KR20120099505A - High-strength hot-dip galvanized steel sheet with excellent processability and impact resistance and process for producing same - Google Patents

Abstract

Hot-dip galvanized steel sheet having a tensile strength TS of 590 MPa or more, excellent in workability and high in absorption energy to a low deformation area of about 5% even without introduction of deformation due to press working, and excellent in crash resistance. It provides a manufacturing method. The component composition is C: 0.04% or more, 0.13% or less, Si: 0.7% or more, 2.3% or less, Mn: 0.8% or more, 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% It contains 0.1% or more of the above, the remainder is made of iron and unavoidable impurities, the structure is, in area ratio, 75% or more ferrite phase, 1% or more bainitic ferrite phase and 1% or more and 10% or less pearlite phase In addition, the area ratio of the martensite phase is 10% or less, satisfies the martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≦ 0.6, and the Mn concentration in the ferrite phase and agent The ratio of Mn concentration in two phases is 0.70 or more.

Description

High-strength hot-dip galvanized steel sheet with excellent workability and impact resistance and its manufacturing method {HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET WITH EXCELLENT PROCESSABILITY AND IMPACT RESISTANCE AND PROCESS FOR PRODUCING SAME}

The present invention relates to a high strength hot dip galvanized steel sheet having excellent impact resistance properties for use as a steel sheet for automobiles.

In recent years, from the standpoint of global environmental conservation, fuel economy improvement of automobiles has become an important problem. For this reason, the movement to seek to reduce the thickness of the vehicle body itself by increasing the strength of the vehicle body material has been actively performed. However, since the increase in strength of the steel sheet causes a decrease in ductility, that is, a decrease in molding processability, development of a material having high strength and high workability is desired. Moreover, since the deformation rate accommodated by each part at the time of an automobile collision reaches about 10 ³ / s, the impact resistance in such a high speed area becomes particularly important. Furthermore, in addition to the recent increase in the demand for improvement of corrosion resistance for automobiles, development of high tensile strength steel sheets subjected to hot dip galvanization has been conducted. In addition, in order to secure pressability, spot weldability, and paint adhesion, many alloyed hot-dip galvanized steel sheets in which Fe of the steel sheet is diffused in the plating layer by heat treatment after plating are used.

As a high strength steel sheet excellent in workability and impact resistance absorption characteristics to such a request, the two-phase structure steel plate (DP steel plate) which consists of a composite structure of ferrite and martensite disclosed by patent document 1 is typical. However, the DP steel sheet with low yield strength inherently exhibits high impact absorption ability because of its large work hardening by press working, and when the work deformation occurs, it causes deformation aging in the coating baking process following it, and the yield strength greatly increases. This is the reason that there is a problem that a sufficient impact absorption capacity is not necessarily exhibited in parts having a small processing amount such as bending work. Also, in the DP steel, 10? It is characterized by high impact absorption energy up to 30% of the high deformation area and excellent impact resistance characteristics.It is suitable for a part that deforms to some extent and absorbs collision energy at the time of a collision, such as a front collision area, but rides like a side collision site. From the standpoint of securing a space for personnel, it is not sufficient to satisfy the characteristics of the site where high absorption energy is required in a small deformation zone.

In addition, Patent Literature 2 discloses a technique for improving impact resistance characteristics in a TRIP steel using a calcined organic transformation of residual γ, which has the same problem as the DP steel.

Japanese Laid-Open Patent Publication 2003-213369 Japanese Unexamined Patent Publication No. 2001-335891

The present invention has a high strength (tensile strength TS of 590 MPa or more), is excellent in workability, and has a high absorption energy to a low deformation area of about 5% even without introducing deformation by press working, and has high impact resistance characteristics. An object of the present invention is to propose an excellent hot dip galvanized steel sheet and a method of manufacturing the same.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly researched from the viewpoint of the composition of a steel plate and a micro structure, in order to achieve the said subject and to manufacture the high strength hot dip galvanized steel plate excellent in workability and impact resistance characteristics. As a result, the main phase is ferrite and the structure including bainitic ferrite, martensite, and pearlite in the second phase has martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≦ 0.6. It turns out that high workability and impact resistance property are obtained by satisfying and setting ratio of Mn concentration in a ferrite phase and Mn concentration in a 2nd phase to 0.70 or more.

