KR20170096207A - Plating steel sheet excellent in plating property, workability and delayed fracture resistance, and manufacturing method thereof - Google Patents

Abstract

Plating ability; Balance of strength and ductility and workability of bending property; And 980 MPa or more of hot-dip galvanized steel sheet and galvannealed hot-dip galvanized steel sheet excellent in delayed fracture resistance. A high-strength plated steel sheet of the present invention is a plated steel sheet having a plated layer on the surface of a base steel sheet, characterized in that it comprises a predetermined steel component and has a thickness of the base steel sheet in order from the interface between the base steel sheet and the plated layer toward the base steel sheet, A soft layer having a Vickers hardness of 90% or less of Vickers hardness at a t / 4 portion of the base steel sheet; Martensite, and a hard layer containing bainite and ferrite in a predetermined range; and the average depth D of the soft layer is not less than 20 占 퐉 and the average depth d of the inner oxide layer is not less than 4 占 퐉.

Description

Plating steel sheet excellent in plating property, workability and delayed fracture resistance, and manufacturing method thereof

The present invention relates to a high strength plated steel sheet having a tensile strength of 980 MPa or more and excellent in workability including both plating ability, balance between strength and ductility and ductility, and delayed fracture resistance, and a production method thereof. The coated steel sheet of the present invention includes both a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet.

The hot-dip galvanized steel sheet and the galvannealed hot-dip galvanized steel sheet which are generally used in the fields of automobiles and transportation are required to have excellent workability such as balance of strength and ductility and ductility and bending resistance in addition to high strength.

It is effective to add a large amount of strengthening elements such as Si and Mn in the steel in order to secure high strength and workability. However, since Si and Mn are easily oxidizable elements, the wettability of the hot dip galvanizing is remarkably deteriorated by the Si oxide, Mn oxide, complex oxide of Si and Mn formed on the surface, and problems such as plating are caused.

Therefore, various techniques have been proposed for improving the workability and the like in a plated steel sheet containing a large amount of Si or Mn.

For example, Patent Document 1 discloses a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent in bendability and corrosion resistance of a processed portion. Specifically, in Patent Document 1, the growth of the decarburized layer is compared with the growth of the internal oxide layer so as to suppress occurrence of bending cracks and damage of the plating film due to the internal oxide layer formed on the steel plate side from the interface between the steel plate and the plating layer It is remarkably fast. Also disclosed is a surface near-surface structure in which the thickness of the internal oxide layer in the ferrite region formed by decarburization is controlled to be thin.

Patent Document 2 discloses a hot-dip galvanized steel sheet having a tensile strength of 770 MPa or more which is excellent in fatigue durability, water-proofing (equivalent to delayed fracture resistance), and excellent bendability. Specifically, in Patent Document 2, the steel plate portion has a structure in which a soft layer directly contacting the interface with the plated layer and a soft layer having a maximum area ratio of ferrite are provided. It is preferable that the thickness D of the soft layer and the depth d of the oxide containing at least one of Si and Mn present in the surface layer of the steel sheet from the plating / A coated steel sheet is disclosed.

Patent Document 3 discloses a high strength cold rolled steel sheet having an excellent tensile strength of 700 MPa or more and excellent bendability. Specifically, Patent Document 3 discloses that a surface layer of a steel sheet can be softened by decarburization treatment, and even if a high-strength cold-rolled steel sheet having a tensile maximum strength of 700 MPa or more, excellent bendability as if it is a steel sheet with low strength is obtained.

Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet which is excellent in ductility and strength, is excellent in resistance to delayed fracture, and has low anisotropy of delayed fracture characteristics even in thin plates. Specifically, Patent Document 4 discloses that a surface layer portion of a base metal sheet is formed of a decarburized layer having a small hard texture and a fine grained layer which functions as a trap site of hydrogen in order to prevent delayed fracture starting from the surface layer portion of the base metal sheet And the oxide is dispersed at a high density.

Patent Document 5 discloses a high strength steel sheet having a maximum tensile strength of 900 MPa or more and excellent moldability and water-proofing property. Specifically, Patent Document 5 discloses that since the steel sheet has a decarburized layer (softened layer) that is softer than the inside of the steel sheet in the surface layer, the steel sheet has excellent water-resisting disintegration properties (delayed fracture resistance) .

Japanese Patent Application Laid-Open No. 2011-231367 Japanese Patent No. 4943558 Japanese Patent No. 5454746 Japanese Patent No. 5352793 Japanese Patent Application Laid-Open No. 11-111675

As described above, various techniques for improving the workability and the like of a plated steel sheet containing a large amount of Si and Mn have heretofore been proposed. However, various characteristics required for the coated steel sheet, i.e., high strength of 980 MPa or more; Plating ability; Balance and workability of strength and ductility; It is desired to provide a technique that combines both the characteristics of the delayed fracture and the delayed fracture.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to provide a plating method, Balance of strength and ductility and workability of bending property; And 980 MPa or more of hot-dip galvanized steel sheet and galvannealed steel sheet excellent in delayed fracture resistance, and a method for producing the same.

A high strength plated steel sheet having a tensile strength of 980 MPa or more according to the present invention capable of solving the above problems is a plated steel sheet having a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer on the surface of a base steel sheet. P: more than 0% to 0.1%, S: more than 0% to less than 0.05%, Al: 0.005 to 1%, and C: 0.08 to 0.30%, Si: 0.25 to 3%, Mn: 1.5 to 4% N: not less than 0% and not more than 0.01%, and the balance of iron and inevitable impurities; (2) from the interface between the base steel sheet and the plating layer toward the base steel sheet in order from the group consisting of Si and Mn An inner oxide layer containing at least one kind of oxide; A soft layer having a Vickers hardness of 90% or less of a Vickers hardness at a t / 4 portion of the base steel sheet when the base steel sheet has the inner oxide layer and the thickness of the base steel sheet is t; Martensite and bainite: at least 20% by area and less than 60% by area, and polygonal ferrite: at least 40% by area and at most 80% by area, wherein the soft layer has an average depth D of not less than 20 μm and The average depth d of the internal oxide layer satisfies the following condition: 4 탆 or more and less than D, and the tensile strength is 980 MPa or more.

In a preferred embodiment of the present invention, the base steel sheet further comprises at least one of the following (a) to (c) in mass%.

(a) at least one member selected from the group consisting of Cr: more than 0% to 1%, Mo: more than 0% to 1%, and B: more than 0% to 0.01%

(b) at least one selected from the group consisting of Ti: more than 0% to 0.2%, Nb: more than 0% to 0.2%, and V: more than 0% to 0.2%

(c) at least one selected from the group consisting of Cu: more than 0% to 1%, and Ni: more than 0% to 1%

In a preferred embodiment of the present invention, the average depth d of the inner oxide layer and the average depth D of the soft layer satisfy a relationship of D > 2d.

The method of manufacturing a high-strength plated steel sheet according to any one of the above is characterized in that the hot-rolled steel sheet satisfying the above-mentioned components in the steel is hot-rolled at a temperature of 600 ° C or higher, A process; A step of pickling and cold rolling so that the average depth d of the internal oxide layer is 4 占 퐉 or more; Oxidizing at an oxidizing zone at an air ratio of 0.9 to 1.4; A step of holding in the range of (Ac ₁ point + 30 ° C) to Ac ₃ point, thereby cracking; And a step of cooling the range from the fracture to the cooling stop temperature at an average cooling rate of 5 deg. C / sec or more in this order.

