JP2007146222A - Method for producing steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high strength steel sheet having fine ferrite structure without a work under large pressure. <P>SOLUTION: The method for producing the steel sheet is performed as the followings: when producing the steel sheet by hot-rolling the steel heated at Ae<SB>3</SB>transformation point, or higher, in the finish pass of this hot-rolling, the elastic-working is applied while cooling this steel so that the working steel temperature becomes (Ae<SB>3</SB>transformation point - 60°C) or higher and the working finish temperature becomes (Ae<SB>3</SB>transformation point - 200°C) or lower. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a high-strength steel sheet.

  In recent years, there has been a demand for increasing the strength of steel materials in response to needs such as weight reduction of automobiles and high-rise buildings. Generally, increasing the strength of a steel material reduces toughness, but in the case of strengthening by grain refinement, it is generally possible to improve strength, particularly the yield point, without reducing ductility, and toughness is also improved. . Therefore, various crystal grain refining techniques have been proposed.

As a conventional means for refining crystal grains, large strain processing has been studied a lot. When steel heated to Ae ₃ points or more and turned into an austenite phase is rapidly cooled, normally, even when the temperature is ₃ points Ae or less, transformation does not occur immediately, and an austenite phase in a supercooled state is obtained. If the temperature is maintained or further cooling is continued, transformation occurs, but depending on the composition and cooling conditions, transformation occurs to a ferrite phase, a martensite phase, or a bainite phase. When processing is performed in a supercooled state immediately before the transformation, the structure rapidly changes to a structure mainly composed of ferrite. This is presumably because transformation is induced and promoted by processing. At that time, it is known that by changing the processing temperature and the processing rate, a ferrite phase in which the strain is released and a structure having a small crystal grain size can be obtained.

For example, a method for producing a high-strength hot-rolled steel sheet having a fine ferrite structure by rolling at a total rolling reduction of 80% or more in the vicinity of the Ar ₃ transformation point, or supercooled austenite at 650 ° C. or lower is shown. In the region, there is shown a method of manufacturing a high-strength steel by performing processing in which the cross-sectional area reduction rate at the end of processing relative to the start of processing is 60% or more (for example, Patent Document 1, Patent Document 2, Patent Document) 3).
JP 58-123823 A JP 11-334881 A JP 2001-98322 A

In the large reduction processing (rolling) in the vicinity of the Ar ₃ transformation point and in the low temperature range of 650 ° C. or less described in Patent Documents 1 to 3 and the like, a rolling mill that can withstand a large rolling load, including the drive system, is necessary. It is difficult to carry out with rolling equipment with specific specifications.

  An object of the present invention is to solve the above-described problems of the prior art and to provide a method for producing a high-strength steel sheet having a fine ferrite structure that does not depend on a large rolling process.

  In order to solve the above-mentioned problems, the present inventors have intensively studied a method for suppressing grain growth and grain growth by means of dynamic transformation during processing as means for further promoting the refinement of steel. .

At that time, the processing is not performed after cooling to the vicinity of the Ar ₃ transformation point or to a low temperature range of 650 ° C. or lower, but by cooling from the temperature of (Ae ₃ transformation point−60 ° C.) or higher, by conventional cooling. It has been found that a ferrite structure having a very small crystal grain size can be produced with a rolling load smaller than that of the technology.

This is a temperature range in which austenite recrystallization occurs when processing starts at a temperature higher than the Ar ₃ point (hereinafter simply referred to as “recrystallization region”) and / or austenite recrystallization does not occur. (Hereinafter, simply referred to as “non-recrystallized region”) In addition to the effect of processing in the process, the transformation progresses during processing due to a rapid temperature change (temperature decrease) during processing, and this transformed ferrite is being processed. Or it is thought to be caused by recrystallization after processing.

In addition, since rolling starts from a state in which the steel plate is at a high temperature, even if the temperature is lowered at the end of rolling, the load is higher than when the rolling starts after the steel plate is cooled to the vicinity of the Ar ₃ transformation point or to a temperature range of 650 ° C. or lower. Can be reduced.

  In addition, as a method of cooling at the same time as performing processing, in addition to means such as water cooling, a processing method such as rolling or forging can utilize a heat transfer phenomenon in contact with a tool.

