JP2001073071A - THICK STEEL PLATE HAVING 570 TO 720 N/mm2 TENSILE STRENGTH AND SMALL IN DIFFERENCE IN HARDNESS BETWEEN WELDING HEAT-AFFECTED ZONE AND BASE MATERIAL AND ITS PRODUCTION - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thick steel plate having 570 to 720 N/mm2 tensile strength and small in the difference in the hardness between the welding heat- affected zone and base material and to provide a method for producing it. SOLUTION: This steel plate has a steel compsn. contg., by weight, 0.01 to 0.1% C, <=0.6% Si, 0.4 to 2.0% Mn, <=0.025% P, <=0.008% S, <=0.06% Al, 0.05 to 0.2% Nb, 0.005 to 0.035% Ti and 0.001 to 0.005% N, contg., at need, Cu, Ni, Cr, Mo, V, Mg, Ca and rare earth metals, and the balance iron with inevitable impurities and simultaneously satisfying PCM=C+Si/30+(Mn+Cu+ Cr)/20+Ni/60+Mo/15+V/10+5B<=0.18%. Furthermore, as the method for producing the thick steel plate, a cast slab or a steel slab having the above steel compsn. is reheated to 1,100 to 1,250 deg.C, is directly quenched after the completion of rolling at >=750 deg.C and is successively subjected to tempering treatment at the temp. of <=Ac1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a tensile strength of 570 to 7 for construction, bridges, shipbuilding, storage tanks and the like.
The present invention relates to a steel for welded structures of 20 N / mm ² (specifically, SM570 grade steel specified in JIS standards) and a method for producing the same.

[0002]

2. Description of the Related Art Generally, it is said that when the strength of steel material is increased, the fatigue strength of a base material is improved, but the fatigue strength of a welded portion is not improved. There has been a problem that the amount of steel used is not reduced, and the advantages of high-tensile steel cannot be fully enjoyed.

[0003] The problem of fatigue of welded steel structures is not limited to a particular field, but exists in all fields. Weld detail is considered to be dominant in the fatigue strength of welds. To improve the fatigue strength, reduction of stress concentration by flattening the shape of the weld (toe) and control of residual stress by peening are widely performed. . As measures for improving the fatigue strength of the welded portion from the steel surface, JP-A-9-227987, JP-A-9-241796, and JP-A-10-1
No. 742, Japanese Unexamined Patent Application Publication No. 10-1743, etc., control of the weld heat affected zone (HAZ) structure to a specific structure and the definition of steel components for that purpose, or further reduce the hardness difference between the weld metal (WM) and the HAZ. Is described. However, the hardness difference described above is intended for a relatively narrow region sandwiching the weld fusion line. This is presumed to be due to the fact that the weld toe is problematic at the weld toe.
There is no description about the hardness difference from Z.

On the other hand, sulfide stress corrosion cracking (SSC) caused by hydrogen sulfide in the vicinity of a weld has become a serious problem in storage tanks for ammonia, LPG, etc., oil / natural gas refining plants, and transportation line pipes. . The SSC susceptibility of steel varies depending on the steel composition, microstructure, the presence or absence of nonmetallic inclusions, etc., but is particularly affected by hardness, and it is said that SSC does not occur at HRC22 (HV conversion: 248 or less). However, in a non-uniform hardness portion such as a welded portion, concentration of stress and strain (heterogeneous distribution) occurs even when the above-described hardness regulation is performed, and S is regarded as one form of SSC.
OIC and the like may occur. The SOHIC prevention measures are described in, for example, Japanese Patent Application Laid-Open Nos. 5-287442 and
It is only an extension of the conventional measures of making SSC prevention measures more strict as disclosed in JP-A-188838, JP-A-9-125136, etc., and describes the effectiveness with respect to the SOHIC resistance characteristics at the welded portion. It has not been.

[0005]

The present invention has a tensile strength of 5%.
70-720 N / mm^TwoOf hardness between weld heat affected zone and base metal of steel
Thick steel plate and a method of manufacturing the same.
You. Weld crack susceptibility composition P _cmAnd other steel compositions
As is obvious, if the limitation is made, the weldability is significantly improved.
In addition, the hardening and softening properties of the heat affected zone are reduced,
It is possible to reduce the hardness difference between the heat affected zone and the base metal
During operation due to unevenness in the hardness (strength) of the weld
Concentration (non-uniform distribution) of stress and strain on
The fatigue strength and stress corrosion cracking resistance of steel
The safety of the structure can be improved.

[0006]

That is, the gist of the present invention is as follows.

