JP4837426B2 - High Young's modulus thin steel sheet with excellent burring workability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high Young's modulus thin steel sheet excellent in burring workability suitable as a steel sheet for automobiles and a method for producing the same.

  In recent years, the application of high-strength steel sheets has been promoted for the purpose of weight reduction for improving safety performance and fuel efficiency of automobiles. However, the vehicle body rigidity related to the safety and handling stability of the automobile as well as the collision safety is generally governed by the Young's modulus, the plate thickness, and the structure of the material. For this reason, improvement in rigidity cannot be expected simply by increasing the strength of a steel sheet as a raw material, and the current situation is that the weight reduction due to the application of high-strength steel sheets has not progressed particularly in structural members that require member rigidity. .

  As a method for improving the rigidity of a member while suppressing an increase in weight due to an increase in plate thickness, a method of employing a light metal such as an Al alloy having a high specific strength has been conventionally proposed. However, light metals including this Al alloy have a problem that they are extremely expensive compared to steel. For this reason, the application of such Al alloys is currently limited to special applications.

  Therefore, in order to promote weight reduction of the entire automobile by applying inexpensive and highly rigid materials to many members in the automobile, it is not a response by using light metals such as Al alloy, but to the last It was necessary to cope by using a steel plate. In order to actually cope with the steel plate, it was necessary to improve the Young's modulus of the steel plate.

  On the other hand, as a method for increasing the member rigidity, there is also a method for optimizing the structure, for example, by applying beads. However, in order to cope with the recent refinement and complexity of automobile members, more excellent moldability and workability, particularly burring workability and stretch flange workability are required. In general, when the strength is increased, material properties such as formability and workability are deteriorated. For this reason, how to increase the strength without deteriorating such material characteristics is a key point for the development of a high-strength steel sheet. In particular, burring workability, flange workability, ductility, fatigue durability, and corrosion resistance are important characteristics required for steel sheets for inner plate members, structural members, and suspension members. How to balance these properties with increasing strength Whether it can be pulled out well is important.

As a technique for improving the burring workability and the flange workability, for example, the microstructure is an acicular ferrite structure, and TiC and / or NbC is further precipitated, whereby the tensile strength is 70 kgf / mm ² or more. However, a technique for obtaining excellent stretch flangeability is disclosed (for example, see Patent Document 1).

  Patent Document 2 discloses an elongated flange in which the main structure is an acicular ferrite structure having a high dislocation density and the cooling rate after hot rolling is controlled to reduce the amount of dissolved C as much as possible. A high-strength hot-rolled steel sheet having excellent properties is disclosed.

  Furthermore, Patent Document 3 discloses a hot-rolled steel sheet having excellent formability that exhibits excellent elongation and stretch flangeability by controlling the size and number of precipitates, in addition to defining the microstructure and chemical composition. Has been.

  However, since the disclosed techniques of Patent Documents 1 to 3 are intended only to improve stretch flangeability, they do not assume anything about improving the Young's modulus of the steel sheet. It is not disclosed at all.

On the other hand, as a technique for improving the Young's modulus of the steel sheet itself, a technique for performing rolling at an α + γ two-phase region temperature below the Ar ₃ transformation point is disclosed. (For example, see Patent Documents 4 and 5) However, although the technique mentions a technique for increasing the Young's modulus, there is no disclosure of a technique for improving workability such as stretch flangeability, and the Ar ₃ transformation point or less. Since rolling is performed at the α + γ two-phase region temperature, the plate thickness accuracy is deteriorated, and the productivity is remarkably deteriorated.

For this reason, there has been a demand for a steel sheet that can be easily formed even in parts that require strict burring workability and stretch flange workability, and that can be further bent and torsionally rigidity after being processed. .
Japanese Patent Laid-Open No. 07-011382 JP 2000-144259 A JP 2004-307919 A Japanese Patent Laid-Open No. 01-011926 JP 09-053118 A

  Therefore, the present invention has been devised in view of the above-described problems, and an object thereof is to provide a high Young's modulus thin steel sheet excellent in burring workability and a method for manufacturing the same.

  With the manufacturing process of a 490-980 MPa grade steel plate produced on an industrial scale by the production equipment that is usually employed at present, the present inventors have excellent burring workability and a high Young compared to conventional steel plates. We worked hard to obtain a steel plate with a high rate.

As a result, in mass%, C: 0.01 to 0.2%, Si: 0.01 to 2%, Mn: 0.1 to 2%, P ≦ 0.1%, S ≦ 0.03%, Al: 0.001 to 0.1%, N ≦ 0.01%, Nb: 0.005 to 0.05%, Ti: 0.001 to 0.2%, the balance being Fe and inevitable impurities The microstructure of the steel plate is a continuous cooling transformation structure, and the average value of the X-ray random intensity ratio of the {100} plane and the X-ray random intensity ratio of the {211} plane is 2.5 or more. It was newly found that it is very effective.

That is, in order to solve the above-mentioned problem, the invention according to claim 1 of the present application is, in mass%, C: 0.01 to 0.2%, Si: 0.003 to 2%, Mn: 0.1. ˜2%, P ≦ 0.1% (exceeding 0%), S ≦ 0.03% (exceeding 0%), Al: 0.001 to 0.1%, N ≦ 0.01% (provided 0 %)), Nb: 0.005 to 0.1%, Ti: 0.001 to 0.2%, and the balance is Fe and unavoidable impurities, and the microstructure is continuously cooled. It is a transformation structure, and the average value of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio of the {211} plane is 2.5 or more.

