JP2003193190A - Galvanized high strength steel sheet having excellent weldability, hole expansibility and corrosion resistance and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvanized high strength steel sheet which has a tensile strength of ≥800 MPa, and excellent weldability, hole expansibility and corrosion resistance, and to provide a production method therefor. <P>SOLUTION: The steel sheet has a composition containing, by mass, 0.01 to 0.20% C, ≤1.5% Si, 0.01 to 3% Mn, 0.0010 to 0.1% P, 0.0010 to 0.05% S, and 0.005 to 4% Al, and further containing one or two kinds selected from 0.01 to 5.0% Mo and 0.001 to 1.0% Nb in the ranges so as to simultaneously satisfy the following inequalities (A) and (B), and the balance Fe with inevitable impurities, and has a microstructure containing bainite or bainitic ferrite by ≥70% in an area ratio, and has a tensile strength of ≥800 MPa: 3.0Nb+2.5Mo+Mn-5C<SP>0.5</SP>>0 (A), and Al+15.0Si<SP>2</SP>+1.5Si-1.8<0 (B). <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to building materials, home electric appliances,
TECHNICAL FIELD The present invention relates to a hot-dip galvanized high-strength steel sheet and a hot-dip galvanized steel sheet having excellent weldability, hole expandability, and corrosion resistance suitable for automobiles, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for high-strength steel sheets with good workability, especially for the purpose of improving fuel efficiency and durability in automobile bodies. In addition, the tensile strength is 800 MPa because of the needs of collision safety and expansion of cabin space.
Steel sheets of higher grade or higher are being used in parts such as reinforcements. The durability of these members includes fatigue durability and corrosion resistance of welded parts and processed parts. When assembling members using such high strength materials, ductility,
Bendability, hole expandability, and the like are more serious problems than high-strength steel sheets having a tensile strength of up to about 590 MPa, and countermeasures against them are necessary.

The following measures are taken for each characteristic. For example, for hole expandability, CAMP
-ISIJ vol.13 (2000) As described in p.395, bainite is used as the main phase to improve the hole expansibility, and the overhanging formability is also improved by forming retained austenite in the second phase. It is disclosed that it exhibits the same bulging property as that of retained austenitic steel. Furthermore, it is also shown that when austenite treatment with Ms temperature or less is performed to generate retained austenite with a volume ratio of 2 to 3%, the tensile strength × hole expansion ratio becomes maximum. But 800 MP
The weldability and the softening behavior in the heat-affected zone which are actualized beyond a are not considered.

Regarding the weldability, the softening behavior (HAZ softening behavior) in the heat-affected zone of welding is often regarded as a problem. On the other hand, for example, as disclosed in Japanese Unexamined Patent Publication No. 2000-87175, by precipitation of Nb and Mo carbides (Nb, Mo) C,
It has been shown to suppress HAZ softening behavior. But,
Although this technique considers fatigue strength, it does not sufficiently consider workability such as hole expandability. Also HA
The effect of suppressing the Z softening behavior is low at 800MP.
It cannot be said that the weldability and workability of extremely high strength materials of a or higher are sufficient. Especially, the tensile strength is 800MP
If it is a or more, welding itself becomes difficult, and it becomes more remarkable at 980 MPa or more. Therefore, in some cases, in addition to conventional welding methods such as spot welding, laser welding and the like are partially applied. However, because of the high strength, the material variation in the welded part and the heat-affected zone is extremely remarkable as compared with the high-strength material of 590 MPa class or higher.

In order to increase the ductility of high strength materials,
It is common to actively utilize complex organization. However, when martensite or retained austenite is used for the second phase, there is a problem that the hole expandability is significantly reduced (for example, CAMP-ISIJ, vol.13 (2000), p.3).
91). Further, this document discloses that the main phase is ferrite and the second phase is martensite, and the hole expansion ratio is improved by reducing the difference in hardness between the two, but the hole expansion ratio is 70%. If it is less than%, it is not a significant improvement. Further, Si or the like is added to these steel sheets, the material is not secured in the hot dip galvanizing process, and the plating properties and the subsequent corrosion resistance are not sufficiently taken into consideration.

[0006]

SUMMARY OF THE INVENTION The present invention solves the problems of the prior art as described above, and has a tensile strength of 800 MPa.
It is an object of the present invention to provide a high-strength steel sheet having improved corrosion resistance and improved weldability and hole expandability of the above high-strength steel sheet, which enables hot-dip galvanizing and hot-dip galvannealing, and a method for producing the same.

