JP2011168876A - High-strength hot-dip galvanized steel sheet excellent in workability and impact resistance and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet which has high strength (tensile strength TS of 590 MPa or more), is excellent in workability, has a large absorption energy up to a low strain region of the order of 5% even when no strain is introduced by pressing, and to provide a method of manufacturing the high-strength hot-dip galvanized steel sheet. <P>SOLUTION: The galvanized steel sheet has a composition containing, by mass, 0.04 to 0.13% C, 0.7 to 2.3% Si, 0.8 to 2.0% Mn, ≤0.1% P, ≤0.01% S, 0.01 to 0.1% Al, and the balance iron with inevitable impurities, and has a structure which comprises a ferrite phase having an area ratio of ≥75%, a bainitic ferrite phase having that of ≥1% and a pearlite phase having that of 1 to 10%, and further a martensite phase having that of ≤10% and satisfies the relation: (area ratio of martensite)/((area ratio of bainitic ferrite)+(area ratio of pearlite))≤0.6, wherein the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is ≥0.70. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of conservation of the global environment. For this reason, a movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body has become active. However, increasing the strength of a steel sheet causes a decrease in ductility, that is, a decrease in forming processability, and therefore development of a material having both high strength and high processability is desired. In addition, since the strain rate received by each part at the time of a car collision reaches about 10 ³ / s, the impact resistance characteristic in such a high speed region is particularly important. Furthermore, taking into account the recent increase in demand for corrosion resistance of automobiles, development of high-tensile steel sheets subjected to hot dip galvanization has been frequently carried out. Further, in order to ensure pressability, spot weldability, and paint adhesion, many alloyed hot-dip galvanized steel sheets that are heat-treated after plating to diffuse Fe of the steel sheet into the plating layer are used.

  As a high-strength steel sheet excellent in workability and shock-absorbing property against such demands, a dual-phase steel sheet (DP steel sheet) composed of a composite structure of ferrite and martensite as disclosed in Patent Document 1 is representative. Is. However, DP steel, which originally has a low yield strength, shows a high impact absorption capacity because of its large work-hardening due to press working, and when processing strain enters, strain aging occurs in the subsequent paint baking process, resulting in a significant increase in yield strength. This is the reason for this, and there is a problem that a part with a small amount of processing such as bending does not necessarily exhibit a sufficient shock absorbing ability. Furthermore, DP steel is characterized by high shock absorption energy up to a high strain range of about 10 to 30% and excellent shock resistance characteristics, and is suitable for parts that deform to some extent during a collision, such as frontal collision parts, and absorb the collision energy. However, it cannot be said that the characteristics are sufficiently satisfied for a portion that requires high absorbed energy in a small strain region from the viewpoint of securing passenger space such as a side collision portion.

  Further, Patent Document 2 discloses a technique for improving impact resistance characteristics in TRIP steel using plasticity-induced transformation of residual γ, but has the same problem as DP steel.

JP 2003-213369 A JP 2001-335891 A

  The present invention has a high strength (tensile strength TS of 590 MPa or more), is excellent in workability, and has a large absorbed energy up to a low strain range of about 5% even without the introduction of strain by press working. An object of the present invention is to propose a hot-dip galvanized steel sheet having excellent impact characteristics and a method for producing the same.

  In order to achieve the above-described problems and to produce a high-strength hot-dip galvanized steel sheet that is excellent in workability and impact resistance characteristics, the present inventors have made extensive studies from the viewpoint of the composition and microstructure of the steel sheet. As a result, the main phase is ferrite and the second phase has a structure containing bainitic ferrite, martensite, and pearlite, and satisfies martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≦ 0.6. Furthermore, it was found that high workability and impact resistance characteristics can be obtained by setting the ratio of the Mn concentration in the ferrite phase and the Mn concentration in the second phase to 0.70 or more.

  Improvement of workability is achieved by improving ductility by improving the work hardening ability of ferrite, which is the main phase by utilizing Si, and expanding holes by reducing the hardness difference between soft ferrite and hard martensite by utilizing bainitic ferrite and pearlite. It became possible due to the improvement of sex.

