EP4656764A1

EP4656764A1 - Method for manufacturing high-strength, hot-dip galvanized steel sheet

Info

Publication number: EP4656764A1
Application number: EP24779669.1A
Authority: EP
Inventors: Daisuke Tahara; Masaki Koba; Katsuya Hoshino; Satoshi Maeda; Masaki Tada; Ryohei Morimoto; Yoshihiko Ono
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-03-28
Filing date: 2024-03-18
Publication date: 2025-12-03
Also published as: WO2024203544A1; JP7694844B2; EP4656764A4; CN120898018A; JPWO2024203544A1; KR20250149763A; WO2024203544A8; MX2025010990A

Abstract

A high-strength hot-dip galvanized steel sheet with a good surface appearance is manufactured from a boron-containing base steel sheet while suppressing the occurrence of gray point defects and black point defects on the coating surface.

In a pre-coating annealing step, the dew point in the range of temperatures from 300 to 500°C is lowered and the atmosphere is rendered reductive for iron to suppress the formation of iron oxide and thereby to reduce the occurrence of ammonia. At temperatures of 500°C or above, ammonia occurs noticeably and nitrogen starts to enter the steel sheet. The formation of ammonia and the nitridation of the steel sheet are suppressed by accelerating heating in the above temperature range. Furthermore, the annealing time is shortened and the dew point is controlled to -55°C or above in the temperature range of 750°C or above so as to fix part of boron as oxide and thereby to suppress the formation of boron nitride stemming from the diffusion of boron to the steel sheet surface during annealing. In this manner, the occurrence of boron nitride that causes gray point defects and black point defects is suppressed.

Description

Technical Field

The present invention relates to a method for manufacturing a high-strength hot-dip galvanized steel sheet having a good surface appearance (coating appearance).

Background Art

Strengthening and thinning of steel sheets for automobiles are pursued to reduce the weight of vehicles and thereby to cut down CO₂ emissions, and also to enhance crashworthiness by increasing the strength of car bodies. For example, high-strength steel sheets with 590 MPa or higher tensile strength (TS) are increasingly applied to main structural parts of automobile cabin frameworks in order to increase the strength of car bodies.
A common approach to increasing the strength of steel is to add hardening elements, such as C, Mn, B, Cr, and Mo. In particular, boron provides high hardenability enhancements at a small dose and advantageously enables strengthening of steel at low cost. In addition, boron has negligible adverse effects in deteriorating bendability or delayed fracture resistance through the formation of inclusions. Thus, boron is widely used as an additive element in high-strength steel sheets.
High-strength hot-dip galvanized steel sheets have been manufactured by adding boron to base steel sheets. For example, Patent Literature 1 discloses a technique in which a base steel sheet with a predetermined chemical composition is continuously annealed and hot-dip galvanized in such a manner that the dew point of the atmosphere during the continuous annealing is controlled to -40°C or below while the in-furnace temperature is in the range of 750°C or above. This lowers the oxygen potential at the interface between the steel sheet and the atmosphere, and eliminates the formation of internal oxides and suppresses the surface enrichment of such elements as Si and Mn. The technique thus provides an excellent coating appearance.
Furthermore, Patent Literature 2 discloses a technique in which the ratio of the amount of Si enrichment to the amount of Mn enrichment on the surface of a base steel sheet is controlled to 0.7 or more and 1.3 or less, and the steel sheet, after being cold-rolled, is annealed in such a manner that the cold-rolled steel sheet is heated to a maximum attainment temperature and is held while controlling the dew point of the atmosphere in that region to -40°C or below. The technique thus provides excellent coatability.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2010-255100
PTL 2: International Publication No. 2020/170542

Summary of Invention

Technical Problem

Patent Literature 1 addresses "surface defects stemming from the formation of Si- and Mn-containing oxides that lower wettability". Specifically, Si- and Mn-containing oxides repel the coating and give rise to bare regions. However, in-depth studies by the present inventors revealed that a hot-dip galvanized steel sheet obtained in Patent Literature 1 suffers very small surface defects that are distinct from the above defects, and the occurrence of such surface defects is a new challenge that should be solved. Even when a steel sheet does not repel a coating in the production of a hot-dip coated steel sheet, the adhesion between the steel sheet and the coating is weak to give rise to a phenomenon in which the coating that has been attached to the steel sheet comes off from the steel sheet. The very small surface defects described above are probably the consequence of such a phenomenon. The present inventors have found that these surface defects stemming from the exfoliation of a coating are frequent particularly in steels containing boron. The present inventors have also found that a hot-dip galvanized steel sheet obtained in Patent Literature 2 frequently suffers similar surface defects stemming from the exfoliation of a coating particularly when the steel contains boron. The surface defects described above are point defects with a diameter of approximately 0.1 to 1.0 mm that look gray or black, written as gray point defects or black point defects respectively hereinbelow.
The present invention has been made in order to solve the new problem described above. It is therefore an object of the present invention to provide a method for manufacturing a high-strength hot-dip galvanized steel sheet from a boron-containing base steel sheet in a way that the high-strength hot-dip galvanized steel sheet attains a good surface appearance (coating appearance) with reduced occurrence of gray point defects and black point defects on the coating surface.

Solution to Problem

The present inventors conducted extensive studies directed to solving the problem described above and have consequently obtained the following findings.

(1) After the dipping treatment, the coating is attached to the steel sheet without being repelled by the steel sheet surface. However, due to the weak adhesion between the coating and the steel sheet, the coating comes off upon contact with delivery rolls and is caught on the rolls. The consequent hollow defects are gray point defects. The coating that has been caught on the rolls falls from the rolls and is attached again to the steel sheet. The consequent protruding defects are black point defects. These surface defects are particular phenomena that occur when boron is added to the base steel sheet and the dew point during pre-coating annealing is low, and are therefore not recognized as a problem to be solved in the conventional art.
(2) The present inventors investigated and studied the reasons why the above surface defects are particular phenomena that occur when boron is added to a base steel sheet and the dew point during pre-coating annealing is low. As a result, the present inventors have found that the phenomena are ascribed to the fact that boron nitride is formed on the steel sheet surface exclusively when the dew point during annealing is low. Specifically, the phenomena are almost absent when the dew point during annealing is high because Si- and Mn-containing oxides are formed preferentially on the steel sheet surface. When boron is not added, annealing at a low dew point takes place while the formation of Si- and Mn-containing oxides is suppressed, and the coating surface attains an enhanced appearance quality. When, in contrast, boron is added and the dew point is low, the atmosphere suppresses the formation of Si- and Mn-containing oxides but allows nitrides to occur stably, thus giving rise to defects caused by boron nitride.
(3) Boron nitride probably occurs mainly due to the following two factors. Respective countermeasures to these factors are available.
1. (i) Nitrogen (N) from ammonia in the atmosphere adsorbs to or finds its way into a steel sheet mainly during a heating step and thereafter reacts with boron in a step where the steel sheet is heated or soaked in an atmosphere having a low oxygen potential. Ammonia occurs in a significantly accelerated manner by the catalytic action of iron oxide or pure iron resulting from the reduction of iron oxide. In view of this fact, the formation of boron nitride can be suppressed by sufficiently suppressing the occurrence of iron oxide from the early stage of heating during annealing so as to lower the ammonia concentration in the atmosphere.
2. (ii) When the oxygen potential is low and boron does not form the oxide, dissolved boron that has been diffused to the superficial layer of a steel sheet forms the nitride at the superficial layer. The occurrence of boron nitride can be suppressed when boron can be fixed as the oxide, which is scarcely detrimental to the coating appearance quality, on the steel sheet surface or within the steel sheet even under low-oxygen potential conditions.
(4) Based on the above findings, the present inventors have ascertained that the formation of boron nitride can be effectively suppressed by performing a pre-coating annealing step while controlling the annealing conditions as described below. The control can thereby suppress the occurrence of gray point defects and black point defects and the hot-dip galvanized steel sheet that is obtained attains a good surface appearance (coating appearance).
1. (i) While the dew point in the range of temperatures from 300 to 500°C was not controlled in the conventional art, an Fe reducing atmosphere is created by lowering the dew point in the above temperature range so as to sufficiently suppress the occurrence of iron oxide and thereby to reduce the amount of ammonia formation, a factor of boron nitride formation, that is formed.
2. (ii) At temperatures of 500°C or above and 750°C or below, ammonia occurs noticeably and nitrogen starts to adsorb to or enter the steel sheet. The adsorption or entry of nitrogen into the steel sheet is suppressed by accelerating heating in this temperature range.
3. (iii) Furthermore, the annealing time is shortened and the dew point is optimized in the temperature range of 750°C or above so as to fix part of boron as oxide and thereby to suppress the formation of boron nitride stemming from the diffusion of boron to the steel sheet surface during annealing.

The present inventors have further found that the addition of antimony is effective for suppressing the adsorption or entry of nitrogen into a steel sheet in the temperature range of 500°C or above and 750°C or below; controlling the Mn to Si content ratio, Mn/Si, to a predetermined range facilitates the formation of Mn-B composite oxide and thus enables fixing boron as oxide; and these contribute to suppressing the occurrence of gray point defects and black point defects.
The present invention has been made based on the findings described above. The gist of the present invention is as follows.

