WO1995004838A1

WO1995004838A1 - High tensile strength steel having superior fatigue strength and weldability at welds and method for manufacturing the same

Info

Publication number: WO1995004838A1
Application number: PCT/JP1994/001297
Authority: WO
Inventors: Katsumi Kurebayashi; Shuji Aihara; Atsushi Seto
Original assignee: Nippon Steel Corporation
Priority date: 1993-08-04
Filing date: 1994-08-04
Publication date: 1995-02-16
Also published as: NO951288D0; EP0666332A4; KR0157540B1; JP3526576B2; CN1040555C; CN1113391A; NO951288L; KR950703661A; EP0666332A1

Abstract

A method for manufacturing a high tensile strength steel by normally hot rolling or control rolling following the hot rolling of a high tensile strength steel or a slab made thereof containing, in terms of weight %, 0.03 to 0.20 % of C, 0.6 to 2.0 % of Si, 0.6 to 2.0 % of Mn, 0.01 to 0.08 % of Al, 0.0020 % or less of B, 0.002 to 0.008 % or less of N, and, as required, at least one of Cu, Mo, Ni, Cr, Nb, V, Ti, Ca and REM, as well as the remaining portion of Fe and unavoidable impurities, wherein generation of fatigue cracks as welded is restrained at portions of this steel which are affected by welding heat, and development of cracks, if generated, is prevented or restrained.

Description

Description High-strength steel with excellent fatigue strength and weldability of welds and method of manufacturing the same

TECHNICAL FIELD The present invention relates to a high-strength steel having excellent fatigue strength and weldability of a weld portion used for shipbuilding, offshore structures, bridges, and the like, and a method for producing the same.

With the increase in the size of structures, reducing the weight of structural members has become an important issue in recent years, and to achieve this, the steel used in structures has been increasing in tension. However, ships, offshore structures, bridges, etc. are subject to repeated loads during the service period, so care must be taken to prevent fatigue damage in such structures. Since the most susceptible site for fatigue fracture is the weld, it is required to improve the fatigue strength of the weld.

To date, a great deal of research has been conducted on the factors that govern the fatigue strength of welds and on the improvement of fatigue strength.The improvement of weld fatigue strength is achieved by grinding / grinding or heating and re-melting the final layer of the weld bead. As a result, compressive stress is applied to the weld toe by reducing the concentration of stress by improving the shape of the weld toe, such as shaping the shape of the weld toe, and by performing shot peening. Most of the improvement was due to mechanical factors, such as those that produced phenomena (JP-A-59-110490, JP-A-11-301823, etc.). The residual stress reduction effect obtained by heat treatment after welding has been well known.

—However, without the special construction and post-weld heat treatment described above, It has been proposed to improve the fatigue strength of welds depending on the situation. In Japanese Patent Application Laid-Open No. 62-10239, by increasing the amount of Si and specifying the amounts of C and P added, even at high C and high Mn levels, it is possible to prevent deterioration of the fatigue properties of spot welds. The purpose is as follows: C: 0.3% or less, Si: 0.7 to 1.1%, Mn: 2.0% or less, P: 0.16% or less, and solAl: 0.02 to 0.1% High strength steel sheets are disclosed.

In Japanese Patent Application Laid-Open No. 3-264645, Si is used to favor the formation of clean polygonal graphite, B is used to strengthen the steel, and by improving the hardenability, it is possible to obtain good elongation flangeability, fatigue properties, In order to obtain resistance weldability, C: 0.01 to 0.2%, Mn: 0.6 to 2.5%, Si: 0.02 to 1.5%, B: 0.0005 to 0.1%, etc. A high-strength thin steel sheet excellent in quality is disclosed.

In Japanese Examined Patent Publication No. 3-56301, the addition of B, etc., is intended to improve the joint fatigue strength of the spot weld by improving the ratio of the components in the steel and the unrecrystallized structure in the steel sheet. For the purpose, C: 0.006% or less, Mn: 0.5% or less, A1: 0.05% or less, and the total of one or two types of solid solution Ti and / or Nb excluding nitrides and sulfides:

An ultra-low carbon steel sheet comprising 0.001 to 0.100% and having good spot weldability is disclosed.

