KR101464844B1 - High-strength hot-dip galvanized steel sheet with excellent processability and impact resistance and process for producing same - Google Patents

Abstract

A hot dip galvanized steel sheet having a tensile strength TS of 590 MPa or more and excellent workability and having a large absorption energy up to a low deformation degree of about 5% And a manufacturing method thereof. Wherein the composition of C is 0.04 to 0.13%, Si: 0.7 to 2.3%, Mn: 0.8 to 2.0%, P: 0.1% or less, S: 0.01% Or more and 0.1% or less, the balance being iron and inevitable impurities, and the structure is composed of 75% or more of a ferrite phase, 1% or more of a bayite ferrite phase, and 1% or more and 10% or less of a pearlite phase And the area ratio of the martensite phase is 10% or less, and the martensite area ratio / (the area ratio of the bayonite ferrite + the pearlite area ratio) The ratio of the Mn concentration in the two phases is 0.70 or more.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance, and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in impact resistance, which is used for use as a steel sheet for automobiles.

Recently, fuel economy improvement of automobile becomes important problem from viewpoint of global environment conservation. For this reason, there has been an active movement to reduce the weight of the vehicle body by reducing the thickness of the vehicle body by increasing the strength of the vehicle body material. However, the increase in the strength of the steel sheet leads to a decrease in ductility, that is, a decrease in molding processability. Therefore, development of a material having both high strength and high porosity has been desired. In addition, since the deformation speed of each part is about 10 ³ / s at the time of a collision of an automobile, the impact resistance characteristic in such a high-speed region becomes particularly important. Further, in addition to the recent demand for improvement in corrosion resistance against automobiles, there has been much development of high tensile steel sheets subjected to hot-dip galvanizing. In order to ensure pressability, spot weldability and paint adhesion, a galvannealed steel sheet in which Fe is diffused into a plating layer by performing heat treatment after plating is widely used.

As a high-strength steel sheet excellent in workability and impact resistance absorption property against such a demand, a two-phase steel sheet (DP steel sheet) having a composite structure of ferrite and martensite disclosed in Patent Document 1 is typical. However, a DP steel sheet originally having a low yield strength exhibits a high shock absorbing ability because it has a large work hardening by press working, and when deformation occurs, a deformation aging is caused in a subsequent baking step, There is a problem that a component having a small processing amount such as a bending process does not necessarily exhibit sufficient shock absorbing ability. In addition, DP steel has a high impact absorption energy up to 10 to 30% in high deformation range, and is excellent in impact resistance characteristics. Therefore, it is suitable for a part that absorbs impact energy when deformed to some extent during a collision such as a front impact part. From the viewpoint of ensuring space for occupant, such as the side impact area, the characteristics are not sufficiently satisfied in a region where high absorption energy is required in a small deformation region.

Patent Document 2 discloses a technique for improving the impact resistance characteristic of a TRIP steel using fired-organic transformation of residual?, Which has the same problem as the DP steel described above.

Japanese Patent Application Laid-Open No. 2003-213369 Japanese Patent Application Laid-Open No. 2001-335891

INDUSTRIAL APPLICABILITY The present invention has high strength (tensile strength TS of 590 MPa or more), excellent workability and absorption energy up to a low deformation degree of about 5% without introduction of deformation by press working, And an object of the present invention is to propose an excellent hot-dip galvanized steel sheet and a manufacturing method thereof.

The present inventors have conducted intensive studies from the viewpoints of the composition of the steel sheet and the microstructure in order to produce the high-strength hot-dip galvanized steel sheet which has achieved the above-mentioned object and is excellent in workability and impact resistance. As a result, it was confirmed that the main phase was ferrite and the second phase was a structure containing bainite ferrite, martensite and pearlite, and the martensite area ratio / (the ratio of the area ratio of the bayonite ferrite to the pearlite area ratio) And that the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is 0.70 or more, whereby high workability and impact resistance can be obtained.

Improvement in workability is due to improvement in ductility due to improvement of work hardening ability of ferrite as main phase by utilization of Si and reduction in hardness difference between soft ferrite and hard martensite due to application of bay nitride ferrite or pearlite The hole expandability can be improved.