Another manufacturing method of the present invention capable of solving the above problems is a method for producing a high strength coated steel sheet according to any one of the above, wherein the hot rolled steel sheet satisfying the above-mentioned components in steel is rolled up at a temperature of 500 캜 or higher A hot rolling process; Keeping at a temperature of 500 DEG C or higher for 80 minutes or longer; A step of pickling and cold rolling so that the average depth d of the internal oxide layer is 4 占 퐉 or more; Oxidizing at an oxidizing zone at an air ratio of 0.9 to 1.4; A step of holding in the range of (Ac ₁ point + 30 ° C) to Ac ₃ point, thereby cracking; And a step of cooling the range from the fracture to the cooling stop temperature at an average cooling rate of 5 deg. C / sec or more in this order.

The coated steel sheet of the present invention comprises an inner oxide layer containing at least one oxide selected from the group consisting of Si and Mn from the interface between the plated layer and the base steel sheet to the base steel sheet side, (Including martensite and bainite: not less than 20% by area and not more than 60% by area and polygonal ferrite: not less than 40% by area and not more than 80% by area) as the parent layer other than the soft layer Particularly, since the average depth d of the internal oxide layer is controlled to be thicker than 4 탆 and used as a hydrogen trap site, hydrogen embrittlement can be effectively suppressed, and workability such as balance of strength and ductility and bendability can be effectively prevented. A high strength plated steel sheet having a tensile strength of 980 MPa or more excellent in all of the delayed fracture characteristics is obtained. Preferably, since the relationship between the average depth d of the inner oxide layer and the average depth D of the soft layer including the region of the inner oxide layer is suitably controlled, the bending property and the delayed-failure resistance property are further enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram schematically illustrating the layer structure extending from the interface between the plated layer and the base steel sheet to the base steel sheet in the plated steel sheet of the present invention. Fig.
2 is an explanatory diagram for measuring the average depth d of the internal oxide layer in the coated steel sheet of the present invention.
Fig. 3 is a view for explaining the measurement position of the Vickers hardness used for determining the average depth D of the soft layer. Fig.

The present inventors have found out that in order to provide a high strength plated steel sheet having a high strength of 980 MPa or more and excellent in plating ability, workability and delayed fracture resistance characteristics and further in impact resistance in a base steel sheet containing a large amount of Si and Mn, The layer structure extending from the interface between the plated layer and the base steel sheet to the base steel sheet has been studied. As a result, as shown in the following schematic diagram of Fig. 1, (A) the layer structure extending from the interface between the plated layer and the base steel sheet to the base steel sheet side is changed to a layer structure comprising at least one oxide selected from the group consisting of Si and Mn A soft layer comprising an inner oxide layer; And a hard layer comprising martensite and bainite: at least 20% by area and less than 60% by area, and polygonal ferrite: at least 40% by area and not more than 80% by area, B) If the average depth d of the internal oxide layer is controlled to be 4 μm or more, the internal oxide layer can function as a hydrogen trap site and hydrogen embrittlement can be effectively suppressed. (C) Preferably, it is found out that the control of the relationship between the average depth d of the inner oxide layer and the average depth D of the soft layer including the region of the inner oxide layer improves the bending property and the anti-delay fracture property, Thus, the present invention has been completed.

In the present specification, the coated steel sheet includes both a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet. In the present specification, the base steel sheet means a steel sheet before the hot-dip galvanized layer and the galvannealed hot-dip galvanized layer are formed, and is distinguished from the coated steel sheet.

In this specification, high strength means that the tensile strength is 980 MPa or more.

In the present specification, excellent workability means that both balance of strength and ductility and bending property are excellent. For details, when these characteristics were measured by the method described in the later examples, those satisfying the acceptance criteria of the examples were called "excellent in workability".

As described above, the plated steel sheet of the present invention has a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer (hereinafter sometimes referred to as a plated layer) on the surface of the base steel sheet. The feature of the present invention resides in that it has the following layers (A) to (C) in order from the interface between the base steel sheet and the plated layer toward the base steel sheet.

(A) Internal oxide layer: A layer containing at least one oxide selected from the group consisting of Si and Mn. The average depth d of the inner oxide layer is 4 占 퐉 or more and less than the average depth D of the soft layer described in (B) described later.

(B) Soft layer: The inner oxide layer, wherein Vickers hardness satisfies 90% or less of the Vickers hardness at t / 4 part of the base steel sheet when the thickness of the base steel sheet is t. The average depth D of the soft layers is 20 占 퐉 or more.

(C) hard layer: martensite and bainite: 20% by area or more and less than 60% by area and polygonal ferrite: 40% by area or more and 80% by area or less.

Hereinafter, the layers constituting the above (A) to (C), which characterize the present invention, will be described in detail with reference to FIG. 1, the layered structure of the coated steel sheet of the present invention on the side of the base steel sheet 2 has a layer structure of (B (1)) from the interface between the plated layer 1 and the base steel sheet 2 toward the base steel sheet 2, And a hard layer 5 in the inside of the soft steel layer 2 side than the soft layer 4. The soft layer (4) of (B) comprises the inner oxide layer (3) of (A). In addition, the soft layer 4 and the hard layer 5 are continuously present.

(A) On the inner oxide layer

First, the portion directly contacting the interface between the plated layer 1 and the base steel sheet 2 has the internal oxide layer 3 having an average depth d of 4 탆 or more. Here, the average depth means an average depth from the interface, and a detailed measurement method thereof will be described with reference to Fig. 2 in the later section of the embodiment.

The internal oxide layer 3 is composed of an oxide containing at least one of Si and Mn and a depletion layer of Si and Mn in which Si and Mn form a solid oxide around which Si and Mn are contained.

In the present invention, the maximum feature is that the average depth d of the internal oxide layer 3 is controlled to be as large as 4 m or more. Thereby, the internal oxide layer can be utilized as a hydrogen trap site, hydrogen embrittlement can be suppressed, and bendability and delayed fracture characteristics are improved.

On the other hand, in the base steel sheet containing a large amount of easy oxidizing elements such as Si and Mn as in the present invention, it is preferable that at the time of annealing (later-described oxidation-reduction process in the continuous hot-dip galvanizing line) An oxide, an oxide film having a complex oxide of Si and Mn tends to be formed, and the plating ability is deteriorated. As a countermeasure therefor, a method is known in which an Fe oxide film is formed by oxidizing the surface of a base steel sheet in an oxidizing atmosphere, followed by annealing (reduction annealing) in an atmosphere containing hydrogen. Further, by controlling the atmosphere in the furnace, the easy oxidizing element is fixed as an oxide in the surface layer of the substrate steel sheet to reduce the easy oxidizing element contained in the surface layer of the substrate steel sheet, thereby preventing the formation of an oxide film on the surface of the substrate sheet Is also known.

However, according to the examination results of the inventors of the present invention, it has been found that in a redox method commonly used for plating a base steel sheet containing a large amount of Si and Mn, hydrogen enters the base steel sheet in a hydrogen atmosphere during reduction, The degradation of scalability occurs; In order to improve these deterioration, it has been found that the utilization of at least one kind of oxide selected from the group consisting of Si and Mn is effective. Specifically, the oxide is useful as a hydrogen trap site capable of preventing intrusion of hydrogen into the inner side of a steel sheet during reduction and improving bending property, hole expandability and delayed fracture resistance, , It has been found that it is indispensable to form an average depth d of the internal oxide layer containing the oxide to 4 mu m or more.