  The present invention has been made based on these findings and has the following characteristics.

[1] When a steel sheet is manufactured by hot rolling steel heated to the Ae ₃ transformation point or higher, in the final pass of the hot rolling, the processing start temperature is (Ae ₃ transformation point −60 ° C.) or more and the processing is completed. A method for producing a steel sheet, comprising subjecting the steel to plastic working while cooling so that the temperature is not higher than (Ae ₃ transformation point -200 ° C).

  [2] The method for producing a steel sheet according to [1], wherein a time required for plastic working in the final pass of the hot rolling is set to be within 2 seconds.

  [3] The method for manufacturing a steel sheet according to [1] or [2], wherein a reduction rate of the cross-sectional area in the final pass of the hot rolling is 40% or more.

  According to the present invention, it is possible to manufacture a high-strength steel sheet having a fine ferrite structure.

In the present invention, when a steel producing steel by hot rolling, the steel according to the material is heated above Ae ₃ transformation point, it is necessary to the austenite single phase. This is because the present invention uses the transformation from austenite to ferrite to refine the crystal grains and increase the strength of the steel.

The manufacturing method of the steel sheet of the present invention, in the final pass of hot rolling, machining start temperature (Ae ₃ transformation point -60 ° C.) or higher and working end temperature is equal to or less than (Ae ₃ transformation point -200 ° C.) Thus, plastic working (hereinafter referred to as plastic working of the present invention) is performed while cooling the steel. Here, the final pass of hot rolling refers to the final pass when multiple hot workings are applied to the steel material, and the single pass when only one hot working is applied. It shall refer to processing. The steel before the start of the plastic working of the present invention may be one in which the raw steel is heated from room temperature to a temperature equal to or higher than the Ae ₃ transformation point in a heating furnace, or the raw steel is higher than the Ae ₃ transformation point. After heating at this temperature, it may be processed into a required shape by rough forging, rough rolling or the like. In addition, since the steel with a fine ferrite structure can be obtained more effectively if the austenite grain size before plastic working of the present invention is fine, the austenite grain size before plastic working of the present invention is more preferably 50 μm or less.

The plastic working of the present invention is performed at a temperature of (Ae ₃ transformation point −60 ° C.) or higher if there is a ferrite structure before the plastic working, the transformation from austenite to ferrite is used to refine the crystal grains. This is because it deviates from the spirit of the present invention. Further, since the cooling is performed while processing to obtain fine grains and the processing start temperature is too low, a large rolling load is required. Therefore, the processing is started at (Ae ₃ transformation point −60 ° C.) or more.

The temperature at the end of the plastic working of the present invention is set to not more than (Ae ₃ transformation point-200 ° C.) by cooling the steel during the working. Because, when the processing is finished at a temperature higher than (Ae ₃ transformation point −200 ° C.), the degree of non-recrystallized region processing is reduced, so that processing strain due to rolling cannot be effectively introduced and accumulated, and a sufficiently fine structure This is because there is a problem that the fraction of ferrite generated at the end of processing is reduced, and a sufficiently fine structure cannot be obtained due to transformation and recrystallization during processing deformation. Moreover, when processing is performed to a temperature lower than (Ae ₃ transformation point−350 ° C.), a homogeneous microstructure cannot be obtained, and processing strain remains. Therefore, it is preferable to finish the processing in a temperature range of (Ae ₃ transformation point−200 ° C.) or less and (Ae ₃ transformation point−350 ° C.) or more.

  Here, as a steel cooling means performed simultaneously with the plastic working of the present invention, a heat transfer phenomenon in contact with a tool can be used in a processing method such as rolling or forging. For example, in the case of rolling, the average temperature change of the rolled material during rolling is generally determined by the sum of the amount of temperature change due to heat generated by processing, heat generated by friction, and heat extracted from the roll. Plastic working can be performed while cooling.

In order to control the cooling temperature at the end of processing by heat removal from the roll, the rolling speed may be controlled. For example, as shown in FIG. 1, when rolling at a processing start temperature of 830 ° C., a roll diameter of 200 mm, a roll temperature of 50 ° C., and a sheet thickness of 5 mm to 2.5 mm at a reduction rate of 50%, the Ae ₃ transformation point is 850 ° C. If steel is rolled at a rolling speed of 6 mpm or less, the temperature at the end of rolling will be 650 ° C. or lower (Ae ₃ transformation point−200 ° C.) or lower. The processing time at this time is 0.16 seconds.

