KR102145293B1 - Plated steel sheet and its manufacturing method - Google Patents

Abstract

A plated steel sheet having very high tensile strength and excellent delayed fracture properties and resistance welding crack resistance is provided. Ferrite by volume fraction of 35% or more and 70% or less, residual austenite by volume fraction of 12% or less, martensite by volume fraction of 15% or more and 60% or less, balance of bainite by volume fraction of 30% or less and non-recrystallization It has a structure containing 5% or less of ferrite by volume, the average grain size of the ferrite is 5 μm or less, the average grain size of the retained austenite is 2 μm or less, and the average grain size of the martensite is 2 μm or less , A plated steel sheet containing 30 or more Ti or Nb-based precipitates having an average grain diameter of 3 μm or less of the bainite and having an average particle diameter of 0.10 μm or less per 100 μm ² is provided.

Description

Plated steel sheet and its manufacturing method

TECHNICAL FIELD The present invention relates to a plated steel plate and a method for manufacturing the same, and in particular, to a plated steel plate suitable as a member of structural parts such as automobiles.

BACKGROUND ART In recent years, regulations on CO ₂ emission have been stricter due to increasing environmental problems, and in the automotive field, weight reduction of a vehicle body aiming to improve fuel efficiency has been a problem. Therefore, by applying a high-strength steel plate to automobile parts, the thickness of automobile parts is progressing, and application of a steel plate having a tensile strength (TS) of 980 MPa or more is proceeding. In addition, from the viewpoint of corrosion resistance, a plated steel sheet having a hot-dip galvanized layer is used in a portion exposed to rainwater.

Patent Document 1 discloses a technique for improving delayed fracture resistance (hydrogen embrittlement resistance) by controlling Si or iron-based carbides containing Si and Al in a steel sheet structure.

In addition, Patent Document 2 discloses a technique for improving surface cracking during resistance welding by controlling the amount of Si, Al, and Mn added.

Japanese Patent Publication No. 4949536 Japanese Patent Publication No. 3758515

However, in the technique described in Patent Literature 1, even if the iron-based carbide can become a hydrogen trap site, not only is there no effect of improving the resistance welding cracking characteristics, and the presence of the iron-based carbide at the grain boundary causes cracking during resistance welding. There is a possibility that it will be promoted.

In addition, in the technique described in Patent Document 2, it is difficult to achieve high strength of 980 MPa or more, and it is difficult to obtain excellent delayed fracture characteristics. As described above, in a plated steel sheet of 980 MPa or more, it is difficult to achieve both the delayed fracture characteristics and the resistance welding cracking characteristics at both ends, and even if a steel sheet other than the plated steel sheet is included, a steel sheet having these characteristics has been developed. The fact is that there is not.

In addition, high-strength steel sheets used for structural members and reinforcing members of automobiles are concerned about delayed destruction (hydrogen embrittlement) due to hydrogen entering from their use environment. In addition, the high-strength steel sheet is often combined with press-formed parts by resistance welding (spot welding). However, due to the melting of zinc on the surface of the steel sheet and the generation of residual stress in the vicinity of the welded portion during this spot welding, there is a concern that liquid metal embrittlement occurs and cracks occur in the steel sheet. Therefore, in order to apply a high-strength hot-dip galvanized steel sheet, it is necessary to have excellent both of delayed fracture characteristics and resistance weld cracking characteristics.

The present invention has been made in view of such circumstances, and an object of the present invention is to solve the problems in the prior art, and to provide a plated steel sheet excellent in delayed fracture characteristics and resistance welding crack resistance, and a method for manufacturing the same.

The inventors of the present invention repeated intensive examinations in order to improve both the delayed fracture characteristics and the resistance welding cracking resistance. As a result, the volume fraction of the steel sheet structure of ferrite, retained austenite, martensite, bainite, and non-recrystallized ferrite is controlled at a specific ratio, and the average grain size of each steel sheet structure is refined, and Ti or It has been found that by generating Nb-based fine carbides, both excellent delayed fracture characteristics and resistance welding cracking resistance can be obtained. The present invention is based on the above recognition.

In delayed fracture, when hydrogen enters the steel sheet, cracks are generated and advanced, thereby causing fracture. For example, in a hot-dip galvanized steel sheet that can also be used as a thin steel sheet for automobiles, there is a possibility that a scratch is generated due to some factor in painting, and the base iron surface is exposed. In this situation, when rainwater or the like adheres to the surface of the steel sheet, since zinc becomes the anode and iron becomes the cathode, the generation of hydrogen is accelerated on the iron surface. Therefore, in the plated steel sheet, it is necessary to consider the delayed fracture characteristics of the steel sheet, not the component or composition of the plating layer.

In addition, for cracks due to liquid metal embrittlement during resistance welding, internal stress is generated due to Zn dissolved (melted) during welding, and cracks are generated in the heat affected zone (HAZ) near the nugget. . Conventionally, in a plated steel sheet, when welding with a high current value such as spatter (spatter) occurs during welding, cracks due to liquid metal brittleness may occur on the surface of the electrode contact side, but spatter If welding with an appropriate current value that does not appear, this problem does not occur. However, even in an appropriate current range in which spatter does not appear, when the tensile strength reaches high strength up to the 980 MPa class, cracks (resistance cracks) may occur on the surface where the steel plates overlap each other. In particular, when the welding electrode is welded at an angle to the steel plate, internal stress increases and cracks are liable to occur. Here, the resistance welding crack in the present invention indicates this crack resistance. When this cracking-resistant occurs, there is a concern, in particular, of a decrease in the fatigue strength of the welding portion. Therefore, in order to be used in automobiles or the like, it is necessary to avoid this cracking resistance. When the crack-resistant part was observed, it became clear that cracks due to intergranular fracture occurred in the place where the martensite single phase was formed after welding of the heat-affected part (HAZ).

