JP2009280900A - METHOD FOR PRODUCING HIGH-STRENGTH HOT-ROLLED STEEL SHEET HAVING 780 MPa OR MORE OF TENSILE STRENGTH - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high-strength hot-rolled steel sheet having ≥780 MPa of TS and excellent stretch flange formability and stably enabling the fluctuation of the quality in the steel sheet to be small. <P>SOLUTION: The method for producing the high-strength hot-rolled steel sheet having ≥780 MPa of the TS, is characterized in that a steel billet having the following element composition, composed by mass% of 0.04-0.15% C, 0.3-1.5% Si, 0.5-2.0% Mn, ≤0.06% P, ≤0.005% S, ≤0.10% Al, 0.05-0.20% Ti and the balance Fe with inevitable impurities, is heated to the heating temperature of 1,150-1,300°C, and after hot-rolling at 800-1,000°C the finish temperature, this hot-rolled steel sheet is cooled at ≥55°C/sec cooling rate and successively, after cooling in the temperature range of at least ≤500°C thereof, at ≥120°C/sec cooling rate under condition of becoming nucleate-boiling cooling, this steel sheet is coiled up at 350-500°C coiling temperature. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a high-strength hot-rolled steel sheet having a thickness of 8 mm or less suitable for automobile undercarriage parts, etc., and in particular, has a tensile strength TS of 780 MPa or more which is excellent in stretch flangeability after processing and small in material variation in the steel sheet The present invention relates to a method for producing a high-strength hot-rolled steel sheet.

  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. In particular, the use of high-strength hot-rolled steel sheets with TS of 590 MPa or higher and even 780 MPa or higher is increasing for undercarriage parts of automobiles in place of high-strength hot-rolled steel sheets with TS of about 440 MPa, which are widely used at present. Tend to.

  Manufacturing automobile undercarriage parts often involves severe stretch flange processing, so these high-strength hot-rolled steel sheets have excellent stretch flangeability, especially due to recent advances in press technology. Since there is an increasing number of cases of drawing (drawing and overhanging) → trimming (hole punching) → wrist like (hole expanding), an excellent stretch flangeability after processing is required.

  Conventionally, several high-strength hot-rolled steel sheets that have a bainite single-phase structure or a composite structure of bainitic ferrite and bainite, have a TS of 780 MPa or more, and have excellent stretch flangeability have been proposed. For example, in Patent Document 1, in mass%, C: 0.02 to 0.15%, Si: 1.5% or less, Mn: 1.0 to 3.0%, P: 0.04 to 0.15%, S: 0.010% or less, Cr and Mo 1 0.1-0.5% of seeds or two kinds, Al: 0.01-0.1%, Ni: 1.2% or less, Cu: 0.6-1.6%, Nb, Ti and V containing one or more of 0.05-0.25%, Welding mainly composed of bainite by making steel that satisfies P + Cu / 10- (Cr + Mo) / 3 ≧ 0 an average cooling rate after finish rolling of 10 ° C / s or higher and a winding temperature of 700 ° C or lower A high-strength steel sheet having excellent fatigue characteristics and a method for producing the same are disclosed. Patent Document 2 includes mass%, C: 0.02 to 0.05%, Si: 0.3 to 1%, Mn: 1.3 to 2.3%, P: 0.1% or less, S <0.0010%, Cr: 0.05 to 0.7%, Mo. : 0.05 to 0.5%, and (P-0.02) / Si> 1/60, Ti: 0.01 to 0.06%, Nb: 0.01 to 0.03%, V: 0.01 to 0.08%, one or more After finish rolling a steel containing steel, it is cooled to 500 to 600 ° C. at a cooling rate of 35 to 65 ° C./s, and then cooled at a cooling rate of 2 to 20 ° C./s and then wound at 300 to 475 ° C. A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics and a method for producing the same are disclosed. Patent Document 3 includes mass%, C: 0.02 to 0.15%, Si: 0.3 to 2.5%, Mn: 0.5 to 2.5%, P: 0.1% or less, S: 0.01% or less, Sol.Al: 0.005 to 0.08. %, N: 0.008% or less, or steel containing 100% or more of Cu, Ni, Mo, Sn, Nb, Ti, V, Zr, B, Cr, W, Ca, REM Disclosed is a method for producing a bainite-based high-strength steel sheet, which is rolled at a constant speed in the range of 850 mpm, cooled to 700 ° C. or lower at a cooling rate of 10 ° C./s or higher after finish rolling, and then wound at 300 to 550 ° C. ing. In Patent Document 4, C: 0.05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.02% or less Including steel, and Ti: 0.005 to 0.2%, Nb: 0.003 to 0.2%, one or two or more types of steel starts cooling within 2 s after finish rolling, and is wound at a cooling rate of 20 to 150 ° C / s A high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized by continuous cooling to a temperature and winding at 300 to 550 ° C., and a method for producing the same are disclosed.

