JPH06306457A - Production of high tensile strength steel excellent in plating crack resistance - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel excellent in strength and toughness and remarkably improved in plating crack resistance in a base material and an HAZ. CONSTITUTION:A steel, having a composition consisting of, by weight, 0.01-0.10% C, <=1.0% Si, 0.6-2.0% Mn, 0.5-2.0% Cu, 0.007-0.07% Nb, 0.005-0.05% Ti, 0.0005-0.005% sol.Al, 0.003-0.014% N, <=0.0035% O (oxygen), and the balance Fe with inevitable impurities, is hot-rolled at 900-700 deg.C finishing temp. and then cooled down to <=550 deg.C at >=3 deg.C/sec cooling rate. In order to improve strength and toughness, <=0.0012% B and one or more kinds among <=2% Ni, <=1% Cr, <=1% Mo, and <=0.12% V can be incorporated simultaneously or separately into the steel as material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high-strength steel having excellent resistance to plating cracks, which is used in buildings, steel towers, bridges and the like.

[0002]

2. Description of the Related Art Conventionally, for the purpose of preventing rusting of steel materials, for example, means for hot dipping zinc (Zn) on the surface of steel materials has been widely used. Such a method is an extremely effective means for rust prevention, but the structure is large,
Depending on the composition of the steel material used in the structure and how it is immersed in the plating bath, when the structure is immersed in the hot dip plating bath, cracks may occur especially in the weld heat affected zone (hereinafter referred to as HAZ). There is. Since such cracks are referred to as plating cracks, the resistance to them is hereinafter referred to as "plating crack resistance".

It is considered that the above-mentioned plating cracks are caused by the infiltration of liquid metal such as zinc into the grain boundaries of the steel, causing the steel material to become brittle and cracking. In particular, a large stress is applied to the HAZ of the steel material. It is known that embrittlement is remarkable when immersed in a liquid metal bath of zinc or the like in this state. As a means for improving plating crack resistance, Japanese Patent Publication No. 2-5814 discloses high strength containing Nb, V and Ti in which contents of C, Si, Mn and the like, which are harmful to zinc plating crack resistance, are restricted. Low alloy steel
Further, JP-A-2-57669 discloses high-strength steel in which the content of B is suppressed to 2 ppm or less in addition to C, Si and Mn.

However, such control of limited chemical components is not sufficient as a measure for preventing plating cracks in large structures and structures using high-strength steel. In other words, the welding residual stress and the thermal stress generated during plating increase with the increase in the size of the structure, so it is not possible to eliminate the occurrence of plating cracks. It was not possible to suppress the deterioration of the plating cracking property.

[0005]

Generally, plating cracks tend to occur as the thickness of the steel material increases and as the strength increases, but the present invention does not impair mechanical properties and economical efficiency. It is an object of the present invention to provide a method for producing high-strength steel in which the plating crack resistance of the base material and HAZ is dramatically improved.

[0006]

As a result of repeated research to solve the above problems, the present inventor has found that Al contained in steel is
By reducing the amounts of (aluminum) and O (oxygen), and adding appropriate amounts of Ti (titanium) and N (nitrogen),
C (carbon), Si (silicon), Mn (manganese), B (boron), etc. which are considered to be harmful to zinc plating cracking resistance by significantly improving plating cracking resistance at grain boundaries, especially at HAZ grain boundaries. It has been found that a steel containing a certain amount can be provided with sufficient plating crack resistance.

Further, an appropriate amount of Cu (copper) is contained in the steel,
It has been found that rapid cooling after hot rolling can remarkably strengthen the base material and at the same time improve the crack resistance to zinc plating.

The present invention has been made on the basis of the above findings, and the gist thereof is the following methods (1) to (4) for producing high-strength steel.

(1) C: 0.01 to 0.10% by weight, Si:
1.0% or less, Mn: 0.6 to 2.0%, Cu: 0.5 to 2.0%, Nb
(Niobium): 0.007 to 0.07%, Ti: 0.005 to 0.05%, so
l.Al: 0.0005 to 0.005%, N: 0.003 to 0.014% and O: 0.0035% or less, the balance being Fe (iron) and unavoidable impurities.
The method for producing high-strength steel excellent in plating cracking resistance, which comprises rolling in the temperature range of 1) and then cooling to a temperature range of 550 ° C or lower at a cooling rate of 3 ° C / sec or higher.

(2) In addition to the components described in (1) above, a steel containing B: 0.0012% or less by weight% and the balance Fe and inevitable impurities is hot-rolled to 900- 700
A method for producing a high-strength steel having excellent resistance to plating cracking, which comprises finishing rolling in a temperature range of ℃, and then cooling to a temperature range of 550 ℃ or lower at a cooling rate of 3 ℃ / sec or higher.

