JP2013044022A - Hot-dip galvanized steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably provide a high strength hot-dip galvanized steel sheet superior in a formability and spot-weldability at low cost.SOLUTION: The steel sheet has a chemical composition composed of 0.02-0.10% C, 0.005-0.5% Si, 1.4-2.5% Mn, ≤0.025% P, ≤0.010% S, 0.001-0.2% sol. Al, ≤0.008% N, ≤0.15% Ti and one or more elements selected from group of ≤0.01% Ca, ≤0.01% Mg and ≤0.01% REM, and satisfying formulae (1): C-(12/48)×Ti-(12/93)×Nb≤0.090, (2): Ti=max[Ti-(48/14)×N-(48/32)×S,0], and (3): 2/3×C+(1/150)×Mn+P+2×S<0.15. Further, this steel sheet has the steel structure containing by area, 50-94% ferrite, 5-49% bainite and 1-20% total of martensite and retained austenite. The hot-dip galvanized steel sheet has mechanical properties of ≥1,500%(El×λvalue) the product of a total extension (El) and a drilling expansion ratio (λ), ≥75% the yield ratio (YR), and ≥490 MPa the tensile strength (TS); and ≥0.55 the extending ratio (CTS/TSS) as the ratio value of a cruciform tension strength (CTS) in the cruciform tension test and a tension shearing strength (TSS) in the shearing test of a resistant spot-welding coupling which is made under direct-current type resistant spot-welding condition of 6 mm as the welding electrode top diameter, 4,410 kN as the pressure force, 9 kA as the welding current and 18 cycle as the turning time of the electricity, is ≥0.55, and the ratio value of Vickers hardnesses between the welded part and the basic material in the resistant spot-welding coupling is ≤2.0.

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet and a method for producing the same. More specifically, the present invention relates to a high-strength hot-dip galvanized steel sheet excellent in resistance spot weldability and stretch flangeability and a method for producing the same.

  In recent years, in order to reduce the weight of automobiles, it has been required to increase the strength of automobile steel sheets. Research has been conducted to increase the strength of steel sheets. For example, solid solution strengthening elements and precipitation strengthening elements have been added, and steel structures have been strengthened by using steel structures as bainite or dual phases. .

  However, for steel plates with a tensile strength of 490 MPa or higher, degradation of stretch flangeability due to reduced ductility and further hole expandability has become a problem, especially for steel plates with a tensile strength of 590 MPa or higher. This is an important issue because of the remarkable deterioration.

  Furthermore, with the increase in strength of steel sheets, it is necessary to add elements that strengthen the steel such as C, Si, Mn, etc., so that there is a problem that the alloy content is high and spot weldability deteriorates. have.

  By the way, depending on the use of the steel sheet for automobiles, not only strength but also corrosion resistance is required. In order to improve the corrosion resistance of the steel sheet, a large amount of hot dip galvanized steel sheet is used, but in particular, a hot dip galvanized steel sheet which is excellent in terms of economy, rust prevention function and performance after coating is widely used.

Therefore, various techniques have been proposed in order to increase the strength of hot-dip galvanized steel sheets while suppressing deterioration of formability and spot weldability.
For example, Patent Document 1 proposes a high-strength alloyed hot-dip galvanized steel sheet that is excellent in hole expansibility, in which the structure ratio of cementite having an equivalent circle radius of 0.1 μm or more is limited to 0.1% or less. ing.

  In Patent Document 2, Si is added as a solid solution strengthening element in an amount of 0.3 to 1.8% (in this specification, “%” means “mass%” unless otherwise specified), There has been proposed a high-tensile hot-dip galvanized steel sheet having a structure containing 20% or more of tempered martensite by area ratio and having excellent workability and spot weldability.

  Patent Document 3 discloses a steel structure comprising a main phase of ferrite having an average particle size of 3.5 μm or less and a second phase, and having a volume ratio of martensite of 70% or more and austenite of 2% or more as the second phase. A high-tensile hot-dip galvanized steel sheet that is considered to have excellent workability has been proposed.

Patent Document 4 includes a bainitic ferrite structure and / or a bainitic ferrite structure containing coarse cementite having an equivalent circle diameter of 0.5 μm or more and a space factor of 0.1% or less. A high-strength hot-rolled steel sheet having a tensile strength of 780 N / mm ² or more and having excellent strength ductility balance and burring properties is disclosed. Further, Patent Document 5 discloses C, Mn, Si, Al A high-strength hot-dip galvanized steel sheet, which has excellent spot weldability and formability, has been proposed in which the additive amount is controlled to have a specific resistance of 28 μΩ · cm to 53 μΩ · cm.

Japanese Patent Laid-Open No. 4-268057 JP 2001-207235 A JP 2000-212686 A JP 7-48648 A JP 2007-146246 A

However, all of these conventional inventions have the problems listed below.
The technique proposed in Patent Document 1 has no element for fixing C as a carbide, and suppresses the formation of cementite under heat treatment conditions. In order to utilize the tissue change in this way, the variation of the tissue is large, and it is difficult to stably obtain excellent performance.

  In addition, a steel sheet containing a large amount of Si has a problem that a red scale called firelight is generated in the hot rolling process, and the appearance and paintability are deteriorated. Furthermore, there is a problem that a Si oxide layer is formed in the pre-oxidation process of the continuous hot dip galvanizing process, making the subsequent alloying process difficult, and requiring an alloying process at a high temperature. With the technique proposed in Patent Document 2, since sufficient countermeasures are not taken for these problems, good plating properties cannot be obtained.

