac1 point and less than Ac3 point, and is tempered at an optional temperature of Ac1 or less. It is a method of manufacturing a plate .

Further, in the above-mentioned production method, Cr: 0.05 to 2.5%, Mo: 0.15% by weight based on the essential basic components.
１．５1.5%, V: 0.005 to 0.10%, Nb: 0.
One of the strength improving element group consisting of 003 to 0.07%
It contains one or more species.

In the above-mentioned production method, Ti: 0.004 to 0.05% and Ca: 0.
0005 to 0.006%, rare earth element: one or more elements of the low temperature toughness improving / homogenizing element group of 0.03% or less.

Further, in the above-mentioned production method, Cr: 0.05 to 2.5%, Mo: 0.15% by weight based on the essential basic components.
１．５1.5%, V: 0.005 to 0.10%, Nb: 0.
One of the strength improving element group consisting of 003 to 0.07%
Seed or two or more, Ti: 0.004 to 0.05%, C
a: 0.0005 to 0.006%, rare earth element: 0.0
It contains one or more of the low-temperature toughness improving / homogenizing element group of 3% or less.

[0015]

The present inventors have found that the tensile strength is 686 MPa class or higher.
Results of the examination of the influence of various alloying elements and heat treatment process on the elongation properties of the high strength steel plate, first of all to improve the Cu addition elongation properties, adding a Cu Secondly, quenching and tempering It has been found that by adding a heat treatment of quenching from the two-phase region in the middle of the steel and suppressing the hydrogen content of the steel material to a low level, a remarkable improvement in elongation properties can be achieved by the synergistic effect of Cu addition and the structure with low hydrogen.

FIG. 1 shows the effect of Cu content on the elongation characteristic value.
5 is a diagram showing the influence of the amount of hydrogen and the heat treatment process.

It can be seen that the addition of Cu improves uniform elongation. In addition, in the case of a steel having a normal H level, a slight improvement in uniform elongation is observed when a quenching treatment is performed from a temperature in the two-phase region, but in a steel in which the H level is suppressed, the uniform elongation is significantly improved.

This is presumably because the ductility of the ferrite in the ferrite-martensite-retained austenite structure containing Cu formed during quenching from the two-phase region temperature is synergistically improved by reducing the hydrogen content.

Further, according to the method of the present invention, as shown in FIG. 2, the strength increases as the amount of Cu increases. Conventionally, strength and elongation characteristics are generally contradictory characteristics, and it is said that elongation decreases as strength increases. However, according to the method of the present invention,
An epoch-making feature that the elongation property can be improved while increasing the strength is also possible.

From this knowledge, it is possible to produce high-strength steel having high strength and good elongation characteristics. The present invention has been made based on such knowledge.

Hereinafter, the present invention will be described in detail together with its operation. First, the reason why the steel applied to the present invention is limited to the above steel components will be described.

C: C is necessary for improving the hardenability and ensuring the strength, and it is necessary to obtain 0.02% or more in order to obtain the effect. However, C is an element that inhibits the weldability, and the upper limit is 0.1%. 11
%.

Si: Si is required as a deoxidizing element on steel making, and is required to be 0.05% or more to ensure strength.
On the other hand, if the content exceeds 0.50%, the weldability and the toughness of the base metal and the weld heat-affected zone (HAZ) are reduced, so that the content is set to 0.
05 to 0.50%.

Mn: Mn is required to be at least 0.3% in order to improve hardenability and secure strength and toughness at low cost.
% Or more impairs the toughness and hardens the HAZ and impairs the weldability, so is limited to 0.3 to 2.0%.

Ni: Ni has the effect of improving the hardenability and the toughness of the ground, but 0.3% or more is required to obtain the effect. On the other hand, if it exceeds 4.0%, it becomes too expensive, so this value is set as the upper limit.

B: B is hardly added to harden the welded HAZ and enhance the weld cracking resistance, but 0.000 2
% Is harmless, and this is the upper limit.

Cu: Cu is the main element of the present invention, and enhances the elongation characteristics together with the addition due to the synergistic effect of the amount of hydrogen and the ferrite in the two-phase quenched structure. Further, it is an element capable of improving strength. If it is less than 0.5%, the effect is small, so the lower limit is made 0.5%. On the other hand, if the content is too large, high-temperature cracking is likely to occur during welding.

Al: Al is an element necessary as a deoxidizing material and is also effective in reducing the grain size. However, to obtain the effect, 0.005% or more is required. Further, if the addition exceeds 0.10%, alumina-based inclusions increase to impair the cleanliness and toughness of the steel sheet, so that the addition is limited to 0.005 to 0.10%.