JP-A-5-331595 Japanese Patent Laid-Open No. 2001-172745 JP 2000-87143 A JP 2000-109951 A JP 2000-42621 A

  In general, for cooling rate control from hot rolling to coiling, forced cooling is performed using water cooling. However, in the production methods described in Patent Document 1 and Patent Document 2, the cooling rate of 500 ° C. or lower, at which the transition boiling state in which film boiling and nucleate boiling are mixed at the time of water cooling, is slow, the transition boiling state at the time of water cooling, and the cooling of the steel sheet Unevenness occurs and material variation in the steel sheet occurs. The steel sheet described in Patent Document 3 tries to reduce the TS fluctuation in the steel sheet by suppressing the uneven cooling by using constant speed rolling, but the material fluctuation in the steel sheet is not necessarily small because the cooling below 500 ° C is not controlled. Can not. In the manufacturing method described in Patent Document 4, it is necessary to strictly control the cooling rate at the time of rapid cooling because the strength fluctuation in the steel sheet increases when attempting to cool at a cooling rate exceeding 150 ° C / s. Thus, it is difficult to strictly control the cooling rate, and particularly when the coiling temperature is 500 ° C. or less, the material fluctuation in the steel sheet cannot always be reduced. In addition, none of the techniques described in Patent Documents 1 to 4 is intended to improve stretch flangeability after processing.

  The present invention has been made to solve such problems, and is a high-strength hot-rolled steel sheet having a TS of 780 MPa or more, which has excellent stretch flangeability after processing and can stably reduce material fluctuations in the steel sheet. An object is to provide a manufacturing method.

  The present inventors examined a method for producing a high-strength hot-rolled steel sheet having a TS of 780 MPa or more with a sheet thickness of about 1 to 8 mm, which is excellent in stretch flangeability after processing and can stably reduce material fluctuations in the steel sheet. As a result, the following was found.

  i) In order to obtain an excellent stretch flangeability after processing with a TS of 780 MPa or more, it is effective to obtain a bainite single phase structure by controlling the cooling conditions after hot rolling. Here, bainite means both bainitic ferrite and bainite.

  ii) In order to stably reduce the material fluctuation in the steel plate, the temperature range of 500 ° C or lower, which is transition boiling cooling in which film boiling cooling and nucleate boiling cooling coexist during water cooling of the steel plate, is set at a cooling rate of 120 ° C / second or more. It is effective to cool under the condition of nucleate boiling cooling.

  The present invention has been completed based on such findings, and in mass%, C: 0.04 to 0.15%, Si: 0.05 to 1.5%, Mn: 0.5 to 2.0%, P: 0.06% or less, S: A steel slab having a component composition comprising 0.005% or less, Al: 0.10% or less, Ti: 0.05-0.20%, the balance consisting of Fe and inevitable impurities, is heated at a heating temperature of 1150-1300 ° C., 800-1000 After hot rolling at a finish temperature of ℃, cool at a cooling rate of 55 ℃ / second or more, and then cool at a temperature range of at least 500 ℃ or less at a cooling rate of 120 ℃ / second or more under nucleate boiling cooling conditions, 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 coiling temperature of 350 to 500 ° C.

