JP6424967B2 - Plated steel sheet and method of manufacturing the same - Google Patents

Description

  The present invention relates to a plated steel plate and a method of manufacturing the same, and more particularly to a plated steel plate suitable as a member of a structural part such as an automobile.

In recent years, CO ₂ emission regulations have become stricter due to the rise of environmental problems, and in the automobile field, weight reduction of a vehicle body for improving fuel consumption has become an issue. Therefore, thinning of automobile parts is advanced by applying a high-strength steel plate to automobile parts, and application of a steel plate having a tensile strength (TS) of 980 MPa or more is promoted. Moreover, the plated steel plate which has a hot dip galvanized layer is used for the site | part exposed to rain water from a corrosion-resistant viewpoint.

Patent Document 1 discloses a technique for improving delayed fracture resistance (hydrogen embrittlement resistance) by controlling iron-based carbides containing Si or Si and Al in a steel sheet structure.
Moreover, in patent document 2, the technique which improves the surface crack at the time of resistance welding is disclosed by controlling the addition amount of Si, Al, and Mn.

Patent 4949536 Patent No. 37 58 515

However, in the technology described in Patent Document 1, even if the iron-based carbide can become a trap site of hydrogen, not only there is no improvement effect of resistance weld cracking resistance characteristics, but the iron-based carbide exists in the grain boundary. There is a possibility that cracking during resistance welding may be promoted.
Moreover, in the technique described in Patent Document 2, it is difficult to achieve high strength of 980 MPa or more, and it is also difficult to obtain excellent delayed fracture resistance. As described above, it is difficult to simultaneously improve both delayed fracture resistance and resistance weld cracking resistance in a plated steel plate of 980 MPa or more, and even if steel plates other than the plated steel plate are included, these characteristics can be combined. The fact is that no steel plate has been developed.

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

  The present invention has been made in view of such circumstances, and solves the problems in the above-mentioned prior art, and provides a plated steel sheet excellent in delayed fracture resistance and resistance weld cracking resistance, and a method of manufacturing the same. With the goal.

  The present inventors diligently studied to improve both the delayed fracture resistance and the resistance weld cracking resistance. As a result, the volume fraction of the steel plate 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 plate structure is refined, It has been found that by generating fine carbides of Ti or Nb, excellent delayed fracture resistance and resistance weld cracking characteristics can be obtained together. The present invention is based on the above findings.

  In delayed fracture, hydrogen penetrates into the steel plate, causing a crack to form and propagate. 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 the coating may be damaged for some reason and the surface of the base iron may be exposed. When rainwater or the like adheres to the surface of the steel sheet in such a situation, zinc serves as an anode and iron serves as a cathode, so that the generation of hydrogen is accelerated on the iron surface. Therefore, in the plated steel sheet, it was necessary to consider the delayed fracture characteristics of the steel sheet, not the composition or composition of the plated layer.

  With regard to cracking due to liquid metal embrittlement at the time of resistance welding, internal stress occurs due to Zn melted (melted) at the time of welding, and cracking occurs in the heat affected zone (HAZ) near the nugget. Conventionally, with plated steel sheets, when welding is performed at a high current value that causes spatter (dust) at the time of welding, cracking due to liquid metal embrittlement may occur on the surface on the electrode contact side. Such problems do not occur if welding is performed. However, even in an appropriate current range in which no spattering occurs, if the tensile strength is as high as 980 MPa, cracks (inner cracks) may occur on the surface where the steel plates overlap. In particular, when the welding electrode is welded at an angle to the steel plate, the internal stress is increased to easily cause a crack. Here, the resistance welding crack in the present invention indicates this internal crack. If this internal crack occurs, there is a concern that the fatigue strength of the welds in particular may be reduced. Therefore, it is necessary to avoid this internal crack in order to use for a car etc. An observation of the inner crack portion revealed that a crack due to intergranular fracture has occurred in the place where it becomes a martensitic single phase after welding of the heat affected zone (HAZ).

Therefore, as a result of intensive investigations, the inventors found that by controlling the size and number of fine Ti or Nb-based carbides, trap sites of hydrogen can be generated to improve delayed fracture resistance. . In addition, it has been found that by refining the grain size of the steel sheet, the susceptibility to liquid metal embrittlement during welding is improved. Furthermore, it became clear by controlling the volume fraction of steel plate structure that strength, delayed fracture resistance, and resistance weld crack resistance are improved. Fine Ti or Nb carbides not only become trap sites of hydrogen but also suppress nucleation of ferrite and austenite during recrystallization in annealing to promote nucleation of ferrite and austenite. For this reason, fine Ti or Nb carbides are very effective in refining the steel sheet structure. Thus, by refining the steel sheet structure, it becomes clear that the toughness of the steel sheet is improved without coarsening of crystal grains even during resistance welding, and therefore resistance welding cracking is also suppressed. The
The present invention has been made based on the above novel findings, and has the following configuration.