(1) In weight%, C: 0.01 to 0.1
%, Si: 0.6% or less, Mn: 0.4 to 2.0%,
P: 0.025% or less, S: 0.008% or less, Al:
0.06% or less, Nb: 0.05 to 0.2%, Ti:
0.005 to 0.035%, N: 0.001 to 0.00
5%, the balance being iron and unavoidable impurities, _{and, P CM = C + Si /} 30 + (Mn + Cu + Cr) / 20 +
A steel plate having a steel composition satisfying Ni / 60 + Mo / 15 + V / 10 + 5B ≦ 0.18% at the same time, and having a small difference in hardness between a weld heat affected zone and a base metal having a tensile strength of 570 to 720 N / mm ^2. .

(2) Further, as a steel composition,
Cu: 0.05-0.5%, Ni: 0.05-0.5
%, Cr: 0.05-0.5%, Mo: 0.05-0.
5%, V: 0.005 to 0.2%, Mg: 0.0002
A tensile strength of 570 to 7 according to the above (1), wherein one or more kinds are contained in a range of from 0.005% to 0.005%.
A thick steel plate with a small difference in hardness between the weld heat affected zone of 20 N / mm ^{2 and} the base metal.

(3) Further, as a steel composition,
Ca: 0.0005-0.004%, REM: 0.00
The heat of welding having a tensile strength of 570 to 720 N / mm ^{2 according} to any one of the above (1) and (2), further comprising at least one of 0.5 to 0.004%. A thick steel plate with a small difference in hardness between the affected zone and the base metal.

(4) Any one of the above (1) to (3)
The slab or the slab having the steel composition described in the above item 1100
After reheating to a temperature of 1250 ° C. and finishing the rolling at a temperature of 750 ° C. or higher, it is immediately quenched, and subsequently Ac ₁
A method for producing a thick steel plate having a small difference in hardness between a weld heat affected zone and a base material having a tensile strength of 570 to 720 N / mm ² , characterized by performing a tempering treatment at the following temperature.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As measures against HIC, reduction of center segregation serving as a trap site for hydrogen, reduction of nonmetallic inclusions such as sulfide (MnS) and control of the form thereof are extremely effective. As a countermeasure, in addition to the above, it is effective to reduce the hardness of the heat affected zone, which is considered to have a great effect on the SSC sensitivity of steel.

First, it is effective to reduce the hardenability of steel to reduce the hardenability in the weld heat affected zone, but at the same time, the strength of the base metal is also reduced. For this reason, it is extremely difficult to achieve both in a well-balanced manner only by optimizing the steel composition. Therefore, the hardness of the weld heat-affected zone is reduced based on the B-free and low C in which C and B, which are most effective for hardenability, are suppressed to the utmost, and at the same time, Nb or, if necessary, V is added. A method utilizing precipitation hardening was devised.

The precipitation hardening itself of Nb (Nb + V) has been widely known, but the greatest feature of the present invention is that Nb is added in a relatively large amount of 0.05 to 0.2%. . Examples of the case where Nb (Nb + V) is added in a relatively large amount and its precipitation hardening is actively used are disclosed in, for example, JP-A-5-25542 and JP-A-5-20922.
No. 2, the tensile strength is 780.
N / mm ² (80 kgf / mm ² ) class high tensile steel.
The precipitation hardening allowance of Nb is very large, and assuming that a large amount is added, the tensile strength 570 to 720 targeted by the present invention is considered.
SM57 specified by the so-called JIS standard of N / mm ²
As for the 0-grade steel (60 kgf / mm ² -grade steel), the strength tends to be excessive, so that a large amount was not added.
Although it is possible to suppress an increase in strength depending on the manufacturing conditions even on the assumption that a large amount of Nb is added, such a case does not fully exhibit the original effect of Nb, and is not appropriate as an added amount.

In the present invention, 0.05 to 0.2%
With Nb addition as an essential requirement
/ Mm ² of steel. Such a relatively high Nb addition not only increases the tensile strength of the base material due to precipitation hardening, but also exerts an effect in preventing the welding heat affected zone from softening. It has the characteristics that the difference in hardness between the weld heat affected zone and the base metal is small as well as the suppression of hardening. Therefore, it is necessary to limit the steel composition as follows.

C most remarkably affects the hardenability of steel, and greatly affects the hardness of the weld heat affected zone. If the C content is too large, the hardenability increases, and the hardness of the weld heat affected zone increases with respect to the strength level targeted by the present invention.
%. On the other hand, the lower limit is required to be 0.01% or more as the minimum amount for ensuring the strength of the base material and the welded portion and exhibiting the precipitation hardening of Nb.