Therefore, as a result of repeated intensive studies, the inventors have found that by controlling the size and number of fine Ti or Nb-based carbides, trap sites for hydrogen can be generated and the delayed fracture characteristics can be improved. Further, it was found that the susceptibility to liquid metal embrittlement during welding was improved by miniaturizing the crystal grains of the steel sheet. Moreover, it became clear that strength, delayed fracture characteristics, and resistance welding cracking resistance were improved by controlling the volume fraction of the steel sheet structure. The fine Ti or Nb-based carbide not only serves as a hydrogen trap site, but also inhibits the nucleation growth of ferrite and austenite during recrystallization in annealing, thereby promoting nucleation of ferrite and austenite. For this reason, fine Ti or Nb-based carbides are very effective in miniaturizing the structure of a steel sheet. In this way, it became clear that the steel sheet structure was made finer so that the crystal grains were not coarse even at the time of resistance welding, and the toughness of the steel sheet was improved, and therefore, resistance welding cracking was also suppressed.

The present invention has been made based on the above novel recognition, and has the following configuration.

1. By mass%,

C: 0.05% or more and 0.22% or less,

Si: 0.05% or more and 1.80% or less,

Mn: 1.45% or more and 3.35% or less,

P: 0.05% or less,

S: 0.005% or less,

Al: 0.01% or more and 0.10% or less,

N: 0.010% or less and

B: 0.0002% or more and 0.0045% or less

Containing, in addition,

Ti: 0.005% or more and 0.090% or less and Nb: 0.005% or more and 0.090% or less, containing at least one selected from, the balance has a component composition of Fe and inevitable impurities,

Ferrite by volume fraction of 35% or more and 70% or less, retained austenite by volume fraction of 12% or less, martensite by volume fraction of 15% or more and 60% or less, balance of bainite by volume fraction of 30% or less and non-recrystallized It has a structure containing 5% or less of ferrite by volume,

The average grain size of the ferrite is 5 μm or less,

The average crystal grain size of the retained austenite is 2 μm or less,

The average crystal grain size of the martensite is 2 μm or less,

The bainite has an average crystal grain size of 3 μm or less,

A plated steel sheet in which the structure contains 30 or more Ti or Nb-based precipitates having an average particle diameter of 0.10 μm or less on an average per 100 μm ² .

2. The above component composition, further,

In mass%,

V: 0.10% or less,

Cu: 0.50% or less,

Ni: 0.50% or less,

Mo: 0.50% or less,

Cr: 0.80% or less and

Ca and/or REM: 0.0050% or less

The plated steel sheet according to 1 above, containing one or two or more selected from among.

3. By mass%,

C: 0.05% or more and 0.22% or less,

Si: 0.05% or more and 1.80% or less,

Mn: 1.45% or more and 3.35% or less,

P: 0.05% or less,

S: 0.005% or less,

Al: 0.01% or more and 0.10% or less,

N: 0.010% or less and

B: 0.0002% or more and 0.0045% or less

Contains, and further, in mass%,

Ti: 0.005% or more and 0.090% or less, Nb: 0.005% or more and 0.090% or less, containing at least one selected from among, the remainder is Fe and inevitable impurities in a steel slab having a component composition, the finish rolling temperature 850°C Hot-rolling is performed under conditions of not less than 950°C to obtain a hot-rolled steel sheet,

The hot-rolled steel sheet is cooled to 680°C or less at a first average cooling rate of 75°C/s or more, and cooled to a range of 400°C or more and 580°C or less at a second average cooling rate of 5°C/s or more, and then winding And cold-rolled to obtain a cold-rolled steel sheet,

The cold-rolled steel sheet is heated to a temperature range of 760°C to 900°C at an average heating rate of 3 to 30°C/s, and preserved for 15 seconds or longer in a temperature range of 760°C to 900°C to crack ( After sintering), annealing is performed to cool down to a temperature range of 600°C or less at an average cooling rate of 3 to 30°C/s,

A method for producing a plated steel sheet in which a plating treatment is performed on the cold-rolled steel sheet after the annealing.

4. The composition of the component, further,

In mass%,

V: 0.10% or less,

Cu: 0.50% or less,

Ni: 0.50% or less,

Mo: 0.50% or less,

Cr: 0.80% or less and

Ca and/or REM: 0.0050% or less

The method for producing a plated steel sheet according to 3 above, containing one or more selected from among.

5. The method for producing a plated steel sheet according to 3 or 4 above, wherein the plating is alloyed in a temperature range of 450°C or more and 600°C or less after performing the plating treatment.

According to the present invention, it has very high tensile strength, has excellent delayed fracture properties that do not cause delayed fracture due to hydrogen entering from the environment even after molding into a member, and cracks do not occur even during resistance welding. It has excellent resistance to welding and cracking properties. For example, tensile strength is 980 MPa or more, and after U-bending processing, no breakage occurs for 100 hours in an environment immersed in hydrochloric acid at 20°C and pH = 1.5, welding with an electrode with an angle (about 0.5 to 10°) with the steel plate Even in the case where resistance welding cracking does not occur, a high-strength plated steel sheet excellent in strength, delayed fracture resistance, and resistance welding cracking resistance can be stably obtained.

(Form for carrying out the invention)

Hereinafter, a plated steel sheet according to an embodiment of the present invention will be described. First, the reasons for limiting the component composition of the steel will be described. In addition, in this specification, "%" which shows the content of each component element means "mass%" unless otherwise stated.

C: 0.05% or more and 0.22% or less

C is an element effective in increasing the strength of the steel sheet, and also contributes to the formation of the second phase (structure other than ferrite as the first phase) of bainite, martensite and retained austenite in the present invention. If it is less than 0.05%, it is difficult to secure the necessary volume ratios of bainite, martensite, and retained austenite, so it is difficult to secure strength. Preferably it is 0.06% or more. More preferably, it is 0.065% or more. On the other hand, if excessively added, the hardness after resistance welding increases, the toughness at the time of resistance welding decreases, and the resistance welding crack resistance deteriorates, so the content is set to 0.22% or less. It is preferably 0.20% or less, and more preferably 0.18% or less.