It is generally known that Mn is concentrated to the second phase during hot rolling or annealing and is distributed in the steel. However, by setting the coiling temperature at hot rolling to a low temperature and appropriately setting the cracking time at annealing, The ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is set to be not less than 0.70 so that the absorption energy up to a low strain degree of about 5% And the collision characteristics can be improved.

The present invention is configured based on the above knowledge.

That is,

(1) The steel sheet according to any one of the above items (1) to (3), wherein the steel sheet has a composition of C: 0.04 to 0.13% : Not less than 0.01% and not more than 0.1%, the balance being iron and inevitable impurities, and the structure is composed of not less than 75% ferrite phase, not less than 1% Of the pearlite phase, and the area ratio of the martensite phase is 10% or less,

The martensite area ratio / (the ratio of the area of the bayonite ferrite to the area of the pearlite)? 0.6, and the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is 0.70 or more. Excellent high strength hot dip galvanized steel sheet.

(2) The steel according to the above item (1), further comprising at least one element selected from the group consisting of Cr: at least 0.05% and not more than 1.0%, V: at least 0.005% and not more than 0.5%, and Mo: at least 0.005% A hot-dip galvanized steel sheet excellent in workability and impact resistance as described in (1).

(3) The steel sheet according to any one of (1) to (3), further comprising, by mass%, Ti: 0.01 to 0.1% (1) or (2), wherein the high-strength hot-dip galvanized steel sheet contains at least one element selected from the group consisting of 0.05% or more and 1.0% or less.

(4) The steel according to any one of (1) to (3), further comprising at least one element selected from the group consisting of Ca in an amount of not less than 0.001% and not more than 0.005%, and REM in an amount of not less than 0.001% ) Having excellent processability and impact resistance.

(5) The steel sheet according to any one of (1) to (4) above, further comprising at least one element selected from the group consisting of 0.001% to 0.010% of Ta and 0.002% ) Having excellent processability and impact resistance.

(6) The high strength hot-dip galvanized steel sheet according to any one of (1) to (5), which further contains Sb in an amount of not less than 0.002% and not more than 0.2% in terms of mass%.

(7) A steel slab having the composition described in any one of (1) to (6) above is hot rolled and then rolled at a temperature of 300 ° C to 570 ° C to pick up the hot rolled plate, And thereafter, at a temperature range of 750 to 900 DEG C, t: retention time (s) satisfies the following formula;

15? T? 47.6 10 ^-10 / exp (-27016 / (T + 273))

T: Annealing temperature (占 폚)

, Then cooled and maintained at a temperature range of 450 to 550 캜 for 10 to 200 s and then subjected to hot dip galvanizing and further annealed at a temperature in the range of 500 to 600 캜 to obtain Tave : Average holding temperature (占 폚) and th: holding time (s)

0.45? Exp [200 / (400-Tave)] ln (th)? 1.0

Wherein the galvannealing treatment is carried out under the condition that the hot-dip galvanized steel sheet satisfies the following conditions:

According to the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet excellent in workability and high in absorption energy up to a low deformation degree of about 5% and excellent in impact resistance even when no deformation is introduced by press working. And the collision safety can be made compatible with each other, thereby exerting an excellent effect of greatly contributing to the high performance of the vehicle body.

Hereinafter, the present invention will be described in detail.

First, the reason why the composition of steel components is limited to the above range in the present invention will be described. The "% " marking on the components means mass% unless otherwise stated.

C: not less than 0.04% and not more than 0.13%

C is an element for stabilizing austenite and is an element necessary for increasing the strength of a steel sheet because it is easy to generate an image other than ferrite. When the C content is less than 0.04%, it is difficult to secure the desired strength even if optimization of the production conditions is attempted. On the other hand, if the C content exceeds 0.13%, the ferrite phase is decreased to lower the workability of the steel sheet, and furthermore, the welded portion and the heat affected portion are hardened and mechanical properties of the welded portion are deteriorated. From this point of view, the C content is 0.04% or more and 0.13% or less.