In the present invention, the upper limit of the average depth d of the inner oxide layer is at least less than the average depth D of the soft layer of (B) described later. The upper limit of d is preferably 30 mu m or less. In order to increase the thickness of the internal oxide layer, it is necessary to maintain the high temperature region for a long time after hot-rolled coils, but the above-mentioned preferable value is obtained due to productivity and facility limitations. More preferably, d is 18 탆 or less, more preferably 16 탆 or less. On the other hand, the value d is preferably 6 m or more, more preferably 8 m or more.

In the present invention, it is preferable to control the average depth d of the internal oxide layer so as to satisfy the relation of D > 2d in relation to the average depth D of the soft layer of (B) The delayed fracture property, particularly the bending property, is further improved. On the other hand, in the above-mentioned Patent Document 2, d / 4? D is satisfied with respect to the depth d of the presence of oxide and the thickness D of the soft layer, which approximately correspond to the average depth d of the inner oxide layer and the average depth D of the soft layer, ? D, and the direction of control is completely different from the relationship (D > 2d) defined in the present invention. In addition, in Patent Document 2, it is described that the range of the depth d of presence of oxide is controlled while satisfying the relationship of d / 4? D? 2d basically, and the average depth d There is no basic idea that the thickness is controlled to 4 μm or more. Of course, the effect of the present invention that the action as a hydrogen trap site is effectively exhibited thereby to improve the bending property and the delayed fracture resistance property is not described.

On the other hand, in the present invention, in order to control the average depth d of the internal oxide layer to 4 탆 or more, it is necessary to control the average depth of the internal oxide layer in the cold-rolled steel sheet before passing through the continuous hot- Do. Details will be described later in the manufacturing method section. That is, as shown in the later examples, the internal oxide layer after pickling and cold rolling is taken over by the internal oxide layer in the finally obtained coated steel sheet after passing through the plating line.

(B) About the soft layer

As shown in Fig. 1, the soft layer 4 in the present invention is a layer including the region of the internal oxide layer 3 of the above (A), and the Vickers hardness is the t / 4 part of the base steel sheet 2 Of 90% or less of the Vickers hardness in the nonwoven fabric. A detailed measurement method of the Vickers hardness will be described in the later section of the embodiment.

The soft layer is a soft structure having a Vickers hardness lower than that of the hard layer (C) described later and has excellent deformability, so that the bendability is particularly improved. That is, at the time of bending, the surface layer portion of the base steel sheet becomes a starting point of cracking, but the bending property is improved particularly by forming a predetermined soft layer on the surface layer of the base steel sheet as in the present invention. Furthermore, by forming the soft layer, it is possible to prevent the oxide in (A) from becoming a starting point of cracking at the time of bending, and thus it is possible to enjoy only the advantages as the hydrogen trap site described above. As a result, not only the bendability but also the delayed fracture characteristics are further improved.

In order to effectively exhibit the effect of forming such a soft layer, the average depth D of the soft layer is set to 20 m or more. The above D is preferably 22 탆 or more, more preferably 24 탆 or more. On the other hand, if the average depth D of the soft layer is excessively large, the strength of the plated steel sheet itself is lowered. Therefore, the upper limit of the average depth D is preferably 100 탆 or less. It is more preferable that D is 60 탆 or less.

(C) For the hard layer

In the present invention, as shown in Fig. 1, the hard layer is formed on the side of the base steel sheet 2 of the soft layer 4 of the above-mentioned (B), and further includes martensite and bainite: % And polygonal ferrite: more than 40% by area and less than 80% by area. The martensite of the hard layer 5 may be tempered. The higher the total area ratio of bainite and martensite (i.e., the smaller the area ratio of ferrite), the higher the strength and the lower the total area ratio of bainite and martensite (i.e., the larger the area ratio of ferrite ) Ductility tends to be improved. Also, when the area ratio of ferrite is decreased, balance between strength and ductility is deteriorated. Therefore, it is recommended that the preferable area ratio of these tissues be appropriately set in consideration of the relationship with the desired characteristics. For example, the total area ratio of bainite and martensite is preferably 45 percent by area or more, and the total area ratio of ferrite is preferably 55 percent by area or less, from the viewpoint of strength improvement. Considering the improvement in balance between strength and ductility, it is preferable that the total area ratio of bainite and martensite is 40% or less and the total area ratio of ferrite is 60% or more.

The hard layer may contain, in addition to the above-described structure, a structure that can be incorporated in the manufacturing process, for example, retained austenite (?), Pearlite and the like within the range not impairing the action of the present invention. The structure is at most 15 area% or less, and the smaller the number is, the better. On the other hand, the above-mentioned organization is described as " Others "

On the other hand, the hard layer in the present invention may contain bainite and martensite in a total area of 20% by area to less than 60% by area, and the ratio of bainite and martensite to each other But it is not limited at all. In the present invention, as long as the above requirements are satisfied, the above-described effect due to the formation of the hard layer is exerted. Therefore, as long as the above requirements are satisfied, the hard layer can satisfy any relationship of bainite> martensite, bainite = martensite, bainite <martensite. Also, a sun consisting of bainite only and not including martensite at all; On the other hand, both of the martensitic solute-free and bainite-free solutes are included in the scope of the present invention. From the above viewpoint, in the later examples, only the total area was measured without distinguishing between bainite and martensite, and the results are shown in Table 3. &lt; tb &gt; &lt; TABLE &gt;

Above, the layer structure from the interface between the plating layer and the base steel sheet, which characterizes the present invention most, toward the base steel sheet has been described.

Next, the components in the steel used in the present invention will be described.

The coated steel sheet according to the present invention contains 0.08 to 0.30% of C, 0.25 to 3% of Si, 1.5 to 4% of Mn, 0.1 to 0.1% of P, 0.1 to 0.05% of S, %, And N: not less than 0% and not more than 0.01%, and the balance of iron and inevitable impurities.

C: 0.08 to 0.30%

C improves the hardenability and is an important element for increasing the strength of steel due to the hardening effect of martensite. In order to effectively exhibit such an effect, the lower limit of the C content is made 0.08% or more. The lower limit of the C content is preferably 0.11% or more, and more preferably 0.13% or more. However, when C is excessively added, the hardness difference between the soft phase and the hard phase becomes large, and the workability and the delayed fracture resistance deteriorate. Therefore, the upper limit of the C content is set to 0.30%. The preferred upper limit of the C content is 0.25% or less, more preferably 0.20% or less.

Si: 0.25 to 3%

Si is an element effective for enhancing the strength of steel by solid solution strengthening and improving workability. It also produces an internal oxide layer and also has an effect of inhibiting hydrogen embrittlement. In order to effectively exhibit such effects, the lower limit of the amount of Si is set to 0.25% or more. The lower limit of the Si content is preferably 0.3% or more, more preferably 0.5% or more, and still more preferably 0.7% or more. However, since Si is a ferrite generating element, if Si is excessively added, generation of ferrite can not be suppressed, and the hardness difference between the soft phase and the hard phase becomes large, and the workability is lowered. Further, since the plating property also deteriorates, the upper limit of the amount of Si is set to 3%. The upper limit of the Si content is preferably not more than 2.5%, more preferably not more than 2.0%.

Mn: 1.5 to 4%

Mn is an element for improving hardenability, suppressing ferrite and bainite, and producing martensite to contribute to high strength. In order to effectively exhibit such effects, the lower limit of the amount of Mn is set to 1.5% or more. The lower limit of the Mn content is preferably 1.8% or more, and more preferably 2.0% or more. However, if Mn is excessively added, the plating ability is lowered and the segregation becomes remarkable. Furthermore, there is a fear that the grain boundary segregation of P is promoted. Therefore, the upper limit of the amount of Mn is set to 4%. The preferable upper limit of the Mn content is 3.5% or less.