Si: 0.05% or more and 1.80% or less

Si is an element effective for solid-solution strengthening of ferrite and increasing strength. In order to obtain the effect, 0.05% or more of addition is required. Preferably it is 0.10% or more. More preferably, it is 0.20% or more. However, since excessive addition of Si results in non-plating due to a decrease in plating properties, the content is set to 1.80% or less. Preferably it is 1.60% or less. More preferably, it is 1.50% or less.

Mn: 1.45% or more and 3.35% or less

Mn is an element that contributes to solid solution strengthening and strengthening by generating a second phase. In addition, it is an element that stabilizes austenite, and is an element necessary for controlling the fraction of the second phase. It is necessary to contain 1.45% or more in order to obtain the effect. It is preferably 1.60% or more. More preferably, it is 1.80% or more. On the other hand, when contained excessively, the volume ratio of the second phase becomes excessive, and when hydrogen infiltrates into the steel sheet, the sliding restraint of the grain boundary increases, and the crack at the grain boundary becomes more likely to propagate. This will degrade. Therefore, the content is set to 3.35% or less. Preferably it is 3.20% or less. More preferably, it is 3.0% or less.

P: 0.05% or less

P contributes to high strength due to solid solution strengthening, but when it is added excessively, segregation at the grain boundary becomes remarkable to embrittle the grain boundary, so that the resistance welding cracking characteristics are deteriorated. Therefore, the content is made 0.05% or less. Preferably it is 0.04% or less. More preferably, it is 0.03% or less. Although there is no lower limit in particular, it is preferable to contain it 0.0005% or more because the extremely low P reduction raises the steelmaking cost. More preferably, it is 0.0008% or more.

S: 0.005% or less

When the S content is large, a large amount of sulfides such as MnS are generated, and cracks are generated from MnS during hydrogen intrusion, so that the delayed fracture characteristics are deteriorated. Therefore, the upper limit of the content is made 0.005%. Preferably, it is 0.0045% or less. More preferably, it is 0.004% or less. Although there is no lower limit in particular, since the steel making cost increases like P, it is preferable to contain it 0.0002% or more of ultra-low S reduction. More preferably, it is 0.0004% or more.

Al: 0.01% or more and 0.10% or less

Al is an element necessary for deoxidation, and it is necessary to contain 0.01% or more in order to obtain this effect. Preferably it is 0.015% or more. On the other hand, even if it contains more than 0.10%, since the effect is saturated, it is set as 0.10% or less. Preferably it is 0.06% or less. More preferably, it is 0.05% or less.

N: 0.010% or less

Since N forms a coarse nitride and deteriorates the delayed fracture characteristic, it is necessary to suppress the content. In particular, when N is more than 0.010%, the content of N is set to 0.010% or less because this tendency becomes remarkable. Preferably it is 0.008% or less. More preferably, it is 0.006% or less.

B: 0.0002% or more and 0.0045% or less

B is an element that improves quenchability and contributes to high strength by generating a second phase, and does not lower the martensite transformation initiation point while securing quenchability. Moreover, since the grain boundary strength is improved by segregation at the grain boundary, it is effective for delayed fracture characteristics. In order to exhibit this effect, it contains 0.0002% or more. Preferably it is 0.0003% or more. However, since excessive addition deteriorates toughness, the resistance welding crack resistance is deteriorated, and the content is set to 0.0045% or less. Preferably it is 0.0035% or less. More preferably, it is 0.0030% or less.

At least one selected from among Ti: 0.005% or more and 0.090% or less and Nb: 0.005% or more and 0.090% or less

Ti is an element that can contribute to an increase in strength by forming a fine carbonitride. Further, the fine carbonitride of Ti serves as a trap site for hydrogen and is effective in refining grains, so it is also effective in suppressing resistance welding cracking. In order to exhibit such an effect, the lower limit of the Ti content is set to 0.005%. The preferred lower limit is 0.008%. A more preferable lower limit is 0.010%. On the other hand, if Ti is added in a large amount, the ductility remarkably decreases, so the content is set to 0.090% or less. Preferably it is 0.080% or less. More preferably, it is 0.070% or less.

Nb also contributes to an increase in strength by forming a fine carbonitride similarly to Ti, and serves as a trap site for hydrogen, and is effective in refining crystal grains. In order to exhibit such an effect, the lower limit of the content of Nb is set to 0.005%. The preferred lower limit is 0.008%. A more preferable lower limit is 0.010%. On the other hand, when Nb is added in a large amount, not only the ductility is remarkably lowered, but the recrystallization rate is remarkably lowered, so that the amount of non-recrystallized ferrite increases. Therefore, the content is set to 0.090% or less. Preferably it is 0.080% or less. More preferably, it is 0.070% or less.

In the above, the basic components of the present invention have been described. The balance other than the above components is Fe and unavoidable impurities, but in the present invention, in addition to the above basic components, one or two or more of the following components may be added.

V: 0.10% or less

V can contribute to an increase in strength by forming a fine carbonitride. In order to have such an action, it is preferable to contain V of 0.01% or more. More preferably, it is 0.02% or more. On the other hand, even when a large amount of V is added, the effect of increasing the strength by exceeding 0.10% is small, and furthermore, an increase in alloy cost is caused. Therefore, the content of V is preferably 0.10% or less. More preferably, it is 0.08% or less.

Cu: 0.50% or less

Cu is an element that contributes to high strength by solid solution strengthening, and also contributes to high strength by generating a second phase, and can be added as necessary. In order to exhibit such an effect, it is preferable to contain 0.05% or more. More preferably, it is 0.08% or more. On the other hand, even if it contains more than 0.50%, the effect is saturated, and since surface defects due to Cu tend to occur, the content is preferably 0.50% or less. More preferably, it is 0.35% or less.

