JP2003221623A - Method for manufacturing high-strength cold-rolled steel sheet and hot-dip galvanized high-strength steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a cold-rolled and hot-dip galvanized steel sheet with high strength, which acquires excellent impact resistance after being press-formed, coated and baked. <P>SOLUTION: This manufacturing method comprises finish rolling a steel comprising 0.02-0.15% C, 0.7% or less Si, 2.0-4.0% Mn, 0.1% or less P, 0.01% or less S, 0.1% or less sol.Al, 0.005% or less N, 0.01-0.1% Nb, and the balance substantially iron, at the Ar<SB>3</SB>point or higher, while total reduction r(%) and an average strain rate v(s<SP>-1</SP>) in the temperature range of 960 to 860°C satisfies the expression: 30≤r≤-150(Nb+C)+76-200(Nb+C)+68≤v≤-200(Nb+C)+128, winding it at 650°C or lower, making the metal structure be a composite structure containing 20-80% of a structure transformed from non-recrystallized austenite, then cold rolling it, and annealing it at 880°C or less for 120-240 seconds to make the composite structure with an average particle diameter of 5 μm or less. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength cold-rolled steel sheet excellent in impact resistance and a method for producing a high-strength galvanized steel sheet.

[0002]

2. Description of the Related Art In recent years, in the manufacture of automobiles, when the automobile collides with an object while it is running, the energy absorption capacity of the members against impacts is increased, and the impact load on the occupants is reduced, thereby increasing the safety of the occupants. It is considered to increase.
In order to improve the collision safety performance of such automobiles and to reduce the weight of the vehicle body, application of high strength steel plates to various automobile parts such as members and lockers is being promoted.

In order to meet the demands of automobiles from the viewpoint of impact resistance, for example, Japanese Patent Laid-Open No. 9-25538 discloses a high-strength cold-rolled steel sheet and a high-strength molten zinc excellent in perforation corrosion resistance and crushing property. A method of manufacturing a plated steel sheet is disclosed. This technology is to specify the chemical composition of the steel sheet and the volume fraction of the microstructure to obtain a steel sheet having a tensile strength of 500 N / mm ² or more and excellent puncture corrosion resistance and crushing properties. .

[0004]

[Problems to be Solved by the Invention] However, JP-A-9-2553
Regarding the crushing property, the technique described in Japanese Patent No. 8 does not explain the test method itself, let alone the shape of the sample used, and does not specifically show the evaluation method. Therefore, it is unclear how the characteristics of the material affect the crushing characteristics of the press-molded parts. In addition, in actual automobile parts, paint baking is often performed after press molding, and it is considered that the impact resistance performance of actual parts can be reflected by the above-mentioned conventional technology that does not assume such a thermomechanical treatment step. Hard to think.

The structure of this cold-rolled steel sheet according to the prior art is composed of at least 15% by volume of ferrite and at least one of the low-temperature transformation structures of martensite, tempered martensite and bainite at the balance. It will be. This is because if the volume ratio of ferrite exceeds 15% by volume, a predetermined strength cannot be obtained, and the difference between the ferrite and the second phase becomes large, and bending workability deteriorates.

In this example of the prior art, the second phase is martensite in all the samples, and the volume fraction of ferrite is mostly 0 to 6%. However, with such a material that can be said to be almost a single phase of martensite, strength can be easily obtained, but press formability cannot be generally expected. Further, it is not necessarily suitable for applications such as impact resistance in which the energy absorption capacity after the start of deformation is a problem.

Therefore, in the present invention, the above problems are solved, excellent impact resistance performance is obtained after press forming and paint baking, and a high strength cold rolled steel sheet and a high strength molten zinc having a tensile strength of 780 MPa or more are obtained. An object is to provide a method for manufacturing a plated steel sheet.

[0008]

The above problems can be solved by the following inventions. The invention is that the chemical composition is mass%
C: 0.02-0.15%, Si: 0.7% or less, Mn: 2.0-4.0%, P: 0.1% or less, S: 0.01% or less, sol.Al: 0.1% or less, N: 0.005% or less, N
b: a step of smelting and casting a steel containing 0.01 to 0.1% and the balance substantially consisting of iron, and rough-rolling the cast slab and then performing finish rolling, at 960 to 860 ° C. The total rolling ratio r (%) and the average strain rate v (s ^-1 ) in the temperature range satisfy the following inequality, and finish rolling is completed at a temperature of Ar ₃ points or higher, and then coiled at a temperature of 650 ° C or lower. Winding, hot rolling process to produce a hot rolled steel sheet consisting of a composite structure containing 20-80% of the structure transformed from unrecrystallized austenite, and the obtained hot rolled steel sheet is subjected to pickling and cold rolling. Applying process and 880 ℃
And an annealing step of annealing for 120 to 240 seconds at the following temperature,
A method for producing a high-strength cold-rolled steel sheet, wherein the structure is a composite structure having an average crystal grain size of 5 μm or less.