Si: not less than 0.7% and not more than 2.3%

Si is a ferrite generating element and also an element effective for solid solution strengthening. In order to improve the balance between strength and ductility and to secure hardness of ferrite, addition of 0.7% or more is required. However, the excessive addition of Si exceeding 2.3% causes deterioration of the surface property and deterioration of the adhesion and adhesion of the plating due to occurrence of red scale or the like. Therefore, Si is set to 0.7% or more and 2.3% or less. Preferably, it is 1.2% or more and 1.8% or less.

Mn: not less than 0.8% and not more than 2.0%

Mn is an effective element for strengthening the steel. It is an element for stabilizing austenite and is an element necessary for adjusting the fraction of the second phase. For this reason, it is necessary to add Mn of 0.8% or more. On the other hand, if it is added in an excess amount exceeding 2.0%, the area ratio of martensite in the second phase increases, and the stretch flangeability decreases. Therefore, Mn is set to be not less than 0.8% and not more than 2.0%. , Preferably not less than 1.0% and not more than 1.8%.

P: not more than 0.1%

P is an effective element for strengthening steel, but when it is excessively added in excess of 0.1%, embrittlement is caused by grain boundary segregation and deteriorates impact resistance. If it exceeds 0.1%, the alloying speed is greatly delayed. Therefore, P should be 0.1% or less.

S: not more than 0.01%

S becomes an inclusion such as MnS or the like, which causes cracks along the deterioration of the impact resistance and the metal flow of the welded portion. Therefore, the S is as low as possible, and S is 0.01% or less in terms of production cost.

Al: not less than 0.01% and not more than 0.1%

Al is preferably added as a deoxidizing agent as an effective element for the cleanliness of the steel as a deoxidizing agent. If the amount of Al is less than 0.01%, the effect of addition becomes insufficient, so that the lower limit is set to 0.01%. However, excessive addition of Al deteriorates the slab quality during steelmaking. Therefore, the content of Al is 0.1% or less.

The high-strength hot-dip galvanized steel sheet according to the present invention has the above-mentioned composition as a base component and the balance of iron and inevitable impurities. Depending on the desired properties, at least one kind of element selected from the following elements It can be appropriately contained.

Cr: not less than 0.05% and not more than 1.0%, V: not less than 0.005% and not more than 0.5%, Mo: not less than 0.005% and not more than 0.5%

Cr, V, and Mo increase the hardenability and are effective elements for reinforcing steel. The effect is obtained at not less than 0.05% of Cr, not less than 0.005% of V, and not less than 0.005% of Mo. However, when the amount of Cr exceeded 1.0%, V: 0.5%, and Mo: 0.5% in excess, the second phase fraction would be excessively large, resulting in a risk of deterioration in workability. Therefore, when these elements are added, the amounts thereof are set to not less than 0.05% and not more than 1.0% of Cr, not less than 0.005% and not more than 0.5% of V, and not less than 0.005% and not more than 0.5% of Mo, respectively.

Further, it may contain at least one element selected from the following Ti, Nb, B, Ni and Cu.

Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less

Ti and Nb are effective for precipitation strengthening of steel, and their effects are obtained at 0.01% or more, respectively, and they can be used for reinforcing steel within the range specified in the present invention. However, if each exceeds 0.1%, the workability is lowered. Therefore, when Ti and Nb are added, the addition amount thereof is set to be not less than 0.01% and not more than 0.1% Ti and not less than 0.01% and not more than 0.1% Nb.

B: not less than 0.0003% and not more than 0.0050%

B has an effect of inhibiting the formation and growth of ferrite from the austenite grain boundaries, and therefore can be added as needed. The effect is obtained at 0.0003% or more. However, if it exceeds 0.0050%, the workability is lowered. Therefore, when B is added, the content is 0.0003% or more and 0.0050% or less.

Ni: 0.05% or more and 1.0% or less, Cu: 0.05% or more and 1.0% or less

Ni and Cu are effective elements for strengthening the steel, and they may be used for strengthening the steel within the range specified in the present invention. In order to obtain these effects, 0.05% or more is required. On the other hand, if both Ni and Cu are added in an amount exceeding 1.0%, the workability of the steel sheet is lowered. Therefore, in the case where Ni and Cu are added, the addition amounts thereof are set to 0.05% or more and 1.0% or less, respectively.