[0007]

Means for Solving the Problems As a result of various studies, the present inventors have found that the tensile strength is 800 MPa or more.
As a method of simultaneously improving weldability and hole expandability, by limiting the microstructure and the composition range and formula (A), the softening behavior of the weld heat affected zone is suppressed while maintaining a high strength of 800 MPa or more. Improves the fatigue durability of the welded part, and further secures a hole expandability of 70% or more if the hole expansion ratio: (hole inner diameter before hole expansion test / hole diameter before hole expansion test-1) x 100 I found that I could do it. Further, it has been found that it is possible to improve the plating property and the corrosion resistance by satisfying the formula (B) or (C).

The present invention has been completed based on the above findings, and the gist thereof is as follows. (1)% by mass, C: 0.01 to 0.20%, S
i: 1.5% or less, Mn: 0.01 to 3%, P: 0.0
010-0.1%, S: 0.0010-0.05%, A
1: 0.005 to 4%, and Mo: 0.0
1 to 5.0%, Nb: 0.001 to 1.0% of one or two are contained in a range that simultaneously satisfies the following formulas (A) and (B), and the balance is Fe and inevitable impurities, A hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance, characterized in that the microstructure contains bainite or bainitic ferrite in an area ratio of 70% or more and a tensile strength of 800 MPa or more. 3.0Nb + 2.5Mo + Mn-5C ^0.5 > 0 ・ ・ ・ (A) Al + 15.0Si ² + 1.5Si-1.8 <0 ・ ・ ・ (B)

(2) Further, in mass%, Cr: 0.0
1-5%, Ni: 0.01-5%, Cu: 0.01-5
%, Containing one or more of
Furthermore, the weldability described in (1) that satisfies the formulas (A) and (C) (however, it is not necessary to satisfy the formula (B) in this section),
Hot-dip galvanized high-strength steel sheet with excellent hole expandability and corrosion resistance. 3.0Nb + 2.5Mo + Mn-5C ^0.5 > 0 ・ ・ ・ (A) Al + 15.0Si ² ＋ 1.5Si ＋ 0.5Cr − (30Ni ＋ 10Cu) −1.8 <0 ・ ・ ・ (C)

(3) Further, in mass%, Co: 0.0
1 to 5%, W: 0.01 to 5% of 1 type or 2 types or more are contained, The meltability excellent in the weldability, the hole expandability, and the corrosion resistance as described in (1) or (2). Galvanized high strength steel plate.

(4) Further, in% by mass, Zr, Hf,
One or two or more of Ta, Ti, and V is 0.00 in total.
The hot-dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance according to (1) to (3), which is characterized by containing 1 to 1%.

(5) Further, in mass%, B: 0.00
The hot-dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance according to (1) to (4), which contains 0.1 to 0.1%.

(6) Furthermore, in% by mass, Ca, Y, Re
One or more of m is 0.0001 to 0.5 in total
%, The hot dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance according to (1) to (5).

(7) A cast slab comprising the components described in (1) to (6) is directly or once cooled and then reheated, and the hot rolled steel sheet rolled after hot rolling is pickled and cold rolled. The atmosphere during heating and annealing has an oxygen concentration of 50 ppm or less and a dew point of -20 ° C or less, and the maximum temperature during annealing is 0.3 ×
(Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) or more Ac ₃ +30
After annealing at (° C.) or lower, it is cooled at a cooling rate of 0.1 to 100 ° C./sec to a temperature range of zinc plating bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C., followed by plating immersion in the same temperature range. The method for producing a hot-dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance, which is characterized by holding for 1 second to 1000 seconds.