P: more than 0% and not more than 0.1%

P is an element to strengthen the steel as an employment strengthening element. In order to effectively exhibit such an effect, the lower limit of the P amount is made to exceed 0%. However, if it is added in excess, there is a fear of deteriorating the weldability and toughness in addition to the workability, so the upper limit thereof is set to 0.1% or less. The amount of P is preferably small, preferably 0.03% or less, and more preferably 0.015% or less.

S: more than 0% and less than 0.05%

S is an element which inevitably contains S, which forms a sulfide such as MnS and becomes a starting point of cracking, which may deteriorate workability. Therefore, the upper limit of the amount of S is set to 0.05% or less. The amount of S is preferably small, preferably 0.01% or less, and more preferably 0.008% or less.

Al: 0.005 to 1%

Al acts as a deoxidizer. Further, Al is combined with N to become AlN, and the workability and delayed fracture characteristics are improved by miniaturization of the austenite grain size. In order to effectively exhibit such an effect, the lower limit of the amount of Al is 0.005% or more. The lower limit of the Al content is preferably 0.01% or more, and more preferably 0.02% or more. However, when Al is excessively added, inclusions such as alumina are increased to deteriorate workability, and toughness also deteriorates. Therefore, the upper limit of the amount of Al is set to 1%. The preferable upper limit of the amount of Al is 0.8% or less, more preferably 0.6% or less.

N: more than 0% and not more than 0.01%

N is an element which inevitably contains, but if it is contained in excess, workability is deteriorated. Further, when B (boron) is added to the steel, BN precipitates are generated to inhibit the effect of improving the hardenability by B, so that N should be reduced as much as possible. Therefore, the upper limit of the amount of N is set to 0.01% or less. The preferable upper limit of the amount of N is 0.008% or less, more preferably 0.005% or less.

The coated steel sheet of the present invention contains the above components, and the balance is iron and unavoidable impurities.

In the present invention, the following optional elements may be contained.

At least one member selected from the group consisting of Cr: more than 0% to 1% or less, Mo: more than 0% to 1% or less, and B: more than 0% to 0.01%

These elements are effective elements for increasing the strength of the steel sheet. These elements may be added alone, or two or more of them may be used in combination.

Specifically, Cr improves hardenability and contributes to increase in strength. Furthermore, Cr inhibits the formation and growth of cementite, and contributes to improvement of bending property. In order to effectively exhibit such an effect, the lower limit of the Cr content is preferably 0.01% or more. However, when Cr is excessively added, the plating ability is lowered. Further, Cr carbide is excessively produced, and the workability is lowered. Therefore, the upper limit of the Cr content is preferably 1% or less. , More preferably not more than 0.7%, and even more preferably not more than 0.4%.

Mo is effective for increasing the strength, and therefore the lower limit of the Mo content is preferably 0.01% or more. However, even if Mo is excessively added, the above action is saturated, and the cost becomes high. Therefore, the preferable upper limit of Mo is 1% or less. , More preferably not more than 0.5%, and still more preferably not more than 0.3%.

B, like Mn, is an element for improving the hardenability, and is an element which inhibits ferrite and bainite and generates martensite to contribute to high strength. In order to effectively exhibit such effects, the lower limit of the amount of B is preferably 0.0002% or more. More preferably, it is 0.0010% or more. However, when the amount of B exceeds the upper limit, the hot workability deteriorates. Therefore, the preferable upper limit of the amount of B is set to 0.01% or less. More preferably not more than 0.0070%, and still more preferably not more than 0.0050%.

At least one selected from the group consisting of Ti: more than 0% to 0.2%, Nb: more than 0% to 0.2%, and V: more than 0% to 0.2%

These elements are effective elements for improving workability and delayed fracture characteristics by microstructure. These elements may be added alone, or two or more of them may be used in combination.

In order to effectively exhibit this action, the lower limit of each of Ti, Nb and V is set to 0.01% or more. However, when the content of each element is excessive, ferrite is generated and the workability is deteriorated. Therefore, the preferable upper limit of each element is set to 0.2% or less. Any element is more preferably 0.15% or less, and more preferably 0.10% or less.

At least one selected from the group consisting of Cu: more than 0% to 1%, and Ni: more than 0% to 1%

Cu and Ni are effective elements for increasing the strength. These elements may be added singly or in combination.

In order to effectively exhibit the above-mentioned action, the preferable lower limit of each of Cu and Ni is 0.01% or more. However, when the content of each element is excessive, the hot workability is deteriorated. Therefore, the preferable upper limit of each element is set to 1% or less. Any element is more preferably 0.8% or less, and more preferably 0.5% or less.

The components in the steel of the present invention have been described above.

Next, a method for manufacturing the coated steel sheet of the present invention will be described. The production method of the present invention includes a first method of pickling immediately after hot rolling without warming, and a second method of warming after hot rolling and pickling the pickled product. Depending on the presence or absence of thermal insulation, the first method (no thermal insulation) and the second method (thermal insulation) differ in the lower limit of the hot-rolled coiling temperature, but the other processes are the same. Hereinafter, it will be described in detail.

[First Manufacturing Method (Insulating)]

The first manufacturing method according to the present invention is roughly divided into a hot rolling process, a pickling and cold rolling process, an oxidation process in a continuous hot galvanizing line (CGL), a reducing process and a plating process. A feature of the present invention resides in a hot-rolling step of obtaining a hot-rolled steel sheet in which an inner oxide layer is formed by winding a steel sheet satisfying the above-mentioned steel components at a temperature of 600 占 폚 or higher; A step of pickling and cold rolling so that the average depth d of the internal oxide layer is 4 占 퐉 or more; Oxidizing at an oxidizing zone at an air ratio of 0.9 to 1.4; A step of holding in the range of (Ac ₁ point + 30 ° C) to Ac ₃ point, thereby cracking; And a step of cooling the crack to a cooling stop temperature at an average cooling rate of 5 deg. C / sec or more, in this order.

Hereinafter, explanation will be given in the order of steps.

First, a hot-rolled steel sheet satisfying the above-mentioned components in the steel is prepared. The hot rolling may be performed according to a conventional method. For example, in order to prevent coarsening of the austenitic grains, the heating temperature is preferably about 1150 to 1300 캜. The finishing rolling temperature is preferably controlled to approximately 850 to 950 占 폚.

In the present invention, it is important to control the coiling temperature after hot rolling to 600 占 폚 or higher. Thereby, an inner oxide layer is formed on the surface of the base steel sheet, and a soft layer is also formed by decarburization, so that a desired inner oxide layer and soft layer can be obtained on the steel sheet after plating. If the coiling temperature is lower than 600 占 폚, the internal oxide layer and the soft layer are not sufficiently generated. Further, the strength of the hot-rolled steel sheet is increased, and the cold-rolling resistance is lowered. The preferred coiling temperature is 620 DEG C or higher, more preferably 640 DEG C or higher. However, if the coiling temperature becomes too high, the blackening scale grows too much and can not be dissolved by pickling, so that the upper limit thereof is preferably 750 캜 or lower.

Next, the hot-rolled steel sheet thus obtained is pickled and cold-rolled so that the average depth d of the inner oxide layer is 4 μm or more. As a result, not only the internal oxide layer but also the soft layer is left, so that a desired soft layer is easily produced after plating. It is known that the thickness of the internal oxide layer is controlled by controlling the pickling conditions. Specifically, the temperature and the time of pickling are appropriately adjusted so that the thickness of the desired internal oxide layer can be ensured depending on the kind and concentration of the pickling solution to be used Control.