1. 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% to 3.35%,
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 is further contained,
Ti: containing at least one selected from 0.005% or more and 0.090% or less and Nb: 0.005% or more and 0.090% or less, and the balance having a component composition which is Fe and an unavoidable impurity,
35% or more and 70% or less of ferrite, 12% or less of retained austenite by volume fraction, 15% or more and 60% or less of martensite, and 30% or less of bainite as a balance Has a structure containing 5% or less by volume fraction of unrecrystallized ferrite,
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,
The average grain size of the martensite is 2 μm or less,
The average grain size of the bainite is 3 μm or less,
The plated steel sheet containing 30 or more of Ti or Nb-based precipitates having an average particle diameter of 0.10 μm or less on average per 100 μm ^{2 in the} structure.

2. The component composition is 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: The plated steel plate as described in 1 above, which contains one or more selected from 0.0050% or less.

3. 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% to 3.35%,
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: containing 0.0002% or more and 0.0045% or less, and further, in mass%,
Ti: containing one or more selected from 0.005% or more and 0.090% or less, Nb: 0.005% or more and 0.090% or less, and the remainder being an end of finish rolling on a steel slab having a component composition of Fe and unavoidable impurities Hot rolling is performed at a temperature of 850 ° C. or more and 950 ° C. or less to form 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 is cooled to a range of 400 ° C. to 580 ° C. at a second average cooling rate of 5 ° C./s or more , Cold rolled and 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, held for 15 seconds or more in a temperature range of 760 ° C. to 900 ° C., and homogenized Annealing to cool to a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C./s,
The manufacturing method of the plated steel plate which performs a plating process to the cold rolled steel plate after this annealing.

4. The component composition is 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: The manufacturing method of the plated steel plate of said 3 containing 1 type (s) or 2 or more types selected from 0.0050% or less.

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

  According to the present invention, it has an extremely high tensile strength, and also has excellent delayed fracture resistance that does not cause delayed fracture due to hydrogen entering from the environment even after being formed into a member, and cracks also occur during resistance welding. Has excellent resistance welding crack resistance characteristics that do not occur. For example, no fracture occurs for 100 hours in a hydrochloric acid immersion environment with a tensile strength of 980 MPa or more and U bending after pH bending at 20 ° C, and welding was performed with an electrode with an angle (about 0.5 to 10 °) attached to a steel plate Also in the case, it is possible to stably obtain a high-strength plated steel sheet having excellent strength, delayed fracture resistance, and resistance weld crack resistance which do not cause resistance weld cracking.

  Hereinafter, a plated steel plate according to an embodiment of the present invention will be described. First, the reasons for limiting the composition of the steel will be described. In the present specification, “%” representing the content of each component element means “% by mass” unless otherwise specified.

C: 0.05% or more and 0.22% or less
C is an element effective for increasing the strength of the steel sheet, and also contributes to the formation of the second phase (structure other than ferrite which is 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 bainite, martensite and retained austenite volume ratio, 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 it is added excessively, the hardness after resistance welding becomes high, the toughness at the time of resistance welding decreases and the resistance weld cracking characteristics deteriorate, so the content is made 0.22% or less. Preferably it is 0.20% or less, More preferably, it is 0.18% or less.

Si: 0.05% or more and 1.80% or less
Si solid solution strengthens ferrite and is an element effective for strengthening. In order to obtain the effect, addition of 0.05% or more is necessary. Preferably, it is 0.10% or more. More preferably, it is 0.20% or more. However, excessive addition of Si lowers the plating property to cause non-plating, so the content thereof is made 1.80% or less. Preferably it is 1.60% or less. More preferably, it is 1.50% or less.

Mn: 1.45% to 3.35%
Mn is an element that contributes to the strengthening by forming solid solution strengthening and the second phase. Moreover, it is an element which stabilizes austenite, and is an element required for fraction control of a 2nd phase. In order to acquire the effect, it is necessary to contain 1.45% or more. Preferably, it is 1.60% or more. More preferably, it is 1.80% or more. On the other hand, when it is contained in excess, the volume fraction of the second phase becomes excessive, and if hydrogen intrudes into the steel sheet, slip restraint at grain boundaries increases and cracks at grain boundaries easily grow. As a result, the delayed fracture resistance is reduced. Therefore, the content is made 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 by solid solution strengthening, but when it is added excessively, segregation to the grain boundaries becomes significant and the grain boundaries become embrittled, so resistance weld cracking resistance characteristics deteriorate. 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, extremely low P is preferable to contain 0.0005% or more because steelmaking cost increases. More preferably, it is 0.0008% or more.