(8) In the manufacturing method described in (7), the zinc plating bath temperature is −20 ° C. to the zinc plating bath temperature +5.
After cooling to a temperature range of 0 ° C. and then holding for 1 second to 1000 seconds including plating immersion in the same temperature range, alloying treatment is performed at 43 ° C.
A method for producing a hot-dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance, which is characterized by performing 0 ° C to 580 ° C.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
The inventors have found that, in mass%, C: 0.01 to 0.2%, S
i: 1.5% or less, Mn: 0.01 to 3%, P: 0.00
10-0.1%, S: 0.0010-0.05%, A
l: 0.005 to 4% by weight, steel sheet consisting of the balance Fe and unavoidable impurities was added to each alloy and melted, as-cast or once cooled and then heated again, and hot-rolled and wound. The hot rolled steel sheet was pickled, cold rolled, and then annealed to prepare a cold rolled annealed sheet. The steel sheet was observed for microstructure, a hole expansion test prescribed by the Iron and Steel Federation, a tensile test in accordance with JIS, laser welding was performed by bringing the steel sheets together, and then a ball head overhanging test was performed to measure the fracture position. In addition, each welded joint was created, and a fatigue test in a tensile-tensile mode was performed for 10 ⁵ to 10 ⁶ cycles, and the fatigue properties of the welded joint of the plated material were compared and evaluated by comparing the fatigue strength with the base metal. . Here, the welding joint was bead-on, and after welding, the end surface and the surface of the test piece were ground to reduce the variation factor of the joint shape due to the welding chance as much as possible. Further, in the plating property test, the appearance was observed and a rating of 5 levels was given. Rating 5: Good appearance without any plating. Rating 4: Very good appearance with very little plating peeling. Rating 3: Non-plating or peeling of plating occurred. Rating 2: Non-plating and frequent peeling of plating. Rating 1: The plating did not get wet.

For the corrosion resistance test after plating, after the plate is bent at a radius of curvature three times the plate thickness, a dry / wet repeated test is carried out by a 100% cycle corrosion test according to the Japanese Society of Automotive Engineers (JASO) standard.
Went up to times. The state of corrosion was evaluated on a 5-grade scale by observing the external appearance and the cross section of the visual field of 200 to 1000 times in 20 fields or more with an optical microscope and observing the progress of corrosion on the inner surface. The scores are as follows. Rating 5: Good appearance with no rust. As for the degree of corrosion progress, the corrosion depth of the plating phase only or the base metal is less than 50 μm. Rating 4: Very good appearance with little rusting. Regarding the degree of progress of corrosion, the corrosion depth of the base metal is 50 μm to less than 100 μm. Rating 3: Rust generation is clearly recognized. Rating 2: Many rust. Rating 1: Rust frequently occurred, some swelling and perforation.

As a result, the finally obtained microstructure control provides a tensile strength of 800 MPa or more, weldability,
It has been found that a hot-dip galvanized steel sheet and a hot-dip galvanized high-strength steel sheet having excellent hole expandability and corrosion resistance can be produced.

Next, a preferable microstructure of the base steel sheet will be described. In order to obtain a tensile strength of 800 MPa or more and to achieve both weldability and hole expandability, bainite or bainitic ferrite is suitable as the main phase.
In order to achieve both weldability and hole expandability, the area ratio is 70% or more.

The bainite referred to here includes both upper bainite in which carbide is formed at the lath boundary and lower bainite in which fine carbide is formed in lath.
In addition, bainitic ferrite means bainite without carbide, and for example, acicular ferrite is one example. In order to improve the hole expandability, it is desirable that the lower bainite in which carbide is finely dispersed or bainitic ferrite without carbide is the main phase and the area ratio exceeds 97%.

On the other hand, prevention of softening in the heat affected zone of welding becomes a problem. On the other hand, by satisfying the formula (A) defining the components as described below, the weldability and the post-welding fatigue strength of the high-strength plated material having a tensile strength of 800 MPa or more can be secured.

From the standpoint of ensuring ductility and increasing strength, ferrite may be included in an area ratio of less than 30%. on the other hand,
The inclusion of austenite and / or martensite is not desirable from the viewpoint of hole expanding workability and softening behavior of the weld heat affected zone, but if the area ratio is less than 3%, no remarkable deterioration of properties is observed. The area ratio may be less than 3%. Further, inclusions such as oxides and sulfides may be inevitably included.

Further, when the formula (A) is not satisfied, the tensile strength of the plated material cannot be secured to 800 MPa or more, the softening of the heat affected zone of the welding cannot be suppressed, and the fatigue characteristics of the welding joint deteriorate. Not only will it happen, but it will also be difficult to secure hole expandability. 3.0Nb + 2.5Mo + Mn-5C ^0.5 > 0 (A) Further, in addition to the above, as the remaining structure of the microstructure, 1 or 2 or more of carbide, nitride, sulfide, and oxide is used as the area ratio 1
Even when the content is less than or equal to%, it can be used in the present invention, and these are included in the area ratio of the main phase.