The improvement of workability is due to the improvement of ductility by the improvement of work hardenability of columnar ferrite by the application of Si and the relaxation of the hardness difference between the soft ferrite and hard martensite by use of bainitic ferrite and pearlite. This was made possible by the improvement of hole expandability.

Moreover, although it is known that Mn is normally concentrated in a 2nd phase at the time of hot rolling and annealing, and distribution in steel is produced, by making winding temperature in hot rolling low temperature and making the crack time at the time of annealing suitable in steel, By uniformizing the distribution of Mn in the mixture and setting the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase to 0.70 or more, the absorbed energy up to about 5% of the low strain region without introducing a deformation by press working. It becomes large and the improvement of a collision resistance characteristic is attained.

The present invention is configured based on the above knowledge.

That is,

(1) Component composition is C: 0.04% or more and 0.13% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.0% or less, P: 0.1% or less, S: 0.01% or less by mass% : 0.01% or more and 0.1% or less, remainder consists of iron and an unavoidable impurity, and a structure consists of an area ratio of 75% or more ferrite phase, 1% or more bainitic ferrite phase, and 1% or more and 10% or less Has a pearlite phase, the martensite phase has an area ratio of 10% or less, and

The martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≦ 0.6 is satisfied and the ratio of the Mn concentration in the ferrite phase and the Mn concentration in the second phase is 0.70 or more. Excellent high strength hot dip galvanized steel sheet.

(2) Further, at least one element selected from mass: Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less as the component composition. A high strength hot dip galvanized steel sheet excellent in workability and impact resistance characteristics as described in (1).

(3) Further, by mass% as the component composition, Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, B: 0.0003% or more and 0.0050% or less, Ni: 0.05% or more and 1.0% or less, Cu: A high strength hot dip galvanized steel sheet excellent in workability and impact resistance properties as described in (1) or (2), characterized by containing at least one element selected from 0.05% or more and 1.0% or less.

(4) Further, (1)? Characterized by containing at least one element selected from Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less by mass% as the component composition? The high strength hot dip galvanized steel sheet excellent in the workability and impact resistance characteristic in any one of (3).

(5) Further, (1)? Characterized by containing at least one element selected from Ta: 0.001% or more and 0.010% or less and Sn: 0.002% or more and 0.2% or less by mass% as the component composition? A high strength hot dip galvanized steel sheet having excellent workability and impact resistance as described in any one of (4).

(6) Furthermore, Sb: 0.002% or more and 0.2% or less are contained by mass% as a component composition, (1)? The high strength hot dip galvanized steel sheet excellent in the workability and impact resistance characteristic in any one of (5).

(7) (1) After hot rolling to the steel slab which has a component composition as described in any one of (6), the hot rolled sheet produced by winding up at the temperature of 300 degreeC or more and 570 degrees C or less is pickled, or cold-rolled further, and thereafter, 750 ? In the temperature range of 900 ° C, t: the holding time (s) is represented by the following formula;

15≤t≤47.6 × 10 ^-10 / exp (-27016 / (T + 273))

T: annealing temperature (℃)

After annealing to satisfy the condition, it is cooled and 450? 10? At a temperature range of 550 ℃. 200 s is maintained, followed by hot dip galvanizing and further 500? In the temperature range of 600 ° C, Tave: average holding temperature (° C) and th: holding time (s) are represented by the following formulas;

0.45≤exp [200 / (400-Tave)] × ln (th) ≤1.0

Process for producing a high strength hot dip galvanized steel sheet excellent in workability and impact resistance characteristics, characterized in that the galvanizing alloying treatment under conditions satisfying the conditions.