30 ≦ r ≦ −150 (Nb + C) + 76-200 (Nb + C) + 68 ≦ v ≦
-200 (Nb + C) +128 Here, the symbol of an element in a formula shows mass% of each.

Further, in the production method of the present invention, as chemical components, mass% Ti: 0.01-0.1%, V: 0.01-0.
3%, B: 0.0002 to 0.002%, Cr: 0.05 to 0.5%, Mo: 0.05 to 0.5%
It is also possible to provide a method for producing a high-strength cold-rolled steel sheet, characterized by containing at least one of these.

The present invention was made on the basis of the finding found as a result of intensive studies in order to obtain a high-strength cold-rolled steel sheet excellent in impact resistance. That is, in the case of a composite microstructure steel sheet containing ferrite and martensite, a hard low-temperature transformation phase such as bainite, or retained austenite, it is possible to uniformly fine-grain those structures, and after press forming, a coating baking treatment is performed. It is effective in improving impact resistance when applied to various automobile parts (structural parts such as members and lockers).

Based on this finding, the present invention makes it possible to obtain a high-strength cold-rolled steel sheet and a high-strength galvanized steel sheet having a tensile strength of 780 MPa or more and excellent in impact resistance after press forming. Below, the reason for adding the steel component of the present invention,
The reasons for limiting the components, the morphology of the structure, and the reasons for limiting the production conditions will be described. The following% indicates mass%.

(1) Steel composition range C: 0.02 to 0.15% C is an element effective for strengthening steel, and an addition amount of 0.02% or more is required to obtain strengthening ability. However, the C content is 0.
If it exceeds 15%, a band structure develops remarkably in the rolling direction of the steel sheet, and such a non-uniform structure in the sheet thickness direction remarkably deteriorates the press formability and the impact resistance performance of the member after baking treatment. Therefore, the amount of C is set within the range of 0.02 to 0.15%.

Si: 0.7% or less Si is an element effective for strengthening steel and can be added as appropriate. However, if the amount of Si exceeds 0.7%, the adhesion of the plating is significantly deteriorated when hot dip galvanizing is performed, and a uniform plating film cannot be obtained. Therefore, the Si content is 0.7% or less.

Mn: 2.0 to 4.0% Mn is an element effective in quench hardening of steel and needs to be added in a sufficient amount. If the addition amount of Mn is less than 2.0%, pearlite is easily generated from austenite in the cooling step after heating, and hard low-temperature transformation phase such as martensite and bainite and retained austenite are difficult to be obtained, and thus 780M
Stable strength over Pa cannot be obtained. On the other hand, the Mn content is 4.0%
If it exceeds, Mn-based inclusions are often formed in the steel, and the ductility of the steel sheet is significantly deteriorated. Therefore, the amount of Mn is 2.0-4.0%
Within the range of.

P: 0.1% or less P is an element effective for strengthening steel and can be added as appropriate. However, if the P content exceeds 0.1%, the toughness of the steel sheet deteriorates significantly. Further, when the hot dip galvanizing treatment is performed, the adhesion of the plating is significantly deteriorated. For this reason,
The amount of P should be 0.1% or less.

S: 0.01% or less If S is excessively present in the steel, it segregates at the austenite grain boundaries during slab heating, so that red hot embrittlement easily occurs during hot rolling. In particular, if the S content exceeds 0.01%, this adverse effect is a concern. Therefore, the S content is 0.01% or less.

Sol.Al: 0.1% or less If Al is excessively present in the steel, a large amount of Al-based inclusions are produced, so that the ductility of the steel sheet deteriorates. Especially when the amount of Al added is
If it exceeds 0.1%, this adverse effect is a concern. For this reason,
The Al content is 0.1% or less.

N: 0.005% or less If N is excessively present in the steel, cracks will occur on the surface of the slab during casting. In particular, if the N content exceeds 0.005%, this adverse effect is a concern. Therefore, the N content is 0.005% or less.