  In the production method of the present invention, it is preferable to cool at least a temperature range of 500 ° C. or less under a condition of nucleate boiling cooling at a cooling rate of 250 ° C./second or more. Also, by mass%, Si: 0.3-1.5%, Cr: 0.1-0.8%, Nb: 0.005-0.1%, V: 0.005-0.2%, W: 0.005-0.2%, Mo: 0.01- It is preferable to use a steel slab having a component composition containing one or more selected from 0.3%.

  The production method of the present invention makes it possible to produce a high-strength hot-rolled steel sheet having a TS of 780 MPa or more that is excellent in stretch flangeability after processing and that can stably reduce the material fluctuation in the steel sheet. The high-strength hot-rolled steel sheet produced by the production method of the present invention is particularly suitable for automobile underbody parts.

  Below, the detail of the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention is demonstrated. Note that “%” representing the content of each component element means “% by mass” unless otherwise specified.

1) Component composition
C: 0.04-0.15%
C is an element necessary for producing bainite and ensuring the necessary strength. In order to obtain a TS of 780 MPa or more, the C amount needs to be 0.04% or more. However, if it exceeds 0.15%, the stretch flangeability after processing is deteriorated. Therefore, the C content is 0.04 to 0.15%, preferably 0.05 to 0.10%.

Si: 0.05-1.5%
Si is an element necessary for increasing the strength by solid solution strengthening. When the Si content is less than 0.05%, it is necessary to increase the amount of the expensive alloy element added in order to obtain TS of 780 MPa or more. On the other hand, if it exceeds 1.5%, the surface properties are lowered and the fatigue properties are deteriorated. Therefore, the Si content is 0.05 to 1.5%, preferably 0.3 to 1.5%, more preferably 0.3 to 1.2%.

Mn: 0.5-2.0%
Mn is an element effective for solid solution strengthening and bainite formation. In order to obtain a TS of 780 MPa or more, the Mn content needs to be 0.5% or more, but if it exceeds 2.0%, the weldability deteriorates. Therefore, the Mn content is 0.5 to 2.0%, preferably 0.8 to 0.18%.

P: 0.06% or less
If the P content exceeds 0.06%, the stretch flangeability after processing is reduced due to segregation. Therefore, P is 0.06% or less, preferably 0.03% or less. Note that P is also an element effective for solid solution strengthening. In order to obtain this effect, P is preferably contained in an amount of 0.005% or more.

S: 0.005% or less
S forms sulfides with Mn and Ti to reduce the stretch flangeability after processing, and causes a reduction in the amount of Mn and Ti effective for increasing the strength. Therefore, the S content is 0.005% or less, more preferably 0.003% or less. Note that the amount of S is preferably reduced as much as possible.

Al: 0.10% or less
Al is an important element as a deoxidizer for steel, but if the Al content in the steel exceeds 0.10%, the surface properties of the steel sheet are lowered. Therefore, the Al content is 0.10% or less, preferably 0.06% or less. In order to secure a sufficient deoxidation effect, the Al content is preferably 0.005% or more.

Ti: 0.05-0.20%
A part of Ti is an element that combines with C to form fine carbides and contributes to strength enhancement and prevention of HAZ (heat affected zone) softening during welding. In order to obtain such an effect, the Ti amount needs to be 0.05% or more. However, if it exceeds 0.20%, the stretch flangeability after processing is lowered. Therefore, the Ti content is 0.05 to 0.20%, preferably 0.07 to 0.14%.

  The balance is Fe and inevitable impurities, but for the following reasons, in addition, in mass%, Cr: 0.1 to 0.8%, Nb: 0.005 to 0.1%, V: 0.005 to 0.2%, W: 0.005 to 0.2%, Mo: It is preferable to contain one or more selected from 0.01 to 0.3%.