  In addition, it is known that Mn usually concentrates in the second phase during hot rolling and annealing, and distribution occurs in the steel. However, the coiling temperature in hot rolling is lowered, and the soaking time during annealing is reduced. By making it appropriate, the distribution of Mn in the steel is made uniform, and the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is 0.70 or more, so that no strain is introduced by press working. Even so, the absorbed energy up to a low strain range of about 5% is large, and the collision resistance can be improved.

The present invention is configured based on the above-described knowledge.
That is, the present invention
(1) Component composition is mass% C: 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 less, Al: 0.01% or more and 0.1% or less, the balance is made of iron and inevitable impurities, and the structure has an area ratio of 75%. Having the above ferrite phase, 1% or more bainitic ferrite phase, and 1% or more and 10% or less pearlite phase, and the martensite phase area ratio is 10% or less, and
It satisfies martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≦ 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. High strength hot-dip galvanized steel sheet with excellent workability and impact resistance.

  (2) Furthermore, as a component composition, Cr: 0.05% to 1.0%, V: 0.005% to 0.5%, Mo: 0.005% to 0.5% in mass% The high-strength hot-dip galvanized steel sheet having excellent workability and impact resistance as described in (1), which contains at least one element selected from the following.

  (3) Further, as a component composition, Ti: 0.01% to 0.1%, Nb: 0.01% to 0.1%, B: 0.0003% to 0.0050% in mass% In the following, at least one element selected from Ni: 0.05% to 1.0% and Cu: 0.05% to 1.0% is contained (1) or (2) High-strength hot-dip galvanized steel sheet with excellent workability and impact resistance as described in 1.

  (4) Furthermore, as a component composition, it contains at least one element selected from Ca: 0.001% to 0.005% and REM: 0.001% to 0.005% by mass%. A high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance according to any one of (1) to (3).

  (5) Further, as a component composition, it contains at least one element selected from Ta: 0.001% to 0.010% and Sn: 0.002% to 0.2% by mass%. A high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance according to any one of (1) to (4).

  (6) Furthermore, as a component composition, Sb: 0.002% or more and 0.2% or less is contained in mass%, and the workability and resistance to resistance according to any one of (1) to (5), High strength hot-dip galvanized steel sheet with excellent impact properties.

(7) After hot rolling the steel slab having the component composition according to any one of (1) to (6), a hot-rolled sheet rolled and manufactured at a temperature of 300 ° C. or higher and 570 ° C. or lower is acidified. Washing or further cold rolling, and then in the temperature range of 750 to 900 ° C., t: retention time (s) is the following formula;
15 ≦ t ≦ 47.6 × 10 ⁻¹⁰ / exp (−27016 / (T + 273))
T: Annealing temperature (℃)
After annealing under the conditions satisfying, it is cooled and held for 10 to 200 s in a temperature range of 450 to 550 ° C., then hot dip galvanized, or further in a temperature range of 500 to 600 ° C., Tave: Average holding temperature ( ° C) and th: retention time (s) is the following formula;
0.45 ≦ exp [200 / (400−Tave)] × ln (th) ≦ 1.0
A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance characteristics, characterized in that galvanizing alloying treatment is performed under conditions satisfying the above conditions.

  According to the present invention, a high-strength hot-dip galvanized steel sheet having excellent workability and large absorbed energy up to a low strain range of about 5% and excellent impact resistance can be obtained without introducing strain by press working. Therefore, it is possible to achieve both the weight reduction of the automobile and the improvement of the collision safety, and the excellent effect of greatly contributing to the improvement of the performance of the automobile body is achieved.

  Hereinafter, the present invention will be specifically described.

  First, the reason why the composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.

C: 0.04% or more and 0.13% or less C is an element that stabilizes austenite, and is an element necessary for increasing the strength of the steel sheet in order to easily generate a phase other than ferrite. If the C content is less than 0.04%, it is difficult to ensure the desired strength even if the production 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 lowered, the hardening of the welded part and the heat-affected zone is remarkable, and the mechanical properties of the welded part are deteriorated. From this viewpoint, the C content is set to 0.04% or more and 0.13% or less.