For example, a mineral acid such as hydrochloric acid, sulfuric acid, nitric acid and the like can be used as the acidic liquid.

Further, in general, when the concentration or temperature of the acid tax solution is high and the pickling time is long, the internal oxide layer tends to be dissolved and thinned. Conversely, if the concentration or temperature of the pickling liquid is low and the pickling time is short, removal of the blackening scale layer by pickling becomes insufficient. Therefore, for example, when using hydrochloric acid, it is recommended to control the concentration to about 3 to 20%, the temperature to 60 to 90 DEG C, and the time to about 35 to 200 seconds.

On the other hand, the number of pickling baths is not particularly limited, and a plurality of pickling baths may be used. Furthermore, acid pickling agents such as amine, such as inhibitors and acid pickling accelerators, may be added to the acid pickling solution.

After the pickling, cold rolling is performed so that the average depth d of the internal oxide layer is maintained at 4 占 퐉 or more. The cold rolling condition is preferably controlled in the range of about 20 to 70% of the cold rolling rate.

Next, oxidation and reduction are performed.

Specifically, the oxidation is first carried out at an air ratio of 0.9 to 1.4 in an oxidation zone. The air ratio means the ratio of the amount of air actually supplied to the amount of air required to completely combust the supplied combustion gas. If the air ratio is higher than 1, oxygen becomes excessive. If the air ratio is lower than 1, oxygen becomes deficient. In the embodiment described later, CO gas is used as the combustion gas.

By oxidizing in an atmosphere in which the air ratio is in the above range, decarburization is promoted, so that a desired soft layer is formed and the bendability is improved. Further, an Fe oxide film can be formed on the surface, and the formation of a complex oxide film or the like which is detrimental to the plating ability can be suppressed. If the air ratio is less than 0.9, the decarburization becomes insufficient and a sufficient soft layer is not formed, so that the bendability is deteriorated. In addition, generation of the Fe oxide film is insufficient, generation of the composite oxide film and the like can not be suppressed, and the plating ability is deteriorated. It is necessary to control the air ratio to 0.9 or more, and preferably to be 1.0 or more. On the other hand, if the air ratio exceeds 1.4, the Fe oxide film is excessively generated and can not be sufficiently reduced in the following reduction furnace, thereby deteriorating the plating ability. It is necessary to control the air ratio to 1.4 or less, and it is preferable to control the air ratio to 1.2 or less.

In the oxidation zone, it is particularly important to control the air ratio, and other conditions may be employed in a commonly used method. For example, the lower limit of the oxidation temperature is preferably 500 DEG C or higher, and more preferably 750 DEG C or higher. The upper limit of the oxidation temperature is 900 DEG C or lower, more preferably 850 DEG C or lower.

Then, in the reduction zone, the oxide film is reduced in a hydrogen atmosphere. In the present invention, in order to obtain a hard layer containing bainite, martensite and ferrite in a predetermined range, the steel sheet is subjected to a crack treatment while keeping it in the range of (Ac ₁ point + 30 ° C) to Ac ₃ point. When the crack temperature is lower than (Ac ₁ point + 30 ° C), the ferrite becomes excessive, while when it exceeds the Ac ₃ point, the ferrite is insufficient. The preferable cracking temperature is (Ac ₁ point + 45 ° C) or higher (Ac ₃ point -20 ° C) or lower.

On the other hand, Ac ₁ point in the present invention is calculated based on the following equation (i). In the formula, [] represents the content (mass%) of each element. This formula is described in "Leslie Steel Materials" (published by Maruzen Co., Ltd., William C. Leslie, p. 273).

As a result, it is possible to obtain the following effects: Ac ₁ (° C) = 723-10.7 × [Mn] -16.9 × [Ni] + 29.1 × [Si] + 16.9 × [Cr] + 290 × [As] + 6.38 × [W]

In the present invention, the Ac ₃ point is calculated based on the following equation (ii). In the formula, [] represents the content (mass%) of each element. This formula is described in "Leslie Steel Materials" (published by Maruzen Co., Ltd., William C. Leslie, p. 273).

_{Ac 3 (℃) = 910-203 ×} [C] 1/2 -15.2 × [Ni] + 44.7 × [Si] + 104 × [V] + 31.5 × [Mo] + 13.1 × [W] - {30 × [Mn] + 11 x [Cr] + 20 x [Cu] -700 x [P] -400 x [Al] -120 x [As] -400 x [Ti]

In the present invention, the holding time at the cracking temperature is preferably 10 seconds or more. If the holding time is less than 10 seconds, the reduction becomes insufficient and the plating ability is impaired. More preferably 30 seconds or more, and even more preferably 50 seconds or more. On the other hand, the holding time at the time of the cracking treatment is not particularly limited in view of the above, but is preferably controlled to about 100 seconds or less, more preferably about 80 seconds or less in consideration of productivity.

In the reducing furnace, it is particularly important to control the cracking temperature and the holding time at the cracking temperature, and other conditions may be employed in a commonly used method. For example, the atmosphere of the reduction zone includes hydrogen and nitrogen, and the hydrogen concentration is preferably controlled within a range of about 5 to 25% by volume. The dew point is preferably controlled to -30 to -60 占 폚.

Then, it is cooled. Specifically, the range from the crack to the cooling stop temperature is cooled to an average cooling rate of 5 deg. C / sec or more. Thereby, the area ratio of the ferrite can be controlled to a predetermined range. Preferably 8 占 폚 / second or more, and more preferably 10 占 폚 / second or more. The upper limit of the average cooling rate is not particularly limited, but it is preferable to control it to about 100 DEG C / second or less in consideration of easiness of control of the base steel sheet temperature, equipment cost, and the like. A more preferable average cooling rate is 50 DEG C / second or less, more preferably 30 DEG C / second or less.

On the other hand, the cooling stop temperature may be up to the temperature range where ferrite is not generated, and it is preferable that the cooling stop temperature is lowered to, for example, 550 캜 or lower. The preferable lower limit of the cooling stop temperature is, for example, 400 占 폚 or higher, more preferably 430 占 폚 or higher, and still more preferably 460 占 폚 or higher.

In the present invention, it is important to appropriately control the average cooling rate to at least the cooling stop temperature, and the subsequent cooling method is not limited to the above. For example, in the case of heating to the plating bath temperature at the time of hot dip galvanizing after cooling, the cooling temperature may be lower than the above-mentioned preferable cooling stop temperature (for example, see No. 25 in Table 2 below). Alternatively, after cooling to a predetermined temperature, it may be water-cooled.

Thereafter, hot-dip galvanizing is performed according to a conventional method. The method of hot dip galvanizing is not particularly limited. For example, the lower limit of the plating bath temperature is preferably 400 占 폚 or higher, and more preferably 440 占 폚 or higher. The preferable upper limit of the plating bath temperature is 500 캜 or lower, more preferably 470 캜 or lower. The composition of the plating bath is not particularly limited, and a known hot-dip galvanizing bath may be used. The cooling condition after the hot dip galvanizing is not particularly limited, and it is preferable to control the average cooling rate to room temperature to, for example, preferably about 1 占 폚 / sec or more, more preferably 5 占 폚 / sec or more Do. Although the upper limit of the average cooling rate is not particularly specified, it is preferable to control it to about 50 DEG C / second or less in consideration of easiness of control of the base steel sheet temperature, equipment cost, and the like. The average cooling rate is preferably 40 占 폚 / second or less, and more preferably 30 占 폚 / second or less.