According to the present invention, a high-strength hot dip galvanized steel sheet having excellent workability and high absorption energy up to about 5% of the low deformation region and excellent impact resistance characteristics is obtained even without introduction of deformation by press working, thereby reducing the weight of automobiles. It is possible to achieve both an improvement in the collision safety and 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 which limited the component composition of steel to the said range in this invention is demonstrated. In addition, unless otherwise indicated, the "%" display regarding a component shall mean the mass%.

C: 0.04% or more and 0.13% or less

C is an element for stabilizing austenite, and is an element necessary for increasing the strength of the steel sheet because it is easy to produce a phase other than ferrite. If the amount of C is less than 0.04%, securing desired strength is difficult even if the manufacturing conditions are optimized. On the other hand, when the amount of C exceeds 0.13%, the ferrite phase is reduced, the workability of the steel sheet is reduced, the hardening of the welded portion and the heat affected zone is remarkable, and the mechanical properties of the welded portion deteriorate. From this viewpoint, the amount of C is made into 0.04% or more and 0.13% or less.

Si: 0.7% or more and 2.3% or less

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

Mn: 0.8% or more and 2.0% or less

Mn is an element effective for reinforcing steel. Moreover, it is an element which stabilizes austenite and is an element which is necessary for the fractionation of a 2nd phase. For this reason, Mn needs 0.8% or more of addition. On the other hand, when it adds excessively exceeding 2.0%, the martensite area ratio in a 2nd phase will increase and a stretch flange property will fall. Therefore, Mn is made into 0.8% or more and 2.0% or less. Preferably they are 1.0% or more and 1.8% or less.

P: 0.1% or less

Although P is an effective element for reinforcing steel, when excessively added in excess of 0.1%, P causes embrittlement due to grain boundary segregation and deteriorates impact resistance. If it exceeds 0.1%, the alloying speed is significantly delayed. Therefore, P is made into 0.1% or less.

S: 0.01% or less

Since S becomes inclusions, such as MnS, and it causes deterioration of impact resistance and the crack along the metal flow of a weld part, S is as low as possible, but S is made into 0.01% or less from a viewpoint of manufacturing cost.

Al: 0.01% or more and 0.1% or less

Al acts as a deoxidizer and is an element effective for the cleanliness of steel, and it is preferable to add Al in a deoxidation process. In addition, when the amount of Al is less than 0.01%, since the addition effect becomes insufficient, the minimum was made into 0.01%. However, excessive addition of Al deteriorates slab quality in steelmaking. Therefore, Al is made into 0.1% or less.

The high strength hot-dip galvanized steel sheet in the present invention has the above-described composition as a basic component, and the balance consists of iron and unavoidable impurities. According to desired characteristics, at least one element selected from the elements described below It can contain suitably.

Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less

Cr, V, and Mo improve the hardenability and are effective elements for reinforcing steel. The effect is obtained at Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. However, when excessively adding Cr: 1.0%, V: 0.5%, and Mo: 0.5%, respectively, the 2nd phase fraction will become excessive and there exists a possibility of the workability fall. Therefore, when adding these elements, the quantity shall be Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, and Mo: 0.005% or more and 0.5% or less, respectively.

Moreover, it can contain 1 or more types of elements from the following Ti, Nb, B, Ni, 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 strengthening precipitation precipitation of steel, and the effects thereof are obtained at 0.01% or more, respectively, and may be used for strengthening steel as long as it is within the range specified in the present invention. However, when each exceeds 0.1%, workability will fall. Therefore, when adding Ti and Nb, the addition amount shall be 0.01% or more and 0.1% or less, and Nb may be 0.01% or more and 0.1% or less.

B: 0.0003% or more and 0.0050% or less

B has a function of suppressing the production and growth of ferrite from the austenite grain boundary and can be added as necessary. The effect is obtained at 0.0003% or more. However, when it exceeds 0.0050%, workability will fall. Therefore, when adding B, you may be 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 reinforcing steel, and may be used for reinforcing steel as long as it is within the range specified in the present invention. In order to acquire these effects, 0.05% or more is required, respectively. On the other hand, when both Ni and Cu are added exceeding 1.0%, the workability of a steel plate will fall. Therefore, when adding Ni and Cu, the addition amount shall be 0.05% or more and 1.0% or less, respectively.

