JP2911725B2 - High-strength steel with excellent weld fatigue strength - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel which is used for shipbuilding, marine structures, bridges, etc. and which has excellent fatigue strength at welds.

[0002]

2. Description of the Related Art With the increase in the size of a structure, reduction of the weight of a structural member has become an important issue in recent years, and in order to realize this, steel used for the structure has been increased in tension. . However, since a ship, a marine structure, a bridge, and the like are repeatedly subjected to a load during a use period, it is necessary to take measures to prevent fatigue failure in such a structure. Since the portion where fatigue fracture is most likely to occur is the welded portion, it is required to improve the fatigue strength of the welded portion.

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. By reducing the degree of stress concentration by improving the shape of the weld toe, such as shaping the shape,
Improvements due to mechanical factors, such as those caused by the generation of compressive stress at the weld toe, such as shot peening, have been mostly found (JP-A-59-110490;
01823). Also, the effect of reducing residual stress by post-weld heat treatment has been well known.

[0004] On the other hand, there has been proposed a method of improving the fatigue strength of a welded portion by using a component of a steel material without using the above-described special construction or heat treatment after welding. JP-A-3-56301
In order to improve the joint fatigue strength of the welded joint by adding B to the gazette to refine the structure of the spot weld, suppress the grain growth of the HAZ, and prevent softening, , C: 0.006 wt% or less, Mn: 0.5
wt% or less, Al: 0.05 wt% or less, N: 0.00
6 wt% or less, P: 0.05% or less, and B: 0.
An extremely low carbon steel sheet composed of 0001 to 0.005 wt% or the like and having good spot weldability is disclosed.

Japanese Patent Application Laid-Open No. Sho 62-10239 discloses that Si
By increasing the amount, even at high C and high Mn levels,
C: 0.3% or less, Si: 0.7-1.1%, M
n: 2.0% or less, P: 0.16% or less, and so
A high-strength thin steel sheet comprising 1Al: 0.02 to 0.1% and having excellent spot weldability fatigue properties is disclosed. In Japanese Patent Application Laid-Open No. Hei 3-264645, good stretch flangeability, fatigue properties, and resistance weldability are obtained by improving the formation of clean polygonal ferrite by Si, strengthening steel by B, and improving hardenability. For that purpose,
C: 0.01-0.2%, Mn: 0.6-2.5%, S
i: 0.02-1.5%, and B: 0.0005-
A high-strength steel sheet comprising 0.1% or the like and having excellent stretch flangeability and the like is disclosed.

[0006]

Of these, Japanese Patent Application Laid-Open No. Sho 59-110490 and Japanese Patent Application Laid-Open No.
In Japanese Patent No. 823, it is necessary to perform special construction after welding, and it is not possible to improve the fatigue strength as it is. The method using heat treatment after welding is also not preferable because the number of steps increases and the welding operation becomes complicated. Also, the effect is limited. The steel sheet disclosed in JP-A-3-56301 is an ultra-low carbon steel (C: 0.
006 wt% or less), which is completely different from the welding method and components used for the thick steel plate targeted by the present invention. Further, the addition of B is not intended to suppress grain boundary ferrite.

The steel sheet disclosed in Japanese Patent Application Laid-Open No. 62-10239 relates to a high-strength thin steel sheet having excellent fatigue properties at spot welds, and this particularly defines the relationship between the amounts of C and P. (C: less than 0.22%, P: 0.16% or less, C: 0.22-0.3%, C + 0.6P ≦ 0.3
1) to improve the fatigue strength, and as in the present invention, to strengthen the solid solution of ferrite,
It is not the one to which i is added, but has a different basic idea.

The steel sheet disclosed in Japanese Patent Application Laid-Open No. 3-264645 is used for a base material of a vehicle wheel or a disk, and is a steel sheet used in shipbuilding and marine structures, which is an object of the present invention. The use is completely different from that described above, and it is not clear whether the fatigue strength of the welded portion is improved. The present invention is not to improve the fatigue strength by additional welding to reduce the stress concentration and to reduce the residual welding stress, but to control the structure of the weld heat affected zone as it is welded, high strength steel sheet It is an object of the present invention to provide a method for improving the fatigue strength of a welded part.

[0009]

The main principles of the present invention for solving the above problems can be summarized as follows. That is, the structure that easily becomes a fatigue crack initiation point is a soft ferrite phase, which can be divided into a grain boundary proeutectoid ferrite and a ferrite matrix. Therefore,
First, in order to suppress the grain boundary proeutectoid ferrite, B is added to improve the hardenability of the grain boundary, thereby suppressing the occurrence of fatigue cracks from the grain boundary and improving the fatigue strength.

In addition, with respect to the ferrite matrix, by adding an appropriate amount of Si, the hardenability is improved, and the solid solution strengthening is performed to improve the strength of the matrix. It is to improve. However, when N is combined with B to form a B nitride, the effect of improving the intergranular quenching property by solid solution B is lost by the reduced amount, and it becomes easy to generate intergranular ferrite, resulting in a decrease in fatigue strength. , N must be suppressed.

