JP3889766B2 - High-strength hot-rolled steel sheet excellent in hole expansion workability and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet used for automobiles such as passenger cars and trucks, industrial machines, and the like, and a method for producing the same. The present invention relates to a high-strength hot-rolled steel sheet that can be effectively used, and a useful method for producing such a hot-rolled steel sheet.

  In recent years, demand for hot-rolled steel sheets with higher strength (for example, 780 MPa or more in tensile strength) against the background of weight reduction of vehicle bodies to improve fuel efficiency of automobiles from the viewpoint of energy saving and ensuring safety of automobile collisions, etc. Has been increasing. In applications where such high-strength hot-rolled steel sheets are used, the hot-rolled steel sheets are required to have excellent hole expansion workability as well as elongation. For these reasons, various techniques for improving hole expansibility in high-strength steel sheets used as raw materials have been proposed.

  As such a high-strength hot-rolled steel sheet for processing, a multi-structure steel sheet having retained austenite and martensite is widely known. For example, Patent Document 1 discloses a composite steel sheet composed of ferrite, bainite, retained austenite, and martensite structure, which is extremely low P steel, control of the microstructure and maximum length of inclusions, and the hardness of the microstructure. A method for improving hole expansibility by control or the like has been proposed.

  Further, for example, Patent Document 2 discloses a ferrite-bainite structure mainly composed of ferrite, an amount of non-fixed carbon that does not react with Ti or Nb in steel, and unprecipitated carbon that precipitates at grain boundaries during aging treatment and increases strength. High-strength steel sheets with controlled amounts have been proposed. Furthermore, Patent Document 3 discloses a technique for improving hole expansion workability by using a high-strength hot-rolled steel sheet having a microstructure mainly composed of ferrite and having a microstructure composed of bainitic ferrite and polygonal ferrite. Has been proposed. Further, in this technique, in order to build the structure, a cooling condition in a step of winding the coil after the hot rolling is completed and a method for controlling the cooling condition are disclosed.

However, with the techniques that have been proposed so far, the fact is that stable and good hole expansion workability cannot be exhibited.
Japanese translation of PCT publication No. 2004-536965, claims, etc. JP 2003-342684 A, claims, etc. JP 2003-342684 A, claims, etc.

  The present invention has been made in order to solve the problems of the above-described conventional high-strength hot-rolled steel sheet, and the purpose thereof is a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, and has excellent elongation. It is another object of the present invention to provide a high-strength hot-rolled steel sheet having hole expandability and a useful method for producing such a high-strength hot-rolled steel sheet.

  The high-strength hot-rolled steel sheet of the present invention that can achieve the above-mentioned object is: C: 0.05 to 0.15% (meaning of mass%, the same applies hereinafter), Si: 1.50% or less (including 0% Mn: 0.5 to 2.5%, P: 0.035% or less (not including 0%), S: 0.01% or less (including 0%), Al: 0.020 to 0 .15%, Ti: 0.05 to 0.2% of each steel sheet, the balance being Fe and unavoidable impurities, the metal structure being 60 to 95% by volume of bainite, solid solution strengthening or precipitation It is a structure containing reinforced ferrite or ferrite and martensite, and has a gist in that the fracture surface transition temperature vTrs obtained by an impact test of the steel sheet is 0 ° C. or less.

  In the hot-rolled steel sheet of the present invention, if necessary, (a) Ni: 1.0% or less (not including 0%), (b) Cr: 1.0% or less (not including 0%), ( c) Mo: 0.5% or less (excluding 0%), (d) Nb: 0.1% or less (not including 0%), (e) B: 0.01% or less (including 0%) (F) Ca: 0.01% or less (not including 0%), (g) Cu: 1.0% or less (not including 0%), etc. are also effective and contained. The properties of the hot-rolled steel sheet are further improved according to the type of element to be used.