  Moreover, the invention according to claim 2 of the present application is the invention according to claim 1, further in mass%, B: 0.0002 to 0.002%, Cu: 0.2 to 1.2%, Ni: It is characterized by containing one or more of 0.1 to 0.6%, Mo: 0.05 to 1%, V: 0.02 to 0.2%, Cr: 0.01 to 1%. To do.

  Further, the invention according to claim 3 of the present application is the invention according to claim 1 or 2, further in mass%, Ca: 0.0005 to 0.005%, REM: 0.0005 to 0.02%. Or it contains 2 types, It is characterized by the above-mentioned.

  The invention according to claim 4 of the present application is characterized in that in the invention according to any one of claims 1 to 3, surface treatment is performed.

When the invention according to claim 5 of the present application is hot-rolled to obtain a thin steel sheet containing the component according to any one of claims 1 to 3, it is heated above the solution temperature of Nb carbide. Further, after rough rolling, the total rolling reduction of the final stage and the preceding stage is set to 25% or more at a rolling speed of 600 mpm or more, and finish rolling is performed in a temperature range of Ar ₃ transformation point temperature or higher and Ar ₃ transformation point temperature + 100 ° C. or lower. The temperature range from the start of cooling to winding is cooled to a temperature range of 400 ° C. or more and 600 ° C. or less at a cooling rate of 20 ° C./sec or more, and winding is performed.

  The invention according to claim 6 of the present application is characterized in that, in the invention according to claim 5, the finish rolling start temperature is 1000 ° C. or higher.

  The invention according to claim 7 of the present application is the invention according to claim 5 or 6, wherein the steel bar, the rough bar after the completion of rough rolling, until the start of finish rolling and / or during the finish rolling of the rough bar. And heating.

  The invention according to claim 8 of the present application is characterized in that, in the invention according to any one of claims 5 to 7, descaling is performed from the end of rough rolling to the start of finish rolling.

The invention according to claim 9 of the present application is characterized in that after the manufacturing process according to any one of claims 5 to 8, the obtained steel sheet surface is surface-treated.

In the present invention, the microstructure is a continuous cooling transformation structure, and the average value of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio of the {211} plane is 2.5 or more, Even parts that require strict burring workability and stretch flange workability can be easily molded, and after being processed, the parts can have higher bending and torsional rigidity than before, so the present invention has high industrial value. It can be said that it is an invention.

  Hereinafter, as a best mode for carrying out the present invention, a method for producing a high Young's modulus thin steel sheet excellent in burring workability will be described in detail.

The present inventor conducted the following experiment on the assumption that the texture of the steel sheet has an influence on the bending rigidity of the parts related to the Young's modulus. In the experiment, slabs of steel components shown in Table 1 were melted. In Table 1, two types of steels A, Nb-added steel A and Nb-free steel B, which will be described later in detail, are prepared. Note that the solution temperature of NbC in steel A to which Nb was added was 987 ° C., and the Ar ₃ transformation point temperature was 828 ° C. Further, the Ar ₃ transformation point temperature of steel B not containing Nb is 845 ° C.

  A steel having a thickness of 252 mm of the components shown in Table 1, a heating temperature of 910 to 1230 ° C., a rolling reduction of 10 to 35%, a rolling speed of 120 to 870 mpm, a rolling temperature of 860 to 1100 ° C., and different manufacturing conditions, and a thickness of 3.2 mm Steel sheets were prepared, and the X-ray surface strength and the bending rigidity of the parts were investigated.

  The evaluation of the bending rigidity of the part was performed with the press-shaped product 1 shown in FIG. This press-shaped product is press-molded so that the groove portion 2 is formed, and the width w1 of the groove portion 2 is 100 mm, the thickness d of the groove portion is more than 80 m, and the length La in the longitudinal direction is 600 mm. . As a result of the diameter of the groove portion 2 being enlarged at an angle θ (= 15 °) in the middle, the width w1 becomes 140 mm.

  From the cut plate sample collected above, the longitudinal direction of the press-formed product 1 is parallel to the rolling direction (0 ° direction in FIG. 1), vertical (90 ° direction in FIG. 1), and further 45 ° direction (see FIG. 1). In order to evaluate the bending rigidity after cutting out the blank so that it becomes either of the above, clamp the longitudinal direction of the part and load the bending moment, and the bending rigidity was obtained from the load and displacement at that time .

FIG. 2 is a diagram in which the relationship between the average value of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio (plane intensity) of the {211} plane and the bending stiffness is arranged based on the measurement results. . As shown in FIG. 2, if the average value of the X-ray random intensity ratio of the {100} plane of the plate and the X-ray random intensity ratio (plane intensity) of the {211} plane is 2.5 or more regardless of the rolling conditions, high Young It has been found that a good part bending rigidity of 300 MNm ² or more can be obtained as a criterion for the rate. However, all of the steels to which the bending rigidity was 300 MNm ² or more were added with Nb. This mechanism is not necessarily clear, but in steel to which Nb is added, the amount of Nb present in a solid solution state in the steel at the time of finish rolling is limited to a specific range, and further, a specific temperature, rolling speed, reduction When the finish rolling is performed under the rate condition, the recrystallization of austenite is suppressed by the Nb salt drag phenomenon, and the crystal orientation of the continuously cooled transformed structure obtained from the unrecrystallized austenite by the γ → α transformation is selected as the texture. In particular, the average value of the X-ray random intensity ratio of the {100} plane and the X-ray random intensity ratio of the {211} plane is increased, the Young's modulus of the specific orientation of the steel sheet is improved, and the bending rigidity as a part is improved. It is estimated to be.

Next, the microstructure will be described. A sample was cut out from a steel sheet having an average value of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio (plane intensity) of the {211} plane of 2.5 or more, and the microstructure was examined.

