[0001]The present invention relates to a high-strength hot-rolled steel sheet and a manufacturing method thereof, particularly to elongation and hole expandability and excellent tensile strength 980MPa or more high-strength hot-rolled steel sheet and a manufacturing method thereof.
Background technique
[0002]Recently, in order to improve and enhance the collision safety of automobile fuel efficiency, efforts to body weight reduction by application of high-strength steel sheet has been actively conducted. However, when increasing the strength of the steel sheet, material properties such as generally moldability (processability) is deteriorated. Therefore, in the development of high-strength steel sheet is an important challenge is to achieve high strength without deteriorating the material properties. In particular, in high-strength steel sheet which is applied to the mobile member, it is important to ensure press formability. Here, Dual Phase steel sheet composed of a composite structure of a soft ferrite phase and a hard martensite phase (hereinafter DP steel) are known to be excellent in uniform elongation. On the other hand, the DP steel, since they produce significantly cracking voids from the interface occurs between the different ferrite phase and martensite phase hardness, there is a problem of poor hole expandability. Thus, DP steel, it is not suitable for applications where high hole expandability of such undercarriage components is required.
[0003]
　In contrast, controlled to be lower as the structural percentage of 3% or more less than 10% and DP steel in Patent Document 1, martensite, adding Ti and Nb as an alternative, hot rolling of ROT (Run Out Table ) by providing the cooling zone during the cooling, to precipitate the carbides of Ti and / or Nb ferrite, precipitated with improved strength by strengthening hot-rolled steel sheet is proposed which is excellent in balance between elongation and hole expandability ing.
　However, in the invention described in Patent Document 1, thereby improving hole expandability by reducing the fraction of martensite. Therefore, in order to obtain a tensile strength 980MPa or more strength it is necessary to further raise the hardness of ferrite, increasing the hardness of the ferrite, there is a problem that elongation is lowered.
[0004]
　Patent Document 2, bainitic ferrite elongation and hole expandability and improved tensile strength 980MPa or more high-strength hot-rolled steel sheet by the area ratio 90% or more have been proposed. Further, after the area ratio of bainite in Patent Document 3 to 90%, hot-rolled steel sheet with improved hole expandability by controlling the average particle diameter and content of cementite dispersed in the tissue have been proposed ing.
　However, in the invention described in Patent Documents 2 and 3, bainitic ferrite is a structure configured close to a single phase of the main, sufficient elongation is obtained.
CITATION
Patent Document
[0005]
Patent Document 1: Japanese Patent 2011-184788 JP
Patent Document 2: Japanese Patent 2008-255484 JP
Patent Document 3: Japanese Patent 2014-205890 JP
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　Recently, increasing demand for further weight reduction of the automobile, the background complexity of parts shaped, high-strength hot-rolled steel sheet is required to have a higher hole expandability and elongation.
　The present invention has been made in view of the above problems, the present invention aims to provide a high-strength hot-rolled steel sheet excellent in the elongation and hole expandability.
Means for Solving the Problems
[0007]
　Conventionally, towards the material improvement of DP steel, it has been made various efforts to suppress the generation of voids at the interface between the martensite and ferrite. The present inventors have found that cracking of martensite generated during processing paying attention to be a factor that degrades the elongation and hole expandability and extensive studies. As a result, it was found that the original hard martensite by thinking of reversing such to soften and improve the properties of the DP steel. Specifically, in the cooling process of hot rolling, by controlling the working ratio of austenite to control the ferrite transformation rate, the fraction of ferrite by controlling the cooling of the run-out table to control the ferrite transformation (ROT) by, C enrichment to austenite is suppressed, the ductility of martensite was found to be greatly improved. Further, it was confirmed that it is possible to suppress the generation of voids that occur during processing by improved ductility of martensite.
[0008]
　The present invention has been made based on the above findings, it is an aspect of the present invention is as follows.
(1) High strength hot rolled steel sheet according to one embodiment of the present invention, C in mass%: 0.02% or more, 0.30% or less, Si: 0.20% or more, 2.0% or less, Mn: 0.5% or more, 3.0% or less, P: 0.10% or less, S: 0.010% or less, Al: 0.10% or more, 1.0% or less, N: 0.010% or less, Ti: 0.06% or more, 0.20% or less, Nb: 0% or more, 0.10% or less, Ca: 0% or more, 0.0060% or less, Mo: 0% or more, 0.50% or less, cr: 0% or more and 1.0% or less, and the balance: an Fe and impurities, tissue, an area ratio of 20% or more, containing 60% or less of martensite, and 40% of ferrite, the and the martensite and the total area ratio of the ferrite is 90% or more, average particle diameter of the martensite is more than 5.0 .mu.m, 5 and a μm or less, the ratio of the hardness of the hardness and the ferrite martensite 0.6 or more and 1.6 or less, a tensile strength of not less than 980 MPa.
(2) In the high-strength hot-rolled steel sheet according to the above (1), Nb mass%: 0.01% or more, 0.10% or less, Ca: 0.0005% or more, 0.0060% or less, Mo: 0.02% or more, 0.50% or less, Cr: 0.02% or more, 1.0% or less, may contain one or more.
Effect of the invention
[0009]
　According to this aspect of the present invention, suitable for press parts requiring large processing, it is possible to provide a high-strength hot-rolled steel sheet excellent in the elongation and hole expandability. According to the high-strength steel, lightweight body such as an automobile, integral molding of parts, it is possible to shorten the processing steps, improvement in fuel consumption, it is possible to reduce the manufacturing cost, the present invention is industrially specific high value.
DESCRIPTION OF THE INVENTION
[0010]
　For high-strength hot-rolled steel sheet according to an embodiment of the present invention (sometimes referred to as hot-rolled steel sheet according to the present embodiment.), It will be described. Hot rolled steel sheet according to the present embodiment, by controlling the ferrite transformation rate generated during after hot finish rolling cooled fraction to control the C enrichment to austenite, the ductility of martensite It is improved. Therefore, the hot rolled steel sheet according to the present embodiment is excellent in the elongation and hole expandability. Specifically, the hot rolled steel sheet according to the present embodiment has a predetermined chemical composition, organization, an area ratio, containing 20% ​​or more, 60% or less of martensite, and 40% ferrite and, wherein is the martensite and the total area ratio of the ferrite is 90% or more, the average particle size of the martensite is more than 5.0 .mu.m, and at 50μm or less, the hardness of the hardness and the ferrite of the martensite ratio 0.6 or more and 1.6 or less, a tensile strength of not less than 980 MPa.
[0011]
　Will be described in detail for each constituent element of the present invention are described below. First, we describe reasons for limiting the chemical composition of the hot rolled steel sheet according to the present embodiment (chemical composition). % Of component content means mass%.
[0012]

