WO2017131055A1

WO2017131055A1 - High-yield ratio high-strength galvanized steel sheet, and method for producing same

Info

Publication number: WO2017131055A1
Application number: PCT/JP2017/002616
Authority: WO
Inventors: 裕美吉冨; 拓弥平島; 弘之増岡; 斉祐津田; 正貴木庭; 康弘西村; 達也中垣内
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2016-01-27
Filing date: 2017-01-26
Publication date: 2017-08-03

Abstract

Provided are: a high-yield ratio high strength galvanized steel sheet having a steel sheet containing Mn as a substrate, having high yield strength, outstanding galvanized appearance, corrosion resistance, in addition to plating peeling resistance when machined heavily; and a method for producing same. The high-yield ratio high-strength galvanized steel sheet comprises steel sheet having a specific constituent composition and a metal structure in which, by surface area ratio, ferrite does not exceed 20%, the total of bainite and tempered martensite is at least 40%, quenched martensite does not exceed 60%, and the average crystal grain size of bainite does not exceed 6.0 µm, and a plating layer formed on said metal sheet, and having a coated plating weight per side of 20-120 g/m², and Mn content not exceeding 0.05g/m². The high-yield ratio high-strength galvanized steel sheet has a yield ratio of at least 65% and tensile strength of at least 950 MPa.

Description

High yield ratio type high strength galvanized steel sheet and method for producing the same

The present invention uses a high-strength steel sheet containing Mn as a base material, a high yield strength type high-strength galvanized steel sheet that is excellent in high yield strength, plating appearance, post-processing corrosion resistance, and anti-plating resistance during high processing, and its manufacture. The present invention relates to a method for manufacturing the method.

In recent years, in the fields of automobiles, home appliances, building materials, etc., surface-treated steel sheets imparted with rust resistance to raw steel sheets, especially hot-dip galvanized steel sheets and galvannealed steel sheets have been used in a wide range. In addition, from the viewpoint of improving the fuel efficiency of automobiles and improving the collision safety of automobiles, there is an increasing demand for reducing the thickness of the vehicle body by increasing the strength of the vehicle body material and reducing the weight of the vehicle body. Therefore, application of high-strength steel sheets to automobiles is accelerating. In particular, a high strength steel sheet having a high yield ratio (YR: YR = YS / TS × 100 (%), YS: yield strength, TS: tensile strength) is required.

In general, a hot dip galvanized steel sheet uses a thin steel sheet obtained by hot rolling or cold rolling a slab as a base material, and the base steel sheet is used in an annealing furnace of a continuous hot dip galvanizing line (hereinafter referred to as CGL). Manufactured by annealing and hot dip galvanizing. In the case of an alloyed hot-dip galvanized steel sheet, it is manufactured after further hot-dip galvanizing treatment.

Here, as a heating furnace type of the CGL annealing furnace, there are a DFF type (direct flame type), a NOF type (non-oxidation type), an all radiant tube type (ART type), and the like. In recent years, there has been an increase in the construction of CGLs equipped with an all-radiant tube-type heating furnace because, for example, it is possible to produce high-quality plated steel sheets at low cost due to ease of operation and difficulty in picking up. However, unlike DFF type (direct flame type) and NOF type (non-oxidation type), all radiant tube type heating furnaces do not have an oxidation step immediately before annealing, so for steel plates containing oxidizable elements such as Mn. It is disadvantageous in terms of securing plating properties.

As a method for producing a hot-dip steel sheet using a high-strength steel sheet containing a large amount of Mn as a base material, Patent Document 1 and Patent Document 2 specify a heating temperature in a reduction furnace by an expression expressed by a water vapor partial pressure, and a dew point. A technique for internal oxidation of the surface layer of the railway is disclosed.

In Patent Document 3, not only the oxidizing gases H ₂ O and O ₂ but also the CO ₂ concentration are simultaneously defined, so that the surface layer immediately before plating is internally oxidized to suppress external oxidation. A technique for improving the appearance is disclosed.

Patent Document 4 also discloses that the dew point of the atmosphere in the annealing furnace is −45 ° C. or lower in the temperature range of 820 ° C. or higher and 1000 ° C. or lower in the annealing process and 750 ° C. or higher in the cooling process. A technique for reducing the oxygen potential, reducing surface enrichment without forming internal oxidation, and improving the plating appearance is disclosed.

JP 2004-323970 A JP 2004-315960 A JP 2006-233333 A JP 2010-255106 A

In the techniques described in Patent Documents 1 and 2, since the area for controlling the dew point is premised on the entire furnace, it is difficult to control the dew point and stable operation is difficult. In addition, when an alloyed hot-dip galvanized steel sheet is manufactured under unstable dew point control, variations in the distribution of internal oxides formed on the base steel sheet are observed. There is a concern that defects such as plating wettability unevenness and alloying unevenness may occur.

In the technique described in Patent Document 3, as in Patent Documents 1 and 2, cracks are likely to occur during processing due to the presence of the internal oxide, and the plating peel resistance deteriorates. Moreover, deterioration of corrosion resistance is also recognized. Furthermore, there is a concern that CO ₂ may cause problems such as furnace contamination and carburizing on the surface of the steel sheet, resulting in changes in mechanical properties.

In the technique described in Patent Document 4, it is very difficult to control the moisture in the atmosphere in the annealing furnace in order to stably maintain the atmospheric dew point in the soaking process at −45 ° C. or less. There is a problem that requires.

Furthermore, recently, the application of high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets to places where processing is severe is progressing. For this reason, the anti-plating peeling property at the time of high processing is regarded as important. Specifically, it is required to suppress plating peeling at the processed part when the plated steel sheet is bent at an angle of more than 90 ° and bent at an acute angle or when the steel sheet is processed by impact.

In order for steel plates with high strength and high yield ratio to satisfy the properties of good plating peelability during high processing, high temperature annealing is performed to ensure the desired steel sheet structure by adding a large amount of Mn to the steel. It is not enough. In the prior art, to produce hot-dip galvanized steel sheet with high yield strength and excellent corrosion resistance after processing with CGL equipped with an all radiant tube type heating furnace in the annealing furnace as a base material I can't.