Cr: 0.1-0.8%
Cr is an element effective for improving hardenability. If the Cr content is less than 0.1%, the effect is small, and if it exceeds 0.8%, the stretch flangeability after processing is lowered. Therefore, the Cr content is 0.1 to 0.8%, preferably 0.3 to 0.7%.

Nb: 0.005-0.1%, V: 0.005-0.2%, W: 0.005-0.2%, Mo: 0.01-0.3%
Nb, V, W, and Mo are all elements that contribute to an increase in strength by combining with C to form fine carbides. However, if the Nb, V, and W amounts are each less than 0.005%, or the Mo amount is less than 0.01%, the amount of carbide generated is small and the strength increase is insufficient, the Nb amount exceeds 0.1%, and the V and W amounts are each 0.2%. Exceeding or if the Mo content exceeds 0.3%, the stretch flangeability after processing is lowered. Therefore, the Nb amount is 0.005 to 0.1%, the V amount is 0.005 to 0.2%, the W amount is 0.005 to 0.2%, and the Mo amount is 0.01 to 0.3%.

  In addition, Cu, Ni, Cr, Sn, Pb, and Sb may be contained in a range of 0.1% or less as trace elements that do not adversely affect the operational effects of the present invention.

2) Manufacturing conditions Heating temperature before hot rolling: 1150 ~ 1300 ℃
From the viewpoint of reducing the rolling load and ensuring good surface properties, the heating temperature needs to be 1150 ° C. or higher. In addition, when Ti carbide, or Nb, V, W, and Mo are added before hot rolling, heating at 1150 ° C. or higher is necessary to dissolve these carbides. On the other hand, when the heating temperature exceeds 1300 ° C., the austenite grains become coarse, and the stretch flangeability after processing decreases. Accordingly, the heating temperature is 1150-1300 ° C.

Hot rolling finishing temperature: 800 ~ 1000 ℃
If the finishing temperature is less than 800 ° C., surface defects increase due to an increase in rolling load. Further, if the finishing temperature is less than 800 ° C., the two-phase rolling of ferrite and austenite may occur, and TS of 780 MPa or more may not be obtained. On the other hand, if the finishing temperature exceeds 1000 ° C., the austenite grains are insufficiently refined and the bainite structure is coarsened, so that the stretch flangeability after processing deteriorates. Therefore, the finishing temperature is 800 to 1000 ° C, preferably 820 to 950 ° C.

Cooling conditions after hot rolling: Cooling at a cooling rate of 55 ° C / second or higher, and then cooling at a temperature range of at least 500 ° C or lower at a cooling rate of 120 ° C / second or higher with nucleate boiling cooling Cooling after hot rolling If the speed is less than 55 ° C./second, ferrite is produced, which causes a decrease in elongation furan after processing. For this reason, the cooling rate needs to be 55 ° C / second or more. In addition, cooling at 70 ° C./second or more is more preferable. Here, when water cooling is used for cooling the steel sheet, the conventional cooling mainly based on film boiling is transition boiling cooling in which film boiling cooling and nucleate boiling cooling coexist in a temperature range of 500 ° C. or less. The occurrence of uneven temperature was inevitable. However, if nucleate boiling cooling is performed at a temperature range of 500 ° C or lower at a cooling rate of 120 ° C / second or higher, preferably 250 ° C / second or higher, temperature unevenness can be reliably eliminated, and material fluctuations in the steel sheet can be stably reduced. Can do. Here, in the case of nucleate boiling cooling, the average cooling rate in the cooling after the hot rolling until the start of nucleate boiling cooling is set to 55 ° C./second or more, and the cooling is performed under the conditions for subsequent nucleate boiling cooling. The cooling rate in the temperature range of 500 ° C. or lower (120 ° C./second or higher) is an average cooling rate up to a coiling temperature of 500 ° C. or lower. In the present invention, nucleate boiling cooling may be performed at least in a temperature range of 500 ° C. or lower, and cooling may be started under conditions of nucleate boiling cooling from a temperature of 500 ° C. or higher after hot rolling.

