JPH0757883B2 - Method for producing 80 kg high-strength steel with excellent galvanizing resistance in weld heat-affected zone - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a steel material used for a steel structure (steel tower, bridge, building, etc.) that is hot dip galvanized after welding, particularly, a hot-dip zinc of a welded portion. The present invention relates to 80 kg high strength steel with excellent galling resistance.

(Prior Art) Conventionally, the above-mentioned steel structures are often hot dip galvanized from the viewpoint of corrosion resistance. However, in these structures, when the members are hot dip galvanized, cracks may occur mainly in the welded portions, and preventive measures are required for the safety of the structures.

This crack is a phenomenon known as galvanized crack due to liquid metal embrittlement, and it is empirically known in the industry that cracks are more likely to occur as the strength of the steel material increases.

Several proposals have been made so far as measures to prevent this galvanization. For example, JP-A-59-
In Japanese Patent No. 50157, measures are taken by limiting the amount of [S] in steel to 0.030 to 0.060%, Japanese Patent Laid-Open Nos. 61-133363, 61-231141, and 62-5044. Have proposed a measure by satisfying a specific relationship with the amount of alloying elements in a steel material.

However, as for steel strength, these proposed technologies are intended for steel materials up to 60 kg high-strength steel, and for steel materials with higher strength, it is not always sufficient to completely prevent galling, Especially for 80 kg steel, there is almost no crack prevention technology, and new technology is required.

(Problems to be Solved by the Invention) As described above, in a steel structure to be hot dip galvanized for the purpose of preventing rust after welding and assembling, zinc embrittlement may occur at the weld toe during plating. However, establishment of preventive measures is also desired from the viewpoint of safety of structures.

It is considered that this zinc embrittlement is governed by the magnitude of the zinc embrittlement susceptibility based on the structure factor near the weld toe, the residual welding stress acting on that portion, and the thermal stress during plating.

It has been conventionally known that the higher the strength of steel, the higher the content of alloying elements, and thus the increase in susceptibility to zinc embrittlement is inevitable. Quenching can be considered as one of the means for improving the base metal strength without increasing the alloy addition amount. However, although it is easy to secure the strength of the as-quenched steel material, it is difficult to stably obtain the required toughness as a structural steel,
It is essential to establish a solution.

On the other hand, it is known that the welding residual stress, which is one of the controlling factors, is reduced by heating the welded portion, and it has been confirmed that the same phenomenon occurs in the plating process. However, since the magnitude of this welding residual stress is governed by the yield strength of the base material, the higher the yield strength of steel, the higher the residual welding stress that remains in the plating bath, and the more likely zinc embrittlement occurs.

The above-mentioned factors are all negative factors for 80 kg steel, and it was considered that it is extremely difficult to prevent zinc embrittlement of 80 kg steel.

In view of the above situation, the present invention proposes a method for producing an 80 kg high strength steel capable of completely preventing hot dip galvanizing.

(Means and Actions for Solving Problems) The present invention has been made to solve the above problems, and the gist thereof is that C: 0.20% or less by weight%, Si: 0.35%
Below, Mn: 1.7% or less, 1.0 in total of one or two of Cr and Mo
% Or less, Al: 0.005% to 0.10%, and if necessary for strength and toughness, Cu: 1.0% or less, Ni: 1.0% or less, V: 0.2% or less, Nb:
0.05% or less, Ti: 0.03% or less, 1 type or 2 or more types, and the balance consists of Fe and impurities, and at the same time Ceq (Z) = C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ A steel material satisfying 0.58 is rolled at a rolling reduction of 30% or more in the austenite unrecrystallized region during rolling, then directly quenched from a temperature range of 900 ° C to 800 ° C, and hot-dip galvanized at 430 ° C to 480 ° C. It is a method for producing a 80 kg high-strength steel having excellent galvanizing resistance in the weld heat-affected zone, which is characterized in that a steel having a tensile strength of 80 kg or more is obtained.