The present invention has been made in view of such circumstances, and uses a steel plate containing Mn as a base material, and has high yield ratio, high yield strength, plating appearance, corrosion resistance, and high plating ratio peeling resistance during high processing. An object is to provide a high-strength galvanized steel sheet and a method for producing the same.

Conventionally, the inside of a steel plate has been actively oxidized for the purpose of achieving a good plating appearance without unplating. However, corrosion resistance and workability deteriorate simultaneously with internal oxidation.

Specifically, until now, attempts to carry out hot dip galvanizing of high-strength steel sheets containing Mn through annealing in an atmosphere at a heating temperature Ac3 or higher in the soaking process have been made in terms of deterioration of the plating appearance. A lot was not done. The reason for this is that when the dew point is −40 ° C. or more and −20 ° C. or less and the hydrogen concentration is 5 vol% or more, which is an industrial atmosphere that is relatively easy to operate industrially, the Mn surface concentration increases with increasing heating temperature. This is because there is common knowledge in the art that the appearance deteriorates and non-plating occurs. Therefore, few attempts have been made to galvanize steel sheets containing Mn after annealing at an Ac3 point or higher in an environment having a dew point of -40 ° C or higher and -20 ° C or lower and a hydrogen concentration of 5 vol% or higher.

However, the present inventors have deliberately examined a heating temperature region of Ac3 point or higher. As a result, the present invention has been achieved.

Specifically, the present inventors examined a method for solving the problem by a new method that is not bound by the conventional idea. As a result, by appropriately controlling the atmospheric dew point and heating temperature in the annealing process to have a specific metal structure and a Mn content in the plating layer within a specific range, high yield strength, plating appearance, corrosion resistance and resistance It was found that a high yield ratio type high-strength galvanized steel sheet with good plating peelability was obtained. This is presumed to be because the formation of internal oxidation and surface enrichment can be suppressed in the surface layer portion of the steel plate immediately below the plating layer. More specifically, the temperature range of the heating temperature T: Ac3 point to 950 ° C is set to a hydrogen concentration of 5 vol. % Or more, and by performing heat treatment including a soaking process in which the dew point D in the furnace satisfies the following formula (1), the specific metal structure and the Mn content in the plating layer are specified. In this range, it was found that the selective surface oxidation reaction (hereinafter referred to as surface concentration) of an easily oxidizable element such as Mn on the steel sheet surface is suppressed.
−40 ≦ D ≦ (T-1137.5) /7.5 (1)
The mechanism by which Mn surface enrichment is suppressed in the high temperature region above the Ac3 point at this stage is unknown, but is estimated as follows.

From the Mn / MnO equilibrium diagram, it is presumed that Mn surface concentration hardly occurs because Mn approaches the reduction region from the oxidation region in the high temperature region above the Ac3 point. By adopting specific manufacturing conditions and a specific metal structure, and by setting the Mn content in the plating layer to a specific range, the surface concentration of Mn can be suppressed without forming internal oxidation on the steel sheet surface, It is expected that a high yield ratio type high strength galvanized steel sheet having high yield strength and excellent corrosion resistance after processing will be obtained. Here, the Mn surface enrichment formed during annealing in the furnace is taken into the plating layer when the plating bath reacts with the steel plate in the plating process, so the Mn surface enrichment is estimated from the Mn content during plating. it can.

That is, it is important to solve the above problems that the Mn content in the plating layer, the area fraction of the metal structure, and the crystal grain size are appropriately controlled by adjusting the component composition and manufacturing conditions. It was revealed.

The present invention is based on the above, and the features thereof are as follows.

[1] By mass%, C: 0.12% or more and 0.25% or less, Si: less than 1.0%, Mn: 2.0% or more, 3% or less, P: 0.05% or less, S: 0 0.005% or less, Al: 0.1% or less, N: 0.008% or less, Ca: 0.0003% or less, and a total of one or more of Ti, Nb, V, and Zr is 0.01. Component composition having a composition of 0.1 to 0.1%, the balance being Fe and inevitable impurities, an area ratio of ferrite of 20% or less, bainite and tempered martensite in total of 40% or more, as-quenched martens A metal structure having a site of 60% or less and an average crystal grain size of bainite of 6.0 μm or less, a steel sheet having the metal structure, and a plating adhesion amount per side of 20 to 120 g / m ² ; because the Mn content is 0.05 g / ^{m 2} or less Comprising a can layer, a high yield ratio high-strength galvanized steel sheet yield ratio is more than 950MPa There tensile strength at least 65%.

[2] The component composition further includes, in mass%, at least one of Mo, Cr, Cu, and Ni in a total of 0.1 to 0.5% and / or B: 0.0003 to 0.005% The high yield ratio type high-strength galvanized steel sheet according to [1].

[3] The high yield ratio type high-strength galvanized steel sheet according to [1] or [2], wherein the component composition further includes Sb: 0.001 to 0.05% by mass.

[4] A steel material having the composition according to any one of [1] to [3] is heated to 1100 ° C. or higher and 1350 ° C. or lower, and then hot rolled at a finish rolling temperature of 800 ° C. or higher and 950 ° C. or lower. Next, a hot rolling step of winding at a temperature of 450 ° C. or higher and 700 ° C. or lower, a cold rolling step of cold rolling the hot rolled steel sheet obtained in the hot rolling step, and the cold rolling With respect to the cold-rolled steel sheet obtained in the process, the heating temperature T: Ac 3 point to 950 ° C., the hydrogen concentration H in the furnace atmosphere in the temperature range: 5 vol% or more, the dew point D in the annealing furnace in the temperature range: 1) satisfying the formula, heating time in the temperature range of Ac3 point to 950 ° C .: 60 s or less, residence time in the temperature range of 450 to 550 ° C .: annealing step for annealing under conditions of 5 s or more, and the annealing step The obtained annealed plate is plated, and the average cooling rate is 5 ° C. then cooled to 50 ° C. or less under the above conditions s, and galvanizing step of forming a plating coating weight per one side is 20 ~ 120g / m ^2, the plating plate after the galvanizing step, 0.1% And a temper rolling step of performing temper rolling at the above elongation rate, and a method for producing a high yield ratio type high strength galvanized steel sheet.
−40 ≦ D ≦ (T-1137.5) /7.5 (1)
In the formula (1), D means an annealing furnace dew point (° C.), and T means an annealing furnace temperature (° C.).