[1] A method for manufacturing a high-strength hot-dip galvanized steel sheet, the method including continuously annealing a steel sheet, hot-dip galvanizing the steel sheet by dipping the steel sheet into a hot-dip galvanizing bath, and optionally subjecting the steel sheet to an alloying treatment, wherein
- the steel sheet has a chemical composition including, in mass%:
  - C: 0.050% or more and 0.300% or less,
  - Si: 1.20% or less,
  - Mn: 2.00% or more and 3.50% or less,
  - P: 0.100% or less,
  - S: 0.0100% or less,
  - sol. Al: 1.00% or less,
  - N: 0.0200% or less,
  - B: 0.0001% or more and 0.0050% or less, and
  - Sb: 0.001% or more and 0.200% or less,
  - [% Mn]/[% Si] being 2.5 or more, and
  - optionally further including one or more selected from:
    - Cr: 1.00% or less,
    - Ti: 0.200% or less,
    - Nb: 0.200% or less,
    - V: 0.200% or less,
    - Mo: 2.000% or less,
    - Cu: 1.000% or less,
    - Ni: 0.500% or less,
    - Sn: 0.200% or less,
    - Mg: 0.0100% or less,
    - Ca: 0.0100% or less,
    - Zn: 0.100% or less,
    - Co: 0.200% or less,
    - Zr: 0.200% or less,
    - REM: 0.0100% or less,
    - Ta: 0.10% or less,
    - Te: 0.10% or less,
    - As: 0.10% or less,
    - Hf: 0.10% or less,
    - Bi: 0.20% or less,
    - Pb: 0.20% or less,
    - Ge: 0.10% or less,
    - Sr: 0.10% or less, and
    - Cs: 0.10% or less,
    - the balance being Fe and incidental impurities, and
- the continuous annealing step includes:
  - heating the steel sheet in a temperature range of 300°C or above and 500°C or below in an atmosphere containing 3 vol% or more hydrogen and 500 ppm by volume or less oxygen and having a dew point of -10°C or below,
  - heating the steel sheet in a temperature range of 500°C or above and 750°C or below in an atmosphere containing 5 vol% or more hydrogen and having a dew point of -40°C or below at an average heating rate of 1°C/s or more, and
  - soaking the steel sheet at a temperature of 750°C or above and 950°C or below in an atmosphere containing 5 vol% or more hydrogen and having a dew point of -55°C or above and -40°C or below for a holding time of 20 to 300 seconds.
[2] The method for manufacturing a high-strength hot-dip galvanized steel sheet according to [1], wherein the chemical composition of the steel sheet satisfies [% Mn]/[% Si] of 12.0 or more.
[3] The method for manufacturing a high-strength hot-dip galvanized steel sheet according to [1] or [2], wherein the atmosphere in which the steel sheet being continuously annealed is soaked at a temperature of 750°C or above and 950°C or below contains 0.010 vol% or less ammonia. Advantageous Effects of Invention

According to the present invention, a high-strength hot-dip galvanized steel sheet with a good surface appearance (coating appearance) can be manufactured from a boron-containing base steel sheet while suppressing the occurrence of gray point defects and black point defects on the coating surface. High-strength hot-dip galvanized steel sheets produced according to the present invention are suited for structural members, such as, for example, automotive parts, and the use thereof in such applications can reduce the weight of car bodies and can enhance fuel efficiency.

Description of Embodiments

High-strength hot-dip galvanized steel sheets to be manufactured in the present invention have a hot-dip galvanized layer on one or both sides of a steel sheet (a base steel sheet) and include hot-dip galvannealed steel sheets. That is, the hot-dip galvanization may be followed by an alloying treatment.
Here, the composition of the hot-dip galvanized layer is not particularly limited and may be conventional. For example, the hot-dip galvanized layer may have a composition including Fe: 20 mass% or less, Al: 0.001 mass% or more and 1.0 mass% or less, and one, or two or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% or more and 3.5 mass% or less, the balance being Zn and incidental impurities. In hot-dip galvanized steel sheets (GI), the Fe content in the coating layer is generally less than 7 mass%. In hot-dip galvannealed steel sheets (GA), the Fe content in the coating layer is generally 7 mass% or more and 15 mass% or less, and preferably 8 mass% or more and 13 mass% or less.
Hereinbelow, the chemical composition of the steel sheet (the base steel sheet) and the reasons why the chemical composition is thus limited will be described. In the following description, "%" indicating the content of a constituent element in the steel sheet means "mass%" unless otherwise specified. Tensile strength is written as TS.

·C: 0.050% or more and 0.300% or less

Carbon is an element that is effective for obtaining desired amounts of quenched martensite and tempered martensite to attain 590 MPa or higher TS and is also effective for achieving excellent dimensional accuracy at the time of forming. If the C content is less than 0.050%, the area fraction of quenched martensite is lowered and the area fraction of ferrite and bainite is increased to make it difficult to obtain 590 MPa or higher TS. If, on the other hand, the C content is more than 0.300%, the carbon concentration in quenched martensite and tempered martensite is increased, which raises the hardness of quenched martensite and tempered martensite. As a result, a large hardness difference is produced between soft ferrite and bainite phases and hard quenched martensite and tempered martensite phases, leading to deterioration in blankability, stretch flangeability, and bendability. Thus, the C content is limited to 0.050% or more and 0.300% or less. In order to obtain 780 MPa or higher TS, the C content is preferably 0.060% or more. To obtain 980 MPa or higher TS, the C content is more preferably 0.090% or more. In view of the fact that a high carbon content deteriorates weldability, the C content is preferably 0.250% or less, and more preferably 0.220% or less.

·Si: 1.20% or less

Silicon is an element that is effective for strengthening steel to obtain a good material quality and is also effective for enhancing ductility. If, on the other hand, the Si content is more than 1.20%, an increased amount of silicon is concentrated at the steel sheet surface during annealing to form silicon oxide that can cause bare spot defects on the steel sheet surface, making it difficult to achieve good coatability. Controlling the Si content to 1.20% or less suppresses the occurrence of Si-Mn composite oxide and allows the manganese in the steel sheet to be effectively used as Mn-B composite oxide, thereby making it possible to suppress the occurrence of gray point defects and black point defects. Thus, the Si content is limited to 1.20% or less. From the above viewpoints, the Si content is preferably 0.80% or less, and more preferably 0.55% or less.
There is no particular lower limit to the Si content. That is, the Si content may be 0%. However, desiliconization to less than 0.01% increases refining costs. Thus, the Si content is preferably 0.01% or more. In order to obtain high strength and enhanced ductility at the same time, the Si content is preferably 0.05% or more, and more preferably 0.10% or more. In order to achieve particularly high ductility, the Si content is still more preferably 0.15% or more.

·Mn: 2.00% or more and 3.50% or less

Manganese is an element that is necessary to obtain a good surface quality by suppressing the occurrence of gray point defects and black point defects. Manganese, by being added in an amount of 2.00% or more, forms Mn-B composite oxide that is scarcely detrimental to the coating appearance quality. In this manner, the amount of boron nitride that is formed can be reduced and the occurrence of gray point defects and black point defects is suppressed. Furthermore, manganese is an element that is effective for obtaining desired amounts of quenched martensite and tempered martensite so as to achieve 590 MPa or higher TS. If the Mn content is less than 2.00%, less manganese is available for forming a composite oxide with boron during annealing. As a result, an increased amount of BN results and the occurrence of black point defects and gray point defects cannot be suppressed sufficiently. If, on the other hand, the Mn content is more than 3.50%, the area fraction of tempered martensite is increased and the area fraction of ferrite and bainite is lowered, which causes a decrease in dimensional accuracy at the time of forming. Furthermore, an increased amount of manganese is concentrated at the steel sheet surface during annealing to form a large amount of manganese oxide that can cause bare spot defects on the steel sheet surface, making it difficult to achieve good coatability. Thus, the Mn content is limited to 2.00% or more and 3.50% or less. From the point of view of suppressing the occurrence of gray point defects and black point defects, the Mn content is preferably 2.30% or more, more preferably 2.50% or more, and still more preferably 2.60% or more. From the above viewpoints, the Mn content is preferably 3.30% or less, and more preferably 3.00% or less.

·P: 0.100% or less

Phosphorus is an element that has solid solution strengthening ability and increases the strength of steel sheets. If, however, the P content is more than 0.100%, phosphorus segregates at prior austenite grain boundaries and makes the grain boundaries brittle, thus deteriorating blankability and stretch flangeability. Thus, the P content is limited to 0.100% or less. From the above viewpoints, the P content is preferably 0.050% or less, and more preferably 0.030% or less.
There is no lower limit to the P content. That is, the P content may be 0%. However, controlling the P content to less than 0.001% increases refining costs. Thus, the P content is preferably 0.001% or more.

·S: 0.0100% or less

In steel, sulfur is present as sulfides. More than 0.0100% sulfur deteriorates the ultimate deformability of steel sheets and lowers blankability, stretch flangeability, and bendability. Thus, the S content is limited to 0.0100% or less. The lower limit of the S content is not particularly specified. That is, the S content may be 0%. However, controlling the S content to less than 0.0001% increases refining costs. Thus, the S content is preferably 0.0001% or more. From the above viewpoints, the S content is preferably 0.0050% or less.