　C is an important element for improving the strength of the steel sheet. To obtain the desired intensity, it is necessary that the C content is 0.02% or more. Preferably 0.04% or more. However, C content deteriorates toughness of the steel sheet to be 0.30 percent. Therefore, the C content to 0.30% or less. Preferably not more than 0.20%.
[0013]
Si suppresses the formation of carbides in the ferrite transformation, is an element having an effect of improving the ductility of the steel sheet. To obtain this effect, the Si content is set to 0.20% or more. Preferably 0.50% or more. On the other hand, when the Si content is 2.0 percent, the toughness of the steel sheet is degraded. Therefore, the Si content be 2.0% or less. Preferably 1.5% or less.
[0014]
Mn is an element effective for improving the strength of the steel sheet by improving and solid solution strengthening of the hardenability. To obtain this effect, the Mn content is 0.5% or more. Preferably 1.0% or more. On the other hand, Mn content is harmful MnS is generated isotropic comes to toughness of 3.0 percent. Therefore, the Mn content is 3.0% or less. Preferably 2.0% or less.
[0015]

　P is an impurity, P content is as low as desired. However, on lowering the workability and weldability and P content is 0.10 percent is significantly, fatigue properties also decreases. The reason P content is limited to 0.10% or less. Preferably 0.05% or less.
[0016]

　S is an impurity, S content is as low as desired. However, S content is generated becomes significant harmful inclusions, such as MnS isotropic toughness exceeds 0.010%. Therefore, the S content is limited to 0.010% or less. When the particularly severe low temperature toughness is required, it is preferable that the S content is 0.006% or less.
[0017]
Al is an important element for controlling the ferrite transformation. To obtain this effect, the Al content is 0.10% or more. Preferably 0.20% or more. However, if the Al content exceeds 1.0%, alumina deposited on the cluster shape is produced, the toughness is degraded. Therefore, the Al content is 1.0% or less. Preferably 0.8% or less.
[0018]