  In the technique proposed in Patent Document 3, since the steel structure is composed of a composite structure, the hard second phase is likely to be the starting point of cracking in press working. In addition, the variation in the structure due to the variation in the fraction of the second phase makes it difficult to stably obtain excellent performance.

  Since the technique proposed in Patent Document 4 needs to add Ti in a large amount of about 0.1 to 0.15% as described in the examples, the appearance of the steel sheet surface due to material flaws. Defects occur. Furthermore, it is necessary to add a large amount of Cu, P, or Ni in order to ensure corrosion resistance, which increases costs.

  The technique proposed in Patent Document 5 has been proposed to ensure a sufficient appropriate welding current range in spot welding with an alloyed hot-dip galvanized mild steel sheet, but the strength and ductility ratio of the welded portion are considered. In some cases, a stable welding strength cannot be secured in actual parts.

As described above, with the conventional technology, it has been difficult to stably produce a high-strength hot-dip galvanized steel sheet excellent in formability and spot weldability at low cost.
The object of the present invention is to stably provide a high-strength hot-dip galvanized steel sheet excellent in formability and spot weldability at low cost.

  As a result of intensive studies to solve the above problems, the present inventors have efficiently adjusted the steel composition and production conditions to an appropriate range and make maximum use of precipitation strengthening by C and N compounds. Strengthening of the steel sheet is achieved, and solid solution C in the steel is reduced, thereby making it possible to achieve excellent formability and spot weldability, and with high formability and spot weldability. It was newly found that an excellent alloyed hot-dip galvanized steel sheet can be obtained.

The present invention has been made based on the above new findings, and the gist thereof is as follows.
(1) A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet,
The steel sheet is C: 0.02% to 0.10%, Si: 0.005% to 0.5%, Mn: 1.4% to 2.5%, P: 0.025% or less , S: 0.010% or less, sol. Al: 0.001% or more and 0.2% or less, N: 0.008% or less and Ti: 0.15% or less, Ca: 0.01% or less, Mg: 0.01% or less, and REM : Containing one or more selected from the group consisting of 0.01% or less, having a chemical composition satisfying the following formulas (1) to (3), and in area%, ferrite: 50% A steel structure containing 94% or less, bainite: 5% or more and 49% or less, and a total of martensite and retained austenite: 1% or more and 20% or less,
The hot-dip galvanized steel sheet has a product (El × λ value) of total elongation (El) and hole expansion rate (λ) of 1500% ² or more, yield ratio (YR) of 75% or more, and tensile strength (TS). Cross-tension test of resistance spot welded joints with mechanical properties of 490 MPa or higher, welding electrode tip diameter: 6 mm, applied pressure: 4410 kN, welding current: 9 kA, and energization time: 18 cycles of direct current resistance spot welding conditions The ductility ratio (CTS / TSS), which is the value of the ratio of the cross tensile force (CTS) in the shear test to the shear force (TSS) in the shear test, is 0.55 or more, and the Vickers between the molten metal part of the resistance spot welded joint and the base material A hot-dip galvanized steel sheet having resistance spot weldability having a hardness ratio value of 2.0 or less.

C− (12/48) × Ti ^* − (12/93) × Nb ≦ 0.090 (1)
Ti ^* = max [Ti− (48/14) × N− (48/32) × S, 0] (2)
2/3 × C + (1/150) × Mn + P + 2 × S <0.15 (3)
Here, each element symbol in the formulas (1) to (3) indicates the content (unit: mass%) of each element, and max [] in the formula (2) is the maximum value among the arguments in []. A function to return.

  (2) The hot-dip galvanized steel sheet according to (1), wherein the chemical composition further contains Bi: 0.005% or less.

  (3) The chemical composition is Nb: 0.15% or less, Cr: 1% or less, V: 0.1% or less, Mo: 0.5% or less, Cu: 1% or less, Ni: 1% or less, and B: The hot-dip galvanized steel sheet according to item (1) or (2), further containing one or more selected from the group consisting of 0.005% or less.

(4) The ferrite has a number density of 100 / μm ² or more of carbides, nitrides, and composites containing Ti or Nb and having a particle size of 1 nm or more and 20 nm or less. The hot-dip galvanized steel sheet according to any one of items 1) to (3).

  (5) The hot-dip galvanized steel sheet according to any one of items (1) to (4), wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

(6) A method for producing a hot-dip galvanized steel sheet comprising the following steps (A) to (D):
(A) A slab having the chemical composition according to any one of items (1) to (5) is subjected to rough hot rolling at 1100 ° C. or higher to obtain a rough bar, and the rough bar is set to 1000 ° C. or higher. After heating, finish hot rolling is completed to complete rolling in a temperature range of ₃ or more points of Ar to form a hot-rolled steel sheet, and primary cooling is performed at an average cooling rate of 35 ° C./second or more to a temperature range of 600 ° C. to 700 ° C. And then secondary cooling at an average cooling rate of 5 ° C./second or more and 40 ° C./second or less to a temperature range of 400 ° C. or more and 650 ° C. or less, followed by a hot rolling step of winding.
(B) a pickling step of subjecting the hot-rolled steel sheet obtained by the hot rolling step to a pickling treatment;
(C) a soaking process for maintaining the hot-rolled steel sheet obtained by the pickling process at a holding temperature satisfying the following formula (4); and (D) hot-plating the hot-rolled steel sheet obtained by the soaking process. Hot-dip plating process.
900-T ₂ × 0.2 ≦ T ≦ 1000 (4)
Here, T ₂ in the formula indicates the end temperature (° C.) of the secondary cooling in the hot rolling process, and T indicates the holding temperature (° C.) in the soaking process.