N: N combines with Ti and Al to form a nitride and is effective in preventing austenite grains from being coarsened. For this reason, 0.0010% or more is required. Further, if the amount increases, the weld HAZ toughness is impaired, so the upper limit is 0.012%.

H: H is also an important element of the present invention. By keeping it low, synergistic with the Cu2 phase quenching structure,
This results in a significant improvement in uniform elongation. But 0.9ppm
If the ratio exceeds the upper limit, the effect is diminished. Therefore, the upper limit is set to 0.9 ppm.

In the present invention, one or more of the following element groups can be selectively contained depending on the required properties of steel, in addition to the above essential basic components.

Cr, Mo, V, and Nb have an equivalent action of improving the strength of the steel, and one or more of them may be contained as needed. 0.05%, Mo: 0.15%, V:
Must be 0.005% and Nb: 0.003%. However, Cr: 2.5%, Mo: 1.5, respectively.
%, V: 0.10% and Nb: more than 0.07% impair weldability and have an adverse effect of being too expensive, so the above value is defined as the upper limit of the component of the strength improving element.

Ti, Ca and rare earth elements have a uniform function of improving and homogenizing the low-temperature toughness of steel, and one or more of them may be contained as necessary. In order to ensure the following, the lower limits of the contents must be set to 0.004% for Ti and 0.0005% for Ca, respectively. However, Ti: 0.10% and Ca:
Even if the content exceeds 0.005% and rare earth element: 0.03%, it is unnecessarily expensive and also impairs weldability and homogeneity. Therefore, the upper and lower limits are determined.

In addition to the above components, P, S, and the like as unavoidable impurity elements are harmful elements that lower the low-temperature toughness, which is the characteristic of the present invention. Preferably each is 0.01% or less.

Next, a heat treatment method which is another gist of the present invention will be described. In order to obtain good properties as a high-strength steel sheet in addition to the above steel components, a heat treatment method must be appropriate. Here, the reasons for limiting the heat treatment conditions will be described.

The heat treatment method is so-called quenching—two-phase quenching—tempering. At the time of quenching, water cooling from the austenite single phase region is performed in order to obtain sufficient hardenability.
0 ° C or less. This quenching treatment may be performed twice if necessary.

Next, in the two-phase quenching, the purpose is to disperse and precipitate ferrite in the ferrite-martensite-retained austenite structure. By this heat treatment,
Dispersed ferrite containing Cu can improve uniform elongation and improve low-temperature toughness, and can also reduce the yield ratio. As the temperature for this purpose, more than Ac1 point, A
c The upper and lower limits of less than 3 points are required.

The tempering treatment is intended to sufficiently precipitate the strengthening element from the quenched structure and to recover and soften the quenched structure to obtain toughness. If the temperature exceeds the Ac1 point, the strength and toughness are significantly reduced. Therefore, the temperature at the Ac1 point is set as the upper limit.

The quenching treatment before the two-phase quenching may be either offline quenching or online quenching (so-called direct quenching). Before the off-line quenching, a so-called double quenching process of performing one on-line quenching or off-line quenching may be performed.

[0040]

EXAMPLES Steels having compositions shown in Tables 1, 2, 3, and 4 were melted, and then steel sheets having a thickness of 35 to 125 mm were manufactured. Subsequently, each of the heat treatment methods of the present invention method and the comparative method shown in Tables 5, 6, and 7 was performed to produce a high-strength steel sheet. The mechanical properties of the base metal were investigated for these materials, and the weldability
An AZ maximum hardness test was performed. Tables 8, 9, and 1 show the results.
0 is shown.

In the example of the present invention, the uniform elongation is sufficiently high, the low-temperature toughness is good, the maximum hardness of the weld HAZ is sufficiently low, and the weldability is greatly improved.

On the other hand, comparative steels (AA, AB, AC, A) deviating from the chemical composition range defined by the present invention.
In D), in Examples AA1 and AB1, the Cu content is small and the uniform elongation is small. In addition, C content is high and B is added, and low temperature toughness is low, HAZ hardness is high, and weldability is not good.

Examples AC1 and AC2 have good weldability because C is small and B is not added, but uniform elongation is not good because Cu is little added, and AC2 has insufficient strength.

Further, since the amount of Cu added in Example AD1 is higher than that in Example AC1, the uniform elongation is increased, but Ni is not added, and the strength-toughness balance is not good.

Next, Example AA in which the chemical composition range deviates and the heat treatment process deviates from the range specified in the present invention.
In 2, AB2, AC3, and AD3, the synergistic effects of Cu addition, microstructure, and low hydrogen are not utilized, so that elongation and toughness are poor.