[0016] Si is an element contained in the deoxidized upper steel, but if added in a large amount, the weldability and the toughness of the heat affected zone deteriorate, so the upper limit is limited to 0.6%. The deoxidation of steel is
And Ti is only sufficiently possible even at 0.25% from the viewpoint of P _CM which will be described later and hardenability less.

Mn is an indispensable element for securing strength and toughness, and its lower limit is 0.4%. However, if the content is too large, the hardenability and PCM increase, increasing the hardness of the weld heat affected zone, deteriorating the weldability and the weld heat affected zone toughness, and further promoting the center segregation of the slab. 0%.

It is widely known that P and S are impurities in the present invention, and that the smaller the characteristic, the better. It is well known that the upper limit of each of P and S is 0.025 in consideration of economical efficiency such as P removal and S removal. %, 0.008%. In particular, S
Forms MnS, and in wet hydrogen sulfide environment, HIC and S
In order to promote the production of SC and further SOHIC, in applications used in such an environment, it is desirable to make S 0.001% or less together with the addition of Ca described later.

Al is an element generally contained in the deoxidized upper steel, but deoxidation is sufficient with only Si or Ti, and in the present invention, the degree of addition is not limited. But A
As the amount of l increases, not only the cleanliness of the steel deteriorates, but also
Since the toughness of the weld metal deteriorates, the upper limit is set to 0.06%.

[0020] Ti is essential for improving the toughness of the base metal and the weld heat affected zone. This is because when the amount of Al is small (for example, 0.003% or less), Ti combines with O to form Ti.
Precipitates mainly composed of ₂ O ₃ are formed and serve as nuclei for the formation of intragranular transformed ferrite to improve the toughness of the heat affected zone of the weld. Also,
Ti combines with N to precipitate finely in the slab as TiN, suppresses coarsening of γ grains during heating, is effective for reducing the rolling structure, and fine TiN present in the steel sheet is welded. This is because the structure of the heat affected zone is sometimes refined. To obtain these effects, at least 0.005% of Ti is required. However, if the content is too large, TiC is formed and the low-temperature toughness and the weldability are deteriorated, so the upper limit is 0.035%.

N, which is contained in steel as an unavoidable impurity, combines with Nb to form a carbonitride to increase the strength, and forms TiN to form a steel as described above. Enhance the nature. Therefore, the amount of N is at least 0.001.
%is necessary. However, an increase in the amount of N is extremely harmful to the toughness and weldability of the heat affected zone, and the upper limit is 0.005% in the present invention.

Next, the reasons for adding Cu, Ni, Cr, Mo, V, and Mg, which can be contained as required, will be described.

The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent features of the present invention. Therefore, the amount added is of a nature that should be naturally restricted.

Unless Cu is added excessively, weldability,
Improves the strength and toughness of the base metal without adversely affecting the toughness of the weld heat affected zone. In order to exert these effects, it is necessary to add at least 0.05% or more.
However, excessive addition causes deterioration of weldability and Cu-cracks during hot rolling, which makes production difficult.
Limited to 50%.

Ni exhibits almost the same effects and phenomena as Cu, and the lower limit should be the minimum amount for obtaining a substantial effect, and is 0.05%. The upper limit is limited to 0.5% because of the same reasons as Cu, and because of the relatively high cost and the possibility of saw-tooth corrosion called a Fischer under stress in a wet hydrogen sulfide environment under stress.

Cr and Mo improve both the strength and toughness of the base material when added at 0.05% or more. However, if the added amount is too large, the toughness and weldability of the base metal and the welded portion are deteriorated, so the upper limit was made 0.5%.

V has almost the same effect as Nb, but its effect is smaller than that of Nb. Further, since V also affects the hardenability, it should be supplemented with Nb. The lower limit is 0.01% at which the effect is observed, and the upper limit is limited to 0.05%.

Mg has the effect of suppressing the growth of austenite grains in the heat affected zone of welding and reducing the size of the grains, thereby toughening the welded portion. In order to enjoy such effects, Mg needs to be 0.0002% or more. On the other hand, as the addition amount increases, the effect cost on the addition amount decreases,
The upper limit is set to 0.005% because it is not advantageous in terms of cost.