(7) A method for producing a hot-dip galvanized steel sheet, comprising the following steps (a) to (e):
(A) A slab having the chemical composition according to any one of items (1) to (5) is subjected to rough hot rolling at 1100 ° C. or higher to obtain a rough bar, and the rough bar is set to 1000 ° C. or higher. After heating, finish hot rolling is completed to complete rolling in a temperature range of ₃ or more points of Ar to form a hot-rolled steel sheet, and primary cooling is performed at an average cooling rate of 35 ° C./second or more to a temperature range of 600 ° C. to 700 ° C. And then secondary cooling at an average cooling rate of 5 ° C./second or more and 40 ° C./second or less to a temperature range of 400 ° C. or more and 650 ° C. or less, followed by a hot rolling step of winding.
(B) a pickling step of performing a pickling treatment on the hot-rolled steel sheet obtained by the hot rolling step;
(C) a cold rolling process in which the hot rolled steel sheet obtained by the pickling process is cold rolled to obtain a cold rolled steel sheet;
(D) a soaking process for maintaining the cold-rolled steel sheet obtained by the cold rolling process at a holding temperature satisfying the following formula (4); and (e) melting in the cold-rolled steel sheet obtained by the soaking process. Hot dipping process for plating.

900-T ₂ × 0.2 ≦ T ≦ 1000 (4)
Here, T ₂ in the formula indicates the end temperature (° C.) of the secondary cooling in the hot rolling process, and T indicates the holding temperature (° C.) in the soaking process.

  (8) In the process of cooling to room temperature after hot dip galvanization, the alloying treatment is performed while maintaining the temperature range of 480 ° C. to 600 ° C. The manufacturing method of the hot-dip galvanized steel sheet of description.

  Here, the cross tension force (CTS) in the cross tension test of the resistance spot welded joint is defined by JIS Z 3137, and the shear force (TSS) in the shear test of the resistance spot welded joint is defined by JIS Z 3136. The hole expansion rate (λ) is defined by the Japan Iron and Steel Federation Standard JFS T 1001.

  According to the present invention, a high-strength hot-dip galvanized steel sheet excellent in spot weldability and formability can be obtained, which is extremely beneficial particularly for weight reduction and rust prevention of automobile bodies, and has a great industrial effect. Industrial value is very large.

1. First, the reason for limiting the chemical composition of the steel sheet will be described.
(1) C: 0.02% or more and 0.10% or less C contributes to precipitation strengthening by solid solution strengthening, transformation strengthening, and bonding with Ti, Nb, etc. to form carbides. It is an effective element for strengthening. In the present invention, steel is strengthened by utilizing transformation strengthening and precipitation strengthening, so the C content is 0.02% or more in order to obtain sufficient strengthening ability. However, if the C content exceeds 0.10%, the decrease in cross tensile strength in spot welding may become significant. Therefore, the C content is 0.10% or less.

(2) Si: 0.005% or more and 0.5% or less Si is an element effective for strengthening steel and improving formability because it has an effect of increasing strength while suppressing a decrease in ductility. It is. If the Si content is less than 0.005%, the effect by the above action may not be sufficiently obtained. Therefore, the Si content is 0.005% or more. Preferably, it is 0.02% or more. On the other hand, if the Si content exceeds 0.5%, the plating property may be significantly impaired. Therefore, the Si content is 0.5% or less.

(3) Mn: 1.4% or more and 2.5% or less Mn is an element effective for strengthening steel. Furthermore, it suppresses the generation of pearlite that lowers the formability of the steel sheet and has the effect of improving the formability of the steel sheet by making the crystal grains fine. If the Mn content is less than 1.4%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 1.4% or more. On the other hand, if the Mn content exceeds 2.5%, the ductility ratio may be significantly reduced due to a decrease in the toughness of the spot weld. In addition, the wettability of the plating may be significantly deteriorated. Therefore, the Mn content is 2.5% or less.

(4) P: 0.025% or less P is an element that is generally contained as an impurity. However, P has an effect of increasing the strength of the steel sheet by solid solution strengthening, and therefore may be actively contained. However, if the P content exceeds 0.025%, the toughness of the spot welded portion may be significantly deteriorated. Therefore, the P content is 0.025% or less.

(5) S: 0.010% or less S is an element contained as an impurity. If the S content exceeds 0.010%, the toughness of the spot welded portion may be significantly deteriorated. In addition, MnS may be formed to significantly deteriorate moldability. Therefore, the S content is 0.010% or less. Preferably it is 0.008% or less, More preferably, it is 0.005% or less.

(6) sol. Al: 0.001% or more and 0.2% or less Al has an action of deoxidizing molten steel to make the steel sound. sol. When the Al content is less than 0.001%, deoxidation is not sufficient. Therefore, sol. The Al content is 0.001% or more. On the other hand, sol. If the Al content exceeds 0.2%, the effect of the above action is saturated and disadvantageous in cost. Therefore, sol. The Al content is 0.2% or less.

(7) N: 0.008% or less N is an element contained as an impurity. If the content exceeds 0.008%, cracks occur on the surface of the slab during casting, or the ductility of the steel sheet decreases. May become noticeable. Therefore, the N content is 0.008% or less.