　N is an impurity. When the N content is 0.010%, the coarse Ti nitrides are formed at high temperatures, toughness of the steel sheet is degraded. Therefore, the N content is 0.010% or less. Preferably 0.006% or less.
[0019]

　Ti together with an element for precipitation strengthening of ferrite is an important element in order to obtain a ferrite fraction of aim by controlling the ferrite transformation. To obtain the excellent elongation and hole expandability by precipitation strengthening and ferrite transformation control, the Ti content is 0.06% or more. Preferably 0.08% or more. On the other hand, if the Ti content is 0.20 percent, inclusions originating from TiN is formed and hole expandability of the steel sheet is degraded. Therefore, the content of Ti to 0.20% or less. Preferably not more than 0.16%.
[0020]
　Hot rolled steel sheet according to the present embodiment contains the above chemical components, and the balance basically in that it consists of Fe and impurities. However, although not required to satisfy the required properties, or to reduce the manufacturing variations, Nb to or further improve the strength, Ca, Mo, it may be contained within the following range Cr. However, Nb, Ca, Mo, because it is not necessary to meet any Cr is required properties, the lower limit of the content thereof is 0%. Here, the impurities, in producing the steel industrially, refers to the components to be mixed ore raw material scraps and other factors. Nb, Ca, Mo, the content of Cr, is less than the lower limit of the content shown below, can be regarded as an impurity, it does not impair the effect of the hot rolled steel sheet according to the present embodiment.
[0021]
Nb is an element having an effect of increasing the strength of the steel sheet by precipitation strengthening of fine and NbC grain size of the hot-rolled steel sheet. To obtain this effect, the Nb content is preferably set to 0.01% or more. On the other hand, Nb content is 0.10 percent, the effect is saturated. Therefore, even if to be contained, and 0.10% the upper limit of Nb content. More preferable upper limit is 0.06% or less.
[0022]
Ca disperses a large number of fine oxides at the time of deoxidation of molten steel, which is an element having an effect of refining the structure of the steel sheet. Also, Ca has the S in the steel is fixed as CaS spherical, is an element improving the to hole expandability suppress the formation of stretched inclusions such as MnS. To obtain these effects, the Ca content is preferably made 0.0005% or more. On the other hand, the effect Ca content be greater than 0.0060% is saturated. Therefore, even if to be contained, the upper limit of the content of Ca and 0.0060%. More preferable upper limit is 0.0040%.
[0023]
Mo is an effective element for precipitation strengthening of ferrite. To obtain this effect, the Mo content is preferably 0.02% or more. More preferably 0.10% or more. On the other hand, when the Mo content is excessive slab cracking susceptibility handling of the slab it becomes difficult to grow. Therefore, even if to be contained, and 0.50% the upper limit of the Mo content. More preferable upper limit is 0.30%.
[0024]
Cr is an element effective for improving the strength of the steel sheet. To obtain this effect, the Cr content is preferably made 0.02% or more. And more preferably 0.1% or more. On the other hand, the ductility is deteriorated when the Cr content is excessive. Therefore, even if to be contained, and 1.0% the upper limit of the Cr content. More preferable upper limit is 0.8%.
[0025]
　Next, a description tissue for the hot rolled steel sheet according to the present embodiment.
　Hot rolled steel sheet according to the present embodiment has a structure consisting mainly of two phases of martensite and ferrite. The mainly consists of two phases, indicating that the total area fraction of the martensite and ferrite is 90% or more. For the remainder, it may contain tissue such as bainite and pearlite. The remaining structure may be 0%. That is, the total area ratio of the martensite and ferrite may be 100%.
[0026]
　The ferrite having excellent elongation with soft steel plate (steel sheet with a composite structure) having a composite structure of the hard tissue is dispersed, such as martensite, can achieve high stretch, yet high strength. However, such a composite structure steel sheet, high strain in the vicinity of the hard tissue is concentrated since crack propagation speed increases, there is a disadvantage that the hole expandability is lowered. Conventionally, to reduce the crack growth rate was aimed, and phase fraction of ferrite and martensite, study relating to the control of the size of the martensite is increased. On the other hand, the hot rolled steel sheet according to the present embodiment is different from the prior art, the martensite to improve the local ductility of martensite by softening and suppressed as much as possible the hole expandability deteriorates due martensite simultaneously, martensite thereby obtaining high strength of 980MPa by increasing the fraction.
[0027]

