JP5556157B2 - Method for producing high-strength hot-rolled steel sheet having excellent elongation and stretch flange characteristics and tensile strength of 780 MPa or more - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet suitable for structural parts of automobiles, and more particularly to a high-strength hot-rolled steel sheet having a tensile strength TS excellent in elongation and stretch flange characteristics of 780 MPa or more and a method for producing the same.

  In recent years, with increasing interest in environmental issues, steel sheets for automobiles are required to have higher strength and thinner wall thickness for the purpose of improving fuel efficiency through weight reduction. Currently, high-strength hot-rolled steel sheets with 440MPa class or 590MPa class TS are mainly used for structural parts such as automobile pillars and members. In the near future, high strength steel with TS of 780MPa or higher will be used. The practical application of hot-rolled steel sheets is predicted.

  For this reason, research and development for high-strength hot-rolled steel sheets with TS of 780 MPa or more is actively conducted, improving workability that deteriorates with increasing strength, in particular, improving elongation and stretch flange characteristics. Various high-strength hot-rolled steel sheets have been proposed.

For example, Patent Document 1 includes a composite structure including a bainite phase in a volume ratio of 5 to 70% and the balance being substantially composed of a ferrite phase, and Ti within a range satisfying the following formula (1) in the ferrite phase. A high-strength hot-rolled steel sheet excellent in workability (elongation and stretch flange characteristics) having a TS of 690 MPa or more, characterized in that precipitates containing Mo and Mo are dispersed and precipitated.
(Mo / 96) / {(Ti / 48) + (Mo / 96)} ≧ 0.25 ... (1)
However, Ti and Mo in the formula (1) represent% by weight of each component in the precipitate.

  Patent Document 2 includes mass%, C: 0.04 to 0.15%, Si: 1.5% or less, Mn: 0.5 to 1.6%, P: 0.04% or less, S: 0.005% or less, Al: 0.04% or less, It contains Ti: 0.03-0.15% and Mo: 0.03-0.5%, and the balance has a chemical composition consisting of Fe and inevitable impurities, and consists of a ferrite phase in which precipitates are present, a bainite phase and / or a martensite phase. And the ferrite phase and the other phase other than the second phase, and the proportion of the ferrite phase in which the precipitate is present is 40 to 95%, the proportion of the other phase Has disclosed a high-strength hot-rolled steel sheet having 5% or less.

  Furthermore, in Patent Document 3, in mass%, C: 0.06 to 0.15%, Si: 1.2% or less, Mn: 0.5 to 1.6%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.03 to 0.20% contained, the remainder has a composition composed of Fe and inevitable impurities, 50 to 90% by volume occupancy is the ferrite phase, and the remainder is substantially the bainite phase, The total volume occupancy of the ferrite phase and the bainite phase is 95% or more, a precipitate containing Ti is precipitated in the ferrite phase, and the average diameter of the precipitate has a structure of 20 nm or less, and A high-strength hot-rolled steel sheet having a TS of 780 MPa or more, which is excellent in elongation characteristics, stretch flange characteristics and tensile fatigue characteristics, characterized by precipitation of 80% or more of the Ti content in the steel is disclosed.

Japanese Patent Laid-Open No. 2003-321739 JP 2004-339606 A Japanese Unexamined Patent Publication No. 2007-9322

  However, the high-strength hot-rolled steel sheets described in Patent Documents 1 to 3 have the following problems. That is, the high-strength hot-rolled steel sheets described in Patent Documents 1 and 2 are expensive because they use Mo. Usually, stretch flangeability is evaluated by a hole expansion test using a conical punch in accordance with the iron standard JFST1001. In the high-strength hot-rolled steel sheets described in Patent Documents 1 to 3, even if such stretch flangeability is good, the strain distribution in the deformed portion is reduced, specifically represented by cylindrical hole expansion. Cracks may occur when subjected to stretch flange processing that reduces the strain gradient of the deformed portion.

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having a TS of 780 MPa or more, which is inexpensive and excellent in elongation and stretch flange characteristics, and a method for producing the same, which solve such problems.