On the other hand, in producing the above-described high-strength hot-rolled steel sheet, a step of heating the steel slab having the chemical components to a temperature range of 1150 to 1300 ° C., and the steel slab after heating the Ar ₃ transformation point A step of hot rolling at the above finishing temperature to form a steel plate, and a step of cooling the steel plate after hot rolling to a temperature range of 400 to 550 ° C. at an average cooling rate of 30 ° C./second or more and winding it on a coil The coil after winding may be manufactured so as to include a step of cooling to a temperature of 300 ° C. or lower at an average cooling rate of 50 to 400 ° C./hour.

  According to the present invention, by appropriately controlling the fracture surface transition temperature vTrs in addition to the chemical composition and microstructure, a hot-rolled steel sheet excellent in elongation and hole expansion workability can be realized. When the thickness is 2 mm, a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, an elongation of 20% or more, and a hole expansion ratio of 60% or more is obtained. In such hot-rolled steel sheets, hot-rolled steel sheets that were not conventionally applied from the viewpoint of formability can be applied to various members such as automobiles and industrial machines, which not only contributes to cost reduction of members, but also various parts. This makes it possible to reduce the plate thickness and improve the collision safety of the automobile, thereby contributing to the enhancement of the performance of the automobile.

  The present inventors examined from various angles in order to realize a high-strength hot-rolled steel sheet excellent in hole expansion workability. As a result, after appropriately adjusting the chemical composition of the steel, the production conditions are regulated, the microstructure of the steel material is made of ferrite and ferrite, the bainite volume fraction is 60 to 95%, and TiC and / or Nb or It was found that a steel sheet having a tensile strength of 780 MPa or more can be realized by finely depositing Mo carbide in this structure. In addition, by controlling the cooling condition of the winding coil after winding the coil, it is possible to control the fracture surface transition temperature vTrs obtained by the impact test. As a result, it was found that the hole expansion workability of the hot-rolled steel sheet can be improved, and the present invention has been completed. Hereinafter, along with the background of the completion of the present invention, the function and effect will be described.

  In order to improve the elongation and hole expansion workability (hereinafter referred to as “hole expansion property”) in a steel sheet having a tensile strength of 780 MPa or more, the C is made as low as possible, the main phase is made a bainite structure, and solid solution strengthening is performed. In addition, it is effective to contain precipitation strengthened ferrite structure at an appropriate volume fraction, and by lowering C, the hardness of bainite is reduced, the ductility of bainite is improved, and solid solution strengthening and precipitation strengthening are performed. It can be considered that high elongation and high hole expansibility can be secured because the difference in hardness from the obtained ferrite can be reduced. However, even if the steel sheet is hot rolled under the same composition and the same conditions, the hole expandability may change depending on the coil.

  Therefore, the present inventors paid attention to the relationship between hole expandability and toughness, and investigated the relationship between the fracture surface transition temperature vTrs and hole expandability required in the impact test, and these have a good correlation, It was found that the fracture surface transition temperature vTrs should be 0 ° C. or lower in order to ensure good hole expandability of 60% or more with a hole expansion rate (measurement method will be described later). (See FIGS. 1 and 3 below).

  The above-mentioned steel sheet with a high fracture surface transition temperature vTrs (that is, a low toughness value) was investigated in more detail, and it was found that grain boundary fracture occurred when subjected to low temperature fracture, and this grain boundary fracture surface was analyzed using an Auger analyzer. Then, it was observed that P grain boundary segregation occurred. On the other hand, in steel sheets with good toughness (that is, fracture surface transition temperature is low), only cleaving fractures are observed even when fractured at low temperatures, and the presence or absence of segregated elements at grain boundaries cannot be confirmed. There was found.

