JP4772722B2 - Method for producing high-strength hot-rolled steel sheet with excellent stretch flangeability - Google Patents

Description

  The present invention relates to a method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability.

  High-strength steel plates are widely used for parts of transportation machinery such as passenger cars, buses, and trucks. Of these, hot-rolled steel sheets are often used for applications where the plate thickness is relatively thick and design is not necessarily regarded as important. The hot rolled steel sheet used for such applications is required to have high ductility and high stretch flangeability.

  As high-strength steel sheets pursuing improved ductility, so-called DP steels and TRIP steels are known. However, the mechanism that makes these steels highly ductile is a deterioration factor in stretch flangeability. For this reason, development for enhancing stretch flangeability without greatly impairing the ductility of high-strength steel sheets has been underway (see, for example, Patent Documents 1 and 2).

Patent Document 1 has a predetermined chemical component, and by using bainitic ferrite as the main microstructure, it is possible to obtain a hot-rolled steel sheet having a tensile strength of 500 N / mm class ² or more and excellent stretch flangeability. Technology is disclosed. Patent Document 2 discloses a hot-rolled steel sheet having a predetermined chemical component, and having granulated bainitic ferrite and / or bainitic ferrite as a main microstructure, which is remarkably excellent in stretch flangeability. Techniques for disclosing are disclosed.

  In recent years, the range of application of high-strength hot-rolled steel sheets has expanded as the properties of steel sheets have improved, and the requirements for flange formability have become stricter. For example, in burring, the shape of a punched hole (referred to as a pilot hole) before flange-up changes from a simple one as in the past to a more complicated one. As the burring process becomes complicated, it is considered that the punching clearance of the pilot hole is inappropriate as a cause of the molding defect that has become apparent.

  It is well known that the stretch flangeability (hole expandability) of a steel sheet is affected by the punching clearance of the pilot hole (see, for example, Non-Patent Document 1), and there is an optimum clearance that is smaller than that range. Even if it is large, it often deteriorates. However, Patent Documents 1 and 2 do not describe stretch flangeability when the punching clearance changes.

  In addition, when rolling is performed with a multi-stage continuous rolling mill of 6 or 7 stages, the rolling rate of the final pass is generally made smaller than the rolling rate of the previous pass (for example, non-patent) Reference 2). However, this document does not describe the relationship between the rolling rate and stretch flangeability.

JP-A-6-172924 JP 2001-20030 A Edition of Steel Sheet Forming Technology Study Group, “Press Forming Difficulty Handbook (Second Edition)”, July 1997, published by Nikkan Kogyo Shimbun, p. 473 Kanichiro Katsura, “Rolling Technology”, October 1974, published by Nikkan Kogyo Shimbun, p. 101

  Conventionally, a punching tool (mold) has been designed so that the clearance is in the optimum range as much as possible. However, when the shape of the pilot hole becomes complicated, a slight positional deviation of the tool, which is not a problem in the case of a simple shape, becomes a problem. Further, in transfer press molding in which multi-stage pressing is continuously performed, there may be a problem that a shape as designed cannot be formed in a process before the punching process. In these cases, there are not a few cases where the clearance is outside the proper range and the burring workability is lowered.

  Such problems can be solved as such by making the management of the molding process considerably strict, but the production efficiency decreases and the cost increases. For this reason, there is a demand for a steel sheet in which stretch flangeability hardly changes even when the punching clearance changes.

  This invention is made | formed in view of such a real condition, and provides the manufacturing method of the high intensity | strength hot-rolled steel plate excellent in stretch flangeability. In the present invention, “excellent in stretch flangeability” means that the variation in stretch flangeability is small relative to the variation in punching clearance.

  The present invention is a method for producing a steel sheet in which the chemical composition and production conditions are appropriately controlled and the degree of change in stretch flangeability with respect to the variation in punching clearance is small, and the gist thereof is as follows.

(1) In mass%,
C: 0.03-0.08%,
Si: 1.0% or less,
Mn: 1.2 to 2.0%,
P: 0.02% or less,
S: 0.01% or less,
Al: 0.01 to 0.05%,
N: 0.01% or less,
Nb: 0.03 to 0.5%,
The balance is hot rolled a steel slab comprising Fe and inevitable impurities, and the final rolling pass of the hot rolling is performed at a temperature of 850 to 950 ° C., a rolling rate of 15 to 25%, a rolling torque [kN M], rolling load [kN], and rolling roll radius [m] are performed under conditions satisfying the following (formula 1), and then slowly cooled to 700 to 800 ° C. at a cooling rate of less than 20 ° C./second. A method for producing a high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized by that.
0.20 ≦ rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) ≦ 0.35 (Formula 1)

  (2) The method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability as described in (1) above, which contains, by mass%, Ti: 0.05 to 0.2%.