(8) Ti: 0.15% or less Ti is combined with C and N in steel to form precipitates, thereby increasing the strength of the steel sheet and reducing the formability of the steel sheet. It has the effect of reducing dissolved N. Moreover, since the increase in the hardness of the nugget part in spot welding is suppressed by reducing the solid solution C, it also has an effect of increasing the ductility ratio. Furthermore, it suppresses the formation of pearlite and cementite, which lowers the formability of the steel sheet, and also facilitates securing bainitic ferrite, which is an effective phase in achieving both high strength and good hole expandability. Also have. Thus, Ti is an element effective for enhancing the formability of the steel sheet and ensuring good spot weldability. Therefore, Ti is contained. However, even if it contains exceeding 0.15%, the effect by the said effect | action will be saturated and it will become disadvantageous in cost. Therefore, the Ti content is 0.15% or less. In order to more reliably obtain the effect of the above action, the Ti content is preferably 0.001% or more.

(9) One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less. All of Ca, Mg, and REM are sulfurized. It has the effect | action which spheroidizes inclusions, such as a thing and an oxide, and improves the moldability of a steel plate. Accordingly, one or more of the above elements are contained. However, even if the content exceeds the above range, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the content of each element is within the above range. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make content of either of the said elements 0.00010% or more.

(10) Bi: 0.005% or less Bi is an optional element, and its inclusion makes the solidified structure finer, and even when a large amount of Mn or the like is contained, solidification segregation is suppressed, the steel structure becomes uniform, and formability. It has the effect | action which suppresses deterioration. In addition, the above effect promotes the refinement of fine precipitates and promotes the formation of precipitates having the effect of strengthening the ferrite phase, so that the purpose of achieving both a high yield ratio and good formability is achieved. It is an effective element in the invention. Therefore, it is preferable to contain Bi from the viewpoint of ensuring better workability. However, if the Bi content is more than 0.005%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the Bi content is 0.005% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make Bi content 0.0001% or more.

(11) Nb: 0.15% or less, Cr: 1% or less, V: 0.1% or less, Mo: 0.5% or less, Cu: 1% or less, Ni: 1% or less, and B: 0.005 One or more selected from the group consisting of% or less These elements are optional elements and may be contained because they have the effect of increasing the strength of the steel sheet. If the content of each element exceeds the above range, the effect of increasing the strength is saturated and the cost increases. For this reason, content of each element is made into the said range. In order to more reliably obtain the effect of increasing the strength, Nb: 0.001% or more, Cr: 0.1% or more, V: 0.01% or more, Mo: 0.05% or more, Cu: 0.1 % Or more, Ni: 0.1% or more, and B: 0.0002% or more are preferably contained.

(12) Formula (1) and Formula (2)
In the present invention, the contents of C, Ti, N, S, and Nb satisfy the following formulas (1) and (2).

C− (12/48) × Ti ^* − (12/93) × Nb ≦ 0.09 (1)
Ti ^* = max [Ti− (48/14) × N− (48/32) × S, 0] (2)
Here, each element symbol in the formulas (1) and (2) indicates the content (unit: mass%) of each element, and max [] in the formula (2) is the maximum value among the arguments in []. A function to return.

  If the amount of solute C is excessive, the area ratio of ferrite is not sufficient and the formability is lowered. Therefore, the chemical composition satisfies the above formula (1) using the left side of the above formula (1), which is an index of the amount of dissolved C. The right side of the formula (1) is preferably 0.065.

(13) Formula (3)
In the present invention, the contents of C, Mn, P and S satisfy the following formula (3).
2/3 × C + (1/150) × Mn + P + 2 × S <0.15 (3)
Here, each element symbol in Formula (3) indicates the content (unit: mass%) of each element.
In order to increase the cross tension force (CTS) in the cross tension test of the resistance spot welded joint, it is important to suppress segregation of C, Mn, P and S that are the starting points of cracks in the nugget. For this reason, in order to restrict | limit the content of the said element about the chemical composition of the steel plate which is a welding base material, a chemical composition shall satisfy the said Formula (3). It is preferable to satisfy the following formula (4), and it is more preferable to satisfy the following formula (5).

2/3 × C + (1/150) × Mn + P + 2 × S ≦ 0.12 (4)
2/3 × C + (1/150) × Mn + P + 2 × S ≦ 0.10 (5)

2. Next, the reason for limiting the steel structure of the steel sheet will be described. In addition, unless otherwise indicated,% regarding steel structure means an area%.

(1) Ferrite area ratio: 50% or more and 94% or less Ferrite is a soft and highly workable phase, and is an effective phase for ensuring good moldability. If the ferrite area ratio is less than 50%, the ductility may be significantly reduced. Therefore, the ferrite area ratio is 50% or more. The upper limit of the ferrite area ratio is 94% or less in order to secure the area ratio of bainite and retained austenite described later. The ferrite in the present invention includes bainitic ferrite. Since bainitic ferrite is harder than polygonal ferrite, the area ratio is preferably as high as possible from the viewpoint of securing a tensile strength of 490 MPa or more.

(2) Bainite area ratio: 5% or more and 49% or less Bainite is a structure effective for increasing strength while suppressing deterioration of hole expansibility. If the bainite area ratio is less than 5%, it may be difficult to ensure a tensile strength of 490 MPa or more. Therefore, the bainite area ratio is set to 5% or more. The upper limit of the bainite area ratio is set to 49% or less in order to ensure the total area ratio of the ferrite described above and residual austenite and martensite described later.

