阅读说明：本技术 高强度热轧镀覆钢板 (High-strength hot-rolled plated steel sheet ) 是由 后藤聪太 山崎和彦 段蒂玄 小野义彦 于 2019-06-10 设计创作，主要内容包括：本发明提供一种TS为980MPa以上、具有优异的胀形成型性和拉伸凸缘成型性、具有优异的镀覆性的高强度热轧镀覆钢板。所述高强度热轧镀覆钢板具备钢板和在钢板表面的镀层或合金化镀层,所述钢板具有如下成分组成和如下钢组织,所述成分组成以质量％计含有C：0.03～0.09％、Si：0.01～1.60％、Mn：2.20～3.60％、P：0.100％以下、S：0.0100％以下、Ti：0.05～0.18％、B：0.0005～0.0050％、Al：0.005～0.40％和N：0.010％以下,剩余部分由Fe和不可避免的杂质构成,33.8[％C][％Mn]+12.4[％Si]/[％Mn]表示的CSM值满足3.3～12.0,所述钢组织含有面积率为85％以上的贝氏体和面积率为2.0％～15.0％的马氏体。(The present invention provides a high-strength hot-rolled plated steel sheet having a TS of 980MPa or more, excellent bulging formability and stretch flange formability, and excellent plating properties. The high-strength hot-rolled plated steel sheet is provided with a steel sheet and a plating layer or an alloyed plating layer on the surface of the steel sheet, and the steel sheet has a composition of components containing, in mass%, C: 0.03-0.09%, Si: 0.01-1.60%, Mn: 2.20-3.60%, P: 0.100% or less, S: 0.0100% or less, Ti: 0.05-0.18%, B: 0.0005 to 0.0050%, Al: 0.005-0.40% and N: less than 0.010%, the remainder being Fe and unavoidable impurities, a CSM value represented by 33.8 [% C ] [% Mn ] +12.4 [% Si ]/[% Mn ] satisfying 3.3 to 12.0, and the steel structure containing bainite having an area ratio of 85% or more and martensite having an area ratio of 2.0% to 15.0%.)

1. A high-strength hot-rolled plated steel sheet comprising a steel sheet and a plating layer or an alloyed plating layer on the surface of the steel sheet, the steel sheet having the following composition and the following steel structure,

the composition comprises, in mass%, C: 0.03-0.09%, Si: 0.01-1.60%, Mn: 2.20-3.60%, P: 0.100% or less, S: 0.0100% or less, Ti: 0.05-0.18%, B: 0.0005 to 0.0050%, Al: 0.005-0.40% and N: 0.010% or less, the balance being Fe and inevitable impurities, and the CSM value represented by the formula (1) satisfying 3.3 to 12.0,

the steel structure contains bainite with an area ratio of 85% or more and martensite with an area ratio of 2.0% to 15.0%,

CSM value 33.8 [% C ] [% Mn ] +12.4 [% Si ]/[% Mn ] (1)

In the formula (1), [% C ], [% Mn ] and [% Si ] represent the contents of the respective elements, in units of mass%.

2. The high-strength hot-rolled plated steel sheet according to claim 1, further comprising Nb: 0.005-0.060% and V: 0.005-0.080% of more than 1.

3. The high-strength hot-rolled plated steel sheet according to claim 1 or 2, further comprising, in mass%, Cr: 0.02 to 0.15%, Mo: 0.02 to 0.5%, Cu: 0.05-0.5% and Ni: 0.05-1.0% of more than 1.

4. The high-strength hot-rolled plated steel sheet according to any one of claims 1 to 3, further comprising Sb: 0.0002 to 0.0200%.

5. The high-strength hot-rolled plated steel sheet according to any one of claims 1 to 4, further comprising Ca: 0.0002 to 0.0100%, Mg: 0.0002 to 0.0100% and REM: 0.0002 to 0.0100% of at least 1 species.

Technical Field

The present invention relates to a high-strength hot-rolled plated steel sheet having a high tensile strength of 980MPa or more and an expansion ratio of 50% or more, which is suitable for use as a material for structural members of automobiles, transportation equipment, building equipment, and the like, and which has excellent press formability.

Background

Reduction of CO with continuous worldwide demand₂In the automobile industry, it is always required to reduce the weight of the vehicle body without reducing the strength of the vehicle body, thereby improving the fuel efficiency. In order to reduce the weight of a vehicle body without reducing the strength of the vehicle body, it is one of effective methods to increase the strength of a steel sheet as a material of a member and reduce the thickness of the steel sheet. In particular, steel sheets having a tensile strength of 980MPa or more are expected to be materials that greatly improve the fuel efficiency of automobiles by reducing the weight.

However, bulging formability and stretch flange formability, which are particularly important for forming chassis parts of automobiles, generally deteriorate with increasing strength of steel sheets. In addition, particularly in a portion where corrosion proceeds in practical use and the thickness of the part is reduced, such as a chassis part, the risk of fatigue fracture increases. Therefore, plating the surface of the steel sheet to improve corrosion resistance and reduce corrosion and thickness reduction is an effective means. However, if the amount of alloying elements added to increase the strength is increased, a portion where no plating layer is formed on the surface of the steel sheet (non-plated portion) may be generated.