The phases of the above-mentioned microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, interfacial oxide phase and the remaining structure are identified, the existing positions are observed, and the area ratio is measured by using Nital reagent and The reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473 corrodes the section in the steel sheet rolling direction or the section perpendicular to the rolling direction and observes it with an optical microscope at a magnification of 500 to 1000 times and an electron microscope at a magnification of 1000 to 100000 times (scanning type and transmission type). ) Can be quantified. It is possible to obtain the area ratio of each tissue by observing each of 20 or more visual fields and by the point counting method or image analysis.

Next, the reasons for limiting the preferred ranges of the steel sheet components in the present invention will be described. C is an element added for the purpose of controlling the ratios of the main phase and the second phase for ensuring a good strength-hole expandability balance. It also affects the fine homogenization of the substrate. The lower limit is 0.01% by mass (hereinafter the same) in order to secure the strength and the area ratio of each second phase.
Was set to 0.20% as the upper limit capable of maintaining weldability and hole expandability. Preferably, C: 0.05 to 0.1
When it is 5%, a good balance between strength and hole expandability can be obtained.

Si is an element added for the purpose of suppressing the formation of a relatively coarse carbide that deteriorates the strength-ductility balance, but it significantly deteriorates the plating property. For this reason 1.5
% Or less. On the other hand, the extremely low drop does not significantly deteriorate the plating property because it causes a rise in manufacturing cost.
It is desirable to add 5 or more. Further, excessive addition has an adverse effect on weldability and, therefore, the upper limit was made 1.5 mass%. Preferably, by setting Si: 0.05% or less, a more remarkable effect can be obtained.

Mn is added for the purpose of increasing the strength. It is also effective in suppressing ferrite transformation and converting the main phase into bainite or bainitic ferrite. Further, it is added for the purpose of suppressing carbide precipitation, which is one of the causes of strength reduction and deterioration of hole expandability, and pearlite formation.
From these things, it was 0.01 mass% or more. on the other hand,
Excessive addition promotes martensite formation and causes a remarkable decrease in ductility, so the upper limit was 3% by mass. Preferably, by setting Mn: 1.5 to 3.0%, a good balance between strength and hole expandability can be obtained.

P is a strengthening element. Further, lowering P improves hole expandability, but extremely lowering is economically disadvantageous, so the lower limit was made 0.0010 mass%. Further, addition of a large amount adversely affects weldability and manufacturability during casting and hot rolling, so 0.1% was made the upper limit.

As for S, lowering S is effective for improving hole expandability. On the other hand, since extremely low cost is economically disadvantageous,
The lower limit of 0010 mass% and the upper limit of 0.05 mass% are that addition of an amount exceeding this adversely affects weldability and manufacturability during casting and hot rolling.

Al is added as a deoxidizing element. Therefore, the addition amount is set to 0.005 mass% or more. On the other hand, excessive addition impairs weldability and plating wettability, so 4% was made the upper limit.

Mo suppresses the formation of carbides and pearlite which deteriorate the strength-hole expandability balance. Further, it suppresses ferrite transformation and is effective in making the main phase bainite or bainitic ferrite, and has excellent strength-hole expandability and weldability of the plated material and extremely high fatigue durability after welding. Since it is an additive element for obtaining a good balance, its lower limit is set to 0.01% by mass. Moreover, since excessive addition causes deterioration of ductility, the upper limit is set to 5.
It was set to 0%.

Nb forms fine carbides, nitrides or carbonitrides and is extremely effective in strengthening steel sheets. It also delays ferrite transformation and promotes the formation of bainite and bainitic ferrite. Furthermore, since it is also effective in suppressing the softening of the weld heat affected zone,
The addition amount is 001% by mass or more, and the addition amount of 0.01% or more is preferable for improving weldability. On the other hand, excessive addition is
Since ductility and hot workability are deteriorated, the upper limit was made 1.0% by mass.