[5] The production of the high yield ratio type high strength galvanized steel sheet according to [4], wherein the plating treatment is a hot dip galvanizing treatment or a hot dip galvanizing treatment and alloying at a temperature of 450 ° C to 600 ° C. Method.

According to the present invention, a high-yield ratio type high-strength galvanized steel sheet excellent in high yield strength, plating appearance, corrosion resistance, and plating peeling resistance during high processing can be obtained.

When the high yield ratio type high strength galvanized steel sheet of the present invention is applied to a skeleton member of an automobile body, it can greatly contribute to improvement of collision safety and weight reduction.

It is a figure which shows an example of the image obtained by structure | tissue observation.

Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<High yield ratio type high strength galvanized steel sheet>
First, the component composition of the high yield ratio type high strength galvanized steel sheet is mass%, C: 0.12% or more and 0.25% or less, Si: less than 1.0%, Mn: 2.0% or more and 3%. P: 0.05% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.008% or less, Ca: 0.0003% or less, Ti, Nb, V One or more of Zr is contained in an amount of 0.01 to 0.1%, with the balance being Fe and inevitable impurities.

In addition, the above component composition further includes, in mass%, at least one of Mo, Cr, Cu, and Ni in a total of 0.1 to 0.5% and / or B: 0.0003 to 0.005%. You may contain.

The component composition may further contain Sb: 0.001 to 0.05% by mass.

Hereinafter, each component will be described. In the following description, “%” representing the content of a component means “% by mass”.

C: 0.12% or more and 0.25% or less C is an element effective for increasing the strength of the steel sheet, and contributes to increasing the strength by forming martensite containing supersaturated C. C also contributes to high strength by forming carbide forming elements such as Nb, Ti, V, and Zr and fine alloy compounds or alloy carbonitrides. In order to obtain these effects, the C content needs to be 0.12% or more. On the other hand, if the C content exceeds 0.25%, the spot weldability of the steel sheet is remarkably deteriorated, and at the same time, the steel sheet is hardened due to an increase in martensite and YR and bending workability tend to be lowered. Therefore, the C content is 0.12% or more and 0.25% or less. From the viewpoint of characteristics, it is preferably 0.23% or less. Moreover, a preferable range for the lower limit is 0.13% or more.

Si: Less than 1.0% Si is an element that contributes to high strength mainly by solid solution strengthening. The decrease in ductility is relatively small as the strength increases, and it improves the balance between strength and ductility as well as strength. Contribute. On the other hand, Si tends to form a Si-based oxide on the surface of the steel sheet and may cause non-plating. Therefore, it is sufficient to add only the amount necessary for securing the strength, but the upper limit of the Si content is set to less than 1.0% from the viewpoint of plating properties. Preferably it is 0.8% or less. The Si content is preferably 0.01% or more. More preferably, it is 0.05% or more.

Mn: 2.0% or more and 3% or less Mn is an element that contributes to high strength by solid solution strengthening and martensite formation. In order to acquire this effect, it is necessary to make Mn content 2.0% or more. Preferably it is 2.1% or more, More preferably, it is 2.2% or more. On the other hand, when the Mn content exceeds 3%, spot welded portion cracking is caused, and the metal structure is likely to be uneven due to segregation of Mn and the like, and various workability is lowered. Further, Mn tends to concentrate as an oxide or composite oxide on the steel sheet surface, which may cause non-plating. Therefore, the Mn content is 3% or less. Preferably it is 2.8% or less, more preferably 2.7% or less.

P: 0.05% or less P is an element that contributes to increasing the strength of a steel sheet by solid solution strengthening. However, when the P content exceeds 0.05%, workability such as weldability and stretch flangeability is deteriorated. For this reason, it is desirably 0.03% or less. The lower limit of the P content is not particularly specified, but if it is less than 0.001%, the production efficiency is lowered and the dephosphorization cost is increased in the production process. In addition, if P content is 0.001% or more, the said effect of high intensity | strength will be acquired.

S: 0.005% or less S is a harmful element that causes hot brittleness, and is present in the steel as sulfide inclusions and reduces workability of the steel sheet such as bendability. For this reason, it is preferable to reduce S content as much as possible. In the present invention, an S content of up to 0.005% is acceptable. Preferably it is 0.002% or less. Although the lower limit is not particularly defined, if the S content is less than 0.0001%, the production efficiency is lowered and the cost is increased in the production process. Therefore, the S content is preferably 0.0001% or more.

Al: 0.1% or less Al is added as a deoxidizer. When it is necessary to obtain the effect, the Al content is preferably 0.01% or more. More preferably, it is 0.02% or more. On the other hand, if the Al content exceeds 0.1%, the raw material cost increases, and excessive Al also causes surface defects of the steel sheet. Therefore, the Al content is 0.1% or less. Preferably it is 0.08% or less.

N: 0.008% or less When the N content exceeds 0.008%, excessive nitrides are generated in the steel to reduce ductility and toughness, and the surface properties of the steel sheet may be deteriorated. Therefore, the N content is set to 0.008% or less, preferably 0.006% or less. From the viewpoint of improving ductility by ferrite cleaning, it is preferable that the N content is as small as possible. On the other hand, if the N content is excessively reduced, the production efficiency is lowered and the cost is increased in the production process, so the N content is preferably 0.0001% or more.

Ca: 0.0003% or less Ca forms sulfides and oxides in steel and lowers the workability of the steel sheet. Therefore, the Ca content is set to 0.0003% or less. Preferably it is 0.0001% or less. A lower Ca content is preferred, and it may be 0%.