　total area fraction of martensite and ferrite balance but the structure consisting mainly two phases is 90% or more, the area ratio of ferrite (tissue fraction) can not be ensured relaxation and workability of the distortion due to the ferrite grains is less than 40%, the elongation and hole expandability There is reduced. Therefore, the area ratio of ferrite is 40% or more. On the other hand, when the area ratio of ferrite is 80%, it can not be ensured the desired martensite area ratio.
　Further, when the area ratio of the martensite phase is less than 20%, distortion at the time of hole expansion processing is concentrated on martensite grains, voids are easily formed, hole expandability is deteriorated. On the other hand, the elongation to become the area ratio of martensite of 60 percent and poor martensite phase mainly ductility decreases. Therefore, the area ratio of martensite of 20% or more and 60% or less. Preferably, 30% or more, 50% or less.
[0028]
　The above tissue samples were cut from the hot rolled steel sheet on which to appear tissue by etching, it can be identified from its structural photograph. Methods of measuring tissue is not limited as far as good measurement method is accurate, for example, each phase of the determination, measurement of the area ratio and average particle size may be carried out as follows. That performs les expeller etching or nital etching the steel sheet, by observing the tissue 1/4 depth position of the hot-rolled cross section with an optical microscope or SEM, determines each phase. Also, it may be measured area ratio and average particle size of each phase by using an image analyzer or the like.
[0029]

　In the hot rolled steel sheet according to the present embodiment, while satisfying the structural fraction of the above, and the average ferrite grain diameter and martensite martensite it is necessary to further satisfy the hardness ratio (hardness of the hardness / ferrite martensite) of.
　Excellent in order to obtain hole expandability, the average particle size of the martensite is more than 5.0 .mu.m, it is necessary that the 50μm or less. The average particle size of the martensite is less than 5.0 .mu.m, hole expandability is deteriorated. On the other hand, the average particle size of the martensite elongation deteriorates at 50μm greater. Thus, for both the elongation and hole expandability, the average particle size of the martensite to 5.0μm or 50μm or less. Preferably, it is 20μm or less.
　Also, the case of obtaining a better elongation and hole expandability, the average particle size of the martensite is within the above range, and martensite grain size 10 - 30 [mu] m is a ratio of the number, with 40% to 55% there it is preferable.
[0030]

　hardness ratio between the martensite and the ferrite is 0.6 or more, it is necessary that 1.6 or less. Hard hardness of ferrite, if the hardness ratio is less than 0.6, the ferrite ductility is deteriorated, the elongation of the steel sheet deteriorates. On the other hand, the hardness of the martensite is high, the hardness ratio is 1.6 greater than the local ductility decreases with plastic deformability of martensite decreases and hole expandability of the steel sheet is degraded. Thus, for both the elongation and hole expandability, the hardness ratio between the martensite and ferrite 0.6 or more and 1.6 or less. The preferred range of the hardness ratio is 0.8 or more and 1.2 or less, more preferably 0.8 to 1.0.
[0031]
　The hardness ratio, the 1/4 depth position of the hot rolling direction cross-section, can be determined by measuring respectively the hardness of ferrite and martensite by Vickers measurements. However, in the measurement of Vickers hardness, it is difficult to determine the hardness of less tissue than the size of the indentation. Therefore, if you can not small particle size Vickers test, may be measured using a nano-indentation or micro hardness testing. In that case, it used after converted into Vickers hardness. Etc. The conversion when the use of standard samples with hardness similar, it is necessary to issue accurately converted value. In order to enhance the measurement precision, martensite, measured hardness with over 100 places each on each structure of ferrite, it is necessary to obtain the average.
[0032]