If necessary, an alloying treatment may be further carried out by a conventional method, whereby an alloyed hot-dip galvanized steel sheet is obtained. The conditions of the alloying treatment are not particularly limited. For example, after performing the hot-dip galvanizing under the above conditions, the temperature is maintained at about 500 to 600 DEG C, particularly about 530 to 580 DEG C for about 5 to 30 seconds, particularly about 10 to 25 seconds . Below the above range, the alloying is insufficient. On the other hand, if it exceeds the above range, the alloying proceeds excessively, and plating peeling may occur at the time of press forming of the plated steel sheet. Furthermore, ferrite is easily generated. The alloying treatment may be carried out by using, for example, a heating furnace, a direct heating furnace, or an infrared heating furnace. The heating means is not particularly limited. For example, gas heating, induction heater heating, or heating by a high frequency induction heating apparatus can be employed.

After the alloying treatment, the alloyed hot-dip galvanized steel sheet is obtained by cooling according to a conventional method. It is preferable to control the average cooling rate to room temperature to about 1 deg. C / second or more.

[Second Manufacturing Method (with keeping warm)]

A second manufacturing method according to the present invention is characterized by comprising: a hot rolling step of winding a hot-rolled steel sheet satisfying the above-mentioned steel components at a temperature of 500 占 폚 or more; Keeping at a temperature of 500 DEG C or higher for 80 minutes or longer; A step of pickling and cold rolling so that the average depth d of the internal oxide layer is 4 占 퐉 or more; Oxidizing at an oxidizing zone at an air ratio of 0.9 to 1.4; A step of holding in the range of (Ac ₁ point + 30 ° C) to Ac ₃ point, thereby cracking; And a step of cooling the range from the cracking to the cooling stop temperature at an average cooling rate of 5 deg. C / sec or more, in this order. In contrast to the first production method described above, the second production method is different from the first production method only in that the lower limit of the coiling temperature after hot rolling is set to 500 DEG C or higher, or the hot insulation step is provided after the hot rolling process . Therefore, only the difference will be described below. The process according to the first production method may be referred to the first production method.

The reason for providing the warming step as described above is that it is possible to maintain a long time at a temperature range that can be oxidized by thermal insulation and to widen the lower limit of the winding temperature range at which desired internal oxide layer and soft layer can be obtained. In addition, there is an advantage that the temperature difference between the surface layer and the inside of the base steel sheet is reduced and the uniformity of the base steel sheet is also increased.

First, in the second production method, the coiling temperature after hot rolling is controlled to 500 캜 or higher. In the second manufacturing method, as described below, since the heat retaining step is provided thereafter, it can be set to be lower than the lower limit of the coiling temperature of 600 ° C in the first manufacturing method described above. The preferred coiling temperature is 540 캜 or higher, more preferably 570 캜 or higher. On the other hand, the preferable upper limit of the coiling temperature is the same as that of the first production method described above, and it is preferable that the coiling temperature is 750 DEG C or less.

Next, the thus obtained hot-rolled steel sheet is kept at a temperature of 500 DEG C or more for 80 minutes or more. Thereby, a desired internal oxide layer can be obtained. It is preferable that the hot-rolled steel sheet is put in a device having a heat insulating property and kept warm so that the above-mentioned effect by the warming can be effectively exhibited. The device used in the present invention is not particularly limited as long as it is made of a heat insulating material. For example, a ceramic fiber is preferably used as such a material.

In order to effectively exhibit the above effect, it is necessary to keep the temperature at 500 ° C or more for 80 minutes or more. The preferred temperature is 540 占 폚 or higher, more preferably 560 占 폚 or higher. Further, the preferable time is 100 minutes or more, and more preferably 120 minutes or more. On the other hand, it is preferable that the upper limit of the temperature and the time is controlled to approximately 700 캜 or less and 500 minutes or less in consideration of pickling performance, productivity, and the like.

The first and second manufacturing methods according to the present invention have been described above.

The coated steel sheet of the present invention obtained by the above-described production method may further be subjected to various coating and coating treatment treatments, such as a chemical treatment such as a phosphate treatment; An organic film treatment such as formation of an organic film such as a film laminate may be performed.

A known resin such as an epoxy resin, a fluorine resin, a silicone acrylic resin, a polyurethane resin, an acrylic resin, a polyester resin, a phenol resin, an alkyd resin, a melamine resin and the like can be used as a coating material for various coatings . From the viewpoint of corrosion resistance, an epoxy resin, a fluororesin, and a silicone acrylic resin are preferable. A curing agent may be used together with the resin. The coating material may contain known additives such as a coloring pigment, a coupling agent, a leveling agent, a sensitizer, an antioxidant, a UV stabilizer, a flame retardant and the like.

In the present invention, the form of the paint is not particularly limited, and any type of paint, for example, solvent-based paint, water-based paint, water-dispersed paint, powder paint, electrodeposition paint and the like can be used. The coating method is not particularly limited, and a dipping method, a roll coater method, a spraying method, a curtain flow coating method, an electrodeposition coating method, or the like can be used. The thickness of the coating layer such as a plating layer, an organic coating, a chemical conversion coating, a coating or the like may be suitably set in accordance with the use.

The high-strength plated steel sheet of the present invention is excellent in machinability (balance between strength and ductility and bending property) and delayed fracture resistance, and therefore can be used as a strength member for automobiles, for example, a side member of a front or rear portion, It can be used for components such as pillars such as center pillar reinforcements, loop rail reinforcements, side seals, floor members, and kick parts, as well as collision parts such as boxes.

This application claims the benefit of priority based on Japanese Patent Application No. 2015-003672 filed on January 9, 2015, and Japanese Patent Application No. 2015-159213 filed on August 11, 2015. The entire contents of Japanese Patent Application No. 2015-003672 filed on January 9, 2015 and Japanese Patent Application No. 2015-159213 filed on August 11, 2015 are incorporated herein by reference in their entirety.

The present invention will now be described in more detail with reference to the following examples. However, it should be understood that the present invention is not limited to the following examples, And are included in the technical scope of the present invention.

Example

The slabs having the composition shown in Table 1 and the balance being iron and unavoidable impurities were heated to 1250 占 폚, hot-rolled at a finish rolling temperature of 900 占 폚 to 2.4 mm, and then rolled at the temperatures shown in Table 2.

Some examples 26 to 28, 37 to 39, and 41 were then placed in a heat insulating device of a ceramic fiber and kept at the conditions shown in Table 2. The holding time of 500 ° C or more was measured using a thermocouple attached to the outer periphery of the coil.

Next, the thus obtained hot-rolled steel sheet was pickled under the following conditions, and cold-rolled at a cold rolling rate of 50%. The plate thickness after cold rolling is 1.2 mm.

Acid solution: 10% hydrochloric acid, temperature: 82 캜, pickling time: same as in Table 2.

Next, annealing (oxidation and reduction) and cooling were performed under the conditions shown in Table 2 in a continuous hot-dip Zn plating line. The temperature of the oxidation furnace provided in the continuous melting Zn plating line was 800 占 폚, the hydrogen concentration in the reducing furnace was 20 vol%, and the balance was nitrogen and inevitable impurities and the dew point was controlled to -45 占 폚. The holding time at the cracking temperature shown in Table 2 was all 50 seconds.

Thereafter, as shown in Fig. 25, the steel sheet was immersed in a galvanizing bath at 460 DEG C and then cooled to room temperature at an average cooling rate of 10 DEG C / sec to obtain a hot-dip galvanized steel sheet (GI) (No. 24). The galvannealed galvanized steel sheet (GA) was immersed in the zinc plating bath and subjected to hot dip galvanizing, then heated to 500 DEG C, maintained at this temperature for 20 seconds to carry out alloying treatment, and then cooled to room temperature at an average cooling rate And cooled to 10 占 폚 / sec (Nos. 1 to 23 and 26 to 41).