Total of one or more of Ti, Nb, V and Zr: 0.01 to 0.1%
Ti, Nb, V, and Zr form carbides and nitrides (sometimes carbonitrides) with C and N to become precipitates. Fine precipitates contribute to increasing the strength of the steel sheet. Further, these elements have the effect of refining the structure of the hot-rolled coil, and by refining the microstructure after the subsequent cold rolling / annealing, it contributes to improving workability such as strength increase and bendability. For this reason, the total content of these elements is set to 0.01% or more. Preferably it is 0.02% or more. However, excessive addition increases the deformation resistance during cold rolling and hinders productivity, and the presence of excessive or coarse precipitates reduces the ductility of ferrite, resulting in ductility, bendability and stretch flangeability of the steel sheet. Reduces workability. Therefore, the total content of these components is 0.1% or less. Preferably it is 0.08% or less.

The remainder other than the above is Fe and inevitable impurities. In addition, the component composition of a steel plate may include the following components.

One or more of Mo, Cr, Cu and Ni in total 0.1 to 0.5% and / or B: 0.0003 to 0.005%
These elements contribute to high strength because they enhance the hardenability and facilitate the formation of martensite. In order to obtain these effects, the total content of one or more of Mo, Cr, Cu, and Ni is preferably 0.1% or more. Moreover, about Mo, Cr, Cu, Ni, the excessive addition leads to saturation of an effect and a cost increase. Further, Cu induces cracks during hot rolling and causes surface defects. Therefore, the total content is 0.5% or less. Since Ni has an effect of suppressing the generation of surface flaws due to the addition of Cu, it is desirable to add it simultaneously with the addition of Cu. It is preferable to make Ni more than 1/2 of the Cu content. As described above, B also enhances hardenability and contributes to high strength. In addition, a lower limit is set for the B content from the viewpoint of obtaining the effect of suppressing the formation of ferrite that occurs in the annealing cooling process and from the viewpoint of improving hardenability. Specifically, 0.0003% or more is preferable. More preferably, it is 0.0005% or more. The excessive addition will set an upper limit because of saturation of the effect. Specifically, 0.005% or less is preferable. More preferably, it is 0.002% or less. Excessive hardenability also has disadvantages such as weld cracking during welding.

Sb: 0.001 to 0.05%
Sb is an element effective in suppressing strength reduction of a steel sheet by suppressing decarburization, denitrification, deboronation, and the like. Further, the Sb content is preferably 0.001% or more because it is also effective for suppressing spot weld cracking. More preferably, it is 0.002% or more. However, excessive addition of Sb deteriorates workability such as stretch flangeability of the steel sheet. For this reason, the Sb content is preferably 0.05% or less. More preferably, it is 0.02% or less.

In addition, even if it contains the said arbitrary component in less than the said lower limit, the effect of this invention is not impaired. For this reason, it is thought that the content below the said lower limit contains the said arbitrary component as an unavoidable impurity.

Subsequently, the metal structure of the high yield ratio type high strength galvanized steel sheet will be described.

The metal structure of the high yield ratio type high strength galvanized steel sheet is 20% or less in ferrite, 40% or more in total for bainite and tempered martensite, and 60% or less in the as-quenched martensite. The crystal grain size is 6.0 μm or less.

Ferrite is not more than 20% The presence of ferrite is not preferable from the viewpoint of steel sheet strength, but up to 20% is allowed in the present invention. Preferably it is 15% or less. Further, the ferrite may be 0%. As the area ratio, a value measured by the method described in Examples is adopted. Here, bainite containing no carbides generated at a relatively high temperature is not distinguished from ferrite by observation with a scanning electron microscope described in Examples described later, and is regarded as ferrite.

Bainite and tempered martensite total 40% or more In order to achieve a high yield ratio in addition to tensile strength, bainite (because bainite containing no carbide is considered to be ferrite, as described above, this bainite contains carbite containing carbide. ) And tempered martensite are 40% or more in total area ratio. In particular, in order to obtain high YS, this bainite and tempered martensite are important in the present invention, and in order to obtain high YS, it is necessary to be 40% or more. Preferably it is 45% or more, More preferably, it is 50% or more, More preferably, it is 55% or more. The upper limit is not particularly limited, but is preferably 90% or less, more preferably 80% or less, and still more preferably 70% or less, from the viewpoint of balance with strength and ductility. As the area ratio, a value measured by the method described in Examples is adopted.

Quenched martensite is 60% or less Quenched martensite is hard and effective for increasing the strength of the steel sheet, and in order to obtain the effect of increasing TS, the area ratio is preferably 20% or more. More preferably, it is 25% or more, and more preferably 30% or more. On the other hand, hard martensite as hardened lowers YR, so the upper limit is made 60% or less. Preferably it is 50% or less, More preferably, it is 40% or less, More preferably, it is 30% or less. As the area ratio, a value measured by the method described in Examples is adopted.

The average crystal grain size of bainite is 6.0 μm or less. As described above, bainite and tempered martensite are important for obtaining high YS in the present invention. In addition to setting the total area ratio within the above range, it is important that the average crystal grain size of bainite is 6.0 μm or less. Preferably, it is 5 μm or less. Although it does not specifically limit about a minimum, 1.0 micrometer or more is preferable substantially, More preferably, it is 2.0 micrometers or more. The average crystal grain size is a value measured by the method described in the examples.

The technical significance of each structure of the metal structure is as described above, but the high yield ratio type high-strength galvanization with high yield strength, plating appearance, corrosion resistance and anti-plating resistance is good by the manufacturing method described later. A steel plate is obtained. This is a manufacturing method to be described later. When adjusting the Mn content in the plating layer to a specific range and adjusting to the above metal structure, formation of internal oxidation and surface enrichment in the surface layer of the steel plate immediately below the plating layer This is considered to be possible to suppress this.