Si: 0.7% or more and 2.3% or less Si is a ferrite-forming element and also an element effective for solid solution strengthening. In order to improve the balance between strength and ductility and to ensure the hardness of the ferrite, it is necessary to add 0.7% or more. However, excessive addition of Si exceeding 2.3% causes deterioration of surface properties, plating adhesion and adhesion due to generation of red scale and the like. Therefore, Si is made 0.7% to 2.3%. Preferably, it is 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 strengthening steel. In addition, it is an element that stabilizes austenite, and is an element necessary for adjusting the fraction of the second phase. For this reason, it is necessary to add 0.8% or more of Mn. On the other hand, when it exceeds 2.0% and it adds excessively, the martensite area rate in a 2nd phase will increase and stretch flangeability will fall. Therefore, Mn is made 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 P is an element effective for strengthening steel. However, when P is added excessively in excess of 0.1%, embrittlement occurs due to segregation at the grain boundaries and impact resistance is deteriorated. If it exceeds 0.1%, the alloying speed is significantly delayed. Therefore, P is set to 0.1% or less.

S: 0.01% or less S is an inclusion such as MnS, which causes deterioration in impact resistance and cracks along the metal flow of the weld. To S is set to 0.01% or less.

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 is preferably added in the deoxidation step. If the amount of Al is less than 0.01%, the effect of addition becomes poor, so the lower limit was made 0.01%. However, excessive addition of Al deteriorates slab quality during steelmaking. Therefore, Al is made 0.1% or less.

  The high-strength hot-dip galvanized steel sheet according to the present invention has the above-described component composition as a basic component, and the balance is composed of iron and unavoidable impurities, but at least one element selected from the elements described below according to desired characteristics Can be appropriately contained.

Cr: 0.05% to 1.0%, V: 0.005% to 0.5%, Mo: 0.005% to 0.5% Cr, V, and Mo increase the hardenability, It is an effective element for strengthening steel. The effect is obtained when Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. However, if the Cr is added in excess of 1.0%, V: 0.5%, and Mo: 0.5%, respectively, the second phase fraction becomes excessive and there is a concern that the workability will be lowered. Therefore, when these elements are added, the amounts thereof are Cr: 0.05% to 1.0%, V: 0.005% to 0.5%, Mo: 0.005% to 0%, respectively. .5% or less.

  Furthermore, one or more elements can be contained 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, Nb is effective for precipitation strengthening of steel, and the effect is obtained at 0.01% or more, If it is within the range specified in the present invention, it may be used for strengthening steel. However, if each exceeds 0.1%, the workability decreases. Therefore, when adding Ti and Nb, the addition amount is set to 0.01% to 0.1% for Ti and 0.01% to 0.1% for Nb.

B: 0.0003% or more and 0.0050% or less B has an action of suppressing the formation / growth of ferrite from the austenite grain boundary, and can be added as necessary. The effect is obtained at 0.0003% or more. However, if it exceeds 0.0050%, the workability decreases. Therefore, when adding B, it is made 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 elements effective for strengthening steel, and steel is within the range specified in the present invention. It can be used for strengthening. In order to obtain these effects, 0.05% or more is required. On the other hand, if both Ni and Cu are added in excess of 1.0%, the workability of the steel sheet is lowered. Therefore, when adding Ni and Cu, the addition amount is 0.05% or more and 1.0% or less, respectively.

  Furthermore, one or more elements can be contained 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 intended to improve the adverse effect of sulfides on spheroidizing and hole expandability. Is an effective element. In order to obtain this effect, 0.001% or more is required for each. However, excessive addition exceeding 0.005% causes an increase in inclusions and the like, and causes surface and internal defects. Therefore, when Ca and REM are added, the addition amounts are 0.001% or more and 0.005% or less, respectively.

  Furthermore, one or more elements can be contained from the following Ta and Sn.

Ta: 0.001-0.010%, Sn: 0.002-0.2%
Ta, like Ti and Nb, forms alloy carbide and alloy carbonitride to contribute to high strength, and partly dissolves in Nb carbide and Nb carbonitride, and (Nb, Ta) ( By forming a composite precipitate such as (C, N), it is considered that the coarsening of the precipitate is remarkably suppressed and the contribution to strength by precipitation strengthening is stabilized. Therefore, when Ta is added, the content is preferably 0.001% or more. However, if added excessively, not only the above-mentioned precipitate stabilization effect is saturated but also the alloy cost increases. 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 μm of the steel sheet surface layer caused by nitridation, oxidation, or oxidation of the steel sheet surface. By suppressing such nitriding and oxidation, the amount of martensite generated on the steel sheet surface is prevented from decreasing, and fatigue characteristics and aging resistance are improved. From the viewpoint of suppressing nitriding and oxidation, when adding Sn, its content is preferably 0.002% or more, and if it exceeds 0.2%, the toughness is reduced, so its content is reduced to 0. .2% or less is desirable.