Nb: 0.01 to 0.1% Nb acts as a solid solution Nb in steel and forms fine carbides with C, thereby delaying recrystallization of austenite during hot rolling to refine austenite. It is effective for refining the structure of hot-rolled sheet of ferrite, pearlite, etc. which has contributed to the transformation. Also, after cold rolling, even in the heating step during annealing, the solid solution Nb or Nb-based carbide is effective for refining the ferrite recrystallized grains, from which the reverse transformed austenite is refined, and further thereafter. It also contributes greatly to the miniaturization of the ferrite + low temperature transformation phase formed during cooling. To get these effects, set Nb to 0.
It is necessary to add more than 01%.

However, if the amount of Nb exceeds 0.1%, the recrystallization temperature of ferrite is significantly increased during annealing, and the work structure is likely to remain in the structure of the steel sheet after annealing, so that the ductility of the obtained steel sheet is remarkably high. to degrade. Therefore, the Nb content is 0.01-0.1%
Within the range of.

Ti: 0.01 to 0.1% when added Ti contributes to the refinement of the hot rolled sheet structure and the steel sheet structure after annealing by forming fine carbides or fine nitrides with C or N in the steel. . To get this effect, set Ti to 0.
It is necessary to add more than 01%. However, if the amount of Ti exceeds 0.1%, the recrystallization temperature of ferrite rises remarkably during annealing, and the work structure tends to remain in the structure of the annealed steel sheet, so the ductility of the obtained steel sheet deteriorates significantly. For this reason,
When Ti is added, its addition amount is within the range of 0.01 to 0.1%.

V: 0.01 to 0.3% when added V is an element effective for strengthening steel, and the nitride formed by V contributes to grain refinement of the annealed steel sheet structure. In order to obtain these effects, V needs to be added in an amount of 0.01% or more.
However, when the V content exceeds 0.3%, these effects saturate. Therefore, when adding V, the addition amount should be 0.01
Within 0.3%.

B: 0.0002 to 0.002% when added B segregates at the austenite grain boundaries during hot rolling and suppresses austenite grain growth, thereby reducing the grain size of the structure such as ferrite and pearlite to be transformed. It works effectively. Also, after cold rolling, even in the heating step during annealing, segregation from pearlite to grain boundaries of reverse transformed austenite contributes to austenite grain refinement, and further low temperature transformation formed in the subsequent cooling process. It is also effective for finer phases. However, when the amount of B exceeds 0.002%, the effect of grain refining becomes saturated. Therefore, when B is added, the addition amount is set to be in the range of 0.0002 to 0.002%.

Cr: 0.05 to 0.5% when added Cr is an element effective in quench hardening of steel, and it is necessary to add 0.05% or more to obtain this effect. However, when the Cr content exceeds 0.5%, this effect saturates. Therefore, Cr
When adding, the addition amount is within the range of 0.05 to 0.5%.

Mo: 0.05 to 0.5% when added Mo is an element effective in quench hardening of steel, and it is necessary to add 0.05% or more to obtain this effect. However, when the amount of Mo exceeds 0.5%, this effect is saturated. Therefore, Mo
When adding, the addition amount is within the range of 0.05 to 0.5%.

Unless the chemical components other than the above-mentioned elements are added excessively, the effects of the present invention are not impaired. In the present invention, the balance is substantially iron,
For other alloying elements or inevitable impurities,
Unless adversely affecting the intended properties of the present invention,
It means that it may be contained.

(2) Microstructure of Steel Plate The microstructure of the steel plate is related to the impact crushing absorbed energy of the member after press forming and paint baking. As will be described later, if the coating baking process is performed under normal conditions of 170 ° C. for 20 min, the average grain size of ferrite and the second phase (low temperature transformation phase, retained austenite) exceeds 5 μm.
High absorbed energy above 0J cannot be obtained. Therefore, the average grain size of the ferrite and the second phase structure is set to 5 μm or less.

The high-strength cold-rolled steel sheet and the high-strength galvanized steel sheet of the present invention are intended to have excellent impact resistance,
It is a steel sheet in which predetermined components are adjusted as in (1) above and the structure is controlled as in (2) above, and can be manufactured by the following manufacturing method.

(3) Manufacturing method of steel sheet Total rolling reduction r (%) in the temperature range of 960 to 860 ° C. during finish rolling: 30
≦ r ≦ −150 (Nb + C) +76 By applying appropriate reduction in this temperature range, it is possible to secure the required volume ratio of the structure transformed from unrecrystallized austenite and to refine the structure. You can Total rolling ratio (total reduction ratio) in this temperature range
When r (%) is less than 30%, the volume fraction of the structure transformed from unrecrystallized austenite in the stage of hot-rolled sheet structure is as low as less than 20%, and cold rolling, ferrite and second phase structure after annealing The average crystal grain size of is not less than 5 μm. Therefore, the impact crush absorption energy of the member after press molding and paint baking treatment at 170 ° C. × 20 min cannot obtain a high characteristic value of 10,000 J or more.