(3) In addition to the component described in (1) above, further, in weight%, Ni (nickel): 2% or less, Cr (chromium): 1% or less, Mo (molybdenum): 1% or less. And V
(Vanadium): Hot-rolled steel containing 0.1% or less of one or more and the balance of Fe and unavoidable impurities
Finish rolling in the temperature range of 900-700 ℃, then 3 ℃ / s
A method for producing a high-strength steel excellent in plating crack resistance, which comprises cooling to a temperature range of 550 ° C or lower at a cooling rate of ec or more.

(4) In addition to the components described in (1) above, further, in weight%, B: 0.0012% or less, and Ni: 2% or less, Cr: 1% or less, Mo: 1% or less and V. : Steel containing at least one of 0.12% or less, the balance of which is Fe and unavoidable impurities, is hot-rolled to finish rolling in the temperature range of 900 to 700 ℃, and then 3 ℃ / sec or more. A method for producing high-strength steel excellent in plating crack resistance, which comprises cooling to a temperature range of 550 ° C or lower at a cooling rate.

[0013]

The following will describe each requirement defined in the present invention.

First, the action and effect of each component constituting the raw steel used in the method of the present invention and the reason for limiting the content thereof will be described. In addition, all "%" of an alloy element mean "weight%."

C: C is an essential element for increasing the strength of steel, and it is necessary to contain 0.01% or more in order to obtain a strength of 60 kg / mm ² or more. On the other hand, if the content exceeds 0.10%, not only the toughness of the base material and the welded portion is impaired, but also the plating crack resistance of the HAZ decreases, so the content of C was made 0.01 to 0.10%.

Si: Si is an element required not only for deoxidation but also for ensuring strength. However, if it is contained in excess of 1.0%, not only the toughness of the base metal and the welded portion is impaired, but also ,
The plating crack resistance of HAZ is also reduced. Therefore, the Si content is set to 1.0% or less.

Mn: Mn is an essential element for securing the strength and toughness of steel, but if its content is less than 0.6%, the effect is insufficient, while if it exceeds 2.0%,
Not only is the toughness of the welded portion impaired, but the HAZ also has reduced plating crack resistance. Therefore, the content of Mn is 0.6 to 2.
It was set to 0%.

Cu: Cu suppresses corrosion of molten metal (hot-dip zinc in the case of hot-dip galvanizing) on grain boundaries that occurs during hot-dip galvanizing, and improves resistance to plating cracks of the base metal and HAZ. When steel is immersed in a hot dip galvanizing bath, it has the effect of precipitating Cu in the steel and strengthening the steel when the temperature of the steel is raised to about 400-500 ° C. Such effects are not sufficient if the content is less than 0.5%. On the other hand, if it exceeds 2.0%, the toughness of the base material and HAZ is significantly deteriorated. Therefore, the content of Cu is set to 0.5 to 2.0%.

Nb: Nb has the effect of increasing the strength and toughness of steel, but if its content is less than 0.007%, the effect is not sufficient, while if it exceeds 0.07%, it becomes a base material. Not only is the toughness of the welded portion impaired, but the HAZ also has reduced plating crack resistance. Therefore, the Nb content is 0.007-
It was set to 0.07%.

Ti: Ti forms fine carbonitrides of Ti in steel and refines the crystal grains, thereby enhancing the toughness of the base material and the weld. In addition, the synergistic effect of limiting the contents of Al, N and O as described later promotes the precipitation of fine Al oxides and improves the resistance to plating cracking of steel materials, especially HAZ. However, if its content is less than 0.005%, the effect of improving plating crack resistance is not exerted, while
If the content exceeds 0.05%, the toughness of the base material and the weld will decrease due to excessive precipitation hardening.
It was set to 0.005 to 0.05%.

Sol.Al: The content of sol.Al is 0.0005 to 0.005.
The definition of% is extremely important for improving the plating crack resistance of HAZ. That is, by controlling the content of sol.Al to 0.005% or less, during welding, the transformation of the austenite phase to the ferrite phase in the weld zone is promoted, the HAZ structure becomes finer, and island-like susceptibility is enhanced. Generation of martensite between grains is suppressed.

Further, the trace amount of Al contained in the steel is T
In combination with limiting the contents of i, N and O as described above or as described below, a fine Al oxide is produced,
By refining the austenite grains of the HAZ, the effect of increasing the strength of grain boundaries is exhibited. This fine Al oxide is obtained by adjusting the sol.Al content to 0.0005 to 0.005%. Therefore, the content of sol.Al is 0.0005 to 0.005%
And

N: N coexists with appropriate amounts of Al, Ti, and O to promote the precipitation of fine carbonitrides of Ti and, as nitrides, B and Nb which adversely affect plating crack resistance. HAZ reduces the adverse effect by capturing
Has the effect of improving the plating crack resistance. If the N content is less than 0.003%, these effects are not sufficient, and if the N content exceeds 0.014%, coarse TiN is produced, which rather reduces the plating crack resistance. It was 0.014%.

