JP5482162B2 - High-strength hot-rolled steel sheet having excellent elongation and stretch flange characteristics and a tensile strength of 780 MPa or more, and a method for producing the same - 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, and the high-strength hot-rolled steel sheets described in Patent Documents 1 to 3 are stably 780 MPa or more. TS may not be secured.

  The present invention has been made to solve such problems, and provides a high-strength hot-rolled steel sheet excellent in elongation and stretch flange characteristics that can obtain TS of 780 MPa or more stably at low cost and a method for producing the same. The purpose is to provide.

  As a result of studying a high strength hot-rolled steel sheet excellent in elongation and stretch flange characteristics that can stably obtain TS of 780 MPa or more stably at low cost, the present inventors have found the following.

  That is, without using Mo, a microstructure is mainly formed of a polygonal ferrite phase mainly composed of a bainite phase and a bainitic ferrite phase and deposited with a precipitate containing Ti having a size of less than 20 nm. As a result, a TS of 780 MPa or more can be secured stably, and excellent elongation and stretch flange characteristics can be obtained.

  The present invention was made based on such findings, and in mass%, C: 0.06 to 0.15%, Si: 1.2% or less, Mn: 0.5 to 2.0%, P: 0.04% or less, S: 0.005% Hereinafter, Al: 0.05% or less, Ti: 0.03 to 0.15% contained, the remainder has a composition composed of Fe and inevitable impurities, the total volume ratio of the bainite phase and bainitic ferrite phase occupying the entire structure Is a microstructure in which the volume fraction of the bainite phase and the total of the bainitic ferrite phase and the polygonal ferrite phase is 95% or more, and the total Ti content in the precipitate having an average diameter of less than 20 nm. However, the present invention provides a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, characterized in that it is 50% or more of the total Ti content in all precipitates precipitated in the steel.

  In the high-strength hot-rolled steel sheet of the present invention, in addition, by mass%, V: 0.005-0.2%, B: 0.0005-0.002%, Ca: 0.0005-0.005%, or a component containing two or more elements simultaneously A composition is preferred.

  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 550 ° C or more and less than 630 ° C at an average cooling rate, followed by air cooling for 0.5 to 10 seconds, followed by secondary cooling at an average cooling rate of 20 ° C / second or more and winding at 300 to 500 ° C Can be manufactured by winding at temperature.

  The present invention makes it possible to produce a high-strength hot-rolled steel sheet excellent in elongation and stretch-flange characteristics that can obtain TS of 780 MPa or more stably at low cost. 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 bainitic ferrite phase or by precipitating as fine Ti carbides (precipitates) in the polygonal 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 0.9% or less.

Mn: 0.5-2.0%
Mn is an element effective for increasing the strength, but if its amount is less than 0.5%, a TS of 780 MPa or more cannot be obtained. On the other hand, when the Mn content exceeds 2.0%, center segregation becomes prominent, and workability such as stretch flange characteristics and weldability deteriorate. Therefore, the Mn content is 0.5 to 2.0%, preferably 0.8 to 1.8%.

P: 0.04% or less
When the P content exceeds 0.04%, segregation occurs at the grain boundaries, resulting in deterioration of workability such as low temperature toughness 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 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: 0.03-0.15%
Ti is an element that mainly binds to C in the polygonal ferrite phase to precipitate fine precipitates and contributes to increasing the strength. In order to obtain such effects, the Ti amount needs to be 0.03% or more. However, if it exceeds 0.15%, the effects are saturated and the cost is increased. Therefore, the Ti content is 0.03 to 0.15%, preferably 0.05 to 0.12%.

  The balance is Fe and unavoidable impurities, but for the following reasons, V: 0.005-0.2%, B: 0.0005-0.002%, Ca: 0.0005-0.005% are contained individually or simultaneously with two or more elements. It is preferable.

V: 0.005-0.2%
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 0.005%, the effect is poor, and if it exceeds 0.2%, the effect is saturated and the cost is increased. Therefore, the V amount is preferably 0.005 to 0.2%.

B: 0.0005-0.002%
B is an element having an action of suppressing the formation and growth of ferrite phase from the austenite grain boundary, and can be contained as necessary. In order to obtain the effect, the B content is preferably 0.0005% or more. On the other hand, when the amount of B exceeds 0.002%, workability such as elongation characteristics is lowered. Therefore, the B amount is preferably 0.0005 to 0.002%.

Ca: 0.0005-0.005%
Ca is an effective element for making the shape of sulfide spherical and improving the stretch flange characteristics. In order to obtain the effect, the Ca content is preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.005%, inclusions and the like increase, causing surface defects and internal defects. Therefore, the Ca content is preferably 0.0005 to 0.005%.