Further, it may contain at least one element from the following Ca and REM.

Ca: 0.001% or more and 0.005% or less, REM: 0.001% or more and 0.005% or less

Ca and REM are effective elements for improving the adverse effect of sulfide on hole expandability by spheroidizing the shape of sulfide. In order to obtain this effect, 0.001% or more is required. However, an excess amount exceeding 0.005% causes an increase in inclusions and the like, resulting in surface and internal defects. Therefore, when Ca and REM are added, the addition amounts thereof are 0.001% or more and 0.005% or less, respectively.

Further, it may contain at least one element selected from among the following Ta and Sn.

Ta: 0.001 to 0.010%, Sn: 0.002 to 0.2%

Ta forms a composite precipitate such as (Nb, Ta) (C, N) by partially solidifying it in Nb carbide or Nb carbonitride as well as contributing to high strength by forming alloy carbide or alloy carbonitride such as Ti or Nb , It is considered that the effect of stabilizing the contribution to the strength due to precipitation strengthening is remarkably suppressed by suppressing coarsening of precipitates. Therefore, when Ta is added, its content is preferably 0.001% or more. However, in the case of excessive addition, the effect of stabilizing the precipitate is not only saturated but also the alloy cost is increased. Therefore, when Ta is added, its content is preferably 0.010% or less.

Sn can be added from the viewpoint of suppressing decarburization in the region of several tens of micrometers in the surface layer of the steel sheet caused by nitridation, oxidation or oxidation of the surface of the steel sheet. By suppressing such nitrification and oxidation, the amount of martensite produced on the surface of the steel sheet is prevented from decreasing, thereby improving fatigue characteristics and anti-aging properties. When Sn is added from the viewpoint of inhibiting nitrification or oxidation, the content thereof is preferably 0.002% or more, and if it is more than 0.2%, the toughness is lowered, and therefore the content thereof is preferably 0.2% .

Further, it may contain the following Sb.

Sb: 0.002 to 0.2%

Sb can also be added from the viewpoint of suppressing decarburization in the region of several tens of micrometers in the surface layer of the steel sheet, which is caused by nitriding, oxidation or oxidation of the surface of the steel sheet, similarly to Sn. By suppressing such nitrification or oxidation, the amount of martensite produced on the surface of the steel sheet is prevented from decreasing, thereby improving fatigue characteristics and anti-aging properties. From the standpoint of suppressing nitrification and oxidation, when Sb is added, the content thereof is preferably 0.002% or more, and if it exceeds 0.2%, the toughness is lowered, so that the content thereof is preferably 0.2% or less Do.

Next, the steel structure will be described.

Area ratio of ferrite phase: 75% or more

In order to secure good ductility, the ferrite phase is required to have an area ratio of 75% or more.

Area ratio of the behenic ferrite phase: 1% or more

In order to secure good hole expandability, that is, in order to alleviate the hardness difference between the soft ferrite and the hard martensite, the area ratio of the honeycomb ferrite phase is 1% or more.

Area percentage of pearlite phase: 1% or more and 10% or less

For ensuring good hole expandability, the area ratio of the pearlite phase is 1% or more. If the pearlite phase area ratio exceeds 10%, the ductility (TS x EL) decreases. Therefore, the area ratio of the pearlite phase is set to 1% or more and 10% or less.

Area ratio of martensite: 10% or less

When the area ratio of the martensite exceeds 10%, the decrease in stretch flangeability becomes remarkable. Therefore, the area ratio of the martensite is set to 10% or less.

Martensite area ratio / (baynitic ferrite area ratio + pearlite area ratio)? 0.6

Although the martensite has a large difference in ferrite strength and low elongation flangeability, the martensite coexists with the bainite ferrite and the pearlite so that the martensite area ratio / (the ratio of the area ratio of the bayonite ferrite + the area percentage of the pearlite) It is possible to suppress deterioration of the hole expandability by the hole. Therefore, the martensite area ratio / (the area ratio of the bayonite ferrite + the area ratio of the pearlite) is set to 0.6.