Further, the metal structure may contain ferrite, bainite, tempered martensite, or other than quenched martensite. Examples of other structures include pearlite and retained austenite. The area ratio of other tissues is preferably 10% or less.

Next, the plating layer will be described. The plating layer has a plating adhesion amount of 20 to 120 g / m ² per side. An adhesion amount of 20 g / m ² or more is necessary to ensure corrosion resistance. Preferably it is 30 g / m ² or more. It must be 120 g / m ² or less in order to improve the plating peel resistance. Preferably it is 90 g / m ² or less.

The Mn content in the plating layer is 0.05 g / m ² or less. The Mn oxide formed in the heat treatment step before plating reacts with the plating bath and the raw steel plate to form the FeAl or FeZn alloy phase. Although it is taken in, if the amount of oxide is excessive, it remains at the plating / base metal interface and deteriorates the plating adhesion. Therefore, there is no lower limit to the amount of Mn oxide formed in the heat treatment step, and the lower the better. On the other hand, if the Mn content in the plating layer exceeds 0.05 g / m ² , the formation reaction of the FeAl or FeZn alloy phase becomes insufficient, resulting in the occurrence of non-plating and a decrease in plating peel resistance. Here, the amount of Mn oxide formed in the heat treatment step can be quantified from the Mn content in the plating layer after the plating step, and the measurement is performed by the method described in the examples. Therefore, by measuring the “Mn content in the plating layer”, it is possible to evaluate the presence or absence and degree of the problem caused by the Mn oxide.

The plating layer is preferably a galvanizing layer. The galvanized layer may be an alloyed galvanized layer subjected to an alloying treatment.

<Manufacturing method of high yield ratio type high strength galvanized steel sheet>
The manufacturing method of the present invention includes a hot rolling process, a cold rolling process, an annealing process, a galvanizing process, and a temper rolling process.

The hot rolling step is to heat a steel material having the above composition to 1100 ° C. or higher and 1350 ° C. or lower, then perform hot rolling at a finish rolling temperature of 800 ° C. or higher and 950 ° C. or lower, and then 450 ° C. or higher. This is a step of winding at a temperature of 700 ° C. or lower. In the following description, the temperature means the steel sheet surface temperature.

Production of slab (slab (steel material)) The steel material used in the production method of the present invention is produced by a continuous casting method generally called a slab. The continuous casting method is used for the purpose of preventing macro segregation of alloy components. The steel material may be manufactured by an ingot-making method or a thin slab casting method.

In addition to the conventional method in which the steel slab is manufactured and then cooled to room temperature and then re-heated, the steel slab is charged with a hot furnace without being cooled to near room temperature and hot-rolled. Any of the method of hot rolling immediately after performing the supplementary heat treatment or the method of hot rolling while maintaining a high temperature state after casting may be used.

The hot rolling conditions are as follows: a steel material having the above component composition is heated at a temperature of 1100 ° C. or higher and 1350 ° C. or lower, hot rolled at a finish rolling temperature of 800 ° C. or higher and 950 ° C. or lower, and 450 ° C. or higher and 700 ° C. The winding condition is as follows.

Slab heating temperature The heating temperature of the steel slab is in the range of 1100 ° C to 1350 ° C. If the temperature is outside the above upper limit temperature range, the precipitates present in the steel slab are likely to be coarsened, which may be disadvantageous when securing strength by precipitation strengthening, for example. Further, if the temperature is outside the upper limit temperature range, there is a possibility that the structure formation may be adversely affected in the subsequent annealing process using coarse precipitates as nuclei. On the other hand, it is beneficial to achieve a smooth steel plate surface by reducing cracks and irregularities on the steel plate surface by scaling off bubbles and defects on the slab surface by appropriate heating. In order to acquire such an effect, it is necessary to set it as 1100 degreeC or more. On the other hand, when the temperature exceeds 1350 ° C., the austenite grains become coarse, and the metal structure of the final product also becomes coarse, which may cause deterioration in workability such as strength, bendability and stretch flangeability of the steel sheet.

Hot rolling Hot rolling including rough rolling and finish rolling is performed on the steel slab obtained as described above. Generally, a steel slab becomes a sheet bar by rough rolling, and becomes a hot-rolled coil (hot-rolled steel plate) by finish rolling and winding. In addition, depending on the milling ability and the like, there is no problem if it becomes a predetermined size regardless of such division. The hot rolling conditions need to be the following conditions.

Finishing rolling temperature: 800 ° C. or more and 950 ° C. or less By setting the finishing rolling temperature to 800 ° C. or more, the structure obtained by the hot rolled coil can be made uniform. The ability to make the tissue uniform at this stage contributes to a uniform structure of the final product. If the structure is not uniform, workability such as ductility, bendability, stretch flangeability, and the like is deteriorated. On the other hand, when the temperature exceeds 950 ° C., the amount of oxide (scale) generated increases, the interface between the base iron and the oxide becomes rough, and the surface quality after pickling and cold rolling deteriorates. Further, when the temperature exceeds 950 ° C., the crystal grain size becomes coarse in the structure, which may cause a decrease in workability such as strength, bendability and stretch flangeability of the steel plate as in the case of the steel slab.

After finishing the hot rolling, in order to refine and homogenize the structure, cooling is started within 3 seconds after finishing rolling, and the temperature range from [Finishing rolling temperature] to [Finishing rolling temperature-100] ° C is set. It is preferable to cool at an average cooling rate of 10 to 250 ° C./s.

Winding temperature: 450-700 ° C
It is necessary from the viewpoint of fine precipitation of NbC or the like that the winding temperature is 450 ° C. or higher as the temperature immediately before winding the hot rolled coil. The coiling temperature of 700 ° C. or lower is necessary from the viewpoint of preventing the precipitates from becoming too coarse. The lower limit is preferably 500 ° C. or higher. The upper limit is preferably 680 ° C. or lower.

Next, a cold rolling process is performed. The cold rolling process is a process in which cold rolling is performed on the hot rolled sheet (hot rolled steel sheet) obtained in the hot rolling process. Normally, after the scale is dropped by pickling, cold rolling is performed to form a cold rolled coil. This pickling is performed as necessary.