2) Micro structure
2-1) The total volume fraction of the bainite phase and bainitic ferrite phase in the entire structure is 50% or more, and the total volume fraction of the bainite phase, bainitic ferrite phase and polygonal ferrite phase is 95% or more.
A bainite phase and a bainitic ferrite phase are mainly used to satisfy both TS of 780 MPa or more and excellent elongation and stretch flange characteristics. That is, in the present invention, a relatively hard bainitic ferrite phase is generated by air cooling at a cooling stop temperature lower than the ferrite phase generation temperature range. The presence of such bainitic ferrite phase reduces the interface where a large hardness difference between these structures, which has been a problem with the mixed structure of the bainite phase and the polygonal ferrite phase, has been reduced. The characteristics can be greatly improved. In the bainitic ferrite phase, lath can be confirmed in the grains, and in the bainite phase, carbide can be confirmed in the grains. However, in the present invention, the bainitic ferrite phase is generated as described above. It is difficult to clearly distinguish the bainite phase formed at the stage from the bainitic ferrite phase. However, it can be confirmed that it is a bainitic ferrite phase or a bainite phase from the shape of crystal grains and the precipitation state of carbides, and can be distinguished from other structures. For this reason, in the present invention, the structure is defined as the sum of the bainite phase and the bainitic ferrite phase. At this time, if the total volume ratio of the bainite phase and bainitic ferrite phase in the entire structure is less than 50%, TS of 780 MPa or more cannot be obtained. Therefore, the total volume ratio of the bainite phase and the bainitic ferrite phase is 50% or more, preferably 55% or more, more preferably 60% or more, and may be 100%. When it has a remaining structure, it is preferable that this is substantially the polygonal ferrite phase, and the total volume fraction of the bainite phase and the bainitic ferrite phase and the polygonal ferrite phase in the entire structure is 95% or more. There is a need. If the structure other than the bainite phase, bainitic ferrite phase and polygonal ferrite phase, specifically the pearlite phase or martensite phase exceeds 5% in total, the elongation and stretch flangeability will be significantly impaired. It is to become.

  Here, the volume fraction of the bainite phase, bainitic ferrite phase and polygonal ferrite phase occupying the entire structure is obtained by sampling a specimen for a scanning electron microscope (SEM) and polishing a plate thickness section parallel to the rolling direction. Corroded with 3% nital solution, SEM photo taken at 1000x magnification with 3 fields of view, extracted bainite phase, bainitic ferrite phase, polygonal ferrite phase by image processing, and the area of each phase by image analysis processing Measured and calculated as a percentage (percentage) of the area of the observation field. In the bainite phase and bainitic ferrite phase, the shape of grains and the precipitation state of carbides were observed, and the structure in which carbides were confirmed or lath was observed in the grains was defined as the bainite phase and bainitic ferrite phase.

2-2) Total Ti content in precipitates with an average diameter of less than 20 nm: 50% or more of the total Ti content in all precipitates precipitated in steel As described above, in the present invention The balance other than bainite and bainitic ferrite is preferably polygonal ferrite. However, if the difference in hardness from the main bainite phase or bainitic ferrite phase is too large, the stretch flange characteristic is deteriorated. Therefore, in the present invention, even when a polygonal ferrite phase is generated, in order to harden this to some extent, fine precipitation containing Ti with an average diameter of less than 20 nm that can be expected to have a large precipitation strengthening ability by suppressing dislocation movement. The product is precipitated so that the total Ti content in all the precipitates of these fine precipitates is 50% or more of the total Ti content in all the precipitates precipitated in the steel. Such fine precipitates containing Ti are mainly TiC, but due to the difference in the solubility product of TiC between the austenite phase and the ferrite phase, they are likely to precipitate when transformed from the austenite phase to the ferrite phase. When a phase is formed, it precipitates in the polygonal ferrite phase and plays a role of hardening the polygonal ferrite phase. Total Ti content in precipitates with an average diameter of less than 20 nm: If the total Ti content in all precipitates precipitated in the steel is less than 50% of the total, the number of precipitates with an average diameter of less than 20 nm is small, Since the polygonal ferrite phase cannot be hardened, good stretch flangeability cannot be secured.

Here, the Ti content in the precipitate having an average diameter of less than 20 nm can be 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). 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 SHMP aqueous solution containing the precipitate was filtered using a filter with a pore size of 20 nm, and the filtrate after filtration was analyzed using an ICP emission spectrometer, and the absolute amount of Ti in the filtrate was measured. . Then, by dividing the absolute amount of Ti by the electrolysis weight, the Ti content (mass% in steel) contained in the precipitate having a size of less than 20 nm is obtained. In addition, electrolysis weight is calculated | required by measuring 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 the steel was measured using an ICP emission spectrometer for the SHMP aqueous solution before filtration using a filter with a pore diameter of 20 nm. Analyze and find as above. From these Ti contents, the ratio of the total Ti content in the precipitates with an average diameter of less than 20 nm to the total Ti content in all the precipitates precipitated in the steel can be calculated.

3) Manufacturing conditions Slab heating temperature: 1150 ~ 1350 ℃
In order to precipitate fine Ti-containing precipitates in the polygonal ferrite phase after hot rolling, it is necessary to dissolve coarse Ti-containing precipitates 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 above 1350 ° C., the yield tends to decrease due to an increase in scale. Therefore, the heating temperature of the slab is 1150 to 1350 ° C, preferably 1170 to 1260 ° C.