Further, in order to control the weldability and the hole expandability in a well-balanced manner at a strength level of 800 MPa or more, the formula (A) must be satisfied. 3.0Nb + 2.5Mo + Mn-5C ^0.5 ＞ 0 ・ ・ ・ (A)

Nb, Mo and Mn improve hardenability, and Nb and Mo are effective in preventing softening of the heat-affected zone, and each coefficient reflects the degree. on the other hand,
Although C is effective for the effect, it promotes the softening behavior during welding. Although the details of the mechanism have not been clarified, it is considered that the softening preventing effect of Nb and Mo is due to the formation of precipitates or the solution drag effect during the welding thermal cycle. On the other hand, since C has a large amount of hardened coins and promotes precipitation coarsening, it is considered that C deteriorates weldability including softening of the heat-affected zone. As shown in FIG. 1, the formula (A)
In the region where the value on the left side of is less than or equal to zero, even if the area ratio of bainite or bainitic ferrite is 70% or more, it is not possible to obtain a good material that satisfies both fatigue strength ratio-tensile strength-hole expansion ratio: λ. You can see that
mark). Further, even in the region where the value on the left side of the formula (A) is more than zero, it is found that good materials cannot be obtained if the area ratio of bainite and bainitic ferrite is low due to the difference in manufacturing method (FIG. 1). Middle *). That is, it is understood that a good material can be obtained only when the structure and the condition of the formula (A) are simultaneously satisfied.

Further, the formula (B) must be satisfied for the purpose of ensuring plating property and better corrosion resistance. Al + 15.0Si ² + 1.5Si-1.8 <0 ・ ・ ・ (B)

Al and Si affect the wettability of the plating and the alloying behavior. In particular, the wettability is deteriorated, and each coefficient reflects the degree. Although the details of the mechanism are not clear, it is considered that the oxide formed on the surface is caused by the morphology. As shown in FIG. 2, in the region where the value on the left side of the formula (B) is zero or more, the appearance score after the corrosion resistance test is low and the corrosion resistance is poor. Even in the region where the value on the left side of the formula (B) is less than zero, the corrosion resistance may be poor due to the difference in manufacturing method.

Further, the steel targeted by the present invention has strength
For the purpose of further improving the balance of hole expandability, Cr,
Ni, Cu 1 type, or 2 or more types can be contained. Cr is
It is an element added for the purpose of strengthening and suppressing the formation of carbides and the purpose of forming bainite and bainitic ferrite, and is 0.01% or more, and when added in an amount exceeding 5%,
This has an upper limit because it adversely affects workability and plating properties. Ni is 0.01% by mass or more for the purpose of improving the hardenability and the plating property, and if added in an amount of more than 5% by mass, it contributes to the workability, especially the increase in hardness associated with the formation of martensite, which has an adverse effect. Therefore, this is set as the upper limit. Cu is 0.01 for the purpose of strengthening and improving plating property.
Addition of more than 5% by mass will adversely affect workability and manufacturability.

Further, with respect to ensuring the plating property, the formula (C) must be satisfied for the purpose of ensuring better corrosion resistance. ^{Al + 15.0Si 2 + 1.5Si + 0.5Cr} - (30Ni + 10Cu) -1.8 <0 ··· (C)

As described above, Cr is similar to Al and Si,
Since it affects the oxide morphology on the surface, it deteriorates the plating property. On the other hand, Ni and Cu are effective in improving the plating property, and each coefficient reflects the degree thereof. Therefore, when Cr, Ni, or Cu is included, the formula (C) must be satisfied instead of the formula (B). As shown in FIG. 3, the plating appearance score is low in the region where the value on the left side of the formula (C) is zero or more, while the plating appearance score is all good in the region where the value on the left side of the formula (C) is less than zero. Is obtained.

Furthermore, one or more of Co and W can be contained. Co is added in an amount of 0.01% by mass or more in order to achieve a good balance of strength and hole expansibility by controlling bainite transformation. On the other hand, although the upper limit of addition is not particularly set,
Since it is an expensive element, adding a large amount impairs economic efficiency.
It is desirable that the content be 5 mass% or less. W has a strengthening effect when it is 0.01 mass% or more, and the upper limit of 5 mass% is that addition of an amount exceeding this adversely affects workability.

Further, the steel targeted by the present invention is Z which is a strong carbide forming element for the purpose of further improving the strength.
It may contain one or more of r, Hf, Ta, Ti and V. These elements form fine carbides, nitrides or carbonitrides and are extremely effective in strengthening the steel sheet. Therefore, if necessary, one kind or two or more kinds in total of 0.00
The addition was 1 mass% or more. On the other hand, since ductility and hot workability are deteriorated, the upper limit of the total addition amount of one or more kinds is set to 1% by mass.