In addition to the ferrite, baynitic ferrite, pearlite and martensite, carbides such as retained austenite and tempering martensite and cementite may be produced. In addition to the ferrite, bainite ferrite, pearlite and martensite, If the area ratio is satisfied, the object of the present invention can be achieved.

The area ratio of ferrite, bay nitride ferrite, pearlite and martensite in the present invention refers to the area ratio of each phase occupied in the observation area.

The microstructure was corroded with 1/3 of the plate thickness in the rolling direction section of the steel sheet in the rolling direction by 3% or more after the polishing and observed with a scanning electron microscope at a magnification of 5000 times. -Pro can be used to calculate the area ratio of each phase.

At this time, since it is difficult to distinguish the martensite from the retained austenite, the obtained hot-dip galvanized steel sheet is tempered at 200 ° C for 2 hours, and thereafter, Was observed by the above-mentioned method, and the area ratio of the tempering martensite determined by the above method was regarded as the area ratio of the martensite.

The content of the retained austenite phase can be obtained by grinding the steel sheet up to a quarter of the plate thickness direction and by the diffracted X-ray intensity of this plate thickness 1/4 surface. At this time, a CoK? Line is used for the incident X-ray to detect the peaks of the {111}, {200}, {220}, {311} planes of the retained austenite phase and the {110}, {200} And the average value of these is regarded as the content of the residual austenite phase and the content thereof can be treated as the area ratio of the retained austenite.

(Mn concentration in the ferrite phase / Mn concentration in the second phase) of the Mn concentration in the ferrite phase and the Mn concentration in the second phase is not less than 0.70

By making the distribution of Mn uniform in the steel, it is possible to increase the absorption energy up to a low deformation degree of about 5% without introduction of deformation by press working, to improve the impact resistance, and the Mn concentration in the ferrite phase The effect of the Mn concentration in the second phase is 0.70 or more. Therefore, the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is set to 0.70 or more.

Next, the manufacturing conditions will be described.

The steel prepared by the composition described above is dissolved in a converter or the like, and is formed into a slab by a continuous casting method or the like. The steel slab is subjected to hot rolling to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is pickled or further subjected to cold rolling to obtain a cold-rolled steel sheet. Hot-rolled steel sheet or cold-rolled steel sheet is subjected to continuous annealing, and then hot-dip galvanizing is performed, or further galvanizing is performed. The reason for limiting each process will be explained.

[Hot rolling condition]

Coiling temperature: 300 ° C or more and 570 ° C or less

When the coiling temperature after hot rolling exceeds 570 캜, the distribution of Mn to the second phase is promoted after winding, and it is difficult to make the ratio of the Mn concentration in the ferrite phase and the Mn concentration in the second phase to 0.70 or more in the final structure It becomes. If the coiling temperature is less than 300 캜, the shape of the hot-rolled sheet deteriorates or the strength of the hot-rolled sheet excessively increases, making cold rolling difficult. Therefore, the coiling temperature is set to 300 ° C or more and 570 ° C or less.

[Continuous annealing condition]

And is annealed at a temperature range of 750 to 900 占 폚 in a condition satisfying the following formula.

15? T? 47.6 10 ^-10 / exp (-27016 / (T + 273))

t: retention time (s)

T: Annealing temperature (占 폚)

When the annealing temperature is less than 750 占 폚 or when the holding (annealing) time is less than 15 s, the formation of austenite at the time of annealing becomes insufficient and the amount of low temperature transformation phase required after annealing cooling can not be secured. On the other hand, if the annealing temperature exceeds 900 DEG C, austenite at annealing remarkably increases, and it becomes impossible to secure a required amount of ferrite after annealing and cooling. When the holding time exceeds 47.6 x 10 < ^-10 > / exp (-27016 / (T + 273)) sec, the concentration of Mn to the austenite phase during annealing is excessively advanced, And the Mn concentration in the second phase to 0.70 or more.

After annealing, it is cooled and held at a temperature range of 450 to 550 ° C for 10 to 200 s.