　hot rolled steel sheet according to the present embodiment, assuming application to improve or body weight of the collision safety of automobiles and the like, the tensile strength and more than 980 MPa. The upper limit of the tensile strength, in order to utilize the excellent ductility of the ferrite is preferably not more than 1450 MPa.
[0033]
　Hot rolled steel sheet according to the present embodiment, regardless of the manufacturing method, the chemical composition, its effect by having a tissue obtained. However, according to the manufacturing method described below is preferable because stably obtained hot rolled steel sheet according to the present embodiment.
　Specifically, the manufacturing method of the hot rolled steel sheet according to the present embodiment preferably comprises the following of (a) ~ (f) step.
(A) a heating step of heating the slab having the chemical composition described above to below 1200 ° C. or higher 1350 ° C.
the slab after (b) heating step, a rolling step of rolling using a rolling mill having a plurality of stands, final stand and the rolling at the preceding stage Ar3 point or more, carried out in the temperature range of 960 ° C. or less, and, based on the sum of the reduction ratio of each stand of the finishing rolling stand for continuous, the sum of the final stand and front stand that ratio of 0.12 or more, 0.30 or less, the final stand and the ratio of the reduction ratio of the previous stage is 0.5 or more, the rolling process to obtain a rolled steel sheets to be less than 1.0
(c) rolling end 1.5 seconds within cool start to 40 ° C. / sec or more cooling rate at 600 ° C. or higher after the primary retirement step of cooling to 750 ° C. or less
after (d) the primary cooling step, more than 2 seconds, 10 seconds or less, 10 ℃ / s or less of Intermediate cooling step of cooling at却速degree
(e) after the intermediate cooling step, 60 ° C. / sec to 300 ° C. or less at a cooling rate higher than the secondary cooling step of cooling
(f) after the secondary cooling step, the take-up to perform winding process.
　Hereinafter, each step will be described.
　In the present embodiment, the cooling rate is an average cooling rate from the cooling starting to cool down. Further, Ar @ 3 points (℃) is the temperature at which austenite during cooling starts transformation, can be determined as appropriate, in a simple manner, on the basis of the content of each element can be calculated by the following formula .
Ar3 = 901-325 × C + 33 × Si-92 × Mn + 287 × P + 40 × Al
[0034]

　before hot rolling (hot rolling), heat against the slab. When heating the slab having the same chemical composition as the hot rolled steel sheet according to the embodiment, obtained by continuous casting or the like, the temperature of the heating is less than 1200 ° C., contained in homogenization, and / or slabs of the slab Ti dissolution of carbide becomes insufficient. In this case, the strength and workability of the steel sheet resulting is reduced. On the other hand, if the heating temperature is above 1350 ° C., in the finally obtained steel sheet by initial austenite grain diameter increases, the tissue is likely to be mixed grain. Also, increase or manufacturing cost, also leads to a decrease in productivity. Therefore, the heating temperature, 1200 ° C. or higher, desirably less than 1350 ° C..
[0035]