On the other hand, when the total rolling reduction in the temperature range of 960 to 860 ° C exceeds [-150 (Nb + C) +76]%, the volume fraction of the structure transformed from unrecrystallized austenite is 80%. And a work structure is observed in the structure after cold rolling and annealing, so that the ductility of the steel sheet is significantly deteriorated. Therefore, the total rolling ratio r (%) in the temperature range of 960 to 860 ° C. during finish rolling is set within the range represented by the inequality formula 30 ≦ r ≦ −150 (Nb + C) +76 (1).

Average strain rate v (s ^-1 ): -200 (Nb + C) + 68≤
v ≦ -200 (Nb + C) +128 When the average strain rate in the above temperature range (960 to 860 ° C) during finish rolling is less than [-200 (Nb + C) +68] s ^-1 , hot rolling The volume fraction of the structure transformed from unrecrystallized austenite in the stage of the plate structure is as low as less than 20%, and the average grain size of the structure after cold rolling and annealing does not fall below 5 μm.

On the other hand, the average strain rate is [-200 (Nb + C) +128]
If it exceeds s ^-1 , the volume ratio of the structure transformed from unrecrystallized austenite will exceed 80%, and there will be a worked structure in the structure after cold rolling and annealing, so the ductility of the steel sheet will increase. Remarkably deteriorates. Therefore, the average strain rate v (s ^-1 ) in the above temperature range (960 to 860 ° C) during finish rolling is calculated by the inequality -200 (Nb + C) + 68≤v≤-200 (Nb + C) +128 (2 ) Within the range indicated by.

The invention of the method for producing a high-strength cold-rolled steel sheet may be a method for producing a high-strength hot-dip galvanized steel sheet, which further comprises a step of performing hot dip galvanizing treatment after the annealing step. it can.

According to the present invention, in the production of the above-mentioned high-strength cold-rolled steel sheet, the surface of the steel sheet is subjected to hot dip galvanizing treatment after the annealing step and during the cooling from the annealing temperature.
Manufacture high strength galvanized steel sheet.

The high-strength cold-rolled steel sheet and the high-strength hot-dip galvanized steel sheet excellent in impact resistance intended by the present invention can be manufactured through the above manufacturing steps. Further, even if the steel sheet thus obtained is subjected to surface treatment such as electroplating and chemical conversion treatment, desired steel sheet characteristics are not impaired.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION In carrying out the invention, first, steel having the above chemical composition is melted and cast. The method for melting and casting steel is not particularly limited, as long as there is no segregation of components, especially nonuniformity of the structure. The cast slab may be hot-rolled immediately after casting, or may be once cooled and heated and then rolled. After rough rolling, finish rolling is performed and wound on a coil.

In the present invention, as a result of diligent examination of the microstructural factors for obtaining a steel sheet having excellent impact resistance, a composite microstructural steel sheet containing ferrite and a second phase (low-temperature transformation phase of austenite or retained austenite) was used. It was found that it is essential to make the structure fine and uniform, and for that purpose, it is important to make the structure finer from the hot-rolled sheet stage.

Next, since the material having a larger hard structure has a higher deformation resistance of the material itself, the deformation load becomes larger even at a high speed crushing deformation of the member after press molding, and the member becomes hard to be crushed. Impact performance is improved. However, since a member such as a rocker, which is required to have energy absorbing ability during crushing, is required to be easily crushed, a soft tissue having high plastic deformability is important as a material. Thus, in order to improve the impact resistance of the member, a composite structure steel sheet containing a low temperature transformation phase of soft ferrite and hard austenite (martensite or bainite) or retained austenite is promising as a material.

Further, during finish rolling, the rolling ratio and the average strain rate are controlled so that the hot-rolled sheet structure becomes a fine structure transformed from unrecrystallized austenite, so that the microstructure of the steel sheet structure after cold rolling and annealing is reduced. Try to change. The hot rolled sheet structure is controlled by slab heating conditions, hot rolling conditions, etc.
In order to reduce the grain size of the structure, it is effective to optimize the rolling conditions such as the rolling rate and strain rate on the trailing pass side of the finish rolling stand mill.

In particular, 960 in the austenite single phase region
By optimizing the rolling conditions (rolling rate, strain rate) in the temperature range of ~ 860 ° C, finishing rolling is completed at Ar ₃ points or more, and unrecrystallized austenite is formed during rolling. It was found that it is extremely important to transform the two-phase structure (pearlite, martensite, bainite, etc.).

