JP2832070B2 - Manufacturing method of steel sheet with excellent galvanizing property - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to a method for producing a steel sheet having excellent galvanizing properties (tensile strength of 50 to 60 kgf / mm grade ² steel), such as bridges, buildings,
It can be used in the field of hot-dip galvanizing after welding structural members for rust prevention such as steel towers.

(Prior art) In recent years, after welding to improve the corrosion resistance of steel structures,
The method of hot-dip galvanizing structures is increasing. However, when the welded steel structure is immersed in the hot-dip galvanizing bath, cracks may occur at the grain boundaries of the heat affected zone (HAZ).
This phenomenon is known as liquid metal embrittlement, and when liquid metal comes into contact with a solid metal under stress, the liquid metal penetrates into the grain boundaries of the solid metal, reducing the grain boundary strength and causing cracking. It is considered.

Therefore, in order to prevent galvanization cracking from the viewpoint of improving the steel material, particularly the HAZ structure, many studies have been made so far (for example, Japanese Patent Application Laid-Open Nos. 01-198449 and 02-57669). Such). However, at present, cracks due to hot-dip galvanizing have not yet been completely prevented because the size and complexity of the welded steel structure increase the residual stress of welding and the thermal stress (deformation) during plating.

(Problems to be Solved by the Invention) An object of the present invention is to provide an inexpensive manufacturing technique for a steel sheet having excellent galvanizing properties. As is well known, whether or not cracks occur during galvanization depends on the stress applied to the HAZ and the properties of the grain boundaries of the HAZ.

The present invention is intended to improve the properties of the HAZ grain boundaries and to improve the galvanizability remarkably compared to conventional steel.
As a result, a welded structure using a steel sheet manufactured according to the method of the present invention is excellent in galvanization even if it is large and complicated, and does not crack.

(Means for Solving the Problems) The gist of the present invention is as follows: C: 0.10 to 0.18%, Si: more than 0.03 to 0.5%, Mn: 0.9 to 1.3%, Nb: 0.005 to 0.02%, Ti: 0.005 to 0.0
25%, Al: 0.06% or less, N: 0.001 to 0.005% V: 0.01 to 0.05%, Ni: 0.05 to 0.30%, Cu: 0.05 to 0.30 as required
%, Cr: Contain one or more of 0.05 to 0.30%,
The balance consists of iron and unavoidable impurities and has a Z value (= Mn +
20Nb + 10V + Ni + Cu + Cr + 3Mo) 1.6% or less is heated to a temperature range of 1100-1250 ° C. Rolling is performed at a cumulative reduction of 950 ° C or less 30% or more and a finish temperature of 750-900 ° C. Accelerated cooling to an arbitrary temperature of 550 ° C. or less at a rate of ° C./sec.

In general, the galvanizability of as-rolled or accelerated cooling steel (base material) is extremely excellent, presumably because the structure is relatively soft and fine, with the former austenite (γ) grain boundary being unclear. Therefore, the present inventors conducted a study to improve the galvanizing property by bringing the HAZ structure closer to the base material.

First, in order to obscure the former γ grain boundary of HAZ and obtain a soft and fine structure, the hardenability except for C was reduced (Z value =
Mn + 20Nb + 10V + Ni + Cu + Cr + 3Mo) and a small amount of Ti were added. Accelerated cooling after rolling was applied to achieve high strength with a low alloy component with a reduced Z value.

By limiting the Z value and adding a small amount of Ti, γ of HAZ even at small heat input
The grain boundaries became unclear, and it was found that the galvanizability was dramatically improved as compared with the conventional steel. The Z value is the hardenability excluding C. If the Z value is 1.6% or less, the former γ grain boundary of HAZ disappears and the zinc plating property is remarkably improved. But Z value is 1.6%
If the HAZ structure becomes coarse even in the following cases, an old γ grain boundary appears.

Therefore, it became clear that it was necessary to add a trace amount of Ti to form TiN to suppress the growth of γ grains during welding.

However, in order to obtain high strength with such low components, it is not enough to limit only the chemical components of steel.
It is necessary to limit the reheating, rolling and cooling conditions of the steel as follows.

First, the reheating temperature is limited to the range of 1100 to 1250 ° C. The reheating temperature must be 1100 ° C. or higher (preferably 1150 ° C. or higher) in order to form a solid solution of precipitates such as Nb and V and secure high strength. Below this temperature, Nb does not form a solid solution and the precipitation hardening and hardenability due to Nb are insufficient, so that sufficient strength cannot be obtained.

However, if the reheating temperature is 1250 ° C. or higher, the γ grains are remarkably coarsened, cannot be completely refined by rolling, and the low-temperature toughness deteriorates. Therefore, the reheating temperature must be 1250 ° C. or lower.