  The microstructure was examined by cutting a sample cut from a 1/4 W or 3/4 W position of the steel plate width to a cross section in the rolling direction, etching using a Nital reagent, and observing at 200 to 500 times magnification using an optical microscope. This was done with a photograph of the field of view at 1/4 t of the thickness of the plate. As a result, it was found that the microstructure was a ferrite structure and a continuous cooling transformation structure.

Here, the continuous cooling transformation structure (Zw) is the Japan Iron and Steel Institute Basic Research Group, Bainite Research Group / Ed; Recent research on the bainite structure and transformation behavior of low carbon steel-Final Report of Bainite Research Group (1994 Japan) The microstructure defined as the transformation structure in the intermediate stage between the microstructure containing polygonal ferrite and pearlite produced by the diffusion mechanism and the martensite produced by the non-diffusion shearing mechanism as described in It is. That is, the continuous cooling transformation structure (Zw) is an optical microscope observation structure as described in the above-mentioned References 125 to 127, and the microstructure is mainly Bainitic ferrite (α ° _B ), Granular ferritic ferrite (α _B ), Quasii. -Polygonal ferrite (α _q ), which is further defined as a microstructure containing a small amount of retained austenite (γ _r ) and Martensite-austenite (MA). The internal structure of α _q does not appear by etching like polygonal ferrite (PF), but the shape is ash and is clearly distinguished from PF. Here, α _q is a grain whose ratio (lq / dq) satisfies lq / dq ≧ 3.5 when the perimeter length lq of the target crystal grain and its equivalent circle diameter is dq.

  The continuous cooling transformation structure (Zw) is a microstructure containing one or more of α0B, αB, αq, γr, MA as described above, and a small amount of γ, MA makes the total amount 3% or less. Is. Note that it is not always necessary that the ferrite structure or the continuous cooling transformation structure is used alone, and it may be a composite structure of these.

  Then, the reason for limitation of the chemical component of this invention is demonstrated. Hereinafter, the mass% in the tissue is simply referred to as%.

C: 0.01 to 0.2%
C is an important element for dissolving a sufficient amount of C in the austenite phase and leaving the desired austenite phase at room temperature, and is useful for increasing the balance between strength and stretch flangeability. One of the most important elements. If it contains more than 0.2%, Fe ₃ C as a carbide that becomes the starting point of burring cracks increases, not only the hole expansion value deteriorates but also the strength increases and the workability deteriorates. 0.2%. Incidentally, less than 0.1% is desirable from the viewpoint of preventing a decrease in ductility. If it is less than 0.01%, the desired strength as a structural material cannot be obtained, so the content is made 0.01% or more.

Si: 0.003 to 2%
Since Si has the effect of suppressing precipitation of iron carbide that becomes the starting point of burring cracks during cooling, it is added in an amount of 0.003% or more, but even if added over 2%, the effect is saturated. Therefore, the upper limit is made 2%. Further, if it exceeds 1%, a tiger-strike-like scale wrinkle is generated, and the aesthetic appearance of the surface may be impaired and the chemical conversion property may be deteriorated. Therefore, the upper limit is desirably set to 1%. Further, from the viewpoint of suppressing scale-like scale defects, the Si content is preferably 0.1% or more.

Mn: 0.1 to 2%
Mn is a so-called austenite stabilizing element, has the effect of expanding the austenite temperature range to the low temperature side and making it easy to obtain a continuous cooling transformation structure, which is one of the constituent requirements of the microstructure of the present invention, during cooling after the end of rolling. There is. In the present invention, 0.1% or more of this Mn is added in order to obtain a low temperature transformation phase from austenite to stably obtain a continuous cooling transformation structure. However, even if Mn is added in excess of 2%, the effect is saturated, so the upper limit is made 2%. In addition, in addition to Mn, when an element that suppresses the occurrence of hot cracking due to S is not sufficiently added, it is desirable to add an amount of Mn that satisfies Mn / S ≧ 20 by mass%.

P ≦ 0.1%,
P is an unavoidable impurity and is preferably as low as possible. If it exceeds 0.1%, the workability and weldability are adversely affected, so 0.1% or less. However, considering hole expansibility and weldability, 0.02% or less is desirable. However, it is assumed that the P content exceeds 0.

S ≦ 0.03%
S is an unavoidable impurity and not only causes cracking during hot rolling, but if it is too much, it generates A-based inclusions such as MnS that degrades the hole expandability, so it should be reduced as much as possible. 0.03% or less is an acceptable range. However, 0.01% or less is desirable when a certain degree of hole expansion is required, and 0.003 or less is desirable when higher hole expansion is required. However, it is assumed that the S content exceeds 0.

Al: 0.001 to 0.1%
Al acts as a deoxidizer, but combines with N in the steel to suppress coarsening of the austenite grain size. In order to deoxidize molten steel, it is necessary to add at least 0.001% or more of Al, but the upper limit is set to 0.1% in order to increase the cost. Further, if added too much, non-metallic inclusions are increased and elongation is deteriorated, so 0.06% or less is desirable.

N ≦ 0.01%
N is an element inevitably mixed in steel, and is an element that forms nitrides such as Ti and Nb. Since this nitride precipitates at a relatively high temperature, it may become coarse and become the starting point of burring cracks. Further, as will be described later, in order to effectively utilize Nb and Ti, a smaller amount is preferable. Therefore, the upper limit is made 0.01%. However, when it is applied to a component in which aging deterioration is a problem, aging deterioration becomes severe when N is added in excess of 0.006%, so 0.006% or less is desirable. Furthermore, when it is assumed that the product is left at room temperature for 2 weeks or more after production and then subjected to processing, 0.005% or less is desirable from the viewpoint of aging resistance. In consideration of exports that exceed the equator when left at high temperatures in summer and transported by ship, it is preferably less than 0.003%. However, the N content exceeds 0.