Of these, JP-A-59-110490 and JP-A-1-1301823 require special work after welding, and the fatigue strength cannot be improved without welding. The method using heat treatment after welding is also not preferable because the number of processes increases and the welding work becomes complicated. The effect is also limited.

Thin steel sheets disclosed in JP-A-62-10239 or JP-A-3-264645 are mainly used for automobile wheels and disk base materials. Since the purpose, thickness, and method of use are completely different from those of steel plates used in shipbuilding and marine structures, which are the subject of the present invention, the findings here are applied to steel plates as they are. It is not possible. Looking at the steel composition, the thin steel sheet disclosed in Japanese Patent Application Laid-Open No. 62-10239 particularly shows that the relationship between the amounts of C and P is C: less than 0.22% and P: 0.16%. In the following, the aim is to improve the fatigue strength of spot welds by specifying the range of C + 0.6 P31 with C 0.22 to 0.3%, and to improve the weld strength by arc welding. No disclosure is made regarding solid solution strengthening of the flight structure.

In other words, spot welding is a type of resistance welding method, and is mainly used for welding thin steel sheets with a thickness of about 0.5 to 3.5 mra and after forming, for example, thin steel sheets for automotive parts. This is done by sandwiching the welded part of a thin steel plate by pressing it with an electrode and applying a large current in a short time.

Therefore, spot welding is the arc welding method used for welding high-tensile steel plates that are used as materials for shipbuilding, marine structures, bridges, etc. with a thickness of 6 mm or more. In addition to differences in welding methods such as welding conditions, welding conditions, etc., the shape of the welded part and welding residual stress also differ, the two factors governing the fatigue strength differ between the two. It is not possible to directly apply the knowledge of welding to arc welding.

—On the other hand, the steel sheet disclosed in Japanese Patent Application Laid-Open No. 264645 is a steel sheet to which B has been added to improve the strength and hardenability of the steel and to obtain a desired structure. Not touched. Furthermore, there is no description on the improvement of the fatigue strength of welds other than the base metal.

The steel sheet disclosed in Japanese Patent Publication No. 56301/1996 relates to spot welds of ultra-low carbon steel sheets and is intended to control the hardness distribution of the spot welds. And grain growth. The upper limit of the amount added is set to suppress deterioration of the material, and no study has been made on weldability.

An object of the present invention is to improve the fatigue strength of a weld of a structural member, particularly, a weld welded by an arc welding method.

Another object of the present invention is to improve the fatigue strength of the structure of a welded portion of a structural member, particularly the structure of a weld heat affected zone (hereinafter, referred to as HAZ) by controlling the structure of the HAZ up to welding. .

Another object of the present invention is to provide a high-tensile steel plate having good weldability in which weld cracks do not occur immediately after welding.

Another object of the present invention is to provide a method for producing a high-strength steel plate that achieves the above object. Disclosure of the invention

The present invention provides the following high-strength steel plate for achieving the above object.

Here, the basic concept of the present invention will be described below.

(1) The inventors of the present invention have observed microscopically the state of crack initiation and propagation in a fatigue test piece of a welded joint. As a result, it was found that fatigue cracks often occur at the boundary between the weld metal and the HAZ where repeated load stresses are concentrated, propagate through the HAZ, and further propagate to the base metal, causing the specimen to fracture. did.

From the above observations, it is considered that the HAZ structure in which this fatigue crack is generated and propagated is greatly related to the fatigue strength. Fatigue is caused by the repeated movement of dislocations.To improve the fatigue strength of the weld, fatigue cracks are generated.The HAZ structure is strengthened so that it is difficult to propagate, and dislocation movement is suppressed. I came to think I needed to. Generally, there are strengthening methods such as solid solution strengthening, precipitation strengthening, and dislocation strengthening. Since the welds are rapidly heated and cooled, the precipitates are also dissolved, so it is not possible to strengthen the HAZ structure up to the weld by precipitation strengthening. Even if the base metal is strengthened by working dislocation, dislocation density is reduced by welding, so dislocation strengthening is not an appropriate strengthening method. Therefore, solid solution strengthening is an effective means to strengthen the HAZ structure. Elements effective for solid solution strengthening are C, N, P, Si, Cu, and Mo in the order of their effect. C and N, which are interstitial elements, have a large effect of solid solution strengthening, but have a greater effect on properties such as hardenability, weldability, and toughness other than solid solution strengthening. However, it cannot achieve the sole purpose of solid solution strengthening of the HAZ structure. Also, P has a great effect of solid solution strengthening, but embrittles grain boundaries, so its content must be reduced. On the other hand, substitutional Si, Cu, and Mo have a smaller ratio of solid solution strengthening to the added amount than C, N, and P, but are effective for solid solution strengthening because they can be added more than these. Furthermore, Si reduces stacking fault energy and reduces cross slip, thereby suppressing localization of deformation during repeated plastic deformation and increasing reversibility of plastic deformation. Has the effect of suppressing the generation of cracks.