Next, in the rolling process, the cumulative reduction amount
%, And the rolling end temperature must be 750 to 900 ° C. This is for obtaining a suitable strength and toughness by refining the γ structure.

Insufficient cumulative rolling reduction or termination of rolling at high temperatures is detrimental to ductility. However, when the rolling end temperature is too low, the effect of the subsequent accelerated cooling decreases, and high strength cannot be obtained. Therefore, the adjustment of the rolling end temperature was limited to 750 ° C.

Furthermore, the rolled steel sheet is 550 ° C at a cooling rate of 5 to 40 ° C / sec.
It must be accelerated and cooled to any of the following temperatures: If the cooling rate is too low (5 ° C./sec or less) or the stop temperature is too high (more than 550 ° C.), the effect of increasing the strength by accelerated cooling cannot be sufficiently obtained. However, the cooling rate is too high and 40 ℃
If the rate exceeds / sec, a hardened structure appears and the old γ grain boundaries become clear, causing not only deterioration in galvanizing property but also deterioration in ductility, so the upper limit of the cooling rate was limited to 40 ° C / sec.

After rolling, tempering the steel sheet at a temperature equal to or lower than the Ac ₁ transformation point for dehydrogenation or improvement of ductility does not impair the features of the present invention.

Hereinafter, the reasons for limiting the individual elements of the present invention will be described.

C is extremely important as a strengthening element of the steel of the present invention. To obtain a strength of 50 kgf / mm ² or more with low components (low hardenability), the minimum amount of C is 0.10%. However, if the amount of C is too large, the galvanizability and the weldability are remarkably deteriorated due to an increase in hardenability. Therefore, the upper limit is set to 0.18%. With the C content in this range, the galvanizing property is good with almost no dependence on C.

Si can be added in excess of 0.03%, but if too much is added, the weldability and HAZ toughness deteriorate, so the upper limit was made 0.5%. Deoxidation of steel is sufficient with only Al and Ti, and Si need not always be added.

Mn is an indispensable element for securing strength and toughness,
The lower limit is 0.9%. However, if the amount of Mn is too large, the hardenability increases and not only the galvanizability but also the weldability and the HAZ toughness deteriorate, so the upper limit was made 1.3%.

Nb is an essential element in the present invention to obtain the strength and low-temperature toughness of the base material, and the lower limit is 0.005%. However, if the addition amount is too large, the hardenability, precipitation hardening will increase and the galvanizing property, HAZ toughness, weldability will be significantly impaired,
The upper limit was set to 0.02%.

Ti is an essential element in the present invention. When Ti has a small amount of Al (for example, 0.003% or less), it combines with O to form an oxide mainly composed of Ti ₂ O ₃ to improve HAZ toughness, and further combines with N to form TiN. , Suppresses coarsening of γ grains in HAZ, γ
Grain boundaries are obscured and zinc plating is greatly improved.
To obtain these hardenings, Ti must be at least 0.005%. However, if too much, TiC is formed and the low-temperature toughness and weldability deteriorate, so the upper limit is 0.025%.

Al is an element generally contained in the deoxidized upper steel, but deoxidation is sufficient with only Si or Ti, and the lower limit is not limited in the steel of the present invention. However, when the amount of Al increases, not only the cleanliness of the steel deteriorates, but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.

N is an element contained in steel as an unavoidable impurity, but forms TiN to enhance galvanization as described above. At least 0.001% of N is required for this purpose. However, excess N is harmful to HAZ toughness and weldability,
This effect is more pronounced for higher strength steels. Therefore the upper limit is 0.005
%.

 Next, the reason for adding V, Ni, Cu, and Cr will be described.

The main purpose of adding these elements to the basic components is to improve the properties such as strength and toughness and to increase the thickness without deteriorating the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is of a nature restricted by itself.

V is an element having almost the same effect as Nb, but hardenability and precipitation hardening ability are slightly weaker than Nb. In order to obtain toughening hardening due to the addition of V, a minimum of 0.01% is required. However, if the V content exceeds 0.05%, the hardenability and precipitation hardening increase to degrade the galvanizing property.
And

Ni improves the strength and toughness without adversely affecting the galvanizing property and weldability, and has a hardening effect for preventing Cu-cracks. However, if it exceeds 0.3%, it is not preferable for galvanization and weldability, so the upper limit is made 0.3%.

Cu also improves the strength without adversely affecting the galvanizability, weldability, and HAZ toughness, and also hardens to improve the contact resistance. However, if it exceeds 0.3%, the galvanizability and weldability are impaired, so the upper limit was made 0.3%.

When Cr is added in an amount of 0.30% or less, it has almost the same effect as Ni and Cu.

The lower limit of the amount of Ni, Cu, and Cr is the minimum amount at which the toughening effect of these elements can be obtained, and is 0.05%.