·sol. Al: 1.00% or less

Aluminum can be used as a deoxidizing agent. In this case, the sol. Al content in the steel is preferably 0.01% or more. In boron-containing steel sheets, aluminum fixes nitrogen in the steel as AlN and thereby boron that has been added can be used as solute boron that is effective for increasing strength. Furthermore, aluminum suppresses the formation of carbides during annealing and increases the volume fraction of retained austenite. Retained austenite has an effect of enhancing ductility. In order to effectively fix nitrogen as AlN, the sol. Al content is preferably 0.02% or more. In order to obtain the ductility enhancing effect, the sol. Al content is more preferably 0.05% or more. If, however, the sol. Al content is more than 1.00%, bare spots occur. Thus, the sol. Al content is limited to 1.00% or less. From the above viewpoints, the sol. Al content is preferably 0.10% or less, and more preferably 0.08% or less.

·N: 0.0200% or less

In steel, nitrogen is present as nitrides. More than 0.0200% nitrogen deteriorates the ultimate deformability of steel sheets and lowers blankability, stretch flangeability, and bendability. Thus, the N content is limited to 0.0200% or less. The lower limit of the N content is not particularly specified. However, due to technical restrictions in production, the N content is preferably 0.0005% or more. From the above viewpoints, the N content is preferably 0.0080% or less.
There is no lower limit to the N content. That is, the N content may be 0%. However, controlling the N content to less than 0.0005% increases refining costs. Thus, the N content is preferably 0.0005% or more.

·B: 0.0001% or more and 0.0050% or less

Boron is an element that segregates at austenite grain boundaries and thereby enhances hardenability. The addition of boron to steel can suppress the occurrence and growth of ferrite during anneal cooling. In order to obtain these effects, 0.0001% or more boron needs to be added. If, on the other hand, the B content is more than 0.0050%, a large amount of nitride is formed on the steel sheet surface to deteriorate coating adhesion, giving rise to a poor appearance resulting from the exfoliation of coating. Thus, the B content is limited to 0.0001% or more and 0.0050% or less. From the above viewpoints, the B content is preferably 0.0002% or more. Similarly from the above viewpoints, the B content is preferably 0.0030% or less.

·Sb: 0.001% or more and 0.200% or less

Antimony is an element known as a nitridation inhibiting element. The addition of antimony suppresses the adsorption or entry of nitrogen into the steel sheet in the temperature range from 500°C to 750°C and thereby makes it possible to suppress the occurrence of boron nitride that causes gray point defects and black point defects. In order to obtain these effects, the Sb content needs to be 0.001% or more. On the other hand, antimony embrittles grain boundaries. If the Sb content is more than 0.200%, the slab will be cracked during hot rolling. Thus, the Sb content is limited to 0.001% or more and 0.200% or less. From the above viewpoints, the Sb content is preferably 0.005% or more. Similarly from the above viewpoints, the Sb content is preferably 0.100% or less.

·[% Mn]/[% Si]: 2.5 or more

[% Mn] is the Mn content and [% Si] is the Si content. Controlling [% Mn]/[% Si] to 2.5 or more suppresses the occurrence of the oxide of silicon alone and thereby suppresses the occurrence of bare spots. Furthermore, the control in combination with the annealing method of the present invention ensures that Mn-B composite oxide will be formed and thereby suppresses the occurrence of gray point defects and black point defects. Thus, [% Mn]/[% Si] is limited to 2.5 or more. From the point of view of suppressing the occurrence of gray point defects and black point defects, [% Mn]/[% Si] is preferably 4.0 or more, more preferably 6.0 or more, and still more preferably 12.0 or more. In particular, controlling [% Mn]/[% Si] to 12.0 or more promotes the formation of Mn-B composite oxide and enables more effective suppression of the occurrence of boron nitride that causes gray point defects and black point defects. Furthermore, controlling [% Mn]/[% Si] to 14.0 or more significantly promotes the formation of Mn-B composite oxide and can suppress the occurrence of microscopic unevenness that can lead to gray point defects and black point defects. Thus, [% Mn]/[% Si] is more preferably 14.0 or more. The upper limit is not particularly specified. However, the ratio is preferably 300.0 or less to avoid excessive formation of Mn-B composite oxide.

