CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2019/014220, filed Mar. 29, 2019, which claims priority to Japanese Patent Application No. 2018-068995, filed Mar. 30, 2018 and Japanese Patent Application No. 2019-037382, filed Mar. 1, 2019, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a high-strength galvanized steel sheet, a high strength member, and a method for manufacturing the same that are excellent in coatability and bendability, and that is suitable for building materials and automotive collision-resistant parts, and a method for manufacturing the same.

BACKGROUND OF THE INVENTION

In these days when collision safety and fuel efficiency improvement of automobiles are strongly required, the strength increase of steel sheets that are materials of parts is being advanced. Further, in view of the fact that automobiles are being widely spread on a global scale and automobiles are used for various uses in diverse areas and climates, steel sheets that are materials of parts are required to have high antirust properties.

In general, when the strength of a steel sheet is enhanced, the formability of the steel sheet is reduced. In particular, a steel sheet provided with coating tends to have poorer formability than a steel sheet not provided with coating.

If a large amount of an alloying element is contained in order to increase strength, it is difficult for a good plated coating quality to be formed on the steel sheet. Further, it is known that, if coating of Zn, Ni, or the like is provided, hydrogen that enters during the manufacturing process is less likely to be released from the interior of the steel.

Steel sheets having excellent bendability have thus far been developed. Based on the features of the method for forming such a steel sheet, how to design a location that is exposed to most severe forming conditions at the time of bending, that is, a location where stress is concentrated is presented as a solution to an issue. In particular, in the case of a steel sheet containing two or more kinds of steel structures with different hardnesses, it is likely that deformation will concentrate and defects of microvoids will occur at the interface between steel structures, and consequently bendability is degraded.

Also the control of an atmosphere in the furnace of an annealing-coating step is attempted in order to deposit good coating quality.

In Non-Patent Literatures 1 and 2, while the steel structure of a steel sheet contains ferrite and martensite, a steel structure of ferrite and martensite is temporarily produced and then tempering is performed to soften the martensite, and bendability is improved.

Patent Literature 1 discloses a high-strength steel sheet in which a structure uniformity index given by a standard deviation of Rockwell hardness of a surface of a steel sheet and serving as an index indicating the uniformity of the steel sheet is 0.4 or less and that is good in ductility and bendability and has a maximum tensile stress of 900 MPa or more, and a method for manufacturing the same. This literature provides a technique obtained as a result of improving, as a factor influencing bendability, the non-uniformity of a solidified structure at the time of casting, and proposes, by this method, a steel sheet that has a maximum tensile stress of 900 MPa or more and is excellent in bendability.

In Patent Literature 1, at this time, an interior of an annealing furnace of a continuous galvanizing line is set to an atmosphere having a hydrogen concentration of 1 to 60 vol % and containing N2, H2O, O2, and incidental impurities as the balance, and the logarithm of the partial pressure of water and the partial pressure of hydrogen in the atmosphere, log (PH2O/PH2), is prescribed to −3≤log(PH2O/PH2)≤−0.5 in order to ensure good coatability quality.

Patent Literature 2 provides a dualphase steel sheet that contains 50% or more of bainite and 3 to 30% of retained austenite and in which the ratio between the hardness Hvs of an outer layer of the steel sheet and the hardness Hvb of a portion of ¼ of the thickness of the steel sheet is prescribed to 0.35 to 0.90. Further, annealing is performed in an atmosphere in which log(partial pressure of water/partial pressure of hydrogen) is −3.0 to 0.0, and thereby coatability is ensured in a high alloy system.

Patent Literature 3 ensures bendability by prescribing a decarburized ferrite layer, and discloses, as a technique for manufacturing a coated steel sheet, a method of adjustment to an atmosphere containing 2 to 20 vol % of hydrogen and the balance containing nitrogen and impurities and having a dew-point temperature of more than −30° C. and 20° C. or less.