In the steel of the present invention, the amounts of P and S as impurities are not particularly limited. Reduction of P prevents grain boundary destruction, and reduction of S content is Mn
Although the toughness is prevented from deteriorating due to S, the steel of the present invention does not need to be extremely reduced. The preferred P and S amounts are 0.02%,
0.006% or less.

(Examples) Steel plates (sheet thickness: 10 to 30 mm) of various steel components were manufactured in a converter-continuous casting-thick plate process, and their galvanizing property, strength, and toughness were investigated.

Table 1 shows examples. In addition, the evaluation of galvanizing property is the first.
Shown in the figure. It was evaluated by a notched round bar tensile test method.

First, a round bar specimen was cooled to a peak temperature of 1400 ° C and 800 to 500 ° C for 8 seconds (after applying a reproducible heat cycle equivalent to small heat input welding with a welding heat input of 17 kJ / cm), and then a notch was formed. A high-temperature tensile test was performed by galvanizing (for comparison, a high-temperature tensile test using a test piece not galvanized was also performed).

FIG. 2 shows a schematic diagram of the stress / temperature history of the high-temperature tensile test.

The temperature of the high temperature tensile test was 470 ° C. (heating time 30 seconds) assuming the actual immersion of the structure in the galvanizing bath, the initial applied stress was changed, and the stress rate was 0.5 kgf / mm ² .

The evaluation of the galvanizing property was made after 400 seconds from the start of the test.
LM (%) = Rupture stress of zinc-plated notched specimen /
The evaluation was made by the rupture stress of a notched test piece without galvanization × 100 (the time of 400 seconds almost corresponds to the actual immersion time of hot-dip galvanizing).

The galvanizing property is better as the SLM (%) is larger, but according to conventional knowledge, if the content is 42% or more, it is considered that galvanizing cracks do not occur in ordinary structures. However, in the present invention, the SLM (%) is targeted at 70% or more in order to eliminate cracks even in a large and complicated structure.

All of the steel sheets manufactured according to the method of the present invention (steel of the present invention) have good galvanizing properties, strength and toughness. On the other hand, the comparative steel not according to the present invention is inferior in galvanizing property or strength / toughness. Steel 9 has a high Mn content and does not contain Ti, so that it has poor galvanizing properties and its toughness is just one step away. Steel 10
Is poor in base material toughness and galvanizing property because it does not contain Ti. Steel 11 has a high Mn content and galvanization is one step away,
In addition, the base material strength is lower than that of the invention steel because it does not contain Nb. Steels 12 and 14 have poor Z-plating properties because of high Z value, and Steel 13 has poor base metal toughness and zinc plating properties because C content is too high. The same applies to the steel of the present invention. However, since steel 15 has not been subjected to accelerated cooling, steels 16 and 17 have insufficient strength because the reheating temperature and rolling end temperature are too low (the tensile strength of the steel of the present invention is 55 kgf / mm ² or more). which it is utmost to satisfy the 50 kgf / mm ² whereas) a.

(Effect of the Invention) According to the present invention, excellent tensile strength of zinc plating of 50,60k
Production of gf / mm ² steel has become possible. As a result, the work efficiency on site and the safety of the welded steel structure were significantly improved.

[Brief description of the drawings]

1 (a) and 1 (b) are explanatory views of the shape of a tensile test piece, and FIG.
The figure is a chart of the stress and temperature history of the high-temperature tensile test.

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Haruo Imai 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside the Kimitsu Works of Nippon Steel Corporation (56) References JP-A-1-294848 (JP, A) (58) Field surveyed (Int.Cl. ⁶ , DB name) C21D 8/02

Claims (2)

(57) [Claims] (1) In weight%, C: 0.10 to 0.18%, Si: more than 0.03 to 0.5%, Mn: 0.9 to 1.3%, Nb: 0.005 to 0.02%, Ti: 0.005 to 0.025%, Al: 0.06% or less , N: 0.001 to 0.005%, the balance being iron and unavoidable impurities, Z value (= Mn +
20Nb + 10V + Ni + Cu + Cr + 3Mo) The steel satisfying 1.6% or less is heated to the temperature range of 1100-1250 ° C, the rolling reduction is 950 ° C or less, 30% or more, and the rolling temperature is 750-900 ° C. A method for producing a steel sheet having excellent galvanizing properties, wherein the steel sheet is accelerated and cooled to an arbitrary temperature of 550 ° C. or less at a rate of 40 ° C./sec. 2. The method according to claim 1, wherein one or more of V: 0.01 to 0.05%, Ni: 0.05 to 0.30%, Cu: 0.05 to 0.30%, Cr: 0.05 to 0.30% by weight%. For producing steel sheets with excellent galvanizing properties.