Nb: 0.005 to 0.1%
Nb is one of the most important elements in the present invention. Nb suppresses recovery / recrystallization and grain growth of austenite during or after rolling by means of a dripping effect in a solid solution state and / or a pinning effect as a carbonitride precipitate, and is effective for improving the Young's modulus. It has an effect of improving the accumulation of {100} plane + {211} plane. However, at least 0.005% of addition is necessary to obtain these effects. Desirably, it exceeds 0.01%. On the other hand, even if added over 0.1%, the effect is saturated, so the upper limit is made 0.1%.

Ti: 0.001 to 0.2%
Ti is one of the most important elements in the present invention. In addition to forming fine carbides and contributing to an increase in strength due to precipitation strengthening, there is an effect of suppressing the nucleation of ferrite in the γ / α transformation and promoting the formation of a continuous cooling transformation structure. However, to obtain this effect, at least 0.001% or more must be added. Desirably, it is 0.005% or more. On the other hand, in order to effectively use Ti, it is necessary to dissolve carbonitride formed at the time of casting in the slab heating in the hot rolling process. Since it deviates from the range, the upper limit is set to 0.2%.

B: 0.0002 to 0.002%
B has the effect of improving the hardenability and facilitating obtaining a continuously cooled transformed structure, so is added as necessary. However, if it is less than 0.0002%, it is insufficient for obtaining the effect, and if added over 0.002%, slab cracking occurs. Therefore, the addition of B is set to 0.0002% or more and 0.002% or less.

  Further, in order to impart strength, one or more of precipitation strengthening or solid solution strengthening elements of Cu, Ni, Mo, V, and Cr may be added. However, the effect cannot be obtained if the content is less than 0.2%, 0.1%, 0.05%, 0.02%, and 0.01%, respectively. Moreover, the effect is saturated even if added exceeding 1.2%, 0.6%, 1%, 0.2%, and 1%, respectively.

  Ca and REM are elements that become destructive by changing the form of non-metallic inclusions that become the starting point of destruction and degrade workability. However, even if added less than 0.0005%, there is no effect, and if Ca is added more than 0.005% and if REM is added more than 0.02%, the effect is saturated, so Ca is 0.0005-0. It is desirable that REM is added within a range of 0.0005% to 0.02% within a range of 0.005%.

  Note that Ti, Nb, Zr, Sn, Co, Zn, W, and Mg may be contained in a total amount of 1% or less in steel containing these as main components. However, Sn is preferably 0.05% or less because wrinkles may occur during hot rolling.

  Next, the reasons for limiting the production method of the present invention will be described in detail below.

  Since the Nb solid solution amount is related to the slab heating temperature before rolling, the relationship between the heating temperature and the solid solution Nb amount was investigated. FIG. 3 shows the relationship between the heating temperature of steel A added with Nb and the amount of solute Nb in Table 1. As shown in FIG. 3, the amount of solute Nb changes with the slab heating temperature below the solution temperature of Nb, but stable solute Nb is obtained above the solution temperature regardless of the heating temperature. In order to obtain the effect of addition, it has been found that it is necessary to heat to a temperature higher than the solution temperature of Nb.

Next, the relationship between the manufacturing conditions other than the slab heating temperature, the average value of the {100} plane X-ray random intensity ratio and the {211} plane X-ray random intensity ratio was investigated.

First, an investigation was conducted on the rolling reduction during finish rolling. Normally, finishing mills are rolled to a predetermined thickness sequentially by a plurality of rolling mill groups, but the final stage of the finishing mill group and the rolling reduction of the preceding stage have a greater influence on the surface strength than the preceding stage of the finishing mill group. Therefore , the average value of the total rolling reduction ratio of the final stage and the preceding stage, the {100} X-ray random intensity ratio of the plate surface, and the X-ray random intensity ratio of the {211} plane was investigated. In the test, steel A in Table 1 was used, and the total rolling reduction ratio of the final stage and the preceding stage was changed, and the other operating conditions were constant (heating temperature 1200 ° C., rolling speed 600 mpm, rolling temperature 900 ° C.). The average value of {100} X-ray random intensity ratio and {211} plane X-ray random intensity ratio was measured. The results are shown in FIG. 4, and the average value of the X-ray random intensity ratio of {100} and the X-ray random intensity ratio of {211} plane when the rolling reduction of the final stage of the finish rolling mill and the preceding stage is 25% or more. It was found that the target was increased significantly beyond 2.5.

Next, the relationship between the average value of the {100} X-ray random intensity ratio of the plate surface and the {211} plane X-ray random intensity ratio according to the finish rolling speed was investigated. In the same manner as described above, {100} X-rays of the obtained steel sheet were obtained by changing the rolling speed using steel A and keeping the other operating conditions constant (heating temperature 1200 ° C, rolling reduction 25%, rolling temperature 900 ° C). The average value of the random intensity ratio and the X-ray random intensity ratio of the {211} plane was measured. As a result, as shown in FIG. 5, when the rolling speed is 600 mpm or more, the average value of the X-ray random intensity ratio of {100} and the X-ray random intensity ratio of the {211} plane significantly increases beyond the target of 2.5. I found out.