For example, when manufacturing a thick steel plate using steel having an Ae ₃ transformation point of 850 ° C., the processing start temperature in the final rolling pass is 830 ° C., the roll diameter is 1200 mm, the roll temperature is 50 ° C., and the plate thickness is from 12 mm to 6 mm. When rolling at a rolling reduction of 5%, from the relationship shown in FIG. 2, if the rolling speed is 5 mpm, the temperature at the end of processing becomes 650 ° C. or lower (Ae ₃ transformation point−200 ° C.) and has a fine ferrite structure. A high strength thick steel plate can be manufactured. The processing time at this time is 0.72 seconds.

  In addition, when cooling during processing is performed by contact heat transfer with the tool, it is desirable that the temperature variation of the tool is small. For example, when roll heat removal is used for cooling during processing, fluctuations in roll temperature cause fluctuations in the temperature of the steel sheet after rolling, and therefore fluctuations in roll temperature are preferably as small as possible. Therefore, in order to cool the roll of the rolling mill and keep it at a constant temperature, it is desirable to provide the rolling mill with a roll cooling device. The same applies to a plate thickness reduction press device such as forging. In the case of simple compression, water cooling from the surroundings may be considered during processing, but even in that case, the temperature fluctuation of cooling water is preferably as small as possible.

  The time required for the plastic working of the present invention performed while cooling the steel is preferably within 2 seconds. This is because if the time is too long, grain growth during processing cannot be ignored. Here, the time required for plastic processing is not the time required for processing the entire target steel, but the time from the start of processing a specific position of steel to the end of processing, that is, the specific position of steel. It refers to the time in contact with a tool such as a rolling roll.

  Furthermore, the plastic working of the present invention may be accompanied by plastic deformation, and a fine structure corresponding to the strain distribution can be obtained. However, in order to obtain a structure having no distribution in the plate thickness direction, the reduction rate of the cross-sectional area is preferably 40% or more. If the amount of deformation is less than 40%, the deformation is insufficient and the fine grain structure is not uniform in the thickness direction, and the release of processing strain due to transformation tends to be insufficient. Here, the reduction rate of the cross-sectional area is substantially the same as the reduction rate in the case of plate rolling.

Next, an example of the manufacturing equipment of the steel plate used for implementation of this invention is demonstrated.
The steel sheet manufacturing facility used in the present invention includes a rolling mill for reducing the thickness of a hot slab directly cast or reheated in a heating furnace to a predetermined thickness after being cast by a continuous casting apparatus. As the rolling mill, one or more rough rolling mills and finishing mills may be provided, or one or more of them may be provided. In addition, it may be provided with a plate thickness reduction press device that can apply a reduction in the plate thickness direction to the hot slab after rolling downstream of the rough rolling mill and the finish rolling mill or instead of the rolling mill. . Then, in the final pass of processing by such a manufacturing facility, the plastic processing of the present invention described above is performed.

  Here, the plate thickness reduction press device is a forging die reduction device that uses a die sandwiched between hot slabs, and repeatedly performs a pressing operation to open and close the die while sequentially feeding the hot slab. It is an apparatus that processes the entire length of the slab to a desired thickness. According to such a plate thickness reduction press apparatus, unlike the conventional rough rolling using a rolling roll, there is no biting limit, so that the reduction rate per one time is not restricted and arbitrary large reduction is possible. A plurality of plate thickness reduction press devices capable of such large reduction may be provided, or only the plate thickness reduction press device may be used without using a rough rolling mill or a finish rolling mill.

  Moreover, the effect of this invention can be exhibited more by controlling the to-be-rolled material before rolling so that the temperature of the to-be-rolled material may become uniform by heating / heat retaining means such as an induction heating device. This effect can be obtained regardless of the presence or absence of heating and heat retention and the method thereof. Therefore, in particular, a heat insulating cover for preventing a temperature drop at the tail end of the material to be rolled may be installed, or a coil box may be used.