S: 0.005% or less
When the content of S is large, a large amount of sulfides such as MnS is formed, and a crack is generated from MnS at the time of hydrogen penetration, so that the delayed fracture resistance is 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. There is no lower limit in particular, but since the steel making cost rises similarly to P as in the case of extremely low S, it is preferable to contain 0.0002% or more. 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 in order to obtain this effect, it is necessary to contain 0.01% or more. Preferably it is 0.015% or more. On the other hand, even if the content exceeds 0.10%, the effect is saturated, so the content is made 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 degrades the delayed fracture resistance, it is necessary to suppress the content. In particular, when N exceeds 0.010%, this tendency becomes remarkable, so the content of N is made 0.010% or less. Preferably it is 0.008% or less. More preferably, it is 0.006% or less.

B: 0.0002% to 0.0045%
B is an element that improves the hardenability and contributes to high strength by generating the second phase, and does not lower the martensitic transformation start point while securing the hardenability. Further, segregation at grain boundaries improves grain boundary strength, which is effective for delayed fracture resistance. In order to exhibit this effect, 0.0002% or more is contained. Preferably it is 0.0003% or more. However, the excessive addition reduces the resistance weld cracking characteristics to deteriorate toughness, so the content is made 0.0045% or less. Preferably it is 0.0035% or less. More preferably, it is 0.0030% or less.

One or more selected from 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 increase in strength by forming fine carbonitrides. In addition, since fine carbonitrides of Ti become trap sites of hydrogen and are effective in grain refinement, they are also effective in suppressing resistance welding cracking. In order to exert such an effect, the lower limit of the content of Ti is made 0.005%. The preferred lower limit is 0.008%. A further preferable lower limit is 0.010%. On the other hand, when a large amount of Ti is added, the ductility is significantly reduced, so the content is made 0.090% or less. Preferably it is 0.080% or less. More preferably, it is 0.070% or less.
By forming fine carbonitrides similarly to Ti, Nb also contributes to increase in strength, and also serves as trap sites for hydrogen, and is effective in grain refinement. In order to exert such an effect, the lower limit of the content of Nb is made 0.005%. The preferred lower limit is 0.008%. A further preferable lower limit is 0.010%. On the other hand, when Nb is added in a large amount, not only the ductility is significantly reduced, but also the non-recrystallized ferrite is increased to significantly reduce the recrystallization rate. Therefore, the content is made 0.090% or less. Preferably it is 0.080% or less. More preferably, it is 0.070% or less.

  The basic components of the present invention have been described above. 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 more of the following components may be added.

V: 0.10% or less
V can contribute to increase in strength by forming fine carbonitrides. In order to have such an effect, it is preferable to contain V 0.01% or more. More preferably, it is 0.02% or more. On the other hand, even if a large amount of V is added, the effect of increasing the strength exceeding 0.10% is small, and moreover, the cost of the alloy is also increased. 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 through solid solution strengthening and 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 the content is more than 0.50%, the effect is saturated, and surface defects caused by Cu are easily generated, so 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 also an element that contributes to high strength through solid solution strengthening and 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. Moreover, since it has an effect which suppresses the surface defect of Cu origin when it adds simultaneously with Cu, it is effective at the time of Cu addition. On the other hand, even if the content is more than 0.50%, the effect is saturated, so 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 high strength by generating the second phase, and is a further element that partially generates carbide to contribute to high strength, 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 the content is more than 0.50%, the effect is saturated, so 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 as needed. 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, if the content is more than 0.80%, the hot-dip galvanizing property is lowered to cause non-plating, so the content is made 0.80% or less. More preferably, it is 0.70% or less.

Ca and / or REM: (in total) less than 0.0050% in total
Ca and REM (rare earth elements) are elements contributing to spheroidizing the shape of the sulfide to improve the adverse effect on delayed fracture resistance, 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, even if the content is more than 0.0050%, the effect is saturated, so the content is made 0.0050% or less. More preferably, it is 0.0035% or less.

  The balance other than the above is Fe and unavoidable impurities. As an unavoidable impurity, Sb, Zn, Co, Sn, Zr etc. are mentioned, for example, As an allowable range of these contents, 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 is not lost.

  Next, the microstructure of the plated steel sheet of the present invention will be described in detail. In the present invention, ferrite is 35% to 70% by volume fraction, retained austenite is 12% or less (including 0%) by volume fraction, martensite is 15% to 60% by volume fraction, and the balance is bainite Of not more than 30% (including 0%) by volume fraction and not more than 5% (including 0%) of unrecrystallized ferrite by volume fraction, average grain size of ferrite is not more than 5 μm, retained austenite The average grain size of martensite is 2 μm or less, the average grain size of martensite is 2 μm or less, and the average grain size of bainite is 3 μm or less. The volume fraction mentioned here is a volume fraction to the whole steel plate, and so on.