Therefore, specific numerical values were obtained by experiments.
The details will be described below.

The steel sheet used in the experiment has a chemical composition of mass%
C: 0.03-0.08%, Si: 0.05-0.30%, Mn: 2.7-3.2%, P: 0.0
1-0.03%, S: 0.001-0.008%, sol.Al:0.04-0.06%, N:
Hot-rolled sheet (sheet thickness: 2.8 mm) obtained by rough rolling and finish rolling of a slab (sheet thickness: 30 mm) of 0.003 to 0.004%, Nb: 0.01 to 0.07%, Cr: 0.2 to 0.4%. After cold rolling to 1.2mm,
This is an annealed plate produced by annealing at 840 ° C and then temper rolling at an elongation of 0.5%.

Here, during finish rolling, the rolling rate and average strain rate in the temperature range of 960 to 860 ° C. are 10 to 80% and 70 to 9 respectively.
After changing to 0 s ^-1 , rolling was completed at 840 ° C. The structure of the hot-rolled sheet or the annealed sheet was observed with a scanning electron microscope in a vertical section parallel to the rolling direction. In the annealed plate,
For 200 randomly selected ferrites, the volume fraction of the second phase and the average grain size (annealed plate) were measured. In addition, the volume ratio of ferrite and the second phase was quantified.

With respect to the hot rolled sheet, the volume fraction of the structure transformed from unrecrystallized austenite was quantified. Next, from this annealed plate, materials with relatively small difference in strength and greatly different average particle diameters were press-formed into the hat shape shown in FIG. 1, and then 170 ° C. × 20 min equivalent to baking. After the heat treatment, the molded material was subjected to an impact crush test.

From the load-stroke curve obtained in the test, the amount of work (displacement energy of impact crushing) at a displacement of 0 to 130 mm was calculated, and the impact resistance performance of the member was evaluated by this characteristic value. The results of quantifying the microstructure are shown in Fig. 2.

The resulting annealed sheet has a TS of 830-900 MPa. Further, as is clear from the results of FIG. 2, the ferrite obtained in the final annealed plate, the average grain size of the second phase structure (low-temperature transformation phase of austenite or retained austenite) is 860 ~ 960 ℃ temperature range during finish rolling. There is a correlation with the rolling ratio and the Nb + C amount at. That is, when the rolling ratio is less than 30%, the transformation structure from unrecrystallized austenite is less than 20% in any hot-rolled sheet, and the average grain size of ferrite after annealing and the second phase is large as 6 to 10 μm. .

As the rolling ratio increases, the volume fraction of the transformed structure from unrecrystallized austenite increases and the average grain size of the annealed sheet structure decreases, but the upper limit of the rolling ratio is Nb + C amount. It is defined by the relational expression r = -150 (Nb + C) +76. In other words, the rolling rate is -15
When it is larger than 0 (Nb + C) +76, the volume fraction of the transformed structure from unrecrystallized austenite is as high as 81 to 90%, and the worked structure remains in the annealed sheet. On the other hand, when the rolling ratio is 30% or more, -150 (Nb
+ C) +76 or less, the volume ratio of the transformation structure from unrecrystallized austenite is 20 to 80% in the hot rolled sheet, and the average grain size of ferrite and the second phase is 5 μm or less in the annealed sheet. A grain structure is obtained.

Next, the results of evaluating the impact resistance performance of the members are shown in FIG. The steel plate used for evaluation has a TS of 830 to 860 MPa. As is clear from the results of FIG. 3, the impact absorption energy of the member is related to the average grain size of the material structure (ferrite and second phase), and the absorbed energy decreases as the grain size increases. In particular, when the particle size exceeds 5 μm, the absorbed energy decreases significantly. On the other hand, when the particle size is 5 μm or less, high absorbed energy is obtained.

Then, an appropriate range of hot rolling conditions (appropriate value of strain rate) for refining the annealed sheet structure was determined by the following experiment. The steel sheet used in the experiment has a mass% chemical composition.
C: 0.04-0.08%, Si: 0.10-0.25%, Mn: 2.8-3.0%, P: 0.0
1-0.03%, S: 0.003-0.005%, sol.Al:0.04-0.06%, N:
Slabs (sheet thickness: 30 mm) of 0.003 to 0.004%, Nb: 0.01 to 0.07%, Cr: 0.1 to 0.3% are rough-rolled and finish-rolled to obtain hot-rolled sheets (sheet thickness 2.8 mm). After cold rolling to 1.2mm,
It is an annealed plate produced by annealing at 830 ° C and then temper rolling at an elongation of 0.5%.