Moreover, 1 or more types of element can be contained from 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 in order to spheroidize the shape of a sulfide, and to improve the bad influence of a sulfide on pore expandability. In order to acquire this effect, it is required 0.001% or more, respectively. However, excessive addition exceeding 0.005% causes an increase in inclusions etc. and causes surface and internal defects. Therefore, when Ca and REM are added, the addition amount shall be 0.001% or more and 0.005% or less, respectively.

Moreover, 1 or more types of element can be contained from following Ta and Sn.

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

Ta, like Ti and Nb, forms alloy carbides and alloy carbonitrides to contribute to high strength, and is partially dissolved in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta) (C, N). By forming, it is thought that the coarsening of a precipitate is suppressed remarkably, and the effect on the intensity | strength accompanying precipitation strengthening is stabilized. Therefore, when adding Ta, it is preferable to make the content into 0.001% or more. However, when excessively added, not only the above-mentioned precipitate stabilization effect is saturated, but also the alloy cost increases, and when Ta is added, the content is preferably 0.010% or less.

Sn can be added from the viewpoint of suppressing the decarburization of several 10 micrometers area | regions of the steel plate surface layer produced by nitriding, oxidation, or oxidation of the steel plate surface. By suppressing such nitriding and oxidation, the production amount of martensite is prevented from decreasing on the surface of the steel sheet, and the fatigue characteristics and the aging resistance are improved. In the case of adding Sn from the viewpoint of suppressing nitriding and oxidation, the content thereof is preferably 0.002% or more, and when the content exceeds 0.2%, the toughness is reduced, so the content thereof is preferably 0.2% or less. .

In addition, the following Sb may be contained.

Sb: 0.002? 0.2%

Sb can also be added in the same manner as Sn in view of suppressing decarburization of several 10 μm regions of the steel sheet surface layer caused by nitriding, oxidation or oxidation of the steel sheet surface. By suppressing such nitriding and oxidation, the production amount of martensite is prevented from decreasing on the surface of the steel sheet, and the fatigue characteristics and the aging resistance are improved. From the viewpoint of suppressing nitriding and oxidation, the content of Sb is preferably 0.002% or more, and if it exceeds 0.2%, the toughness is lowered, so the content is preferably 0.2% or less. Do.

Next, the river organization will be described.

Area ratio of ferrite phase: 75% or more

In order to ensure satisfactory ductility, the ferrite phase is required 75% or more in area ratio.

Area ratio of bainitic ferrite phase: 1% or more

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

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

In order to ensure good hole expandability, the area ratio of the pearlite phase is 1% or more. Ductility (TSxEL) falls when the area ratio of a pearlite phase exceeds 10%. Therefore, the area ratio of a pearlite phase is made into 1% or more and 10% or less.

Area ratio of martensite phase: 10% or less

When the area ratio of the martensite phase exceeds 10%, the deterioration of the elongation flange property becomes remarkable. Therefore, the area ratio of martensite phase is made into 10% or less.

Martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≤ 0.6

Martensite has a large difference in strength between ferrites and reduces elongation flangeability, but martensite is coexisted with bainitic ferrite and pearlite to make martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≦ 0.6 It becomes possible to suppress the fall of the hole expandability by a trap. Therefore, martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) is set to 0.6.

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

In addition, the area ratio of the ferrite bainitic ferrite pearlite martensite in this invention means the area ratio of each phase to occupy an observation area.

The microstructure was corroded with 3% nital after polishing to a sheet thickness 1/4 of the sheet thickness in the rolling direction cross section of the steel sheet, and was observed at a magnification of 5000 times using a scanning electron microscope, and the image of Media Cybernetics. -Pro can be used to find the area ratio of each phase.

