JP2959898B2 - High tensile steel for Zn-Al plating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing molten Zn-A
In steel structures (bridges, steel towers, buildings, etc.) that are to be plated with alloys, 4
High strength steel of 90MPa class or higher.

[0002]

2. Description of the Related Art Conventionally, the above-mentioned steel structures are often hot-dip plated due to corrosion resistance. Currently generally molten Zn
Although plating is often performed, Zn plating is inferior in corrosion resistance at the shore, and Zn-Al alloy plating excellent in shore corrosion resistance is being studied.

[0003] However, if the structural member becomes large, cracks may occur mainly in the welded portion during the hot-dip plating, and measures for preventing the crack are necessary for the safety of the structure. Until now, there has been no disclosure of a steel material taking measures against cracking of Zn-Al alloy plating, and a new technology is required.

[0004]

Conventionally, in the case of hot-dip Zn plating, a welded structure is subjected to hot-dip Zn plating after being welded and assembled in accordance with the intended use of a steel material. In the same way, a welded structure is manufactured using the current high-strength steel,
-Al alloy plating may cause cracks due to liquid metal embrittlement at the weld toe. The present invention has been made in view of the above situation, and provides a high-strength steel that does not cause cracks due to liquid metal embrittlement at a weld in a hot-dip Zn-Al plating step.

[0005]

SUMMARY OF THE INVENTION The present invention is directed to a steel comprising Zn-A
l If cracking susceptibility during plating is greatly affected by trace B as an impurity and if B is 0.0002% or less,
CE = C + Si / 30 + Mn / 4 + (Ni + Cu) / 1
This is because it has been found that if the value calculated by 0+ (Cr + Mo + Nb + V) / 3 is 0.54% or less, no crack occurs. Further, it has been found that a quenching critical diameter Di (cal) is required to be 18 mm or more in order to obtain strength, and it is possible to obtain a steel material that can be used as a structure only after satisfying both of them.

That is, C: 0.10-0.
20%, Si: 0.35% or less, Mn: 0.5 to 1.2
%, Ni: 0.7% or less, Cu: 0.8 to 1.5%, A
l: 0.05% or less, Ti: 0.005 to 0.02%,
N: 0.003% to 0.01% Further, Cr: 0.3% or less, Mo: 0.4% or less, N
b: 0.03% or less, V: 0.1% or less, one or two
At least the seeds and the remainder consist of Fe and impurities,
= C + Si / 30 + Mn / 4 + (Ni + Cu) / 10 +
(Cr + Mo + Nb + V) /3≦0.54% and B as an impurity: 0.0002% or less, and the quenching critical diameter Di (cal) is 18 mm or more, and the Zn-Al alloy-resistant cracking is characterized in that: High tensile strength steel with excellent resistance. However

(Equation 2)

[0007]

The present invention will be described below in detail. First, in the present invention, Zn-Al alloy plating is 20 to 60% by weight.
% Of Al, 40 to 80% of Zn, and if necessary, 10% or less of Mg, Si, Cr or the like is added to the hot-dip galvanized steel having excellent corrosion resistance to the welded structure. In addition, the crack here means that the above-mentioned welded structure is Zn-A
This refers to cracks mainly generated at the weld toe due to liquid metal embrittlement when alloy plating.

Hereinafter, the reasons for limitation in the present invention will be described. As described above, the limiting requirements of the present invention are that the content of various alloying elements and the restriction of CE by their combination,
i (cal) is limited. The effects of the present invention are exhibited only when all of these requirements are satisfied. When any of the requirements is not satisfied, the effects are not exhibited.

First, the reason why the alloy element content is limited to the above range will be described. C is necessary to obtain strength,
If it is less than 0.10%, the effect is insufficient. If it exceeds 0.20%, not only the toughness and weldability of the steel material are impaired, but also the melting
-Al plating cracking property is significantly impaired, so 0.10 to
20%.

[0010] Si is added as a deoxidizing element at the time of steel making, but if it exceeds 0.35%, the toughness is reduced and the soundness of the plated surface is impaired.

Mn is an element effective for ensuring hardenability.
If it is less than 5%, the effect is insufficient, and if it exceeds 1.2%, the plating cracking property is significantly impaired.

Although Ni is effective in preventing surface damage due to Cu in Cu-added steel, the effect is saturated even if it exceeds 0.7%, so the upper limit is set to 0.7%.

[0013] Cu is an element that is remarkably age-hardened and precipitates and hardens during plating, and is most effective in ensuring the strength of this type of plated steel. If it is less than 0.8%, the age hardening is small, and even if it exceeds 1.5%, it becomes small.
To 1.5%.

Further, Al is not only effective for deoxidation, but also an effective alloying element that fixes N to become AlN and also plays a role of grain refinement. Since the effect is sufficient at 0.05%, the upper limit is set to 0.05%.

[0015] Ti is effective in reducing the crystal grains in the heat affected zone by bonding with N, and improves the plating crack resistance. If less than 0.005%, the effect is insufficient,
Since the amount is sufficient at 0.02%, 0.005%
0.02%.

N combines with Ti to perform the same function as Ti. If the amount is less than 0.003%, the effect is insufficient, and the amount is 0.01%.
To 0.01%.