During the finish rolling, the rolling rate and the average strain rate in the temperature range of 960 to 860 ° C were changed to 45% and 20 to 190 s ^-1 , respectively, and the rolling was completed at 840 ° C. The volume fraction of the second phase and the average crystal grain size of the structures of the hot rolled sheet and the annealed sheet were determined by the same method as above. The results are shown in Figure 4.

The annealed sheet has a TS of 850-950 MPa. Further, as is clear from the results of FIG. 4, the average grain size of the annealed sheet structure correlates with the average strain rate and Nb + C amount during rolling in the temperature range of 960 to 860 ° C, and the average strain rate and Nb The appropriate particle size range can be specified by the relational expression with the + C amount. That is, when the average strain rate v is -200 (Nb + C) +68 or more and -200 (Nb + C) +128 or less, in the hot-rolled sheet, the volume fraction of the transformed structure from unrecrystallized austenite is The annealed plate has a fine grain structure in which the average grain size of ferrite and the second phase is 5 μm or less.

As described above, according to the present invention, when finish rolling is performed in the austenite single-phase region, particularly, by appropriately controlling the rolling conditions (rolling rate, average strain rate) in the temperature range of 860 to 960 ° C., A fine hot rolled sheet structure transformed from recrystallized austenite can be obtained. Furthermore, since a fine grain structure is obtained even after cold rolling and annealing, the impact resistance performance of the member after molding and heat treatment can be improved. Further, the finish rolling finish temperature is set to Ar in order to suppress coarsening of the structure of the steel sheet surface layer portion.
It is better to have ₃ or more points.

After finish rolling, the coil is wound up. However, in order to suppress the coarsening of the structure of the steel sheet surface layer and to obtain a uniform structure in the plate thickness direction, the coil winding temperature is preferably 650 ° C. or less. good. Next, this hot rolled coil is pickled and cold rolled. The cold rolling rate is set according to a predetermined product sheet thickness, but the rolling rate is set to 90% or less in order to reduce the rolling load.

After that, through a continuous annealing or continuous hot dip galvanizing process, a fine ferrite and a second phase structure are obtained.
A composite structure steel sheet having (low temperature transformation phase of austenite or residual austenite) is manufactured. At this time, since the ferrite is recrystallized and the low-temperature transformation phase from austenite (martensite or bainite) or retained austenite is stably formed, the heating temperature is
It is preferably 750 ° C or higher.

Further, by suppressing the coarsening of these tissues,
In order to obtain a fine structure with an average grain size of 5 μm or less, the upper limit of the heating temperature is 880 ° C., and the heating time is 120 to 240 seconds. In both cases of continuous annealing and continuous hot dip galvanizing process, cooling conditions after heating do not require particularly strict control, but in order to suppress the transformation from austenite to pearlite, the average cooling rate up to 500 ° C after heating is controlled. It is better to set it to 3 ℃ / s or more.

Further, when the hot dip galvanizing treatment is performed, the steel sheet is heated, cooled, immersed in a hot dip bath, and then cooled again. In this case, the plating treatment conditions are not particularly limited, and after the galvanization, the plating layer may be subjected to an alloying treatment, if necessary. After this,
The obtained steel sheet is temper-rolled at an elongation of 0.1 to 1.0%,
Wind up in a coil.

[0058]

EXAMPLES Examples of the present invention will be shown below.

(Example 1) Steels having the components shown in Table 1 (steel Nos. 1 to 1)
(0: Steel of the present invention, Steel Nos. 11 to 16: Comparative steel) was melted in a laboratory and then cast to form a slab having a plate thickness of 50 mm. After slab rolling this slab to a plate thickness of 30 mm, it was heated at 1270 ° C in an atmospheric furnace.
It was heated for 1 hr and subjected to hot rolling. 960 during finish rolling
Rolling rate is 45% and average strain rate is 80
s ^-1 , rolling at a finishing temperature of 840 ° C, and then heat treatment equivalent to winding at 550 ° C x 1 hr, with a plate thickness of 3.0 m
A hot rolled sheet of m was prepared.

[0060]

[Table 1]

Next, this hot-rolled sheet was pickled and cold-rolled to a sheet thickness of 1.2 mm. Then, this cold-rolled sheet was heated at 820 ° C for 180se
After being soaked and cooled at an average cooling rate of 5 ° C./s and immersed in a hot dip galvanizing bath at 460 ° C., the galvanized layer was alloyed at 550 ° C. Then, this galvanized steel sheet was temper-rolled at an elongation of 0.5%. Using the galvanized steel sheet thus obtained, microstructure observation, evaluation of tensile properties, and evaluation of impact resistance performance of members were carried out.