  As a result of intensive studies to achieve the above object, the present inventors have found the following.

  B) In order to improve the workability in stretch flange processing so that the strain gradient of the deformed portion becomes small, it is important to improve the uniform stretch characteristics of the steel sheet.

  (B) For that purpose, the ferrite composition is composed mainly of a ferrite phase containing 2-9% bainite phase by area ratio after optimizing the component composition, and in the ferrite phase, the ferrite phase of crystal grains having an aspect ratio of less than 3.0 The ratio of the total Ti content in precipitates with an average diameter of less than 20 nm to the total Ti content in all precipitates in steel is 70% or more and the average diameter is less than 20 nm. It is effective to satisfy the condition that the area ratio of the crystal grains where the precipitates are present to the entire ferrite phase is 50% or more.

The present invention was made based on such findings, and in mass%, C: 0.06 to 0.15%, Si: 1.2% or less, Mn: 1.0 to 1.8%, P: 0.04% or less, S: 0.005% Below, Al: 0.05% or less, Ti: 0.06-0.13% is contained, the remainder has a component composition consisting of Fe and inevitable impurities, the area ratio of the ferrite phase occupying the entire structure is 85% or more, bainite phase Provided is a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, wherein the area ratio is 2 to 9% and the ferrite phase has a microstructure satisfying the following conditions i) to iii).
i) The area ratio of crystal grains with an aspect ratio of less than 3.0 in the entire ferrite phase is 80% or more
ii) The ratio of the total Ti content in precipitates with an average diameter of less than 20 nm to the total Ti content in all precipitates in steel is 70% or more
iii) The area ratio of the crystal grains in which precipitates having an average diameter of less than 20 nm are present in the entire ferrite phase is 50% or more In the high-strength hot-rolled steel sheet of the present invention, V: 0.03 to 0.15%, Cr : It is preferable to have a component composition containing at least one element selected from 0.01 to 0.5%.

  The high-strength hot-rolled steel sheet of the present invention is a steel slab having the above component composition, heated at a heating temperature of 1150 to 1350 ° C., hot-rolled at a finishing temperature of 850 to 950 ° C., and then at least 30 ° C./second. Primary cooling to a cooling stop temperature of 650 to 750 ° C at an average cooling rate, followed by air cooling for 0.5 to 5 seconds, followed by secondary cooling at an average cooling rate of 20 ° C / second or more and a winding temperature of 400 to 500 ° C Can be manufactured by winding.

  According to the present invention, it is inexpensive and has good uniform elongation characteristics. As a result, it has excellent workability in stretch flange processing such that the strain gradient of the deformed portion becomes small. TS with excellent elongation and stretch flange characteristics is 780 MPa or more. High-strength hot-rolled steel sheets can be manufactured. If the high-strength hot-rolled steel sheet of the present invention is applied to structural parts such as automobile pillars and members, it is possible to reduce the wall thickness while ensuring the safety of passengers, and to reduce the environmental burden of automobiles. .

  Details of the present invention will be described below. Note that “%” representing the content of each component element means “% by mass” unless otherwise specified.

1) Component composition
C: 0.06-0.15%
C is an element effective for increasing the strength by promoting the formation of a hard bainite phase or by depositing as fine Ti carbide (precipitate) having a diameter of less than 20 nm in the ferrite phase. In order to obtain a TS of 780 MPa or more, the C content needs to be 0.06% or more. On the other hand, if the amount of C exceeds 0.15%, the weldability decreases. Therefore, the C content is 0.06 to 0.15%, preferably 0.07 to 0.12%.

Si: 1.2% or less
Si is an element that contributes to solid solution strengthening, but if its amount exceeds 1.2%, the surface properties are remarkably deteriorated and the corrosion resistance is lowered. Therefore, the Si content is 1.2% or less, preferably 0.3 to 0.9%.