·Optional elements

The chemical composition of the high-strength steel sheet used in the present invention may further include one or more selected from, in mass%, Cr: 1.00% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Mo: 2.000% or less, Cu: 1.000% or less, Ni: 0.500% or less, Sn: 0.200% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Zn: 0.100% or less, Co: 0.200% or less, Zr: 0.200% or less, REM: 0.0100% or less, Ta: 0.10% or less, Te: 0.10% or less, As: 0.10% or less, Hf: 0.10% or less, Bi: 0.20% or less, Pb: 0.20% or less, Ge: 0.10% or less, Sr: 0.10% or less, and Cs: 0.10% or less. These elements are optional elements added as required and may represent 0%. The advantageous effects of the present invention can be obtained even when the content of each of the above elements is 0%.
Chromium is an element that enhances hardenability. This element is effective for obtaining desired amounts of quenched martensite and tempered martensite to attain 590 MPa or higher TS and is also effective for achieving excellent dimensional accuracy at the time of forming. If, however, the Cr content is more than 1.00%, the coating appearance quality is deteriorated and the dimensional accuracy at the time of forming is lowered due to the increase in the area fraction of quenched martensite and tempered martensite and the decrease in the area fraction of ferrite and bainite. Thus, when chromium is added, the content thereof is preferably controlled to 1.00% or less. From the point of view of enhancing the coating appearance quality, the Cr content is more preferably 0.75% or less. In order to obtain the hardenability enhancing effect conferred by chromium, the Cr content is preferably 0.02% or more.
Titanium forms fine carbide, nitride, or carbonitride at the time of hot rolling or annealing, thereby increasing TS. If, however, the Ti content is more than 0.200%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when titanium is added, the content thereof is preferably controlled to 0.200% or less. From the above viewpoints, the Ti content is more preferably 0.100% or less. In order to obtain the above effects, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more.
Niobium also forms fine carbide, nitride, or carbonitride at the time of hot rolling or annealing, thereby increasing TS. If, however, the Nb content is more than 0.200%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when niobium is added, the content thereof is preferably controlled to 0.200% or less. From the above viewpoints, the Nb content is more preferably 0.100% or less. In order to obtain the above effects, the Nb content is preferably 0.005% or more, and more preferably 0.010% or more.
Vanadium also forms fine carbide, nitride, or carbonitride at the time of hot rolling or annealing, thereby increasing TS. If, however, the V content is more than 0.200%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when vanadium is added, the content thereof is preferably controlled to 0.200% or less. From the above viewpoints, the V content is more preferably 0.100% or less. In order to obtain the above effects, the V content is preferably 0.005% or more, and more preferably 0.010% or more.
Molybdenum is an element that enhances hardenability and is effective for controlling the area fraction of quenched martensite and tempered martensite to a more preferred range so as to achieve a further increased TS and enhanced dimensional accuracy at the time of forming. If, however, the Mo content is more than 2.000%, the area fraction of quenched martensite and tempered martensite is so increased that 590 MPa or higher TS is hardly achieved and the dimensional accuracy at the time of forming is lowered. Furthermore, coarse precipitates and inclusions occur in increased amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when molybdenum is added, the content thereof is preferably controlled to 2.000% or less. From the above viewpoints, the Mo content is more preferably 0.500% or less. In order to obtain the above effects, the Mo content is preferably 0.005% or more, and more preferably 0.020% or more.
Copper is an element that enhances hardenability and is effective for controlling the area fraction of quenched martensite and tempered martensite to a more preferred range so as to achieve a further increased TS and enhanced dimensional accuracy at the time of forming. If, however, the Cu content is more than 1.000%, the area fraction of quenched martensite and tempered martensite is so increased that it is difficult to obtain 590 MPa or higher TS and excellent dimensional accuracy at the time of forming. Furthermore, coarse precipitates and inclusions occur in increased amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when copper is added, the content thereof is preferably controlled to 1.000% or less. From the above viewpoints, the Cu content is more preferably 0.200% or less. In order to obtain the above effects, the Cu content is preferably 0.005% or more, and more preferably 0.020% or more.
Nickel is an element that enhances hardenability and is effective for controlling the area fraction of quenched martensite and tempered martensite to a more preferred range so as to achieve a further increased TS and enhanced dimensional accuracy at the time of forming. If, however, the Ni content is more than 0.500%, the area fraction of quenched martensite and tempered martensite is so increased that TS and the dimensional accuracy at the time of forming are lowered. Furthermore, coarse precipitates and inclusions occur in increased amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when nickel is added, the content thereof is preferably controlled to 0.500% or less. From the above viewpoints, the Ni content is more preferably 0.200% or less. In order to obtain the above effects, the Ni content is preferably 0.005% or more, and more preferably 0.020% or more.
Tin is an element that suppresses the oxidation of the base steel sheet surface during annealing to effectively improve coatability. If, however, the Sn content is more than 0.200%, coarse precipitates and inclusions occur in increased amounts. When the base steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when tin is added, the Sn content is preferably controlled to 0.200% or less. From the above viewpoints, the Sn content is more preferably 0.050% or less. In order to obtain the above effects, the Sn content is preferably 0.001% or more, and more preferably 0.005% or more.
Magnesium is an element that makes the shape of inclusions into a sphere, such as sulfides and oxides, so as to effectively enhance the ultimate deformability of steel sheets and to enhance stretch flangeability. If, however, the Mg content is more than 0.0100%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when magnesium is added, the content thereof is preferably controlled to 0.0100% or less. From the above viewpoints, the Mg content is more preferably 0.0050% or less. In order to obtain the above effects, the Mg content is preferably 0.0001% or more, and more preferably 0.0005% or more.
Calcium is present as inclusions in the base steel sheet. When the base steel sheet contains diffusible hydrogen and if the Ca content is more than 0.0100%, such inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, the Ca content is preferably controlled to 0.0100% or less. While the lower limit of the Ca content may be 0.0000%, the Ca content is preferably 0.0001% or more due to technical restrictions in production. From the above viewpoints, the Ca content is more preferably 0.0020% or less.
Zinc is an element that makes the shape of inclusions into a sphere so as to effectively enhance the ultimate deformability of steel sheets and thereby to enhance stretch flangeability. If, however, the Zn content is more than 0.100%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when zinc is added, the content thereof is preferably controlled to 0.100% or less. From the above viewpoints, the Zn content is more preferably 0.020% or less, and still more preferably 0.010% or less. In order to obtain the above effects, the Zn content is preferably 0.001% or more, and more preferably 0.002% or more.
Cobalt is also an element that makes the shape of inclusions into a sphere so as to effectively enhance the ultimate deformability of steel sheets and thereby to enhance stretch flangeability. If, however, the Co content is more than 0.200%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when cobalt is added, the content thereof is preferably controlled to 0.200% or less. From the above viewpoints, the Co content is more preferably 0.010% or less. In order to obtain the above effects, the Co content is preferably 0.001% or more, and more preferably 0.005% or more.
Zirconium is also an element that makes the shape of inclusions into a sphere so as to effectively enhance the ultimate deformability of steel sheets and thereby to enhance stretch flangeability. If, however, the Zr content is more than 0.200%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when zirconium is added, the content thereof is preferably controlled to 0.200% or less. From the above viewpoints, the Zr content is more preferably 0.010% or less. In order to obtain the above effects, the Zr content is preferably 0.001% or more, and more preferably 0.005% or more.
Rare earth metals are elements that make the shape of inclusions into a sphere so as to effectively enhance the ultimate deformability of steel sheets and thereby to enhance stretch flangeability. If, however, the total REM content is more than 0.0100%, coarse precipitates and inclusions occur in large amounts. When the steel sheet contains diffusible hydrogen, such precipitates and inclusions serve as the origin of cracking at the time of bending test. That is, bendability is lowered. Thus, when a rare earth metal(s) is added, the total content thereof is preferably controlled to 0.0100% or less. From the above viewpoints, the total REM content is more preferably 0.0080% or less. In order to obtain the above effects, the total REM content is preferably 0.0001% or more, and more preferably 0.0005% or more.
Tantalum is an element that is effective for increasing the strength of the base steel sheet and may be added as required. The strengthening effect is obtained by adding 0.005% or more tantalum. When tantalum is added, the Ta content is preferably 0.10% or less to avoid an increase in cost.
Addition of 0.001% or more of tellurium can control the morphology of the sulfide and consequently improve ductility and toughness. When tellurium is added, the Te content is preferably 0.10% or less to avoid an increase in cost.
Addition of 0.001% or more of arsenic can control the morphology of the sulfide and consequently improve ductility and toughness. When arsenic is added, the As content is preferably 0.10% or less to avoid an increase in cost.
Addition of 0.01% or more of hafnium can control the morphology of the sulfide and consequently improve ductility and toughness. When hafnium is added, the Hf content is preferably 0.10% or less to avoid an increase in cost.
By adding 0.001% or more bismuth, grain boundary segregation can be suppressed so as to attain enhancements in ductility and toughness. Furthermore, bismuth is effective in enhancing machinability so as to improve the smoothness of the cut end face and also enhances the delayed fracture resistance of the machined cross section. When bismuth is added, the Bi content is preferably 0.10% or less to avoid an increase in cost.
By adding 0.001% or more lead, grain boundary segregation can be suppressed so as to attain enhancements in ductility and toughness. Furthermore, lead is effective in enhancing machinability so as to improve the smoothness of the cut end face and also enhances the delayed fracture resistance of the machined cross section. When lead is added, the Pb content is preferably 0.10% or less to avoid an increase in cost.
Germanium does not have significant impacts on mechanical properties or surface quality even when contained at 0.001% or more. When germanium is added, the Ge content is controlled to 0.10% or less to avoid an increase in cost.
Strontium does not have significant impacts on mechanical properties or surface quality even when contained at 0.001% or more. When strontium is added, the Sr content is controlled to 0.10% or less to avoid an increase in cost.
Cesium does not have significant impacts on mechanical properties or surface quality even when contained at 0.001% or more. When cesium is added, the Cs content is controlled to 0.10% or less to avoid an increase in cost.
The balance after the above components is Fe and incidental impurities.
Next, manufacturing conditions in the method of the present invention will be described.
In the manufacturing method of the present invention, a steel sheet (a cold-rolled steel sheet or a hot-rolled steel sheet) having the chemical composition described hereinabove is introduced into continuous hot-dip galvanizing equipment, and is continuously annealed, hot-dip galvanized, and optionally subjected to an alloying treatment in the equipment, thus giving a hot-dip galvanized steel sheet.
The continuous hot-dip galvanizing equipment is generally composed of, for example, an annealing furnace and a hot-dip galvanizing equipment disposed downstream of the annealing furnace. The hot-dip galvanizing equipment includes a hot-dip galvanizing bath, and a snout that is connected to the steel strip exit side of the annealing furnace and has a front-end portion submerged in the hot-dip galvanizing bath. Such continuous hot-dip galvanizing equipment may be a general continuous galvanizing line (CGL) configured to perform continuously a series of treatments including heating, cooling, hot-dip galvanizing, and alloying treatment of the hot-dip zinc coating. Of the treatments, the alloying treatment of the hot-dip zinc coating is performed as required and may be omitted.
The steel sheet that has been introduced in the continuous hot-dip galvanizing equipment is annealed by being passed through the annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order. Specific annealing conditions are as described later. The number of annealing passes is not particularly limited but is preferably one (one-pass annealing) because the present invention can suppress the occurrence of gray point defects and black point defects by one-time annealing.
The occurrence of surface defects (gray point defects and black point defects) addressed in the present invention is a particular phenomenon that is encountered when boron is added to the base steel sheet and the dew point during pre-coating annealing is low, and is not recognized as a problem to be solved in the conventional art. The present inventors were the first to find out the phenomenon as a problem to be solved. As already mentioned, the present inventors conducted extensive studies directed to manufacturing a hot-dip galvanized steel sheet having no or less surface defects and exhibiting a good surface appearance (coating appearance), and have consequently obtained the following findings.

(1) When the base steel sheet contains boron and the dew point during annealing is low, boron nitride occurs on the steel sheet surface to cause gray point defects and black point defects. Thus, the occurrence of gray point defects and black point defects can be prevented by suppressing the formation of boron nitride.
(2) Boron nitride occurs probably due to the following two main factors. (i) Nitrogen (N) from ammonia in the atmosphere adsorbs to or finds its way into the steel sheet and reacts with boron under low-oxygen potential conditions. (ii) When the oxygen potential is low and boron does not form the oxide, dissolved boron is diffused to the superficial layer of the steel sheet and forms the nitride on the superficial layer. Regarding the factor (i), ammonia forms from N₂ and H₂ in the atmosphere. This reaction is significantly accelerated in the temperature range of 500°C or above and 750°C or below by the catalytic action of iron oxide or pure iron resulting from the reduction of iron oxide, and more nitrogen adsorbs to or finds its way into the steel sheet. In view of this fact, the formation of boron nitride can be suppressed by suppressing the occurrence of iron oxide at temperatures of 500°C or below so as to reduce the amount of ammonia that is formed. Regarding the factor (ii), the occurrence of boron nitride can be suppressed when boron can be fixed as oxide, which is scarcely detrimental to the coating appearance quality, on the steel sheet surface or within the steel sheet even under low-oxygen potential conditions.
(3) The formation of boron nitride can be effectively suppressed by performing a pre-coating continuous annealing step while controlling the annealing conditions as described below. The control can thereby suppress the occurrence of gray point defects and black point defects effectively.
1. (i) While the dew point in the range of temperatures from 300 to 500°C was not controlled in the conventional art, an Fe reducing atmosphere is created by lowering the dew point in the above temperature range so as to sufficiently suppress the occurrence of iron oxide and thereby to reduce the amount of ammonia that is a factor of boron nitride formation.
2. (ii) At temperatures of 500°C or above and 750°C or below, ammonia occurs noticeably and nitrogen starts to enter or adsorb to the steel sheet. The nitridation of the steel sheet is suppressed by accelerating heating in this temperature range so as to reduce the amount of boron nitride that is formed at temperatures of 750°C or above.
3. (iii) Furthermore, the annealing time is shortened and the dew point is optimized in the temperature range of 750°C or above (the soaking zone) so as to fix part of boron as oxide and thereby suppress the formation of nitride stemming from the diffusion of boron to the steel sheet surface during annealing.

The present invention performs continuous annealing under conditions optimized so that the above effects (3) will be obtained. The optimized annealing conditions are a very important feature in the present invention.
The continuous annealing step of the present invention starts with heating the steel sheet in a temperature range of 300°C or above and 500°C or below in an atmosphere containing 3 vol% or more hydrogen and having a dew point of -10°C or below and an oxygen concentration of 500 ppm by volume or less. In this temperature range, the atmosphere is rendered reductive for iron in order to suppress the formation of iron oxide and thereby to reduce the occurrence of ammonia that can give rise to the formation of boron nitride (in particular, to reduce the noticeable occurrence of ammonia at temperatures of 500°C or above).