35% or more and 70% or less by volume fraction of ferrite When the volume fraction of ferrite is more than 70%, it is difficult to achieve a tensile strength of 980 MPa or more. Therefore, the volume fraction of ferrite is 70% or less. Preferably it is 65% or less, More preferably, it is 60% or less. When the volume fraction is less than 35%, the second phase with a high dislocation density is increased, and the delayed fracture resistance is degraded. Therefore, the volume fraction of ferrite is 35% or more. In order to improve elongation, it is preferably 40% or more.

When the average grain size of ferrite is 5 μm or less and the average grain size of ferrite is more than 5 μm, the crystal grains are further coarsened during resistance welding, whereby the toughness is degraded and internal cracking occurs. Therefore, the crystal grain size of ferrite is 5 μm or less. Preferably, it is 4 μm or less. In order to improve elongation, it is preferably 0.5 μm or more.

12% or less of retained austenite by volume fraction Retained austenite contributes to strength by processing-induced martensitic transformation. In addition, since it is a hydrogen trap site, it is also effective for delayed fracture resistance. However, when martensitic transformation is performed, a crack is formed due to hydrogen penetration in order to maintain a high dislocation density, and the delayed fracture resistance becomes inferior. Therefore, the volume fraction of retained austenite is 12% or less. Preferably it is more than 0% and 10% or less. More preferably, it is 1% or more. More preferably, it is 7% or less. The volume fraction of retained austenite may be 0%.

The average grain size of retained austenite is 2 μm or less The average grain size of retained austenite is likely to form martensite at the time of press forming due to the influence of C distribution in retained austenite, and the delayed fracture resistance is deteriorated. The upper limit is 2 μm. Although the lower limit is not particularly specified, the contribution to the elongation becomes large when the lower limit is 0.3 μm or more, and the lower limit is preferably 0.3 μm or more.

The volume fraction of martensite is 15% or more and 60% or less In order to ensure the desired strength, the volume fraction of martensite is 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 the crack formation is likely to occur at the time of hydrogen penetration, but also the crack growth rate increases, so the upper limit is made 60%. Preferably it is 57% or less. More preferably, it is 55% or less.

When the average grain size of martensite is 2 μm or less and the average grain size of martensite is more than 2 μm, the toughness is deteriorated by the coarsening of crystal grains during resistance welding, and internal cracking occurs. Therefore, the average grain size of martensite is 2 μm or less. Preferably, it is 1.8 μm or less. The term "martensite" as used herein refers to martensite formed after annealing, but is self-tempered (auto-tempered) martensite transformed after martensitic transformation during annealing cooling, tempered martensite tempered after martensitic transformation. And fresh martensite transformed from austenite to martensite without being tempered.

As the balance, bainite is 30% or less by volume fraction. Although bainite contributes to high strength, it contains high dislocation density, so the delayed fracture resistance is deteriorated if the volume fraction is more than 30%. Therefore, the upper limit is 30%. Preferably it is more than 0% and 25% or less. More preferably, it is 5% or more. More preferably, it is 20% or less. The bainite volume fraction may be 0%.

When the average grain size of bainite is 3 μm or less, when the average grain size of bainite is more than 3 μm, the toughness is deteriorated due to the further coarsening of crystal grains during resistance welding, and internal cracks occur, so the average grain size of bainite is 3 μm or less I assume. Preferably, it is 2.5 μm or less.

5% or less by volume fraction of non-recrystallized ferrite as the remaining portion Non-recrystallized ferrite also contributes to high strength, but since it includes high dislocation density like bainite, its upper limit is 5%. Preferably it is more than 0% and 3% or less. More preferably, it is 1% or less. The volume fraction of unrecrystallized ferrite may be 0%.

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

In the present invention, at least 30 Ti or Nb-based precipitates having an average particle size of 0.10 μm or less per 100 μm ^{2 In the} present invention, 30 or more Ti / Nb-based precipitates having an average particle size of 0.10 μm or less per 100 μm ² Needs to be included. This is because Ti or Nb-based precipitates become trap sites of hydrogen to improve delayed fracture resistance, and are effective for grain refinement, and improve resistance weld cracking resistance. If the particle size is more than 0.10 μm, or if the number of the precipitates is less than 30 per 100 μm ^{2 on} average, the delayed fracture resistance and the resistance weld cracking resistance are deteriorated. Preferably it is 50 or more per 100 μm ² . More preferably, it is 60 or more per 100 μm ² . Specific examples of Ti or Nb-based precipitates include carbides.

Next, the manufacturing conditions of the plated steel plate according to the present invention will be described.
The steel slab having the above-mentioned composition (chemical composition) is subjected to hot rolling under conditions of finish temperature of 850 ° C. or more and 950 ° C. or less at finish rolling temperature, and 680 at a first average cooling rate of 75 ° C./s or more as primary cooling. After cooling to a temperature of at most 2 ° C, it is cooled in a range of 400 ° C to 580 ° C at a second average cooling rate of 5 ° C / s or more as secondary cooling and then wound, and the hot rolled steel sheet is pickled, Subsequently, cold rolling is performed, and then in the annealing step, the cold-rolled steel plate is heated to a temperature range of 760 ° C. to 900 ° C. at an average heating rate of 3 to 30 ° C./s. After holding for 15 seconds or more in a temperature range of 900 ° C. or less, the steel sheet is cooled to a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C./s, annealed, galvanized, and cooled to room temperature.