  Furthermore, the following Sb can be contained.

Sb: 0.002 to 0.2%
Sb can also be added from the viewpoint of suppressing decarburization in the region of several tens of μm of the steel sheet surface layer caused by nitridation, oxidation, or oxidation of the steel sheet surface, similarly to Sn. By suppressing such nitriding and oxidation, the amount of martensite generated on the steel sheet surface is prevented from decreasing, and fatigue characteristics and aging resistance are improved. From the viewpoint of suppressing nitriding and oxidation, when Sb is added, its content is preferably 0.002% or more, and if it exceeds 0.2%, the toughness is reduced, so the content is reduced to 0. .2% or less is desirable.

  Next, the steel structure will be described.

Area ratio of ferrite phase: 75% or more In order to ensure good ductility, the ferrite phase needs to have an area ratio of 75% or more.

Area ratio of bainitic ferrite phase: 1% or more In order to ensure good hole expansibility, that is, to reduce the hardness difference between soft ferrite and hard martensite, the area ratio of bainitic ferrite phase is 1 % Or more is necessary.

Area ratio of pearlite phase: 1% or more and 10% or less In order to ensure good hole expansibility, the area ratio of the pearlite phase is 1% or more. When the area ratio of the pearlite phase exceeds 10%, the ductility (TS × EL) decreases. Therefore, the area ratio of the pearlite phase is 1% or more and 10% or less.

Martensite phase area ratio: 10% or less When the martensite phase area ratio exceeds 10%, the stretch flangeability deteriorates significantly. Therefore, the area ratio of the martensite phase is 10% or less.

Martensite area ratio / (Bainitic ferrite area ratio + pearlite area ratio) ≤ 0.6
Martensite has a large strength difference from ferrite and reduces flangeability, but coexists with bainitic ferrite and pearlite, martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≤ 0.6 By doing so, it becomes possible to suppress the fall of the hole expansibility by martensite. Accordingly, the martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≦ 0.6.

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

  Further, the area ratio of ferrite, bainitic ferrite, pearlite, and martensite in the present invention is the area ratio of each phase in the observation area.

  For the microstructure of the steel sheet 1/4 section of the cross section in the rolling direction of the steel sheet, after polishing, it was corroded with 3% nital, and was observed with a scanning electron microscope in a field of view of 5000 times. Image--Media Cybernetics, Inc. The area ratio of each phase can be obtained using Pro.

  At that time, since it is difficult to distinguish between martensite and retained austenite, the obtained hot-dip galvanized steel sheet was subjected to tempering treatment at 200 ° C. for 2 hours, and then the structure of the plate thickness section parallel to the rolling direction of the steel sheet was described above. The area ratio of the tempered martensite phase obtained by the above method was defined as the area ratio of the martensite phase.

  Further, the content of the retained austenite phase can be obtained from the diffraction X-ray intensity of the ¼ surface thickness after polishing the steel plate to ¼ surface in the thickness direction. At that time, CoKα rays are used as incident X-rays, and the {111}, {200}, {220}, {311} planes of the retained austenite phase and the {110}, {200}, {211} planes of the ferrite phase Intensity ratios can be obtained for all combinations of peak integrated intensities, and the average value of these can be used as the content of retained austenite phase, and the content can be treated as the area ratio of retained austenite.

The ratio of the Mn concentration in the ferrite phase to 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 making the distribution of Mn uniform in the steel, Even without the introduction of strain due to press working, the absorbed energy up to a low strain range of about 5% is large, and the impact resistance can be improved. The ratio between the Mn concentration in the ferrite phase and the Mn concentration in the second phase The effect is acquired by making 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, manufacturing conditions will be described.

  Steel adjusted to the above component composition is melted in a converter or the like and is made into a slab by a continuous casting method or the like. The steel slab is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is pickled or cold-rolled to obtain a cold-rolled steel sheet. After the hot-rolled steel sheet or the cold-rolled steel sheet that has been pickled is subjected to continuous annealing, it is subjected to a hot dip galvanizing process, or an alloying process of galvanizing. The reason for limitation of each process will be described.