  (3) The method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability according to the above (1) or (2), wherein B: 0.0001 to 0.0030% in mass% .

  (4) The method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability according to any one of (1) to (3) above, wherein the thickness of the hot-rolled steel sheet is 2 to 7 mm. .

  (5) After slow cooling as described in (1) above, rapidly cooling at 20 to 70 ° C./s and winding up at 600 ° C. or less, described in any one of (1) to (4) above Of high strength hot-rolled steel sheet with excellent stretch flangeability.

  In the hot-rolled steel sheet of the present invention, even if the clearance at the time of punching changes slightly, the influence on the subsequent stretch flange formability is small. The industrial contribution is remarkable, such as reduction in production and improvement in manufacturing yield.

  When punching clearance is changed, the hole expansibility (hole expanding limit value; λ) changes as a result of changes in the shape of the punched section (such as sagging and burr size changes), changes in the roughness of the punched surface, and Changes in the degree of work hardening around the hole are known. Of these, the inventors focused on the importance of changes in work hardening around the hole.

  The present inventors conducted a hole expansion test using a vaneitic ferritic steel plate that is generally known to be excellent in stretch flangeability, and confirmed the punching clearance of the pilot hole before the hole expansion and the processing effect around the hole. The relationship between the change and the hole-expandability was investigated in detail. That is, the hardness around the punched hole is measured two-dimensionally for the cross-section including the center line of the hole, and an isohardness diagram is created. The punching clearance and the two-dimensional hardness (work hardening) The relationship between morphology (distribution) and λ was examined.

  As a result, it was found that the ultimate hardness of the fracture surface decreases with increasing clearance, but is not directly related to λ, and the change in λ is rather influenced by the hardness form of the shear plane. . As for the cause, in the hole expansion deformation with the burr as the die side (outer peripheral side), the vicinity of the burr is the most strongly deformed, and the generation and progression of cracks is the most advanced part. It was presumed that the difference in the degree of hardening in the vicinity of the sag (on the shear surface side) had a strong influence on λ without affecting the λ, and the degree of hardening was lower than that near the burr.

  Furthermore, more hot-rolled steel sheets were investigated, and it was investigated whether there were any two-dimensional work-hardening forms on the shear surface side that were not easily affected by the punching clearance. As a result, the chemical component contains Nb, and the hot rolling condition is when the rolling reaction force (load) is relatively high in the final rolling pass and the cooling rate after the final pass is compared. It was concluded that such a tendency was prominent when it was late.

  The present invention further advances such studies and clarifies desirable chemical components and hot rolling cooling conditions.

  The reasons for limitation will be described below.

  First, the reasons for limiting chemical components will be described.

  C is an essential element for securing the strength of the steel sheet, and 0.03% or more is necessary to obtain a vanite / ferrite phase or a bainitic structure. On the other hand, if it is contained excessively, the ductility is remarkably lowered and there is a concern about deterioration of weldability, so 0.08% is made the upper limit.

  Si is useful as a solid solution strengthening element and can be added as necessary. However, if it exceeds 1.0%, the surface properties may deteriorate, so the upper limit is set to the same value.

  Since Mn has a function of relaxing conditions for obtaining a vanetic ferrite phase or a bainite structure, 1.2% or more is added. However, excessive addition adversely affects weldability, so the upper limit is made 2.0%.

  P is useful for securing the strength of the steel sheet, but reduces toughness. Therefore, the upper limit is 0.02%.

  S forms inclusions such as MnS and deteriorates stretch flangeability, so it is necessary to make it 0.01% or less.

  Al is useful as a deoxidizing element, and it is necessary to add 0.01% or more. However, if it exceeds 0.05%, there is a concern that the cleanliness of the steel sheet is lowered and the stretch flangeability is lowered, so the same value is made the upper limit.

  N is an element that should be suppressed to ensure non-aging properties, but is acceptable up to 0.01%.