  In the hot rolling process, after casting, the steel slab is started hot rolling at 1150 ° C. or more and 1300 ° C. or less without reheating, or after reheating to 1150 ° C. or more and 1300 ° C. or less, hot rolling is performed. It is preferable to start. The steel slabs used are preferably produced by continuous casting to prevent macrosegregation of the components, but can also be produced by ingot casting or thin slab casting. In the present invention, after the steel slab is manufactured, it is cooled to room temperature and then added to the conventional method of reheating, without being cooled, after being inserted into the heating furnace as a hot piece, or after performing heat retention It is also possible to apply an energy saving process such as direct feed rolling or direct rolling in which rolling is performed immediately or after rolling as it is, without any problem.

[Hot rolling process]
Finishing rolling finish temperature: 850 ° C. or more and 950 ° C. or less Hot rolling is to improve delayed fracture resistance characteristics after annealing and resistance welding crack resistance characteristics by homogenizing the structure in the steel sheet and reducing the anisotropy of the material, It is necessary to finish in the austenite single phase region. Therefore, the finish rolling end temperature is set to 850 ° C. or more. On the other hand, when the finish rolling finish temperature exceeds 950 ° C., the hot-rolled structure becomes coarse, and the crystal grains after annealing also become coarse. Therefore, the finish rolling end temperature is set to 850 ° C. or more and 950 ° C. or less.

・ Cooling conditions after finish rolling
After cooling to 680 ° C or less at a first average cooling rate of 75 ° C / s or more as primary cooling, cooling to a range of 400 ° C to 580 ° C at a second average cooling rate of 5 ° C / s or more as secondary cooling In the present invention, cooling after finish rolling is an important step in order to control the steel sheet structure after annealing by controlling the precipitation form of Ti or Nb precipitates during hot rolling. After the completion of hot rolling, austenite undergoes ferrite transformation in the cooling process, but at high temperatures, the ferrite becomes coarse. Therefore, by quenching the steel sheet after completion of the hot rolling, the structure is homogenized as much as possible, and at the same time, the formation of precipitates is suppressed. Therefore, cooling is performed to 680 ° C. or less at a first average cooling rate of 75 ° C./s or more as primary cooling.

If the first average cooling rate is less than 75 ° C./s, the ferrite is coarsened, so that the steel sheet structure of the hot rolled steel sheet becomes inhomogeneous, and the resistance weld cracking characteristics deteriorate. In addition, the desired average grain size can not be obtained for ferrite, martensite and bainite. Preferably, the temperature is 85 ° C./s or more.
When the temperature to be cooled by the primary cooling exceeds 680 ° C., pearlite in the steel sheet structure of the hot rolled steel sheet is generated excessively and the steel sheet structure becomes inhomogeneous, so that the resistance weld cracking characteristics deteriorate. In addition, a desired average grain size can not be obtained for martensite. Preferably, the temperature is 650 ° C. or less. In addition, since martensite excessively increases in the steel sheet structure of the heat-rolled steel sheet, the temperature is preferably 400 ° C. or higher.

The subsequent secondary cooling is performed at a second average cooling rate of 5 ° C./s or more to a range of 400 ° C. or more and 580 ° C. or less. When cooling to less than 5 ° C./s or more than 580 ° C., ferrite or pearlite is excessively formed in the steel sheet structure of the hot-rolled steel sheet, and resistance weld cracking resistance characteristics after annealing are degraded. Further, with regard to martensite, a desired average crystal grain size can not be obtained, and 30 or more Ti or Nb-based precipitates having an average crystal grain size of 0.10 μm or less can not be obtained on average per 100 μm ² . Preferably, it is 10 ° C./s or more. In addition, Ti and Nb are dissolved in excess, so the temperature is preferably 65 ° C./s or less.

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

[Pickling process]
After the hot rolling step, it is preferable to carry out a pickling step to remove the scale on the surface of the hot rolled sheet. The pickling step is not particularly limited, and may be carried out according to a conventional method.

[Cold rolling process]
A cold rolling process is performed to roll a cold-rolled sheet having a predetermined thickness. The cold rolling process is not particularly limited and may be carried out by a conventional method. The preferable range of the rolling reduction in cold rolling is 30% or more and 95% or less.