Furthermore, since the finish rolling temperature FT is considered to have a large influence on the surface strength, the average value of the {100} X-ray random intensity ratio of the plate surface and the X-ray random intensity ratio of the {211} surface due to the finish rolling temperature The relationship was investigated. The finish rolling temperature FT of steel was changed, and the average value of the {100} X-ray random intensity ratio of the obtained steel sheet and the X-ray random intensity ratio of the {211} plane was measured. As a result, as shown in FIG. 6, it was found that when the finishing temperature was 930 ° C. or less, the random intensity ratio of X-rays significantly increased beyond the target of 2.5.

  A preferable manufacturing method will be described below for the hot rolling method other than the above and the steps before and after the hot rolling step. In the present invention, the production method preceding hot rolling is not particularly limited.

  In the present invention, after casting, after hot rolling, after cooling or after hot rolling, or with hot-rolled steel sheets subjected to heat treatment in a hot dipping line, these steel sheets are separately subjected to hot dip galvanizing. Can also be obtained. Thereby, it becomes possible to improve corrosion resistance more.

  In the present invention, the production method preceding hot rolling is not particularly limited. In other words, following smelting with a blast furnace, converter, electric furnace, etc., the components are adjusted so that the desired component content is obtained by various secondary scouring, and then, in addition to normal continuous casting, casting by ingot method, thin slab What is necessary is just to cast by methods, such as casting. Scrap may be used as a raw material. In the case of a slab obtained by continuous casting, it may be directly sent to a hot rolling mill as it is a high-temperature slab, or may be hot-rolled after being reheated in a heating furnace after being cooled to room temperature.

The slab reheating temperature is
SRT (° C.) = 6670 / (2.26-log [% Nb] [% C])-273
The solution temperature is equal to or higher than the solution temperature of Nb carbide calculated by the above. If the temperature is lower than this temperature, the carbide of Nb is not sufficiently dissolved, and in the subsequent rolling process, the austenite recovery / recrystallization by Nb and the suppression of grain growth and the effect of accumulating crystal orientation due to the delay of γ / α transformation cannot be obtained. . Therefore, the slab reheating temperature (SRT) is not less than the temperature calculated by the above equation. However, if it is 1400 ° C. or higher, the scale-off amount becomes large and the yield decreases, so the reheating temperature is preferably less than 1400 ° C. In addition, heating below 1000 ° C significantly impairs the operation efficiency in terms of schedule, so the slab reheating temperature is desirably 1000 ° C or higher. Furthermore, since the scale-off amount is small when heating at less than 1100 ° C., inclusions on the surface of the slab may not be removed together with the scale by subsequent descaling, so the slab reheating temperature is preferably 1100 ° C. or higher. The slab heating time is not particularly defined, but it is desirable to hold it for 30 minutes or more after reaching the temperature in order to sufficiently dissolve the carbide of Nb. However, this is not the case when the cast slab is directly fed and rolled at a high temperature.

  In the hot rolling process, after the rough rolling is finished, finish rolling is performed. In order to obtain a continuous cooling transformation structure that is more uniform in the thickness direction, the finish rolling start temperature is set to 1000 ° C. or higher. Further, 1050 ° C. or higher is desirable. For that purpose, a rough bar or a rolling material is heated as needed from the end of rough rolling to the start of finish rolling and / or during finish rolling. In particular, it is effective to suppress fine precipitation of MnS and the like in order to stably obtain excellent elongation at break in the present invention. The heating device in this case may be of any type, but if it is a transverse type, it is desirable to use the transverse type because it can soak in the thickness direction. Usually, precipitates such as MnS are re-dissolved by slab reheating at about 1250 ° C. and finely precipitated during subsequent hot rolling. Therefore, ductility can be improved if the slab reheating temperature is controlled to about 1150 ° C. and re-solution of MnS or the like can be suppressed. However, in order to set the rolling end temperature within the range of the present invention, heating of the rough bar or rolled material from the end of rough rolling to the start of finish rolling or / and during finish rolling is an effective means.

  Moreover, a sheet bar may be joined between rough rolling and finish rolling, and finish rolling may be performed continuously. At that time, the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again before joining.

The finish rolling finish temperature (FT) is set to a temperature range of Ar ₃ transformation point temperature or higher and Ar ₃ transformation point temperature + 100 ° C. or lower. Here, the Ar ₃ transformation point temperature is simply shown in relation to the steel component by the following calculation formula, for example. That is, Ar ₃ = 910-310 ×% C + 25 ×% Si-80 ×% Mneq
However, Mneq = Mn + Cr + Cu + Mo + Ni / 2 + 10 (Nb−0.02)
Or, Mneq = Mn + Cr + Cu + Mo + Ni / 2 + 10 (Nb−0.02) +1: When B is added

If the finish rolling finish temperature (FT) is lower than the Ar ₃ transformation point temperature, α + γ two-phase region rolling may occur, and the processed structure remains in the ferrite grains after rolling and the ductility deteriorates, so the Ar ₃ transformation point temperature That's it. On the other hand, when the Ar ₃ transformation temperature exceeds + 100 ° C., the effect of suppressing the recovery / recrystallization and grain growth of austenite by dripping and / or pinning due to the addition of Nb is lost. There is a possibility that the effect of accumulating crystal orientation due to the delay of / α transformation cannot be obtained. Although the effect of the present invention can be obtained even if the rolling pass schedule in each stand of finish rolling is not particularly limited, the rolling rate in the final stand is preferably less than 10% from the viewpoint of plate shape accuracy.