As a plated steel sheet suitable for stretch flange working, patent document 1 discloses a high-strength hot-dip Zn — Al — Mg-based plated steel sheet having a tensile strength of 400MPa or more, which contains, in mass%, C: 0.005-0.08%, Si: 0.8% or less, Mn: 0.1-1.8%, P: 0.05% or less, S: 0.02% or less, N: 0.001 to 0.005%, Ti: 0.02-0.2%, B: 0.0005 to 0.01%, Al: 0.1% or less, the balance being Fe and unavoidable impurities, Ti and C being contained so as to satisfy a Ti/C equivalent ratio of (Ti/48)/(C/12), a microstructure comprising a main phase comprising a bainitic ferrite single phase or a bainitic ferrite phase and a ferrite phase, a hard 2 nd phase of 3% or less, and cementite, a ratio of 2 to 15 DEG low-angle grain boundaries having different crystal orientations being 30 to 75%, and Ti-containing carbides having an average grain diameter of 20nm or less being dispersed and precipitated.

Patent document 2 discloses a high-strength hot-rolled steel sheet having a product of tensile strength (MPa) and hole expansion ratio (%) of 35000, which contains, in mass%, C: 0.03-0.2%, Mn: 0.1-3.0%, P: 0.10% or less, S: 0.03% or less, Al + Si: 0.2-3.0%, N: more than 0 and 0.01% or less, O: more than 0 and 0.01% or less, the balance being Fe and impurities, the microstructure being mainly composed of bainite, 3% or more and less than 20% in area percentage of a hard phase composed of martensite and/or austenite, 60% or more of a portion having an aspect ratio of 3 or more of the hard phase present in the central portion of the sheet thickness, a length of the hard phase present in the central portion of the sheet thickness in the rolling direction of less than 20 μm, a sum of random intensity ratios of X-rays of < 011 > orientation and < 111 > orientation as seen in the rolling direction of 3.5 or more, and a random intensity ratio of X-rays of < 001 > orientation as seen in the rolling direction of 1.0 or less.

Patent document 3 discloses a high-strength hot-rolled steel sheet having a specific amount of C, Mn, P, S, Al, N, Ti, Cr, and B as a component composition, and having the following structure: a bainite phase having an area ratio of 85% or more is used as a main phase, a martensite phase or a martensite-austenite mixed phase having an area ratio of 15% or less is used as a 2 nd phase, the remainder is composed of a ferrite phase, the 2 nd phase has an average grain diameter of 3.0 [ mu ] m or less, and the prior austenite grains have an average aspect ratio of 1.3 to 5.0, the area ratio of the recrystallized prior austenite grains to the non-recrystallized prior austenite grains is 15% or less, precipitates having a diameter of less than 20nm are 0.10% or less by mass%, and the tensile strength is 980MPa or more, and the excellent tensile flange characteristic is exhibited with an expansion ratio of 60% or more.

Patent document 4 discloses an alloyed hot-dip zinc-plated steel sheet having a tensile strength of 780MPa or more, which contains, in mass%, C: 0.03-0.30%, Si: 0.005-2.5%, Mn: 1.9-3.5%, P: 0.1% or less, S: 0.01% or less, sol.Al: 0.001-1.5% and N: the hot-rolled steel sheet is heated to 720 ℃ or higher by 0.02% or less, cooled to 450 to 600 ℃ at a rate of 2 to 200 ℃/sec, cooled to 200 ℃ or lower by 5 ℃/sec or higher from the alloying temperature after hot-dip galvanization, and further tempered for 1 second to 10 minutes or less in a temperature range of 200 to 600 ℃, and has a microstructure containing 3% or more by volume of tempered martensite and 1% or more by volume of retained austenite.

Patent document 5 discloses a hot-rolled steel sheet having a tensile strength of 900MPa or more, which contains, in mass%, C: more than 0.050% and 0.10% or less, Si: 0.1-2.0%, Mn: 1.0-3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.005-0.05%, N: 0.01% or less, Ti: 0.10 to 0.20%, Nb: 0-0.06%, B: 0-0.03%, Ca: 0 to 0.005% and the balance Fe and impurities, an average crystal grain diameter of 7.0 μm or less, and an X-ray random intensity ratio of {211} < 011 > orientation parallel to a rolling surface and parallel to a rolling direction of 2.5 or less. Patent document 5 discloses a method for producing a plated steel sheet, in which a hot-rolled steel sheet is wound up and then heated to 500 to 650 ℃ in a reducing atmosphere to activate the surface, thereby forming a plated layer on the surface.

Documents of the prior art

Patent document

Patent document 1: WO2015/093596

Patent document 2: WO2016/010004

Patent document 3: WO2017/017933

Patent document 4: japanese patent laid-open publication No. 2013-144830

Patent document 5: WO2014/097430

Disclosure of Invention

However, patent document 1 cannot obtain a tensile strength of 980MPa or more.