Hot rolling finishing temperature: 850-950 ° C
When the finishing temperature is less than 850 ° C., rolling is performed in a two-phase region of ferrite and austenite, so that the processed structure remains after rolling and the stretch flange characteristics deteriorate. On the other hand, when the finishing temperature exceeds 950 ° C., the amount of precipitates containing Ti decreases, the necessary precipitation strengthening ability cannot be obtained, and the stretch flange characteristics deteriorate. Therefore, the finishing temperature of hot rolling is 850 to 950 ° C.

Primary cooling conditions after hot rolling: average cooling rate of 30 ° C / second or more, cooling stop temperature of 550 ° C or more and less than 630 ° C When the average cooling rate of 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 primary cooling is less than 550 ° C, no bainitic ferrite phase is generated, and when it is 630 ° C or more, polygonal ferrite phase is excessively generated, and a microstructure mainly composed of bainite phase and bainitic ferrite phase is obtained. Absent. Therefore, the cooling stop temperature of the primary cooling is set to 550 ° C. or higher and lower than 630 ° C.

Air-cooling conditions after primary cooling: Air-cooling time 0.5 to 10 seconds After primary cooling, it is necessary to air-cool in order to promote precipitation of precipitates containing fine Ti in the polygonal ferrite phase. At this time, if the air cooling time is less than 0.5 seconds, precipitation of precipitates is not sufficient, and even if it exceeds 10 seconds, the effect of precipitation strengthening is saturated. Therefore, the air cooling time after the primary cooling is 0.5 to 10 seconds, preferably 0.75 to 5 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 generated, and the stretch flange characteristic is deteriorated.

Winding temperature: 300 ~ 500 ℃
When the coiling temperature is less than 300 ° C., an extremely hard martensite phase is generated. Therefore, the coiling temperature is 300 to 500 ° C, preferably 380 to 500 ° 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 H having the composition shown in Table 1 were melted in a converter, and a steel slab was obtained by continuous casting. After these steel slabs were heated to 1250 ° C., hot-rolled steel sheets Nos. 1 to 14 having a thickness of 3.4 mm were produced under the hot-rolling conditions shown in Table 2.

  And, by the above method, the total volume fraction of the bainite phase and the bainitic ferrite phase, and the total content of Ti in the precipitate with an average diameter of less than 20 nm in all the precipitates precipitated in the steel The ratio (%) to the total Ti content was determined.

  Also, a JIS No. 5 tensile test piece (parallel to the rolling direction) was collected, a tensile test was performed at a strain rate of 10 mm / min according to JIS Z2241, TS and total elongation El were measured, TS was 780 MPa or more, El If the ratio is 21% or more, the target of the present invention is achieved.

  Furthermore, in order to evaluate the stretch flange characteristics, a 130 mm square hole expansion test specimen was collected and subjected to a hole expansion test in accordance with the iron standard JFST 1001, to determine the hole expansion ratio λ, where λ was 80% or more. If so, the objective of the present invention was achieved.

  The results are shown in Table 2. In the example of the present invention, a TS of 780 MPa or more is obtained, El is 21% or more, λ is 80% or more, and it can be seen that the elongation and stretch flange characteristics are excellent.

Claims (5)

  In mass%, C: 0.06-0.15%, Si: 1.2% or less, Mn: 0.5-2.0%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.03-0.15% However, the balance is composed of Fe and inevitable impurities, the total volume fraction of the bainite phase and bainitic ferrite phase in the entire structure is 50% or more, and the bainite phase, bainitic ferrite phase and The total volume fraction of the null ferrite phase has a microstructure of 95% or more, and the total Ti content in the precipitates with an average diameter of less than 20 nm is the Ti content in all the precipitates precipitated in the steel. A high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, characterized by being 50% or more of the total content.   2. The high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more according to claim 1, wherein the high-strength steel sheet has a component composition containing V: 0.005-0.2% by mass%.   3. The high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more according to claim 1, wherein the high-strength steel sheet has a component composition containing B: 0.0005 to 0.002% by mass%.   4. The high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more according to any one of claims 1 to 3, further comprising a component composition containing Ca: 0.0005 to 0.005% by mass%. A microstructure in which the total volume fraction of the bainite phase and bainitic ferrite phase in the entire structure is 50% or more, and the total volume fraction of the bainite phase, bainitic ferrite phase and polygonal ferrite phase is 95% or more. Producing high-strength hot-rolled steel sheets that have a total Ti content of precipitates with an average diameter of less than 20 nm that is 50% or more of the total Ti content of all precipitates precipitated in the steel. A way to
The steel slab having the component composition according to any one of claims 1 to 4 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 550 ° C or more and less than 630 ° C at a cooling rate, followed by air cooling for 0.5 to 10 seconds, followed by secondary cooling at an average cooling rate of 20 ° C / second or more, and a coiling temperature of 300 to 500 ° C A method for producing a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, which is characterized by winding at a high temperature.