　In the rolling step, the tandem rolling rolling a continuously steel with a rolling mill having a plurality of stands, a final stand and the rolling temperature at the preceding stage (stand before one than the last), and pressure it is important to control the rate. In the rolling of the final stand and the previous stage, rolling temperature, by controlling the reduction ratio, it is possible to optimize the dislocation density of austenite. The dislocation density of austenite in the subsequent step, a large effect on the C enrichment rate to ferrite transformation rate and austenite.
　Specifically, rolling in the final stand and the preceding stage should be carried out in a temperature range of austenite single phase. Therefore, it is rolling in the final stand and the front above Ar3 point. Further, in order to suppress the recovery of dislocations stored by rolling, rolling in the final stand and the previous stage carried out at 960 ° C. or less. The 960 ° C. greater is promoted and austenite recovery and recrystallization, it is impossible to store dislocation.
　Further, with respect to the reduction ratio of each stand of the finishing rolling stand for continuous summation, the final stand and a ratio of the total rolling reduction at the preceding stage of the stand (subsequent reduction ratio) is 0.12 or more and 0.30 or less . Is less than the ratio of the reduction ratio is 0.12, the recrystallization promoted preceding the finish rolling, it is impossible to accumulate strain to the subsequent stage. In this case, the ferrite transformation is delayed in the cooling process of the next process. On the other hand, the ratio of the reduction ratio at 0.30 greater than insufficient preceding reduction ratio, causing coarsening of the tissue. Preferably 0.20 or more and 0.25 or less. Here, the sum of the reduction ratio, the total reduction rate is the sum of the reduction ratio, when performing 2 times 20% of the rolling for example, a 20 + 20 = 40%.
　Further, the reduction ratio of the final stand, the ratio preceding rolling reduction (final stand of rolling reduction / front rolling reduction) is 0.5 or more and less than 1.0. Final stand and the ratio of the reduction ratio of the previous stage (the final stand of the rolling reduction / front rolling reduction) is insufficient strain is less than 0.5, the ferrite transformation is delayed in the cooling step of the next process. In this case, it is not possible to obtain a ferrite and martensite area ratio as a target. Further, coarse martensite is formed, the average particle size of the martensite becomes 50μm greater. On the other hand, the ratio of the reduction ratio of the final stand and its previous stage is too fast ferrite transformation is 1.0 or more, it is impossible to obtain a ferrite and martensite area ratio as a target. Furthermore, C enrichment into austenite proceeds to the diffusion rate of C is increased, the average particle diameter of hard martensite of less than 5.0μm is formed.
　In the present embodiment, the reduction ratio of the final stand among the stand rolling reduction is subjected to reduction of 5% or more with respect to the steel sheet, it refers to the reduction ratio in the stands of the final stage. That is, reduction ratio is not no rolling state of 5% or more, for example, is not included when the rolling roll and steel sheet, such as simply contact. Rolling reduction at the final stand, in order to perform accumulation of dislocations in the austenitic enough, more than 20%, preferably not more than 45%.
[0036]

　After the end of rolling, in order to effectively utilize the dislocations stored by rolling, starting the primary cooling within 1.5 seconds. After rolling (after reduction at the final stand), dislocations in austenite recovery, reduced by recrystallization in the time until the cooling is 1.5 seconds greater. In this case, it is not possible to obtain the objectives of the organization.
　In the primary cooling, the cooling rate 40 ° C. / s or higher at 600 ° C. or higher, cooled to 750 ° C. or less. Also, it performed after the primary cooling completed, two seconds or more, during the following 10 seconds, cooling the average cooling rate is less than 10 ° C. / s (the intermediate air cooling). Intermediate air cooling may be a so-called natural cooling. With ferrite is formed during the intermediate cooling, by the diffusion of C, C enrichment of the untransformed austenite occurs. Ferrite is improved ductility by generating, C was concentrated to austenite contributes to the strength of the martensite produced by subsequent cooling. If it is less than the primary cooling of the cooling rate is 40 ° C. / s, also occurs ferrite transformation during cooling, C diffusion rate into austenite is accelerated at elevated temperatures. As a result, the hard martensite is formed, hole expandability is deteriorated. When the primary cooling stop temperature (intermediate air cooling start temperature) exceeds 750 ° C., ferrite area ratio is insufficient. Intermediate air cooling start temperature is less than 600 ° C., the cooling rate of the primary cooling is 40 ° C. / sec, or greater than the intermediate air cooling time is not predetermined ferrite fraction is obtained in less than 2 seconds, the higher the fraction of martensite. Diffusion of C into austenite when the intermediate air cooling time exceeds 10 seconds becomes excessive, hole expandability is deteriorated. While maintaining the structural percentage of interest, in order to suppress the enrichment of C austenite appropriate range, it is desirable that the cooling time is less than 8 seconds.
　The upper limit of the cooling rate of the primary cooling is not necessarily limited, considering equipment constraints, etc., also achieve a uniform tissue distribution in the thickness direction, it is preferable cooling rate is less than 200 ° C. / s .
[0037]