Patent document 2 cannot achieve a balance between high strength and hole expansion ratio. In addition, the plating property is not discussed in patent document 2.

Patent document 3 discloses a hot-rolled steel sheet having high strength of 980MPa or more and excellent hole expansion characteristics. However, in patent document 3, the plating property has not been sufficiently studied.

In patent document 4, after hot dip galvanizing is performed on a hot-rolled steel sheet, further tempering treatment is required, and there is a problem in terms of economy.

In patent document 5, in order to obtain a desired microstructure, it is necessary to set the hot rolling end temperature to 960 ℃ or higher. However, when the hot rolling end temperature is raised to a high level, scale on the surface of the steel sheet excessively grows to cause die-cutting marks, or the scale remains after pickling, which may deteriorate the plating property.

In summary, in the prior art, there has been no established technique for a high-strength hot-rolled steel sheet having a tensile strength of 980MPa or more and excellent press formability and plating property.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a high-strength hot-rolled plated steel sheet having excellent bulging formability, stretch-flange formability, and plating properties while maintaining a tensile strength of 980MPa or more.

The inventors have intensively studied to achieve the above object and to improve the bulging formability and the stretch flange formability of a hot-rolled plated steel sheet while securing a tensile strength of 980MPa or more. As a result, the following findings were obtained: a high strength of 980MPa or more, excellent bulging formability and stretch flange formability are obtained by making the main phase a bainite structure, making the 2 nd phase a martensite structure, and controlling the area ratio of the 2 nd phase to 2.0 to 15.0%.

The bainite structure referred to herein is an intermediate temperature between a polygonal ferrite structure formed by diffusion transformation and a martensite structure formed by non-diffusion transformationThe structure of the region generation means that the average dislocation density is 5X 10¹⁴～5×10¹⁵m^-2The tissue of (1). The bainite structure is in a lath-shaped structure form. Therefore, the bainitic structure can be clearly distinguished from the polygonal ferrite structure by using a Scanning Electron Microscope (SEM), for example. The martensite structure can also be distinguished from the bainite structure by observing its lower structure in detail using SEM. The martensite structure and the bainite structure may be distinguished by an Electron Back Scattering Diffraction (EBSD) analyzer installed in the SEM. That is, since the martensite structure has a higher dislocation density than the bainite structure, the Image Quality value (IQ value) of EBSD is lower than the IQ value of the bainite structure. When an EBSD map is taken under the conditions of an acceleration voltage of 15kV and a focal length of 19mm, the IQ value of the martensite structure is 80000 or less.

Regarding the bulging formability, a steel sheet having a high yield ratio (ratio of yield strength to tensile strength) has a low strain dispersion capability, and necking and cracking are likely to occur at a position where strain is concentrated, resulting in a bulging failure. In addition, when the single-phase structure and the complex-phase structure have the same strength and ductility, the yield ratio of the single-phase structure is higher than that of the complex-phase structure. Therefore, in the present invention, in order to ensure the bulging formability, the bulging formability is improved by reducing the yield ratio by making the phase 2 structure having strength and ductility different from those of the main phase exist in the steel sheet. In addition, when the 2 nd phase, which becomes a starting point of void generation in the hole expansion test, is small, the stretch flange formability is improved. The present invention ensures stretch flangability without deteriorating stretch flangability by controlling the area ratio of the phase 2 within an appropriate range.

Based on the above findings, the present invention has examined the influence of the amount of alloy added on the plating property and the steel structure, and as a result, the gist is as follows.

[1] A high-strength hot-rolled plated steel sheet comprising a steel sheet and a plating layer or an alloyed plating layer on the surface of the steel sheet, the steel sheet having the following composition and steel structure, and containing C in mass%: 0.03-0.09%, Si: 0.01-1.60%, Mn: 2.20-3.60%, P: 0.100% or less, S: 0.0100% or less, Ti: 0.05-0.18%, B: 0.0005 to 0.0050%, Al: 0.005-0.40% and N: 0.010% or less, and the balance of Fe and inevitable impurities, wherein the CSM value represented by the formula (1) satisfies 3.3 to 12.0, and the steel structure contains bainite having an area ratio of 85% or more and martensite having an area ratio of 2.0% to 15.0%.

CSM value 33.8 [% C ] [% Mn ] +12.4 [% Si ]/[% Mn ] (1)

In the formula (1), [% C ], [% Mn ] and [% Si ] represent the contents (mass%) of the respective elements.

[2] The high-strength hot-rolled plated steel sheet according to [1], wherein the composition further contains, in mass%: 0.005-0.060% and V: 0.005-0.080% of more than 1.

[3] The high-strength hot-rolled plated steel sheet according to [1] or [2], wherein the above-described composition further contains, in mass%, Cr: 0.02 to 0.15%, Mo: 0.02 to 0.5%, Cu: 0.05-0.5% and Ni: 0.05-1.0% of more than 1.

[4] The high-strength hot-rolled plated steel sheet according to any one of [1] to [3], wherein the composition further contains, in mass%, Sb: 0.0002 to 0.0200%.