[Annealing process]
The annealing step is carried out to promote recrystallization and to form fine bainite, retained austenite and martensite in the steel sheet structure for high strength. Therefore, in the annealing step, after heating to a temperature range of 760 ° C. or more and 900 ° C. or less 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. or more and 900 ° C. or less It cools to a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C./s.
In addition, you may implement temper rolling after annealing. The preferred range of elongation is 0.05% to 2.0%.

・ Average heating rate: 3 to 30 ° C / s
By setting the average heating rate to 3 to 30 ° C./s, it is possible to refine the crystal grains after annealing. Rapid heating makes it difficult for recrystallization to proceed. In addition, a desired average grain size can not be obtained for bainite, and a desired volume fraction for non-recrystallized ferrite, and an average of 30 or less Ti or Nb-based precipitates having an average crystal grain size of 0.10 μm or less per 100 μm ² You can not get more than one. Therefore, the upper limit of the average heating rate is 30 ° C./s. In order to increase unrecrystallized ferrite, the temperature is preferably 25 ° C./s or less.
Also, if the heating rate is too low, ferrite and martensite grains become coarse and a predetermined average grain size can not be obtained. In addition, 30 or more Ti or Nb-based precipitates having an average crystal grain size of 0.10 μm or less can not be obtained on average per 100 μm ² . Therefore, an average heating rate of 3 ° C./s or more is required. Preferably it is 5 degrees C / s or more.

Soaking temperature (holding temperature): 760 ° C. or more and 900 ° C. or less As the soaking temperature, soaking is performed in a temperature range that is a two-phase region or austenite single-phase region of ferrite and austenite. If the temperature is less than 760 ° C., it is difficult to secure strength because the ferrite fraction increases. In addition, the desired average grain size can not be obtained for ferrite and martensite. Therefore, the lower limit of the soaking temperature is 760 ° C. Preferably, the temperature is 780 ° C. or higher. When the soaking temperature is too high, crystal grain growth of ferrite, martensite and austenite becomes remarkable, and the crystal grain becomes coarse, whereby the resistance welding crack characteristic is deteriorated. In addition, 30 or more Ti or Nb-based precipitates having an average crystal grain size of 0.10 μm or less can not be obtained on average per 100 μm ² . Therefore, the upper limit of the soaking temperature is 900 ° C. Preferably it is 880 degrees C or less.

Soaking time: 15 seconds or more The soaking time needs to be maintained for 15 seconds or more in order to cause austenite transformation of the progress and part or all of recrystallization at the above soaking temperature. In order to increase the volume fraction of unrecrystallized ferrite, preferably 20 seconds or more. The upper limit is not particularly limited, but is preferably within 600 seconds.

Cooling conditions during annealing: cooling to a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C./s After the above soaking, from the soaking temperature to a temperature range of 600 ° C. or less (cooling stop temperature), It is necessary to cool at an average cooling rate of 3 to 30 ° C./s. If the average cooling rate is less than 3 ° C./s, ferrite transformation proceeds during cooling, and the volume fraction of the second phase decreases, so it is difficult to secure strength. Also, the desired average grain size can not be obtained for martensite, retained austenite and bainite. On the other hand, when the average cooling rate exceeds 30 ° C./s, not only martensite is generated in excess but it is also difficult to achieve this on the equipment. Further, when the cooling stop temperature exceeds 600 ° C., pearlite is excessively generated, so that a predetermined volume fraction in the microstructure of the steel sheet can not be obtained, and it is difficult to secure strength. In addition, 30 or more Ti or Nb-based precipitates having an average crystal grain diameter of 0.10 μm or less can not be obtained on average per 100 μm ² , and the delayed fracture resistance and the resistance weld cracking resistance are degraded. The above-mentioned average cooling rate is the average of the cooling rates in the range up to immersing in the plating bath of 600 ° C. or less, and an average cooling rate of 3 to 30 ° C./s may be maintained in this temperature range. .

[Plating treatment]
After the above-mentioned annealing, plating treatment is performed and it is cooled to room temperature. The plating treatment includes hot-dip galvanizing treatment, galvanizing treatment and the like. In the case of the hot dip galvanization process, for example, it is preferable to set the steel plate temperature immersed in a plating bath to (hot dip galvanization bath temperature-40) ° C-(hot dip galvanization bath temperature + 50) ° C. When the temperature of the steel sheet immersed in the plating bath falls below (hot-dip galvanization bath temperature-40) ° C, when the steel sheet is immersed in the plating bath, part of the molten zinc solidifies and the plating appearance is degraded There is. Therefore, the lower limit is set to (hot-dip galvanizing bath temperature-40) ° C. In addition, if the temperature of the steel sheet immersed in the plating bath exceeds (hot-dip galvanization bath temperature + 50) ° C, the temperature of the plating bath rises, which causes a problem in mass productivity. For the other conditions, the conditions used in normal plating can be used.