　thickening austenite of C in the primary cooling step and the intermediate cooling step, in order to martensite transformation, to 300 ° C. or less at 60 ° C. / s or more cooling rate after the intermediate air cooling cooling (secondary cooling), and wound up. Secondary cooling stop temperature (the coiling temperature) is bainite or pearlite is produced during winding exceeds 300 ° C., elongation of the hot-rolled steel sheet is reduced. Further, when the cooling rate of the secondary cooling is lower than 60 ° C. / s bainite or pearlite phase is formed during cooling, the composite structure can not be obtained consisting mainly ferrite and martensite.
　The upper limit of the cooling rate of the secondary cooling is not necessarily limited, considering equipment constraints, etc., also achieve a uniform tissue distribution in the thickness direction, that the cooling rate is less than 200 ° C. / s preferable.
Example
[0038]
　Hereinafter, the high-strength hot-rolled steel sheet of the present invention will be specifically described by way of examples. However, conditions in the examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to the following examples. Without departing from the gist of the present invention, as long as they achieve the object of the present invention, it is also possible to put into practice after appropriate modifications or variations within the scope adaptable to the gist. Accordingly, the present invention can employ various conditions so that they are all included in the technical features of the present invention.
[0039]
　Steels having the chemical compositions shown in Table 1 were melted in a converter furnace, and the slab thickness 230mm at continuous casting. Thereafter, the slab is heated to a temperature of 1200 ° C. ~ 1250 ° C., the billet was subjected to rough rolling, finish rolling under the conditions shown in Table 2, the primary cooling, intermediate cooling, secondary cooling, subjected to winding, the hot rolled steel sheet It was produced. Cooling rate of the intermediate air cooling was 3 ~ 8 ℃ / s.
[0040]
　Table 2 shows using the steel type and finish rolling conditions, the thickness of the steel sheet. In Table 2, with respect to "subsequent reduction ratio" of rolling reduction of each stand of the rolling stand finishing successive summation, the final stand and the total reduction rate of the preceding stand ratios, "F5 reduction ratio" is the final stand preceding rolling reduction in the stand, "FT5" rolling temperature of the previous stand of the final stand, "F6 rolling reduction" is the reduction ratio of the final stand, "FT6" is the final stand rolling temperature, "reduction ratio", of the final stand the ratio for the preceding rolling reduction of rolling reduction, "cooling start" time to the primary cooling start from the end of finish rolling, "primary cooling" is an average cooling from the end of the finish rolling to an intermediate air cooling start temperature speed, "air temperature" will stop after the primary cooling, temperature that initiated the intermediate air cooling, "air time" intermediate air cooling time, "secondary cooling" is put in the secondary cooling of the wound up after the intermediate air cooling The average cooling rate, "the coiling temperature" is the coiling temperature after the secondary cooling end.
[0041]
[Table 1]

[0042]
[Table 2]

[0043]
　Thus it was at the position of the sheet thickness 1/4 of the steel sheet of the obtained steel sheet to randomly selects a field of view, ferrite using an optical microscope at a minimum five fields, structural fraction of martensite, martensite and ferrite hardness It was investigated ratio.
[0044]
　Of ferrite and martensite of the steel plate, for tissue fraction and particle size, randomly 5 viewing photographs a view of 500 [mu] m × 500 [mu] m using an optical microscope after nital corrosion, the average area ratio of the 5-field using image analysis and to determine the average particle size.
　The hardness of the martensite and ferrite performs micro Vickers test in each tissue, martensite, and Vickers hardness (Hv) determined for more than 100 locations by the respective structure of ferrite was calculated the average.
　The tensile test of the steel sheet, a JIS5 test piece No. rolling width direction (C direction) of the steel sheet was taken, according to JISZ2241, yield strength: YP (MPa), tensile strength: TS (MPa), elongation: EL ( %) were evaluated.
　The hole expansion ratio λ (%), were evaluated by a method specified in JISZ2256.
　The evaluation results of the tissue and the material in Table 3. In Table 3, the area ratio of ferrite, martensite "each tissue area ratio of" other tissue, the average particle size of "M size" martensite "hardness ratio" hardness / ferrite (martensite a hardness ratio obtained in hardness).
[0045]
[table 3]