Cold rolling is preferably performed at a reduction rate of 20% or more. This is to obtain a uniform fine microstructure (metal structure) in the subsequent annealing. If it is less than 20%, if it tends to become coarse grains during annealing, it may tend to become a non-uniform structure, and as described above, there is a concern that the strength and workability of the final product plate will be reduced. Although the upper limit of the rolling reduction is not particularly specified, since it is a high-strength steel sheet, a high rolling reduction may cause a shape defect in addition to a decrease in productivity due to a rolling load. The rolling reduction is preferably 90% or less.

With respect to the cold-rolled steel sheet after the cold pressure step, the heating temperature T: Ac 3 point to 950 ° C., the hydrogen concentration H in the furnace atmosphere in the temperature range: 5 vol% or more, the furnace dew point D in the temperature range: (1) The expression is satisfied, and annealing is performed under the conditions of a heating time in a temperature range of Ac3 to 950 ° C .: 60 s or less and a residence time in a temperature range of 450 to 550 ° C .: 5 s or more (annealing step).

Heating temperature T: Ac3 point to 950 ° C
When the heating temperature (annealing temperature) is less than Ac3 point and exceeds 950 ° C., a predetermined microstructure cannot be obtained and high yield strength cannot be obtained. In addition, Ac3 = 937-477C + 56Si-20Mn-16Cu-27Ni-5Cr + 38Mo + 125V + 136Ti + 35Zr-19Nb + 198Al + 3315B. In addition, the element symbol in said formula means content of each element, and the component which does not contain is set to 0.

Hydrogen concentration H in the temperature range of Ac 3 to 950 ° C .: 5 vol% or more If the volume fraction of hydrogen gas in the atmosphere is less than 5 vol%, the activation effect by reduction cannot be obtained and the plating appearance deteriorates. The upper limit is not particularly specified, but if it exceeds 20 vol%, the effect of improving the plating appearance is saturated and the cost is increased. Therefore, the volume fraction of hydrogen gas is preferably 5 vol% or more and 20 vol% or less. The gas component in the furnace is composed of nitrogen gas and unavoidable impurity gas other than hydrogen gas. Other gas components may be included as long as the effects of the present invention are not impaired. The hot dip galvanizing treatment can be performed by a conventional method. Further, except for the above temperature range, the hydrogen concentration may not be in the range of 5 vol% or more.

Dew point D in the temperature range of Ac 3 to 950 ° C .: range of formula (1) In order to improve the plating appearance, corrosion resistance, and plating peeling resistance during high processing while adopting the above heating temperature range, It is necessary to properly control the dew point within the range of equation (1). In the annealing with the atmospheric dew point not satisfying the formula (1), non-plating occurs, resulting in poor plating appearance. Specifically, when the dew point D exceeds the upper limit, alloy elements such as Si are easily picked up during operation in the present invention. The lower limit of the dew point is not particularly defined, but it is difficult to control the dew point below -40 ° C, and there is a problem that enormous equipment costs and operation costs are required.
−40 ≦ D ≦ (T-1137.5) /7.5 (1)
In the formula (1), D means furnace dew point (° C.), and T means heating temperature (° C.).

Heating time in the temperature range from Ac3 point to 950 ° C .: 60 s or less The heating time in the temperature range from Ac3 point to 950 ° C. is controlled to 60 s or less. When it exceeds 60 s, the amount of Si, Mn, or a complex oxide thereof formed on the steel sheet surface is concentrated, which causes poor appearance. The heating time is preferably 20 s or less. The heating time in this temperature range is preferably 3 seconds or more from the viewpoint of material stabilization (homogenization of the microstructure (metal structure)).

Residence time in the temperature range of 450 to 550 ° C: 5 s or longer By retaining for 5 s or more in the temperature range of 450 to 550 ° C before the plating process, the microstructure, especially bainite, can be obtained stably, and the plate before entering the plating bath Stabilize the temperature. That is, in order to obtain a desired microstructure and good plating quality, the high YS and the plate temperature before entering the plating bath can be stabilized by retaining for 5 s or more in a predetermined temperature range. When the temperature is lower than 450 ° C., the temperature at which the liquid is normally retained is the cooling stop temperature, so that plating is performed at a temperature lower than 450 ° C., and the quality of the plating bath is deteriorated in the galvanizing process. Therefore, the lower limit of the temperature range is set to 450 ° C. On the other hand, if the plate temperature before entering the bath exceeds 550 ° C., the temperature of the plating bath is increased, and dross is likely to occur, and the generated dross adheres to the surface, which causes poor appearance. Therefore, the upper limit of the temperature range is set to 550 ° C. About cooling from heating temperature to this temperature range, it is preferable to set it as a cooling rate (average cooling rate) of 3 degrees C / s or more from a viewpoint of obtaining a desired characteristic. There is no specific upper limit. The cooling stop temperature may be 450 to 550 ° C., but may be once cooled to a temperature lower than this and retained in a temperature range of 450 to 550 ° C. by reheating.

In addition, it is also possible to pass through a heating process for the purpose of softening the coil and other purposes before the annealing.

Next, a galvanizing process is performed. A galvanization process is a process of giving the plating process to the annealing plate obtained by annealing, and cooling to 50 degrees C or less on conditions with an average cooling rate of 5 degrees C / s or more.

The plating treatment may be performed so that the amount of plating attached on one side is 20 to 120 g / m ² . An adhesion amount of 20 g / m ² or more is necessary to ensure corrosion resistance, and 120 g / m ² or less is necessary to improve plating resistance.