(3) Total area ratio of retained austenite and martensite: 1% or more and 20% or less Residual austenite and martensite are effective phases for ensuring good ductility. Therefore, in order to ensure the good ductility targeted by the present invention, the total area ratio of retained austenite and martensite is set to 1% or more. On the other hand, since martensite produced by transformation of retained austenite due to processing strain and martensite present from the beginning are hard, if the area ratio is excessive, deterioration of formability becomes remarkable. For this reason, the total area ratio of retained austenite and martensite is 20% or less.

(4) Number density in ferrite of carbides, nitrides and composites thereof containing Ti or Nb and having a particle size of 1 nm to 20 nm: 100 / μm ² or more By precipitating Ti and Nb in ferrite, Ferrite is strengthened, the hardness difference from other phases and structures is reduced, and the hole expandability can be improved. Since the particle size of the precipitate capable of effectively strengthening ferrite is 1 nm or more and 20 nm or less, in the present invention, carbides, nitrides and composites thereof containing Ti or Nb and having a particle size of 1 nm or more and 20 nm or less are used. The number density in the ferrite is preferably 100 pieces / μm ² or more. The upper limit of the number density is not particularly defined, it is preferable to 10000 / [mu] m ² or less because if the ferrite cause deterioration in stretch flangeability Excessive harden.

3. Next, the reason for limiting the mechanical properties of the hot-dip galvanized steel sheet will be described.

(1) Product El × λ value of total elongation (El) and hole expansion ratio (λ): 1500% ² or more In the present invention, since it aims at obtaining good moldability, total elongation (El) The product El × λ value with the hole expansion rate (λ) is 1500% ² or more.

(2) Yield ratio (YR): 75% or more, Tensile strength (TS): 490 MPa or more When the yield ratio or tensile strength is low, it is necessary to avoid plastic deformation at the time of parts requiring high impact resistance or large inputs. It becomes difficult to apply to uses such as certain parts. Therefore, in the present invention, the yield ratio (YR) is 75% or more, and the tensile strength (TS) is 490 MPa or more.

4). Next, the reason for limiting the weldability of the hot dip galvanized steel sheet will be described.

  The resistance spot welded joint was prepared under the conditions of a direct current resistance spot welding of welding electrode tip diameter: 6 mm, pressure: 4410 kN, welding current: 9 kA, and energization time: 18 cycles.

(1) Ductility ratio (CTS / TSS), which is the value of the ratio between the cross tension force (CTS) in the cross tension test of the resistance spot welded joint and the shear force (TSS) in the shear test: 0.55 or more As an index indicating reliability, there are a cross tension force (CTS) in a cross tension test of a resistance spot welded joint and a shear force (TSS) in a shear test. Usually, since the cross tension force (CTS) is smaller than the shear force (TSS), the ductility ratio (CTS / TSS) which is the value of the ratio between the cross tension force (CTS) and the shear force (TSS) is high. It can be said that the spot weldability is excellent. Therefore, in the present invention, the ductility ratio (CTS / TSS) is set to 0.55 or more.

(2) Ratio value of the Vickers hardness ratio between the molten metal part of the resistance spot welded joint and the base metal: 2.0 or less When the hardness ratio between the molten metal part of the resistance spot welded joint and the base material is large, the ductility ratio ( It becomes difficult to set CTS / TSS to 0.55 or more. Therefore, the smaller the value of the ratio of the Vickers hardness between the molten metal part of the resistance spot welded joint and the base material, the better.

5. Manufacturing method Next, the suitable manufacturing method of the hot dip galvanized steel plate concerning this invention is demonstrated.

(1) Hot rolling step The slab having the above chemical composition is subjected to rough hot rolling at 1100 ° C. or higher to form a rough bar, and the rough bar is heated to 1000 ° C. or higher and then rolled in a temperature range of ₃ or more points of Ar. Finished hot rolling is performed to obtain a hot-rolled steel sheet, which is cooled to a temperature range of 600 ° C. or higher and 700 ° C. or lower at an average cooling rate of 35 ° C./second or higher, and then to a temperature range of 400 ° C. or higher and 650 ° C. or lower. Cooling and winding at an average cooling rate of not less than 40 ° C / second and not more than 40 ° C / second.

  The temperature of the slab used for rough hot rolling is 1100 ° C. or higher. If the temperature of the slab to be subjected to rough hot rolling is less than 1100 ° C, the final product remains coarse as the Ti and Nb carbides, nitrides, and carbonitrides precipitated during casting do not sufficiently re-dissolve. It may remain on a certain hot-dip galvanized steel sheet, leading to a decrease in hole expansibility and formability. Moreover, when the tensile strength is improved by precipitation strengthening with re-dissolved Ti or Nb, the amount of Ti or Nb re-dissolving decreases, so that precipitation strengthening with re-dissolved Ti or Nb is insufficient. Thus, the target tensile strength may not be obtained. Therefore, the temperature of the slab used for rough hot rolling is 1100 ° C. or higher. The upper limit of the temperature of the slab to be subjected to rough hot rolling need not be specified, but if it is excessively high, the yield decreases due to the scale, so it is preferably 1400 ° C. or lower.

  A coarse bar obtained by rough hot rolling is heated to 1000 ° C. or higher. By heating the coarse bar to 1000 ° C. or higher, precipitation of Ti and Nb in the high temperature range is suppressed, and fine precipitates can be sufficiently generated in the low temperature range after finish hot rolling described later. Further, the steel can be strengthened by precipitation strengthening of Nb. Moreover, the scale production | generation of the steel plate surface is accelerated | stimulated, the peeling becomes easy, and the steel plate which has a beautiful surface can be obtained. Therefore, the coarse bar heating temperature is set to 1000 ° C. or higher. Although the upper limit of the heating temperature of the coarse bar is not particularly defined, it is preferably 1300 ° C. or less from the viewpoint of productivity.