The structure of the hot-rolled sheet or the annealed sheet was observed with a scanning electron microscope in a cross section parallel to the rolling direction and perpendicular to the sheet surface. Randomly extracted 200 ferrites, volume fraction of second phase and average grain size in annealed sheet
(Annealed plate) was measured. In addition, the volume ratio of ferrite and the second phase was quantified. Regarding the hot-rolled sheet, the volume ratio of the structure transformed from unrecrystallized austenite was quantified.

In the annealed sheet, the ferrite and the second phase
(Martensite, bainite, or retained austenite) when the average grain size is 5 μm or less, the structure is good, and when the average grain size of these structures exceeds 5 μm, pearlite, a processed structure, etc. were observed. In that case, the structure was considered to be poor. Also, the tensile property values are Japanese Industrial Standard JIS Z220
Using the No. 5 test piece described in 1, a tensile test was performed in the direction perpendicular to the rolling, and measurement was performed.

The impact resistance performance of the press-molded member was determined by subjecting the hat-shaped member shown in FIG. 1 to a heat treatment at 170 ° C. for 20 minutes, and then subjecting this to an impact crushing test. Work load (impact crush absorption energy) from 0 to 130 mm was calculated and evaluated. When the absorbed energy is 10,000 J or more, the characteristics are good, and
When the absorbed energy was less than 10,000 J, it was evaluated as characteristic deterioration.

Table 2 shows the results of evaluation of these characteristics.

[0066]

[Table 2]

Inventive Examples Nos. 1 to 10 (Steel Nos. 1 to 10) are all in the composition range of the present invention, and the hot rolled sheet contains 30 to 70% of the transformation phase from unrecrystallized austenite. In the annealed plate, fine ferrite having an average grain size of 1 to 4 μm and a second phase structure are obtained. Also, in all cases, TS satisfies 780 MPa or more, and the absorbed energy of the member is as high as 11000 to 13000J.

On the other hand, Comparative Examples Nos. 11 to 16 have chemical components outside the scope of the present invention and therefore do not satisfy any of the steel sheet structure, tensile properties, and impact crushing properties of members. Comparative Examples Nos. 11 and 12 are both insufficient in strength. Further, in Comparative Example No. 13, since C is high and the band structure is developed in the rolling direction, the impact absorption energy of the member is as low as 9500J. Comparative Example Nos. 14 and 15
Has a processed structure, so El is low. In Comparative Example No. 16, the average particle size of both ferrite and the second phase is as large as 7 μm, so the impact crush absorption energy of the member is as low as 8300J.

Example 2 A slab having a plate thickness of 60 mm was produced from the steel No. 2 (inventive steel) of Table 1. After slab rolling this slab to a thickness of 30 mm, it was heated at 1290 ° C. for 1 hr in an atmospheric furnace to carry out hot rolling. Finish rolling was carried out at two levels of finishing temperatures of 850 ° C and 975 ° C. 9 in the case of the former (850 ℃ finish)
Rolling is performed by changing the rolling rate in the temperature range of 60 to 860 ° C to 20 to 60% and the average strain rate to 40 to 130 s ^-1 , respectively, and in the latter case (975 ° C finishing), finishing is performed. Final pass (975 ℃)
Was rolled at a rolling rate of 40% and a strain rate of 90 s ^-1 .

After finish rolling, a coiling simulation of 570 ° C. × 1 hr was carried out to produce a hot rolled plate having a plate thickness of 3.0 mm. continue,
The hot-rolled sheet was pickled, cold-rolled to a thickness of 1.2 mm, and then annealed. Annealing is performed at 840 ~ 895 ℃ (975 ℃ finish rolling material 840 ℃) 18
After soaking for 0 sec, cool at an average speed of 10 ° C / s to 400
Continuous annealing was carried out by holding at 550 ° C for 550 seconds and then cooling to room temperature at an average rate of 5 ° C / s.

Further, after subjecting this annealed plate to temper rolling at an elongation of 0.5%, the microstructure was observed, the tensile properties were evaluated, and the impact resistance performance of the member was evaluated in the same manner as above. . The results are shown in Table 3.

[0072]

[Table 3]

Inventive Examples Nos. 26, 27, 30, and 31 have manufacturing conditions within the scope of the present invention, the hot rolled sheet has a transformation structure from unrecrystallized austenite of 35 to 70%, and the annealed sheet has an average grain size of 35 to 70%. Diameter 1 ~
A fine ferrite of 3 μm and a second phase structure are obtained. In addition, TS is 780MPa or more, and the impact crush absorption energy of the member is as high as 11000-12200J.