B can also be added if necessary. B
Is effective in strengthening the grain boundaries and increasing the strength of steel by adding 0.0001 mass% or more, but if the addition amount exceeds 0.1 mass%, not only the effect is saturated, but also the workability is improved. Therefore, the upper limit was made 0.1 mass%.

When Ca, Y and Rem are added in appropriate amounts, they contribute to the control of inclusions, particularly to fine dispersion, so 0.000.
1% or more, while excessive addition lowers manufacturability such as castability and hot workability and the ductility of steel sheet products.
The upper limit was 5% by mass. Examples of unavoidable impurities include N and Sn, but the inclusion of these elements in the range of 0.02 mass% or less does not impair the effects of the present invention.

A method for producing a high-strength steel sheet having such a structure and excellent in weldability and hole expandability will be described below. When the steel sheet of the present invention is manufactured by cold rolling and annealing after hot rolling, the slab adjusted to have the predetermined components is directly or once cooled and then reheated to perform hot rolling.

The reheating temperature at this time is 1100 ° C. or higher 1
It is desirable that the temperature be 300 ° C. or lower. When the reheating temperature becomes high, coarse grains and thick oxide scale are formed, while
Rolling resistance increases with low temperature heating. Further, after hot rolling, if the surface scale is removed by a high pressure descaling device or pickling, the surface of the product can be cleaned well, which is advantageous when plating is applied. After that, the product is finished by cold rolling and annealing. In addition, electroplating, hot dip galvanizing, and hot dip galvanizing do not hinder the present invention. Further, the hot rolling completion temperature is generally set to an Ar ₃ transformation temperature or higher determined by the chemical composition of the steel, but if the temperature is from Ar ₃ to a low temperature of about 10 ° C., the properties of the final steel sheet will not be deteriorated. In addition, the coiling temperature after cooling is higher than the bainite transformation start temperature, which is determined by the chemical composition of the steel, to prevent the load during cold rolling from being increased more than necessary, but when the total reduction rate of cold rolling is small. Is not limited to this, and the properties of the final steel sheet will not be deteriorated even if wound at a temperature below the bainite transformation temperature of the steel. Further, the total reduction ratio of cold rolling is set from the relationship between the final plate thickness and the cold rolling load, but if it is 40% or more, it is sufficient for recrystallization,
Does not deteriorate the properties of the final steel sheet.

When annealing after cold rolling, at the temperatures Ac ₁ and Ac ₃ where the annealing temperature is determined by the chemical composition of the steel (for example, "Steel Material Science": W. C. Leslie, translated by Shigeyasu Koda, Maruzen P273). , Expressed as 0.3 × (Ac ₃
If it is less than −Ac ₁ ) + Ac ₁ (° C.), the amount of austenite obtained at the annealing temperature is small, so that bainite or bainitic ferrite cannot be mainly produced in the final steel sheet. In addition, the higher the annealing temperature is, the larger the grain size and surface oxidation are promoted, and the upper limit of the annealing temperature is set to Ac in order to increase the manufacturing cost.
_{It was set to 3} + 30 (° C). The annealing time in this temperature range needs to be 10 seconds or more to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 30 minutes, the generation of the grain boundary oxidized phase is promoted and the cost is increased. Here, the atmosphere at the time of temperature rising and annealing had an oxygen concentration of 50 ppm or less and a dew point of -20 ° C or less. If the oxygen concentration exceeds 50 ppm or the dew point exceeds −20 ° C., the plateability of the steel sheet, particularly the wettability, deteriorates, causing non-plating.

The subsequent primary cooling is important for producing bainite or bainitic ferrite while suppressing the transformation from the austenite phase to the ferrite phase to some extent. Setting this cooling rate to less than 0.1 ° C./sec
The lower limit of the cooling rate was set to 0.1 ° C./second because there is a concern that the generation of ferrite or pearlite may be promoted and the strength may be reduced. On the other hand, when the cooling rate is more than 100 ° C./sec, the hard steel plate has a large amount of hard phase such as martensite phase in the final steel plate, and it is difficult in operation.