  Nb is the most important element in the present invention. First, it has the function of expanding the non-recrystallized region of the austenite phase and expanding the appropriate range of hot rolling conditions described later. It also has the function of forming nitrides with N and ensuring non-aging properties easily. Such a function is manifested at 0.03% or more. On the other hand, if it exceeds 0.5%, an effect commensurate with the cost cannot be obtained, so the same value is made the upper limit.

  Ti has the effect of suppressing the formation of MnS by forming TiS, and also has the function of refining crystal grains, so it can be added as necessary. The effect appears at 0.05% or more. On the other hand, if it exceeds 0.2%, an effect commensurate with the cost cannot be obtained, so the same value is made the upper limit.

  B has a function of stably generating a vanetic ferrite phase or a bainite structure, and can be added as necessary. The effect appears at 0.0001% or more, but if it exceeds 0.0030%, the ductility is remarkably deteriorated, so 0.0030% is made the upper limit.

  The balance other than the above is Fe and inevitable impurities.

Next, hot rolling conditions will be described.
The cast ingot may be directly rolled, or may be reheated and rolled. The conditions for rough rolling are not particularly limited.

  When rolling is performed with a multi-stage continuous rolling mill having 6 or 7 stages, the rolling rate of the final pass is generally made smaller than the rolling rate of the previous pass. This is because the effect of the roll gap (variation) on the product sheet thickness (accuracy) gradually increases toward the later stage in addition to the fact that the volume of the rolled material passing through each rolling mill is constant. For example, it is advantageous to use a light rolling rate in order to perform rolling while ensuring the necessary sheet thickness accuracy against fluctuations in the temperature of the rolled material, that is, fluctuations in deformation resistance. Light rolling is also selected for the purpose of reducing the possibility of damage to surface quality such as wrinkles and improving shape stability.

  The technology of the present invention generally focuses on the rolling rate in the final pass operated at a light rolling rate, and examines the rolling torque, rolling load [kN], roll radius [m], and clarifies the optimum range. This makes it clear that a steel sheet having a small variation in λ with respect to the variation in punching clearance can be obtained.

  Specifically, at the time of finish rolling, the final rolling pass is set to 0.20 ≦ rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) ≦ 0.35, rolling rate of 15 to 25. %, And completed at a temperature of 850-950 ° C., followed by cooling to 700-800 ° C. at a cooling rate of less than 20 ° C./second. When the above conditions are satisfied, a steel sheet having a small variation in λ with respect to the variation in punching clearance can be obtained. These conditions are determined based on the experimental results described in the examples.

  Details will be described in Examples, but as shown in FIG. 1, in order to appropriately introduce strain due to shear deformation, rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) Is required to be 0.2 or more. On the other hand, even if the rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) exceeds 0.35, the same effect as in the case of 0.2 to 0.35 can be obtained. However, it is limited to 0.35 or less in order to reduce the wear of the roll and not to deteriorate the surface quality of the rolled material.

  Although the mechanism is not clear at present, rolling torque [kN · m] / (rolling load [kN] × roll radius [m] in a temperature range of 850 to 950 ° C. corresponding to the non-recrystallized region of the austenite phase. ) Is 0.2 to 0.35 and rolling at a rolling rate of 15 to 25%, strain due to shear deformation is introduced into the surface layer of the steel sheet and the region immediately below the surface layer. Furthermore, when the film is slowly cooled after the final pass (less than 20 ° C./second), the plane orientation of the region where the strain due to the shear deformation is introduced forms some unique integrated form, which makes it more processed than in other conditions. It is presumed that the punching clearance dependence of the cured form is insensitive.

  If the completion temperature of the final rolling pass exceeds 950 ° C., the crystal grains become too coarse and excellent stretch flangeability cannot be obtained. When the temperature is less than 850 ° C. and when the cooling rate after the pass is 20 ° C./second or more, the processed structure remains, so that excellent stretch flangeability cannot be obtained. These are also the reasons for limiting the hot rolling conditions.