Mn: 1.0-1.8%
Mn is an element effective for increasing the strength, but if its amount is less than 1.0%, a TS of 780 MPa or more cannot be obtained. On the other hand, when the amount of Mn exceeds 1.8%, center segregation becomes prominent, and workability and weldability such as elongation and stretch flange characteristics deteriorate. Therefore, the Mn content is 1.0 to 1.8%, preferably 1.2 to 1.6%.

P: 0.04% or less
When the amount of P exceeds 0.04%, it segregates at the grain boundaries and lowers the workability such as low temperature toughness, elongation and stretch flange characteristics. Therefore, the P content is 0.04% or less, but it is preferable to reduce it as much as possible.

S: 0.005% or less
S forms sulfides with Mn and Ti, and degrades workability such as elongation and stretch flange characteristics. Therefore, the S amount is 0.005% or less, but it is preferable to reduce it as much as possible.

Al: 0.05% or less
Al is added as a steel deoxidizer and is an effective element for improving the cleanliness thereof, so it is preferably contained in an amount of 0.001% or more. However, if the Al content exceeds 0.05%, a large amount of inclusions are formed, causing surface defects. Therefore, the Al content is 0.05% or less, preferably 0.01 to 0.04%.

Ti: 0.06-0.13%
Ti is an element that combines with C in the ferrite phase to precipitate as fine precipitates and contributes to increasing the strength. In order to obtain a TS of 780 MPa or more, the Ti amount needs to be 0.06% or more. However, if it exceeds 0.13%, the effect is saturated and the cost is increased. Therefore, the Ti content is 0.06 to 0.13%, preferably 0.09 to 0.12%.

  The balance is Fe and inevitable impurities, but at least one element selected from V: 0.03 to 0.15% and Cr: 0.01 to 0.5% can be contained for the following reasons.

V: 0.03-0.15%
V is an element that contributes to increasing strength and improving fatigue strength by precipitation strengthening or solid solution strengthening of steel. However, if the amount is less than 0.03%, the effect is poor, and if it exceeds 0.15%, the effect is saturated, resulting in an increase in cost. Therefore, the V amount is preferably 0.03 to 0.15%.

Cr: 0.01-0.5%
Cr, like V, is an element that contributes to increasing strength and improving fatigue strength by precipitation strengthening or solid solution strengthening of steel. However, if the amount is less than 0.01%, the effect is poor, and if it exceeds 0.5%, the effect is saturated, resulting in an increase in cost. Therefore, the Cr content is preferably 0.01 to 0.5%.

2) Microstructure In the present invention, TS of 780 MPa or more is achieved by a ferrite phase hardened by precipitates containing fine Ti having an average diameter of less than 20 nm and a hard bainite phase. That is, Ti is finely precipitated such that the ratio of the total Ti content in the precipitates having an average diameter of less than 20 nm to the total Ti content in all the precipitates in the steel is 70% or more. Such fine Ti precipitates are mainly precipitated in a ferrite phase having an area ratio of 85% or more in the entire structure, thereby precipitating and strengthening the ferrite phase. In addition, the strength is increased by including a hard bainite phase having an area ratio of 2 to 9%. When the ratio of the total Ti content in precipitates with an average diameter of less than 20 nm to the total Ti content in all precipitates in steel is less than 70% or the area ratio of the bainite phase is less than 2%, TS of 780MPa or more cannot be obtained. Further, when the area ratio of the bainite phase exceeds 9%, the elongation characteristics are deteriorated.

  Excellent elongation characteristics, the area ratio of the entire structure is 85% or more to make the ferrite phase rich in ductility, and the area ratio of grains with an aspect ratio of less than 3.0 in the ferrite phase is 80% or more This is achieved mainly by the recrystallized ferrite phase. In particular, if the area ratio of the crystal grains containing precipitates with an average diameter of less than 20 nm in the entire ferrite phase is set to 50% or more, that is, if fine precipitates are deposited more uniformly in the ferrite phase, the uniform elongation characteristics are remarkably improved. In addition, it is possible to prevent the occurrence of cracks at the time of stretch flange processing that reduces the strain gradient of the deformed portion. Such fine precipitates observed in the ferrite phase are mainly precipitates containing Ti. If the area ratio of the ferrite phase occupying the entire structure is less than 85%, or the area ratio of the crystal grains having an aspect ratio of less than 3.0 in the ferrite phase is less than 80%, the elongation characteristics are deteriorated. Further, if the area ratio of the crystal grains in which precipitates having an average diameter of less than 20 nm are present in the entire ferrite phase is less than 50%, the uniform elongation characteristics are deteriorated. The area ratio of the crystal grains having an aspect ratio of less than 3.0 in the ferrite phase is preferably 90% or more.