If the subsequent cooling is carried out until the plating bath temperature is lower than -20 ° C., there is a problem in operation such as a large heat removal during the immersion of the plating bath. Further, when the cooling stop temperature exceeds + 50 ° C. in the plating bath, in addition to problems in operation, carbides are generated during the subsequent holding, which causes a decrease in strength, so this was made the upper limit. Further, in order to promote the progress of bainite transformation, the temperature is maintained in this temperature range. It is desirable to set the retention time to 1000 seconds or less, because if the retention time is long, it is not preferable in terms of productivity and carbide is generated. Further, in order to allow the bainite transformation to proceed, it is desirable to hold for 1 second or more, preferably 15 seconds to 10 minutes. If the plating bath temperature is lower than -20 ° C, bainite transformation is unlikely to occur, and if the plating bath temperature exceeds -50 ° C, carbides are generated and the material is deteriorated. Moreover, when performing an alloying process, it was 430 degreeC or more and 580 degreeC or less. If the alloying temperature is lower than 430 ° C., the progress of alloying is slow and the productivity is poor. Further, if the temperature exceeds 580 ° C, the material is deteriorated along with the precipitation of carbide. Also, regarding the welding method,
Welding methods that are commonly used, such as arc, TIG, MIG,
It is within the scope of the present application even if welding such as mash and laser is performed.

[0049]

The present invention will be described in more detail with reference to the following examples. A steel plate having a composition as shown in Table 1 is 1200 ° C.
After completion of hot rolling at the Ar ₃ transformation temperature or higher, after cooling, the steel strip wound at the bainite transformation start temperature or higher determined by the chemical composition of each steel is pickled and then cold rolled to a thickness of 1.2 mm. did. Then, the Ac ₁ and Ac ₃ transformation temperatures were calculated from the components (mass%) of each steel according to the following formulas. Ac ₁ = 723-10.7 × Mn% + 29.1 × Si
%, Ac ₃ = 910−203 × (C%) ^{1 /} 2−15.2 ×
Ni% + 44.7 × Si% + 104 × V% + 31.5 ×
Mo% -30 x Mn% -11 x Cr% + 400 x Al
%, The annealing temperature calculated from these Ac ₁ and Ac ₃ transformation temperatures is raised and held in a 10% H ₂ —N ₂ atmosphere, and then 200 to 450 ° C. at a cooling rate of 3 to 150 ° C./sec. It was cooled and then held for 1-3000 seconds and then cooled.

JIS No. 5 tensile test pieces were taken from these steel sheets and their mechanical properties were measured. Further, a hole expansion test was conducted in accordance with the Iron and Steel Federation standards to obtain the hole expansion ratio. Regarding weldability, each weld was performed on the steel plate in a bead-on form,
A ball head overhanging test was carried out with a fluorine resin (Teflon brand name) lubrication, and the overhanging height and breaking position relative to the base material were measured. Also, make each welding joint and pull
Perform tensile mode fatigue test for 10 ⁵ to 10 ⁶ cycles,
By comparing the fatigue strength with the base material, the fatigue characteristics of the welded joint of the plated material were comparatively evaluated. Where the weld joint is
The method was bead-on, and after welding, the end surface and surface of the test piece were ground to minimize the variation factor of the joint shape due to the welding chance. Further, in the plating property test, the appearance was observed and a rating of 5 levels was given. For the corrosion resistance test after plating, the plate was bent at a radius of curvature three times the plate thickness, and then a dry-wet repeated test was performed, and the degree of rusting was also evaluated in five stages.

Table 2 shows the microstructure and each material, and Table 3 shows each manufacturing condition and material. Inventive steel satisfying the outline of the present invention has weldability, ductility, strength (tensile strength
800 MPa or more), which shows that the hole expandability is excellent. On the other hand, in Comparative Examples that deviate from the conditions of the present invention, the fracture position in the ball head overhanging process of the welded part becomes the weld heat affected zone, the fatigue strength is low, and the hole expandability, plating property and corrosion resistance are inferior.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

According to the present invention, the tensile strength is 800 MPa.
It is possible to obtain the hot-dip galvanized high-strength steel sheet and the manufacturing method thereof in which the weldability, hole expandability, and corrosion resistance of the above high-strength steel sheet are simultaneously improved.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between fatigue strength ratio × tensile strength × λ (hole expansion ratio) and the value on the left side of the expression (A).

[Fig. 2] Appearance score after dry and wet repeated corrosion test and (B)
It is a graph which shows the relationship with the value of the left side of a formula.

FIG. 3 is a graph showing the relationship between the plating appearance rating and the value on the left side of the expression (C).