At that time, since it is difficult to distinguish between martensite and residual austenite, the obtained hot-dip galvanized steel sheet is subjected to a tempering treatment at 200 ° C. for 2 hours, and thereafter, the structure of the plate thickness cross section parallel to the rolling direction of the steel sheet. Was observed by the above method, and the area ratio of the tempered martensite phase determined by the above method was taken as the area ratio of the martensite phase.

In addition, content of the retained austenite phase can be calculated | required by the diffraction X-ray intensity | strength of the plate | board thickness 1/4 surface by grind | polishing a steel plate to the 1/4 surface of a plate thickness direction. At this time, CoKα rays are used for incident X-rays, and the peaks of the surfaces of the 111-, 200-, 220-, and 311-planes of the retained austenite phase, and the 110-, 200-, and 211-plane surfaces of the ferrite phase, respectively. The strength ratios were determined for all combinations of the integrated strengths of the above, and their average values were taken as the residual austenite phase content, and the content could be treated as the area ratio of the residual austenite.

The ratio of the Mn concentration in the ferrite phase and the Mn concentration in the second phase (Mn concentration in the ferrite phase / Mn concentration in the second phase) is 0.70 or more.

By uniformizing the distribution of Mn in the steel, the absorption energy to the low deformation region of about 5% is large and the impact resistance can be improved even without the introduction of deformation by press working, and the Mn concentration in the ferrite phase The effect is acquired by making ratio of Mn concentration in a 2nd phase into 0.70 or more. Therefore, ratio of Mn concentration in a ferrite phase and Mn concentration in a 2nd phase shall be 0.70 or more.

Next, manufacturing conditions are demonstrated.

The steel adjusted to the above-mentioned component composition is dissolved in a converter, etc., and the slab is formed by a continuous casting method or the like. The steel slab is hot rolled to form a hot rolled steel sheet, and the hot rolled steel sheet is pickled, or further cold rolled to form a cold rolled steel sheet. After performing continuous annealing on the pickled hot rolled steel sheet or cold rolled steel sheet, hot dip galvanizing treatment is performed, or further galvanizing alloying treatment is performed. The reason for limitation of each process is demonstrated.

[Hot rolling condition]

Winding temperature: 300 ℃ or more and 570 ℃ or less

When the coiling temperature after hot rolling exceeds 570 ° C, the distribution of Mn to the second phase is promoted after the coiling, 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. Become. If the coiling temperature is less than 300 ° C, the shape of the hot rolled sheet deteriorates or the strength of the hot rolled sheet rises excessively, making cold rolling difficult. Therefore, winding temperature shall be 300 degreeC or more and 570 degrees C or less.

[Continuous Annealing Condition]

750? It annealed on conditions which satisfy | fill the following formula in the temperature range of 900 degreeC.

15≤t≤47.6 × 10 ^-10 / exp (-27016 / (T + 273))

t: holding time (s)

T: annealing temperature (℃)

When the annealing temperature is less than 750 ° C., or when the holding (annealing) time is less than 15 s, the formation of austenite during annealing becomes insufficient, and the amount of low temperature transformation phase required after the annealing cooling cannot be secured. On the other hand, when the annealing temperature exceeds 900 ° C, the austenite at the time of annealing is remarkably increased, so that the ferrite in the required amount after the annealing cooling cannot be secured. In addition, when the holding time exceeds 47.6 × 10 ⁻¹⁰ / exp (-27016 / (T + 273)) seconds, the concentration of Mn in the austenite phase during annealing proceeds excessively, and the concentration of Mn in the ferrite phase in the final structure is increased. It becomes difficult to make ratio of Mn concentration in a 2nd phase and 0.70 or more.

After annealing and cooling, 450? 10? In the temperature range of 550 ℃. Keep 200 s

When the holding temperature is higher than 550 ° C. or when 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 is not obtained. In addition, when the holding temperature is less than 450 ° 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 generated by bainite transformation promotion, and the desired pearlite area ratio is achieved. This is not obtained, and the hard martensite area ratio is increased, and good hole expandability and material stability are not obtained.