[5] The high-strength hot-rolled plated steel sheet according to any one of [1] to [4], wherein the composition further contains, in mass%, Ca: 0.0002 to 0.0100%, Mg: 0.0002 to 0.0100% and REM: 0.0002 to 0.0100% of at least 1 species.

According to the present invention, a high-strength hot-rolled plated steel sheet having a tensile strength of 980MPa or more and excellent press formability is obtained. Further, the high-strength hot-rolled plated steel sheet has excellent plating properties, and thus can be produced economically and stably without any unplated portion.

When the high-strength hot-rolled plated steel sheet of the present invention is used for automobile chassis parts, structural parts, frame parts, and truck frame parts, the reliability of an automobile can be ensured and the weight of the automobile body can be reduced, thus providing industrially significant effects.

In the present invention, the excellent press formability means that the yield ratio as the bulging formability is 0.93 or less and the hole expansion ratio λ as the stretch flange formability is 50% or more. In consideration of balance with other characteristics such as tensile strength, the yield ratio is preferably 0.70 or more, more preferably 0.75 or more. From the viewpoint of balancing with other characteristics as well, the hole expansibility is preferably 95% or less, and more preferably 90% or less.

Detailed Description

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

The high-strength hot-rolled plated steel sheet of the present invention has a steel sheet and a plated layer or an alloyed plated layer. First, a steel sheet will be explained.

The steel sheet had the following composition. In the following description, "%" which is a unit of the content of an element in the component composition means "% by mass".

C：0.03～0.09％

C is an element that promotes the formation of the bainite structure and the 2 nd phase structure by improving the strength of the steel and improving hardenability. In the present invention, the content is required to be 0.03% or more. Preferably 0.04% or more. On the other hand, if the content exceeds 0.09%, the strength of the 2 nd phase becomes too high, and stretch flange formability deteriorates even if the area ratio of the 2 nd phase is set to 15.0% or less. Therefore, the C content is 0.09% or less. Preferably 0.08% or less.

Si：0.01～1.60％

Si is an element effective for promoting the generation of phase 2. Therefore, the Si content is 0.01% or more. Preferably 0.10% or more. On the other hand, if the Si content exceeds 1.60% in the plated steel sheet, adhesion of the plating layer is inhibited, and the corrosion resistance of the steel sheet is deteriorated. Therefore, the Si content is limited to 1.60% or less. Preferably 1.20% or less, more preferably 1.00% or less, and further preferably 0.70% or less.

Mn：2.20～3.60％

Mn is an element that contributes to increase in strength of steel by solid solution, and promotes formation of a bainite structure and a phase 2 structure by improving hardenability. Therefore, the Mn content is 2.20% or more. Preferably 2.40% or more, more preferably 2.60% or more. On the other hand, if the Mn content exceeds 3.60%, the adhesion of the plating layer is inhibited, and the corrosion resistance of the steel sheet is deteriorated. Therefore, the Mn content is 3.60% or less. Preferably 3.40% or less, more preferably 3.20% or less.

P: less than 0.100%

P is an element contained as an impurity. P segregates to the prior austenite grain boundaries to reduce toughness. Therefore, when the slab is heated, breakage occurs during hot rolling. Therefore, it is preferable to reduce the P content as much as possible. The content of the organic solvent may be 0.100% or less. Preferably 0.050% or less, and more preferably 0.020% or less. It should be noted that the P content is zero and there is no problem.

S: 0.0100% or less

S is an element which forms coarse sulfides with Ti and Mn and deteriorates the bulging formability and stretch flange formability of the high-strength hot-rolled plated steel sheet. Therefore, it is preferable to reduce the S content as much as possible. The content of the compound may be 0.0100% or less. Preferably 0.0050% or less, more preferably 0.0035% or less.

Ti：0.05～0.18％

Ti is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. In addition, the nitride is formed in the casting stage to fix N. This suppresses precipitation of BN, and B is present in the steel in a solid solution state, thereby achieving hardenability necessary for the formation of a bainite structure. In order to obtain these effects, the Ti content needs to be 0.05% or more. Preferably 0.07% or more, more preferably 0.09% or more. On the other hand, if the Ti content exceeds 0.18%, the precipitation strengthening amount becomes large, the yield ratio becomes high, and the bulging formability becomes poor. Therefore, the Ti content is limited to 0.18% or less. Preferably 0.16% or less.

B：0.0005～0.0050％

B is an element which segregates at the prior austenite grain boundary, suppresses the formation of ferrite, promotes the formation of a bainite structure, and contributes to the improvement of the strength of the steel sheet. In order to exhibit these effects, the B content is set to 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, hardenability is excessively improved, and an excessive martensite structure is formed, resulting in deterioration of stretch flange formability. Therefore, the B content is limited to 0.0050% or less. Preferably 0.0040% or less, and more preferably 0.0030% or less.