[Alloying treatment]
After the plating, the plating can be alloyed in a temperature range of 450 ° C. or more and 600 ° C. or less. By performing the alloying treatment in a temperature range of 450 ° C. to 600 ° C., the Fe concentration in the plating becomes 7 to 15 mass%, and the adhesion of the plating and the corrosion resistance after coating are improved. If the temperature is less than 450 ° C., alloying does not proceed sufficiently, resulting in a decrease in sacrificial corrosion resistance and sliding property, and at temperatures higher than 600 ° C., the progress of alloying becomes excessive and the powdering resistance decreases. .

  The conditions of the other manufacturing methods are not particularly limited, but from the viewpoint of productivity, a series of treatments such as the above annealing, plating treatment, and alloying treatment of plating are continuous melting when hot dip galvanizing treatment is performed. It is preferable to carry out by a zinc plating line (CGL). Moreover, it is preferable to use the zinc-plating bath which contains 0.10-0.20 mass% of Al for hot dip galvanization. After plating, wiping can be performed to adjust the plating amount of plating.

(Example)
Hereinafter, examples of the present invention will be described.
However, the present invention is of course not limited by the following examples, and can be implemented with appropriate modifications as long as they are compatible with the spirit of the present invention, and all of them are technical of the present invention. It is included in the range.
The steels with the composition shown in Table 1 are melted and cast to produce a slab, and hot rolling is performed under the conditions that the hot rolling heating temperature is 1250 ° C and the finish rolling end temperature (FDT) is shown in Table 2 Plate thickness: 3.2 mm hot-rolled steel plate, and after cooling to the first cooling temperature at the first average cooling rate (cold rate 1) shown in Table 2, the second average cooling rate (cold rate 2) 2 Cooled to cooling temperature and wound at winding temperature (CT). Next, the hot-rolled sheet obtained was pickled and cold-rolled to produce a cold-rolled sheet (thickness: 1.4 mm).

The cold-rolled steel sheet thus obtained is annealed in a continuous galvanizing line according to the manufacturing conditions shown in Table 2 and subjected to hot-dip galvanizing treatment, and then alloying treatment is carried out at the temperature shown in Table 2. An alloyed galvanized steel sheet (GA) was obtained. Here, the plating process is performed using a zinc plating bath temperature: 460 ° C., a zinc plating bath Al concentration: 0.14 mass% (in the case of alloying treatment), 0.18 mass% (in the case of no alloying treatment), plating adhesion per side The amount was 45 g / m ² (double-sided plating). In addition, in some steel plates, it was set as the non-alloy hot-dip galvanized steel plate (GI), without alloying of galvanization.

From the manufactured steel plate, a JIS No. 5 tensile test specimen 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)).
For delayed fracture test, using the test piece obtained by cutting the obtained cold-rolled steel plate to 30 mm × 100 mm with the rolling direction as the longitudinal direction and grinding the end face, bending the test piece at 180 ° with a curvature radius of 10 mm of the punch tip gave. The springback generated in this bent test piece is tightened by bolts so that the inner gap is 20 mm, stress is applied to the test piece, and then it is immersed in hydrochloric acid at 20 ° C and pH = 1.5, and breakage occurs The time until it occurred was measured up to 100 hours. In the case where a crack did not occur in the test piece within 100 hours, it was regarded as good (○), and in the case where a crack occurred in the test piece, it was regarded as inferior (×).

  For the test of resistance welding cracking, resistance welding (spot welding) was performed using a test piece cut into 50 × 150 mm with the direction perpendicular to the rolling direction of the obtained cold rolled steel sheet as a longitudinal direction. For welding, resistance spot welding is performed on a plate set in which two steel plates are stacked, using a servomotor pressure type single-phase direct current (50 Hz) resistance welder attached to a welding gun with the plate set inclined at 4 °. Carried out. The welding conditions were a pressure of 3.5 kN and a hold time of 0.36 seconds. The welding current and welding time were adjusted so that the nugget diameter was 5.9 mm. After welding, the test piece is cut in half, the cross section is observed with an optical microscope, and those with no cracks of 0.2 mm or more are found to have good resistance crack resistance characteristics (○), cracks of 0.2 mm or more are found The resistance cracking resistance was evaluated as poor (×).

  The volume fraction of ferrite, martensite and non-recrystallized ferrite of the steel plate corroded with 3 vol% nital after polishing the plate thickness section parallel to the rolling direction of the steel plate, and 2000 times using SEM (scanning electron microscope) The area ratio was measured by a point count method (based on ASTM E562-83 (1988)), and the area ratio was defined as a volume fraction. The average grain size of ferrite and martensite is the area of each phase by taking in the photograph which identified each ferrite and martensitic crystal grain beforehand from the steel plate structure photograph using Image-Pro of Media Cybernetics. The circle equivalent diameter was calculated, and those values were averaged to obtain.