After the above holding, the surface is subjected to a hot dip galvanizing treatment for the purpose of improving the rust prevention performance during actual use.

In order to secure press property, spot weldability, and paint adhesiveness, many alloying hot dip galvanized steel sheets which heat-treated after plating and diffused Fe of the steel sheet in the plating layer are used. When manufacturing an alloying hot dip galvanized steel plate, after galvanizing, an alloying process is further performed on condition of the following.

[Alloy Processing Conditions]

500? In the temperature range of 600 degreeC, Tave: average holding temperature (degreeC) and th: holding time (s) are a following formula;

0.45≤exp [200 / (400-Tave)] × ln (th) ≤1.0

The alloying treatment of the plating layer is carried out under the conditions satisfying the following conditions.

In addition, exp (X) and ln (X) represent the exponential function and natural logarithm of X, respectively.

The alloying process of a plating layer is 500? In order to obtain an appropriate Fe% in the plating layer. It is made into the range of 600 degreeC.

When exp [200 / (400-Tave) × ln (th) is less than 0.45, there are many martensites in the final structure, and since the hard martensite is adjacent to the soft ferrite, there is a large hardness difference between the above. The hole expandability falls. When exp [200 / (400-Tave) × ln (th) is greater than 1.0, most of the unmodified austenite is transformed into cementite or pearlite, and as a result, a desired balance between strength and ductility is not obtained.

In addition, in a series of heat processing in the manufacturing method of this invention, if it exists in the temperature range mentioned above, a holding temperature does not need to be constant, and if it exists in a prescribed range, it does not impair the meaning of this invention. In addition, as long as only the thermal history is satisfied, the steel sheet may be heat treated by any equipment. In addition, temper rolling on the steel sheet of the present invention for shape correction after heat treatment is also included in the scope of the present invention. In addition, in the present invention, it is assumed that a steel material is manufactured through each process of ordinary steelmaking, casting, and hot rolling. For example, thin steel is used to omit part or all of the hot rolling process. It may be the case.

Although the other manufacturing method does not specifically limit, it shows below about a preferable example.

[Casting condition]

The steel slab to be used is preferably manufactured by a continuous casting method in order to prevent macro segregation of the components, but may be produced by a coarse method or a thin slab casting method. In addition, after the steel slab is manufactured, in addition to the conventional method of cooling to room temperature once and then heating again, it is not immediately cooled to room temperature and inserted into the heating furnace as it is, or immediately rolled after performing some heat retention. Energy saving processes such as direct rolling and direct rolling can also be applied without problems.

[Hot rolling condition]

Slab heating temperature: above 1100 ℃

The slab heating temperature is preferably energy low temperature, but when the heating temperature is less than 1100 ° C, carbides cannot be sufficiently dissolved or the risk of troubles during hot rolling due to an increase in rolling load is increased. There is a problem. Moreover, it is preferable that slab heating temperature shall be 1300 degrees C or less in the case of increase of the scale loss with increase of oxidation amount, etc.

Moreover, you may utilize what is called a sheet bar heater which heats a sheet bar from a viewpoint of preventing the trouble at the time of hot rolling, even if slab heating temperature is made low.

Finish rolling temperature: above Ar ₃ transformation point

If the finish rolling finish temperature is less than the Ar ₃ transformation point, α and γ are formed during rolling, and band-like structures are easily formed in the steel sheet, and such band-like structures remain after cold rolling or after annealing, resulting in anisotropy in material properties. This may cause deterioration of workability. Therefore, the finish rolling temperature is preferably not less than Ar ₃ transformation point.

In addition, in the hot rolling process in this invention, in order to reduce the rolling load at the time of hot rolling, you may perform lubrication rolling part or all of finish rolling. Implementing lubrication rolling is effective also from the viewpoint of the uniformity of the steel plate shape and the uniformity of the material. In addition, the friction coefficient at the time of lubrication rolling is 0.25? It is preferable to set it as the range of 0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet bars which are mixed back and front, and finish-rolls continuously. It is preferable to apply a continuous rolling process also from the viewpoint of operation stability of hot rolling.