Cr, Mo, Nb, and V are elements added as needed for the purpose of improving strength and toughness.
Cr: 0.3% or less, Mo: 0.4% or less, Nb: 0.
03% or less, V: 0.1% or less added by one or more kinds
Ru pressurized to, but because resistance to plating cracking when added beyond the limited range is impaired, so this was made the upper limit.

The above are the basic components of the steel to be used in the present invention. The content of B contained as an unavoidable impurity in the present invention is 0.0002% or less, and
Must satisfy 0.54% or less.

The CE equation quantifies the effect of various alloying elements on the cracking property of the weld heat-affected zone during hot-dip Zn-Al alloy plating. The lower this value, the more the liquid metal embrittlement by the molten Zn-Al alloy Is less likely to occur, and the steel component desirably has a lower CE as long as the strength is satisfied.

These amounts and the cracking property of the Zn—Al alloy plating were determined by the following experiments. The experimental method is shown in FIG. In the figure, 1 is a test piece, 2 is a test bead, and 3 is a restraining bead for applying a residual stress to the test bead. In this experiment, stress was applied to the toe of the test bead by the constrained bead of 3, and then the test piece was immersed in a molten Zn-Al alloy bath to reduce liquid metal embrittlement at the toe of the test bead. The purpose of the present invention is to evaluate the plating crack resistance of steel materials based on the presence or absence of cracks based on the results. It should be noted that plating cracks are more likely to occur as the applied stress is higher for the same steel material.

In this experiment method, since the residual stress corresponding to the yield strength of the test piece can be applied to the vicinity of the test bead toe with the number of restraining beads in 20 passes, the number of restraining beads in this experiment is all 20 passes. And Table 1 shows the welding conditions for the test and the constraint bead.

[0022]

[Table 1] Under the above conditions, B is 0.0002% or less and CE is 0.5%.
It was found that if it was 4% or less, no cracking occurred.

Next, in the present invention, the quenching critical diameter Di is further determined.
The fact that (cal) satisfies 18 (mm) or more is one of the main points. Di (cal) is a formula for regression calculation of the maximum diameter of the maximum diameter (50% martensite at the center) where the round bar is quenched to the center when water cooled as quickly as possible.
Shown in units.

In this case, if the Di (cal) is less than 18, the strength is too low, and it becomes difficult to produce a high-tensile steel of 490 MPa class or more. in this case,

(Equation 3) This formula is derived from the formula proposed by Grossman on September 25, 1979, “Hardenability”, first edition of Nikkan Kogyo Shimbun, page 34, line 5, C% and grain size. (In this case, Nγ = 8)
The influence of various additional elements is obtained by multiplying the multiple of each element by the amount of the element.

[0025]

EXAMPLE A JIS No. 4 tensile test piece was sampled from 1/4 t of a steel sheet of 40 mm in steel A to J having the chemical components shown in Table 2 and a tensile test was conducted. Further, the cracking test described above was performed.

[0026]

[Table 2] A to E are steels of the present invention, and their chemical components, CE, and Di
(cal) was satisfied, the tensile strength was 490 MPa or more, and no cracks occurred in any of the crack tests.

F to J are comparative steels. F is Cu, Ni,
Since Ti was not added, Di (cal) was also low at 15.4, and the strength was insufficient. G also has no added Cu, Ni and Ti, but has a high Mn and a strength of 490 MPa or more.
CE became 0.55% and cracks occurred. In H, C,
Cu is low, and Mn and B are high. As a result, cracks occurred. In I, cracks occurred because Mn and B were high. In J, Cr, Al and B were high and cracks occurred.

[0028]

According to the present invention, it is possible to provide a high-strength steel with a significantly improved cracking property in hot-dip Zn-Al alloy plating as compared with the conventional material, and the effect is extremely industrial. Notable.

[Brief description of the drawings]

FIG. 1 is a diagram showing a test method for evaluating hot-dip Zn—Al alloy plating cracking properties.

[Description of Signs] 1 Test piece 2 Test bead 3 Restraint bead

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C23C 2/12 C23C 2/12 (72) Inventor Kazuhiro Masuda 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division (56) References JP-A-1-198449 (JP, A) JP-A-5-65593 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38 / 16 C22C 38/50 C23C 2/06 C23C 2/12

Claims (2)

(57) [Claims] 1. Weight (%) of C: 0.10 to 0.20%, Si: 0.35% or less, Mn: 0.5 to 1.2%, Ni: 0.7% or less, Cu : 0.8 to 1.5%, Al: 0.05% or less, Ti: 0.005 to 0.02%, N: 0.003 to 0.01% , the balance being Fe and impurities , At the same time CE
= C + Si / 30 + Mn / 4 + (Ni + Cu) / 10 +
(Cr + Mo + Nb + V) /3≦0.54% and B as an impurity: 0.0002% or less, and the quenching critical diameter Di (cal) is 18 mm or more, and the Zn-Al alloy-resistant cracking is characterized in that: High tensile steel with excellent properties. However, 2. One or more of the following by weight (%) : Cr: 0.3% or less, Mo: 0.4% or less, Nb: 0.03% or less, V: 0.1% or less. Claims characterized by containing
High tensile strength excellent in crack resistance of Zn-Al alloy plating described in 1
steel.