Al：0.005～0.40％

Al is an element that acts as a deoxidizer and is effective in improving the cleanliness of steel. In order to obtain this effect, the Al content needs to be 0.005% or more. Preferably 0.010% or more. On the other hand, if the Al content exceeds 0.40%, Al oxide inclusions increase, and the stretch flangeability deteriorates. Therefore, the Al content is limited to 0.40% or less. Preferably 0.10% or less, more preferably 0.06% or less.

N: 0.010% or less

N is easily bonded to Ti at high temperature to form coarse nitrides, and deteriorates stretch flangability. Therefore, the N content is limited to 0.010% or less. Preferably 0.008% or less. More preferably 0.006% or less. Note that, although there is no problem even if the N content is zero, it is preferably 0.0005% or more.

CSM value: 3.3 to 12.0

In the present invention, the contents of C, Si and Mn are adjusted so that CSM value represented by the following formula (1) is 3.3 to 12.0 in order to ensure bulging formability, stretch flange formability and plating property. If the CSM value is less than 3.3, the formation of the phase 2 structure is insufficient and the bulging formability is deteriorated. Therefore, the CSM value is limited to 3.3 or more. Preferably 3.5 or more, more preferably 4.0 or more. On the other hand, if the CSM value exceeds 12.0, the 2 nd phase structure is excessively generated, the stretch flangeability is deteriorated, and the plating property is also deteriorated. Therefore, the CSM value is limited to 12.0 or less. Preferably 10.8 or less, more preferably 10.0 or less.

CSM value 33.8 [% C ] [% Mn ] +12.4 [% Si ]/[% Mn ] (1)

In the formula (1), [% C ], [% Mn ] and [% Si ] represent the contents (mass%) of the respective elements.

In the present invention, the above essential elements are used to obtain the desired properties, but any of the following elements may be contained as necessary for higher strength and further improvement in press formability. When any of the following elements is contained below the lower limit, any element contained below the lower limit is contained as an inevitable impurity.

Nb: 0.005-0.060% and V: 0.005-0.080%

In addition to the above components, Nb: 0.005-0.060%, V: 0.005-0.080% of more than 1. Nb is an element that improves the strength of the steel sheet by precipitation strengthening, and this effect is exhibited when it is contained by 0.005% or more. Preferably 0.010% or more. When the Nb content exceeds 0.060%, the phase 2 area ratio increases, and the stretch flangability deteriorates. Therefore, when Nb is contained, the Nb content is limited to 0.060% or less. Preferably 0.050% or less.

V is also an element contributing to the high strength of the steel sheet by precipitation strengthening. When the content is 0.005% or more, the effect is exhibited. Preferably 0.010% or more. When the V content exceeds 0.080%, the area ratio of the 2 nd phase increases, and the stretch flange formability deteriorates. Therefore, when V is contained, the V content is limited to 0.080% or less. Preferably 0.060% or less.

Cr: 0.02 to 0.15%, Mo: 0.02 to 0.5%, Cu: 0.05-0.5% and Ni: 0.05-1.0% of more than 1

In the present invention, the alloy may further contain, in mass%, Cr: 0.02 to 0.15%, Mo: 0.02 to 0.5%, Cu: 0.05 to 0.5%, Ni: 0.05-1.0% of more than 1. Cr, Mo, Cu, and Ni are added for the purpose of improving hardenability of steel to obtain a bainite structure. Contains Cr: 0.02% or more, Mo: 0.02% or more, Cu: 0.05% or more, Ni: this effect is exhibited at 0.05% or more. On the other hand, if the Cr content exceeds 0.15%, the plating property is remarkably lowered, so that when Cr is contained, the Cr content is limited to 0.02 to 0.15%. Is Mo: more than 0.5%, Cu: more than 0.5%, Ni: if the content exceeds 1.0%, hardenability is excessively improved, the phase 2 structure increases, and stretch flange formability is deteriorated. Therefore, the content of Mo is limited to 0.02 to 0.5%, the content of Cu is limited to 0.05 to 0.5%, and the content of Ni is limited to 0.05 to 1.0%.

Sb：0.0002～0.0200％

Sb has an effect of suppressing nitriding of the slab surface in the slab heating stage, and suppresses precipitation of BN in the slab surface layer portion. Further, the presence of solid solution B can provide hardenability necessary for bainite formation even in the surface layer portion of the hot-rolled steel sheet, thereby improving the strength of the hot-rolled steel sheet. In order to exhibit such an effect, the Sb content needs to be 0.0002% or more. Preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if the Sb content exceeds 0.0200%, the rolling load may increase, and the productivity may decrease. Therefore, when Sb is contained, the Sb content is 0.0200% or less. The Sb content is preferably 0.0180% or less, and more preferably 0.0150% or less.

Ca: 0.0002 to 0.0100%, Mg: 0.0002 to 0.0100%, REM: 0.0002-0.0100% of more than 1

Ca is effective in controlling the shape of oxide and sulfide inclusions and improving the stretch flange formability of hot-rolled plated steel sheet. In order to exhibit these effects, the Ca content is set to 0.0002% or more. Preferably 0.0004% or more. Among them, if the Ca content exceeds 0.0100%, surface defects of the steel sheet may be caused. Therefore, when Ca is contained, the Ca content is limited to 0.0100% or less. Preferably 0.0050% or less.