Ni: 0.50% or less

Like Cu, Ni is an element that contributes to high strength by solid solution strengthening and also contributes to high strength by generating a second phase, and can be added as needed. In order to exhibit such an effect, it is preferable to contain 0.05% or more. More preferably, it is 0.08% or more. In addition, when added simultaneously with Cu, since it has an effect of suppressing surface defects attributable to Cu, it is effective when adding Cu. On the other hand, since the effect is saturated even if it contains more than 0.50%, the content is preferably 0.50% or less. More preferably, it is 0.35% or less.

Mo: 0.50% or less

Mo is an element that contributes to increase in strength by generating the second phase, and also contributes to increase in strength by generating some carbides, and can be added as necessary. In order to exhibit such an effect, it is preferable to contain 0.05% or more. More preferably, it is 0.08% or more. On the other hand, since the effect is saturated even if it contains more than 0.50%, the content is preferably 0.50% or less. More preferably, it is 0.35% or less.

Cr: 0.80% or less

Cr is an element that contributes to high strength by generating the second phase, and can be added if necessary. In order to exhibit such an effect, it is preferable to contain 0.10% or more. More preferably, it is 0.13% or more. On the other hand, when it contains more than 0.80%, since hot-dip galvanizing property falls, it becomes non-plating, and the content is set to 0.80% or less. It is more preferably 0.70% or less.

Ca and/or REM: 0.0050% or less in total (total)

Ca and REM (rare earth elements) are elements that spheroidize the shape of a sulfide and contribute to the improvement of the adverse effect on the delayed fracture characteristics, and can be added as necessary. In order to exhibit such an effect, it is preferable to contain 0.0005% or more. More preferably, it is 0.0008% or more. On the other hand, since the effect is saturated even if it contains more than 0.0050%, the content is made 0.0050% or less. More preferably, it is 0.0035% or less.

The balance other than the above is made of Fe and unavoidable impurities. Examples of the inevitable impurities include Sb, Zn, Co, Sn, Zr, and the like, and the allowable range of their content is Sb: 0.01% or less, Zn: 0.01% or less, Co: 0.10% or less, Sn: 0.10% or less, Zr: 0.10% or less. Further, in the present invention, even if Ta and Mg are contained within the range of a normal steel composition, the effect does not disappear.

Next, the microstructure of the plated steel sheet of the present invention will be described in detail. In the present invention, ferrite is 35% or more and 70% or less by volume fraction, retained austenite is 12% or less (including 0%) by volume fraction, martensite is 15% or more and 60% or less by volume fraction, and bainite is the balance. It has a structure containing 30% or less (including 0%) by volume fraction and 5% or less (including 0%) of unrecrystallized ferrite, and the average grain size of ferrite is 5 μm or less, and retained austenite The average crystal grain size of is 2 µm or less, the average crystal grain size of martensite is 2 µm or less, and the average crystal grain size of bainite is 3 µm or less. The volume fraction described here is the volume fraction with respect to the whole steel sheet, and is the same as follows.

Ferrite in volume fraction of 35% or more and 70% or less

When the volume fraction of ferrite exceeds 70%, it is difficult to achieve a tensile strength of 980 MPa or more. Therefore, the volume fraction of ferrite is set to 70% or less. It is preferably 65% or less, and more preferably 60% or less. In addition, when the volume fraction is less than 35%, the second phase having high dislocation density increases, and the delayed fracture characteristics deteriorate. Therefore, the volume fraction of ferrite is made 35% or more. In order to improve elongation, it is preferably 40% or more.

Ferrite has an average grain size of 5 μm or less

When the average grain size of ferrite exceeds 5 µm, the grains become coarse during resistance welding, resulting in deterioration of toughness and cracking resistance. Therefore, the crystal grain size of ferrite is 5 µm or less. It is preferably 4 μm or less. In order to improve elongation, it is preferably 0.5 µm or more.

12% or less of retained austenite in volume fraction

The retained austenite contributes to the strength by transforming the process-induced martensite. In addition, since it becomes a hydrogen trap site, it is also effective for delayed fracture characteristics. However, since martensitic transformation retains a high dislocation density, cracks are generated due to hydrogen intrusion, resulting in deterioration in terms of delayed fracture characteristics. Therefore, the volume fraction of retained austenite is set to 12% or less. It is preferably more than 0% and 10% or less. More preferably, it is 1% or more. More preferably, it is 7% or less. In addition, the volume fraction of retained austenite may be 0%.

The average grain size of retained austenite is 2㎛ or less

Since the average crystal grain size of retained austenite is influenced by the C distribution in retained austenite, martensite is easily generated during press molding, and the delayed fracture property is lowered, so the upper limit is set to 2 µm. The lower limit is not specifically defined, but 0.3 µm or more is preferably 0.3 µm or more, since the contribution to elongation increases.

Martensite in volume fraction 15% or more and 60% or less

In order to ensure the desired strength, the volume fraction of martensite is set to 15% or more. Preferably it is 20% or more. More preferably, it is 23% or more. On the other hand, when the volume fraction of martensite is more than 60%, not only is cracking easily generated during hydrogen intrusion, but also the crack propagation rate increases, so the upper limit thereof is set to 60%. Preferably it is made into 57% or less. More preferably, it is 55% or less.

Martensite has an average crystal grain size of 2 μm or less

When the average particle diameter of martensite is more than 2 µm, the crystal grains become coarse at the time of resistance welding, resulting in deterioration of toughness and crack resistance. Therefore, the average crystal grain size of martensite is set to 2 µm or less. Preferably it is 1.8 μm or less. In addition, martensite here refers to martensite generated after annealing, but self-tempering (autotempered) martensite, which is transformed into martensite upon cooling of annealing, is tempered after martensite transformation. Martensite and fresh martensite transformed from austenite without tempering.

Bainite as the balance is 30% or less in volume fraction

Bainite contributes to high strength, but since it contains a high dislocation density, when the volume fraction exceeds 30%, the delayed fracture characteristics deteriorate. Therefore, the upper limit is set to 30%. It is preferably more than 0% and 25% or less. More preferably, it is 5% or more. More preferably, it is 20% or less. Further, the volume fraction of bainite may be 0%.