  Excellent stretch flange properties are achieved by hardening the ferrite phase with fine precipitates and reducing the hardness difference from the bainite phase. This is presumably because the stress concentration at the interface between the ferrite phase and the bainite phase is alleviated by reducing the hardness difference. In addition, as described above, when fine precipitates are deposited more uniformly in the ferrite phase, the uniform elongation characteristics are remarkably improved, and cracks do not occur even during stretch flange processing where the strain gradient of the deformed portion is reduced. Excellent stretch flange characteristics can be obtained.

  Here, the area ratio of ferrite phase and bainite phase occupying the whole structure is obtained by corroding the plate thickness section parallel to the rolling direction after sampling a scanning electron microscope (SEM) test piece and then corroding with 3% nital solution. In addition, three fields of view of the SEM photograph were taken at a magnification of 1000 times, the area of each phase was measured by image analysis processing, and the ratio (percentage) in the area of the observation field was obtained. Similarly, the area ratio of the crystal grains having an aspect ratio of less than 3.0 in the ferrite phase was determined by measuring the area of the crystal grains having an aspect ratio of less than 3.0 and occupying the area of the ferrite phase. The aspect ratio of the crystal grains is defined by (maximum grain size in the rolling direction) / (maximum grain size in the plate thickness direction) in the crystal grains.

The Ti content in the precipitate having an average diameter of less than 20 nm was determined by the following method. In other words, the sample was subjected to constant current electrolysis at a current density of 20 mA / cm ² in a 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol), and deposits adhered to the surface. Remove the sample from the electrolyte, immerse it in a sodium hexametaphosphate aqueous solution (500 mg / l) (hereinafter referred to as SHMP aqueous solution), apply ultrasonic vibration, peel off the deposit from the sample, and remove the SHMP aqueous solution. Extracted in. Next, the aqueous solution of SHMP containing the precipitate is filtered using a filter with a pore size of 20 nm, and the filtrate after filtration is analyzed using an ICP emission spectroscopic analyzer, an ICP mass spectrometer, an atomic absorption spectrometer, etc. The absolute amount of Ti in the filtrate was measured. Then, the absolute amount of Ti was divided by the electrolytic weight to obtain a Ti content (mass% in steel) contained in a precipitate having a size of less than 20 nm. In addition, the electrolysis weight was calculated | required by measuring a weight with respect to the sample after deposit peeling, and subtracting from the sample weight before electrolysis. In addition, the Ti content (% by mass in steel) in all precipitates precipitated in steel was analyzed using an ICP emission spectroscopic analyzer for the SHMP aqueous solution before filtration using a filter with a pore size of 20 nm. And obtained in the same manner as described above. From these Ti contents, the ratio of the total Ti content in precipitates having an average diameter of less than 20 nm to the total Ti content in all precipitates in steel was calculated.

  Further, the area ratio of the crystal grains in which precipitates having an average diameter of less than 20 nm are present in the entire ferrite phase was determined by the following method. First, the ferrite crystal grains confirmed in a visual field with a magnification of 10,000 times by TEM are observed at a high magnification (about 50,000 times), and the area of the crystal grains where precipitates of less than 20 nm are present is obtained. The same operation is repeated in 3 to 5 fields of view to determine the total area of the grains having precipitates of less than 20 nm, and the total area of grains having deposits of less than 20 nm relative to the total area of the ferrite phase in the observed field of view. The ratio (percentage) was defined as the area ratio.