[Cold rolling condition]

Subsequently, when cold rolling is performed, Preferably the oxidation scale of the surface of a hot rolled sheet steel is removed by pickling, it is provided to cold rolling, and it is set as the cold rolled sheet steel of predetermined plate | board thickness. Pickling conditions and cold rolling conditions are not specifically limited here, According to a conventional method. It is preferable to make the rolling reduction rate of cold rolling into 40% or more.

[Molten Zinc Plating Conditions]

The plating treatment is 0.08? Bath temperature 440? In a plating bath having a dissolved Al content of 0.18%? A steel plate is made to penetrate in a plating bath in 500 degreeC plating bath, and an adhesion amount is adjusted by gas wiping etc.

In addition, you may perform temper rolling on the steel plate after a hot dip galvanization process for adjustment of shape correction, surface roughness, etc. Moreover, there is no problem at all even if it processes resin, oil-fat coating, various coatings, etc.

Example

It had the component composition shown in Table 1, and the remainder melted in the converter the steel which consists of Fe and an unavoidable impurity (in Table 1, N is an unavoidable impurity), and made it cast by the continuous casting method.

The obtained cast steel was hot rolled at a plate thickness of 3.0 mm under the conditions shown in Tables 2 and 3. Subsequently, after pickling, it cold-rolled to 1.4 mm of plate | board thickness, the cold rolled sheet steel was produced, and it provided to the annealing. In addition, a part of the hot rolled steel sheet hot rolled to a plate thickness of 2.3 mm was subjected to annealing as it was.

Next, these cold-rolled steel sheets or hot-rolled steel sheets were subjected to annealing and plating treatment under the conditions shown in Tables 2 and 3 in a continuous hot dip galvanizing line. Plating amount is 35? It was 45 g / m <2>.

The microstructure, tensile property, extension flange property, and impact resistance property of the obtained steel sheet were investigated, and the results are shown in Tables 4 and 5.

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

The Mn concentration in the ferrite phase and the second phase was measured by performing a line analysis of Mn at 0.1 μm intervals by EPMA. The average value of Mn concentration of each particle | grain was made into the Mn density | concentration of the particle | grain, and it measured about 10 particle | grains in each of a ferrite phase and a 2nd phase, and made the average value the Mn concentration of the ferrite phase and a 2nd phase.

Workability evaluated ductility and hole expandability (elongation flange property).

Ductility was carried out by a tensile test at a strain rate of 10 ⁻³ / s, using a JIS No. 5 test piece taken from a rolling direction and a direction perpendicular to the raw steel sheet, to measure TS (tensile strength) and EL (total elongation). And it determined with the case of TSxEL≥19000 Mpa%% as favorable.

Extension flange performance was performed based on Japanese Steel Federation Standard JFST1001. After cutting the obtained steel plate to 100 mm x 100 mm, after punching the hole of diameter 10mm or more with plate | board thickness 2.0mm or more at clearance 12% +/- 1%, plate | board thickness below 2.0mm with clearance 12% +/- 2%, and Using a die with a 75 mm inner diameter, a punch of 60 ° cone was pushed into the hole with the blank holder force suppressed at 9 ton, and the hole diameter at the crack generation limit was measured, and the limit hole expansion ratio (λ) was calculated from the following equation. ) (%) Was calculated | required and the elongation flange property was evaluated from the value of this limit hole expansion rate.

Limit hole expansion rate (λ) (%) = ｛(D _f -D ₀ ) / D ₀ ｝ × 100

However, _f D is the pore diameter of the crack occurrence (㎜), D ₀ is the initial hole diameter (㎜). In the present invention, the case of λ ≧ 70 (%) was determined to be good.