When the holding temperature exceeds 550 占 폚 or the holding time is less than 10 s, the bainite transformation is not promoted and the bainitic ferrite is hardly obtained, so that the desired hole expandability can not be obtained. When the holding temperature is lower than 450 DEG C or the holding time exceeds 200 s, most of the second phase becomes austenite and bainitic ferrite having a large amount of solid carbon produced by bainite transformation promotion, and the desired pearlite area ratio And the area ratio of hard martensite is increased, so that good hole expandability and material stability can not be obtained.

After carrying out the above-described holding, the surface is subjected to hot-dip galvanizing treatment for the purpose of improving the green tea intelligence in actual use.

A galvannealed steel sheet in which Fe is diffused into a plating layer by performing heat treatment after plating is often used in order to ensure pressability, spot weldability and paint adhesion. In the production of the galvannealed hot-dip galvanized steel sheet, galvannealing is further performed under the following conditions after hot-dip galvanizing.

[Alloying treatment conditions]

Tave: average holding temperature (占 폚) and th: holding time (s) in the temperature range of 500 to 600 占 폚 satisfy the following formula:

0.45? Exp [200 / (400-Tave)] ln (th)? 1.0

The alloying treatment of the plating layer is performed.

In addition, exp (X) and ln (X) denote exponential functions of X and natural logarithm, respectively.

The alloying treatment of the plating layer is performed in the range of 500 to 600 占 폚 in order to obtain an appropriate Fe% in the plating layer.

When martensite is abundant in the final structure and the hard martensite is adjacent to the soft ferrite when exp [200 / (400-Tave) xln (th) is less than 0.45, a large hardness difference And hole expandability is deteriorated. When exp [200 / (400-Tave) xln (th) is more than 1.0, most of the untransformed austenite is transformed into cementite or pearlite, resulting in a balance of desired strength and ductility.

In the series of heat treatment in the production method of the present invention, the holding temperature does not have to be constant if it is within the above-mentioned temperature range, and the object of the present invention is not impaired if it is within the specified range. If the heat history is satisfied, the steel sheet may be heat-treated with any equipment. In addition, it is within the scope of the present invention to perform temper rolling on the steel sheet of the present invention for shape correction after heat treatment. In the present invention, it is assumed that a steel material is manufactured through various steps such as ordinary steelmaking, casting, and hot rolling. For example, some or all of the hot rolling process may be omitted by thin casting, .

Other production methods are not particularly limited, and preferable examples are shown below.

[Casting condition]

The steel slab to be used is preferably produced by continuous casting in order to prevent macro segregation of the components, but it may be manufactured by rough casting or thin slab casting. In addition to the conventional method in which a steel slab is once cooled to a room temperature and then heated again, the steel slab is cooled without being cooled to room temperature, inserted into a heating furnace as it is, or after a slight boiling, The energy saving process such as direct rolling and direct rolling can be applied without any problem.

[Hot rolling condition]

Slab heating temperature: 1100 ℃ or higher

When the heating temperature is less than 1100 DEG C, the carbide can not be sufficiently solidified, or the risk of occurrence of trouble during hot rolling due to an increase in rolling load is increased There is a problem. In addition, the slab heating temperature is preferably 1300 占 폚 or less in the case of an increase in scale loss accompanying an increase in the oxidation amount and the like.

Further, from the viewpoint of preventing trouble during hot rolling even if the slab heating temperature is lowered, a so-called sheet bar heater for heating the sheet bar may be used.

Finishing rolling temperature: Ar ₃ transformation point or more

When the finish rolling finish temperature is lower than the Ar ₃ transformation point,? And? Are generated during rolling, and a band-like structure is likely to be formed on the steel sheet. Such band-like structure remains after cold rolling or annealing and causes anisotropy Or cause deterioration in processability. For this reason, it is preferable that the finishing rolling temperature is equal to or higher than the Ar ₃ transformation point.

In the hot rolling step in the present invention, a part or all of the finish rolling may be lubricated by rolling to reduce the rolling load during hot rolling. Performing lubrication rolling is effective also from the viewpoints of uniformity of the steel sheet shape and uniformity of materials. The friction coefficient at the time of lubrication rolling is preferably in the range of 0.25 to 0.10. It is also preferable to form a continuous rolling process in which front and rear sheet bars are bonded to each other and are continuously subjected to finish rolling. Application of the continuous rolling process is also preferable from the standpoint of operational stability of hot rolling.