  The P segregating at the ferrite grain boundaries as described above could be thought to be that P diffused and segregated at grain boundaries that were unstable compared to the inside of the grains due to the slow cooling of the winding coil. . From the viewpoint that the toughness can be improved by preventing the segregation of P as described above, the present inventors have further studied the means, and as a result, thought that it would be effective to shorten the diffusion time. Based on the above, we examined the specific means for that from various angles. As a result, after winding the steel sheet in a coil, the fracture surface transition temperature vTrs is obtained by cooling at an average cooling rate of 50 ° C./hour (hereinafter referred to as “° C./hr”) to a temperature range of 300 ° C. or lower. It has been found that the toughness value can be improved by lowering (see FIGS. 2 and 4 below).

  In the hot-rolled steel sheet of the present invention, in order to provide the basic mechanical properties (yield strength YS, tensile strength TS, elongation EL, etc.), it is necessary to appropriately adjust the chemical composition, The reasons for limiting the range of the chemical composition defined in the present invention are as follows.

C: 0.05 to 0.15%
C is a basic component as a strength improving element, and it is necessary to contain 0.05% or more in order to ensure a tensile strength of 780 MPa or more of the steel sheet. However, if the C content exceeds 0.15%, the second phase other than ferrite (for example, martensite and the like) is generated and increased in the microstructure, and the hole expandability is deteriorated. In addition, the minimum with preferable C content is 0.06%, and a preferable upper limit is 0.10%.

Si: 1.5% or less (excluding 0%)
Si is an element effective in promoting the formation of polygonal ferrite and ensuring strength without deteriorating elongation and hole expansibility. These effects increase as the content increases. However, if the content is excessive, the surface properties are significantly deteriorated, the hot deformation resistance is increased, and the manufacture of the steel sheet becomes difficult. Should be less than 1.5%. In addition, the minimum with preferable Si content is 0.2%, and a preferable upper limit is 1.0%.

Mn: 0.5 to 2.5%
Mn is an element useful for solid solution strengthening of steel, and in order to ensure a tensile strength of 780 MPa or more, it is necessary to contain at least 0.5% or more. However, if Mn is contained excessively, the hardenability becomes too high and a large amount of transformation product is generated, and it becomes difficult to ensure a high hole expansion rate, so it should be 2.5% or less. . In addition, the minimum with preferable Mn content is 1.4%, and a preferable upper limit is 2.3%.

P: 0.035% or less (excluding 0%)
P is an element effective for solid solution strengthening of steel without deteriorating ductility, and is an especially important element in the present invention. When the content of P is excessive, segregation occurs in the grain boundary during cooling after coil winding, which deteriorates toughness and raises the fracture surface transition temperature vTrs. For these reasons, the P content is preferably 0.035% or less. In addition, the upper limit with preferable P content is 0.025%.

S: 0.01% or less (including 0%)
S is an element inevitably mixed in the manufacturing process, but it forms sulfide inclusions that adversely affect hole expansibility, so it is preferable to reduce it as much as possible. From such a viewpoint, the S content is preferably suppressed to 0.01% or less. In addition, the upper limit with preferable S content is 0.008%, It is good to set it as 0.005% or less more preferably.

Al: 0.02-0.15%
Although Al is added as a deoxidizing element during melting, it is an effective element for improving the cleanliness of steel. In order to exert such an effect, it is necessary to contain Al by 0.02% or more. However, if the content is excessive, a large amount of alumina inclusions are generated and cause surface defects. It is necessary to make it 15% or less. In addition, the minimum with preferable Al content is 0.025%, and a preferable upper limit is 0.06%.

Ti: 0.05 to 0.2%
Ti is an element effective for precipitation strengthening by using C and N in ferrite as precipitates, strengthening ferrite, reducing the amount of dissolved C and cementite in ferrite, and improving hole expandability. , An element important in securing a tensile strength of 780 MPa or more. In order to exert these effects, the Ti content needs to be 0.05% or more. However, if the Ti content is excessive, the ductility deteriorates and the above effect is saturated, so it is necessary to make it 0.2% or less. In addition, the minimum with preferable Ti content is 0.08%, and a preferable upper limit is 0.18%.