·Hydrogen concentration in the atmosphere: 3 vol% or more

Hydrogen is a reducing gas and thus can suppress the oxidation of the steel sheet surface at the time of annealing. In order to obtain the oxidation suppressing effect sufficiently, the hydrogen concentration in the atmosphere is limited to 3 vol% or more, and is preferably 5 vol% or more. While the upper limit of the hydrogen concentration is not particularly limited, the hydrogen concentration is preferably 30 vol% or less to avoid an increase in cost.

·Dew point in the atmosphere: -10°C or below

If the dew point in the low temperature range from 300 to 500°C is above -10°C, iron is oxidized at the steel sheet surface and the resultant iron oxide promotes the formation of ammonia at temperatures of 500°C and above. The oxidation of iron at the steel sheet surface can be suppressed by controlling the dew point to -10°C or below. Thus, the dew point is limited to -10°C or below. To suppress the formation of iron oxide sufficiently, the dew point is preferably -40°C or below. While the lower limit is not particularly specified, the dew point is preferably - 60°C or above to avoid an increase in cost associated with lowering the dew point.

·Oxygen concentration in the atmosphere: 500 ppm by volume or less

To suppress the oxidation of iron during annealing, the oxygen concentration is limited to 500 ppm by volume or less, and is preferably 200 ppm by volume or less.
The balance (95 vol% or less) other than hydrogen, H₂O, and oxygen in the atmosphere gas is preferably N₂ gas and incidental impurities. Part of the N₂ gas may be substituted with one or more of CO gas, CO₂ gas, and Ar gas. In this case, the substitute gas preferably represents 30 vol% or less of the atmosphere gas. While the lower limit is not particularly specified, 0.01 vol% or more is preferable to avoid an increase in cost associated with removing incidental impurities.

·Average heating rate in the temperature range from 300°C to 500°C

As already mentioned, ammonia as the source of boron nitride is formed in an accelerated manner under the catalytic action of iron oxide or pure iron resulting from the reduction of iron oxide. From the point of view of suppressing the occurrence of gray point defects and black point defects, the average heating rate is preferably controlled to 10°C/s or less to ensure a sufficient annealing time under the Fe-reducing atmosphere and thereby to reduce the amount of iron oxide present on the steel sheet surface. By virtue of this control, Mn-B oxide is formed on the steel sheet surface at a very early stage of the subsequent heating at temperatures of 500°C or above so as to cover the steel sheet surface, and thus can prevent the pure iron layer from being exposed on the steel sheet surface during heating and soaking/holding. On the other hand, the average heating rate is preferably 1°C/s or more in view of productivity. From the above viewpoints, the average heating rate is more preferably 2°C/s or more and 7°C/s or less.
In the continuous annealing step, the steel sheet is subsequently heated in the temperature range of 500°C or above and 750°C or below in an atmosphere containing 5 vol% or more hydrogen and having a dew point of -40°C or below at an average heating rate of 1°C/s or more. In this temperature range, ammonia occurs noticeably and nitrogen starts to adsorb to or enter the steel sheet. The formation of ammonia and the entry of nitrogen in this temperature range are suppressed to reduce the amount of boron nitride that will be formed in the subsequent soaking process.

·Hydrogen concentration in the atmosphere: 5 vol% or more

Hydrogen is a reducing gas and thus can suppress the formation of silicon and manganese oxides during annealing, thereby preventing the occurrence of bare spot defects caused by the oxides. In order to suppress the formation of oxides sufficiently, the hydrogen concentration in the atmosphere is limited to 5 vol% or more, and is preferably 6 vol% or more. The upper limit of the hydrogen concentration is not particularly limited, but is preferably 30 vol% or less to avoid an increase in cost.

·Dew point of the atmosphere: -40°C or below

If the dew point is above -40°C, silicon and manganese oxides are formed in large amounts on the steel sheet surface to cause bare spot defects. In addition, iron oxidation newly occurs to catalyze the formation of ammonia during this heating process at 500°C or above and during the soaking process at 750°C or above, resulting in a failure to sufficiently suppress the occurrence of gray point defects and black point defects. Thus, the dew point is limited to -40°C or below. From the point of view of suppressing the occurrence of gray point defects and black point defects, the dew point of the atmosphere is more preferably -42°C or below. While the lower limit is not particularly specified, the dew point is preferably -60°C or above to avoid an increase in cost associated with lowering the dew point.
The balance (95 vol% or less) other than hydrogen, H₂O, and oxygen in the atmosphere gas is preferably N₂ gas and incidental impurities. Part of the N₂ gas may be substituted with one or more of CO gas, CO₂ gas, and Ar gas. In this case, the substitute gas preferably represents 30 vol% or less of the atmosphere gas. While the lower limit is not particularly specified, 0.01 vol% or more is preferable to avoid an increase in cost associated with removing incidental impurities.

·Average heating rate: 1°C/s or more

The nitridation through the formation of ammonia occurs easily in the temperature range from 500 to 750°C. Thus, heating in this temperature range needs to be accelerated from the point of view of suppressing the occurrence of gray point defects and black point defects. At a heating rate of less than 1°C/s, an increased amount of time is required to reach the predetermined temperature and more nitrogen is allowed to enter the steel sheet and will form boron nitride in the downstream soaking process to cause a poor appearance with gray point defects and black point defects. Thus, the average heating rate is limited to 1°C/s or more, and is preferably 1.5°C/s or more, and more preferably 5°C/s or more. While the upper limit is not particularly specified, an average heating rate feasible in a general annealing furnace, specifically, 20°C/s or less is preferable.
In the continuous annealing step, the steel sheet is soaked at a temperature of 750°C or above and 950°C or below in an atmosphere containing 5 vol% or more hydrogen and having a dew point of -55°C or above and -40°C or below for a holding time of 20 to 300 seconds. This soaking treatment suppresses the nitridation of boron by ammonia and nitrogen that has entered the steel and also fixes part of boron as oxide on the steel sheet surface or within the steel sheet so as to suppress the nitridation of boron that has been diffused to the steel sheet surface during annealing. In this manner, the amount of boron nitride that is formed is reduced.

·Hydrogen concentration in the atmosphere: 5 vol% or more

·Dew point of the atmosphere: -55°C or above and -40°C or below

If the dew point is above -40°C, silicon and manganese oxides are formed in large amounts on the steel sheet surface to cause bare spot defects. If, on the other hand, the dew point is below -55°C, the treatment fails to sufficiently fix part of boron as oxide so as to suppress the nitridation of boron that has been diffused to the steel sheet surface during annealing. Furthermore, such an annealing atmosphere has a low oxygen potential and an increased nitrogen potential; hence, nitride formation is stable and boron nitride is formed on the steel sheet surface in an accelerated manner. Consequently, the occurrence of gray point defects and black point defects cannot be suppressed appropriately. Thus, the dew point is limited to -55°C or above, and is preferably -50°C or above. Furthermore, the dew point is limited to -40°C or below, and is preferably -45°C or below.

·Holding time: 20 seconds or more and 300 seconds or less

If the holding time is less than 20 seconds, austenite is not formed with a sufficient proportion during the heating in the ferrite-austenite two-phase region, and consequently the area fraction of ferrite and bainite is increased to make it difficult to obtain 590 MPa or higher TS. If, on the other hand, the holding time is more than 300 seconds, boron is partly fixed as oxide but part of the remaining boron is nitrided. Consequently, an increased amount of boron nitride is formed on the steel sheet surface, and the occurrence of gray point defects and black point defects cannot be suppressed appropriately. Thus, the holding time is limited to 20 seconds or more, and is preferably 30 seconds or more. Furthermore, the holding time is limited to 300 seconds or less, and is preferably 100 seconds or less. Incidentally, the holding time indicates the amount of time for which the steel sheet resides in the atmosphere (the amount of time in which the steel sheet is passed through the atmosphere) at a temperature of 750°C or above and 950°C or below.