Bainite has an average grain size of 3㎛ or less

When the average grain size of bainite is more than 3 µm, the grains become coarse during resistance welding, resulting in deterioration of toughness and cracking resistance. Therefore, the average grain size of bainite is set to 3 µm or less. It is preferably 2.5 μm or less.

5% or less by volume fraction of non-recrystallized ferrite as the balance

Moreover, although non-recrystallized ferrite also contributes to the increase in strength, since it contains a high dislocation density like bainite, its upper limit is 5%. It is preferably more than 0% and 3% or less. More preferably, it is 1% or less. Further, the volume fraction of non-recrystallized ferrite may be 0%.

In the present invention, in addition to ferrite, bainite, martensite, retained austenite and non-recrystallized ferrite, pearlite may be produced, but the volume fraction of ferrite, bainite, martensite, retained austenite and non-recrystallized ferrite, If the average crystal grain size of ferrite, bainite, martensite, and retained austenite, and distribution of Ti or Nb-based precipitates (carbide) satisfies the range defined as the present invention, the effects of the present invention can be obtained. However, the volume fraction of pearlite is preferably 5% or less, and more preferably 3% or less.

More than 30 Ti or Nb precipitates with an average particle diameter of 0.10㎛ or less on average per 100㎛ ²

In the present invention, it is necessary to contain 30 or more Ti or Nb-based precipitates having an average particle diameter of 0.10 µm or less on an average per 100 µm ² . This is because Ti or Nb-based precipitates become trap sites for hydrogen to improve delayed fracture characteristics, and are effective for grain refinement, and improve resistance welding cracking characteristics. When the particle diameter is more than 0.10 µm or the amount of precipitates is less than 30 on average per 100 µm ² , the delayed fracture characteristics and resistance welding cracking resistance deteriorate. It is preferably 50 or more per 100 μm ² . More preferably, it is 60 or more per 100 μm ² . Specific examples of the Ti or Nb-based precipitate include carbides.

Next, the manufacturing conditions of the plated steel sheet according to the present invention will be described.

A steel slab having the above component composition (chemical composition) is hot-rolled under conditions of a finish rolling end temperature of 850°C or more and 950°C or less, and is 680°C or less at a first average cooling rate of 75°C/s or more as primary cooling. After cooling to, after cooling in the range of 400°C or more and 580°C or less at a second average cooling rate of 5°C/s or more as secondary cooling, the hot-rolled steel sheet is pickled, and then cold-rolled. Then, in the annealing step, the cold-rolled steel sheet is heated to a temperature range of 760°C to 900°C at an average heating rate of 3 to 30°C/s, and the first soaking temperature is in a temperature range of 760°C to 900°C. After storage and maintenance for 15 seconds or more, it is cooled to a temperature range of 600° C. or less at an average cooling rate of 3 to 30° C./s, annealing, then hot-dip galvanizing, and cooling to room temperature.

In the hot rolling step, it is preferable to start hot rolling after casting the steel slab at 1150°C or more and 1300°C or less without reheating after casting, or after reheating to 1150°C or more and 1300°C or less. The steel slab to be used is preferably manufactured by a continuous casting method in order to prevent macro segregation of components, but it is also possible to manufacture by an ingot casting method or a thin slab casting method. In the present invention, after producing a steel slab, in addition to the conventional method of once cooling to room temperature and then reheating, it is charged into a heating furnace without cooling, but is charged into a heating furnace, or heat preservation. An energy saving process such as direct rolling or direct rolling in which rolling immediately after performing the process or rolling as it is after casting can also be applied without a problem.

[Hot rolling process]

·Finish rolling end temperature: 850℃ or more and 950℃ or less

Hot rolling needs to be terminated in the austenite single-phase region in order to improve the delayed fracture characteristics after annealing and the resistance welding cracking resistance after annealing due to uniformity of the structure in the steel sheet and reduction of anisotropy of the material. Therefore, the finish rolling end temperature is set to 850°C or higher. On the other hand, when the finish rolling end temperature exceeds 950°C, the hot-rolled structure becomes coarse, and crystal grains after annealing also coarsen. Therefore, the finish rolling end temperature is 850°C or more and 950°C or less.

·Cooling conditions after finishing rolling

After cooling down to 680°C at a first average cooling rate of 75°C/s or higher as primary cooling, and then cooling to a range of 400°C to 580°C at a second average cooling rate of 5°C/s or higher as secondary cooling

In the present invention, since the steel sheet structure after annealing is controlled by controlling the form of precipitation of precipitates of Ti or Nb during hot rolling, cooling after finish rolling is an important step. After the completion of hot rolling, austenite transforms into ferrite in the cooling process, but the ferrite coarsens at high temperatures. Therefore, by quenching the steel sheet after completion of hot rolling, the structure is homogenized as much as possible and formation of precipitates is suppressed. Therefore, as primary cooling, it cools to 680 degreeC or less at the 1st average cooling rate of 75 degreeC/s or more.

When the first average cooling rate is less than 75°C/s, since the ferrite becomes coarse, the steel sheet structure of the hot-rolled steel sheet becomes heterogeneous, and the resistance welding cracking resistance decreases. Further, for ferrite, martensite and bainite, a desired average crystal grain size cannot be obtained. It is preferably 85°C/s or more.

When the temperature to be cooled in the primary cooling exceeds 680°C, pearlite in the steel sheet structure of the hot-rolled steel sheet is excessively generated and the steel sheet structure becomes uneven, so that the resistance welding cracking resistance decreases. Further, for martensite, a desired average crystal grain size cannot be obtained. Preferably, it is set at 650°C or less. Further, since martensite excessively increases in the steel sheet structure of the hot-rolled steel sheet, it is preferably set to 400°C or higher.