  In the hot-rolled steel sheet of the present invention, in addition to the above components, it consists of Fe and inevitable impurities (for example, V, Sn, etc.). Ni, Cr, Mo, Nb, B, Ca, Cu, etc. It is also effective to contain. The reason for defining the range when these elements are contained is as follows.

Ni: 1% or less (excluding 0%)
Ni is an element effective for solid solution strengthening of steel, but if its content is excessive, its effect is saturated and economically disadvantageous, so it should be 1% or less. The above-mentioned effect due to the addition of Ni increases as the content thereof increases. However, from the viewpoint of securing a tensile strength of 780 MPa or more in the ferrite single-phase structure steel, it is preferable to contain Ni at least 0.1%. More preferably, the content is 0.2% or more. Moreover, the upper limit with preferable Ni content is 0.8%, More preferably, it is good to set it as 0.5% or less.

Cr: 1.0% or less (excluding 0%)
Cr is an element effective for strengthening the precipitation by strengthening the ferrite with C in the steel as a precipitate, but even if its content is excessive, its effect is saturated and economically disadvantageous. It is good to set it as 1.0% or less. The above effect due to the addition of Cr increases as the content thereof increases, but in order to effectively exhibit the above effect, it is preferable to contain the Cr at least 0.1%, more preferably 0.2% or more. It is good to contain. Moreover, the upper limit with preferable Cr content is 0.8%, More preferably, it is good to set it as 0.5% or less.

Mo: 0.5% or less (excluding 0%)
Mo precipitates in the ferrite as a carbide and is an extremely effective element for strengthening the precipitation of ferrite. Further, when the winding coil is cooled, it effectively acts to prevent P from segregating at the ferrite grain boundaries, lowering the toughness value, and increasing the fracture surface transition temperature vTrs. Such an effect increases as the content thereof increases. However, if the Mo content becomes excessive, the effect is saturated, so 0.5% or less is preferable.

Nb: 0.1% or less (excluding 0%)
Nb is an element that refines the ferrite produced from austenite after hot end and contributes to improvement of hole expansibility. Moreover, it is effective for strengthening the precipitation by making C and N in steel into precipitates and strengthening the ferrite. Such an effect increases as the content increases. However, even if the content is excessive, the effect is saturated and disadvantageous economically. In order to effectively exhibit the above-described effect by Nb, it is preferable to contain 0.01% or more, and more preferably 0.02% or more. In addition, the upper limit with preferable Nb content is 0.08%, More preferably, it is good to set it as 0.07% or less.

B: 0.01% or less (excluding 0%)
B is an element effective for reducing the grain boundary energy of steel and suppressing the grain boundary segregation of P. Such an effect increases as the content thereof increases. However, since the effect is saturated even if the content is excessive, the content is preferably 0.01% or less. In addition, the minimum with preferable B content is 0.001%, and a more preferable upper limit is 0.005%.

Ca: 0.01% or less (excluding 0%)
Ca is an element effective for spheroidizing the sulfides in the steel sheet and improving the hole expansibility. However, when the content is excessive, the effect is saturated, so 0.01% or less is preferable. In order to effectively exhibit the effect of addition of Ca, Ca is preferably contained in an amount of 0.001% or more. A more preferable upper limit of Ca is 0.005%.

Cu: 1.0% or less (excluding 0%)
Cu, when added together with Ti and Nb, is an effective element because it promotes uniform fine precipitation of TiC and NbC and improves strength and further expandability due to fine precipitation, but its content becomes excessive. However, the effect is saturated and economically disadvantageous, so 1.0% or less is preferable. The above effect due to the addition of Cu increases as the content thereof increases. However, in order to effectively exhibit the above effect, it is preferable to contain Cu at least 0.1%, more preferably 0.3% or more. It is good to contain. Moreover, the upper limit with preferable Cu content is 0.8%.