In addition, after slow cooling to 700-800 degreeC, you may cool at the same cooling rate, but considering productivity, it is desirable to cool at 20-70 degreeC / second and to wind up at 600 degrees C or less.
The rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) is set to 0.2 to 0.35 in the steady portion of the rolled material (the portion excluding the start and end of biting), and It is preferable to control the injection amount (or / and the rolling oil concentration) of the lubricant. The lubricant is preferably water mainly having a temperature of 20 to 70 ° C., and hot rolling oil added thereto. Hot rolling oil can be selected from mineral oil, polymer, grease, etc. mainly for roll wear and rough skin suppression, but it may be the same as that used for finish rolling rolls other than the final rolling pass. . For example, a method of supplying 0.1 to 1 ml / m ² of mineral oil having a viscosity (40 ° C.) of 100 to 150 mm ² / sec per unit area of the rolled material can be employed.

  For the lubricant injection, the water injection method is desirable from the uniformity of the injection amount and the control response.

  It is desirable to use a high-speed roll or a nickel glen roll as the work roll, and manage the surface roughness in the range of 0.05 to 5 μm in terms of centerline average roughness (Ra).

  The plate thickness after finish rolling is 2 to 7 mm. If the plate thickness exceeds 7 mm, the microstructure becomes extremely non-uniform in the plate thickness direction, and favorable characteristics may not be obtained. On the other hand, if it is less than 2 mm, it may not be easy to perform slow cooling. Therefore, it is limited in this way.

  Finally, the microstructure of the steel sheet is described.

  In order to increase the absolute value of λ, it is desirable to use a vanetic ferrite phase, a polygonal ferrite phase, or a bainitic structure. These may be combined. A pearlite structure or cementite may be contained.

  In mass%, C: 0.04%, Si: 1.0%, Mn: 1.45%, P: 0.01%, S: 0.001%, Al: 0.03%, N0.0025 %, Nb: 0.24%, and the balance of the steel ingot consisting of Fe and inevitable impurities was melt cast to obtain a steel plate having a plate width of 1260 mm and a plate thickness of 3.2 mm by hot rolling. The reheating temperature was 1250 ° C., roughly rolled to 30 mm, and then subjected to 6-pass finish rolling. The conditions for the final pass (6th pass) of the finish rolling are as follows: roll radius 0.31 [m], rolling rate 20%, completion temperature 910 ° C., and cooling to 720 ° C. at an average cooling rate of 14 ° C./second after rolling. Furthermore, it cooled to 550 degreeC with the average cooling rate of 45 degreeC / sec, and wound up.

  In the final pass, a lubricant in which hot water of 70 ° C. and a commercially available hot-rolled lubricating oil (Kyodo HS-40B manufactured by Kyodo Yushi Co., Ltd.) are mixed at various ratios is used for the upper and lower work rolls by the water injection method. Rolling was performed while controlling the rolling load by spraying uniformly from the biting side to the region completely including the plate path (width through which the material to be rolled passes).

  The rolling torque [kN · m] and the rolling load [kN] were measured, and the rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) was obtained. A continuous region having a fluctuation range of rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) within ± 0.25% was sampled and used as a test material having a median condition. Table 1 shows the rolling load [kN] and the rolling torque [kN · m] for each condition.

  The obtained steel sheet was pickled and then subjected to a hole expansion test. The test was conducted in accordance with Japan Iron and Steel Federation standard JFS T 1001. The initial hole diameter was fixed at 10.0 mm punch diameter, and the clearance was adjusted by using dies with different hole diameters. The clearance (%) is defined by (die inner diameter (mm) -10.0) / (2 × plate thickness (mm)) × 100, 7.0%, 12.5%, 18.8%, and 20 .3% was run. After punching, the hole was expanded, and when the crack penetrated the plate thickness, the diameter expansion was completed, and the hole expansion limit value λ was obtained from the change in the inner diameter.

  FIG. 1 shows the relationship between the punching clearance (%) and the hole expansion limit value λ (%) for each condition. From FIG. 1, the change in the punching clearance by setting the rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) to 0.2 (condition 3) or more in the conditions 1 to 5. It is clear that the variation of λ with respect to becomes small. Table 1 shows conditions 1-5.

  In mass%, C: 0.035%, Si: 0.75%, Mn: 1.5%, P: 0.005%, S: 0.0004%, Al: 0.03%, N: 0 A steel ingot containing .0015%, Nb: 0.28% and the balance being Fe and inevitable impurities was hot rolled into a steel plate having a plate width of 1000 mm and a plate thickness of 3.0 mm. The reheating temperature was roughly rolled to 1250 ° C. and 30 mm, and subjected to 6-pass finish rolling. The final rolling pass schedule was adjusted to change the rolling rate of the final pass in the range of 7.5-25%. By controlling the rolling load as shown in Table 2, the rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) in the steady portion was set to 0.27. Completion temperature was 880 degreeC, it cooled to 700 degreeC with the average cooling rate of 12 degreeC / second after rolling, and also cooled to 500 degreeC with the average cooling rate of 50 degreeC / second, and wound up. The roll radius is 0.31 [m].