[Cold rolling conditions]

Next, in the case of performing cold rolling, preferably, the oxide scale on the surface of the hot-rolled steel sheet is removed by pickling and then subjected to cold rolling to obtain a cold-rolled steel sheet having a predetermined thickness. The pickling conditions and the cold rolling conditions are not particularly limited, but can be performed according to a conventional method. The reduction ratio of the cold rolling is preferably 40% or more.

[Hot-dip galvanizing]

The plating treatment is performed by immersing the steel sheet in a plating bath at a bath temperature of 440 to 500 ° C in a plating bath of 0.08 to 0.18% of dissolved Al, and adjusting the amount of adhesion by gas wiping or the like.

The steel sheet after the hot dip galvanizing treatment may be subjected to temper rolling to adjust the shape correction, surface roughness, and the like. In addition, there is no problem even if processing such as resin, oil coating, and various coatings is applied.

Example

A steel having the composition shown in Table 1 and the balance Fe and unavoidable impurities (N in Table 1) was inevitably dissolved in a converter, and the casting was carried out by a continuous casting method.

The obtained cast steel was hot-rolled under the conditions shown in Table 2 and Table 3 to a plate thickness of 3.0 mm. Subsequently, after pickling, cold-rolled steel sheet having a thickness of 1.4 mm was produced to provide a cold-rolled steel sheet for annealing. A part of the hot-rolled steel sheet, which had been hot-rolled at a thickness of 2.3 mm, was picked up and was directly provided to the annealing.

Then, these cold-rolled steel sheets or hot-rolled steel sheets were annealed and plated in the continuous hot-dip galvanizing line under the conditions shown in Tables 2 and 3. The plating amount was set at 35 to 45 g / m 2 per one side.

The microstructure, tensile properties, stretch flangeability and impact resistance characteristics of the steel sheet thus obtained were examined. The results are shown in Tables 4 and 5. < tb > < TABLE >

Further, the microstructure was observed at a magnification of 5000 times in the field of view by using a scanning electron microscope with respect to 1/4 sheet thickness of the cross section of the steel sheet in the rolling direction, and the area ratio of each phase was obtained by the above-described method.

Mn concentration in the ferrite phase and the second phase was measured by EPMA using line analysis of Mn at intervals of 0.1 占 퐉. The average value of the Mn concentration of each particle was taken as the Mn concentration of the particles, and 10 particles of each of the ferrite phase and the second phase were measured. The average value was determined as the ferrite phase and the Mn concentration of the second phase.

The workability was evaluated by ductility, hole expandability (elongation flangeability).

The tensile test was conducted at a deformation rate of 10 ^-3 / s using a JIS No. 5 specimen taken from a direction perpendicular to the rolling direction of a non-processed steel sheet, and TS (tensile strength) and EL (total elongation) , And it was judged that the case of TS EL = 19000 MPa.% Was good.

The elongation flangeability was measured in accordance with JFST1001 of Japan Steel Federation. After cutting the obtained steel sheet to 100 mm x 100 mm, a hole having a diameter of 2.0 mm or more was punched out with a clearance of 12% ± 1% and a hole having a thickness of less than 2.0 mm with a clearance of 12% The pore of the conical 60 ° was pushed into the hole and the pore diameter at the crack generation limit was measured using a die having an inner diameter of 75 mm and the blank holder force was suppressed to 9 tons. ) (%) Was obtained, and the stretch flangeability was evaluated from the value of the critical hole expansion ratio.

(%) = {(D _f -D ₀ ) / D ₀ } × 100

D _f is the hole diameter (mm) at the time of cracking, and D ₀ is the initial hole diameter (mm). In the present invention, the case of? 70 (%) was judged to be good.