The reasons for limitation in the present invention will be described below.

As described above, the limiting requirements of the present invention are the individual contents of various alloying elements, the limitation of Ceq (Z)% by the combination thereof, and the limitation of manufacturing conditions, and the effect of the present invention is to meet all of these requirements. It will be exhibited only when satisfied, and its effect will not be exhibited unless one of the requirements is met.

First, the reason why the content of each alloying element is limited to the above range will be described.

C is added to secure the strength, but if it exceeds 0.20%, not only the toughness and weldability of the steel material are impaired, but also the galvanizing resistance is significantly impaired, so 0.2% was made the upper limit.

Si is added to secure strength and deoxidize, but if it exceeds 0.35%, the toughness deteriorates and the soundness of the plated surface is impaired, so this was made the upper limit.

Mn is added to secure the strength, but if added in excess of 1.7%, the weldability and galvanizing resistance will be significantly impaired, so this was made the upper limit.

Cr and Mo are extremely effective elements for increasing the hardenability and ensuring strength by adding a small amount. However, if the total amount of one type or two types added exceeds 1.0%, the galvanizing resistance is significantly impaired, so this was made the upper limit.

Al is a range that is usually used as a deoxidizing element 0.00
Limited to 5 to 0.100%.

Cu, Ni, V, Nb, and Ti are elements added for the purpose of improving strength and toughness, but if added in excess of the above limits, the weldability and galvanizing resistance will be impaired, so this is the upper limit. And

In the present invention, as described above, while limiting the individual alloy element addition amount, the total addition amount combining these is
The effect is exhibited only when a specific formula is satisfied, and this point will be described below with experimental results.

The reason is that the chemical composition of the steel material used is limited by the formula of Ceq (Z) = C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ 0.58 as the total addition amount. The equivalent formula is
The effect of various alloying elements on the zinc embrittlement of the weld heat affected zone is quantified and formed. The lower this value, the less likely the zinc embrittlement due to the above-mentioned structural factors near the weld toe occurs.

Therefore, the steel composition should be within Ceq
It is desirable to lower the (Z) value.

First, the experimental method and experimental results that obtained this new knowledge
Shown in FIG.

The experimental method is shown in FIG.

In FIG. 1, 1 is a test plate, 2 is a test bead, and 3 is a restraining bead for giving a residual stress to the test bead.

In this experiment, after applying stress to the toe of the test bead 2 with the restraining bead 3, the test piece was dipped in a zinc bath, and the presence or absence of galvanized cracks at the toe of the test bead,
This is to evaluate the galvanizing resistance of steel materials.

Note that galvanized cracks are more likely to occur as long as the applied stress is higher if the same steel material is used.

According to the present experimental method, it is possible to apply residual stress equivalent to the yield strength of the test plate at room temperature to the vicinity of the toe of the test bead with 5 passes of the restraining bead. All were 5 passes. Table 1 shows the welding conditions for the test and restraining beads.

Under the above conditions, test pieces were made from steel materials having different amounts of various alloy elements added, and after immersing the test pieces in a zinc bath, the presence or absence of cracking at the toe of the test bead was investigated.

The experimental results are shown in FIG. 2 in relation to Ceq (Z).

As is clear from the figure, it was confirmed that the occurrence of galvanization can be completely prevented if the content of various alloy elements is within the above-mentioned limited component range and the Ceq (Z)% is 0.58% or less.

Next, the reasons for limiting the manufacturing conditions will be described.

As mentioned above, to prevent galvanizing, Ceq
It is necessary to set (Z) to 0.58% or less, but the steel material satisfying this condition is a low-component steel compared with the conventional 80 kg steel, so it is necessary to secure the strength by quenching.

Although it is easy to secure the strength of the base metal by quenching, it is difficult to secure the toughness of the as-quenched steel for use as structural steel, and it is common to improve the toughness by tempering.