The finishing hot rolling completion temperature is ₃ points or more for Ar. When the finish hot rolling completion temperature is less than Ar ₃ points, the steel structure becomes a non-uniform band structure, and the product formability deteriorates. The upper limit of the finish hot rolling completion temperature does not need to be specified in particular, but if the finish hot rolling completion temperature is extremely high, the scale may be excessively generated to induce surface defects. It is preferable.
After the finish hot rolling is completed, the ferrite area ratio is increased by rapidly cooling and holding to the temperature range where the ferrite transformation is activated. For this reason, the primary cooling which cools to the temperature range of 600 degreeC or more and 700 degrees C or less with an average cooling rate of 35 degrees C / sec or more is performed. If the average cooling rate of the primary cooling is less than 35 ° C./second, it may be difficult to secure sufficient time for ferrite transformation due to equipment constraints, and it may be difficult to secure a predetermined ferrite area ratio. .

  After the primary cooling, an average cooling rate of 5 ° C./second to 40 ° C./second up to a temperature range of 400 ° C. to 650 ° C. to promote the formation of carbides of Ti and Nb while suppressing excessive ferrite transformation. Secondary cooling is performed for 5 seconds or more. If the average cooling rate of the secondary cooling is less than 5 ° C / second or the end temperature of the secondary cooling exceeds 650 ° C, the ferrite transformation proceeds excessively, and the area ratio of bainite and residual austenite is In some cases, the desired strength cannot be obtained. On the other hand, if the average cooling rate of the secondary cooling exceeds 40 ° C./second, the end temperature of the secondary cooling is less than 400 ° C., or the secondary cooling time is less than 5 seconds, Since the formation of carbides does not proceed sufficiently, even with a chemical composition that satisfies the above formula (1), a large amount of C remains in the steel in a solid solution state, which may reduce resistance spot weldability.

(2) Pickling process The pickling process is performed on the hot-rolled steel sheet obtained by the hot rolling process.
Pickling may be performed according to a conventional method. In addition, before or after pickling, skin pass rolling may be performed for flattening correction or scale peeling promotion, which does not affect the effect of the present invention. There is no particular need to define the elongation in the case of performing the skin pass rolling, and it may be, for example, 0.3% or more and less than 3.0%.

(3) Cold rolling process The hot rolled steel sheet obtained by the pickling process may be cold rolled to form a cold rolled steel sheet. Cold rolling may follow a conventional method.

(4) Soaking process and hot-dip galvanizing process The hot-rolled steel sheet obtained by the pickling process or the cold-rolled steel sheet obtained by the cold rolling process is maintained at a holding temperature satisfying the following formula (4). After performing soaking, the hot dip galvanizing process is performed. It is preferable from the viewpoint of productivity that the soaking process and the hot dip galvanizing process are continuously performed in a continuous hot dip galvanizing facility.

900-T ₂ × 0.2 ≦ T ≦ 1000 (4)
Here, T ₂ in the formula indicates the end temperature (° C.) of the secondary cooling in the hot rolling process, and T indicates the holding temperature (° C.) in the soaking process.

If the soaking temperature is less than (900-T ₂ × 0.2) ° C., the transformation to austenite is insufficient, and the desired retained austenite and martensite area ratio cannot be obtained. It becomes difficult to ensure ductility. Therefore, the soaking temperature is set to (900−T ₂ × 0.2) ° C. or higher. On the other hand, when the soaking temperature exceeds 1000 ° C., crystal grains grow excessively, and it may be difficult to obtain the intended strength. In addition, coarse pearlite and cementite are likely to precipitate, and the target moldability may not be obtained. Therefore, the soaking temperature is set to 1000 ° C. or less.

  The cooling conditions after the soaking process need not be specified. From the viewpoint of promoting the precipitation of Ti carbide and ensuring better spot weldability, the average cooling rate is preferably 10 ° C./second or less.

The hot dip galvanizing treatment may be performed according to a conventional method. Further, an alloying treatment may be further performed to obtain an alloyed hot-dip galvanized steel sheet. In this case, the alloying treatment temperature is preferably 400 ° C. or more and 600 ° C. or less. The coating weight is generally may be set to 25 g / ^{m 2} or more 70 g / ^{m 2} or less range, which is used as a product. Moreover, you may give a skin pass rolling to the steel plate of this invention for shape correction.

  Thus, the hot-dip galvanized steel sheet according to the present invention is provided by the manufacturing method according to the present invention.

Specific examples of the present invention will be described below.
Steel having 1 to 12 chemical components shown in Table 1 was melted in a converter and continuously cast using a continuous casting tester to obtain a slab having a width of 1400 mm and a thickness of 250 mm. The obtained slab was hot-rolled under the conditions shown in Table 2. The heating temperature of the coarse bar was 1030 ° C. The obtained hot-rolled steel sheet was pickled. Some steel plates were cold-rolled at a rolling reduction of 45%. The obtained hot-rolled steel sheet and cold-rolled steel sheet were subsequently subjected to galvannealing treatment under the conditions shown in Table 3. Some steel plates were also alloyed at 510 ° C. after plating.