Patent Document 5 discloses a technique for reducing temperature unevenness in a steel sheet surface caused by transition boiling cooling by performing a transition boiling cooling temperature range in which film boiling cooling and nucleate boiling cooling coexist with nucleate boiling cooling. . However, according to the study by the present inventors, it is not always possible to eliminate the temperature unevenness in the steel sheet surface by simply replacing the transition boiling cooling with the nucleate boiling cooling. It was found that it was necessary to increase the cooling rate of 120 ° C./second or more. The cooling rate during nucleate cooling is preferably 200 ° C./second or more, and more preferably 250 ° C./second or more. The reason for this is not necessarily clear, but it is considered that when the cooling rate is low, a portion where the water film on the steel sheet surface is not sufficiently broken is generated. In order to reliably perform nucleate boiling cooling at a cooling rate of 120 ° C./sec or more, the water density is preferably 2000 L / min.m ² or more. In order to carry out nucleate boiling cooling, a conventional method, that is, laminar or jet cooling excellent in linearity with respect to the upper surface of the steel sheet is preferable. As the shape of the nozzle, there are generally a circular tube and a slit nozzle, but there is no problem even if either is adopted. The laminar or jet cooling flow rate is preferably 4 m / second or more. This is because it is necessary to obtain a momentum for stably breaking through the liquid film formed on the steel plate during cooling by laminar or jet cooling. In addition, since cooling water falls with respect to the steel plate lower surface by gravity, since a liquid film cannot be formed on the steel plate surface, there is no problem even if spray cooling is used. Of course, the same laminar and jet cooling as in the case of the upper surface of the steel plate can be employed.

Winding temperature: 350-500 ° C
In order to transform the retained austenite maintained until after cooling into bainite, it is necessary to wind it in a coil shape at a coiling temperature of 350 to 500 ° C, preferably 400 to 500 ° C. This is because martensite harder than bainite is generated when the coiling temperature is less than 300 ° C., and pearlite is generated when the coiling temperature exceeds 500 ° C., and stretch flangeability after processing is deteriorated. When the coiling temperature is less than 500 ° C, transition boiling cooling occurs at 500 ° C or less. Therefore, as described above, the temperature range from 500 ° C to the coiling temperature is nucleate boiling cooled at a cooling rate of 120 ° C / second or more. There is a need. When the coiling temperature is 500 ° C, water cooling below 500 ° C is unnecessary, so there is no need to consider nucleate boiling cooling. If the average cooling rate to the coiling temperature is 55 ° C / second or more, Good.

  The structure of the high-strength hot-rolled steel sheet obtained in this way is a bainite single-phase structure, bainite exceeding 95% in volume ratio, or other phases that occur inevitably less than 5% (martensite and retained austenite). It consists of and.

  Other manufacturing conditions can be performed under normal 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, hot dip galvanizing, electroplating, and chemical conversion treatment can be performed.

Steel pieces having the chemical composition shown in Table 1 were heated to 1250 ° C. and hot-rolled at a finishing temperature shown in Table 2 to form a hot-rolled sheet having a thickness of 3.2 mm, and then water-cooled under the cooling conditions shown in Table 2. The sample was cooled using a coil and wound into a coil at the winding temperature shown in Table 2. Here, the coiling temperature is a value obtained by measuring the coiling temperature at the center in the width direction of the steel strip in the longitudinal direction of the steel strip and averaging them. At this time, a radiation thermometer [NEC Sanei Model TH7800], which can measure the steel sheet surface temperature in two dimensions, was installed just before the winding device, and the temperature unevenness of the steel sheet surface was evaluated as follows. did.
Temperature non-uniformity: Obtain the area of the low-temperature part where the coiling temperature is locally measured by a radiation thermometer, and the ratio of the total area of the steel sheet S [= (area of the low-temperature part) / (steel sheet Total area) × 100 (%)], and if S <5%, there was no temperature unevenness.