Further, Mg is effective for controlling the shape of oxide and sulfide-based inclusions and improving the stretch flange formability of hot-rolled plated steel sheet, similarly to Ca. In order to exhibit these effects, the Mg content is set to 0.0002% or more. Preferably 0.0004% or more. When the Mg content exceeds 0.0100%, the steel becomes poor in cleanliness and the stretch flange formability is deteriorated. Therefore, when Mg is contained, the Mg content is 0.0100% or less. Preferably 0.0050% or less.

REM is effective for controlling the shape of oxide and sulfide-based inclusions and improving the low-temperature toughness of the hot-rolled plated steel sheet, as with Ca and Mg. In order to exhibit these effects, the REM content is set to 0.0002% or more. Preferably 0.0004% or more. When the REM content exceeds 0.0100%, the cleanliness of the steel is deteriorated and the low-temperature toughness is deteriorated. Therefore, when REM is contained, the REM content is 0.0100% or less. Preferably 0.0050% or less.

In the present invention, the balance other than the above is Fe and inevitable impurities. The inevitable impurities include Zr, Co, Sn, Zn, W, and the total content thereof is as small as 0.5% or less.

Next, the reason for limiting the steel structure of the steel sheet of the present invention will be described.

The high-strength hot-rolled plated steel sheet of the present invention has a steel structure as follows: the area ratio of the bainite structure as the main phase is 85% or more in total, and the area ratio of the martensite structure as the 2 nd phase is 2.0% to 15.0%. The steel structure referred to herein means the steel structure at the center of the thickness of the sample. The sample was collected so that the 1/4 point in the thickness of the high-strength hot-rolled plated steel sheet was at the center of the sample thickness.

The high-strength hot-rolled plated steel sheet of the present invention has a bainite structure as a main phase in order to improve high strength and stretch flange formability of 980MPa or more. The area ratio of the bainite structure is 85% or more. Preferably 87% or more, more preferably 90% or more. The bainite structure referred to herein is a structure formed in an intermediate temperature region between a polygonal ferrite structure formed by diffusion transformation and a martensite structure formed by non-diffusion transformation, and means that the average dislocation density is 5 × 10¹⁴～5×10¹⁵m^－2The tissue of (1). In the present invention, the bainite structure formed by cooling from the austenite phase is not particularly distinguished from the tempered bainite structure obtained by annealing the bainite structure at the Ac1 point or less. The average dislocation density was determined from an EBSD spectrum obtained under conditions of an acceleration voltage of 15kV, a focal length of 19mm, and a measurement interval of 0.25 μm using an EBSD detector manufactured by EDAX, Inc., which is mounted on a field emission scanning electron microscope (FE-SEM SU5000), manufactured by Hitachi high and New technologies. Specifically, the Average value of Kernel Average Misorientation (KAM value) was obtained from the obtained EBSD map by using data analysis software (OIM analysis ver.7.3), and the Average dislocation density was obtained using the obtained Average KAM value. When the average KAM value is obtained, an aggregate of measurement points whose Image Quality (IQ value) is 80000 or less is regarded as the 2 nd phase, and the aggregate is removed and calculated. Dislocation density ρ (m)^-2) By using the average KAM value θ (rad), the dislocation component constant α (═ 1.5), the berfield vector b (═ 2.48 × 10)^-10m) and measurement interval d (═ 2.5 × 10)^-7m) is calculated from the formula (2).

ρ＝2αθ/bd (2)

In the present invention, the 2 nd phase is a structure containing 2.0% to 15.0% of a martensite structure in order to improve the bulging formability. In order to improve the balance between the bulging formability and the hole expanding formability, the area ratio of the 2 nd phase is preferably 2.0% to 10.0%. In order to ensure the bulging formability, the steel sheet has a phase 2 structure having strength and ductility different from those of the main phase, and the yield ratio is reduced to improve the bulging formability. In order to obtain this effect, the area ratio of the 2 nd phase needs to be 2.0% or more. Preferably 3.0% or more, more preferably 5.0% or more. On the other hand, if the area ratio of the 2 nd phase exceeds 15.0%, the connection of the fine voids generated at the interface between the main phase bainite structure and the 2 nd phase structure in the pore-widening test becomes easy, and the stretch flange formability is deteriorated. Therefore, the area ratio of the 2 nd phase is 15.0% or less. Preferably 13.0% or less, more preferably 10.0% or less. The martensite structure can be distinguished from the bainite structure by observing the lower structure in detail using SEM, or can be distinguished by further reducing the Image Quality (IQ value) of EBSD. Specifically, when an EBSD map was taken using FE-SEM SU5000 under conditions of an acceleration voltage of 15kV and a focal length of 19mm, the IQ value of the martensite structure was 80000 or less. The 2 nd phase area ratio is calculated by extracting an aggregate of measurement points whose Image Quality (IQ value) is 80000 or less by using the Highlighting function of oimamalysis, and calculating the aggregate as a ratio of the total area of the measurement points whose IQ value is 80000 or less with respect to the measurement area. As described above, the phase 2 structure in the present invention is essentially a structure having a low IQ value and a high dislocation density, and the name of the phase 2 structure is not specified. That is, if the IQ value is 80000 or less, the 2 nd phase may be a tempered martensite structure or a lower bainite structure.