·Ammonia concentration in the atmosphere

In the present invention, the ammonia concentration in the atmosphere during the soaking process is preferably lowered to 0.010 vol% or less. For example, ammonia is mixed in the soaking zone atmosphere by the introduction of ammonia gas from the heating zone into the soaking zone, and/or by the reuse of ammonia-containing exhaust gas in the soaking zone. To reduce the adverse effects caused by the mixing or occurrence of ammonia, it is important to enhance the sealability of the partition walls between the heating zone and the soaking zone, to reduce the reuse ratio of exhaust gas, and to supply new high-purity gas in an increased flow rate from the backward to the forward in the direction of the travel of the steel sheet (from the exit side to the entry side of the soaking zone). Reducing the ammonia concentration in the soaking zone and the consequent further suppression of the formation of boron nitride can further reduce the chance of a poor appearance with gray point defects and black point defects. Thus, the ammonia concentration in the atmosphere is preferably lowered to 0.010 vol% or less. While the lower limit is not particularly specified, the ammonia concentration is preferably 0.0001 vol% or more to avoid an increase in cost associated with removing ammonia.
In the present invention, the steel sheet that has been continuously annealed under the above-described conditions is cooled and is hot-dip galvanized by being dipped into a hot-dip galvanizing bath. Preferably, the cooling is performed at a temperature of 200 to 520°C, and the steel sheet is dipped into a hot-dip galvanizing bath after being heated as required. The bath temperature of the hot-dip galvanizing bath is generally about 440 to 500°C. The hot-dip galvanizing bath is not particularly limited and may be, for example, one having a composition including 0.10 mass% or more and 0.23 mass% or less Al, and one, or two or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total content of 0 mass% or more and 3.5 mass% or less, the balance being Zn and incidental impurities. To avoid a change in coating bath temperature, the temperature of the steel sheet before the coating treatment (the temperature of the sheet being dipped) is preferably equal to or higher than the coating bath temperature and 50°C or less above the coating bath temperature.
The hot-dip galvanization may be followed by an alloying treatment of the zinc coating to form a hot-dip galvannealed layer. The alloying treatment is preferably performed at a temperature of 480°C or above and 570°C or below. If the alloying temperature is below 480°C, the Zn-Fe alloying rate is excessively retarded and the alloying is very difficult to accomplish. If, on the other hand, the alloying temperature is above 570°C, non-transformed austenite is transformed into pearlite and may cause a decrease in TS and a decrease in El. The alloying treatment is more preferably performed at a temperature of 490°C or above and 560°C or below, still more preferably 490°C or above and 530°C or below.
The coating mass per side of the hot-dip galvanized steel sheet (GI) or the hot-dip galvannealed steel sheet (GA) is preferably 20 to 80 g/m². The coating mass may be controlled by, for example, performing gas wiping after the hot-dip galvanization.
The hot-dip galvanized or galvannealed steel sheet is cooled to a temperature of 350°C or below and room temperature or above. In order to enhance ductility, it is preferable that the steel sheet be cooled to a temperature of 350°C or below and 150°C or above, subsequently heated as required, and held at a predetermined temperature. On the other hand, cooling is preferably performed to room temperature in order to enhance strength. When the hot-dip galvanized or galvannealed steel sheet is cooled to a temperature of 350°C or below and 150°C or above, the cooling rate is not particularly specified. In order to ensure high TS and to enhance ductility, it is preferable that the rate of cooling be to 350°C be 3°C/s or more and 20°C/s or less. When the hot-dip galvanized or galvannealed steel sheet is cooled to room temperature, the cooling rate is not particularly specified but the average rate of cooling to 50°C is preferably 5°C/s or more in order to further increase TS. On the other hand, due to technical restrictions in production, the average rate of cooling to 50°C is preferably 40°C/s or less. The average rate of cooling to 50°C is more preferably 7°C/s or more and 30°C/s or less. The cooling rate below 50°C is not particularly limited and the steel sheet may be cooled to a predetermined temperature in any manner.
The hot-dip galvanized or galvannealed steel sheet may be cooled appropriately by, for example, gas jet cooling, mist cooling, water cooling, or air cooling. The high-strength hot-dip galvanized steel sheets are usually traded after being cooled to room temperature.
The hot-dip galvanized or galvannealed steel sheet that has been cooled to 350°C or below after the hot-dip galvanization may be rolled with a predetermined rolling ratio. The rolling ratio in this rolling is preferably 0.05% or more and 1.00% or less. This rolling at a rolling ratio of 0.05% or more can introduce cracks into the galvanized layer. The introduction of cracks into the galvanized layer can reduce the amount of diffusible hydrogen in the steel sheet and thus can enhance bendability and flangeability. If, on the other hand, the rolling ratio in the rolling is more than 1.00%, the rolling results in an increase in YS and the dimensional accuracy at the time of forming is lowered. The rolling ratio in this rolling is more preferably 0.70% or less and 0.10% or more.
The above rolling may be on-line rolling performed with an equipment connected to the continuous hot-dip galvanizing equipment or may be performed off-line from the continuous hot-dip galvanizing equipment. The target rolling ratio (for example, 0.05% or more and 1.00% or less) may be reached in one rolling pass, or the steel sheet may be rolled a plurality of times until the target rolling ratio is reached.
Temper rolling is generally performed as the above rolling. Other rolling, such as leveler working, may be adopted as long as the same level of rolling ratio as temper rolling can be applied.
After the hot-dip galvanized or galvannealed steel sheet is cooled to 350°C or below and is rolled as required in the above manner, the steel sheet may be held at room temperature or may be hot-idled at a temperature of above room temperature and 450°C or below. Holding at room temperature or retaining at a temperature of above room temperature and 450°C or below can reduce the amount of diffusible hydrogen in the steel sheet and thereby can improve bendability and flangeability. When, as described above, the steel sheet is cooled to a temperature of 350°C or below and 150°C or above in order to enhance ductility, it is preferable that the cooled steel sheet, after being heated as required, be held (retained) in the range of temperatures of 300°C or above and 450°C or below. Here, the holding time at room temperature is usually about 3 days to 10 months, and the hot-idling time at above room temperature is usually about 1 minute to 14 days.
The manufacturing conditions other than those described above may be conventional.
The high-strength hot-dip galvanized steel sheet manufactured in the present invention can achieve 590 MPa or higher TS. When higher strength is desired, TS can be increased to 780 MPa or more, or to 980 MPa or more. TS is measured as follows in accordance with JIS Z2241. A JIS No. 5 test specimen is sampled from the hot-dip galvanized steel sheet so that the longitudinal direction will be perpendicular to the rolling direction of the steel sheet. The TS of the test specimen is measured by a tensile test at a cross head displacement velocity Vc of 1.67 × 10^-1 mm/s.
The thickness of the hot-dip galvanized steel sheet manufactured in the present invention is not particularly limited but is usually about 0.3 mm or more and 2.8 mm or less.

EXAMPLES

[Example 1]

Steel materials having a chemical composition described in Table 1 (the balance was Fe and incidental impurities) were smelted in a converter and were continuously cast to give steel slabs. The steel slabs were each heated to 1250°C, rough rolled, finish rolled at a finish rolling temperature of 900°C, and coiled at a coiling temperature of 400 to 600°C. Hot-rolled steel sheets were thus obtained. The hot-rolled steel sheets were pickled and cold-rolled to give 1.4 mm thick cold-rolled steel sheets.
The cold-rolled steel sheets were each annealed on CGL under conditions described in Tables 2 to 5 and subsequently hot-dip galvanized under conditions described in Tables 2 to 5. Some of the steel sheets were subjected to an alloying treatment after the hot-dip galvanization. The steel sheets were then cooled to 50°C or below and were subsequently temper rolled with a rolling ratio of 0.1%. High-strength hot-dip galvanized steel sheets (GI) and high-strength hot-dip galvannealed steel sheets (GA) were thus obtained. The hyphen "-" in the column Alloying treatment in Tables 2 to 4 means that the alloying treatment was not performed.
The hot-dip galvanizing bath used in the manufacturing of GI contained Al: 0.20 mass%, the balance being Zn and incidental impurities. The hot-dip galvanizing bath used in the manufacturing of GA contained Al: 0.14 mass%, the balance being Zn and incidental impurities. In the manufacturing of GI, the coating mass was about 45 to 72 g/m² per side (both sides were coated). In the manufacturing of GA, the coating mass was about 45 to 55 g/m² per side (both sides were coated).
The coating layer of GI had a composition including Fe: 0.1 to 1.0 mass% and Al: 0.2 to 1.0 mass%, the balance being Fe and incidental impurities. The coating layer of GA had a composition including Fe: 7 to 15 mass% and Al: 0.1 to 1.0 mass%, the balance being Fe and incidental impurities.
The high-strength hot-dip galvanized steel sheets and the high-strength hot-dip galvannealed steel sheets obtained as described above were evaluated for tensile properties and coating appearance as follows. The results together with the manufacturing conditions are described in Tables 2 to 5.

·Tensile strength

The tensile test was performed in accordance with JIS Z2241. A JIS No. 5 test specimen was sampled from the steel sheet so that the longitudinal direction would be perpendicular to the rolling direction of the steel sheet. The TS of the test specimen was measured by the tensile test at a cross head displacement velocity Vc of 1.67 × 10^-1 mm/s.

·Coating appearance

The appearance of the coated steel sheet was visually observed to determine the presence or absence of gray point defects and black point defects. Five specimens (N = 5) having an area of 1000 mm in coil width direction × 1000 mm in coil longitudinal direction were sampled from the steel sheet, and the front side and the backside of the specimens were observed. The scores were 3 points when the observed regions were free from any defects; 2 points when 0.2 mm or smaller fine gray point defects or black point defects were found; 1 point when larger than 0.2 mm gray point defects or black point defects were found; and 0 point when conventional bare spots were present. The coating appearance of the steel sheet was rated better with increasing points. Those steel sheets that scored 2 points or above were accepted. The score was 3+ points when the surface did not have gray point defects or black point defects and was particularly beautiful without any microscopic coating unevenness that could lead to such defects.

As described in Tables 2 to 5, all the hot-dip galvanized steel sheets of Inventive Examples had a good coating appearance and achieved high strength with 590 MPa or higher TS. That is, hot-dip galvanized steel sheets with high strength and excellent coating quality were obtained. In contrast, the hot-dip galvanized steel sheets of Comparative Examples were poor in appearance due to the presence of gray point defects and black point defects.