  The preferable chemical components of the steel used in the method for producing a steel sheet of the present invention are as follows. In addition, in this specification, all% which shows the component of steel is the mass%.

  C is an element mainly required to ensure the strength of the hot-rolled steel sheet, and is required to be at least 0.06% in order to produce the high-strength steel targeted by the present invention. On the other hand, C exceeding 0.3% deteriorates the toughness, workability and weldability of the steel material. Therefore, it is preferable that C is in the range of 0.06 to 0.3%.

  Si is an element contributing to solid solution strengthening, and can be added in accordance with a target strength level. However, if Si exceeds 1%, the toughness, weldability and surface properties as a hot-rolled steel sheet having fine ferrite are deteriorated. Therefore, it is preferable that Si is 1% or less.

  Mn is an element that enhances hardenability, and 0.5% is necessary to ensure the strength of the steel sheet. On the other hand, if Mn exceeds 3%, not only the effect is saturated, but also a band-like structure is formed due to solidification segregation, and workability and delayed fracture resistance are deteriorated. Therefore, it is preferable to set Mn within the range of 0.5 to 3%.

  Al is used as a deoxidizer and at the same time has an effect of fixing N contained as an inevitable impurity and improving workability. However, even if Al is added in excess of 0.1%, the effect is saturated, and the cleanliness is deteriorated and the workability is deteriorated. Therefore, it is preferable that Al is 0.1% or less with sol.Al.

  Furthermore, you may contain each element of Nb, Ti, V, Cr, and Mo which contributes to obtaining an ultrafine fine grain structure stably in the range shown below.

  When Nb or Ti is contained, the microstructure can be made sufficiently stable even when processing is performed at a temperature slightly higher than the temperature at which the low temperature phase starts to precipitate. This is presumably because the growth of crystal grains after transformation is suppressed by the precipitation of fine carbonitrides. In order to obtain this effect sufficiently, it is desirable to contain 0.005% or more of Nb and 0.005% or more of Ti. However, if these elements are excessive, the toughness is lowered, so Nb should be 0.05% or less and Ti should be 0.05% or less. That is, when contained, Nb is preferably in the range of 0.005 to 0.05% and Ti is preferably in the range of 0.005 to 0.05%.

  By containing V, Cr and Mo, a fine grain structure can be obtained stably. These elements form carbides, and the precipitates act to suppress the growth of crystal grains as in the case of Nb or Ti, but the effect is not great. Rather, these elements have a strong effect of delaying transformation, lowering the precipitation of the low-temperature phase at a lower temperature, delaying the precipitation time, and expanding the range of austenite at low temperatures in the supercooled state. There is an effect of facilitating generation of a grain structure. In order to obtain such an effect, it is desirable to contain 0.008% or more for V, 0.05% or more for Cr, and 0.05% or more for Mo, respectively. However, these elements tend to delay transformation, and if the content is increased more than necessary, it becomes difficult to obtain a structure mainly composed of ferrite. Therefore, it is preferable that V is 0.08% or less, and Cr and Mo are each 1% or less. That is, when V is contained, the content is preferably 0.008 to 0.08% for V, 0.05 to 1% for Cr, and 0.05 to 1% for Mo.

  Other elements may be contained within a range that does not hinder the effects of the present invention, that is, the remainder may be substantially iron. Note that inevitable impurities such as P, S, O, and N are preferably as low as possible, but may be included as long as they are within the range of a normal high-strength hot-rolled steel sheet.

  As chemical components in terms of mass%, C: 0.14%, Si: 0.02%, Mn: 0.64%, Sol. Steels A and C containing Al: 0.02%: 0.1%, Si: 0.4%, Mn: 0.15%, Sol. Steel B containing Al: 0.02% and Nb: 0.03% was melted. These steels were heated to 1300 ° C. and rolled to obtain test materials having a width of 100 mm, a length of 550 mm, and a thickness of 5 mm. These test materials were heated to the temperatures shown in Table 1, then air-cooled to a predetermined temperature, hot-rolled, and air-cooled after rolling. Table 1 shows the numbers used for the rolling, the respective rolling processing conditions, the ferrite average particle diameter, the fraction, the rolling load, and the like.