  In the hot-rolled steel sheet of the present invention, the structure of the metal structure is an important requirement in order to have high strength, high hole expansibility, and excellent ductility. In order to realize high strength and high hole expansibility, it is necessary to use bainite, which has high strength but has a hardness difference smaller than that of martensite, as the main phase, and to contain ferrite in order to ensure ductility. . From such a point of view, by setting the bainite phase in the metal structure in the range of 60 to 95% by volume, a steel sheet having high strength and good workability can be obtained.

  The metal structure in the steel sheet of the present invention is basically (bainite + ferrite), but part of the ferrite may be martensite. In the present invention, the term “ferrite” includes polygonal ferrite and pseudopolygonal ferrite, and structures having a high dislocation density such as ashular ferrite and bainitic ferrite are included in “bainite” in the present invention. Is.

  Next, the manufacturing method of this invention is demonstrated. In order to produce the high-strength steel sheet of the present invention, it is necessary to appropriately control at least the cooling rate after coil winding as described above, and other conditions (hot rolling conditions) may be in accordance with normal conditions. However, the basic production conditions in the production method of the present invention are as follows.

  In producing the high-strength hot-rolled steel sheet of the present invention, first, the steel sheet controlled to the chemical component composition as described above is made into a slab slab by a conventional method, and will be subjected to hot rolling. The slab heating temperature at this time needs to be 1150 ° C. or higher. This is the temperature at which TiC and Nb (C, N) begin to dissolve in austenite. By heating above this temperature, the added Ti and Nb can be effectively dissolved in the steel. Solid solution Ti or Nb reacts with solid solution C or solid solution N in the ferrite during the formation of ferrite after completion of hot rolling, precipitates as a compound, and obtains a desired tensile strength by precipitation strengthening the steel sheet. Can do. However, if this heating temperature becomes too high, the heating furnace will be damaged and the energy cost will be increased.

In hot rolling, it is basically sufficient to follow normal hot rolling conditions and there are no special restrictions, but the hot rolling finishing temperature is higher than the Ar ₃ transformation point which is the austenite single phase temperature range. Need to be temperature. When the hot rolling temperature falls below the Ar ₃ transformation point, the hot rolling ends with a two-phase structure of ferrite-austenite, so that the processed ferrite (meaning of the processed structure) remains, ductility and hole expansion. Will deteriorate. Moreover, a coarse structure is formed in the surface layer portion, and the elongation is lowered. Furthermore, solute Nb and solute Ti precipitate as carbonitride during hot rolling, but this precipitate does not contribute to the increase in strength. As a result, it precipitates in the ferrite and cannot participate in increasing the strength of the ferrite, the amount of precipitation strengthening, which is the original purpose of addition, decreases, and the desired strength of the steel material cannot be obtained.

  In cooling after the end of hot rolling, it is necessary to cool at an average cooling rate of 30 ° C./second (hereinafter referred to as “° C./s”) or more to a coiling temperature range of 400 to 550 ° C., This is because the bainite structure generated from austenite is made into a uniform fine-grained structure, and the ductility and hole expansibility are improved. That is, when the average cooling rate at this time becomes slower than 30 ° C./s, the ferrite after the transformation becomes coarse, and the agglomeration and growth of carbides precipitated inside the bainite progresses to produce coarse carbides. Hole expandability will deteriorate.

  The reason for setting the coiling temperature in the temperature range of 400 to 550 ° C. is that the microstructure of the steel is a bainite-based structure. That is, when the coiling temperature is lower than 400 ° C., a martensite structure is generated, and the hole expandability is lowered. Moreover, the precipitation strengthening amount of carbonitride is insufficient, and a desired strength cannot be obtained.

  On the other hand, when the coiling temperature is higher than 550 ° C., cementite is precipitated and pearlite structure is mixed, and the strength is decreased instead. Moreover, the hole expandability is also lowered. For these reasons, the coiling temperature needs to be in the temperature range of 400 to 550 ° C, and preferably in the temperature range of 400 to 500 ° C.