Further, Ca and REM control the morphology of MnS and improve the low-temperature toughness of the base material, and also cause hydrogen-induced cracking (HIC, SSC, SOC) in a wet hydrogen sulfide environment.
HIC) reduces susceptibility. To achieve these effects, a minimum of 0.0005% is required. However, too much addition will conversely increase the cleanliness of the steel, increase base metal toughness and hydrogen-induced cracking in wet hydrogen sulfide environments (HIC, SSC, SSC).
OHIC) To increase the sensitivity, the upper limit of the amount added is 0.00
Limited to 4%. Since Ca and REM have almost the same effect, one of them may be added in the above range.

In the present invention, in addition to limiting the addition amount of each element, P _CM = C + Si / 30 + (Mn + Cu + C
r) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B
The reason for limiting the value defined as 0.18% or less is primarily to prevent welding cold cracking as a welded structural steel and to ensure excellent weldability. In addition, at least 0.05% of Nb, which is the most important feature of the present invention, is added, and in order to utilize the precipitation hardening of Nb, a limited tensile strength (570 to 720 N / mm ² ) is required. This is because the hardenability is not increased more than necessary. Conversely, when utilizing the precipitation hardening maximally 0.05% of Nb addition, 0 in P _CM.
If the content exceeds 18%, the strength becomes excessive as SM570 grade steel.

In order to reduce the difference in hardness between the weld heat affected zone and the base metal, which is a feature of the present invention, various components including Nb are limited as described above. Should be appropriately limited. Hereinafter, the limitation range and the reason will be described.

First, a slab or a slab having components controlled within the above-mentioned limited range must be reheated to a temperature of 1100 to 1250 ° C. The lower limit temperature is a reason from the viewpoint of utilizing the precipitation hardening of Nb that it is necessary to once turn Nb into a solution during reheating. Once the solution is formed, the precipitate of Nb can be finely dispersed and precipitated by making the subsequent process as in the present invention, and the precipitation hardening phenomenon can be effectively exhibited. On the other hand, the upper limit of the reheating temperature is to prevent the austenite grains at the time of reheating from being excessively coarsened. Since the austenite grains during reheating have a considerable effect on the refinement of the structure after rolling, the reheating temperature is preferably as low as possible, but the upper limit is limited to 1250 ° C. from the viewpoint of solutionizing Nb. It is.

In the hot rolling after reheating, it is necessary to finish the rolling at 750 ° C. or higher. This is to avoid the danger of ferrite being precipitated during rolling and rolling it. The processed ferrite is likely to promote toughness degradation and material anisotropy, and is not preferred.

After hot rolling, it must be immediately quenched. The intentional waiting until cooling or water cooling after rolling is because Nb may precipitate and coarsen during that time. The coarse precipitate during cooling does not exert any effect as precipitation hardening, not only may not be able to secure the strength of the base material, but even if it is reheated and quenched, unless Nb is solutionized, Precipitation hardening cannot be reused. It should be noted that “immediate quenching” is, of course, acceptable for a transfer time to a cooling device without intentional waiting after rolling, and the time is quenched within approximately 1 to 2 minutes. Means that. Further, “quenching” itself does not only mean rapid cooling from the austenitic single phase region defined by metallurgy, but also includes forced cooling from a state in which ferrite is slightly precipitated.
The following is performed. Such "quenching"
This is indispensable for preventing the Nb precipitate from becoming coarse.

After quenching, it is necessary to temper at a temperature of Ac ₁ or lower for fine precipitation of Nb, that is, precipitation hardening and toughening by tempering of the quenched structure.

[0036]

EXAMPLES In order to exemplify the usefulness of the present invention, steels having the chemical components shown in Table 1 were melted in a converter and subjected to plate rolling under the conditions shown in Table 2.

Among the mechanical properties, regarding the strength, in the direction perpendicular to the rolling direction, a steel sheet having a thickness of 50 mm or less
As a IS5 tensile test piece and a steel plate having a thickness of more than 50 mm, a JIS No. 4 round bar tensile test piece sampled from a quarter plate thickness position was used. In the impact test, a JIS No. 4 Charpy test piece cut out from the 1/4 sheet thickness position in the rolling direction was used to determine the transition temperature between brittle and brittle fracture surfaces (vTrs).

Further, in order to examine the difference in hardness between the weld heat affected zone and the base metal, a bead-on-hardness test was performed in accordance with the maximum hardness test method for the weld heat affected zone specified in JIS Z 3101.
A plate was prepared, the highest hardness and the lowest hardness of the weld heat affected zone at a position 0.5 mm below the steel plate surface were examined, and the base metal hardness was set at 10 points at a position 0.5 mm below the steel plate surface sufficiently separated from the weld metal. Was the average value. The test plate was kept at its original thickness, and was not preheated during welding.