[Hot rolling conditions]
Winding temperature: 300 ° C. or more and 570 ° C. or less When the winding temperature after hot rolling exceeds 570 ° C., the distribution of Mn to the second phase is promoted after winding, and the Mn concentration in the ferrite phase in the final structure It becomes difficult to set the ratio of the Mn concentration in the second phase to 0.70 or more. On the other hand, when the coiling temperature is less than 300 ° C., the shape of the hot rolled sheet deteriorates, the strength of the hot rolled sheet increases excessively, and cold rolling becomes difficult. Therefore, the coiling temperature is set to 300 ° C. or more and 570 ° C. or less.

[Continuous annealing conditions]
Annealing is performed in a temperature range of 750 to 900 ° C. under the conditions satisfying the following formula.
15 ≦ t ≦ 47.6 × 10 ⁻¹⁰ / exp (−27016 / (T + 273))
t: Retention time (s)
T: annealing temperature (° C)
When the annealing temperature is less than 750 ° C., or when the holding (annealing) time is less than 15 s, austenite is not sufficiently generated during annealing, and a necessary amount of low-temperature transformation phase cannot be secured after annealing cooling. On the other hand, if the annealing temperature exceeds 900 ° C., the austenite during annealing increases remarkably, and a necessary amount of ferrite cannot be secured after annealing cooling. Further, if the holding time exceeds 47.6 × 10 ⁻¹⁰ / exp (−27016 / (T + 273)) seconds, the concentration of Mn into the austenite phase during annealing proceeds excessively, and the final structure contains the ferrite phase. It becomes difficult to set the ratio of the Mn concentration and the Mn concentration in the second phase to 0.70 or more.

  It cools after annealing and hold | maintains for 10 to 200 s in the temperature range of 450-550 degreeC.

  When the holding temperature exceeds 550 ° C. or when the holding time is less than 10 s, the bainite transformation is not promoted and bainitic ferrite is hardly obtained, so that the desired hole expandability cannot be 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 with a large amount of dissolved carbon generated by promoting bainite transformation, and a desired pearlite area ratio is obtained. In addition, the hard martensite area ratio increases, and good hole expansibility and material stability cannot be obtained.

  After the above holding, the surface is hot dip galvanized for the purpose of improving the rust prevention ability during actual use.

  In order to ensure pressability, spot weldability, and paint adhesion, an alloyed hot-dip galvanized steel sheet that is heat-treated after plating and diffuses Fe of the steel sheet in the plating layer is often used. When producing an alloyed hot-dip galvanized steel sheet, after the hot-dip galvanizing, an alloying treatment is further performed under the following conditions.

[Alloying conditions]
In the temperature range of 500 to 600 ° C., Tave: average holding temperature (° C.), th: holding time (s) is the following formula:
0.45 ≦ exp [200 / (400−Tave)] × ln (th) ≦ 1.0
An alloying treatment of the plating layer is performed under conditions that satisfy the above conditions.
Here, exp (X) and ln (X) represent the exponential function and natural logarithm of X, respectively.

  The alloying treatment of the plating layer is set to a range of 500 to 600 ° C. in order to obtain an appropriate Fe% in the plating layer.

  When exp [200 / (400-Tave)] × ln (th) is less than 0.45, a lot of martensite is present in the final structure, and the hard martensite is adjacent to the soft ferrite. A large hardness difference occurs between them, and the hole expansibility decreases. When exp [200 / (400-Tave)] × ln (th) exceeds 1.0, most of the untransformed austenite is transformed into cementite or pearlite, and as a result, a desired balance between strength and ductility cannot be obtained.

  In the series of heat treatments in the production method of the present invention, the holding temperature does not have to be constant as long as it is within the above-described temperature range, and the spirit of the present invention is not impaired as long as it is within the specified range. Further, as long as the thermal history is satisfied, the steel sheet may be heat-treated by any equipment. In addition, temper rolling of the steel sheet of the present invention for shape correction after heat treatment is also included in the scope of the present invention. In the present invention, it is assumed that the steel material is manufactured through normal steelmaking, casting, and hot rolling processes, but the manufacturing process is performed by omitting part or all of the hot rolling process by thin casting, for example. You may do it.