(1) Evaluation of steel structure About the cross section parallel to the rolling direction of a steel plate, the area ratio of phases and structures other than retained austenite was determined by image processing using an optical microscope or an electron microscope. The area ratio of residual austenite was obtained by subjecting each steel plate to chemical polishing for reducing 25% of the plate thickness and calculating the amount of residual austenite by X-ray diffraction on the surface after chemical polishing.

(2) Tensile test JIS No. 5 tensile test specimens were collected from various steel sheets in the direction perpendicular to the rolling direction, and the yield strength (YS), tensile strength (TS), and total elongation (El) were investigated.

(3) Formability Formability was evaluated using the yield ratio (YR) and total elongation (El) determined by a tensile test, and the hole expansion rate (λ) determined by a hole expansion test. The hole expansion test was conducted in accordance with Japan Iron and Steel Federation Standard JFS T 1001. When the yield ratio (YR) was 75% or more and the product El × λ value with the total elongation (El) was 1500% ² or more, it was determined that the moldability was good.

(4) Ductility ratio Spot weldability was performed under the conditions of a tip diameter of the welding electrode of 6 mm, a direct current power source, a pressure of 450 kg, a current of 9 kA, and an energization time of 18 cycles. After spot welding, the cross tension force (CTS) by the cross tension test of JIS Z 3137 and the shear force (TSS) by the shear test of JIS Z 3136 were measured.

(5) Ratio of Vickers hardness between the molten metal part of the resistance spot welded joint and the base material The Vickers hardness of the molten metal part of the resistance spot welded joint and the base material is determined by the Vickers hardness test described in JIS Z 2244. investigated. The test force was Hv 0.5. The hardness at the 1/4 depth position of the plate thickness from the steel plate surface was measured for the cross section parallel to the rolling direction. The hardness of the molten metal was measured at 5 points at 0.2 mm pitch from the center of the nugget diameter after spot welding, and the hardness of the base metal was measured at 10 points at 0.2 mm pitch on the steel plate before welding. The value obtained by dividing the average hardness of the molten metal portion by the average hardness of the base material was defined as the ratio of the Vickers hardness between the molten metal portion and the base material.

<Invention>
Specimen No. which is the present invention. 1-19, the product of the total elongation (El) and the hole expansion ratio (λ) (El × λ value) is 1500% ² or more, the yield ratio (YR) is 75% or more, and the tensile strength (TS) is 490 MPa or more. It has the following mechanical properties, high strength and good moldability. The resistance spot welded joint has a ductility ratio (CTS / TSS) of 0.55 or more, which is the value of the ratio between the cross tensile force (CTS) in the cross tensile test and the shear force (TSS) in the shear test. The value of the ratio of Vickers hardness between the molten metal part and the base material is 2.0 or less, and it also has good weldability.

<Comparative example>
Specimen No. No. 20 was outside the present invention because no coarse bar heating was performed. Precipitates were not formed finely, the density was 100 pieces / μm ² or less, the intended tensile strength was not obtained, and the moldability was insufficient.

Specimen No. No. 21 was outside the scope of the present invention because the slab heating temperature was insufficient. At the time of slab heating, fine precipitates were not sufficiently formed after hot rolling, the density was 100 / μm ² or less, and the intended tensile strength was not obtained, and the moldability was insufficient.

Specimen No. No. 22 was outside the present invention because the finish rolling completion temperature was Ar ₃ or less. Due to the reduction of the rolling temperature, the structure was not mainly composed of ferrite, and the formability was insufficient.
Specimen No. No. 23 was outside the scope of the present invention because the primary cooling rate did not satisfy 35 ° C./second. Since the primary cooling rate is slow, the time in the ferrite transformation temperature range is not sufficient, the structure is not mainly ferrite, and the moldability is insufficient.

  Specimen No. No. 24 was outside the present invention because the secondary cooling rate was less than 5 ° C./second. Sufficient tensile strength could not be obtained because the transformation of ferrite progressed excessively in the high temperature range and the desired metal structure could not be obtained.

  The sample material N0.25 had a secondary cooling rate of 40 ° C./second or more, and was outside the scope of the present invention. Precipitation of Ti and Nb was not sufficient, and the amount of solid solution C in the steel sheet increased, so that good weldability could not be obtained.

  Specimen No. No. 26 was outside the scope of the present invention because the soaking temperature was out of formula (4). The austenite transformation was insufficient, residual austenite and martensite were not obtained, and the moldability deteriorated. Moreover, since the production | generation of the precipitate was inadequate and the amount of solid solution C in a steel plate increased, favorable weldability was not obtained.

  Specimen No. No. 27 had a C content of less than 0.02% and was outside the scope of the present invention. The area ratio of bainite, austenite and martensite was insufficient, and the intended strength could not be obtained.

Specimen No. 28 was outside the scope of the present invention, with the C content exceeding 0.1. Good weldability could not be obtained due to the high amount of C in the steel sheet.
Specimen No. No. 29 did not satisfy the formula (1) and was outside the scope of the present invention. Since the amount of solute C is high, the formability is not sufficient, and further good weldability cannot be obtained.

  Furthermore, the test material No. 30 did not satisfy the formula (3) and was outside the scope of the present invention. The amount of Mn and the amount of S were high, the moldability was not sufficient, and even better weldability was not obtained.