[0046]
　Invention Examples As shown in Table 3 in the tensile strength above 980 MPa, ferrite structure fraction of 40% or more, and the fractions of the structures martensite is 20% or more and 60% or less, the hardness of the martensite and ferrite the ratio is 0.6 or more, it was 1.6 or less. Further, as a result, in the present invention embodiment, elongation of 10% or more, hole expandability is 50% were excellent in balance between elongation and hole expandability.
[0047]
　In contrast, structural fraction of interest in the test No. 2 (area ratio of each organization) is not obtained. This, F5 and the ratio of the reduction ratio of the F6 (F6 / F5) is small, ferrite transformation is considered to be caused by delayed. Further, in Test No. 2, the austenite grain size becomes coarse, martensite with an average grain diameter of martensite grains increases is softened, the hardness ratio is reduced. As a result, growth was inferior.
　Test No. 5 is not obtained target tissue fraction, and the elongation and hole expandability was inferior. This low subsequent reduction ratio, also the finishing rolling temperature is higher, ferrite transformation is believed to be due to delayed.
　Test No. 8 is not obtained target tissue fraction, and the elongation and hole expandability was inferior. This high air temperature, ferrite transformation is believed to be due to delayed during air cooling.
　Test No. 12 as well as coarse average particle size of martensite grains, the hardness ratio is less than 0.6, elongation and hole expandability was inferior. This cooling start time after the rolling is long, the austenite grain size is believed to be due to the coarsened.
　Test No. 16 is the hardness ratio is 1.6 super, and the hole expandability was inferior. This is a slow primary cooling, that progress in concentration of C to austenite, martensite is considered to be due to the hardening.
　Test No. 17, the hardness ratio is 1.6 super, and the hole expandability was inferior. This, F5 because the ratio of the reduction ratio is 1.0 or more F6, that ferrite transformation proceeds excessively, C enrichment is promoted, martensite is considered to be due to the excessively hardened.
　Test No. 20 has a low area ratio of martensite, elongation was inferior. This is, air cooling time is as long as 15 seconds, is considered to be due to progress in bainite transformation during air cooling.
　Test No. 22 has a low area ratio of ferrite, elongation was inferior. This air temperature is low, is considered to ferrite transformation is because it was not sufficiently proceed.
　Test No. 24 is not the target tissue is obtained, and were inferior elongation and hole expandability. This is believed to be due to the winding temperature was high.
　With the test numbers 27 to form coarse martensite, lower hardness ratio of tissue elongation were inferior. This reduction ratio of the rear stage is high, austenitic structure for preceding reduction was insufficient presumably because a coarse.
　Test No. 31 is not the target tissue is obtained, and the elongation and hole expandability was inferior. This is believed to be due to the air cooling time was short.
　Test No. 33 is due to insufficient Al content, not the area ratio of the target of the ferrite is obtained, elongation was inferior.
　Test No. 34 because of insufficient Ti content, insufficient amount of precipitation strengthening by Ti, 980 MPa in tensile strength was not obtained.
Industrial Applicability
[0048]
　According to the present invention, it is possible to provide high processing suitable for press parts requiring a high strength hot-rolled steel sheet excellent in the elongation and hole expandability. According to the high-strength steel, lightweight body such as an automobile, integral molding of parts, it is possible to shorten the processing steps, improvement in fuel consumption, it is possible to reduce the manufacturing cost. Therefore, the present invention has a high industrial value.

claims

[Claim 1]By mass%C: 0.02% or more, 0.30% or
　less, Si: 0.20% or more, 2.0% or
　less, Mn: 0.5% or more, 3.0% or
　less, P: 0.10 % or
　less, S: 0.010% or
　less, Al: 0.10% or more, 1.0% or
　less, N: 0.010% or
　less, Ti: 0.06% or more, 0.20% or
　less, Nb: 0 % or more, 0.10% or less,
　Ca: 0% or more, 0.0060% or
　less, Mo: 0% to 0.50% or
　less, Cr: 0% to 1.0% or less,
the balance: Fe and impurities and a,
　tissue, an area ratio of 20% or more, 60% or less of martensite, containing 40% or more of ferrite, the total area ratio of the ferrite and the martensite is 90% or more,
　the martensite average grain size more than 5.0μm, and is at 50μm or less,
　the martensite Hardness and the ratio of the hardness of the ferrite is 0.6 or more and 1.6 or less,
　a tensile strength of more than 980MPa
high-strength hot-rolled steel sheet, characterized in that.
[Claim 2]
　Mass%
　Nb: 0.01% or more, 0.10% or
　less, Ca: 0.0005% or more, 0.0060% or
　less, Mo: 0.02% or more, 0.50% or
　less, Cr: 0.02 % or more, 1.0% or less,
containing one or more
hot-rolled steel sheet according to claim 1, characterized in that.