  Other manufacturing methods are not particularly limited, but a preferred example is shown below.

[Casting conditions]
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab casting method. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then heating again, without cooling to room temperature, insert it into a heating furnace as it is, or carry out slight heat retention Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately, can be applied without any problem.

[Hot rolling conditions]
Slab heating temperature: 1100 ° C or higher Low temperature heating is preferable in terms of energy for the slab heating temperature, but if the heating temperature is less than 1100 ° C, carbides cannot be sufficiently dissolved or during hot rolling due to an increase in rolling load. Problems such as an increased risk of trouble occur. Note that the slab heating temperature is desirably 1300 ° C. or lower because of an increase in scale loss accompanying an increase in the amount of oxidation.

  From the viewpoint of preventing troubles during hot rolling even if the slab heating temperature is lowered, a so-called sheet bar heater that heats the sheet bar may be used.

Finishing rolling temperature: Ar ₃ transformation point or higher If the finishing rolling finish temperature is less than the Ar ₃ transformation point, α and γ are generated during rolling, and a band-like structure is easily formed on the steel sheet. It may remain after rolling or after annealing, causing anisotropy in material properties or reducing workability. For this reason, it is desirable that the finish rolling temperature is not less than the Ar ₃ transformation point.

  In the hot rolling process of the present invention, part or all of finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.

[Cold rolling conditions]
Next, when cold rolling is performed, 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. Here, pickling conditions and cold rolling conditions are not particularly limited, and may be in accordance with conventional methods. The rolling reduction of cold rolling is preferably 40% or more.

[Hot galvanizing conditions]
The plating process is performed by injecting the steel sheet into the plating bath with a plating bath having a bath temperature of 440 to 500 ° C. in a plating bath having a dissolved Al amount of 0.08 to 0.18%, and adjusting the adhesion amount by gas wiping or the like.
In addition, you may perform temper rolling to the steel plate after a hot dip galvanization process for adjustment, such as shape correction and surface roughness. In addition, there is no inconvenience even if treatments such as resin or oil coating and various paintings are applied.

  A steel having the composition shown in Table 1 and the balance being Fe and unavoidable impurities (in Table 1, N is an unavoidable impurity) is melted in a converter, and a slab is formed by a continuous casting method. did. The obtained slab was hot-rolled to a thickness of 3.0 mm under the conditions shown in Tables 2 and 3. Next, after pickling, the steel sheet was cold-rolled to a thickness of 1.4 mm to produce a cold-rolled steel sheet and subjected to annealing. In addition, a part of the hot-rolled steel sheet hot-rolled to a plate thickness of 2.3 mm and pickled was subjected to annealing as it was.

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

  The microstructure, tensile properties, stretch flangeability and impact resistance properties of the obtained steel sheet were investigated, and the results are shown in Tables 4 and 5.

  Note that the microstructure was observed with a scanning electron microscope at a thickness of ¼ part in the cross section in the rolling direction of the steel sheet, and the area ratio of each phase was determined by the method described above.

  The Mn concentration in the ferrite phase and the second phase was measured by EPN analysis of Mn at intervals of 0.1 μm. The average value of the Mn concentration of each particle was taken as the Mn concentration of the particle, 10 particles were measured for each of the ferrite phase and the second phase, and the average value was taken as the Mn concentration of the ferrite phase and the second phase.

  As for workability, ductility and hole expandability (stretch flangeability) were evaluated.

For ductility, a tensile test was performed at a strain rate of 10 ⁻³ / s using a JIS No. 5 specimen taken from a direction perpendicular to the rolling direction of the unprocessed steel sheet, and TS (tensile strength) and EL (total elongation) were measured. The case of TS × EL ≧ 19000 MPa ·% was determined to be good.

Stretch flangeability was performed in accordance with Japan Iron and Steel Federation Standard JFST1001. After cutting the obtained steel plate into 100 mm × 100 mm, after punching a 10 mm diameter hole with a clearance of 12% ± 1% for a thickness of 2.0 mm or more and a clearance of 12% ± 2% for a thickness less than 2.0 mm In a state where the inner diameter is 75 mm and the pressing force is 9 ton, a 60 ° conical punch is pushed into the hole to measure the hole diameter at the crack initiation limit. From the following formula, the critical hole expansion ratio λ (% ), And the stretch flangeability was evaluated from the value of the critical hole expansion rate.
Limit hole expansion ratio λ (%) = {(D _f −D ₀ ) / D ₀ } × 100
However, _{D f} hole diameter at crack initiation (mm), _{D 0} is the initial hole diameter (mm).
In the present invention, the case of λ ≧ 70 (%) is determined to be good.