In the present invention, the microstructure included in the steel sheet includes, in addition to the above-described microstructure, a retained austenite phase, a pearlite structure, a ferrite structure, and the like. When the steel sheet contains a retained austenite phase, a pearlite structure, and a ferrite structure, the effects of the present invention can be sufficiently obtained if the area ratio of the remaining portion is 0 to 3% in total.

The steel sheet of the present invention has a plating layer or an alloyed plating layer on the surface thereof for the purpose of improving corrosion resistance, in order to produce a hot-rolled steel sheet which is preferable as a material for automobile parts exposed to a severe corrosive environment. The type of the plating layer is not particularly limited, and may be a plating layer or a hot-dip plating layer, and if the plating layer is a hot-dip plating layer, a hot-dip galvanized layer is given as a suitable example. The plating layer may be an alloyed plating layer subjected to alloying treatment.

Next, a manufacturing method for obtaining the hot-rolled plated steel sheet of the present invention will be described. In the description, the temperature "c" represents the temperature of the steel sheet surface.

First, a steel material having the above composition is subjected to hot rolling including rough rolling and finish rolling, and after the finish rolling is completed, the steel material is cooled and wound to produce a hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet is annealed. Thereafter, a plating layer is attached.

In the present invention, the method of melting the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be used. Further, after melting, it is preferable to produce a slab (steel billet) by a continuous casting method because of problems such as segregation. However, a slab may be produced by a known casting method such as an ingot-cogging rolling method or a thin slab continuous casting method. When the cast slab is hot-rolled, the slab may be reheated in a heating furnace and then rolled, or when the slab is kept at a temperature equal to or higher than a predetermined temperature, the slab may be directly rolled without reheating.

The slab thus obtained is subjected to heating, rough rolling and finish rolling. In the present invention, it is necessary to dissolve the carbide of the slab before rough rolling. In the present invention containing Ti, the heating temperature of the slab is preferably 1150 ℃ or higher. However, if the heating temperature is too high, TiO is formed by excessive surface oxidation₂However, Ti is consumed, and the strength of the surface layer is liable to decrease when the steel sheet is produced. Therefore, the heating temperature is preferably 1350 ℃ or less.

As described above, when the slab before rough rolling is kept at a temperature equal to or higher than the predetermined temperature and the carbide in the slab is sufficiently dissolved, the step of heating the slab before rough rolling can be omitted. The rough rolling conditions are not particularly limited.

Subsequently, the steel sheet is finish-rolled, subjected to accelerated cooling after finish rolling, and coiled to produce a hot-rolled steel sheet.

The finish rolling is preferably performed in a temperature range of 840 ℃ or higher. If the finish rolling temperature is less than 840 ℃, ferrite transformation easily proceeds during rolling, and the desired bainite structure area ratio cannot be obtained. Further, if the finish rolling temperature exceeds 950 ℃, the seizure may be caused, or the scale may remain after pickling, thereby deteriorating the plating property. Therefore, the finish rolling finishing temperature is preferably set to 950 ℃ or lower.

In the cooling after the finish rolling, the steel sheet is cooled to a cooling stop temperature (coiling temperature) at a cooling rate of 50 ℃/s or more in average from the end of the finish rolling. If the average cooling rate is less than 50 ℃/s, ferrite transformation proceeds during cooling, and a desired bainite structure area ratio cannot be obtained. The upper limit of the average cooling rate is not particularly limited, and if the average cooling rate is too high, the control of the cooling stop temperature becomes difficult, and it becomes difficult to perform winding at a desired winding temperature. Therefore, the average cooling rate is preferably set to 300 ℃/s or less. The cooling after the finish rolling is preferably started within 2.0 seconds after the finish rolling.

The cooling stop temperature (coiling temperature) is preferably 350 ℃ to 600 ℃. When the coiling temperature is less than 350 ℃, the martensite structure with extremely high dislocation density in the metal structure becomes the main phase. When the coiling temperature is less than 350 ℃, a desired area ratio of the 2 nd phase cannot be secured, and the bulging formability is deteriorated. On the other hand, if the coiling temperature exceeds 600 ℃, a ferrite phase and a pearlite phase are formed, and a tensile strength of 980MPa or more cannot be secured.

The hot-rolled steel sheet produced in the hot rolling step may be subjected to temper rolling by a conventional method, or may be subjected to pickling to remove scale formed on the surface.

Next, the hot-rolled steel sheet is annealed.