Therefore, it is considered that the addition of Si is effective in improving the fatigue strength. Based on the above study, various high-strength steels solid-solution-strengthened with Si are considered. The T-shape shown in Fig. 1 A fillet welded joint was prepared and subjected to a fatigue test. As a result, the findings described in the present invention were obtained.

(2) When preparing T-shaped fillet welded joints, low-temperature cracking was observed in HAZ in high-strength steel with a large amount of B added. Cold cracking should not occur in welds of high-strength steel, and it is expected that fatigue cracks will easily occur starting from these cracks when repeated loads are applied. . Where, the carbon Indicates equivalent P era.

P cm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + o _/ ^/ 15 + V / 10 + 5 B (1)

As can be seen from this equation, B has the highest cold cracking susceptibility compared to other elements (the larger the coefficient, the higher the cracking susceptibility).

However, B has the function of suppressing grain boundary fluoride, which is a source of fatigue cracks. Therefore, when considering the susceptibility to low-temperature cracking, the effect must be suppressed to 0.0020% or less, at which the effect of saturation is saturated. In addition, when P cm is high due to the combination of elements, it is preferable to suppress the addition amount to less than 0.0005%, which is an addition amount that does not substantially affect the cold cracking susceptibility.

Therefore, suppressing B and ensuring weldability is a prerequisite for improving the fatigue strength of the weld.

In order to ensure good weldability without low-temperature cracking, it is necessary to control the carbon equivalent P cm in consideration of elements other than B as described above. For example, when welding a steel plate having a thickness of 15 mm as shown in the embodiment of the present application, by setting the value of −P cm to 0.26 or less, favorable welding can be performed at room temperature. If the value of P cm is larger than this, additional steps such as suppression of the amount of invading hydrogen and residual heat of the steel sheet are required.

(3) In addition, the inventors of the present invention observed microscopically the appearance of crack initiation and propagation in fatigue test specimens of welded joints, and obtained knowledge on the relationship between HAZ structure and fatigue strength. . The HAZ structure is classified into ferrite structure, bainite structure, and martensite structure according to the hardenability of steel. Usually, the HAZ structure of commercially available high-strength steel is often the veneer structure. Here, the payinite organization was defined as both the upper bainite organization and the lower payinite organization, and the ratio of the payinite organization to the total organization was determined as the bainite organization fraction by microscopic observation.

Low hardenability of HAZ structure Higher fraction of ferrite structure than 20% When the payinite microstructure fraction is less than 80%, fatigue cracks are likely to occur from soft lite structures such as grain boundary ferrite ヱ ite 'side plates, resulting in lower fatigue strength. Does not improve. On the other hand, when the hardenability is high and the martensite microstructure fraction is higher than 20% and the payneite microstructure fraction is lower than 80%, fatigue cracks occur from the grain boundaries at the interface of the hard martensite microstructure. Again, the fatigue strength does not improve.

Based on these findings, it was confirmed that the bainite tissue improved the fatigue strength, and that the effect of improving the fatigue strength was remarkable when the tissue fraction was 80% or more.

In order to make the HAZ structure mainly composed of bainite, it is effective to add appropriate amounts of Ni, Cr and V as elements that improve the hardenability of the structure.

The present invention provides a high-strength steel sheet having improved fatigue strength and weldability by the effects of the above (1) and (2). Further, by combining (3), a higher strength steel sheet can be obtained. Achieved high-strength steel sheets can be provided.

In addition, it is also beneficial to further add Cu and Mo in order to further strengthen the solid structure of the HAZ in the solid solution and to improve the hardenability. It is also effective for the present invention to add Nb to suppress the recrystallization of the light and to improve the hardenability, and to add Ti to suppress the coarsening of the austenite grain size.

It is also effective to add Ca and REM to fix sulfides that cause fatigue cracks and improve ductility.

That is, in the present invention, C: 0.03 to 0.20%, Si: 0.6 to

2.0 Mn: 0.6 to 2.0%, A1: 0.01 to 0.08%, N: 0.002 to 0.008%, B: 0.0020% or less, with the balance being Fe and unavoidable impurities It is a high-strength steel, and if necessary, Cu: 0.1-1.5 Mo: 0.05-0.5%, Νί: 0.1-3.0%, Cr: 0.1-1.0%, V: 0.01-0.10%, Nb: It is a high-tensile steel containing at least one element in the range of 0.005 to 0.06%, Ti: 0.005 to 0.05%, Ca: 0.0005 to 0.0050%, REM: 0.0005 to 0.0050%. It is a high-strength steel with excellent fatigue strength and weldability in the weld zone with a bainite structure fraction of 80% or more.o Brief description of the drawings

FIG. 1A is a plan view showing a fatigue test piece of a T-shaped fillet welded joint. FIG. 1B is a side view of the fatigue test piece shown in FIG. 1A. BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the present invention will be described in detail below.

First, the reasons for limiting the components of the steel serving as the base material in the present invention will be described.

C is an element that increases the strength of the base material, and it is desirable to add a large amount of C to increase the strength of the base material. However, the addition of more than 0.20% of C lowers the toughness of the base metal and the weld, and deteriorates the weldability. Therefore, the upper limit of C was set to 0.20%. On the other hand, if C is too low, it becomes difficult to secure the strength of the base metal, and the hardenability of the weld decreases, resulting in the formation of grain boundary pro-eutectites that are harmful to the fatigue strength. As described above, if C is less than 0.03%, a desirable structure for improving fatigue strength cannot be obtained, so the lower limit of C was set to 0.03%.

Si is a solid solution strengthening element that does not significantly enhance hardenability, and solid solution strengthens the structure. It suppresses the movement of dislocations and the generation of fatigue cracks. Also, Si is known to reduce the eyebrow defect energy of the steel sheet structure and reduce cross slip. As a result, plastic deformation It has the effect of suppressing the occurrence of cracks by suppressing cross-localization of dislocation slip lines and increasing the reversibility of plastic deformation when the shape is repeatedly loaded. Therefore, Si is an essential element for improving fatigue strength.

If Si is less than 0.6%, the effect of solid solution strengthening and reduction of stacking fault energy is small, and improvement in fatigue strength cannot be expected. Therefore, the lower limit was set to 0.6%. Conversely, if more than 2.0% of Si is added, red scale is generated to deteriorate the surface properties and increase the number of fatigue crack sources, as well as the toughness. Therefore, the upper limit was set to 2.0%.

Mn is an element that increases the strength of the base material without significantly reducing toughness. If Mn is less than 0.6%, sufficient base metal strength cannot be obtained, so the lower limit was set to 0.6%. When Mn is added in excess of 2.0%, not only the toughness of the weld is reduced, but also the weldability and ductility are deteriorated, so the upper limit was set to 2.0%.

A1 is required as a deoxidizing element, and unless added at 0.01% or more, deoxidizing action cannot be expected. On the other hand, if added in excess of 0.08%, large amounts of A1 oxides and nitrides are generated, deteriorating the toughness of the weld. Therefore, the upper limit was set to 0.08%.

When Ti is added, N combines with Ti and suppresses austenite grain growth of HAZ. Since this effect cannot be expected if N is less than 0.002%, the lower limit of N is set to 0.002%. Conversely, if a large amount is added, the amount of dissolved N increases and the HAZ toughness decreases, so the upper limit is set to 0.008%.

B has the effect of improving the hardenability of the HAZ structure and also has the function of suppressing grain boundary fluoride, which is the source of fatigue cracks, but greatly deteriorates the susceptibility to weld cracking and reduces weldability. It is an element that, when added, causes weld cracks such as root cracks and toe cracks. The above effects As the fruits are saturated at 0.0020%, the upper limit of the amount of B added is set to 0.0020% .When the amount of alloying elements other than B is large and P cm is high, the effect on cold cracking susceptibility is substantially negligible. The upper limit was set to less than 0.0005%.

In addition, P and S are the more preferable impurity elements as they are lower. P should take into account the toughness of the base metal and the weld, and S should also take into account the toughness of the base metal and the weld as well as the ductility in the thickness direction, and the upper limit should be 0.020%. Is desirable.

Cu and Mo improve the hardenability of the base metal and HAZ, but these elements are rather effective in strengthening the fiber matrix by solid solution strengthening like Si. However, stacking fault energy does not decrease as much as Si. The effect is not remarkable unless 0.1% and 0.05% or more are added, respectively. Also, if the addition exceeds 1.5% or 0.5%, the hardenability is too high, and the fatigue strength is adversely reduced due to the formation of martensite.

Ni, Cr, and V are all elements that improve the hardenability of the base metal and HAZ. The lower limits of 0.1%, 0.1%, and 0.01%, respectively, were set as the additive amounts at which the effect was exhibited for each element. In addition, excessive addition facilitates the formation of a lower bainite-martensite structure and rather lowers the fatigue strength of the welded portion. Therefore, the upper limits of the respective contents are set to 3.0%, 1.0% and 0.10%.

Nb is an element that has the effect of increasing base metal strength and also has an effect on hardenability.In addition, if controlled rolling and controlled cooling are applied during steel sheet manufacturing, the non-recrystallization temperature range increases. Therefore, it is desirable to add 0.005% or more in order to control recrystallization during rolling and to enable controlled rolling in a wide temperature range. However, high Nb content lowers HAZ toughness. Therefore, the upper limit of Nb was set to 0.06%. Ti combines with N to form TiN, which improves the toughness of HAZ by refining the structure of HAZ. For that purpose, 0.005% or more is necessary, but if it exceeds 0.05%, no further effect is seen, so the lower limit was set to 0.005% and the upper limit to 0.05%.

Ca has the effect of fixing sulfide, which is the source of fatigue cracks, and improving ductility. Also, the occurrence of fatigue fracture originating from sulfides can be suppressed. If the amount is less than 0.0005%, the effect cannot be expected.

If it exceeds 0.0050%, the toughness decreases. Therefore, the lower limit was set to 0.0005% and the upper limit to 0.0050%.

REM has the same effect as Ca in fixing sulfides that cause fatigue cracks and improving ductility. Also, the occurrence of fatigue fracture originating from sulfides can be suppressed. REM is considered to have the same effect as any other rare earth element. Among them, La, Ce and Y are mentioned as typical examples. In order for the effect of adding REM to be exerted, it is necessary to add a total of 0.0005% or more. Even if 0.0050% or more is added, the effect saturates and is not economical. Therefore, the lower limit was 0.0005% and the upper limit was 0.0050%.

Next, a method for producing the high-tensile steel of the present invention will be described.

The present invention is mainly intended for high-tensile steel having a tensile strength of 490 MPa or more. By applying the following manufacturing method, it is possible to obtain thick steel plates having various strengths.

In either case, before hot rolling, the ingot must first be 100% austenitized. Heating to austenite may be achieved by heating to Ac ₃ or higher.However, when heating exceeds 1250 ° C, austenite grains become coarse and the crystal grain size after rolling increases, resulting in a base material such as strength and toughness. Since the characteristics deteriorate, the heating temperature was set to Ar ₃ or higher and 1250 ° C or lower. In order to obtain good base metal properties, austenite grain size Need to be smaller. Since the austenite grain size is very large by heating the ingot, hot rolling is performed in the recrystallization temperature range where the austenite grain size can be reduced (normal rolling: about 900 to 1250 ° C) At a temperature of 10 to 95%.

According to the above-described production method by ordinary rolling, high-strength steel can be stably obtained at low cost. In this case, hot rolling is completed in the recrystallization temperature range, and natural cooling is performed. However, the strength may be insufficient when the plate thickness is large or when the added elements are small.

Controlled rolling (rolling in the non-recrystallization temperature range, approximately 750 to 900 ° C for high-strength steel) can produce high-strength steel with high strength and toughness. In this case, it is effective to introduce a deformation zone by rolling into the austenite grains to increase the number of ferrite-forming nuclei, and then to perform natural cooling. In order to introduce a deformation zone, hot rolling with a cumulative reduction of 40% or more in the non-recrystallization temperature region is required.However, if the cumulative reduction exceeds 90%, the base material toughness will decrease. Cumulative rolling reduction was set to 40 to 90%.

According to a manufacturing method combining controlled rolling and accelerated cooling, a high-tensile steel having a higher strength than a manufacturing method using only controlled rolling can be obtained. In this case, it is effective to accelerate the cooling to a temperature at which the transformation ends while keeping the C concentration in the fly high. In order to maintain the C concentration in the ferrite, it is necessary to cool at 1 ° CZ sec or more.However, when the temperature exceeds 60 ° CZ sec, the increase in strength peaks out, and the toughness decreases. The cooling rate was 1-60 ° C / sec. The temperature at which the transformation is completed is 600 ° C or lower, but the cooling stop temperature is usually set at 600 ° C to room temperature because the liquid above room temperature is usually used as the cooling medium.

The production method using controlled rolling, accelerated cooling and tempering heat treatment is even higher than the production method using a combination of controlled rolling and accelerated cooling. High strength steel having strength and toughness can be obtained. In this case, it is effective to recover the processed structure by dissipating dislocations or reducing the density of lattice defects due to coalescence. If the tempering temperature is lower than 300 ° C, these effects cannot be expected.At temperatures above the Ad point, transformation rather than recovery starts, so the tempering temperature and time are set to SOiTC Ac, point, 10 ~ 120 minutes. Example

Hereinafter, examples of the present invention will be described.

In order to investigate the effect of each element addition amount, 16 steels of the present invention and 16 comparative steels were melted, and a total of 24 steels were melted, and a 50 kg slab of 90 × 200 × 380 mm was manufactured in a laboratory. Table 1 shows the chemical composition and carbon equivalent of the test steel. The carbon equivalent was calculated according to the above equation.

Table 2 shows the manufacturing conditions of each steel (heating temperature, cumulative reduction rate in the recrystallization area, cumulative reduction rate in the non-recrystallization area, finishing temperature, cooling start temperature, cooling rate, cooling stop temperature, and tempering temperature). .

Here, the cumulative reduction in the recrystallization region is (h 0 — h 1) Zh 0, and the cumulative reduction in the non-recrystallization region is the reduction ratio defined by (h 1 — h 2) / h 1 . Where h0 is the slab thickness (mm), hi is the thickness after rolling in the recrystallization temperature range or the thickness before rolling in the non-recrystallization temperature range (mm), and h2 is the rolling in the non-recrystallization temperature range. It is the rear plate thickness (mm).

Each slab is heated to ₃ points or more of Ac and 1250 ° C or less, maintained for 60 minutes, hot-rolled in the recrystallization temperature range, and then cooled naturally or unrecrystallized without natural cooling After hot rolling with a cumulative rolling reduction of 40% to 90% in the temperature range, naturally cool or not cool, and cool at a cooling rate of 1 to SiTCZsec. Cool or temper by heating to the SOiTC Ad point With this, it was manufactured to a finished plate thickness of 15 mm.

As a result of measuring the mechanical properties of these hot-rolled sheets, the values of the yield stress, tensile strength, elongation at break, and Charpy impact value are also shown in Table 2.

Using this steel sheet, a T-shaped fillet weld fatigue test piece 1 shown in Fig. 1 was prepared. In the figure, 2 is a flat plate, 3 is a rib plate, and a corner 4 is formed by both plates, and this corner is welded. 5 is a weld metal. The shape of the test piece 1 was: a = 50 mm, b = 200 mm ^ c = 15nrai, d = 30 °, e = 15 mm. The welding method was sheathed arc welding and the heat input was 18 kJZcm. Table 3 shows the results of a three-point bending fatigue test of this test piece 1 with a stress ratio R (minimum stress Kano maximum stress) = 0.1. This table shows the values of the stress range when the number of repetitions was 1 × 10 + ⁵ and 2 × 10 + ⁶ . Table 4 shows the fraction of bainite in the HAZ structure of each steel and the crack arresting temperature by oblique y-shaped cracking test (JIS Z3158).

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· Οοοοοο ο ο ο ο ο οοοοοοοοο οοο Table 2 Manufacturing conditions Mechanical properties Area Speed Burning j Yield pull Shearing down Stress transition

CO rate (%) Down rate (%) (° C)

(° C / sec) l (.c) CO (MPa) | (MPa) CO

1 950 83 0 954 Shire 432 508 92

2 1100 83 0 1001 448 535 73

3 1100 72 40 858 496 583 47

4 1200 67 50 826 808 40 50 550 490 573 96 5 1230 72 40 857 841 10 500 516 588 98

6 1200 58 60 849 817 20 580 504 594 92

7 1160 72 40 851 829 40 100 600 473 569 61

8 1200 50 67 800 783 35 150 450 496 584 95 Light 9 1240 67 50 810 785 10 450 517 605 45

10 1150 72 40 823 Shiro ^ ¾ϊ 441 519 97 Steel 11 1200 72 40 810 439 533 73

12 1190 50 67 841 471 535 85

13 1210 72 40 866 445 524 71

14 1150 72 40 837 807 20 550 521 592 86

15 1100 50 67 851 824 15 70 500 479 563 84

16 1100 83 0 843 829 30 500 487 573 83

1 960 83 0 891 Legs 421 498 98 Ratio 2 1230 72 40 841 487 582 61 3 1200 67 50 844 826 40 120 550 470 553 93 4 1150 72 40 850 839 30 550 545 605 86 5 1130 50 67 868 841 20 500 505 587 94 Steel 6 1200 67 50 827 805 30 440 533 592 85 7 1200 58 60 816 797 50 50 500 469 562 65 8 1220 83 0 1050 mm 421 505 78

Breaking 222 33222 3222222223223232

4997337998748880 3985 1111% rejected 74 27 227 69733632925561111 Table 3 Fatigue test results (MPa) Steel grade 1 X 10 ⁵ times 2 X 10 ⁶ times Fatigue strength Fatigue strength

1 354 224

2 368 231

3 371 238

4 395 266 bottles 5 £ / U

6 388 258

7 388 258

8 375 247 Light 9 372 249

10 381 251 Steel 11 385 257

12, 383 252

13 387 259

14 one

a y o

15 388 251-

1 C

丄 0 394 268

1 271 167 Ratio 2 321 194

3 291 178 Comparison 4 303 189

5 286 173 Steel 6 308 184

7 323 191

8 327 199

Table 4

Steels 1, 2, and 3 of the present invention are examples in which the amount of added Si is three levels. Compared to the normal rolling steels 1 and 2 of the present invention, the steel 3 of the present invention, which has been subjected to controlled rolling at an unrecrystallized region cumulative rolling reduction of 40%, has a higher yield stress and tensile strength. Also, with the addition of Si, the fatigue strength increases, but the Charpy transition temperature also increases, indicating that there is an optimum amount of Si for practical use.

The steels 4 to 16 of the present invention to which at least one of the groups Cu, Mo, Ni, Cr, Nb, V, Ti, B, Ca, and REM were added also exhibited a solid solution with Cu and Mo, in addition to the effect of Si. Strengthening, effect of improving hardenability of Ni, Cr, V, suppression of recrystallization of Nb, suppression of coarsening of grains by Ti, N, grain boundary finishing by B The fatigue strength increased further than the steels 1 to 3 of the present invention due to the synergistic effect due to the sulfide suppression effect due to Ca and REM, and the sulfide suppression. Here, each manufacturing method of normal rolling, controlled rolling, controlled rolling + accelerated cooling, controlled rolling + accelerated cooling + tempering heat treatment is used, but compared to normal rolling, rolling by combining controlled rolling is performed. However, a high strength steel having higher strength was obtained even with the same carbon equivalent. In addition, the fatigue strength of a welded joint does not depend on the yield stress and tensile strength of the base metal. To improve the fatigue strength, the above-mentioned effects including solid solution strengthening of Si described in the present invention are considered. Is indispensable.

On the other hand, Comparative Steel 1 is an example in which the amount of Si added is smaller than the composition range of the steel of the present invention. Fatigue strength is improved when the amount of Si added is within the composition range of the steel of the present invention.

Comparative steels 2 to 8 with excessive addition of Cu, Mo, Ni, Cr, Nb, V, and B have fatigue strengths higher than those of comparative steel 1 because the amount of Si added is within an appropriate range. Although it is a high value, as can be seen from the payite structure fraction in Table 4, the comparative steels 2 to 8 have too high hardenability to form a martensite structure, and , The fatigue strength was lower than that of the steel of the present invention.

Also, when B was added excessively, the crack stop temperature in the oblique y-crack test also increased, and the weldability was extremely deteriorated. On the other hand, the crack stop temperature of the steel of the present invention was low and the weldability was good. Industrial applicability

According to the steel of the present invention, with regard to high tensile strength steel used in shipbuilding, offshore structures, bridges, etc., by controlling the structure of the heat-affected zone by adding a specific element while ensuring the weldability of the steel sheet. It is possible to improve the fatigue strength of the welded structure by using the steel of the present invention. It has become possible to improve reliability against fatigue failure.

Claims

The scope of the claims

1. In% by weight, C: 0.03 to 0.20%. Si: 0.6 to 2.0%, Mn: 0.6 to 2.0%, A1: 0.01 to 0.08%, N: 0.002 to 0.008%, B: 0.0020% or less, balance Fe and High strength steel with excellent weld strength and weldability characterized by inevitable impurities.

2. The high-strength steel according to claim 1, wherein the high-strength steel contains at least one member selected from the group consisting of: by weight% (11: 0.1 to 1.5% and Mo: 0.05 to 0.5%).

3. Claim 1 containing at least one selected from the group consisting of Ni: 0.1 to 3.0%, Cr: 0.1 to 1.0%, V: 0.01 to 0.10% and Nb: 0.005 to 0.06% by weight. Or High-tensile steel according to 2.

4. The high-strength steel according to claim 1, 2 or 3, comprising at least one selected from the group of 0.005 to 0.05% by weight, 0.0005 to 0.0050% of Ca, and 0.0005 to 0.0050% of REM by weight. .

5. The high tensile strength steel according to claim 1, which contains B in an amount of less than 0.0005% by weight.

6. By weight%, C: 0.03-0.20%, Si: 0.6-2.0%, Mn: 0.6-2.0%, A1: 0.01-0.08%, N: 0.002-0.008%, B: 0.0020% or less, balance Fe and Consisting of unavoidable impurities, Ni: 0.1 to 3.0%, Cr: 0.1 to 1.0%, V: 0.01 to 0.10%, Cu: 0.1 to: I.5%, Mo 0.05 to 0.5%, and Nb: 0.005 to 0.06 % Of at least one selected from the group consisting of the following components, and the heat-affected zone structure of the welded portion has a bainite structure fraction of 80% or more. High tensile steel with excellent properties.

7. By weight%, C: 0.03-0.20%, Si: 0.6-2.0%, Mn: 0.6-2.0%, A1: 0.01-0.08%, N: 0.002-0.008%, B: 0.0020% or less, balance Fe and A slab consisting of unavoidable impurities is Ac ₃ points or more A method for producing high-strength steel with excellent fatigue strength and weldability in welds, characterized by heating to 1250 ° C or lower, hot rolling in the recrystallization temperature range, and then cooling naturally.

8. The production of a high-tensile steel according to claim 7, wherein after hot rolling in the recrystallization temperature range, hot rolling with a cumulative rolling reduction of 40 to 90% is performed in the non-recrystallization temperature range, followed by natural cooling. Method.

9. After hot rolling in the recrystallization temperature range, subsequently perform hot rolling with a cumulative reduction of 40 to 90% in the non-recrystallization temperature range, and at a cooling rate of 1 to 6086, 600 ° C The method for producing a high-strength steel according to claim 7, wherein the cooling is stopped at room temperature to allow natural cooling.

10. After hot rolling in the recrystallization temperature range, subsequently perform hot rolling at a cumulative rolling reduction of 40 to 90% in the non-recrystallization temperature range, and after completion of hot rolling, cooling rate of 1 to 60 ° C / sec. 8. The method for producing a high-tensile steel according to claim 7, wherein after cooling in a temperature range of 600 ° C. to room temperature and natural cooling, the material is further heated to SOtTC Ac and tempered.

11. By weight%, Cu: 0.1 ~ 1.5%, Mo: 0.05 ~ 0.5%, Ni: 0.1 ~ 3.0%, Cr: 0.1 ~ 1.0%, V: 0.01 ~ 0.10%, Nb: 0.005 ~ 0.06% The method for producing a high-tensile steel according to any one of claims 7 to 10, comprising at least one selected from the group consisting of: Ti: 0.005 to 0.05%, Ca: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0050%.