3) Manufacturing conditions Slab heating temperature: 1150 ~ 1350 ℃
In order to precipitate precipitates containing fine Ti in the ferrite phase after hot rolling, it is necessary to dissolve precipitates containing coarse Ti precipitated in the slab before hot rolling. For this purpose, it is necessary to heat the slab to 1150 ° C or higher. On the other hand, when the slab is heated at a temperature exceeding 1350 ° C., the ferrite phase crystal grains become coarse and the strength tends to be lowered. Therefore, the heating temperature of the slab is 1150 to 1350 ° C, preferably 1170 to 1260 ° C.

Hot rolling finishing temperature: 850-950 ° C
If the finishing temperature is less than 850 ° C, it is rolled in the two-phase region of ferrite and austenite. Therefore, the ferrite phase grains after rolling extend and the area ratio of grains with an aspect ratio of less than 3.0 is increased to 80% or more. It becomes difficult. On the other hand, when the finishing temperature exceeds 950 ° C., the precipitate diameter increases, so that the necessary precipitation strengthening ability cannot be obtained. Therefore, the finishing temperature of hot rolling is 850 to 950 ° C, preferably 880 to 920 ° C.

Primary cooling conditions after hot rolling: average cooling rate of 30 ° C / second or more, cooling stop temperature of 650-750 ° C
When the average cooling rate of the primary cooling is less than 30 ° C./second, a pearlite phase is formed, and elongation and stretch flange characteristics deteriorate. Therefore, the average primary cooling rate is 30 ° C./second or more. The upper limit of the average cooling rate is not particularly limited, but a cooling rate of about 100 ° C./second is preferable to stop cooling in the next cooling stop temperature range. The primary cooling method is not particularly limited, and for example, water cooling by known laminar cooling can be used.

  When the cooling stop temperature of the primary cooling is less than 650 ° C, precipitates containing fine Ti cannot be sufficiently precipitated in the ferrite phase during the subsequent air cooling, and there are precipitates with an average diameter of less than 20 nm above 750 ° C. The area ratio of the crystal grains in the entire ferrite phase is less than 50%. Therefore, the cooling stop temperature of the primary cooling is 650 to 750 ° C, preferably 680 to 720 ° C.

Air cooling condition after primary cooling: Air cooling time 0.5 to 5 seconds Air cooling after primary cooling is an important point in the present invention. In other words, the precipitation of fine Ti-containing precipitates in the ferrite phase is promoted by air cooling, and the area ratio of the crystal grains in which these precipitates are present in the entire ferrite phase is 50% or more. It can be greatly improved, and cracking when subjected to stretch flange processing that reduces the strain distribution of the deformed portion can be prevented.

  However, when the air cooling time is less than 0.5 seconds, precipitation of fine Ti-containing precipitates is not sufficient, and when it exceeds 5 seconds, the area ratio of the crystal grains containing precipitates having an average diameter of less than 20 nm to the entire ferrite phase is 50%. Less than%. Therefore, the air cooling time after the primary cooling is 0.5 to 5 seconds, preferably 1 to 4 seconds.

  The cooling rate during air cooling is approximately 15 ° C./second or less.

Secondary cooling conditions after air cooling: average cooling rate of 20 ° C / second or more After air cooling, secondary cooling is performed at an average cooling rate of 20 ° C / second or more until winding. This is because when the average cooling rate is less than 20 ° C./second, a pearlite phase is formed, and elongation and stretch flange characteristics are deteriorated.

Winding temperature: 400-500 ° C
When the coiling temperature is less than 400 ° C., a remarkably hard martensite phase is formed. Therefore, the coiling temperature is 400 to 500 ° C, preferably 420 to 480 ° C.