Patent Literature

Patent Literature 1: JP 2011-111670 APatent Literature 2: JP 2013-163827 APatent Literature 3: JP 2017-048412 A

Non-Patent Literature

Non-Patent Literature 1: Kohei Hasegawa, and five others, “980MPa-kyu Cho-ko-kyodo Kohan no Mage-kako-sei ni Oyobosu Kinzoku-soshiki no Eikyo” (Influence of Metal Structure on the Bending Formability of an Ultrahigh-strength Steel Sheet of the 980-MPa Class), CAMP-ISIJ, vol. 20 (2007), p. 437, published by The Iron and Steel Institute of JapanNon-Patent Literature 2: Nobuyuki Nakamura, and three others, “Cho-ko-kyodo Reien Kohan no Nobi-furanji-seikei-sei ni Oyobosu Soshiki no Eikyo” (Influence of Structure on the Stretch Flange Moldability of an Ultrahigh-strength Cold Rolled Steel Sheet), CAMP-ISIJ, vol. 13 (2000), p. 391, published by The Iron and Steel Institute of Japan

SUMMARY OF THE INVENTION

Thus far, to improve the bendability of a steel sheet, mainly the optimization of steel structure has been made; however, this provides only a certain level of improvement, and further improvement is required. Further, it is presumed that, in the case where a high alloy-based steel sheet is subjected to coating, hydrogen in the atmosphere in the coating step becomes hydrogen in steel remaining in the steel sheet product. It is presumed that improvement in bendability is hindered by the hydrogen in steel. It is also necessary to achieve both improvement in bendability and coatability.

Aspects of the present invention improve the bendability of a coated steel sheet from a new point of view, and an object according to aspects of the present invention is to provide a high-strength galvanized steel sheet and a high strength member excellent in coatability and bendability, and a method for manufacturing them.

The high strength referred to in the present specification means that tensile strength (TS) is 1100 MPa or more.

The present inventors conducted extensive studies in order to solve the issue mentioned above. As a result, it has been found out that a high-strength galvanized steel sheet having good bendability and coatability is obtained by adjusting a galvanized steel sheet in such a manner that the steel sheet has a specific chemical composition and has a steel structure in which inclusions existing in an area extending from a surface to a position of ⅓ of the sheet thickness are in a prescribed existence state, and appropriately adjusting the amount of hydrogen remaining in the steel. Further, it has been found out that a high-strength galvanized steel sheet according to aspects of the present invention can be manufactured by appropriately adjusting conditions of manufacturing steps, such as conditions of an atmosphere in the furnace during annealing. Specifically, aspects of the present invention provide the following.

[1] A high-strength galvanized steel sheet including:

a steel sheet having a chemical composition containing, in mass %,

C: 0.08% or more and 0.20% or less,

Si: less than 2.0%,

Mn: 1.5% or more and 3.5% or less,

P: 0.02% or less,

S: 0.002% or less,

Al: 0.10% or less, and

N: 0.006% or less, and the balance: Fe and incidental impurities, and

a steel structure in which an average grain size of inclusions containing at least one of Al, Si, Mg, and Ca and existing in an area extending from a surface to a position of ⅓ of a sheet thickness is 50 μm or less, and an average nearest distance between the inclusions is 20 μm or more; and

a galvanized layer provided on a surface of the steel sheet and having a coating weight per surface of 20 g/m2or more and 120 g/m2or less,

in which an amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm, and

a tensile strength is 1100 MPa or more.

[2] The high-strength galvanized steel sheet according to [1], in which a mass ratio of a content of Si to a content of Mn in the steel (Si/Mn) is less than 0.1.

[3] The high-strength galvanized steel sheet according to [1] or [2], in which the chemical composition further contains, in mass %, at least one of (1) to (3) below,

(1) one or more of Ti, Nb, V, and Zr: 0.005% or more and 0.1% or less in total,

(2) one or more of Mo, Cr, Cu, and Ni: 0.01% or more and 0.5% or less in total, and

(3) B: 0.0003% or more and 0.005% or less.

[4] The high-strength galvanized steel sheet according to any one of [1] to [3], in which the chemical composition further contains, in mass %, at least one of Sb: 0.001% or more and 0.1% or less and Sn: 0.001% or more and 0.1% or less.