Other conditions for plating treatment are not particularly limited. For example, in mass%, Fe: 0.1-18.0%, Al: 0.001% -1.0%, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, A plating layer containing 0 to 30% in total of one or more selected from Ca, Cu, Li, Ti, Be, Bi and REM, with the balance being Zn and inevitable impurities is obtained by the above method. It is the process of forming on the surface of the obtained annealing board. The plating method is hot dip galvanizing. Conditions may be set as appropriate. Moreover, you may perform the alloying process heated after hot dip galvanization. The alloying process is, for example, a process of holding in a temperature range of 450 to 600 ° C. for about 1 to 60 seconds. When the alloying treatment is performed, the Fe content of the plating layer is preferably 7.0 to 15.0% by mass. If it is less than 7% by mass, unevenness in alloying and flaking properties may be deteriorated. On the other hand, if it exceeds 15% by mass, the plating peel resistance may deteriorate.

After the above plating treatment (after that, when alloying treatment is performed), it is cooled to 50 ° C. or less at an average cooling rate of 5 ° C./s or more. This is to obtain martensite which is essential for increasing the strength. It is difficult to obtain martensite necessary for strength at 5 ° C / s or higher, and if cooling is stopped at a temperature higher than 50 ° C, the martensite is excessively tempered to become tempered martensite and obtain the necessary strength. This is because it becomes difficult. The average cooling rate is preferably 30 ° C./s or less in order to obtain a suitably tempered martensite for obtaining a high YR.

Next, the temper rolling process is performed. The temper rolling step is a step of subjecting the plated plate after the galvanizing step to temper rolling at an elongation rate of 0.1% or more. In addition to the purpose of shape correction and surface roughness adjustment, the plated plate is subjected to temper rolling at an elongation rate of 0.1% or more for the purpose of stably obtaining high YS. For shape correction and surface roughness adjustment, leveling may be performed instead of temper rolling. Excessive temper rolling introduces excessive strain on the steel sheet surface and lowers the evaluation value of bendability and stretch flangeability. Moreover, excessive temper rolling reduces ductility and increases the equipment load due to the high strength steel sheet. Therefore, the rolling reduction of temper rolling is preferably 3% or less.

After molten steel having the composition shown in Table 1 is melted in a converter and made into a slab by a continuous casting machine, hot rolling, cold rolling, annealing and plating treatment, temper rolling (SKP) under various conditions shown in Table 2 The high yield ratio type high-strength galvanized steel sheet (product board) was manufactured. In the annealing, a cold-rolled steel sheet was charged into a CGL equipped with an all-radiant tube type heating furnace in the annealing furnace. Specific conditions are shown in Table 2. The gas components in the atmosphere consisted of nitrogen gas, hydrogen gas, and inevitable impurity gas, and the dew point was controlled by absorbing and removing moisture in the atmosphere. The hydrogen concentration in the atmosphere is shown in Table 2. Moreover, GA used 0.14 mass% Al containing Zn bath, and GI used 0.18 mass% Al containing Zn bath. The adhesion amount was adjusted by gas wiping, and GA was alloyed. In addition, in cooling (cooling after plating treatment), it was cooled to 50 ° C. or lower by passing through a water bath having a water temperature of 40 ° C.

A sample of the galvanized steel sheet obtained as described above was collected and subjected to a tensile test by the following method. Yield strength (YS), tensile strength (TS), yield strength ratio (YR = YS / TS × 100% ) Was measured and calculated. Further, the appearance was visually observed to evaluate the plating property (surface property). Moreover, the corrosion resistance after processing and the plating peel resistance during processing were evaluated. The evaluation method is as follows.

Tensile test A JIS No. 5 tensile test piece (JISZ2201) was sampled from a galvanized steel sheet in a direction perpendicular to the rolling direction, and a tensile test was performed at a constant tensile speed (crosshead speed) of 10 mm / min. Yield strength (YS) is the value obtained by reading the 0.2% proof stress from the slope of the stress 100-200 MPa stress range, and the tensile strength is the value obtained by dividing the maximum load in the tensile test by the initial cross-sectional area of the parallel part of the specimen. It was. As the plate thickness in calculating the cross-sectional area of the parallel portion, the plate thickness value including the plating thickness was used.

Surface properties (appearance)
Visually observe the appearance after plating. “○” indicates that there are no unplating defects, “×” indicates that there are no plating defects, and “△” indicates that there are no plating defects but plating unevenness occurs. " The ripple pattern is of a level that is acceptable for automobile interior panel parts. The non-plating defect means a region having a size of several μm to several mm, where no plating is present and the steel plate is exposed.

Resistance to Plating Peeling Peeling resistance during high processing is required for GA (alloyed) to suppress plating peeling at the bent portion when bent at an acute angle exceeding 90 °. In this example, the cellophane tape was pressed against the processed portion bent by 120 ° to transfer the peeled material to the cellophane tape, and the amount of the peeled material on the cellophane tape was determined by the fluorescent X-ray method as the Zn count number. At this time, the mask diameter is 30 mm, the fluorescent X-ray acceleration voltage is 50 kV, the acceleration current is 50 mA, and the measurement time is 20 seconds. In light of the following criteria, those with ranks 1 and 2 were evaluated as having good plating peel resistance (symbol “◯”), and those with 3 or more were evaluated as having poor plating peel resistance (symbol “x”).
Fluorescent X-ray Zn count number Rank 0 to less than 500: 1 (good)
500 or more and less than 1000: 2
1000 or more and less than 2000: 3
2000 or more and less than −3000: 4
3000 or more: 5 (poor)
In GI (thing which has not been alloyed), resistance to plating peeling during an impact test is required. A ball impact test was performed, a processed part (corresponding to a processed part when highly processed) was peeled off with tape, and the presence or absence of peeling of the plating layer was visually determined. Ball impact conditions are a ball weight of 1000 g and a drop height of 100 cm.
○ (Good): no peeling of plating layer × (NG): peeling of plating layer Corrosion resistance after processing Prepare the test piece that does not peel off the tape by performing the same processing as the plating peeling resistance test. Using an agent: FC-E2011, a surface conditioner: PL-X, and a chemical conversion treatment agent: Palbond PB-L3065, the chemical conversion treatment film adhesion amount is 1.7 to 3.0 g / m ^{2 under} the following standard conditions. Chemical conversion treatment was performed.
<Standard conditions>
・ Degreasing process; treatment temperature 40 ° C., treatment time 120 seconds ・ spray degreasing, surface adjustment process; pH 9.5, treatment temperature room temperature, treatment time 20 seconds ・ Chemical treatment treatment process; 35 ° C, treatment time 120 seconds Electrodeposition coating was applied to the surface of the test piece subjected to the above chemical conversion treatment using an electrodeposition paint V-50 manufactured by Nippon Paint Co., Ltd. so that the film thickness was 25 μm. The following corrosion test was used.
<Salt spray test (SST)>
The test piece subjected to chemical conversion treatment and electrodeposition coating is provided with a cut wrinkle that reaches the plating with a cutter on the surface of the bent part in GA and on the ball impact part in GI. After performing a salt spray test for 240 hours in accordance with a neutral salt spray test prescribed in JIS Z2371: 2000, a tape peel test was performed on the cross-cut collar part, and the cut collar part left and right were combined. The maximum peel width was measured. If the maximum peel width is 2.0 mm or less, the corrosion resistance in the salt spray test can be evaluated as good.
○ (Good): Maximum bulge full width from cut ridge 2.0 mm or less × (NG): Maximum bulge full width from cut ridge exceeds 2.0 mm Table 3 shows the results obtained.