  The volume fraction of retained austenite was obtained by grinding a steel plate to a quarter surface in the plate thickness direction, and determining it from the diffracted X-ray intensity of the quarter surface of the plate thickness. A {200} plane, a {211} plane, a {220} plane, and austenite of iron ferrite by X-ray diffraction method (apparatus: RINT 2200 manufactured by Rigaku) at an accelerating voltage of 50 keV using a Kα ray of Mo as a radiation source Measure the integrated intensities of X-ray diffraction lines of {200}, {220}, and {311} planes of the surface, and use these measured values to read “X-ray Diffraction Handbook” (2000) Rigaku Denki Co., Ltd. The volume fraction of retained austenite was determined from the formulas described on pages 26 and 62-64.

The average grain size of retained austenite is observed at a magnification of 5000 using EBSD (electron beam backscattering diffraction), the equivalent circle diameter is calculated using Image-Pro, and those values are averaged. I asked for.
Further, the steel sheet structure was observed by SEM, TEM (transmission electron microscope) and FE-SEM (field emission scanning electron microscope), and bainite was observed to determine the volume fraction in the same manner as described above. The average crystal grain size of bainite was also calculated by calculating the equivalent circle diameter from the steel sheet structure photograph using the above-mentioned Image-Pro, and averaging those values.

  The particle size of Ti or Nb precipitates is observed at a magnification of 5000 times, 10000 times and 20000 times using SEM and TEM, and the particle size is determined by calculating the equivalent circle diameter using Image-Pro. The The number of Ti or Nb-based precipitates was observed at a magnification of 5000 times, 10000 times and 20000 times using SEM and TEM, and the average number of ten spots was determined.

The measured results of the measured tensile properties, delayed fracture properties, resistance weld cracking properties and steel sheet structure are shown in Table 3.
From the results shown in Table 3, each of the examples of the present invention has ferrite with an average crystal grain size of less than 5 μm by 35 to 70% by volume fraction and residual austenite of average crystal grain size of 2 μm or less by 12% by volume 15% to 60% by volume fraction of martensite having an average particle size of 2 μm or less and 30% or less by volume fraction of bainite having an average particle size of 3 μm or less and 5% or less by volume fraction of non-recrystallized ferrite Containing a composite structure, and as a result, a tensile strength of 980 MPa or more can be obtained, no fracture occurs for 100 hours in the delayed fracture property evaluation test, and excellent delayed fracture resistance properties, and even after resistance welding It was confirmed that no internal cracking occurred and that excellent resistance weld cracking was obtained.

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% to 3.35%,
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 is further contained,
Ti: containing at least one selected from 0.005% or more and 0.090% or less and Nb: 0.005% or more and 0.090% or less, and the balance having a component composition which is Fe and an unavoidable impurity,
35% or more and 70% or less of ferrite, 12% or less of retained austenite by volume fraction, 15% or more and 60% or less of martensite, and 30% or less of bainite as a balance Has a structure containing 5% or less by volume fraction of unrecrystallized ferrite,
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,
The average grain size of the martensite is 2 μm or less,
The average grain size of the bainite is 3 μm or less,
The plated steel sheet containing 30 or more of Ti or Nb-based precipitates having an average particle diameter of 0.10 μm or less on average per 100 μm ^{2 in the} structure. The component composition is 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: The plated steel plate according to claim 1, containing one or more selected from 0.0050% or less. 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% to 3.35%,
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: containing 0.0002% or more and 0.0045% or less, and further, in mass%,
Ti: containing one or more selected from 0.005% or more and 0.090% or less, Nb: 0.005% or more and 0.090% or less, and the remainder being an end of finish rolling on a steel slab having a component composition of Fe and unavoidable impurities Hot rolling is performed at a temperature of 850 ° C. or more and 950 ° C. or less to form 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 is cooled to a range of 400 ° C. to 580 ° C. at a second average cooling rate of 5 ° C./s or more , Cold rolled and 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, held for 15 seconds or more in a temperature range of 760 ° C. to 900 ° C., and homogenized Annealing to cool to a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C./s,
Subjecting the cold-rolled steel sheet after the annealing to plating treatment;
35% or more and 70% or less of ferrite, 12% or less of retained austenite by volume fraction, 15% or more and 60% or less of martensite, and 30% or less of bainite as a balance It has a structure containing 5% or less of non-recrystallized ferrite in volume fraction, 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, the average grain size of the martensite Is 2 μm or less, and the average grain size of the bainite is 3 μm or less, and the structure contains 30 or more Ti or Nb-based precipitates having an average grain size of 0.10 μm or less on average per 100 μm ² Method of manufacturing steel plate. The component composition is 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: The manufacturing method of the plated steel plate of Claim 3 containing 1 type (s) or 2 or more types selected from 0.0050% or less. The manufacturing method of the plated steel plate of Claim 3 or 4 which performs the alloying process of plating in the temperature range of 450 degreeC or more and 600 degrees C or less after performing the said plating process.