[5] The high-strength galvanized steel sheet according to any one of [1] to [4], in which the chemical composition further contains, in mass %, Ca: 0.0005% or less.

[6] The high-strength galvanized steel sheet according to any one of [1] to [5],

in which the steel structure contains 30% or more and 85% or less of martensite, 60% or less (including 0%) of ferrite, 15% or less (including 0%) of bainite, and less than 5% (including 0%) of retained austenite in terms of area ratio, and

an average grain size of ferrite is 15 μm or less.

[7] A method for manufacturing a high-strength galvanized steel sheet, including:

a casting step of casting steel having the chemical composition according to any one of [1] to [5] under a condition where a flow velocity of molten steel at a solidification interface in vicinity of a meniscus of a casting mold is 16 cm/s or more, and producing a steel raw material;

a hot rolling step of hot rolling the steel raw material after the casting step;

a pickling step of pickling a steel sheet after the hot rolling step;

a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction ratio of 20% or more and 80% or less;

a pre-treatment step of heating the steel sheet after the cold rolling step at a pre-treatment heating temperature of 720° C. or more and 880° C. or less, then performing cooling at an average cooling rate of 2° C./s or more from the pre-treatment heating temperature to 500° C. and at an average cooling rate of 3° C./s or more from 499° C. to 200° C., and performing pickling after the cooling;

an annealing step of heating the steel sheet after the pre-treatment step in a continuous annealing line at an annealing temperature of 740° C. or more and (Ac3+20°) C. or less, with a hydrogen concentration of an atmosphere in the furnace in a temperature region of 500° C. or more set to more than 0 vol % and 12 vol % or less, and then performing cooling at an average cooling rate of 3° C./s or more from the annealing temperature to 600° C.; and

a coating step of subjecting the steel sheet after the annealing step to coating treatment, and after the coating treatment, performing cooling at an average cooling rate of 3° C./s or more through a temperature region of 450° C. to 250° C.

[8] The method for manufacturing a high-strength galvanized steel sheet according to [7],

in which, when it is assumed that a dew-point temperature of an atmosphere of 600° C. or more and the pre-treatment heating temperature or less is Y° C. or more at a time of performing heating at the pre-treatment heating temperature in the pre-treatment step,

a dew-point temperature in a temperature region of 700° C. or more in the continuous annealing line is Z° C. or less at a time of performing heating at the annealing temperature in the annealing step, and

the pre-treatment heating temperature is X° C., the X, Y, and Z satisfy relation (i), (ii), or (iii) below,

(i) 720≤X≤800, and Y−Z≥−5,

(ii) 800<X≤840, and Y−Z≥0, or

(iii) 840<X≤880, and Y−Z≥5.

[9] The method for manufacturing a high-strength galvanized steel sheet according to [7] or [8], further including, after the coating step, a width trimming step of performing width trimming.

[10] The method for manufacturing a high-strength galvanized steel sheet according to any one of [7] to [9], further including, after the annealing step or after the coating step, a post-treatment step of performing heating in a temperature region of 50 to 400° C. for 30 seconds or more in an atmosphere with a hydrogen concentration of 5 vol % or less and a dew-point temperature of 50° C. or less.

[11] The method for manufacturing a high-strength galvanized steel sheet according to any one of [7] to [10], in which alloying treatment is performed immediately after the coating treatment in the coating step.

[12] A high strength member, obtained by subjecting the high-strength galvanized steel sheet according to any one of [1] to [6] to at least either one of forming and welding.

[13] A method for manufacturing a high strength member, including a step of performing at least either one of forming and welding on a high-strength galvanized steel sheet manufactured by the method for manufacturing a high-strength galvanized steel sheet according to any one of [7] to [11].

According to aspects of the present invention, a high-strength galvanized steel sheet and a high strength member excellent in coatability and bendability and a method for manufacturing them can be provided. In the case where a high-strength galvanized steel sheet according to aspects of the present invention is used for a framework member of an automobile body, the high-strength galvanized steel sheet can make a great contribution to improvement in collision safety and weight reduction.