  In cooling the coil after winding, in order to prevent segregation of P in the steel to the ferrite grain boundary, the average cooling rate from the coiling temperature to a temperature range of 300 ° C. or less needs to be 50 ° C./hr or more. There is. If it becomes slower than this average cooling rate, precipitation of P occurs at the ferrite grain boundary during cooling, and the fracture surface transition temperature vTrs obtained by the impact test becomes high, and good hole expansibility cannot be obtained.

  The means for ensuring the cooling rate after winding the coil as described above is not particularly limited. For example, a method of cooling the winding coil by using a blower, a method of blasting a mist And a method of cooling by using a water spray nozzle in a winding coil, a method of immersing the winding coil in a water tank, and the like.

  Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.

Example 1
After holding various steel slabs having the chemical composition shown in Table 1 below at a slab heating temperature of 1250 ° C. for 30 minutes, the final rolling temperature is 900 ° C. and the thickness is 4 mm by a normal hot rolling process. A rolled steel sheet was obtained. Then, cooling at an average cooling rate of 30 ° C./s, and after a winding process of 30 minutes at a winding temperature of 600 ° C. using an electric heating furnace, the cooling rate is controlled in order to change the subsequent cooling rate. After taking out from the furnace, cooling was performed by cooling, blast cooling, (blast + mist) cooling, shower cooling, immersion in a water tank, and the like to obtain various hot-rolled steel sheets.

  The hot-rolled steel sheet thus obtained is subjected to an impact test in the direction perpendicular to the rolling direction (C direction) with a JIS No. 5 test piece to obtain mechanical properties (yield strength YS, tensile strength TS, elongation EL, etc.). In addition to the measurement, the hole expandability was evaluated by the hole expansion rate λ measured by the following method, and the fracture surface transition temperature vTrs was measured by the following method. The microstructure of each steel sheet was subjected to nital corrosion, and then ferrite, bainite, and martensite were identified with a scanning electron microscope, and the area ratio of bainite was measured with an image analyzer. In addition, the impact test piece ground both surfaces of the obtained hot-rolled steel plate, and tested it with the subsize test piece of thickness: 2.5mm.

[Method for measuring hole expansion rate λ]
Initial hole diameter: 10 mm (d ₀ ) punched hole is punched out from the punched side with a 60 ° conical punch, and the hole diameter d (mm) is measured when the crack penetrates in the plate thickness direction. The rate λ was measured.
λ = [(d−d ₀ ) / d ₀ ] × 100 (%) [d ₀ = 10 mm]

[Measurement method of fracture surface transition temperature vTrs]
Using a JIS No. 4 impact test piece produced by machining, an impact test is conducted by a test method in accordance with JIS Z2242, and a brittle fracture surface ratio (or “ductile fracture surface ratio”) is obtained by a method in accordance with JIS. The transition temperature vTrs at which the brittle fracture surface ratio becomes 50% was determined from the curve of (temperature vs. brittle fracture surface ratio).

  These results are shown in Table 2 below together with the production conditions. Based on these results, the relationship between the fracture surface transition temperature vTrs and the hole expansion ratio λ is shown in FIG. 1, and the relationship between the cooling rate after coil winding and the fracture surface transition temperature vTrs is shown in FIG.

  As is clear from FIG. 1, a good correlation is observed between the fracture surface transition temperature vTrs and the hole expansion rate λ, and in order to ensure the target good hole expansion rate λ (λ = 60%), It can be seen that the fracture surface transition temperature vTrs should be 0 ° C. or lower. In addition, as a criterion for judging whether or not the hole expandability is good, “hole expansion ratio λ: 60% or more” is required, but this is required when processing into various members to which a high-strength hot-rolled steel sheet is applied. It is a characteristic level.