Subsequent secondary cooling cools to a range of 400°C or more and 580°C or less at a second average cooling rate of 5°C/s or more. In cooling to less than 5°C/s or more than 580°C, ferrite or pearlite is excessively generated in the steel sheet structure of the hot-rolled steel sheet, and the resistance welding cracking resistance after annealing decreases. Further, for martensite, a desired average crystal grain size cannot be obtained, and no more than 30 Ti or Nb-based precipitates having an average crystal grain size of 0.10 µm or less cannot be obtained on an average per 100 µm ² . Preferably, it is 10°C/s or more. Further, since Ti and Nb are excessively dissolved in solid solution, it is preferably 65°C/s or less.

Winding temperature: 400℃ or more and 580℃ or less

When the coiling temperature exceeds 580°C, ferrite and pearlite in the steel sheet structure of the hot-rolled steel sheet are excessively generated. In addition, with respect to retained austenite, a desired average crystal grain size cannot be obtained, and even 30 or more Ti or Nb-based precipitates having an average crystal grain size of 0.10 µm or less cannot be obtained on an average per 100 µm ² . Therefore, it is preferable that the upper limit of the coiling temperature is 580°C. More preferably, it is 550 degreeC or less. Further, when the coiling temperature is less than 400° C., the precipitates of Ti and Nb are not sufficiently precipitated and become a solid solution, so that the effect of miniaturization after annealing cannot be expected. In addition, 30 or more Ti or Nb-based precipitates having an average grain size of 0.10 µm or less cannot be obtained on average per 100 µm ² . Therefore, it is preferable that the coiling temperature is 400°C or higher. More preferably, it is set to 420°C or higher.

[Pickling process]

After the hot rolling process, it is preferable to perform the pickling process to remove the scale of the surface layer of the hot rolled sheet. The pickling process is not particularly limited, and may be performed according to a conventional method.

[Cold rolling process]

A cold rolling step of rolling on a cold rolled sheet having a predetermined thickness is performed. The cold rolling process is not particularly limited and may be performed by a conventional method. A preferable range of the reduction ratio in cold rolling is 30% or more and 95% or less.

[Annealing process]

In the annealing process, it is carried out in order to advance recrystallization and to form fine bainite, retained austenite, and martensite in the steel sheet structure for high strength. Therefore, in the annealing process, it is heated to a temperature range of 760°C to 900°C at an average heating rate of 3 to 30°C/s, and the soaking temperature is maintained for 15 seconds or more in a temperature range of 760°C to 900°C. After that, it is cooled to a temperature range of 600°C or less at an average cooling rate of 3 to 30°C/s.

Moreover, you may perform temper rolling after annealing. The preferable range of the elongation rate is 0.05% to 2.0%.

·Average heating rate: 3∼30℃/s

By setting the average heating rate to 3 to 30°C/s, it becomes possible to refine the crystal grains after annealing. If heated rapidly, recrystallization becomes difficult to proceed. In addition, a desired average crystal grain size for bainite cannot be obtained, and 30 or more Ti or Nb-based precipitates having a desired volume fraction for non-recrystallized ferrite and an average crystal grain size of 0.10 μm or less are obtained on an average per 100 μm ² . Can't. Therefore, the upper limit of the average heating rate is 30°C/s. Since the amount of non-recrystallized ferrite increases, it is preferably 25°C/s or less.

In addition, when the heating rate is too small, ferrite or martensite grains become coarse, and a predetermined average particle diameter cannot be obtained. Further, more than 30 Ti or Nb-based precipitates having an average crystal grain size of 0.10 µm or less cannot be obtained on average per 100 µm ² . Therefore, an average heating rate of 3°C/s or more is required. It is preferably 5°C/s or more.

Cracking temperature (preservation and maintenance temperature): 760℃ to 900℃

As a cracking temperature, it cracks in the temperature range which is a two-phase region of ferrite and austenite or a single phase region of austenite. At less than 760°C, since the ferrite fraction increases, it becomes difficult to secure strength. Further, for ferrite and martensite, a desired average crystal grain size cannot be obtained. Therefore, the lower limit of the soaking temperature is 760°C. Preferably, it is set at 780°C or higher. When the soaking temperature is too high, crystal grain growth of ferrite, martensite, and austenite becomes remarkable, and the crystal grains become coarse, thereby deteriorating resistance welding cracking characteristics. Further, more than 30 Ti or Nb-based precipitates having an average crystal grain size of 0.10 µm or less cannot be obtained on average per 100 µm ² . Therefore, the upper limit of the soaking temperature is set to 900°C. It is preferably 880°C or less.

Cracking time: 15 seconds or more

In the above soaking temperature, in order to undergo recrystallization and to transform austenite for some or all, the soaking time needs to be maintained for 15 seconds or longer. Since the volume fraction of non-recrystallized ferrite increases, it is preferably 20 seconds or longer. The upper limit is not particularly limited, but within 600 seconds is preferable.

Cooling conditions during annealing: cooling down to a temperature range of 600°C or less at an average cooling rate of 3 to 30°C/s

After the above-described soaking, it is necessary to cool at an average cooling rate of 3 to 30°C/s from the soaking temperature to a temperature range of 600°C or less (cooling stop temperature). When the average cooling rate is less than 3°C/s, since ferrite transformation proceeds during cooling and the volume fraction of the second phase decreases, it is difficult to secure strength. Further, for martensite, retained austenite, and bainite, a desired average crystal grain size cannot be obtained. On the other hand, when the average cooling rate exceeds 30° C./s, not only is martensite excessively generated, but also it is difficult to realize this in the facility. In addition, when the cooling stop temperature exceeds 600°C, since pearlite is excessively generated, a predetermined volume fraction in the microstructure of the steel sheet cannot be obtained, and it is difficult to secure strength. Further, more than 30 Ti or Nb-based precipitates having an average crystal grain size of 0.10 µm or less cannot be obtained on an average per 100 µm ² , and the delayed fracture characteristics and resistance welding cracking characteristics are deteriorated. In addition, the above average cooling rate is the average of the cooling rate in the range of 600°C or less until immersion in the plating bath, and if the average cooling rate of 3 to 30°C/s is maintained in this temperature range good.