[0046]

[Table 1] Composition of test steel (Part 1)

[0047]

[Table 2] Composition of test steel (No. 1. Continuing from Table 1)

[0048]

[Table 3] Composition of test steel (Part 2)

[0049]

[Table 4] Composition of test steel (Part 2. Continued from Table 3)

In Tables 1 to 4, Ceq, Pcm,
Ar3 point, Ac1 point, and Ac3 point were calculated by the following equations.

[0051]

## EQU1 ## Ceq = C + Mn / 6 + Si / 24 + Cr / 5
+ Ni / 40 + Mo / 4 + V / 14

[0052]

## EQU2 ## Pcm = C + Si / 30 + Mn / 20 + Cu /
20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10
+ 5B

[0053]

Ar3: -396C (%) + 24.6Si
(%)-68.1Mn (%)-36.1Ni (%)-2
0.7Cu (%)-24.8Cr (%) + 29.6Mo
(%) +868 (° C)

[0054]

## EQU4 ## Ac1: -16.3C (%) + 34.9Si
(%)-27.5Mn (%)-15.9Ni (%) + 1
2.7Cr (%)-5.5Cu (%) + 3.4Mo
(%) + 751 (° C)

[0055]

## EQU5 ## Ac3: -206C (%) + 53.1Si
(%)-15.0Mn (%)-20.1Ni (%)-
0.7Cr (%)-26.5Cu (%) + 41.1Mo
(%) +881 (° C)

[0056]

[Table 5] Heat treatment method (1)

[0057]

[Table 6] Heat treatment method (Part 2)

[0058]

[Table 7] Heat treatment method (Part 3)

[0059]

[Table 8] Test results (No. 1)

[0060]

[Table 9] Test results (No. 2)

[0061]

[Table 10] Test results (part 3)

[0062]

As described above, the composition range and the production method of the present invention make it possible to produce high-toughness and high-strength steels that are homogeneous and have excellent elongation characteristics and low-temperature toughness. By applying to steel with a tensile strength of 686MPa class or higher used for bridges, offshore structures, pressure vessels, etc., the quality of the structure, the efficiency of welding work are improved, and the safety of the structure is further improved. Can be.

[Brief description of the drawings]

FIG. 1 Low H material (H = 0.7 ppm) when Cu addition amount is changed
4 is a drawing showing a change in uniform elongation due to a heat treatment process of a high H material (H = 1.2 ppm).

FIG. 2 is a graph showing a change in tensile strength when the amount of Cu added is changed.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-1455881 (JP, A) JP-A-3-264614 (JP, A) JP-A-3-232923 (JP, A) JP-A-3-232 130345 (JP, A) JP-A-62-205227 (JP, A) JP-A-5-9570 (JP, A) JP-B-57-40887 (JP, B2) (58) Fields investigated (Int. ⁷ , DB name) C21D 6/00 C22C 38/00-38/60

Claims (4)

(57) [Claims] C: 0.02 to 0.11% Si: 0.05 to 0.50% Mn: 0.3 to 2.0% Ni: 0.3 to 4.0% B by weight% : 0.000 2% Cu: 0.5~4.0% Al: 0.005~0.10% N: 0.0010~0.012% H: 0.9ppm contained the following, the balance being iron And once or twice a quenching treatment of rapidly cooling an austenitic single phase of 1050 ° C. or less to a steel sheet composed of unavoidable impurity elements, and then Ac1
High toughness and high strength with excellent elongation characteristics of tensile strength of 686 MPa class or more characterized by quenching treatment of quenching from the two-phase region of more than Ac and less than 3 points and tempering at an arbitrary temperature of Ac1 or less. process for the preparation of steel plate. 2. In% by weight, Cr: 0.05 to 2.5% Mo: 0.15 to 1.5% V: 0.005 to 0.10% Nb: 0.003 to 0.07% excellent preparation of high tenacity and high strength steel plate according to claim 1 <br/> elongation properties tensile strength of more than 686MPa grade according containing one or two or more of the respective intensity improvement group of elements. 3. A low-temperature toughness improving / homogenizing element group of Ti: 0.004 to 0.05%, Ca: 0.0005 to 0.006%, rare earth element: 0.03% or less in weight%. The tensile strength according to claim 1, which contains one or more kinds of 686 MPa class.
Excellent preparation of high tenacity and high strength steel plate of the above elongation characteristics. 4. In% by weight, Cr: 0.05 to 2.5% Mo: 0.15 to 1.5% V: 0.005 to 0.10% Nb: 0.003 to 0.07% One or two or more of the following strength improving element groups: Ti: 0.004 to 0.05% Ca: 0.0005 to 0.006% Rare earth element: 0.03% or less The tensile strength according to claim 1, which contains one or more of the homogenizing element group.
Excellent preparation of high tenacity and high strength steel plate of the above elongation characteristics.