On the other hand, Comparative Examples Nos. 21-25, 28, 29, 32-34
Since the manufacturing conditions are outside the scope of the present invention, the microstructure,
Either the tensile characteristics or the impact absorption energy of the member is not satisfied. Comparative Examples Nos. 21 to 25, 29, 33 and 34 have an average particle size of 5 μm.
A fine structure of m or less is not obtained and the absorbed energy of the member is low. Comparative Examples Nos. 28 and 32 have a processed structure and their ductility is deteriorated.

[0075]

EFFECTS OF THE INVENTION According to the present invention, the chemical composition of steel is regulated, and the rolling conditions (rolling rate, strain rate) in the predetermined temperature range during finish rolling are optimized, whereby the microstructure of the hot-rolled sheet structure is improved. Further, it is possible to produce a cold-rolled steel sheet or hot-dip galvanized steel sheet having fine ferrite and a second phase (martensite or bainite, or retained austenite) even after cold rolling and annealing.

With this steel sheet, the impact resistance performance of the member after press forming and heat treatment can be improved. In this way, by specifying the chemical composition and production conditions of steel, it is possible to stably produce cold-rolled steel sheet or hot-dip galvanized steel sheet having excellent impact resistance having a strength of 780 MPa or more. Since it can be applied to automobile structural parts and the like that require good impact resistance after molding, it has great utility value in the automobile industry.

[Brief description of drawings]

FIG. 1 is a diagram showing a method of evaluating impact resistance performance of a member. (A) Impact test member shape (b) Impact test method

[Fig. 2] Rolling ratio r (86) affecting the structure of hot rolled sheet and annealed sheet
The figure which shows the influence of 0-960 degreeC) and the amount of Nb + C. Hot rolled sheet structure: Volume fraction annealed sheet structure transformed from unrecrystallized austenite: Average grain size of ferrite and second phase

FIG. 3 is a diagram showing an influence of an average grain size of an annealed plate structure (ferrite, second phase) on crushing absorbed energy of a member.

[Fig. 4] Hot rolling sheet structure and annealed sheet structure during rolling (860
The figure which shows the influence of the average strain rate of (-960 degreeC) and the amount of Nb + C. Hot rolled sheet structure: Volume fraction annealed sheet structure transformed from unrecrystallized austenite: Average grain size of ferrite and second phase

Continued front page    (72) Inventor Toshiaki Urabe             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F term (reference) 4K037 EA01 EA05 EA06 EA16 EA18                       EA19 EA23 EA25 EA27 EB05                       EB09 FB07 FC07 FE01 FE02                       FJ04 FJ05 FJ06 GA05

Claims (3)

[Claims] 1. The chemical composition is C: 0.02 to 0.15% in mass% and S
i: 0.7% or less, Mn: 2.0 to 4.0%, P: 0.1% or less, S: 0.01% or less, sol.Al: 0.1% or less, N: 0.005% or less, Nb: 0.01 to 0.1%, the balance When melting and casting steel consisting essentially of iron, and roughly rolling the cast slab, and then performing finish rolling, the total rolling ratio r (%) in the temperature range of 960 to 860 ° C. And the average strain rate v (s ^-1 ) satisfy the following inequality, finish rolling is completed at a temperature of Ar ₃ points or higher, and then wound into a coil at a temperature of 650 ° C or lower, and a structure transformed from unrecrystallized austenite A hot-rolling step of producing a hot-rolled steel sheet composed of a composite structure containing 20 to 80%, a step of subjecting the obtained hot-rolled steel sheet to pickling, and cold rolling, and at a temperature of 880 ° C. or lower 120 ~
It has an annealing step of annealing for 240 seconds and the average grain size is 5μ.
A method for producing a high-strength cold-rolled steel sheet, which has a composite structure of m or less. 30≤r≤-150 (Nb + C) + 76-200 (Nb + C) + 68≤v≤-200 (Nb + C) +1
28 Here, the element symbols in the formulas represent the respective mass%. 2. The method for producing a high-strength cold-rolled steel sheet according to claim 1, further comprising Ti: 0.01% by mass% as a chemical component.
0.1%, V: 0.01 to 0.3%, B: 0.0002 to 0.002%, Cr: 0.05 to 0.5
%, Mo: 0.05 to 0.5%, at least one kind is contained, The manufacturing method of the high strength cold rolled steel sheet characterized by the above-mentioned. 3. The high-strength hot-dip galvanized steel sheet according to claim 1, further comprising a hot dip galvanizing treatment step after the annealing step. Manufacturing method.