JPS5831065A - Steel for welded structure - Google Patents

Abstract

PURPOSE:To obtain a steel for a welded structure causing little deterioration of the toughness of the heat-affected zone even in high heat input welding by adding a restricted amount of B to a steel contg. C, Si and Mn with a specified relation and very small amounts of Al and Ti. CONSTITUTION:This steel for a welded structure consists of <=0.09% C, <=0.50% Si, <=1.30% Mn, <=0.006% N, 0.002-0.005% B, 0.005-0.100% Al, 0.005-0.020% Ti and the balance Fe with inevitable impurities while satisfying Ceq(C+1/24Si+ 1/6Mn)<=0.34%. The added B precipitates at the grain boundaries and enhances the hardenability of the grain boundaries, and simultaneously fine B nitride is precipitated at the grain boundaries to improve the toughness of the steel. In case of Ceq<=0.34%, the effect of B is increased, and by adding said very small amount of Al, it is further increased.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to welded structural steel used as shipbuilding steel, bridge steel, and pressure vessel steel.

Recently, many structures such as ships and bridges are manufactured using welded structures rather than riveted structures. Compared to riveted structures, welded structures are made of plate materials that are easier to obtain, and structures with various shapes and cross sections can be easily manufactured with a simple design.
The manufacturing precision has also been improved by the development of advanced welding and cutting techniques. However, welding is a method of joining the steel materials to be joined using molten steel as a medium, and rivets are inserted into holes drilled in the steel materials to be joined. Compared to riveted joints, which are a mechanical connection that is tightened with heat, some parts of the steel become brittle due to the effects of high heat, and there is a risk of destroying the structure, so care had to be taken in selecting the steel material.

Therefore, for welding joints, a steel material with guaranteed weldability that does not deteriorate the toughness of the so-called weld heat-affected zone must be selected, and the welding work must be carried out carefully, which can be dangerous and interfere with on-site work. .

The present inventors have conducted many experiments with the aim of solving the above-mentioned problems and providing a steel that does not significantly deteriorate the toughness of the weld heat affected zone even during high heat input welding due to welding automation. Ta. As a result, it was found that the desired steel could be obtained by adding 0.002 to 0.0051 B at C@q O, 84 or less. The present invention is a steel obtained based on this result, and the composition of the half steel is C: 0.0'1 or less, 1iii: 0.501 or less, Mu
: 1.804 or less, N: 0.006 or less, B: 0
．． 002~0.0051, AA:0.005~0.1
004, Tl: Contains 0.005 to 0.020 tatami,
C@q: It is a welded structural steel with a coefficient of 0.34 or less, the remainder consisting of iron and unavoidable impurities.

The reason why the steel components of the present invention are limited as described above will be explained in detail below with reference to Examples.

C is an effective component for providing the required strength, but on the other hand, it has the property of increasing the transition temperature and making the steel brittle. However'
Therefore, for the purpose of the present invention, the upper limit of C was set to 0.09, and the strength reduced by this limitation was guaranteed at 81% of 0.504 or less and 1.30% or less. In particular, Mn is an effective surface component that lowers the transition temperature and improves toughness, but its excessive content tends to increase strength and reduce toughness, similar to St. N is the toughness of the weld heat affected zone, and K
has a detrimental effect on the toughness after weld heat-affected zone post-treatment (stress relief annealing) and also suppresses the activation of B. Therefore, the upper limit of N contained in steel was set at 9.006%.

The smaller the amount, the better.B precipitates at the grain boundaries and improves the hardenability of the grain boundaries, and at the same time, fine nitrides precipitate within the grains and precipitate from the grain boundaries as transformation nuclei of ferrite. This suppresses coarse pro-eutectoid ferrite and precipitates fine ferrite within the grains, improving the toughness of steel.

The effect like this is C・q, that is, C@q = C+
Furthermore, a large amount of aluminum can be obtained by containing a small amount of aluminum.

That is, B is 0.002 to 0.005% from the toughness of the steel,
Further, aluminum is contained in an amount of 0.005 to 0.1001 for activating B and refining ferrite. , contents that deviate from these ranges have weak effects. 0
．． Tl of 0.0051 or more is an effective component for activating B by forming T1 nitrides, and an excessive content of more than 0.02096 precipitates T1 carbides and deteriorates toughness. Therefore, the Ti content was estimated to be 0.005 to 0.020.

In addition, PlS and the like that are inevitably mixed into steel as impurities deteriorate various properties of steel such as cracking, deterioration of toughness, and reduction of strength during the manufacturing process or use process, so it is necessary to reduce it as much as possible.

The steel of the present invention is melted in a normal melting furnace such as a converter or an electric furnace,
Furthermore, after being subjected to vacuum degassing treatment, it is made into a steel billet by ingot, blooming or continuous casting, and then hot rolled or further tempered, thus obtaining a steel material with high toughness.

Next, examples of the present invention will be described.

Table 1 shows the chemical composition of the steel plate obtained by ingot-forming, blooming and hot-rolling molten steel produced in a converter, and Table 2 shows the mechanical properties of the steel plate after hot rolling and welding. .

As is clear from the experimental results in Table 2 and above, the steels of the present invention containing B (A-F) have higher strength and toughness than the comparative steels (G, H) that do not contain B, and are particularly resistant to welding heat. Uke also has excellent tenacity at nine o'clock.

Claims (1)

[Claims] C: 0.091 or less, st: o, so or less, Mn: 1
．． 30 or less, N: 0.006 or less, B: 0.002
~o, oos, At: 0. O05~o, 1001. T
i:0.005 to 0.0204 and C*q:
Steel for welded structures with a modulus of 0.34 or less and the remainder consisting of iron and unavoidable impurities.