The impact absorption characteristics were measured using a test piece having a width of 5 mm and a length of 7 mm obtained from the direction perpendicular to the rolling direction of the untreated steel sheet and measuring the absorbed energy up to the deformation amount when the tensile test was conducted at a deformation rate of 2000 / (See Iron and Steel, vol. 83 (1997), p. 748), and the shock absorption characteristics were evaluated by the ratio of absorbed energy to static TS (AE / TS). Also, the absorbed energy was obtained by integrating the stress-strain curve in the range of strain amount 0 to 5%.

In the present invention, the TS is 590 MPa or more, the softness and stretch flangeability are excellent, and the ratio of absorbed energy to static TS (AE / TS) at a deformation rate of 2000 / s to 5% is 0.050 , And a high-strength alloyed hot-dip galvanized steel sheet having high impact resistance characteristics was obtained by processing at a small deformation rate at a high deformation rate. On the other hand, in the comparative example, since the AE / TS is less than 0.050, high impact resistance is deteriorated due to processing at a small deformation rate at a high deformation rate, or at least some characteristics of ductility and elongation flangeability are deteriorated.

Industrial availability

The high-strength hot-dip galvanized steel sheet of the present invention has excellent workability and excellent impact resistance. The high-strength hot-dip galvanized steel sheet of the present invention can be used not only as a front impact portion of an automobile but also as a steel sheet applied to a side impact portion and also as a steel sheet used for a portion having a small processing amount such as bending.

Claims (7)

Wherein the composition of C is 0.04 to 0.13%, Si: 0.7 to 2.3%, Mn: 0.8 to 2.0%, P: 0.1% or less, S: 0.01% Or more and 0.1% or less, the balance being iron and inevitable impurities, and the structure is composed of 75% or more of a ferrite phase, 1% or more of a bayite ferrite phase, and 1% or more and 10% or less of a pearlite phase And the area ratio of the martensite phase is 10% or less,
And the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is not less than 0.70, wherein the martensite area ratio / (the ratio of the area of the bayonite ferrite to the area of the pearlite) High strength hot-dip galvanized steel sheet with excellent characteristics. The method according to claim 1,
Further comprising at least one element selected from the group consisting of Cr: at least 0.05% and not more than 1.0%, V: at least 0.005% and not more than 0.5%, and Mo: at least 0.005% and not more than 0.5% High strength hot-dip galvanized steel with excellent processability and impact resistance. 3. The method according to claim 1 or 2,
The steel sheet according to any one of claims 1 to 3, wherein the steel has a composition of Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, B: 0.0003 to 0.0050%, Ni: 0.05 to 1.0% And 1.0% or less. The hot-dip galvanized steel sheet has excellent processability and impact resistance. 3. The method according to claim 1 or 2,
Further comprising at least one element selected from the group consisting of Ca in an amount of 0.001 to 0.005% and REM in an amount of 0.001 to 0.005% in terms of mass%. Plated steel plate. 3. The method according to claim 1 or 2,
Further comprising at least one element selected from the group consisting of Ta in an amount of 0.001 to 0.010% and Sn in an amount of 0.002 to 0.2% by mass in terms of the composition of the composition, and a high-strength hot- Plated steel plate. 3. The method according to claim 1 or 2,
A high strength hot-dip galvanized steel sheet excellent in processability and impact resistance, characterized by containing Sb in an amount of from 0.002% to 0.2% by mass as a component composition. A hot-rolled steel slab having the composition described in claim 1 or 2 is subjected to hot rolling and then rolled at a temperature of 300 ° C or higher and 570 ° C or lower to pick up hot rolled steel sheets or cold rolled further, , In a temperature range of 750 to 900 DEG C, t: retention time (s) satisfies the following formula:
15? T? 47.6 10 ^-10 / exp (-27016 / (T + 273))
T: Annealing temperature (占 폚)
, Then cooled and maintained at a temperature range of 450 to 550 캜 for 10 to 200 s and then subjected to hot dip galvanizing and further annealed at a temperature in the range of 500 to 600 캜 to obtain Tave : Average holding temperature (占 폚) and th: holding time (s)
0.45? Exp [200 / (400-Tave)] ln (th)? 1.0
Wherein the galvannealing treatment is carried out under the condition that the hot-dip galvanized steel sheet satisfies the following conditions: