JP5640898B2 - Hot rolled steel sheet - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet. More specifically, the present invention relates to a hot-rolled steel sheet having high strength and excellent ductility and stretch flangeability.

  In recent years, from the viewpoint of protecting the global environment, we are working to reduce carbon dioxide emissions in many fields. Automakers are also actively developing technology to reduce vehicle weight for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of the vehicle body because the emphasis is also placed on improving the anti-collision characteristics in order to ensure passenger safety.

Therefore, in order to achieve both the weight reduction of the vehicle body and the anti-collision property, it has been studied to reduce the thickness of the member using a high-strength steel plate. For this reason, a steel sheet having both high strength and excellent formability is strongly desired, and several techniques have been proposed in order to meet these requirements.
Among these, many studies have been made on steel sheets containing retained austenite because they exhibit excellent ductility due to the transformation induced plasticity (TRIP) phenomenon.

  For example, in Patent Document 1 (Japanese Patent Laid-Open No. 11-323494), ferrite is the main phase, the balance is generally bainite and retained austenite, and the average hardness of bainite is 240 to 400 Hv, which is excellent in formability. A high strength hot rolled steel sheet is disclosed.

  In Patent Document 2 (Japanese Patent Laid-Open No. 2008-285748), C: 0.10 to 0.25%, Si: 0.5 to 3.0%, Mn: 0.2 to 1.0%, Cr: 1.0 to 2.5%, Ni: 0.02 to 0.50%, the balance is composed of iron and inevitable impurities, the size of residual austenite grains is 1 μm or less, and the residual austenite structure A high-strength hot-rolled steel sheet having a ratio of 5% to 20%, a martensite structure ratio of 5% or less, and the balance of a ferrite structure and a bainite structure is disclosed.

JP 11-323494 A JP 2008-285748 A

  Since there are various types of processing for automobile parts, the required formability differs depending on the applied member, but ductility and stretch flangeability are positioned as important indicators of formability. It is desirable to have a high level. Furthermore, in recent years, it is desired to have higher strength than before. Moreover, since surface treatments, such as chemical conversion treatment, may be given in order to provide corrosion resistance, it is desired that the workability of these surface treatments is good.

  As described above, although the steel sheet containing retained austenite has excellent ductility, the retained austenite is transformed into hard martensite by transformation-induced plasticity (TRIP), and this hard martensite reduces stretch flangeability. Therefore, it is usually difficult to ensure excellent stretch flangeability.

In the invention described in Patent Document 1, since the steel structure is mainly composed of ferrite, it is difficult to meet the recent demand for higher strength.
Since the invention described in Patent Document 2 requires that the Cr content be 1.0% or more, there is a problem in chemical conversion treatment. Furthermore, stretch flangeability, which is one of the important moldability indexes, is not taken into consideration.

  This invention is made | formed in view of the prior art mentioned above, and it aims at providing the hot-rolled steel plate which has the high ductility and stretch flangeability while having high intensity | strength.

In view of the above situation, the present inventors have conducted extensive research on the chemical composition of hot-rolled steel sheets and the relationship between the steel structure and mechanical properties, and as a result, obtained the following knowledge and completed the present invention.
(A) The steel structure is preferably hard to obtain high strength, and the steel structure is preferably homogeneous to obtain excellent stretch flangeability. Therefore, in order to combine high strength and excellent stretch flangeability, bainite, which is a hard and homogeneous structure, is most suitable, and it is important to have a steel structure mainly composed of bainite.

(B) However, bainite is a structure with poor ductility. For this reason, it is difficult to ensure excellent ductility simply by using a steel structure mainly composed of bainite.
(C) Therefore, by including an appropriate amount of polygonal ferrite and retained austenite, excellent ductility is also achieved.

  (D) That is, by including an appropriate amount of polygonal ferrite, the work hardening index at the initial stage of deformation can be increased, and carbon concentration to retained austenite is promoted as a reflective effect. The curing index can also be increased. As a result, ductility and stretch flangeability are improved.

(E) Moreover, by including a suitable amount of retained austenite, ductility is enhanced by transformation-induced plasticity (TRIP).
(F) Here, retained austenite increases the ductility by transformation-induced plasticity (TRIP), while it transforms to hard martensite by transformation-induced plasticity (TRIP) and reduces stretch flangeability. For this reason, if the retained austenite is simply contained, the effect of improving the stretch flangeability by making the steel structure mainly composed of bainite is diminished, and it becomes difficult to ensure excellent stretch flangeability.

  (G) Therefore, the reduction in stretch flangeability due to martensite is caused by the concentration of strain around the martensite due to the hardness difference between the martensite and the surrounding structure. By dispersing, martensite generated by transformation from retained austenite is finely dispersed, and by dispersing the strain, a decrease in stretch flangeability is suppressed.

  (H) By doing so, it is possible to enjoy the positive effect of improving the ductility while suppressing the negative effect of reducing the stretch flangeability due to retained austenite. The area ratio can be increased, and the effect of improving ductility by retained austenite can be sufficiently exhibited.

  (I) Here, the retained austenite is mainly formed between grains having a crystal orientation difference of 15 ° or more and between bainite laths, but the former tends to be coarser than the latter. Therefore, it is important to finely disperse the former retained austenite. For this reason, it is effective to reduce the average grain size of grains having a crystal orientation difference of 15 ° or more and increase the generation site of retained austenite.

  (J) Since the generation site of retained austenite is increased by reducing the average grain size of grains having a crystal orientation difference of 15 ° or more, the retained austenite area ratio is also increased.

  (K) By making the retained austenite area ratio in the steel sheet surface layer portion higher than the retained austenite area ratio in the steel plate interior, the uniform elongation of the steel sheet surface layer portion is improved, so that the strain amount of the steel sheet surface layer portion is larger than that in the steel plate interior. Formability in large stretch flange molding or bending molding can be improved.

The gist of the present invention based on the above findings is as follows.
(1) By mass%, C: more than 0.08% and less than 0.30%, Mn: 1.0% or more and 4.0% or less, Si: 0.10% or more and less than 3.0%, sol. Al: 0.01% or more and 3.0% or less, provided that Si and sol. The total content of Al is 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.01% or less and N: 0.01% or less, with the balance being Fe and impurities. Having the chemical composition
The steel structure at the 1/4 depth position of the plate thickness from the steel sheet surface is mainly bainite and is in area%, bainite (V _B ): 40% or more, polygonal ferrite (V _α ): 2.0% or more 50 % And residual austenite (V _γ ): 3% or more, the remainder being 15.0% or less, and surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite The average particle size (D) of the particles to be obtained is 15 μm or less,
A hot-rolled steel sheet having a thickness of more than 1.2 mm and not more than 6 mm.

  (2) The chemical composition is selected from the group consisting of Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.50% or less in mass% instead of a part of the Fe. The hot-rolled steel sheet according to (1) above, which contains one or more kinds.

  (3) The chemical composition is mass% in place of part of the Fe, Cr: less than 1.0%, Mo: 0.5% or less, Ni: 1.0% or less, and B: 0.0050. The hot rolled steel sheet according to the above (1) or (2), which contains one or more selected from the group consisting of% or less.

  (4) The chemical composition is selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less in mass% instead of a part of the Fe. The hot-rolled steel sheet according to any one of the above (1) to (3), which contains one or more kinds.

(5) The hot-rolled steel sheet according to any one of (1) to (4), wherein the chemical composition contains Cu: 1.0% by mass or less instead of a part of the Fe.
(6) The hot rolled steel sheet according to any one of (1) to (5), wherein the chemical composition contains Bi: 0.020% by mass or less instead of a part of the Fe.

(7) The residual austenite area ratio (V _γs ) at a depth of 100 μm from the steel sheet surface and the residual austenite area ratio (V _γ ) at a position of ¼ depth from the steel sheet surface satisfy the following formula (1). The hot rolled steel sheet according to any one of (1) to (6) above.

V _γs > V _γ (1)

  Since the hot-rolled steel sheet according to the present invention has high strength and excellent ductility and stretch flangeability, it is suitable as a material used for automobile members, machine structural members, and building members.

  The chemical composition and steel structure of the hot-rolled steel sheet of the present invention will be specifically described below. In the following description, all percentages relating to the chemical composition of the steel sheet are mass% unless otherwise specified.

<Chemical composition>
(C: more than 0.08% and less than 0.30%)
C has an action of promoting the formation of bainite and an action of stabilizing retained austenite. When the C content is 0.08% or less, it is difficult to secure the target bainite area ratio and the retained austenite area ratio. Therefore, the C content is more than 0.08%. Preferably it is 0.10% or more. On the other hand, when the C content is 0.30% or more, pearlite is preferentially generated. As a result, the generation of bainite and retained austenite becomes insufficient, and it becomes difficult to secure the target bainite area ratio and retained austenite area ratio. Therefore, the C content is less than 0.30%. Preferably it is 0.25% or less, more preferably 0.22% or less.

(Mn: 1.0% to 4.0%)
Mn has the effect of suppressing the ferrite transformation and promoting the formation of bainite. If the Mn content is less than 1.0%, it is difficult to ensure the target bainite area ratio. Therefore, the Mn content is 1.0% or more. Preferably it is 1.5% or more, More preferably, it is 1.8% or more. On the other hand, if the Mn content exceeds 4.0%, ferrite transformation is excessively suppressed, and it becomes difficult to secure the target polygonal ferrite area ratio. In addition, since the completion of the bainite transformation is delayed, carbon concentration to austenite is not promoted, and the generation of retained austenite becomes insufficient, making it difficult to secure the desired retained austenite area ratio. Furthermore, it becomes difficult to increase the carbon concentration in the retained austenite. Therefore, the Mn content is 4.0% or less. Preferably it is 3.6% or less, More preferably, it is 3.2% or less.

(Si: 0.10% or more and less than 3.0%)
Si has the effect of deoxidizing steel to make the steel plate sound and also has the effect of promoting the formation of retained austenite by suppressing the precipitation of cementite from austenite. Moreover, it has the effect | action which raises an intensity | strength by solid solution strengthening. If the Si content is less than 0.10%, it is difficult to obtain the effect by the above action. Therefore, the Si content is set to 0.10% or more. Preferably it is 0.5% or more. On the other hand, when the Si content is 3.0% or more, the A ₃ point is significantly increased, and stable hot rolling may be difficult. In addition, ductility and weldability are deteriorated. Therefore, the Si content is less than 3.0%. Preferably it is less than 2.5%.

(Sol.Al: 0.01% or more and 3.0% or less)
Al, like Si, has the effect of deoxidizing steel to make the steel plate sound and also has the effect of promoting the formation of retained austenite by suppressing the precipitation of cementite from austenite. sol. If the Al content is less than 0.01%, it is difficult to obtain the effect by the above action. Therefore, sol. Al content shall be 0.01% or more. Preferably it is 0.10% or more, more preferably 0.20% or more. On the other hand, sol. If the Al content exceeds 3.0%, the A ₃ point may be significantly increased, and stable hot rolling may be difficult. Therefore, sol. Al content shall be 3.0% or less. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

(Total content of Si and sol.Al: 0.8% to 3.0%)
As described above, both Si and Al have an action of promoting the formation of retained austenite, and therefore, from the viewpoint of securing the desired retained austenite area ratio, Si and sol. The total content of Al is specified.

Si and sol. If the total content of Al is less than 0.8%, it is difficult to ensure the intended retained austenite area ratio. In addition, it is difficult to increase the carbon concentration in the retained austenite. Therefore, Si and sol. The total content of Al is 0.8% or more. Preferably it is 1.0% or more, More preferably, it is 1.2% or more, Most preferably, it is 1.5% or more. On the other hand, Si and sol. If the total content of Al exceeds 3.0%, the A ₃ point may be significantly increased, and stable hot rolling may be difficult. Therefore, Si and sol. The total content of Al is 3.0% or less. Preferably it is 2.5% or less, More preferably, it is 2.0% or less, Most preferably, it is 1.9% or less.

(P: 0.050% or less)
P is an element generally contained as an impurity, but is also an element having an effect of increasing strength by solid solution strengthening. Therefore, P may be positively included. However, P is an element that easily segregates, and when its content exceeds 0.050%, the deterioration of formability and toughness due to grain boundary segregation becomes significant. Therefore, the P content is set to 0.050% or less. Preferably it is 0.030% or less, More preferably, it is 0.020% or less. The lower limit of the P content need not be specified, but is preferably 0.001% or more from the viewpoint of refining costs.

(S: 0.010% or less)
S is an element contained as an impurity, and forms sulfide inclusions in the steel and lowers formability. If the S content exceeds 0.010%, the moldability is significantly reduced. Therefore, the S content is set to 0.010% or less. Preferably it is 0.0050% or less, more preferably 0.0003% or less, and most preferably 0.0010% or less. The lower limit of the S content is not particularly required, but is preferably 0.0001% or more from the viewpoint of refining costs.

(N: 0.010% or less)
N is an element contained as an impurity and has an effect of reducing the formability of the steel sheet. If the N content exceeds 0.010%, the moldability is remarkably reduced. Therefore, the N content is set to 0.010% or less. Preferably it is 0.0008% or less, More preferably, it is 0.0070% or less. The lower limit of the N content does not need to be specified, but considering the case where one or more of Ti, Nb, and V are included to refine the steel structure as described later, In order to promote precipitation, the content is preferably 0.0010% or more. More preferably, it is 0.0001% or more.

(Ti: 0.10% or less, Nb: 0.10% or less, and V: one or more selected from the group consisting of 0.50% or less)
Ti, Nb and V produce fine carbonitrides in the hot rolling process, refine the austenite after hot rolling is completed and before transformation by cooling by the pinning effect, and the steel of the hot rolled steel sheet as the final product It also has the effect of refining the tissue. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if the content exceeds the above upper limit, the above action is saturated and disadvantageous in terms of cost. Therefore, the content of each element is as described above. The contents of Ti and Nb are each preferably 0.05% or less, and more preferably 0.03% or less. The V content is preferably 0.30% or less, and more preferably 0.20% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make content of any element 0.005% or more.

(Cr: less than 1.0%, Mo: 0.5% or less, Ni: 1.0% or less, and B: one or more selected from the group consisting of 0.0050% or less)
Cr, Mo, Ni, and B have an effect of improving hardenability. Moreover, Cr and Ni have the effect | action which stabilizes a retained austenite, and Mo has the effect | action which precipitates a carbide | carbonized_material in steel and raises an intensity | strength. Moreover, Ni has the effect | action which suppresses effectively the grain boundary crack of the slab resulting from Cu, when containing Cu so that it may mention later. Therefore, you may contain 1 type, or 2 or more types of these elements.

  However, when the Cr content is 1.0% or more, the chemical conversion property is remarkably lowered. Therefore, the Cr content is less than 1.0%. In order to more reliably obtain the effect of the above action, the Cr content is preferably set to 0.05% or more.

  Even if the Mo content exceeds 0.5%, the effect of the above action is saturated and disadvantageous in cost. Therefore, the Mo content is 0.5% or less. Preferably it is 0.2% or less. In order to more reliably obtain the effect of the above action, the Mo content is preferably set to 0.02% or more.

  Since Ni is an expensive element, a large amount is disadvantageous in terms of cost. Therefore, the Ni content is 1.0% or less. In order to more reliably obtain the effect of the above action, the Ni content is preferably set to 0.05% or more.

  When the B content exceeds 0.0050%, the moldability is remarkably deteriorated. Therefore, the B content is 0.0050% or less. In order to more reliably obtain the effect of the above action, the B content is preferably set to 0.0002% or more.

(One or more selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less)
Ca, Mg, and REM have an effect of improving moldability by adjusting the shape of inclusions. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the content of these elements exceeds the above upper limit, the inclusions in the steel become excessive, and the formability may be lowered on the contrary. Therefore, the content of each element is as described above. Each element is preferably 0.010% or less, more preferably 0.005% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain 0.0005% or more of any of the above elements.

(Cu: 1.0% or less)
Cu has the effect of increasing the strength by precipitation at a low temperature, so it may be contained in the steel. However, if the Cu content exceeds 1.0%, grain boundary cracking of the slab may occur. Therefore, the Cu content is 1.0% or less. Preferably it is less than 0.5%, more preferably less than 0.3%. In order to more reliably obtain the effect of the above action, the Cu content is preferably set to 0.05% or more.

(Bi: 0.020% or less)
Bi has the effect of improving the formability by refining the solidified structure, so it may be contained in the steel. However, even if the Bi content exceeds 0.020%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the Bi content is set to 0.020% or less. Preferably it is 0.010% or less. In order to more reliably obtain the effect of the above action, the Bi content is preferably set to 0.0005% or more.

<Steel structure>
About the structure | tissue of the hot-rolled steel plate which concerns on this invention, the steel structure in the 1/4 depth position of plate | board thickness from a steel plate surface is prescribed | regulated as follows. The position is an intermediate point between the surface of the steel plate and the center of the plate thickness, and shows the average steel structure of the hot-rolled steel plate.

(Bainite area ratio V _B : 40% or more)
As described above, bainite is a hard and homogeneous structure, and is the most suitable structure for combining high strength and excellent stretch flangeability. If the bainite area ratio is less than 40%, it is difficult to combine the steel sheet with high strength and excellent stretch flangeability. Therefore, the bainite area ratio is set to 40% or more. Preferably it is 50% or more, more preferably 60% or more. The upper limit of the bainite area ratio need not be specified. However, the bainite area ratio is 95% or less from the lower limit values of the area ratios of other phases and structures described later. The bainite of the present invention includes both upper bainite and lower bainite.

(Polygonal ferrite area ratio V _α : 2.0% or more and less than 50%)
By including soft polygonal ferrite, the work hardening index at the initial deformation of the steel sheet is improved. Furthermore, since the carbon concentration to retained austenite is promoted as a reflective effect, the work hardening index in the later stage of deformation is also improved. As a result, the ductility and stretch flangeability of the steel sheet are improved. If the polygonal ferrite area ratio is less than 2.0%, it is difficult to obtain the effect by the above action. Therefore, the polygonal ferrite area ratio is set to 2.0% or more. Preferably it is 4.0% or more, More preferably, it is 6.0% or more.

  On the other hand, when the polygonal ferrite area ratio is 50% or more, the formation of steel sheets is caused by the increase in the interface between polygonal ferrite and martensite and the interface between polygonal ferrite and pearlite, which are likely to be the origin of voids. May decrease. Moreover, since the ratio of the soft polygonal ferrite which occupies for a steel structure increases, it becomes difficult to ensure high intensity | strength. Therefore, the polygonal ferrite area ratio is less than 50%. Preferably it is 45% or less, More preferably, it is 40% or less.

(Residual austenite area ratio _Vγ : 3% or more)
Residual austenite has the effect of increasing ductility by transformation induced plasticity (TRIP). When the retained austenite area ratio is less than 3%, it is difficult to obtain the effect by the above action. Therefore, the retained austenite area ratio is set to 3% or more. Preferably it is 4% or more, More preferably, it is 6% or more. The upper limit of the retained austenite area ratio need not be specified, but the retained austenite area ratio that can be secured in the above chemical composition is generally less than 40%.

In addition, by setting the carbon concentration (C _γ ) in the retained austenite to 0.4 mass% or more, the retained austenite is appropriately stabilized so that a large amount of transformation-induced plasticity (TRIP) is generated in the high strain region at the later stage of deformation. Therefore, ductility and stretch flangeability are further improved. Therefore, C _γ is preferably 0.4% by mass or more. More preferably, it is 0.6 mass% or more, Most preferably, it is 0.8 mass% or more. Moreover, by making _Cγ 2.0% by mass or less, excessive stabilization of retained austenite can be suppressed, and transformation-induced plasticity (TRIP) can be expressed more reliably. Therefore, C _γ is preferably 2.0% by mass or less. More preferably, it is 1.8 mass% or less.

  The method for quantifying retained austenite includes X-ray diffraction, EBSP (Electron Back Scattering Pattern, Electron Back Scattering Pattern) analysis, magnetic measurement, and the like, and the quantification value may differ depending on the method. The area ratio of retained austenite specified in the present invention is a value measured by X-ray diffraction.

(Remaining area ratio excluding bainite, polygonal ferrite and retained austenite: 15% or less)
The structure of the hot-rolled steel sheet of the present invention is preferably composed of the above-described bainite, polygonal ferrite and retained austenite from the viewpoint of formability, but even if a structure other than the above, such as martensite, pearlite, and cementite, is mixed. If the area ratio is 15% or less, it is acceptable. The area ratio of the remainder is preferably 10% or less.

(Average grain size (D) of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite: 15 μm or less)
As described above, retained austenite is formed mainly between grains having a crystal orientation difference of 15 ° or more and between bainite laths. And since the former tends to be coarser than the latter, it is important to finely disperse the former retained austenite. For this purpose, it is effective to reduce the average grain size (D) of grains having a crystal orientation difference of 15 ° or more and increase the generation site of retained austenite.

  When D exceeds 15 μm, it is insufficient to finely disperse the retained austenite, and it is difficult to effectively suppress the effect of reducing the stretch flangeability due to the retained austenite. Therefore, D is set to 15 μm or less. The thickness is preferably less than 12 μm, more preferably less than 10 μm, particularly preferably less than 8 μm. Since D is preferably as small as possible, the lower limit of D need not be specified.

  The average particle diameter (D) is a value calculated by the equation shown in [Equation 1] below. In the formula, N is the number of grains included in the average grain size evaluation region, Ai is the area of the i-th grain (i = 1, 2,..., N), and di is the equivalent circle diameter of the i-th grain. Indicates. These data are easily obtained by EBSP analysis. Specifically, phases are distinguished using the crystal structure definition of iron face-centered cubic lattice (FCC) and body-centered cubic lattice (BCC), and only those phases recognized as body-centered cubic lattice (BCC) It is calculated | required by analyzing.

  The grains having a crystal orientation difference of 15 ° or more are mainly ferrite grains and bainite blocks. In the method for measuring the ferrite grain size according to JIS G0552, the grain size is calculated even for grains having a crystal orientation difference of less than 15 °, and the bainite block is not calculated. It cannot be specified. Therefore, in the present invention, a value obtained by EBSP analysis is adopted.

_(V γs> V _γ)
The structure of the hot-rolled steel sheet according to the present invention preferably has the following steel structure in addition to defining the steel structure at a position at a quarter depth of the sheet thickness from the steel sheet surface as described above.

  In a forming method in which the amount of strain in the steel plate surface layer portion is large compared to the inside of the steel plate, such as stretch flange forming and bending forming, it is preferable that the uniform elongation in the steel plate surface layer portion is high. Therefore, it is preferable to have a tilted structure containing more retained austenite in the steel sheet surface layer than in the steel sheet. That is, it is preferable to satisfy the following formula (1).

V _γs > V _γ (1)
Here, V _γs is the retained austenite area ratio at a depth of 100 μm from the steel sheet surface, and V _γ is the retained austenite area ratio at a position of ¼ depth from the steel sheet surface.

<Thickness: more than 1.2mm and less than 6mm>
If the thickness of the hot-rolled steel sheet is 1.2 mm or less, it may be difficult to ensure the rolling completion temperature, and the rolling load may be excessive, which may make hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet of the present invention is more than 1.2 mm. Preferably it is 1.4 mm or more. On the other hand, if the plate thickness exceeds 6 mm, it is difficult to refine the steel structure, and it becomes difficult to secure the above-described steel structure. In addition, it becomes difficult to obtain the above-described inclined structure. Therefore, the plate thickness is 6 mm or less. Preferably it is 5 mm or less.

<Others>
(Plating layer)
The surface of the hot-rolled steel sheet according to the present invention having the above-described chemical composition and steel structure may be provided with a plating layer for the purpose of improving corrosion resistance or the like to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. Examples of the hot dip plating layer include hot dip galvanizing, alloying hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

<Production conditions>
The hot-rolled steel sheet of the present invention only needs to have the above-described chemical composition, steel structure and plate thickness, and its production method is not particularly limited, but a suitable production method for obtaining the hot-rolled steel sheet of the present invention is as follows. Explained. Note that the steel structure and mechanical properties in the following description are not the steel structure in production but the steel structure and mechanical properties of the hot-rolled steel sheet in the final product stage, unless otherwise specified. Further, the steel structure is an average steel structure at a 1/4 depth position of the plate thickness.

In order to obtain the hot-rolled steel sheet of the present invention, it is preferable to introduce an appropriate strain by hot rolling and promote transformation by efficiently using the driving force due to the strain.
Specifically, the hot rolling is multi-pass rolling, the rolling reduction in the final rolling pass is 5% to 50%, and rolling is completed in a temperature range of 860 ° C. or higher and 1050 ° C. or lower, and within 1 second after completion of rolling. After cooling to a temperature range of 720 ° C. or lower and staying in a temperature range of 500 ° C. or higher and 720 ° C. or lower for a time of 1 second or longer and 15 seconds or shorter, winding is preferably performed in a temperature range of 350 ° C. or higher and 500 ° C. or lower. . It is more preferable that the rolling reduction of the rolling pass immediately before the final rolling pass is 20% or more and 50% or less.

(Slab, slab temperature during hot rolling, hot rolling mode)
The slab to be used for hot rolling can be a slab obtained by continuous casting or a slab obtained by casting / splitting, and if necessary, a slab obtained by adding hot working or cold working. be able to.

  The temperature of the slab to be subjected to hot rolling may be heated to a temperature that becomes an austenite single-phase region in order to perform hot rolling in the austenite region, and it is not necessary to specifically limit it, but a suitable rolling completion temperature described later is ensured. From the viewpoint of reducing the scale loss, it is preferable to set the temperature to 1050 ° C. or higher, and from the viewpoint of suppressing the scale loss. In addition, when the slab to be subjected to hot rolling is a slab obtained by continuous casting or a slab obtained by partial rolling and is in a high temperature state, it may be subjected to hot rolling without heating.

  In hot rolling, it is preferable to use a lever mill or a tandem mill as multi-pass rolling. In particular, from the viewpoint of industrial productivity, at least the final several stages are more preferably rolled using a tandem mill.

(Rolling ratio of final rolling pass: 5% to 50%)
The rolling reduction of the final rolling pass is preferably 5% or more and 50% or less.
By setting the rolling reduction ratio of the final rolling pass to 5% or more, moderate strain is introduced, and by appropriately performing subsequent cooling treatment, ferrite transformation and bainite transformation that efficiently use the driving force due to the strain are performed. It is realized and the desired steel structure can be easily obtained for the hot rolled steel sheet as the final product. Therefore, the rolling reduction of the final rolling pass is preferably 5% or more. More preferably, it is 10% or more.

  In addition, by setting the rolling reduction ratio of the final rolling pass to 50% or less, the amount of strain introduced in the final rolling pass is moderately suppressed, and excessive generation of ferrite and pearlite is suppressed, and the intended steel It becomes easier to get the organization. Therefore, the rolling reduction of the final rolling pass is preferably 50% or less. More preferably, it is 45% or less.

(Rolling completion temperature: 860 ° C or higher and 1050 ° C or lower)
The rolling completion temperature is preferably 860 ° C. or higher and 1050 ° C. or lower.
By setting the rolling completion temperature to 860 ° C. or higher, deformation resistance during rolling is reduced and rolling becomes easy. Therefore, the rolling completion temperature is preferably 860 ° C. or higher. More preferably, it is 880 degreeC or more, Most preferably, it is 900 degreeC or more, Most preferably, it is 910 degreeC or more.

  Moreover, by setting the rolling completion temperature to 1050 ° C. or less, release of strain introduced by rolling is suppressed, and by appropriately performing subsequent cooling treatment, ferrite transformation that efficiently uses the driving force due to the strain and The bainite transformation is realized, and the intended steel structure can be easily obtained for the hot rolled steel sheet as the final product. Therefore, the rolling completion temperature is preferably 1050 ° C. or lower. More preferably, it is 1030 degrees C or less, Most preferably, it is 1000 degrees C or less, Most preferably, it is 980 degrees C or less.

(Cooling time from the completion of rolling to a temperature range of 720 ° C. or less: within 1 second)
The cooling time from the completion of rolling to the temperature range of 720 ° C. or lower is preferably within 1 second.

As described above, the amount of strain introduced by the final rolling pass is preferably limited to some extent. For this reason, it is preferable to efficiently utilize the driving force due to the strain introduced by rolling.
By cooling to a temperature range of 720 ° C. or less within 1 second after the completion of rolling, release of strain introduced by rolling is effectively suppressed, and ferrite transformation and bainite transformation that efficiently use the driving force due to the strain are performed. It is realized and the desired steel structure can be easily obtained for the hot rolled steel sheet as the final product.

  Therefore, it is preferable that the time required for cooling to a temperature range of 720 ° C. or less after the completion of rolling is within 1 second. This time is more preferably within 0.6 seconds, and particularly preferably within 0.4 seconds. The cooling at this time is preferably water cooling, and the cooling rate is preferably 400 ° C./second or more.

(Dwelling time in the temperature range of over 500 ° C and below 720 ° C: 1 second to 15 seconds)
It is preferable that the residence time in the temperature range of more than 500 ° C. and not more than 720 ° C. is 1 second or more and 15 seconds or less.

  In order to secure the above-described ferrite area ratio, it is preferable that the cooling is temporarily stopped in the temperature range where the ferrite transformation is active or the cooling rate is lowered to stay in the temperature range for an appropriate time. In the hot-rolled steel sheet having the above chemical composition and subjected to the rolling and cooling treatment, the temperature range in which the ferrite transformation becomes active is a temperature range of more than 500 ° C. and 720 ° C. or less. It is preferable to specify.

  By setting the time for staying in the above temperature range to be 1 second or longer, the ferrite transformation using the driving force due to strain introduced by rolling efficiently proceeds appropriately, and a ferrite area ratio of 2.0 area% or more is secured. In addition, it becomes easy to make the polygonal ferrite fine. Therefore, it is preferable that the time for staying in the temperature range is 1 second or longer.

  Further, by setting the time for staying in the temperature range to 15 seconds or less, excessive progress of the ferrite transformation is suppressed, and the ferrite area ratio is easily set to 50% or less. Further, excessive generation of cementite and pearlite is suppressed, and it becomes easy to make the steel structure other than bainite, polygonal ferrite and retained austenite 15% or less. Therefore, the time for staying in the temperature range is preferably within 15 seconds.

(Winding temperature: 350 ° C or more and 500 ° C or less)
The winding temperature is preferably 350 ° C. or higher and 500 ° C. or lower.
By setting the coiling temperature to 350 ° C. or higher, bainite transformation that efficiently uses the driving force due to strain introduced by rolling is realized while suppressing the formation of martensite, and a bainite area ratio of 40 area% or more is achieved. It is easy to secure a fine austenite area ratio of 3% by area or more while ensuring the fine bainite. Moreover, it becomes easy to raise the carbon concentration in a retained austenite. Therefore, the winding temperature is preferably 350 ° C. or higher.

  In addition, by controlling the coiling temperature to 500 ° C. or less, the bainite transformation that efficiently uses the driving force due to the strain introduced by rolling is realized while suppressing the formation of pearlite, and the bainite area ratio is 40 area% or more. It is easy to secure a retained austenite area ratio of 3% by area or more, and to make bainite fine. Therefore, the winding temperature is preferably 500 ° C. or lower.

(Rolling ratio in the rolling pass immediately before the final rolling pass: 20% or more and 50% or less)
The rolling reduction in the rolling pass immediately before the final rolling pass is preferably 20% or more and 50% or less.

  As described above, it is preferable to limit the strain introduced by the final rolling pass to some extent. For this reason, it is preferable to refine the steel structure by using a means other than the strain introduced by the final rolling pass.

  By setting the rolling reduction ratio in the rolling pass immediately before the final rolling pass to 20% or more, the austenite grains in the stage used for the final rolling pass are refined, and the steel structure is refined by strain introduced by the final rolling pass. Combined with the action, the steel structure can be further refined.

  Furthermore, since the shear strain due to friction between the rolling roll and the steel sheet increases as the rolling reduction increases, by setting the rolling reduction of the rolling pass immediately before the final rolling pass to 20% or more, the final rolling pass In the step of providing, an inclined structure in which the austenite grains are finer is formed closer to the steel plate surface. As a result, the steel structure of the hot-rolled steel sheet, the final product, becomes finer as it gets closer to the steel sheet surface, and as the steel structure becomes finer, the retained austenite generation site increases and the retained austenite area ratio increases. The closer the surface, the higher the retained austenite area ratio.

Therefore, the rolling reduction of the rolling pass immediately before the final rolling pass is preferably 20% or more. More preferably, it is 25% or more, and particularly preferably 30% or more.
In addition, when the rolling reduction of the rolling pass immediately before the final rolling pass is 50% or less, an increase in rolling load can be suppressed, and rolling becomes easy. Therefore, the rolling reduction of the rolling pass immediately before the final rolling pass is preferably 50% or less.

  A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and made into a 30 mm thick steel piece by hot forging. The steel slab was then heated to a temperature of 1250 ° C., and hot rolled under the conditions shown in Table 2 in a small test tandem mill to finish a steel plate having a thickness of 2 mm. After completion of rolling, primary cooling is performed by water cooling to a temperature range of more than 500 ° C. to 720 ° C. or less, and after staying in a temperature range of more than 500 ° C. to 720 ° C. for a predetermined time, cooling to a predetermined coiling temperature The steel sheet was placed in a furnace set at a coiling temperature, held for 30 minutes, and then cooled in a furnace to obtain a hot rolled steel sheet. These conditions are also shown in Table 2.

  The obtained hot-rolled steel sheet is mirror-polished on the thickness cross section perpendicular to the rolling direction of the steel sheet, corroded with a nital corrosion liquid or a repelling corrosion liquid, and then observed with an optical microscope or a scanning electron microscope (SEM). Went. Furthermore, the crystal orientation was measured and analyzed by the EBSP method using a sample prepared by electrolytic polishing after mirror polishing.

  In observation images obtained with an optical microscope or SEM, it may be difficult to distinguish bainite, retained austenite, and martensite. Therefore, the area ratio of each phase and structure was quantified by the following method.

  First, the total area ratio of bainite, martensite and retained austenite was measured by image analysis using the SEM observation image and the EBSP analysis result. Next, the total area ratio of the retained austenite and martensite is measured from the repeller-corroded structure, and the value obtained by subtracting this total area ratio from the total area ratio of bainite, martensite and residual austenite previously measured is the bainite area ratio. did. The polygonal ferrite area ratio was measured by image analysis using SEM observation images and EBSP analysis results. The residual austenite area ratio was measured by X-ray diffraction, and at the same time, the carbon concentration in the residual austenite was also calculated. The total area ratio of bainite, polygonal ferrite and retained austenite measured above was subtracted from 100%, and the area ratio of the remaining structure was used.

The average grain size (D) of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite was determined by EBSP analysis.
As mechanical properties, tensile properties and stretch flangeability were evaluated. As for the tensile properties, a tensile test was performed according to JIS Z 2201 and JIS Z 2241, and tensile strength (TS) and total elongation (El) were measured. For stretch flangeability, a hole expansion test was performed in accordance with the Japan Iron and Steel Federation standard JFS T 1001, and the hole expansion ratio (λ) was determined.

  Table 3 summarizes the steel structure and mechanical properties of the obtained steel sheet. Test Nos. 2 to 10, 12, 13, 15 to 19, 21 to 23, which are invention examples, have high tensile strength (TS), excellent strength-ductility balance (TS × El), and excellent strength-elongation. And flange balance (TS × λ). On the other hand, the comparative example outside the range defined by the present invention is inferior in the characteristics of TS × E1 or TS × λ or both.

Claims (7)

In mass%, C: more than 0.08% and less than 0.30%, Mn: 1.0% to 4.0%, Si: 0.10% to less than 3.0%, sol. Al: 0.01% or more and 3.0% or less, provided that Si and sol. The total content of Al is 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.01% or less and N: 0.01% or less, with the balance being Fe and impurities. Having the chemical composition
The steel structure at the 1/4 depth position of the plate thickness from the steel sheet surface is mainly bainite and is in area%, bainite (V _B ): 40% or more, polygonal ferrite (V _α ): 2.0% or more 50 % And residual austenite (V _γ ): 3% or more, the remainder being 15.0% or less, and surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite The average particle size (D) of the particles to be obtained is 15 μm or less,
A hot-rolled steel sheet having a thickness of more than 1.2 mm and not more than 6 mm.   The chemical composition is selected from the group consisting of Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.50% or less in mass% instead of a part of the Fe. Or the hot-rolled steel plate of Claim 1 containing 2 or more types.   Instead of a part of the Fe, the chemical composition is, in mass%, Cr: less than 1.0%, Mo: 0.5% or less, Ni: 1.0% or less, and B: 0.0050% or less. The hot-rolled steel sheet according to claim 1 or 2, comprising one or more selected from the group consisting of:   The chemical composition is one selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less in mass%, instead of a part of the Fe. Or the hot-rolled steel plate in any one of Claims 1-3 containing 2 or more types.   The hot-rolled steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains Cu: 1.0% by mass or less in place of a part of the Fe.   The hot-rolled steel sheet according to any one of claims 1 to 5, wherein the chemical composition contains Bi: 0.020 mass% or less in place of a part of the Fe. The residual austenite area ratio (V _γs ) at a position of 100 μm depth from the steel sheet surface and the residual austenite area ratio (V _γ ) at a position of ¼ depth from the steel sheet surface satisfy the following formula (1): The hot-rolled steel sheet according to any one of claims 1 to 6.
V _γs > V _γ (1)