After finishing rolling, the temperature range from 400 ° C. to 600 ° C. is cooled at an average cooling rate of 20 ° C./sec or more. Although the cooling start temperature is not particularly limited, the microstructure mainly becomes a continuous cooling transformation structure when cooling is started from the Ar ₃ transformation point temperature or higher, and polygonal ferrite is contained in the microstructure when cooling is started below the Ar ₃ transformation point temperature. Will come to be. In any case, if it is less than the above cooling rate, there is a possibility that the effect of accumulating crystal orientation cannot be obtained due to recovery / recrystallization of austenite and grain growth. The upper limit of the cooling rate is not particularly defined, but the effect of the present invention can be obtained, but at 500 ° C./sec or more, the yield ratio may increase, so 500 ° C./sec or less is desirable. Furthermore, since there is a concern about plate warpage due to thermal strain, it is desirable to set it to 250 ° C./sec or less. Further, a uniform microstructure is desirable for improving the burring workability, and 130 ° C./sec or more is desirable for obtaining such a microstructure. On the other hand, when cooling is stopped at 600 ° C. or higher, a phase containing coarse carbides such as pearlite, which is undesirable for workability, may be generated. Therefore, the temperature range for cooling is up to 600 ° C. However, if cooling is not started within 5 seconds after finishing rolling, recovery of austenite, recrystallization, and grain growth may result in failure to obtain crystal orientation accumulation. Cooling within 5 seconds after finishing rolling is finished. It is desirable to initiate cooling.

  Winding treatment is performed after cooling is completed, but if the winding temperature exceeds 600 ° C., a phase containing coarse carbides such as pearlite which is not preferable for workability may be generated in the temperature range, and further precipitation strengthening of TiC or the like may occur. The coiling temperature is set to 600 ° C. or lower because it may be lost due to overaging and the strength may decrease. On the other hand, if the temperature is lower than 400 ° C., precipitation strengthening such as TiC does not occur and the intended strength may not be obtained, so the coiling temperature is 400 ° C. or higher.

  In the present invention, after casting, after hot rolling, after cooling or after hot rolling, or with hot-rolled steel sheets subjected to heat treatment in a hot dipping line, these steel sheets are separately subjected to hot dip galvanizing. can get. Thereby, it becomes possible to improve corrosion resistance more.

  After completion of the hot rolling process, pickling may be performed as necessary, and then a skin pass with a reduction rate of 10% or less or cold rolling to a reduction rate of about 40% may be performed inline or offline.

In this cold rolling, the total rolling reduction of cold rolling after pickling subsequent to hot finish rolling may be less than 80%. This is because when the total rolling reduction of cold rolling is 80% or more, the X direction of the {111} plane or {554} plane parallel to the plate surface, which is a general cold rolling-recrystallization texture This is because the line diffraction integrated surface intensity ratio becomes high, and an average value of the X-ray random intensity ratio of the {100} plane of the target plate surface and the X-ray random intensity ratio of the {211} plane cannot be 2.5 or more. . Moreover, it is 70% or less desirably. Although the lower limit of the cold rolling rate is not particularly defined, the effect of the present invention can be obtained. However, in order to control the strength of the crystal orientation to an appropriate range, it is desirable to set it to 3% or more.

  The heat treatment of the steel sheet thus cold-rolled is premised on a continuous annealing process.

First, the lower limit temperature of the heat treatment temperature is set to the Ac1 transformation point temperature or higher. When this lower limit temperature is lower than the Ac1 transformation point temperature, the intended continuous cooling transformation structure cannot be obtained. Here, when aiming at coexistence with ductility without significantly degrading the burring property, the temperature range is higher than the Ac1 transformation point temperature in order to increase the volume fraction of α _q among the microstructures constituting the continuous cooling transformation structure. Ac3 The temperature range is below the transformation point temperature (two-phase region of ferrite and austenite). In order to obtain better burring properties, in order to increase the volume fraction of α ° _B and α _{B in} the continuously cooled transformation structure, a temperature range of Ac3 transformation point temperature to Ac3 transformation point temperature + 100 ° C or less is desirable. .

Next, the cooling step is not particularly defined in the present invention, but when the heat treatment temperature is not lower than the Ac1 transformation point temperature and not higher than the Ac3 transformation point temperature, a temperature range of 600 to 400 ° C at a cooling rate of 20 ° C / s or higher. Allow to cool. This is because if the cooling rate is less than 20 ° C./s, there is a risk of bainite containing a large amount of carbide or nose of pearlite transformation. Further, the cooling end temperature is preferably more than 400 ° C. because a large amount of γ _r and MA, which are considered to be harmful to the burring property, may be generated at 400 ° C. or less. Furthermore, the end temperature of the cooling step is set to 600 ° C. or lower because aging properties may deteriorate if it exceeds 600 ° C. Further, the subsequent cooling is not particularly defined. However, at a cooling rate of 20 ° C./s or more, γ _r and MA, which are considered to be harmful to the burring property, may be produced in large quantities. A cooling rate is desirable. In addition, the lower limit of cooling at this time is preferably 50 ° C. or higher because when cooling with water or mist, if the coil is in a wet state for a long time, there is a concern about poor appearance due to rust.

  Thereafter, skin pass rolling is performed as necessary. However, in this case, in order to obtain the effect of reducing the friction coefficient, the skin pass reduction ratio is controlled so that the arithmetic average roughness Ra of at least one of the front and back surfaces of the steel plate after skin pass is 1 to 3.5 μm. It is desirable.

  In order to galvanize the hot-rolled steel sheet after pickling or the cold-rolled steel sheet after completion of the recrystallization heat treatment, it may be immersed in a galvanizing bath and alloyed as necessary.

  In order to improve the ductility by correcting the shape of the steel sheet or introducing movable dislocations, it is desirable to perform skin pass rolling of 0.1% or more and 2% or less.