[Plating treatment]

After the annealing, a plating treatment is performed and the mixture is cooled to room temperature. Examples of the plating treatment include hot-dip galvanizing treatment and electrogalvanizing treatment. The temperature of the steel sheet immersed in the plating bath is, for example, in the case of hot-dip galvanizing treatment, preferably from (melted zinc plating bath temperature -40)°C to (melted zinc plating bath temperature +50)°C. When the temperature of the steel sheet immersed in the plating bath is lower than (the temperature of the molten zinc plating bath -40)°C, when the steel sheet is immersed in the plating bath, a part of the molten zinc solidifies and the appearance of the plating may be deteriorated. Therefore, the lower limit is set to (melted zinc plating bath temperature -40)°C. In addition, when the temperature of the steel sheet immersed in the plating bath exceeds (melted zinc plating bath temperature +50)° C., the temperature of the plating bath rises, causing problems in mass production. As for other conditions, the conditions performed in a normal plating process can be used.

[Alloy treatment]

After the plating, the plating may be alloyed in a temperature range of 450°C or more and 600°C or less. By alloying in a temperature range of 450°C or more and 600°C or less, the Fe concentration in the plating becomes 7 to 15% by mass, and the adhesion of the plating and the corrosion resistance after coating are improved. If the temperature is lower than 450°C, alloying does not sufficiently proceed, resulting in a decrease in sacrificial corrosion resistance and a decrease in sliding properties, and at a temperature higher than 600°C, the progression of alloying becomes excessive and the powdering resistance decreases.

The conditions of other manufacturing methods are not particularly limited, but from the viewpoint of productivity, the series of treatments such as annealing, plating treatment, and alloying treatment of plating described above are continuous hot-dip galvanizing lines when hot-dip galvanizing treatment is performed. It is preferable to do it in (CGL). In addition, it is preferable to use a zinc plating bath containing 0.10 to 0.20 mass% of Al for hot dip galvanizing. After plating, in order to adjust the weight per unit area of plating, wiping can be performed.

(Example)

Hereinafter, an embodiment of the present invention will be described.

However, the present invention is not originally limited by the following examples, and it is also possible to carry out appropriate modifications within a range suitable for the gist of the present invention, and all of these are included in the technical scope of the present invention.

The steel of the component composition shown in Table 1 was melted and cast to produce a slab, and hot rolling was performed under the conditions shown in Table 2 with a hot rolling heating temperature of 1250°C and a finish rolling end temperature (FDT), and plate thickness: 3.2 After setting it as a mm hot-rolled steel sheet, after cooling to the first cooling temperature at the first average cooling rate (cooling speed 1) shown in Table 2, cooling to the second cooling temperature at the second average cooling rate (cooling speed 2), and winding It wound up at temperature (CT). Subsequently, after pickling the obtained hot-rolled sheet, cold rolling was performed to manufacture a cold-rolled sheet (board thickness: 1.4 mm).

The thus-obtained cold-rolled steel sheet was annealed according to the production conditions shown in Table 2 in a continuous hot-dip galvanizing line, subjected to hot-dip galvanizing treatment, and then further alloyed at a temperature shown in Table 2 to be alloyed. A hot-dip galvanized steel sheet (GA) was obtained. Here, the plating treatment is the zinc plating bath temperature: 460°C, the zinc plating bath Al concentration: 0.14 mass% (when alloying treatment), 0.18 mass% (when alloying treatment is not performed), and the amount of plating deposited per side 45 g/ It was set as m2 (double-sided plating). In addition, in some of the steel sheets, a non-alloy hot-dip galvanized steel sheet (GI) was obtained without alloying with zinc plating.

From the manufactured steel sheet, a JIS No. 5 tensile test piece was taken so that the rolling perpendicular direction became the longitudinal direction (tensile direction), and the tensile strength (TS) was measured by a tensile test (JIS Z2241 (1998)).

Regarding the delayed fracture test, the obtained cold-rolled steel sheet was cut into a length of 30 mm x 100 mm with the rolling direction as the length, and the test piece was subjected to 180° bending with a radius of curvature of 10 mm at the tip of the punch using a test piece whose end face was ground. Carried out. The spring back generated on the test piece subjected to this bending process is tightened with bolts so that the inner gap is 20 mm, and the test piece is immersed in hydrochloric acid at 20°C and pH = 1.5 after stress is applied to it, and the time until failure occurs. Was measured for up to 100 hours. It was said that the crack did not occur in the test piece within 100 hours as good ((circle)), and when the crack occurred in the test piece, it was said to be defective (x).

Regarding the test of resistance welding cracks, resistance welding (spot welding) was performed using a test piece cut into 50×150 mm with the obtained cold-rolled steel sheet as a length in the direction perpendicular to the rolling direction. For welding, resistance spot welding was performed on a plate frame in which two steel plates were stacked, using a single-phase direct current (50 Hz) resistance welding machine with a servo motor pressure type attached to a welding gun, while the plate plate was inclined by 4°. As for the welding conditions, the pressing force was 3.5 kN and the hold time was 0.36 seconds. The welding current and welding time were adjusted so that the nugget diameter became 5.9 mm. After welding, the test piece was cut in half, and the cross section was observed with an optical microscope. If no cracks of 0.2 mm or more were observed, the resistance welding cracking characteristics were good (○), and the cracking resistance of 0.2 mm or more was confirmed. It was said to be defective (×).

The volume fraction of ferrite, martensite, and non-recrystallized ferrite of the steel sheet is determined by polishing the cross section of the sheet thickness parallel to the rolling direction of the steel sheet, then corroding it with 3 vol% nital, and using a scanning electron microscope (SEM). It observed at the magnification of 2000 times and 5000 times, the area ratio was measured by the point count method (according to ASTM E562-83 (1988)), and the area ratio was taken as the volume fraction. The average grain size of ferrite and martensite can be calculated by taking a photograph of each ferrite and martensite grains identified in advance from a steel sheet structure photograph using Media Cybernetics' Image-Pro. The circle equivalent diameter was calculated, and these values were averaged.