[Table 1]

Steels	Chemical composition (mass%)										[%Mn] /[%Si]	Remarks
Steels	C	Si	Mn	P	S	sol. Al	N	B	Sb	Other elements	[%Mn] /[%Si]	Remarks
A	0.102	0.20	2.90	0.012	0.0009	0.018	0.0030	0.0029	0.011		14.5	Inv. steel
B	0.098	0.20	2.90	0.010	0.0010	0.014	0.0037	0.0038	0.000		14.5	Comp. steel
C	0.090	1.15	3.20	0.011	0.0010	0.048	0.0020	0.0019	0.009		2.8	Inv. steel
D	0.092	1.15	3.20	0.012	0.0013	0.020	0.0025	0.0025	0000		2.8	Comp. steel
E	0.075	0.80	2.05	0.010	0.0012	0.050	0.0033	0.0038	0.013		2.6	Inv. steel
F	0.077	0.80	2.05	0.012	0.0010	0.039	0.0012	0.0015	0000		2.6	Comp. steel
G	0.088	0.15	2.40	0.012	0.0011	0.035	0.0036	0.0011	0.008	Cr:0.60, Ti:0.020	16.0	Inv. steel
H	0.085	0.20	2.80	0.010	0.0010	0.040	0.0035	0.0016	0.011	Ti:0.020, Nb:0.020	14.0	Inv. steel
I	0.118	0.55	2.45	0.009	0.0005	0.032	0.0028	0.0018	0.022	Cr:0.61, Ti:0.021, Nb:0.044	4.5	Inv. steel
J	0.078	0.20	2.70	0.010	0.0009	0.075	0.0016	0.0008	0.002		13.5	Inv. steel
K	0.220	0.21	3.10	0.006	0.0002	0.040	0.0040	0.0044	0.015	Ti:0.018, Nb:0.008, Cu:0.18, Ni:0.15	14.8	Inv. steel
L	0.089	0.02	2.50	0.012	0.0010	0.040	0.0045	0.0049	0.200		125.0	Inv. steel
M	0.058	0.20	3.45	0.012	0.0013	0.040	0.0019	0.0021	0.050	Ti:0.005, V:0.110, Cu:0.10, Ni:0.05, Mg:0.0009, Ca:0.0008	17.3	Inv. steel
N	0.098	0.31	2.40	0.012	0.0010	0.015	0.0020	0.0002	0.100	Ti:0.024, Nb:0.015, Mg:0.0009, Ca:0.0005	7.7	Inv. steel
O	0.100	0.70	2.30	0.012	0.0010	0.012	0.0040	0.0015	0.030	Ti:0.018, Nb:0.008, Cu:0.18, Ni:0.15	3.3	Inv. steel
P	0.140	0.10	3.00	0.009	0.0006	0.025	0.0032	0.0007	0.150	Ti:0.018, Nb:0.025, Cu:0.05	30.0	Inv. steel
Q	0.100	0.70	2.30	0.012	0.0010	0.012	0.0040	0.0015	0.110	Ti:0.014, Nb:0.045, Mo:0.1, V:0.006	3.3	Inv. steel
R	0.165	0.18	2.85	0.012	0.0005	0.025	0.0040	0.0016	0.043	Ti:0.010, Mo:0.060, Ca:0.0004	15.8	Inv. steel
S	0.097	0.22	2.70	0.010	0.0010	0.040	0.0040	0.0016	0.080	Ti:0.045, Cu:0.45, Ni:0.06, Zn:0.007, Co:0.004, Zr:0.004, REM:0.0008	12.3	Inv. steel
T	0.070	0.71	3.10	0.012	0.0015	0.048	0.0022	0.0026	0.005	Sn:0.009, Co:0.180, Zr:0.025, REM:0.0033	4.4	Inv. steel
U	0.098	0.32	2.40	0.012	0.0010	0.015	0.0020	0.0002	0.030	Ti:0.024, Nb:0.015, Mg:0.0009, Ca:0.0005	7.5	Inv. steel
V	0.055	1.30	3.20	0.012	0.0010	0.055	0.0013	0.0018	0.019		2.5	Comp. steel
W	0.098	0.20	1.90	0.011	0.0008	0.053	0.0018	0.0029	0.013		9.5	Comp. steel
X	0.056	1.00	3.60	0.012	0.0010	0.055	0.0013	0.0018	0.010		3.6	Comp. steel
Y	0.088	0.60	2.70	0.012	0.0010	0.040	0.0040	0.0100	0.010		4.5	Comp. steel
Z	0.090	1.10	2.20	0.012	0.0010	0.040	0.0030	0.0030	0.012		2.0	Comp. steel
a	0.077	0.10	2.60	0.011	0.0010	0.036	0.0031	0.0014	0.032	Zn:0.018, Ta:0.07, Cs:0.07	26.0	Inv. steel
b	0.088	0.02	2.60	0.010	0.0011	0.031	0.0033	0.0021	0.022	Te:0.05, Sr:0.08	130.0	Inv. steel
c	0.085	0.20	2.60	0.012	0.0009	0.027	0.0023	0.0011	0.021	Bi:0.10, Ge:0.07	13.0	Inv. steel
d	0.071	0.22	2.90	0.010	0.0010	0.037	0.0033	0.0017	0.019	As:0.10, Ge:0.08	13.2	Inv. steel
e	0.082	0.18	2.60	0.009	0.0011	0.027	0.0028	0.0014	0.031	Hf:0.10, Pb:0.08	14.4	Inv. steel

[Table 2]

Nos.	Steels	Annealing conditions*1										Cooling	Coating treatment		Alloying treatment		TS (MPa)	Coating appearance (scores)	Remarks
		Temperatures from 300 to 500°C				Temperatures from 500 to 750°C			Soaking treatment (750 to 950°C)			Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
		Hydrogen (%)	Dew point (°C)	Oxygen (ppm)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Holding time (sec)	Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
1	A	6	-51	50	2.0	8	-48	6	7	-50	250	510	490	470	550	20	982	3+	Inv. Ex.
2	A	1	-38	50	2.0	10	-45	6	7	-52	250	510	490	470	550	20	980	1	Comp. Ex.
3	A	7	0	90	2.0	7	-45	6	8	-48	250	510	490	470	550	20	974	1	Comp. Ex.
4	A	9	-30	600	2.0	8	-45	6	8	-47	250	510	490	470	550	20	977	1	Comp. Ex.
5	A	9	-35	80	2.0	2	-47	6	7	-50	250	510	490	470	550	20	980	1	Comp. Ex.
6	A	9	-35	10	2.0	8	-30	6	8	-48	250	510	490	470	550	20	985	1	Comp. Ex.
7	A	9	-35	20	2.0	8	-50	0.4	8	-52	250	510	490	470	550	20	985	1	Comp. Ex.
8	A	9	-50	40	2.0	8	-48	6	2	-50	250	510	490	470	550	20	982	1	Comp. Ex.
9	A	9	-40	40	2.0	8	-48	6	8	-60	250	510	490	470	550	20	983	1	Comp. Ex.
10	A	9	-40	40	2.0	8	-50	6	8	-35	250	510	490	470	550	20	980	0	Comp. Ex.
11	A	9	-40	25	2.0	8	-48	2	8	-50	350	510	490	470	550	20	998	1	Comp. Ex.
12	A	3	-25	300	6.0	7	-48	6	8	-45	50	510	490	470	550	20	993	3+	Inv. Ex.
13	A	15	-10	20	10.0	7	-50	9	7	-42	50	510	490	470	550	20	988	3+	Inv. Ex.
14	A	8	-42	450	10.0	7	-52	9	7	-52	50	510	490	470	550	20	982	3+	Inv. Ex.
15	A	8	-42	100	6.0	5	-52	10	7	-43	50	510	490	470	550	20	980	3+	Inv. Ex.
16	A	8	-42	100	6.0	7	-52	1	5	-50	50	510	490	470	550	20	983	3+	Inv. Ex.
17	A	8	-42	100	6.0	7	-52	10	7	-55	20	510	490	470	550	20	1001	3+	Inv. Ex.
18	A	8	-42	100	6.0	7	-57	6	7	-45	280	510	490	470	550	20	995	3+	Inv. Ex.
19	A	8	-42	20	6.0	7	-57	6	7	-45	280	510	490	470	-	-	1007	3+	Inv. Ex.

*1 Hydrogen (%): Hydrogen concentration (vol%) in the atmosphere. *Underlined symbols and values are outside the range of the present invention.
Oxygen (ppm): Oxygen concentration (ppm by volume) in the atmosphere.
Dew point (°C): Dew point of the atmosphere.