The annealing temperature is preferably 800 ℃ or lower. If the annealing temperature exceeds 800 ℃, the 2 nd phase present in the hot-rolled steel sheet decomposes or reverse-transforms into austenite, and a desired fraction of the 2 nd phase cannot be obtained. In order to ensure tensile strength, bulging formability, and stretch flange formability of 980MPa or more, the lower the annealing temperature, the better. However, when the hot dip galvanizing treatment is performed after the annealing treatment, the annealing temperature is preferably 650 ℃ or higher in order to activate the surface of the steel sheet. The holding time during annealing is preferably 5s to 300 s.

The hot-rolled steel sheet subjected to the annealing is subjected to plating treatment. The plating treatment may be either electroplating or hot dip plating. For example, the plating treatment may be a hot-dip galvanizing treatment, or an alloying treatment may be further performed after the hot-dip galvanizing treatment. At this time, the plating bath temperature and the alloying treatment temperature are preferably temperatures not exceeding the annealing temperature.

The hot-rolled plated steel sheet obtained as described above can be subjected to temper rolling according to a conventional method.

Examples

Molten steel having the composition shown in Table 1 was melted in a converter and formed into a slab having a thickness of 250mm by a continuous casting method. These slabs (steel materials) were heated under the conditions shown in Table 2, and then hot rolled, cooled, and coiled under the conditions shown in Table 2 to produce hot rolled steel sheets having a thickness of 2.0 to 2.6mm and a width of 1000 mm. Then, pickling and temper rolling with a temper rolling ratio of 0.8% were performed. Thereafter, annealing was performed under the conditions shown in table 2. Then, the steel sheet was immersed in a molten zinc bath at 450 ℃ to form a zinc plating layer on the surface of the steel sheet. Further, alloying treatment of the plated layer was performed on a part of the steel sheet under the conditions of 500 ℃ for 100 seconds.

Test pieces were collected from the obtained hot-rolled plated steel sheets, and subjected to structure observation, tensile test, hole expansion test, and test for confirming plating property. The test method is as follows.

(1) Tissue observation

The thickness of the resulting high-strength hot-rolled plated steel sheet parallel to the rolling directionAfter polishing the cross section, the steel structure was exposed to a 3 mass% aqueous solution of nitric acid, and SEM observation was performed at a position 1/4 in the plate thickness. SEM images of 10 visual fields were obtained at a magnification of 3000 times, and each phase (bainite structure, martensite structure, tempered martensite structure, pearlite structure, and ferrite structure) was analyzed by image processing to determine an area fraction. In addition, the sample used for SEM observation was mirror-finished with a colloidal silica solution, and an Electron beam backscattering diffraction pattern (EBSD) spectrum was obtained with an EBSD Detector (EDAX) attached to a scanning Electron microscope. Measurement with EBSD Detector was carried out on each sample at a position of 1/4 μm from the plate thickness²The above-described regions were selected from arbitrary 2 fields and were performed under the condition that the irradiation interval (measurement interval) of the electron beam was 0.25. mu.m. The measured EBSD map was analyzed using Analysis software OIM Analysis manufactured by TSL corporation, measurement points having an IQ value of 80000 or less were extracted, and an area ratio (%) of the 2 nd phase (martensite) was calculated by image processing. Further, a portion identified as an austenite phase by analysis of the EBSD map was defined as retained austenite, and the area ratio (%) of the retained austenite was determined.

The average dislocation density of bainite as a main phase is determined by using the average value of KAM values determined from the obtained EBSD spectrum.

(2) Tensile test

From the high-strength hot-rolled plated steel sheet thus obtained, tensile test pieces of JIS 5 were sampled so that the tensile direction was perpendicular to the rolling direction, and tensile tests were carried out in accordance with the regulations of JIS Z2241 to determine the Yield Strength (YS), Tensile Strength (TS), and total elongation (El). The test was conducted 2 times, and the average value of the tests was defined as the tensile property value of the steel sheet. Further, the Yield Ratio (YR) calculated by equation (3) is calculated from YS and TS.

YR＝YS/TS (3)

In the present invention, when YR obtained in the tensile test is 0.93 or less, the bulging formability is evaluated to be good.

(3) Hole expansion test

A100 mm square test was conducted on the obtained hot-rolled plated steel sheetAnd (6) testing the film. The central part of the test piece is provided with a punchThe flat bottom type of (1) is subjected to blanking processing under the condition that the blanking clearance is 12 +/-1%, and a conical punch with an apex angle of 60 degrees is pushed up from the punch side to enlarge the hole. When a clear crack penetrating the thickness of the plate appeared, the conical punch was stopped, and the hole diameter at that time was measured. The difference between the hole diameter after hole expansion and the hole diameter before hole expansion is divided by the hole diameter before hole expansion and multiplied by 100 to obtain a figure as a hole expansion ratio (λ) as an index of stretch flange formability. In the present invention, the stretch flange formability was evaluated to be good when λ obtained in the hole expansion test was 50% or more.

(4) Plating property

The plating properties of the obtained high-strength hot-rolled plated steel sheet were visually evaluated by visual inspection. In table 3, the case where the plating layer was formed over the entire length and the entire width of the hot-rolled plated steel sheet is indicated as "o", and the case where a partially non-plated portion was observed is indicated as "x".