[0039]

[Table 1]

[0040]

[Table 2]

Table 3 shows the measured values of the mechanical properties, the weld heat affected zone and the hardness of the base metal by the above method. The steel sheets according to the components and the production method specified by the present invention are all SM5
It can be seen that the steel has sufficient strength as a 70-grade steel and also has excellent toughness, and the difference in hardness between the weld heat affected zone and the base metal, which is a feature of the present invention, is suppressed to a small range.

On the other hand, in all of the steels shown in the comparative examples, either the composition or the production conditions are out of the ranges specified by the present invention, and therefore, the hardening or softening of the weld heat affected zone is large. The hardness difference between the weld heat affected zone and the base metal, which is the maximum effect brought by the above, is relatively large.
To specifically illustrate, steel 8 of the comparative example has a high C content,
P _CM even greater degree hardening of the heat affected zone due to the high. Further, since the reheating temperature prior to rolling is low, the precipitation hardening of Nb cannot be sufficiently utilized, and the strength is slightly lower. Steel 9 has a low Nb addition amount and also contains B, so that the softening and hardening of the heat affected zone is large. In addition, since the rolling end temperature is low, the starting temperature at the time of direct quenching is inevitably lowered, and during that time, ferrite is precipitated and the strength and toughness of the base material are poor. Steel 10 does not only contain Ti as a component, but since reheating and quenching are performed instead of direct quenching after rolling, the precipitation hardening of Nb cannot be fully utilized, and The strength of the material is low. The effect of the absence of Ti is that austenite is not sufficiently refined during reheating, and the base material toughness is slightly inferior. Steel 11 does not contain Nb and has a large softening of the weld heat affected zone. Also, C, P
_{Since CM} is high and B is added, the hardening of the weld heat affected zone is large.

[0043]

[Table 3]

[0044]

According to the present invention, the tensile strength is from 570 to 720.
Hardness difference HAZ and the base material of the N / mm ² will be able to provide large quantities and at low cost the small steel plate, including welding crack susceptibility composition P _CM, to limit as the present invention the steel composition In this way, the weldability is remarkably excellent, and the hardening and softening properties of the heat affected zone are reduced, and the difference in hardness between the heat affected zone and the base metal can be reduced. ) The concentration (non-uniform distribution) of stress and strain during operation due to the heterogeneity of () is alleviated, the fatigue strength and stress corrosion cracking resistance of the weld are improved, and the safety of the welded steel structure is improved. did it.

Continued on the front page (72) Inventor Akihiko Kojima 1 Kimitsu, Kimitsu-shi Nippon Steel Corporation Kimitsu Works F-term (reference) 4K032 AA00 AA01 AA04 AA08 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 AA BA01 CA02 CA03 CB02 CC03 CD06 CF01 CF02

Claims (4)

[Claims] 1. C .: 0.01 to 0.1% by weight, S
i: 0.6% or less, Mn: 0.4 to 2.0%, P: 0.
025% or less, S: 0.008% or less, Al: 0.06
% Or less, Nb: 0.05 to 0.2%, Ti: 0.005
0.035%, N: 0.001 to 0.005%, the balance being iron and unavoidable impurities, _{and, P CM = C + Si /} 30 + (Mn + Cu + Cr) / 20 +
A steel plate having a steel composition satisfying Ni / 60 + Mo / 15 + V / 10 + 5B ≦ 0.18% at the same time, and having a small difference in hardness between a weld heat affected zone and a base metal having a tensile strength of 570 to 720 N / mm ^2. . 2. The steel composition further comprises Cu:
0.05-0.5%, Ni: 0.05-0.5%, C
r: 0.05 to 0.5%, Mo: 0.05 to 0.5%,
V: 0.005-0.2%, Mg: 0.0002-0.
The tensile strength of 570-720 N / according to claim 1, wherein one or more kinds are contained in a range of 005%.
A thick steel plate with a small difference in hardness between the weld heat affected zone of ² mm and the base metal. 3. The steel composition further comprises Ca:
0.0005 to 0.004%, REM: 0.0005 to 0.005%
The weld heat-affected zone having a tensile strength of 570 to 720 N / mm ² and a base material according to any one of claims 1 to 2, further comprising at least one of 0.004%. Steel plate with small hardness difference. 4. A slab or a slab made of the steel composition according to any one of claims 1 to 3, which is heated to 1100 to 1250 ° C.
After completion of rolling at a temperature of 750 ° C. or higher, quenching immediately, followed by tempering at a temperature of Ac ₁ or lower.
A method for producing a thick steel plate having a small difference in hardness between a weld heat-affected zone of 20 N / mm ^{2 and} a base metal.