  In Table 2, No. 1 had a coiling temperature of 500 ° C., and no cooling was performed under conditions for nucleate boiling cooling.

Next, after pickling the hot-rolled sheet after winding, the bainite volume ratio at the center in the width direction of the steel sheet at a position 30 m in the longitudinal direction from the coil tip, and the width direction at a position 30 m from the coil tip JIS No. 5 tensile test specimens (perpendicular to the rolling direction) and hole expansion test specimens were collected from the center, width direction 1/4 and 3/4, and TS and hole expansion ratio λ after processing were obtained. Measurement was performed as follows.
Bainite volume fraction: A specimen for a scanning electron microscope (SEM) was collected, the thickness cross section parallel to the rolling direction was polished, then it was corroded by Nital, and SEM photographs were taken at 1000 magnifications with 10 fields of view, and bainite was imaged. The area of bainite and the area of the observation visual field are measured by image processing, and the area ratio of bainite [= (area of bainite) / (area of the observation visual field) × 100 (%)] is obtained. The volume ratio was used.
TS: A tensile test was performed on the three tensile test pieces at a strain rate of 10 mm / min to obtain the tensile strength TS, and the average value of the three was set as TS.
λ: After performing cold rolling with a reduction rate of 10% on the three test specimens for hole expansion test, a 130 mm square plate was cut out, a 10 mmφ hole was punched in the center of the plate, and then a 60 ° conical punch was Pushed up from the opposite side of the burr, the hole diameter dmm was measured when the crack penetrated the plate thickness, calculated from the following equation, and λ was evaluated by the average value of the three.
λ (%) = [(d-10) / 10] × 100
Furthermore, in order to investigate the material fluctuations in the steel sheet, the width of the steel sheet, with the direction parallel to the rolling direction being the longitudinal direction of the test piece at each position 100, 200, 400, 600, 700 m in the longitudinal direction from the coil tip. In the direction, 25 test pieces from the inner side of 25 mm in the width direction are collected at regular intervals, and a total of 125 JIS No. 5 tensile test pieces (the direction parallel to the rolling direction is the tensile direction) are collected. TS was calculated | required by the same method and the standard deviation (sigma) was computed.

  The results are shown in Table 3. In the example of the present invention, TS is 780 MPa or more, the average λ after processing is 80% or more and excellent in stretch flangeability after processing, and there is almost no temperature unevenness in the coil, so the standard deviation σ of TS is It can be seen that the material fluctuation in the steel sheet is small, as small as 15 MPa or less.

Claims (4)

  In mass%, C: 0.04 to 0.15%, Si: 0.05 to 1.5%, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.10% or less, Ti: 0.05 to 0.20% The steel slab having a component composition including Fe and inevitable impurities is heated at a heating temperature of 1150 to 1300 ° C, hot-rolled at a finishing temperature of 800 to 1000 ° C, and then cooled to 55 ° C / second or more. 780MPa, characterized in that it is cooled at a speed, and subsequently cooled at a temperature range of at least 500 ° C under the condition of nucleate boiling cooling at a cooling rate of 120 ° C / second or more, and then wound at a coiling temperature of 350 to 500 ° C A method for producing a high-strength hot-rolled steel sheet having the above tensile strength.   2. The method for producing a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more according to claim 1, wherein the steel piece has a Si content of 0.3 to 1.5% by mass.   The high-strength hot rolling having a tensile strength of 780 MPa or more according to claim 1 or 2, wherein cooling is performed at a temperature range of at least 500 ° C or less under conditions of nucleate boiling cooling at a cooling rate of 250 ° C / second or more. A method of manufacturing a steel sheet.   The steel slab is further selected by mass% from Cr: 0.1-0.8%, Nb: 0.005-0.1%, V: 0.005-0.2%, W: 0.005-0.2%, Mo: 0.01-0.3%. 4. The method for producing a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more according to any one of claims 1 to 3, which has a component composition containing one or more kinds.