  In order to galvanize the hot-rolled steel sheet after pickling, it may be immersed in a galvanizing bath and alloyed as necessary.

In the present invention, descaling may be performed between the end of rough rolling and the start of finish rolling. When descaling is performed between the end of rough rolling and the start of finish rolling, it is desirable to satisfy the condition of high-pressure water collision pressure P (MPa) × flow rate L (liters / cm ² ) ≧ 0.0025 on the steel sheet surface. . The collision pressure P of high-pressure water on the steel sheet surface is described as follows. (Refer to "Iron and Steel" 1991 vol. 77 No. 9 p1450)
P (MPa) = 5.64 × P ₀ × V / H ²
However,
P ₀ (MPa): Liquid pressure V (liter / min): Nozzle flow rate H (cm): Distance between the steel plate surface and the nozzle

The flow rate L is described as follows.
L (liter / cm ² ) = V / (W × v)
However,
V (liter / min): Nozzle flow rate W (cm): Width of spray liquid per nozzle hitting steel plate surface v (cm / min): Plate passing speed

  The upper limit of the collision pressure P × the flow rate L is not particularly required to obtain the effects of the present invention, but increasing the nozzle flow rate causes problems such as severe wear of the nozzle, and therefore is 0.02 or less. Is desirable.

  Furthermore, it is desirable that the maximum height Ry of the steel sheet surface after finish rolling is 15 μm (15 μm Ry, l2.5 mm, ln12.5 mm) or less. This is clear from the fact that the fatigue strength of a hot-rolled or pickled steel sheet correlates with the maximum height Ry of the steel sheet surface, as described in, for example, Metal Material Fatigue Design Handbook, edited by the Japan Society of Materials Science, page 84. is there. Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent the scale from being generated again after descaling.

  Next, the Example of the high Young's modulus thin steel plate to which the present invention is applied will be described in detail. After melting steel Nos. A to K having chemical components shown in Table 2 in a converter and directly feeding or reheating after continuous casting, the thickness of the steel plate is 3.2 mm by finish rolling following rough rolling. Winded up.

  In Table 2, steel numbers A to E, I, and K are within the range of chemical components limited in the present invention, but steel number F is Nb and Ti, steel number G is C, and steel number H is C and Ti, steel number J deviates C, Nb and Ti from the chemical components defined in the present invention.

  Details of the manufacturing conditions are shown in Table 3.

In this Table 3, “heating temperature results” is the result of slab heating extraction temperature, and “rough bar heating” is from the end of rough rolling to the start of finish rolling or / and during induction rolling by induction heating of the rough bar or rolled material. “FT” is the finish rolling end temperature, “Rolling speed” is the sheet passing speed at the end of finish rolling, “Rolling ratio” is the rolling rate in the last stage and the preceding stage, “Cooling speed” "Represents the average cooling rate from the cooling start temperature to the winding temperature range, and" CT "represents the winding temperature. The X-ray random intensity ratio indicates an average value of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio of the {211} plane.

Here, Steel Nos. A-1 to A-9 have different production conditions for the components of Steel No. A in Table 1. Steel No. A-1 is included in the range of manufacturing conditions defined in the present invention, but Steel No. A-2 shows the actual heating temperature, and A-3 and 4 show the finish rolling end temperature (FT), A-5 is the rolling speed, A-6 is the rolling reduction, A-7 is the cooling speed, and A-8 and 9 are the CTs deviating from the scope defined in the present invention. Steel number D was descaled after rough rolling under conditions of a collision pressure of 2.7 MPa and a flow rate of 0.001 liter / cm ² . Further, Steel No. I was immersed in a galvanizing bath and galvanized as usual. Here, “NbC solution temperature” indicates the heating temperature required for dissolution of NbC calculated from the above SRT (° C.) = 6670 / (2.26-log [% Nb] [% C])-273. . However, it was set to "-" in the steel numbers F and J to which Nb was not substantially added.

  In the tensile test of the thin steel plate thus obtained, a sufficient number of samples were collected so that a statistical tendency could be discriminated in the coil width and longitudinal direction, and processed into a No. 5 test piece described in JIS Z 2201, and JIS Z The test method described in 2241 was performed. The burring workability was evaluated by the hole expansion value according to the hole expansion test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996.

Among the steels of steel numbers A to K, those satisfying the requirements defined in the present invention are steel numbers A-1, B, C, D, E, I, and K, which are seven steels. And the microstructure is a ferrite structure, a continuous cooling transformation structure or a mixed structure thereof, and the hole expansion value is 60% or more, and the {100} plane X-ray random intensity ratio and the {211} plane A thin steel plate is obtained in which the average value of the X-ray random intensity ratio is 2.5 or more.