  The impact absorption characteristic is that up to the amount of strain when a tensile test is performed at a strain rate of 2000 / s using a test piece having a width of 5 mm and a length of 7 mm of a parallel portion taken from a direction perpendicular to the rolling direction of the unprocessed steel plate. Absorption energy was obtained (see iron and steel, vol. 83 (1997), p. 748), and the impact absorption characteristics were evaluated by the ratio of the obtained absorption energy to static TS (AE / TS). The absorbed energy was determined by integrating the stress-true strain curve within the strain range of 0 to 5%.

  In the present invention example, the TS is 590 MPa or more, the ductility and the stretch flangeability are excellent, and the ratio (AE / TS) of the absorbed energy up to 5% of the strain rate and the static TS (AE / TS) is 2000 / s. A high-strength galvannealed steel sheet having a high impact resistance is obtained by processing in a small strain 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 is inferior in processing in a small strain region at a high strain rate, or at least any of ductility and stretch flangeability. One characteristic is inferior.

  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 according to the present invention can be used as a steel sheet applied not only to a frontal collision part of an automobile but also to a side collision part, and can also be used as a steel sheet used in a part with a small amount of processing such as bending. .

Claims (7)

The component composition is C: 0.04% to 0.13%, Si: 0.7% to 2.3%, Mn: 0.8% to 2.0%, P: 0.00% by mass. 1% or less, S: 0.01% or less, Al: 0.01% or more and 0.1% or less, the balance is made of iron and inevitable impurities, and the structure is ferrite with an area ratio of 75% or more Having a phase, a bainitic ferrite phase of 1% or more and a pearlite phase of 1% or more and 10% or less, and the area ratio of the martensite phase is 10% or less,
In addition, 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 to the Mn concentration in the second phase is 0.70 or more. A high-strength hot-dip galvanized steel sheet with excellent workability and impact resistance. Further, the component composition is selected from mass%, Cr: 0.05% to 1.0%, V: 0.005% to 0.5%, Mo: 0.005% to 0.5%. The high-strength hot-dip galvanized steel sheet having excellent workability and impact resistance characteristics according to claim 1, wherein the steel sheet contains at least one element selected from the above. Furthermore, as a component composition, in mass%, 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 The processability according to claim 1 or 2, comprising at least one element selected from: 0.05% to 1.0% and Cu: 0.05% to 1.0%. And high strength hot-dip galvanized steel sheet with excellent impact resistance. Furthermore, as a component composition, it contains at least one element selected from Ca: 0.001% to 0.005% and REM: 0.001% to 0.005% in mass%. A high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance characteristics according to any one of claims 1 to 3. Furthermore, as a component composition, it contains at least one element selected from Ta: 0.001% to 0.010% and Sn: 0.002% to 0.2% by mass%. The high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance according to any one of claims 1 to 4. Furthermore, it has excellent workability and impact resistance according to any one of claims 1 to 5, characterized in that, as a component composition, Sb: 0.002% or more and 0.2% or less in mass%. High strength hot dip galvanized steel sheet. After hot rolling the steel slab having the component composition according to any one of claims 1 to 6, the hot-rolled sheet wound and manufactured at a temperature of 300 ° C or higher and 570 ° C or lower is pickled, or further Cold rolling, and then in the temperature range of 750 to 900 ° C., t: retention time (s) is the following formula;
15 ≦ t ≦ 47.6 × 10 ⁻¹⁰ / exp (−27016 / (T + 273))
T: Annealing temperature (℃)
After annealing under the conditions satisfying, it is cooled and held for 10 to 200 s in a temperature range of 450 to 550 ° C., then hot dip galvanized, or further in a temperature range of 500 to 600 ° C., Tave: Average holding temperature ( ° C) and th: retention time (s) is the following formula;
0.45 ≦ exp [200 / (400−Tave)] × ln (th) ≦ 1.0
A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance characteristics, characterized in that galvanizing alloying treatment is performed under conditions satisfying the above conditions.