However, since the steel of the present invention has a low composition as described above, strength reduction is large when ordinary tempering is performed, and it is difficult to secure the desired strength.

Therefore, in the steel of the present invention, it was examined to secure strength and toughness by low temperature tempering applying heating during hot dip galvanizing for tempering. Here, the temperature of the hot-dip galvanized by normal hot-dip galvanizing is 430 ° C to 480 ° C. However, in the case of such low temperature tempering, it was found that the toughness after tempering fluctuates significantly depending on the quenched structure.

The steel of the present invention has excellent galvanizing resistance due to the limitation of the addition amount of alloying elements, and at the same time, it is easy to secure the strength by low temperature tempering by hot dip galvanizing at 430 ° C to 480 ° C. Since it does not meet the requirements as a kilo steel, the optimum manufacturing conditions were examined for improving the toughness of the base metal.

As a result, when the martensite and the lower bainite structure ratio are high in the as-quenched state, the recovery of toughness by tempering is small, and it is difficult to secure sufficient toughness as a structural steel,
It has been found that lowering the structure fraction is an essential condition for ensuring the toughness in low temperature tempering.

The manufacturing conditions limited in the present invention were made on the basis of such findings, and the quenching start temperature after the end of rolling and the application of a reduction ratio of 30% or more in the austenite unrecrystallized region is 90%.
By limiting the range to 0-800 ℃, the quenching structure will be a fine upper bainite and a small amount of martensite + lower bainite, so even if it is low-temperature tempered by hot dip galvanizing at 430 ℃ -480 ℃, it will be structural 80kg steel. As a result, it was found that sufficient strength and toughness can be obtained.

When the rolling reduction in the austenite unrecrystallized region is less than 30% and the quenching start temperature is over 900 ° C, the martensite + lower bainite structure fraction is high and the toughness is poor, and when the quenching start temperature is less than 750 ° C. It has a ferrite + upper bainite structure and cannot secure strength.

(Examples) The effects of the present invention will be specifically shown by the following examples.

The resistance to galvanizing was determined by the test method shown in FIG.

Table 2 shows the composition of the tested steel, Ceq (Z), manufacturing conditions, strength / toughness of the base material after hot dip galvanizing, and the results of the galvanizing resistance evaluation test. Here, the hot dip galvanizing temperature was 450 ° C.

It is apparent that the steel satisfying the requirements of the present invention has sufficient strength and toughness as a structural 80 kg steel and excellent galvanizing resistance.

(Effects of the Invention) As is clear from the above description, by limiting the amount of each alloying element added and the total amount of addition thereof, and by limiting the production conditions, excellent galvanizing resistance is obtained. 80kg high strength steel can be manufactured.

Therefore, it can be said that the present invention has a great effect industrially.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a small-sized galvanized crack resistance evaluation test method, and FIG. 2 is a table showing changes in galvanized resistance due to various alloy element addition amounts and Ceq (Z) values.

Claims (1)

[Claims] 1. C: 0.20% or less in weight% Si: 0.35% or less Mn: 1.7% or less 1.0% or less in total of one or two of Cr and Mo Al: 0.005% to 0.10% Further required strength and toughness Depending on the above, Cu: 1.0% or less Ni: 1.0% or less V: 0.2% or less Nb: 0.05% or less Ti: 0.03% or less 1 type or 2 types or more and the balance consists of Fe and impurities, and at the same time Ceq (Z) = After rolling a steel material satisfying C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ 0.58 at a rolling reduction of 30% or more in the austenite unrecrystallized region during rolling, 900 ° C to 800 ° C Excellent in galvanizing resistance of weld heat-affected zone, characterized by obtaining steel satisfying tensile strength of 80 kg or more after hot-dip galvanizing at 430 ° C to 480 ° C by directly quenching from temperature range of ℃ 80 kg high strength steel manufacturing method.