The impact absorption characteristics are absorbed energy up to the deformation amount when the tensile test was performed at a strain rate of 2000 / s using a test piece having a width of 5 mm and a length of 7 mm in the parallel portion taken from the direction perpendicular to the rolling direction of the raw steel sheet. (See Iron and Steel, vol. 83 (1997), p. 748), and the impact absorption characteristics were evaluated by the ratio of absorbed energy to static TS (AE / TS). In addition, the absorbed energy is calculated from the strain-strain curve 0? It calculated | required by integrating in 5% of range.

In the example of the present invention, TS is 590 MPa or more, is excellent in ductility and elongation flangeability, and the ratio (AE / TS) of absorbed energy and static TS up to 5% of deformation rate at strain rate of 2000 / s is 0.050. As described above, a high strength alloyed hot dip galvanized steel sheet having high impact resistance characteristics was obtained by processing a small deformation region at a high strain rate. On the other hand, in the comparative example, since the AE / TS is less than 0.050, the high impact resistance deteriorates due to the processing of a small deformation region at a high deformation rate, or at least any of the ductility and the elongation flange properties.

Industrial availability

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

Claims (7)

The component composition is C: 0.04% or more, 0.13% or less, Si: 0.7% or more, 2.3% or less, Mn: 0.8% or more, 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% 0.1% or less, the remainder consisting of iron and unavoidable impurities, and the structure is, in area ratio, 75% or more ferrite phase, 1% or more bainitic ferrite phase, and 1% or more and 10% or less pearlite phase. And the area ratio of the martensite phase is 10% or less,
In addition, the martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≤ 0.6 is satisfied, and the ratio of the Mn concentration in the ferrite phase and the Mn concentration in the second phase is 0.70 or more. High strength hot dip galvanized steel with excellent properties. The method of claim 1,
Furthermore, as a component composition, it contains at least 1 sort (s) of element chosen from Cr: 0.05% or more, 1.0% or less, V: 0.005% or more, 0.5% or less, Mo: 0.005% or more and 0.5% or less. High strength hot dip galvanized steel with excellent workability and impact resistance. The method according to claim 1 or 2,
Furthermore, in mass% as a component composition, Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, B: 0.0003% or more and 0.0050% or less, Ni: 0.05% or more and 1.0% or less, Cu: 0.05% or more A high strength hot dip galvanized steel sheet excellent in workability and impact resistance characteristics, characterized by containing at least one element selected from 1.0% or less. The method according to any one of claims 1 to 3,
Furthermore, high-strength molten zinc excellent in the workability and impact resistance characteristics characterized by containing at least 1 sort (s) of elements selected from Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less as a component composition. Plated steel plate. The method according to any one of claims 1 to 4,
Further, high-strength molten zinc excellent in workability and impact resistance characteristics, characterized by containing at least one element selected from Ta: 0.001% or more and 0.010% or less and Sn: 0.002% or more and 0.2% or less as the mass composition. Plated steel plate. 6. The method according to any one of claims 1 to 5,
Furthermore, Sb: 0.002% or more and 0.2% or less by mass% as a component composition, The high strength hot dip galvanized steel plate excellent in the workability and impact resistance characteristics characterized by the above-mentioned. After hot-rolling to the steel slab which has a component composition as described in any one of Claims 1-6, the hot rolled sheet produced by winding up at the temperature of 300 degreeC or more and 570 degrees C or less is pickled, or cold-rolled further. And after that, 750? In the temperature range of 900 ° C, t: the holding time (s) is represented by the following formula;
15≤t≤47.6 × 10 ^-10 / exp (-27016 / (T + 273))
T: annealing temperature (℃)
After annealing to satisfy the condition, it is cooled and 450? 10? In the temperature range of 550 ℃. 200 s is maintained, followed by hot dip galvanizing and further 500? In the temperature range of 600 ° C, Tave: average holding temperature (° C) and th: holding time (s) are represented by the following formulas;
0.45≤exp [200 / (400-Tave)] × ln (th) ≤1.0
Process for producing a high strength hot dip galvanized steel sheet excellent in workability and impact resistance characteristics, characterized in that the galvanizing alloying treatment under conditions satisfying the conditions.