Furthermore, in order to reduce the volume fraction of ferrite, which is likely to cause fatigue cracks, and to obtain a bainite-based structure capable of obtaining a higher fatigue strength, Ni, Cr are used as elements for improving the hardenability of the structure. , V, and Cu and Mo as elements for further strengthening the solid solution of the ferrite structure and improving the hardenability, and Nb as an element for suppressing recrystallization of the ferrite. Ti is added as an element for suppressing the grain size from becoming coarse.

That is, the gist of the present invention is that, by weight%, C: 0.03 to 0.20, Si: 0.6 to 2.0.
0, Mn: 0.6 to 2.0, P: 0.02 or less, S
: 0.01 or less, Al: 0.02 to 0.05, B:
0.0005 to 0.0020, N: 0.008 or less, the balance being high-strength steel composed of Fe and unavoidable impurities, and further, Ni: 0.3 to 1.
0, Cr: 0.3 to 1.0, V: 0.01 to 0.10 Cu: 0.3 to 1.5, Mo: 0.1 to 0.5, Nb:
It is in steel containing at least one element in the range of 0.01 to 0.06 and Ti: 0.005 to 0.05.

[0013]

The reasons for limiting the components in the present invention will be described below. C is an element that increases the strength of the base material, and is desirably added in a large amount to increase the strength of the base material. However, the addition of more than 0.20% of C lowers the toughness of the base material and the welded portion, and deteriorates the weldability. Therefore, the upper limit of C is 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 furthermore, the hardenability of the welded portion is reduced, and the formation of grain boundary proeutectoid ferrite harmful to fatigue strength is caused. As described above, if C is less than 0.03%, a structure desirable for improving fatigue strength cannot be obtained.
3%.

Si is a ferrite solid solution element, and ferrite, which is liable to cause fatigue cracking when dissolved,
Strengthen the matrix. Also, Si reduces stacking fault energy and reduces cross-slip,
It has the effect of suppressing localization of deformation during repeated plastic deformation and increasing the reversibility of plastic deformation, thereby suppressing the occurrence of cracks. Therefore, to improve fatigue strength,
Si is an essential element. If Si is less than 0.6%, the deoxidizing effect at the time of smelting is reduced, the effect of solid solution strengthening and the effect of reducing stacking fault energy are small, and improvement in fatigue strength cannot be expected. Therefore, the lower limit is set to 0.6%. Conversely, if more than 2.0% of Si is added, the surface properties are deteriorated due to the generation of red scale, and this becomes a crack generation source, so that the fatigue strength is deteriorated. Therefore, the upper limit is set to 2.0%.

Mn is an element that increases the strength of the base material without significantly reducing the toughness. If Mn is less than 0.6%, sufficient base material strength cannot be obtained and S embrittlement is likely to occur, so the lower limit was set to 0.6%. Further, when the content of Mn exceeds 2.0%, not only does the toughness of the welded portion decrease, but also the weldability and ductility deteriorate, so the upper limit was made 2.0%. P is preferably as low as possible, and if it exceeds 0.02%, the toughness of the base metal and the welded portion is significantly reduced. Therefore, the upper limit is made 0.02%, preferably 0.01%. S is also preferably as low as possible, and if it is contained more than 0.01%, A-based inclusions become remarkable, impairing the toughness of the base material and the welded portion,
It also reduces ductility in the thickness direction. Therefore, the upper limit is set to 0.0
1%.

Al is necessary as a deoxidizing element, and 0.02
If not more than 10%, deoxidizing action cannot be expected. on the other hand,
If it is added in an amount of 0.05% or more, a large amount of Al oxide or nitride is generated, and the toughness of the weld is deteriorated. Therefore, the upper limit is set to 0.05%. B is an element essential for improving the fatigue strength of the welded portion, because it increases the hardenability of the grain boundaries and suppresses the formation of grain boundary proeutectoid ferrite, which is a crack generation source. However, even if only the grain boundary pro-eutectoid ferrite is suppressed, if the matrix is not strengthened, cracks will be generated from the matrix side, so it is necessary to add Si and B at the same time. The amount of B added is 0.000.
If it is less than 5%, the effect of increasing grain boundary hardenability is small, so the lower limit was made 0.0005%. Further, when the content exceeds 0.0020%, B nitrides and borocarbides are precipitated and the toughness is reduced. Therefore, the upper limit is made 0.0020%. N is preferably as small as possible. In particular, since the addition of B is indispensable in the steel of the present invention, if the content exceeds 0.008%, the amount of solute N increases to form B precipitates, thereby generating grain boundary ferrite and adversely affecting the fatigue strength. The upper limit was limited to 0.008%.

Ni, Cr, and V are all elements that improve the hardenability of the base metal and the weld. For each element, 0.3%,
0.3% and 0.01% were defined as lower limits. Further, excessive addition tends to form lower bainite or martensite structure, and rather lowers the fatigue strength of the welded portion. Therefore, the upper limit of each is 1.0%, 1.0%, 0.10%. And

Although Cu and Mo also improve the hardenability of the base material and the welded portion, these elements are effective for strengthening the matrix by solid solution strengthening rather than Si. However, stacking fault energy does not decrease as much as Si. The effect is not remarkable unless 0.3% and 0.1% or more are added, respectively. Also, 1.5
% Or more than 0.5%, the hardenability is too high, and the formation of martensite causes the fatigue strength to decrease, so the upper limit was set.

Nb is an element having an effect on increasing the strength of the base material. Further, when the TMCP process is applied during the production of a steel sheet, Nb is added to suppress recrystallization during rolling.
It is desirable to add 1% or more. However, Nb
When a large amount is contained, the toughness of the welded portion is reduced. Therefore, the upper limit of Nb is set to 0.06%. Ti combines with N to form TiN, which improves the fatigue strength and the HAZ toughness by reducing the structure of the heat-affected zone. For that purpose, 0.005% or more is necessary,
No further effect is seen with the addition of 0.05% or more, so the lower limit was set to 0.005% and the upper limit was set to 0.05%.

[0020]

Embodiments of the present invention will be described below. In Table 1,
The chemical composition of the steel material manufactured by vacuum melting is shown. Thirteen steel grades of the present invention and eleven steel grades for comparison were dissolved in a total of 24 grades.
After melting in vacuum, heating at 1100 ° C for 30 minutes
mm, which was used as a material for a fatigue test. Table 2
2 shows the yield point (YS), tensile strength (TS), and uniform elongation (UEL) as a result of the mechanical test of the base material. This was welded to prepare a T-type fatigue test piece as shown in FIG. Table 3 shows the results of a three-point bending fatigue test performed on the test piece at a stress ratio R (minimum stress / maximum stress) = 0.1.
Shown in In this table, the number of repetitions is 10e ^{+ 5} times, 10
e The value of the stress range when the number of times reaches ⁺⁶ is shown.

First, the comparative steel 1, which contains less C and Mn, only Si and B are less than the steel 1 of the present invention, and the other elements are comparative steel 2, which is within the range of the inventive steel. The comparative steel 3 which is out of the range of the inventive steel and the comparative steel 4 in which only the addition amount of Si is out of the range were clearly superior in the fatigue strength of the inventive steel 1. This is due to the synergistic effect of solid solution strengthening of Si and improvement of grain boundary hardenability of B, as described above.
This is because the fatigue strength of the weld was improved.

Further, Cu, Ni, Cr, Mo, Nb,
Inventive steels 2-1 containing one or more of V and Ti
No. 1, in addition to the effect of Si and B, the effect of improving the hardenability of Ni, Cr and V, the solid solution strengthening by Cu and Mo,
Due to the synergistic effect of suppressing the recrystallization of Nb and suppressing the coarsening of crystal grains by Ti, the fatigue strength was improved as compared with Comparative Steels 5 to 11 in which the above elements were added without adding Si and B. Furthermore, the steels 12 and 13 of the present invention to which a large amount of the impurity element P was added also had improved fatigue strength.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

As described in detail above, according to the steel of the present invention, the structure of the heat-affected zone is controlled by adding a specific element to high-strength steel used for shipbuilding, marine structures, bridges, and the like. Thereby, the fatigue strength can be improved, and the reliability of the welded structure against fatigue fracture can be improved by using the steel of the present invention.
The significance of the steel of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a view showing the shape of a fatigue test piece of a T-shaped fillet welded joint.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-43980 (JP, A) JP-A-3-14037 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/06

Claims (5)

(57) [Claims] C: 0.03 to 0.20; Si: 0.6 to 2.0; Mn: 0.6 to 2.0; P: 0.02 or less; 01 or less, Al: 0.02 to 0.05, B: 0.0005 to 0.0020, N: 0.008 or less as a basic component, with the balance being Fe and unavoidable impurities, the fatigue strength of the welded portion Excellent high tensile steel. 2. The composition according to claim 1, wherein at least one of Ni: 0.3 to 1.0, Cr: 0.3 to 1.0, and V: 0.01 to 0.10. The high-tensile steel according to claim 1, which has excellent fatigue strength at the welded portion. 3. The method according to claim 1, wherein at least one of Cu: 0.3 to 1.5 and Mo: 0.1 to 0.5 is contained in% by weight. High strength steel with excellent fatigue strength at welds. 4. The welded joint according to claim 1, wherein the alloy contains at least one of Nb: 0.01 to 0.06 by weight. Tension steel. 5. The fatigue strength of a welded part according to claim 1, characterized in that it contains at least one kind of Ti: 0.005 to 0.05 by weight%. High strength steel.