Metallographic observation A specimen for microstructure observation was taken from a hot dip galvanized steel sheet, and after polishing the L cross section (thickness cross section parallel to the rolling direction), it was corroded with a nital liquid and 1 / from the surface with a scanning electron microscope (SEM). An image obtained by observing a position near 4t (t is a plate thickness) at three times or more at a magnification of 3000 was analyzed (an area ratio was measured for each observation field to calculate an average value). Further, the bainite average crystal grain size was calculated by a method of taking an arithmetic average of diameter lengths in two perpendicular directions (horizontal direction and vertical direction on the paper surface) using the above image. An example of the image is shown in FIG.

Mn content in galvanized layer The Mn content in the galvanized layer was measured by dissolving the plated layer in dilute hydrochloric acid to which an inhibitor was added and using ICP emission spectroscopy.

The steel sheet of the example of the present invention obtained with the components and production conditions within the scope of the present invention is a steel sheet having TS ≧ 950 MPa and YR ≧ 65% and having a predetermined plating quality.

The hot-dip galvanized steel sheet of the present invention not only has high tensile strength, but also has high yield strength and good surface properties, so that it is mainly used for skeletal parts of automobile bodies, especially around cabins that affect crash safety. When applied, it can contribute to environmental aspects such as CO ₂ emission by improving the safety performance and contributing to weight reduction of the vehicle body due to the effect of thinning high strength. In addition, since it has good surface properties and plating quality, it can be actively applied to places where there is concern about corrosion due to rain and snow, such as undercarriage, and the performance of rust prevention and corrosion resistance of the car body is also improved. Can be expected. Such characteristics are not limited to automobile parts, but are also effective materials in the fields of civil engineering / architecture and home appliances.

Claims

% By mass
C: 0.12% or more and 0.25% or less,
Si: less than 1.0%,
Mn: 2.0% to 3%,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.1% or less,
N: 0.008% or less,
Ca: 0.0003% or less,
Contains one or more of Ti, Nb, V and Zr in a total of 0.01 to 0.1%,
A component composition having a component composition of the balance consisting of Fe and inevitable impurities,
A metal structure having an area ratio of ferrite of 20% or less, bainite and tempered martensite in total of 40% or more, as-quenched martensite of 60% or less, and an average crystal grain size of bainite of 6.0 μm or less; A steel plate having,
A plating layer formed on the steel sheet and having a plating adhesion amount of 20 to 120 g / m 2 on one side and a Mn content of 0.05 g / m 2 or less;
A high yield ratio type high strength galvanized steel sheet having a yield ratio of 65% or more and a tensile strength of 950 MPa or more.
The component composition further contains, in mass%, at least one of Mo, Cr, Cu, and Ni in a total of 0.1 to 0.5% and / or B: 0.0003 to 0.005%. The high yield ratio type high strength galvanized steel sheet according to claim 1.
The high yield ratio type high strength galvanized steel sheet according to claim 1 or 2, wherein the component composition further contains Sb: 0.001 to 0.05% by mass.
The steel material having the component composition according to any one of claims 1 to 3 is heated to 1100 ° C or higher and 1350 ° C or lower, and then subjected to hot rolling at a finish rolling temperature of 800 ° C or higher and 950 ° C or lower, A hot rolling step of winding at a temperature of 450 ° C. or higher and 700 ° C. or lower;
A cold rolling process in which cold rolling is performed on the hot-rolled steel sheet obtained in the hot rolling process;
With respect to the cold-rolled steel sheet obtained in the cold rolling step, the heating temperature T: Ac 3 point to 950 ° C., the hydrogen concentration H in the furnace atmosphere in the temperature range H: 5 vol% or more, the dew point in the annealing furnace in the temperature range D: An annealing step that satisfies the following formula (1) and performs annealing under conditions of heating at a temperature of Ac3 to 950 ° C .: 60 s or less, residence time at a temperature of 450 to 550 ° C .: 5 s or more,
The annealed plate obtained in the annealing step is subjected to a plating treatment and cooled to 50 ° C. or less under the condition that the average cooling rate is 5 ° C./s or more, and the plating adhesion amount per side is 20 to 120 g / m 2 . A galvanizing process for forming plating;
And a temper rolling step of subjecting the plated plate after the galvanizing step to temper rolling at an elongation rate of 0.1% or more.
−40 ≦ D ≦ (T-1137.5) /7.5 (1)
In the formula (1), D means an annealing furnace dew point (° C.), and T means an annealing furnace temperature (° C.).
The method for producing a high yield ratio type high-strength galvanized steel sheet according to claim 4, wherein the plating treatment is a hot dip galvanizing treatment or a hot dip galvanizing treatment and alloying at a temperature of 450 ° C or higher and 600 ° C or lower.