The Ae ₃ transformation points of these steels A and B are about 875 ° C. and about 880 ° C., respectively, (Ae ₃ transformation point−60 ° C.) are about 815 ° C., about 820 ° C., and (Ae ₃ transformation point−), respectively. 200 ° C.) is approximately 675 ° C. and 680 ° C., respectively. Therefore, both the pre-rolling temperature 830 ° C. for steel A and the pre-rolling temperature 1050 ° C. for steel B in the examples of the present invention shown in Table 1 are both (Ae ₃ transformation point−60 ° C.) or higher. The rolling end temperatures of 650 ° C., 600 ° C., 550 ° C., 500 ° C., and the rolling end temperature 650 ° C. of steel B are all (Ae ₃ transformation point−200 ° C.) or less. Moreover, roll temperature control was performed so that the roll temperature fluctuation | variation at the time of hot rolling might be less than 100 degreeC.

  In the example according to the production method of the present invention (example of the present invention), a steel sheet having a microstructure with an average ferrite grain size of 2 μm or less is obtained, which shows high yield strength.

In Comparative Example 1, since the temperature after rolling was 700 ° C. higher than (Ae ₃ transformation point−200 ° C.), transformation / recrystallization at the time of work deformation was insufficient and an ultrafine structure was not obtained.

In Comparative Example 2, hot rolling was performed after cooling to 600 ° C. lower than (Ae ₃ transformation point−60 ° C.). As a result, a rolling load about 2.3 times that of the inventive example was required.

  In Comparative Example 3, since the cooling during the processing was hardly performed, a fine structure was not obtained.

  Moreover, it replaced with the said rolling mill and implemented the same test using the plate thickness reduction press apparatus. At that time, temperature control was performed so that the temperature variation of the press mold was within 100 ° C.

  When steel A was heated to 900 ° C and processed at a press speed of 50 cpm (cycles / min), cooling by the press mold was effective, and the temperature reached 830 ° C before pressing and 600 ° C after pressing, and the ferrite average grain size A steel sheet having a microstructure with a diameter of 1.2 μm was obtained and exhibited high yield strength.

  On the other hand, when the pressing speed was 100 cpm, the temperature was 830 ° C. before pressing and 800 ° C. after pressing, and the average ferrite particle size was 10 μm. That is, since the cooling during processing was hardly performed, a fine structure was not obtained.

  In the above, only one-pass processing (rolling and pressing) is used as an example. However, the processing (rolling and pressing) only needs to be cooled during processing of only the final pass. You may implement, and you may implement multipass processing (rolling, press). For example, the example at the time of implementing multipass rolling is shown below.

  A 250 mm thick slab of Steel A was heated to 1100 ° C. in a heating furnace, and the slab was reduced to 5 mm by applying 8 passes of rolling by a rolling mill. Then, the thickness was reduced to 2.5 mm in the final pass of rolling. At this time, the temperature before final rolling was 830 ° C., and the finishing temperature was 600 ° C. In addition, roll temperature control was performed so that the roll temperature fluctuation | variation at the time of hot rolling might be less than 100 degreeC. As a result of carrying out under the above-mentioned conditions, which are within the scope of the present invention, a steel sheet having a fine structure with an average ferrite grain size of 1.2 μm was obtained, indicating a high yield strength.

  As described above, according to the present invention, it is possible to manufacture a high-strength steel sheet having a fine ferrite structure under conditions of a lower load than conventional.

A diagram showing the relationship between the rolling temperature and the rolling speed (roll diameter 200 mm) The figure which shows the relationship between the temperature after rolling and the rolling speed (roll diameter 1200mm)

Claims (3)

When the steel heated to the Ae ₃ transformation point or higher is hot-rolled to produce a steel plate, the processing start temperature is (Ae ₃ transformation point -60 ° C) or more and the processing end temperature is (at the final pass of the hot rolling) ( Ae ₃ transformation point−200 ° C.) The steel plate is plastically processed while being cooled so that the temperature is equal to or lower than Ae ₃ transformation point−200 ° C.   The method for producing a steel sheet according to claim 1, wherein a time required for plastic working in a final pass of the hot rolling is set to 2 seconds or less.   The method for manufacturing a steel sheet according to claim 1 or 2, wherein a reduction rate of a cross-sectional area in the final pass of the hot rolling is 40% or more.

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