  In the same manner as in Example 1, the punching clearance was changed to 7.5%, 12.5%, 20.0%, and 21.7%, and the variation state of λ was examined. Here, a value obtained by dividing the difference between the maximum value and the minimum value of λ by the average value of λ is defined as a coefficient of variation, and the relationship between the coefficient of variation and the rolling rate of the final pass is arranged, the result shown in FIG. From FIG. 2, it is clear that when the rolling ratio of the final pass is 15% or more, the coefficient of variation is greatly reduced, that is, the variation of λ with respect to the change of the punching clearance becomes small.

  Steel ingots having the chemical components shown in Table 3 were melt-cast, and steel sheets having a plate width of 820 mm and a plate thickness of 2.6 mm were obtained under the conditions shown in Table 4. Rolling in the final pass is performed at a rolling rate of 25% using a roll having a radius of 0.31 [m], and the rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) is 0.35. The rolling load was controlled so that

  The λ of the obtained steel sheet was measured while the punching clearance was changed to 8.7%, 12.5%, 19.2%, and 25.0%, and the coefficient of variation was examined. Table 5 shows the results. The table also shows the final pass rolling torque and rolling load, and the tensile strength (σB) and elongation (δ) obtained from JIS No. 5 test piece (the tensile direction was taken perpendicular to the rolling direction). It was.

  As is apparent from Table 5, if the steel sheet satisfies the chemical components of the present invention and the hot rolling conditions, λ is large (the value of punching clearance is 12.5%) and the coefficient of variation is small, that is, punching. The variation of λ with respect to the clearance change is small.

  In the hot rolling conditions (Table 4), the reheating temperature, the final pass completion temperature of finish rolling, and the winding temperature are denoted as SRT, FT, and CT, respectively. Further, the first cooling rate after the final pass is expressed as CR1, the next cooling rate is expressed as CR2, and the temperature at which CR1 is changed to CR2 is expressed as MT. In the case of the condition of cooling between FT and CT at one kind of cooling rate, “-” is described in the MT and CR2 columns.

3 is a graph showing the relationship between the punching clearance and the hole expansion limit value λ for each condition described in Table 1. It is a graph which shows the relationship between the rolling rate of a last pass, and a variation coefficient.

Claims (5)

% By mass
C: 0.03-0.08%
Si: 1.0% or less Mn: 1.2-2.0%
P: 0.02% or less S: 0.01% or less Al: 0.01 to 0.05%
N: 0.01% or less Nb: 0.03-0.5%
The balance is hot rolled a steel slab comprising Fe and inevitable impurities, and the final rolling pass of the hot rolling is performed at a temperature of 850 to 950 ° C., a rolling rate of 15 to 25%, a rolling torque [kN M], rolling load [kN], and rolling roll radius [m] are performed under conditions satisfying the following (formula 1), and then slowly cooled to 700 to 800 ° C. at a cooling rate of less than 20 ° C./second. A method for producing a high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized by that.
0.20 ≦ rolling torque [kN · m] / (rolling load [kN] × roll radius [m]) ≦ 0.35 (Formula 1) % By mass
Ti: 0.05 to 0.2%
The manufacturing method of the high strength hot-rolled steel plate excellent in stretch flangeability of Claim 1 characterized by the above-mentioned. % By mass
B: 0.0001 to 0.0030%
The manufacturing method of the high strength hot-rolled steel plate excellent in stretch flangeability of Claim 1 or 2 characterized by the above-mentioned.   The thickness of a hot-rolled steel sheet is 2-7 mm, The manufacturing method of the high strength hot-rolled steel sheet excellent in stretch flangeability of any one of Claims 1-3 characterized by the above-mentioned.   5. After the slow cooling according to claim 1, the film is rapidly cooled at 20 to 70 ° C./s and wound up at 600 ° C. or less and excellent in stretch flangeability according to claim 1. Manufacturing method of high-strength hot-rolled steel sheet.

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