  On the other hand, as is apparent from FIG. 2, it is understood that the fracture surface transition temperature vTrs that affects the hole expansion ratio λ varies depending on the cooling rate that simulates the cooling of the winding coil. At this time, in order to secure the fracture surface transition temperature vTrs at a target of 0 ° C. or lower, it is understood that the average cooling rate needs to be cooled at a cooling rate of 50 ° C./hr or higher.

  When the fracture surface of the impact test piece at this time was observed with an SEM, a grain boundary fracture surface was observed on the brittle fracture surface of the test piece having a high fracture surface transition temperature vTrs. On the other hand, only the cleaved fracture surface was observed in the brittle fracture surface of the specimen having a low fracture surface transition temperature vTrs. Therefore, as a result of measuring the grain boundary fracture surface portion using an Auger electron spectroscopic analyzer, a high concentration of P was detected at the grain boundary. Therefore, it can be considered that P segregated at the ferrite grain boundaries lowers the toughness value of the base material and prevents crack propagation during the hole expansion test, resulting in low characteristics. That is, it can be seen that by controlling the cooling rate after winding the coil, the diffusion of P segregated at the ferrite grain boundaries is suppressed, and a characteristic with a high hole expansion ratio λ value can be obtained.

Example 2
After holding various steel slabs having the chemical composition shown in Table 3 below at a slab heating temperature of 1250 ° C. for 30 minutes, the finish rolling temperature is 900 to 930 ° C. and the thickness is 4 mm by a normal hot rolling process. A hot rolled steel sheet was obtained. Thereafter, the cooling rate was controlled at an average cooling rate of 30 ° C./second, and after 30 minutes of winding at 450 to 650 ° C. using an electric heating furnace, the cooling rate was controlled in order to change the subsequent cooling rate. After cooling from the furnace, taking out from the furnace, cooling, blast cooling, (blast + mist) cooling, shower cooling, cooling by immersion in a water tank, etc. were performed to obtain various hot rolled steel sheets.

  The hot-rolled steel sheet thus obtained is subjected to a tensile test perpendicular to the rolling direction with a JIS No. 5 test piece to measure mechanical properties (yield strength YS, tensile strength TS, elongation EL, etc.) Hole expansibility and fracture surface transition temperature were measured in the same manner as in Example 1. The results are shown in Table 4 below together with the production conditions (rolling finishing temperature, winding temperature, cooling rate after winding). Based on these results, the relationship between the fracture surface transition temperature vTrs and the hole expansion ratio λ is shown in FIG. 3, and the relationship between the average cooling rate after coil winding and the fracture surface transition temperature vTrs is shown in FIG.

  As is clear from FIG. 3, as in Example 1, there is a good correlation between the fracture surface transition temperature vTrs and the hole expansion rate λ, and the target good hole expansion rate λ (λ = 60%). It can be seen that the fracture surface transition temperature vTrs should be 0 ° C. or lower in order to ensure the above. As can be seen from FIG. 4, the fracture surface transition temperature vTrs that affects the hole expansion ratio λ varies depending on the cooling rate that simulates the cooling of the winding coil. At this time, in order to ensure the target fracture surface transition temperature vTrs of 0 ° C. or less, it is understood that the average cooling rate needs to be cooled at a cooling rate of 50 ° C./hr or more. In addition, the part enclosed with the broken line of FIG. 4 is what the fracture surface transition temperature vTrs rose because the chemical component composition deviated from the range prescribed | regulated by this invention.

  From these results, it can be considered as follows. Test No. Those of 12-15, 17, 18, 20-25, 27, 28, 30, 31 satisfy all of the requirements defined in the present invention, and both the mechanical properties and the hole expansion rate are good. It can be seen that a hot-rolled steel sheet having high strength and good workability can be realized.

  In contrast, test no. The ones of 16, 19, 26, 29, and 32-39 lack any of the requirements defined in the present invention, and at least one of mechanical characteristics and hole expansibility is deteriorated.