[Table 3]

Nos.	Steels	Annealing conditions*1										Cooling	Coating treatment		Alloying treatment		TS (MPa)	Coating appearance (scores)	Remarks
		Temperatures from 300 to 500°C				Temperatures from 500 to 750°C			Soaking treatment (750 to 950°C)			Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
		Hydrogen (%)	Dew point (°C)	Oxygen (ppm)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Holding time (sec)	Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
20	B	4	-15	400	5.0	8	-49	6	6	-51	250	510	490	470	550	20	981	1	Comp. Ex.
21	C	7	-30	80	5.0	9	-45	6	7	-50	100	510	490	470	550	20	1020	2	Inv. Ex.
22	C	7	-30	80	5.0	9	-45	6	7	-50	100	510	490	470	-	-	1045	2	Inv. Ex.
23	C	7	-30	80	5.0	9	-45	6	7	-50	300	510	490	470	550	20	1031	2	Inv. Ex.
24	C	7	-30	80	5.0	9	-45	6	7	-50	330	510	490	470	550	20	1038	1	Comp. Ex.
25	D	6	-52	80	5.0	8	-45	6	6	-51	150	510	490	470	550	20	1045	1	Comp. Ex.
26	E	4	-15	50	5.0	8	-52	6	8	-48	80	510	490	470	550	20	780	2	Inv. Ex.
27	E	2	-15	50	6.0	7	-52	6	7	-48	80	510	490	470	550	20	785	1	Comp. Ex.
28	E	4	0	200	6.0	7	-47	6	7	-50	80	510	490	470	550	20	790	1	Comp. Ex.
29	E	4	-15	550	6.0	7	-47	6	7	-50	80	510	490	470	550	20	795	1	Comp. Ex.
30	E	4	-15	50	13.0	2	-47	6	7	-50	80	510	490	470	550	20	794	1	Comp. Ex.
31	E	4	-15	50	6.0	7	-20	6	7	-50	80	510	490	470	550	20	788	1	Comp. Ex.
32	E	4	-15	50	6.0	7	-50	6	4	-50	80	510	490	470	550	20	790	1	Comp. Ex.
33	E	4	-15	50	6.0	7	-50	6	7	-60	80	510	490	470	550	20	793	1	Comp. Ex.
34	F	4	-15	50	5.0	8	-50	6	8	-49	80	510	490	470	550	20	780	1	Comp. Ex.
35	G	6	-20	50	5.0	7	-45	6	7	-48	200	510	490	470	550	20	840	3+	Inv. Ex.
36	H	3	-20	300	10.0	5	-45	15	6	-45	80	510	490	470	550	20	995	3+	Inv. Ex.
37	I	3	-20	150	5.0	8	-44	6	7	-41	250	510	490	470	550	20	1187	2	Inv. Ex.

[Table 4]

Nos.	Steels	Annealing conditions*1										Cooling	Coating treatment		Alloying treatment		TS (MPa)	Coating appearance (scores)	Remarks
		Temperatures from 300 to 500°C				Temperatures from 500 to 750°C			Soaking treatment (750 to 950°C)			Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
		Hydrogen (%)	Dew point (°C)	Oxygen (ppm)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Holding time (sec)	Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
38	J	3	-40	10	10.0	12	-40	10	5	-40	100	510	490	470	550	20	920	3	Inv. Ex.
39	K	3	-40	50	10.0	12	-44	10	7	-50	100	510	490	470	550	20	1481	3+	Inv. Ex.
40	L	5	-10	50	1.0	12	-44	5	5	-42	50	510	490	470	550	20	830	3+	Inv. Ex.
41	M	5	-20	50	1.0	12	-50	5	7	-50	50	510	490	470	550	20	784	3+	Inv. Ex.
42	N	5	-20	50	1.0	12	-50	6	7	-54	20	510	490	470	550	20	1025	2	Inv. Ex.
43	O	10	-25	20	5.0	12	-50	6	7	-54	20	510	490	470	550	20	1045	2	Inv. Ex.
44	P	7	-25	20	5.0	12	-50	6	7	-50	50	510	490	470	550	20	1190	3+	Inv. Ex.
45	Q	6	-45	40	5.0	6	-49	2	7	-52	180	510	490	470	550	20	1067	2	Inv. Ex.
46	R	6	-45	40	5.0	6	-44	2	7	-52	180	510	490	470	550	20	1250	3+	Inv. Ex.
47	R	6	-45	40	5.0	6	-44	2	7	-52	180	200	490	470	550	20	1182	3+	Inv. Ex.
48	S	6	-45	40	5.0	6	-44	2	7	-52	180	525	490	470	530	20	997	3	Inv. Ex.
49	T	6	-45	40	5.0	6	-44	2	7	-52	180	500	490	470	520	20	835	2	Inv. Ex.
50	U	6	-45	40	5.0	6	-44	2	7	-52	180	510	490	470	550	20	997	2	Inv. Ex.
51	V	7	-50	20	5.0	12	-50	6	7	-50	50	510	490	470	550	20	800	1	Comp. Ex.
52	W	7	-50	20	6.0	7	-50	6	7	-50	50	510	490	470	550	20	850	1	Comp. Ex.
53	X	7	-50	20	6.0	7	-50	6	7	-50	50	510	490	470	550	20	840	1	Comp. Ex.
54	Y	7	-50	20	6.0	7	-50	6	7	-50	50	510	490	470	550	20	880	1	Comp. Ex.
55	Z	7	-50	20	6.0	7	-50	6	7	-50	50	510	490	470	550	20	920	1	Comp. Ex.

[Table 5]

Nos.	Steels	Annealing conditions*1										Cooling	Coating treatment		Alloying treatment		TS (MPa)	Coating appearance (scores)	Remarks
		Temperatures from 300 to 500°C				Temperatures from 500 to 750°C			Soaking treatment (750 to 950°C)			Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
		Hydrogen (%)	Dew point (°C)	Oxygen (ppm)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Average heating rate (°C/s)	Hydrogen (%)	Dew point (°C)	Holding time (sec)	Cooling temp. (°C)	Temp. of sheet being dipped (°C)	Coating bath temp. (°C)	Temp. (°C)	Time (sec)
56	a	8	-41	29	5.0	8	-45	2	7	-50	140	510	490	470	550	20	812	3+	Inv. Ex.
57	b	7	-42	28	5.0	8	-46	2	7	-51	140	510	490	470	550	20	806	3+	Inv. Ex.
58	c	8	-40	30	5.0	7	-49	2	7	-49	140	510	490	470	550	20	987	3+	Inv. Ex.
59	d	7	-44	35	5.0	7	-46	2	7	-49	140	510	490	470	550	20	980	3+	Inv. Ex.
60	e	7	-40	33	5.0	8	-46	2	7	-50	140	510	490	470	550	20	997	3+	Inv. Ex.

[Example 2]

From the point of view of further reducing the ammonia concentration in the atmosphere in the soaking process (the soaking zone), hot-dip galvanized steel sheets were manufactured while supplying a high-purity gas at an increased flow rate in the soaking zone. The steel materials A, C, G, I, S, and T described in Table 1 were treated (hot rolling, cold rolling, continuous annealing, hot-dip galvanization, alloying treatment, and temper rolling) under the manufacturing conditions according to Example 1 to give hot-dip galvanized steel sheets. Tensile properties and the coating appearance of the hot-dip galvanized steel sheets obtained were evaluated in the same manner as in Example 1. The results together with the manufacturing conditions are described in Table 6.
The concentration of ammonia gas was measured at an upper part of the path within the furnace that included the lengthwise central portion of the CGL soaking zone. The measurement was performed by ion chromatography.
As described in Table 6, it has been shown that the surface quality (the coating appearance) of the hot-dip galvanized steel sheet is further enhanced by reducing the ammonia concentration in the soaking process (the soaking zone) to 0.010 vol% or less.

Claims

A method for manufacturing a high-strength hot-dip galvanized steel sheet, the method comprising continuously annealing a steel sheet, hot-dip galvanizing the steel sheet by dipping the steel sheet into a hot-dip galvanizing bath, and optionally subjecting the steel sheet to an alloying treatment, wherein
the steel sheet has a chemical composition comprising, in mass%:
C: 0.050% or more and 0.300% or less,

Si: 1.20% or less,

Mn: 2.00% or more and 3.50% or less,

P: 0.100% or less,

S: 0.0100% or less,

sol. Al: 1.00% or less,

N: 0.0200% or less,

B: 0.0001% or more and 0.0050% or less, and

Sb: 0.001% or more and 0.200% or less,

[% Mn]/[% Si] being 2.5 or more, and

optionally further comprising one or more selected from:
Cr: 1.00% or less,

Ti: 0.200% or less,

Nb: 0.200% or less,

V: 0.200% or less,

Mo: 2.000% or less,

Cu: 1.000% or less,

Ni: 0.500% or less,

Sn: 0.200% or less,

Mg: 0.0100% or less,

Ca: 0.0100% or less,

Zn: 0.100% or less,

Co: 0.200% or less,

Zr: 0.200% or less,

REM: 0.0100% or less,

Ta: 0.10% or less,

Te: 0.10% or less,

As: 0.10% or less,

Hf: 0.10% or less,

Bi: 0.20% or less,

Pb: 0.20% or less,

Ge: 0.10% or less,

Sr: 0.10% or less, and

Cs: 0.10% or less,

the balance being Fe and incidental impurities, and

the continuous annealing step comprises:
heating the steel sheet in a temperature range of 300°C or above and 500°C or below in an atmosphere containing 3 vol% or more hydrogen and 500 ppm by volume or less oxygen and having a dew point of -10°C or below,

heating the steel sheet in a temperature range of 500°C or above and 750°C or below in an atmosphere containing 5 vol% or more hydrogen and having a dew point of -40°C or below at an average heating rate of 1°C/s or more, and

soaking the steel sheet at a temperature of 750°C or above and 950°C or below in an atmosphere containing 5 vol% or more hydrogen and having a dew point of -55°C or above and -40°C or below for a holding time of 20 to 300 seconds.
The method for manufacturing a high-strength hot-dip galvanized steel sheet according to claim 1, wherein the chemical composition of the steel sheet satisfies [% Mn]/[% Si] of 12.0 or more.
The method for manufacturing a high-strength hot-dip galvanized steel sheet according to claim 1 or 2, wherein the atmosphere in which the steel sheet being continuously annealed is soaked at a temperature of 750°C or above and 950°C or below contains 0.010 vol% or less ammonia.