O (oxygen): O reacts with sol.Al to form fine particles.
Since the precipitation of Al oxide is promoted and the structure of HAZ is refined, it is particularly effective in significantly improving the plating crack resistance of HAZ. However, if the O content exceeds 0.0035%, the effect is remarkably reduced, so the O content is 0.0035%.
% Or less. Note that the O content is preferably 0.0007% or more in order to suppress the deoxidation cost and ensure an effective oxide.

The raw material steel used in the invention of (1) above is a steel in which the balance other than the above components is substantially Fe. The raw material steel used in the invention of (2) is a steel whose strength is increased by further adding B having the following action and effect to the raw material steel used in the invention of (1).

B: B has the action of improving the hardenability of the steel and increasing the strength of the steel by adding a very small amount. However, if the content exceeds 0.0012%, the hardenability of the HAZ becomes too high, which not only lowers the plating crack resistance but also reduces the toughness of the base material and HAZ. Therefore, the content should be 0.0012% or less. did.

The material steel used in the invention of (3) above is the same as the material steel used in the invention of (1), which is one of Ni, Cr, Mo and V.
It is a steel that contains more than one type of steel to enhance its strength and toughness.
The raw steel used in the invention of (4) is a steel obtained by further adding one or more of Ni, Cr, Mo and V to the raw steel used in the invention of (2).

Ni, Cr, Mo and V: Ni, Cr, Mo and V
Like Nb, has the effect of increasing the strength and toughness of steel.

However, Ni is 2%, Cr is 1%, Mo is 1%,
If the V content exceeds 0.12%, not only the toughness of the base material and the welded portion is impaired, but also the plating crack resistance of the HAZ decreases. Therefore, the contents of these elements are
Was 2% or less, Cr and Mo were each 1% or less, and V was 0.12% or less.

Next, a method of manufacturing the high-strength steel of the present invention using the above-mentioned raw material steel will be described.

In this manufacturing method, the steel having the above chemical composition is hot-rolled, and then accelerated cooling to a predetermined temperature range is performed to refine the structure and strengthen the steel without impairing plating crack resistance. However, in order to bring out the effect sufficiently, it is necessary to limit the production conditions as follows.

Finishing temperature of hot rolling: When the temperature for finishing hot rolling (finishing temperature) exceeds 900 ° C., the structure of the steel becomes coarse, the toughness of the base material is lowered, and at the same time, the plating crack resistance is significantly deteriorated. . On the other hand, when the finishing temperature is lower than 700 ° C, not only the microstructure of the steel does not become finer even when accelerated cooling described later is performed, but also the strengthening action of the steel due to the precipitation of Cu is significantly reduced. Therefore, the finishing temperature during hot rolling is 900-
The temperature shall be 700 ° C.

Cooling rate: 3 ° C./s after completion of hot rolling
By accelerating cooling at a cooling rate of ec or more, the structure of the steel is refined, the toughness of the base material and the plating crack resistance are improved, and at the same time, the precipitation of Cu is promoted when the temperature of the steel sheet is increased due to hot dip Steel can be strengthened without impairing plating cracking. However, when the cooling rate is less than 3 ° C./sec, the micronization of the structure is insufficient, the toughness of the base material is low, and the strengthening due to the precipitation of Cu cannot be expected. Therefore, the cooling rate after completion of hot rolling is 3 ° C./sec or more. The upper limit of the cooling rate is not specified, but it is 5 in actual operation from the viewpoint of ensuring the uniformity of the refined structure and economical efficiency.
It is preferable to cool at a cooling rate in the range of -50 ° C / sec.
In the case of thick steel plates, the cooling rate obtained with ordinary accelerated cooling equipment or direct quenching equipment is sufficient.

Cooling end temperature: Even if accelerated cooling is performed at a cooling rate of 3 ° C./sec or more, if the cooling is finished in a temperature range exceeding 550 ° C., the structure of steel is not sufficiently refined, and However, since effective Cu precipitation strengthening cannot be obtained, the toughness of the base material decreases and the plating crack resistance also deteriorates. Therefore, the end temperature of accelerated cooling shall be 550 ℃ or less.

[0035]

[Example] Steel having the chemical composition shown in Table 1 was melted, and
After heating to 50 ° C, hot rolling was performed, and after the rolling was completed at the finishing temperature shown in Table 2, accelerated cooling was also performed under the conditions shown in Table 2, and the tensile strength was approximately 60 to 80 kg / mm ² (590 to 780N Steel plate with a thickness of 40 mm was finished in the range of / mm ² ).