  First, test no. In the samples of 16, 19, 26, and 29, the average cooling rate after coil winding is small, the fracture surface transition temperature vTrs is high, and good hole expansibility is not obtained. In addition, Test No. Those of 32 and 33 are steel sheets with excessive Si content (steel type J in Table 3), the fracture surface transition temperature vTrs is increased, and good hole expansibility is not obtained.

  Test No. 34 and 35 are steel sheets with an excessive Mn content (steel type K in Table 3), the ductility (elongation) is lowered, the fracture surface transition temperature vTrs is increased, and good hole expansibility is obtained. It is not done. Test No. No. 36 is a steel sheet with an excessive P content (steel type L in Table 3), the fracture surface transition temperature vTrs is increased, and good hole expansibility is not obtained.

  Test No. Nos. 37 and 38 are steel sheets with excessive Ti and C contents (steel types M and N in Table 3), respectively, and the ductility (elongation) is lowered. Test No. In No. 39, the C content is insufficient (steel type O in Table 3), and the tensile strength is lowered.

6 is a graph showing the relationship between the fracture surface transition temperature vTrs and the hole expansion rate λ in Example 1. 6 is a graph showing the relationship between the cooling rate after coil winding in Example 1 and the fracture surface transition temperature vTrs. 6 is a graph showing a relationship between a fracture surface transition temperature vTrs and a hole expansion rate λ in Example 2. It is the graph which showed the relationship between the cooling rate after coil winding in Example 2, and the fracture surface transition temperature vTrs.

Claims (9)

  C: 0.05 to 0.15% (meaning of mass%, the same shall apply hereinafter), Si: 1.50% or less (not including 0%), Mn: 0.5 to 2.5%, P: 0.00. 035% or less (excluding 0%), S: 0.01% or less (including 0%), Al: 0.02 to 0.15%, Ti: 0.05 to 0.2%, respectively. The balance is a steel plate made of Fe and inevitable impurities, and the metal structure is a structure containing ferrite or ferrite and martensite which is solid solution strengthened or precipitation strengthened in addition to bainite having a volume of 60 to 95% by volume. A high-strength hot-rolled steel sheet excellent in hole expansion workability, characterized by having a fracture surface transition temperature vTrs obtained by a test of 0 ° C. or lower.   The high-strength hot-rolled steel sheet according to claim 1, further comprising Ni: 1.0% or less (not including 0%).   The high-strength hot-rolled steel sheet according to claim 1 or 2, further comprising Cr: 1.0% or less (not including 0%).   The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, further comprising Mo: 0.5% or less (not including 0%).   Furthermore, Nb: 0.1% or less (0% is not included) is contained, The high intensity | strength hot-rolled steel plate in any one of Claims 1-4.   The high-strength hot-rolled steel sheet according to any one of claims 1 to 5, further comprising B: 0.01% or less (not including 0%).   The high strength hot rolled steel sheet according to any one of claims 1 to 6, further comprising Ca: 0.01% or less (not including 0%).   The high-strength hot-rolled steel sheet according to any one of claims 1 to 7, further comprising Cu: 1.0% or less (not including 0%). In producing the high-strength hot-rolled steel sheet according to any one of claims 1 to 8, a step of heating the steel slab having the chemical component to a temperature range of 1150 to 1300 ° C, and a steel slab after heating to Ar ₃ The step of hot rolling at a finishing temperature equal to or higher than the transformation point to form a steel sheet, and the hot-rolled steel sheet is cooled to a temperature range of 400 to 550 ° C at an average cooling rate of 30 ° C / second or more and wound on a coil. Production of a high-strength hot-rolled steel sheet excellent in hole expansion workability, comprising a step and a step of cooling the coil after winding to a temperature of 300 ° C. or lower at an average cooling rate of 50 to 400 ° C./hour Method.

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