  Normal conditions can be applied to other manufacturing conditions. For example, steel having a desired component composition is manufactured by melting in a converter or electric furnace and then performing secondary refining in a vacuum degassing furnace. The subsequent casting is preferably performed by a continuous casting method from the viewpoint of productivity and quality. After casting, hot rolling is performed according to the method of the present invention. After hot rolling, the properties of the steel sheet do not change even if the scale is attached to the surface or the scale is removed by pickling. Further, after hot rolling, temper rolling may be performed, or hot dip galvanizing, electrogalvanizing, or chemical conversion treatment may be performed. Here, zinc-based plating is plating mainly composed of zinc and zinc (that is, containing about 90% or more of zinc), and plating or zinc-based plating containing alloy elements such as Al and Cr in addition to zinc. It is the plating which performed the alloying process later.

  Steel Nos. A to D having the composition shown in Table 1 were melted in a converter, and a steel slab was obtained by continuous casting. From these steel slabs, hot-rolled steel sheets Nos. 1 to 6 having a thickness of 3.4 mm were produced under the hot-rolling conditions shown in Table 2.

  And by the above method, the ferrite phase area ratio (F phase area ratio), bainite phase area ratio (B phase area ratio), ferrite phase aspect ratio of less than 3.0 Ti content in precipitates with an average diameter of less than 20 nm with respect to the total area ratio (area ratio of crystal grains with an aspect ratio of less than 3.0), total Ti content in all precipitates precipitated in steel The ratio of the total amount [(Ti amount in precipitates less than 20 nm) / (Ti amount in all precipitates) × 100], and accounts for precipitates having an average diameter less than 20 nm in the entire ferrite phase The area ratio (area ratio of crystal grains where precipitates of less than 20 nm exist) was determined.

  In addition, JIS No. 5 tensile test specimen (parallel to the rolling direction) was collected and subjected to a tensile test at a strain rate of 10 mm / min in accordance with JIS Z2241 to measure TS, total elongation El, and uniform elongation U-El. If the TS is 780 MPa or more, the El is 24% or more, and the U-El is 12% or more, the target of the present invention is achieved.

  Furthermore, a 130 mm square hole expansion test specimen was collected and subjected to a hole expansion test in accordance with the Iron Federation Standard JFST 1001 to obtain a hole expansion ratio λ. If λ is 75% or more, the stretch flangeability is good. It was.

  The results are shown in Table 3. In the example of the present invention, it can be seen that TS is 780 MPa or more, El is 24% or more, excellent elongation characteristics, high λ of 75% or more is obtained, and stretch flange characteristics are also excellent. Furthermore, in the example of the present invention, it can be seen that the U-El is 12% or more and the uniform elongation property is also excellent.

Claims (2)

The area ratio of the ferrite phase occupying the entire structure is 85% or more, the area ratio of the bainite phase is 2 to 9%, and the ferrite phase has a microstructure satisfying the following conditions i) to iii): A method of manufacturing a steel sheet,
In mass%, C: 0.06-0.15%, Si: 1.2% or less, Mn: 1.0-1.8%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.06-0.13% The steel slab having a composition composed of Fe and inevitable impurities as the balance is heated at a heating temperature of 1150 to 1350 ° C., and after hot rolling at a finishing temperature of 850 to 950 ° C., an average of 30 ° C./second or more Primary cooling to a cooling stop temperature of 650 to 750 ° C at a cooling rate, followed by air cooling for 0.5 to 5 seconds, followed by secondary cooling at an average cooling rate of 20 ° C / second or more and 50 ° C / second or less to 400 to 500 ° C A method for producing a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, characterized by winding at a winding temperature.
i) The area ratio of the crystal grains having an aspect ratio of less than 3.0 to the entire ferrite phase is 80% or more,
ii) The ratio of the total Ti content in precipitates with an average diameter of less than 20 nm to the total Ti content in all precipitates in steel is 70% or more,
iii) The area ratio of the crystal grains in which precipitates having an average diameter of less than 20 nm are present in the entire ferrite phase is 50% or more The tensile strength according to claim 1, further comprising a component composition containing at least one element selected from V: 0.03-0.15% and Cr: 0.01-0.5% in mass%. Is a manufacturing method of high-strength hot-rolled steel sheets of 780 MPa or more.