A test plate having a thickness of 15 mm was cut out from this steel plate and, as shown in FIG. 1, 20 passes of constraint welding was performed to prepare a constraint joint test material. In Figure 1, a test plate (thickness t = 15 mm, length l = 150 mm, a width w = 50mm), b the test bead (length l _b = 50mm), a restraining bead c 20 pass. Three constrained joint test materials were produced for each steel type. Further, the welding was carried out by using the covered arc welding method (without preheating), the restraining bead was welded under the conditions of 220A, 26V and 150mm / min, and the test bead was welded under the conditions of 170A, 24V and 150mm / min.

After this constrained joint test material was immersed in a hot dip galvanizing bath at 470 ° C. for 10 minutes, the test bead b portion was inspected by magnetic flaw detection to check for cracks.

Table 2 also shows the crack inspection results by the restraint joint test. The inspection was conducted three times using three test pieces for each steel type. In addition, the same table also shows the tensile strength of each steel type and the fracture surface transition temperature obtained by the Charpy impact test. As is clear from these results, in all of the comparative examples, at least one constrained joint test piece cracked, whereas in the present invention example, 590 N / mm.
^No cracking was observed in one restraint joint test piece while having a tensile strength of ² (60 kg / mm ² ) or more.

[0039]

[Table 1]

[0040]

[Table 2 (1)]

[0041]

[Table 2 (2)]

[0042]

EFFECTS OF THE INVENTION According to the method of the present invention, it is possible to produce a high-strength steel which is excellent in mechanical properties such as strength and toughness, and in which the plating crack resistance of the base material and HAZ is dramatically improved. This steel is particularly suitable as a steel material for large-sized welded structures that are used after being subjected to hot dip coating, such as buildings, steel towers and bridges.

[Brief description of drawings]

FIG. 1 is a view showing a structure of a restraint joint test piece, in which (a) is a front view and (b) is a side view.

Claims (4)

[Claims] 1. By weight%, C: 0.01 to 0.10%, Si: 1.0%
Below, Mn: 0.6-2.0%, Cu: 0.5-2.0%, Nb: 0.0
07-0.07%, Ti: 0.005-0.05%, sol.Al: 0.0005-0.
Steel containing 005%, N: 0.003 to 0.014%, and O: 0.0035% or less, the balance of which is Fe and inevitable impurities, is hot-rolled to finish rolling in a temperature range of 900 to 700 ° C, and then, A method for producing high-strength steel having excellent plating crack resistance, which comprises cooling to a temperature range of 550 ° C or lower at a cooling rate of 3 ° C / sec or higher. 2. By weight%, C: 0.01 to 0.10%, Si: 1.0%
Below, Mn: 0.6-2.0%, Cu: 0.5-2.0%, Nb: 0.0
07-0.07%, Ti: 0.005-0.05%, sol.Al: 0.0005-0.
Steel containing 005%, N: 0.003 to 0.014%, O: 0.0035% or less, and B: 0.0012% or less, the balance of which is Fe and inevitable impurities, is hot-rolled and rolled in the temperature range of 900 to 700 ° C. And then 550 at a cooling rate of 3 ° C / sec or more.
A method for producing a high-strength steel excellent in plating crack resistance, which comprises cooling to a temperature range of ℃ or less. 3. By weight%, C: 0.01 to 0.10%, Si: 1.0%
Below, Mn: 0.6-2.0%, Cu: 0.5-2.0%, Nb: 0.0
07-0.07%, Ti: 0.005-0.05%, sol.Al: 0.0005-0.
005%, N: 0.003 to 0.014%, O: 0.0035% or less, and Ni: 2% or less, Cr: 1% or less, Mo: 1% or less and V: 0.12% or less, and the balance is included. Is hot-rolled to steel consisting of Fe and unavoidable impurities at 900-700 ℃
The method for producing high-strength steel excellent in plating cracking resistance, which comprises rolling in the temperature range of 1) and then cooling to a temperature range of 550 ° C or lower at a cooling rate of 3 ° C / sec or higher. 4. C: 0.01 to 0.10%, Si: 1.0% by weight.
Below, Mn: 0.6-2.0%, Cu: 0.5-2.0%, Nb: 0.0
07-0.07%, Ti: 0.005-0.05%, sol.Al: 0.0005-0.
005%, N: 0.003 to 0.014%, O: 0.0035% or less,
B: 0.0012% or less, Ni: 2% or less, Cr: 1
%, Mo: 1% or less and V: 0.12% or less, and the balance is Fe and inevitable impurities, the steel is hot-rolled and rolled in the temperature range of 900 to 700 ℃. And then cooling to a temperature range of 550 ° C or less at a cooling rate of 3 ° C / sec or more, which is a method for producing high-strength steel excellent in plating crack resistance.