On the other hand, 25, the steel sheet was cooled to a cooling stop temperature of 250 deg. C as shown in Table 2, and then heated to 460 deg. C, followed by dipping in a zinc plating bath to obtain a GA steel sheet in the same manner as described above.

The thus-obtained coated steel sheet, that is, GI or GA, was evaluated for the following characteristics. On the other hand, the mean depth of the inner oxide layer was measured in the same manner not only for the coated steel sheet but also for the base steel sheet after pickling and cold rolling, for reference, as shown below. This is to confirm that the average depth of the desired internal oxide layer is already obtained in the cold-rolled steel sheet before annealing by controlling the coiling temperature and the pickling conditions after hot rolling.

(1) Measurement of the average depth d of the internal oxide layer in the coated steel sheet

(GD-OES (Glow-Discharge-Optical Emission Spectroscopy)] after collecting a test piece having a size of 50 mm x 50 mm from the W / 4 portion when the plate width of the plated steel sheet is W, Specifically, the surface of the test piece was irradiated with a high frequency wave in the Ar glow discharge region by using a GD-OES apparatus of GD-PROFILER 2 type GDA750 manufactured by Horiba Manufacturing Co., Each element amount profile was measured in the depth direction of the base steel sheet by continuously sputtering and continuously spectroscopying the luminous line in the Ar plasma of each element of sputtered O, Fe, and Zn. The sputtering conditions were as follows And the measurement area was from the surface of the plating layer to a depth of 50 mu m.

(Sputtering condition)

Pulse sputtering frequency: 50Hz

Anode diameter (analysis area): Diameter 6 mm

Discharge power: 30W

Ar gas pressure: 2.5 hPa

The results of the analysis are shown in Fig. As shown in Fig. 2, the position where the amount of Zn and Fe amount from the surface of the plating layer became equal was defined as the interface between the plated layer and the base steel sheet. Further, the average value of the O content at each measurement position at a depth of 40 to 50 占 퐉 from the surface of the plating layer is defined as an average value of the O content of the bulk, which is higher by 0.02% ) Is defined as an inner oxide layer, and the maximum depth is defined as the inner oxide layer depth. The same test was carried out using three specimens, and the average was defined as the average depth d of the inner oxide layer.

(2) Measurement of internal oxide layer depth after pickling and cold rolling (Reference)

The average depth of the internal oxide layer was calculated in the same manner as in the above (1) except that the base steel sheet after pickling and cold rolling was used.

(3) Measurement of the average depth D of the soft layer

A test piece having a size of 20 mm x 20 mm was taken out and then buried in the resin, and the thickness t of the base steel sheet was measured from the interface between the plated layer and the base steel sheet The Vickers hardness was measured. The measurement was carried out at a load of 3 gf using a Vickers hardness meter. Specifically, as shown in FIG. 3, measurements were made from the interface between the plated layer 1 and the base steel sheet 2 at a pitch of 5 占 퐉 from the measurement position at a depth of 10 占 퐉 to the inside of the plate thickness, . Spacing between measurement points; That is, in Fig. 3, the distance between x and x is at least 15 μm or more. The Vickers hardness was measured at n = 1 at each depth, and the hardness distribution in the thickness direction inward was examined. The Vickers hardness at t / 4 parts of the base steel sheet was measured at a load of 1 kgf using a Vickers hardness meter (n = 1). A region having a Vickers hardness of 90% or less as compared with t / 4 portions of the base steel sheet was used as a soft layer, and the depth was calculated. The same treatment was performed at 10 points on the same test piece, and the average was defined as the average depth D of the soft layer.

(4) Method of measuring the tissue fraction of the coated steel sheet

W / 4 part which is a section perpendicular to the plate width W direction of the plated steel sheet was exposed, and this section was polished, further subjected to electrolytic polishing, and then corroded with a deviation, and it was observed with an SEM (Scanning Electron Microscope). The observation position was t / 4 when the plate thickness of the base steel sheet was taken as t, the observation magnification was 2000 times, and the observation area was 40 占 퐉 x 40 占 퐉. A photograph of the metal structure taken by SEM was analyzed by image to measure the area ratio of martensite and bainite (not distinguished between them) and ferrite, respectively. In Table 3,? = Ferrite and (B + M) = (bainite + martensite). In Table 3, the area fraction of the "other" structure was calculated by subtracting each area ratio of martensite, bainite, and ferrite from 100 area%. Observation was arbitrarily performed for the three fields of view, and an average value was calculated.

(5) Measurement method of tensile test

JIS No. 13 B tensile test specimens were taken such that the direction perpendicular to the rolling direction of the coated steel sheet and the longitudinal direction of the test specimen were parallel and the tensile strength (TS), yield stress (YS) and elongation (EL) in the C direction according to JIS Z2241 ) Were measured. The yield ratio YR (YS / TS) was calculated from TS and YS.

In the present example, a material having a tensile strength TS of 980 MPa or more was evaluated as high strength (acceptable).

Further, TS 占 EL was calculated from the tensile strength and elongation obtained as described above. In the present embodiment, it was evaluated that TS × EL was 17000 or more, and that the balance between strength and ductility was excellent (acceptable).

(6) Bending test

Test specimens of 20 mm x 70 mm cut out from the plated steel sheet were prepared so that the direction perpendicular to the rolling direction of the plated steel sheet was parallel to the longitudinal direction of the test specimen and the 90 ° V bending test was performed so that the bending ridgeline was in the long direction. The test was carried out by appropriately changing the bending radius R to obtain a minimum bending radius Rmin capable of bending without causing cracks in the test piece.

The bending property was evaluated for each tensile strength TS on the basis of Rmin / t obtained by dividing Rmin by the plate thickness t of the base steel sheet. Details are as follows. On the other hand, evaluation of the bending property is not carried out for the TS not satisfying the acceptance criterion of 980 MPa or more (in Table 3, marked with -).

When TS is 980 MPa or more and less than 1080 MPa, it passes Rmin / t <2.0

If TS is less than 1080 MPa and less than 1180 MPa, pass Rmin / t <3.0

If TS is more than 1180 MPa, pass Rmin / t <3.7

(7) Delayed fracture characteristics test

(W) × 30 mm (L) was cut out, U bending was carried out with a minimum bending radius, then the test piece was tightly fastened with a bolt, U tensile stress of 1000 MPa was applied to the outer surface of the U-bending test piece. The tensile stress was measured by attaching a strain gauge to the outside of the U-bending test piece and converting the stress into tensile stress. Thereafter, the edge portion of the U-bending test piece was masked and electrochemically occupied hydrogen. Hydrogen charge was performed by immersing the test piece in a mixed solution of 0.1M H ₂ SO ₄ (pH = 3) and 0.01M KSCN under the conditions of a room temperature and a constant current of 100 μA / mm ² .

As a result of the hydrogen charge test, it was evaluated that the case of not breaking for 24 hours passed, that is, the delayed fracture characteristic was excellent.

(8) Hole Expansion Test

A hole expansion test was carried out in accordance with JFST1001 of Japan Steel Federation, and the? Was measured. Specifically, after a hole having a diameter of 10 mm was struck on a plated steel sheet, a punch of a cone of 60 ° was pushed into the hole while restricting the circumference, and the diameter of the hole at the crack occurrence limit was measured. From the following equation, the limiting hole expanding ratio? (%) Was determined.