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 2/28 C23C 2/28 (72) Inventor Kenichiro Matsumura 5-3 Tokai-cho, Tokai-shi, Aichi Pref. (72) Inventor Takuya Hara, Steel Works Co., Ltd. (72) Tatsuya Hara 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Co., Ltd. Technology Development Division (72) Inventor Toshiki Nonaka 5-3 Shinkai, Tokai City, Aichi Prefecture Nippon Steel Co., Ltd.Nagoya Steel F-Term (Reference) 4K027 AA05 AA23 AB02 AB05 AB13 AB42 AC73 4K037 EA01 EA02 EA05 EA06 EA09 EA10 EA11 EA13 EA15 EA16 EA17 EA35 EA35 EA35 EA35 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA32 EA31 EB09 EB11 FA00 FB00 FG00 FH00 FJ02 FJ04 FJ05 FK01 FK02 FK03 GA05 GA08 JA01

Claims (8)

[Claims] 1. In mass%, C: 0.01 to 0.20%, Si: 1.5% or less, Mn: 0.01 to 3%, P: 0.0010 to 0.1%, S: 0.0010 to 0.05%, Al: 0.005 to 4% is contained, and further, Mo: 0.01 to 5.0%, Nb: 0.001 to 1.0%, one or two kinds. In the range that simultaneously satisfies the following formulas (A) and (B), with the balance being Fe and inevitable impurities, the microstructure containing 70% or more in area ratio as bainite or bainitic ferrite, and having a tensile strength A hot-dip galvanized high-strength steel sheet with excellent weldability, hole expandability, and corrosion resistance that is characterized by 800 MPa or more. 3.0Nb + 2.5Mo + Mn-5C ^0.5 > 0 ・ ・ ・ (A) Al + 15.0Si ² + 1.5Si-1.8 <0 ・ ・ ・ (B) 2. Further, in mass%, one or more of Cr: 0.01 to 5%, Ni: 0.01 to 5%, and Cu: 0.01 to 5% are contained. A hot-dip galvanizing height which is excellent in weldability, hole expansibility and corrosion resistance as set forth in claim 1 (but it is not necessary to satisfy the formula (B) in this paragraph), which further satisfies the formulas (A) and (C). Strength steel plate. 3.0Nb + 2.5Mo + Mn-5C ^0.5 > 0 ・ ・ ・ (A) Al + 15.0Si ² ＋ 1.5Si ＋ 0.5Cr − (30Ni ＋ 10Cu) −1.8 <0 ・ ・ ・ (C) 3. The composition according to claim 1, further comprising, in mass%, one or more of Co: 0.01 to 5% and W: 0.01 to 5%. The hot-dip galvanized high-strength steel sheet excellent in weldability, hole expandability, and corrosion resistance described in. 4. Further, 0.001 to 1% of Zr, Hf, Ta, Ti, and V in total of 0.001 to 1% by mass is contained. The hot-dip galvanized high-strength steel sheet excellent in weldability, hole expandability, and corrosion resistance described in. 5. Further, B: 0.0001 to% by mass.
The hot-dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance according to any one of claims 1 to 4, which contains 0.1%. 6. Further, in mass%, 1 of Ca, Y and Rem is used.
Type or two or more types in total 0.0001 to 0.5% is contained, The hot-dip galvanized high-strength steel sheet excellent in weldability, hole expandability and corrosion resistance according to claim 1 to 5, . 7. A cast slab comprising the components of claims 1 to 6 is directly or once cooled and then reheated, the hot rolled steel sheet is hot rolled after pickling, cold rolled after pickling, and then raised. The temperature and the atmosphere during annealing have an oxygen concentration of 50 ppm or less and a dew point of -20 ° C or less, and the maximum temperature during annealing is 0.3 x.
(Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) or more Ac ₃ +30
After annealing at (° C.) or lower, it is cooled at a cooling rate of 0.1 to 100 ° C./sec to a temperature range of zinc plating bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C., followed by plating immersion in the same temperature range. The method for producing a hot-dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance, which is characterized by holding for 1 second to 1000 seconds. 8. The manufacturing method according to claim 7, wherein the temperature is cooled to a temperature range of zinc plating bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C., and subsequently 1 second including plating immersion in the same temperature region. After holding for 1000 seconds, alloying treatment is performed at 430 ° C to 58
A method for producing a hot-dip galvanized high-strength steel sheet having excellent weldability, hole expandability, and corrosion resistance, which is characterized by performing 0 ° C.