The volume fraction of retained austenite was obtained by grinding the steel sheet to a quarter surface in the plate thickness direction, and obtained by diffraction X-ray intensity at the 1/4 surface thickness. Using the Kα-ray of Mo as a source, with an acceleration voltage of 50 keV, by an X-ray diffraction method (apparatus: RINT2200 manufactured by Rigaku), the {200} plane, the {211} plane, the {220} plane of iron ferrite, and the Austere The integral intensity of the X-ray diffraction lines on the {200} plane, {220} plane, and {311} plane of the Knight was measured, and using these measured values, "X-ray Diffraction Handbook" (2000) Rigaku Denki Co., Ltd. , p.26, the volume fraction of retained austenite was calculated from the formula described in 62-64.

The average crystal grain size of retained austenite was observed at a magnification of 5000 times using EBSD (electron beam back scattering analysis method), and the equivalent circle diameter was calculated using Image-Pro, and these values were averaged and determined.

Further, by SEM, TEM (transmission electron microscope), and FE-SEM (field emission scanning electron microscope), the steel plate structure was observed, bainite was observed, and the volume fraction was calculated in the same manner as described above. Also about the average crystal grain size of bainite, the equivalent circle diameter was calculated from the steel plate structure photograph using the above-described Image-Pro, and these values were averaged and determined.

The particle diameter of the Ti or Nb-based precipitate was observed at 5000 times, 10000 times, and 20000 times magnification using SEM and TEM, and the particle diameter was calculated by calculating its circular equivalent diameter using Image-Pro. The number of Ti or Nb-based precipitates was observed at 5000 times, 10000 times, and 20000 times magnification using SEM and TEM, and the average number of 10 locations was obtained.

Table 3 shows the measured tensile properties, delayed fracture properties, resistance welding cracking properties, and measurement results of the steel plate structure.

From the results shown in Table 3, in the examples of the present invention, ferrite having an average crystal grain size of less than 5 µm is 35 to 70% by volume, residual austenite having an average crystal grain size of 2 µm or less is 12% or less by volume, and the average grain size is It has a composite structure containing 15% to 60% by volume fraction of martensite of 2 μm or less, 30% of bainite having an average particle diameter of 3 μm or less by volume fraction, and 5% or less of unrecrystallized ferrite by volume fraction. , Tensile strength of 980 MPa or more is obtained, in the delayed fracture property evaluation test, no fracture occurs for 100 hours, has excellent delayed fracture characteristics, and crack resistance does not occur even after resistance welding, and excellent resistance welding cracking It was confirmed to get.

Claims (5)

In mass%,
C: 0.05% or more and 0.22% or less,
Si: 0.05% or more and 1.80% or less,
Mn: 1.45% or more and 3.35% or less,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01% or more and 0.10% or less,
N: 0.010% or less and
B: 0.0002% or more and 0.0045% or less
Containing, in addition,
Ti: 0.005% or more and 0.090% or less and Nb: 0.005% or more and 0.090% or less, containing at least one selected from, the balance has a component composition of Fe and inevitable impurities,
Ferrite by volume fraction of 35% or more and 70% or less, retained austenite by volume fraction of 12% or less, martensite by volume fraction of 15% or more and 60% or less, balance of bainite by volume fraction of 30% or less and non-recrystallized It has a structure containing 5% or less of ferrite by volume,
The average grain size of the ferrite is 5 μm or less,
The average crystal grain size of the retained austenite is 2 μm or less,
The average crystal grain size of the martensite is 2 μm or less,
The bainite has an average crystal grain size of 3 μm or less,
A plated steel sheet in which the structure contains 30 or more Ti or Nb-based precipitates having an average particle diameter of 0.10 µm or less on an average per 100 µm ² . The method of claim 1,
The component composition, further,
In mass%,
V: 0.10% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Mo: 0.50% or less,
Cr: 0.80% or less and
Ca, or REM, or Ca and REM: 0.0050% or less
A plated steel sheet containing one or two or more selected from among. In mass%,
C: 0.05% or more and 0.22% or less,
Si: 0.05% or more and 1.80% or less,
Mn: 1.45% or more and 3.35% or less,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01% or more and 0.10% or less,
N: 0.010% or less and
B: 0.0002% or more and 0.0045% or less
Contains, and further, in mass%,
Ti: 0.005% or more and 0.090% or less, Nb: 0.005% or more and 0.090% or less, containing at least one selected from among, the remainder is Fe and inevitable impurities in a steel slab having a component composition, the finish rolling temperature 850°C Hot-rolling is performed under conditions of not less than 950°C to obtain a hot-rolled steel sheet,
The hot-rolled steel sheet is cooled to 680°C or less at a first average cooling rate of 75°C/s or more, and cooled to a range of 400°C or more and 580°C or less at a second average cooling rate of 5°C/s or more, and then winding And cold-rolled to obtain a cold-rolled steel sheet,
The cold-rolled steel sheet is heated to a temperature range of 760°C to 900°C at an average heating rate of 3 to 30°C/s, and holding for 15 seconds or more in a temperature range of 760°C to 900°C to crack ( After sintering), annealing is performed to cool down to a temperature range of 600°C or less at an average cooling rate of 3 to 30°C/s,
A method for producing a plated steel sheet in which a plating treatment is performed on the cold-rolled steel sheet after the annealing. The method of claim 3,
The component composition, further,
In mass%,
V: 0.10% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Mo: 0.50% or less,
Cr: 0.80% or less and
Ca, or REM, or Ca and REM: 0.0050% or less
A method for producing a plated steel sheet containing one or two or more selected from among. The method according to claim 3 or 4,
A method for producing a plated steel sheet, wherein the plating is alloyed in a temperature range of 450°C or more and 600°C or less after performing the plating treatment.