Limit hole expansion factor λ (%) = {(Df-D0) / D0} × 100

(Mm), D0 is the diameter (mm) of the initial hole,

(9) Plating appearance

The outer appearance of the plated steel sheet was visually observed to evaluate the plating ability based on the presence or absence of occurrence of non-plating.

The results are shown in Tables 2 and 3.

[Table 1A]

[Table 1B]

[Table 2A]

[Table 2B]

[Table 3A]

[Table 3B]

From the table, it can be considered as follows.

First, 1 to 11, 15, 17, 24 to 26 and 29 to 41 are examples satisfying the requirements of the present invention, and they are examples of strength and workability (balance between strength and ductility (TS x EL) and bendability) All of the castles were good. Particularly, when the average depth d of the inner oxide layer and the average depth D of the soft layer satisfy the relation of D &gt; 2d (i.e., the value of &quot; D / 2d &quot; 1 &quot; (D / 2d = 1.13), the number of the No. 17 (D / 2d = 0.96).

On the contrary, 12 shows an example using the steel type L in Table 1 having a large amount of C, and the bending property and the delayed fracture characteristics were degraded.

No. 13 is an example using the steel type M in Table 1 having a small amount of Si. Since the average depth of the internal oxide layer after pickling and cold rolling is shallow, the internal oxide layer is not sufficiently generated and the balance of bending property, strength and ductility, .

No. 14 used the steel grade A of Table 1 in which the steel component satisfies the requirements of the present invention. However, since the average depth of the inner oxide layer after pickling and cold rolling is shallow, The average depth d, and the average depth D of the soft layer became shallow. As a result, the bending property, the delayed fracture resistance property and the plating ability were deteriorated.

No. 16 used the steel grade A in Table 1 in which the steel component satisfied the requirements of the present invention. However, the steel grade was poor in the air ratio in the oxidation furnace, and the iron oxide film was not sufficiently generated, and the plating performance was deteriorated. In addition, since the soft layer was not sufficiently formed, the bending property and the delayed-failure resistance property were deteriorated.

No. 18 is an example in which the steel component of Table 1 satisfying the requirements of the present invention is used but the coiling temperature at hot rolling is low and the air ratio in the oxidation furnace is low. The average depth d of the inner oxide layer after plating and the average depth D of the soft layer became shallow. As a result, the bending property, the delayed fracture resistance property and the plating ability were deteriorated.

No. 19 used the steel grade A in Table 1 in which the steel component satisfies the requirements of the present invention. However, since the average depth of the inner oxide layer after pickling and cold rolling is shallow as an example in which the coiling temperature at hot rolling is low, The average depth d is shallow. As a result, the delayed fracture characteristics and the plating ability were lowered.

No. 20 and No. 21 all use the steel grade A of Table 1 in which the steel component satisfies the requirements of the present invention. However, since the ferrite is not sufficiently generated and the total area ratio of (B + M) is increased, The balance between strength and ductility has deteriorated.

No. 22 uses the steel type A of Table 1 in which the steel component satisfies the requirements of the present invention. However, as an example where the crack temperature is low, excessive ferrite is produced and the total amount of (B + M) Was not obtained. As a result, TS was lowered.

No. 23 is an example in which the steel component of Table 1 satisfies the requirements of the present invention, but the average cooling rate after cracking is slow. The ferrite is excessively produced during cooling and the total amount of (B + M) , A desired hard layer could not be obtained. As a result, the TS decreased.

No. 27 is an example in which the steel component of Table 1 satisfies the requirements of the present invention. However, since the average depth of the inner oxide layer after pickling and cold rolling is shallow, as an example in which the coiling temperature at hot rolling is low, The average depth d, and the average depth D of the soft layer became shallow. As a result, the bending property, the delayed fracture resistance property and the plating ability were deteriorated.

No. 28 used the steel grade A of Table 1 in which the steel component satisfies the requirements of the present invention. However, since the average depth of the inner oxide layer after pickling and cold rolling is shallow, the average depth of the inner oxide layer after plating d, and the average depth D of the soft layer became shallow. As a result, the bending property, the delayed fracture resistance property and the plating ability were deteriorated.

1: Plating layer
2: Substrate steel
3: internal oxide layer
4: soft layer
5: Hard layer

Claims (6)

A coated steel sheet having a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer on the surface of a ground steel sheet,
(1) The above-mentioned ground steel sheet comprises, by mass%
C: 0.08 to 0.30%
Si: 0.25 to 3%
Mn: 1.5 to 4%
P: more than 0% and not more than 0.1%
S: not less than 0% and not more than 0.05%
Al: 0.005 to 1%, and
N: not less than 0% and not more than 0.01%
The balance being iron and inevitable impurities,
(2) from the interface between the base steel sheet and the plating layer toward the base steel sheet,
An inner oxide layer containing at least one oxide selected from the group consisting of Si and Mn,
A soft layer having a Vickers hardness of 90% or less of a Vickers hardness in a t / 4 portion of the base steel sheet when the thickness of the base steel sheet is t,
Martensite and bainite: at least 20% by area and less than 60% by area, and polygonal ferrite: at least 40% by area and not more than 80% by area,
, And
An average depth D of the soft layer is not less than 20 mu m, and
Wherein the average depth d of the internal oxide layer is 4 탆 or more and less than D
And a tensile strength of 980 MPa or more. The method according to claim 1,
The base steel sheet according to claim 1, further comprising at least one of the following (a) to (c) in mass%.
(a) at least one member selected from the group consisting of Cr: more than 0% to 1%, Mo: more than 0% to 1%, and B: more than 0% to 0.01%
(b) at least one selected from the group consisting of Ti: more than 0% to 0.2%, Nb: more than 0% to 0.2%, and V: more than 0% to 0.2%
(c) at least one selected from the group consisting of Cu: more than 0% to 1%, and Ni: more than 0% to 1% The method according to claim 1,
Wherein the average depth d of the inner oxide layer and the average depth D of the soft layer satisfy the relation of D &gt; 2d. 3. The method of claim 2,
Wherein the average depth d of the inner oxide layer and the average depth D of the soft layer satisfy the relation of D &gt; 2d. A method for producing the high strength coated steel sheet according to any one of claims 1 to 4,
A hot rolling step of winding a steel sheet satisfying the constituents of the steel in the above-mentioned ground steel sheet at a temperature of 600 DEG C or higher;
A step of pickling and cold rolling so that the average depth d of the internal oxide layer is not less than 4 탆,
A step of oxidizing in an oxidizing zone at an air ratio of 0.9 to 1.4,
A step of holding in the range of (Ac ₁ point + 30 ° C) to Ac ₃ point in the reduction zone,
And cooling the steel sheet at an average cooling rate of 5 deg. C / sec or more after the cracking to the cooling stop temperature in this order. A method for producing the high strength coated steel sheet according to any one of claims 1 to 4,
A hot rolling step of winding a steel sheet satisfying the constituents of the steel in the above-mentioned ground steel sheet at a temperature of 500 DEG C or higher;
A step of keeping at a temperature of 500 DEG C or higher for 80 minutes or longer,
A step of pickling and cold rolling so that the average depth d of the internal oxide layer is not less than 4 탆,
A step of oxidizing in an oxidizing zone at an air ratio of 0.9 to 1.4,
A step of holding in the range of (Ac ₁ point + 30 ° C) to Ac ₃ point in the reduction zone,
A step of cooling the range from the crack to the cooling stop temperature at an average cooling rate of 5 DEG C /
In the above-described order.

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