Claims (8)

A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet,
The steel sheet is, in mass%, C: 0.02% to 0.10%, Si: 0.005% to 0.5%, Mn: 1.4% to 2.5%, P: 0 0.025% or less, S: 0.010% or less, sol. Al: 0.001% or more and 0.2% or less, N: 0.008% or less and Ti: 0.15% or less, Ca: 0.01% or less, Mg: 0.01% or less, and REM : Containing one or more selected from the group consisting of 0.01% or less, having a chemical composition satisfying the following formulas (1) to (3), and in area%, ferrite: 50% A steel structure containing 94% or less, bainite: 5% or more and 49% or less, and a total of martensite and retained austenite: 1% or more and 20% or less,
The hot-dip galvanized steel sheet has a product (El × λ value) of total elongation (El) and hole expansion rate (λ) of 1500% ² or more, yield ratio (YR) of 75% or more, and tensile strength (TS). Cross-tension test of resistance spot welded joints with mechanical properties of 490 MPa or higher, welding electrode tip diameter: 6 mm, applied pressure: 4410 kN, welding current: 9 kA, and energization time: 18 cycles of direct current resistance spot welding conditions The ductility ratio (CTS / TSS), which is the value of the ratio of the cross tensile force (CTS) in the shear test to the shear force (TSS) in the shear test, is 0.55 or more, and the Vickers between the molten metal part of the resistance spot welded joint and the base material A hot-dip galvanized steel sheet having resistance spot weldability having a hardness ratio value of 2.0 or less.
C− (12/48) × Ti ^* − (12/93) × Nb ≦ 0.090 (1)
Ti ^* = max [Ti− (48/14) × N− (48/32) × S, 0] (2)
2/3 × C + (1/150) × Mn + P + 2 × S <0.15 (3)
Here, each element symbol in the formulas (1) to (3) indicates the content (unit: mass%) of each element, and max [] in the formula (2) is the maximum value among the arguments in []. A function to return.   The hot-dip galvanized steel sheet according to claim 1, wherein the chemical composition further contains, by mass%, Bi: 0.005% or less.   The chemical composition is% by mass, Nb: 0.15% or less, Cr: 1% or less, V: 0.1% or less, Mo: 0.5% or less, Cu: 1% or less, Ni: 1% or less The hot-dip galvanized steel sheet according to claim 1 or 2, further comprising one or more selected from the group consisting of B and 0.005% or less. The ferrite includes Ti, Nb-containing carbides, nitrides, and composites having a particle diameter of 1 nm to 20 nm in a number density of 100 / μm ² or more. The hot-dip galvanized steel sheet according to any one of claims 3 to 4.   The hot-dip galvanized steel sheet according to any one of claims 1 to 4, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer. A method for producing a hot-dip galvanized steel sheet, comprising the following steps (A) to (D):
(A) The slab having the chemical composition according to any one of claims 1 to 5 is subjected to rough hot rolling at 1100 ° C or higher to obtain a rough bar, and the coarse bar is heated to 1000 ° C or higher. A hot rolled steel sheet is obtained by performing finish hot rolling to complete rolling in a temperature range of Ar ₃ or higher afterwards, and primary cooling is performed at an average cooling rate of 35 ° C./second or more to a temperature range of 600 ° C. to 700 ° C., Next, a hot rolling process in which secondary cooling is performed at an average cooling rate of 5 ° C./second or more and 40 ° C./second or less to a temperature range of 400 ° C. or more and 650 ° C. or less, and then wound.
(B) a pickling step of subjecting the hot-rolled steel sheet obtained by the hot rolling step to a pickling treatment;
(C) a soaking process for maintaining the hot-rolled steel sheet obtained by the pickling process at a holding temperature satisfying the following formula (4); and (D) hot-plating the hot-rolled steel sheet obtained by the soaking process. Hot-dip plating process.
900-T ₂ × 0.2 ≦ T ≦ 1000 (4)
Here, T ₂ in the formula indicates the end temperature (° C.) of the secondary cooling in the hot rolling process, and T indicates the holding temperature (° C.) in the soaking process. A method for producing a hot-dip galvanized steel sheet, comprising the following steps (a) to (e):
(A) The slab having the chemical composition according to any one of claims 1 to 5 is subjected to rough hot rolling at 1100 ° C or higher to obtain a rough bar, and the rough bar is heated to 1000 ° C or higher. A hot rolled steel sheet is obtained by performing finish hot rolling to complete rolling in a temperature range of Ar ₃ or higher afterwards, and primary cooling is performed at an average cooling rate of 35 ° C./second or more to a temperature range of 600 ° C. to 700 ° C., Next, a secondary rolling at a temperature range of 400 ° C. to 650 ° C. at an average cooling rate of 5 ° C./second to 40 ° C./second for 5 seconds or more, followed by a hot rolling step of winding;
(B) a pickling step of performing a pickling treatment on the hot-rolled steel sheet obtained by the hot rolling step;
(C) a cold rolling process in which the hot rolled steel sheet obtained by the pickling process is cold rolled to obtain a cold rolled steel sheet;
(D) a soaking process for maintaining the cold-rolled steel sheet obtained by the cold rolling process at a holding temperature satisfying the following formula (4); and (e) melting in the cold-rolled steel sheet obtained by the soaking process. Hot dipping process for plating.
900-T ₂ × 0.2 ≦ T ≦ 1000 (4)
Here, T ₂ in the formula indicates the end temperature (° C.) of the secondary cooling in the hot rolling process, and T indicates the holding temperature (° C.) in the soaking process.   The hot dip galvanizing according to claim 6 or 7, wherein in the process of cooling to room temperature after the hot dip galvanizing, the alloying treatment is performed while maintaining the temperature range of 480 ° C to 600 ° C. A method of manufacturing a steel sheet.