Steels other than the above are outside the scope of the present invention for the following reasons. That is, Steel No. A-2 has a “heating temperature record” exceeding the NbC solution temperature and out of the scope of the present invention, so the {100} plane X-ray random intensity ratio of the plate surface and the {211} plane X The average value of the line random intensity ratio is not 2.5 or more. In Steel No. A-3, since “FT” is less than the Ar3 transformation point temperature and outside the scope of the present invention, the target microstructure cannot be obtained, and the hole expansion value is not 60% or more. In Steel No. A-4, since “FT” exceeds the Ar3 transformation temperature +100 and is outside the scope of the present invention, the {100} plane X-ray random intensity ratio of the plate surface and the {211} plane X-ray random intensity The average value of the ratio is not 2.5 or more. Steel No. A-5 has a “rolling speed” of less than 600 mpm and is outside the scope of the present invention, so the average of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio of the {211} plane The value is not 2.5 or more. Steel No. A-6 has a “rolling ratio” of less than 25% and is outside the scope of the present invention. Therefore, the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio of the {211} plane are The average value is not 2.5 or more. Steel No. A-7 has a “cooling rate” of less than 20 ° C./sec and is outside the scope of the present invention, so that the target microstructure cannot be obtained, and the hole expansion value is not 60% or more. . In Steel No. A-8, since “CT” exceeds 600 ° C. and is outside the scope of the present invention, the target microstructure cannot be obtained, and the hole expansion value is not 60% or more. Steel No. A-9 has a “CT” of less than 400 ° C. and is outside the scope of the present invention. Therefore, the target microstructure cannot be obtained, and the hole expansion value is not 60% or more. Steel No. F does not contain the Nb and Ti contents of 0.005% by mass or more and 0.001% by mass or more, respectively, and is outside the scope of the present invention. The hole expansion value is 60% or more, and the average value of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio of the {211} plane is not 2.5 or more. Steel No. G has a C content of less than 0.01% by mass and is outside the scope of the present invention, so that the desired microstructure cannot be obtained. Further, the X-ray random intensity ratio of the {100} plane of the plate surface And the average value of the X-ray random intensity ratio of the {211} plane is not 2.5 or more. Steel No. H has a C content of more than 0.2% by mass, and further, Ti is not contained in an amount of 0.001% by mass or more and is outside the scope of the present invention. The hole expansion value is not over 60%. Steel No. J has a C content exceeding 0.2% by mass and Nb and Ti contents not exceeding 0.005% by mass and 0.001% by mass, respectively, and are outside the scope of the present invention. Therefore, the target microstructure cannot be obtained, and the hole expansion value is 60% or more. The average value of the X-ray random intensity ratio of the {100} plane of the plate surface and the X-ray random intensity ratio of the {211} plane Is not more than 2.5.

It is a figure which shows the press part for bending rigidity evaluation. It is a figure which shows the relationship between the average value of the X-ray random intensity ratio of {100} plane of a plate surface, and the X-ray random intensity ratio of {211} plane, and bending rigidity. It is a figure which shows the relationship between slab heating temperature and the amount of solute Nb. It is a figure which shows the relationship between a rolling reduction, the average value of {100} X-ray random intensity ratio of a plate surface, and the X-ray random intensity ratio of {211} surface. It is a figure which shows the relationship between a rolling speed, the average value of {100} X-ray random intensity ratio of a plate surface, and X-ray random intensity ratio of {211} surface. It is a figure which shows the relationship between the average value of {100} X-ray random intensity ratio of FT and a plate surface, and X-ray random intensity ratio of {211} surface.

Explanation of symbols

1 Press-shaped product 2 Groove

Claims (9)

% By mass
C: 0.01-0.2%
Si: 0.003 to 2%,
Mn: 0.1 to 2%,
P ≦ 0.1% (however, over 0%),
S ≦ 0.03% (however, over 0%),
Al: 0.001 to 0.1%,
N ≦ 0.01% (however, over 0%),
Nb: 0.005 to 0.1%,
Ti: 0.001 to 0.2%,
And the balance is Fe and inevitable impurities, the microstructure of which is a continuous cooling transformation structure , the {100} plane X-ray random intensity ratio and the {211} plane X-ray A high Young's modulus thin steel sheet excellent in burring workability, characterized in that the average value of the random strength ratio is 2.5 or more. In addition,
B: 0.0002 to 0.002%,
Cu: 0.2 to 1.2%,
Ni: 0.1 to 0.6%,
Mo: 0.05 to 1%
V: 0.02-0.2%,
Cr: 0.01-1%,
The high Young's modulus thin steel sheet excellent in burring workability according to claim 1, comprising one or more of the following. In addition,
Ca: 0.0005 to 0.005%,
REM: 0.0005 to 0.02%,
The high Young's modulus thin steel sheet excellent in burring workability according to claim 1 or 2, characterized by containing one or two of the following.   The high Young's modulus thin steel sheet excellent in burring workability according to any one of claims 1 to 3, wherein hot dip galvanizing is applied. When hot-rolling to obtain a thin steel plate from a steel slab containing the component according to any one of claims 1 to 3, the steel slab is heated to a temperature equal to or higher than a solution temperature of Nb carbide, and further rough rolling is performed. The subsequent rough bar is rolled at a speed of 600 mpm or more, and the total rolling reduction of the final stage and the preceding stage is 25% or more,
Finish rolling is finished in a temperature range of Ar ₃ transformation point temperature or higher and Ar ₃ transformation point temperature + 100 ° C. or lower, and the temperature range from the start of cooling after finish rolling to winding is 400 ° C. or higher at a cooling rate of 20 ° C./sec or higher. A method for producing a high Young's modulus thin steel sheet excellent in burring workability, wherein the steel sheet is cooled to a temperature range of 600 ° C. or lower and wound.   6. The method for producing a high Young's modulus thin steel sheet excellent in burring workability according to claim 5, wherein the finish rolling start temperature is 1000 ° C. or higher.   The steel slab is heated to 1000 ° C. or more until the rough bar after finishing the rough rolling is finished and / or during the finish rolling of the rough bar. A method for producing a high Young's modulus thin steel sheet with excellent burring workability.   Descaling is performed between the end of rough rolling and the start of finish rolling, The method for producing a high Young's modulus thin steel sheet excellent in burring workability according to any one of claims 5 to 7.   The manufacturing method of the high Young's modulus thin steel plate excellent in burring workability characterized by galvanizing the obtained steel plate surface after the manufacturing process of any one of Claims 5-8.

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