JP3511272B2 - Manufacturing method of high Young's modulus steel sheet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet having a high Young's modulus for use in buildings and other steel structures or welded steel pipes. 2. Description of the Related Art When stress is applied to a structure or a component using steel, strength until deformation starts, ie, yield strength, and strength until fracture, ie, tensile strength or breaking strength, are required. However, rigidity is often important. Yield strength and tensile strength of steel can be improved by alloy components, manufacturing methods, heat treatment, etc., whereas Young's modulus, which is an index of the rigidity of this steel, is usually
It hardly changes due to its components, manufacturing method or heat history, and shows a substantially constant value of about 21000 to 21500 kgf / mm ² . Therefore, in the case of a structure or a component that absolutely requires rigidity, the area of a cross section perpendicular to the direction in which the stress is applied cannot be reduced. For example, considering a beam made of steel pipes having the same outer diameter, increasing the strength of the steel has no effect in reducing the deflection, and the thickness must be increased. However, if the Young's modulus of the steel material can be increased, it is possible to further reduce the weight of the steel material used. In many metals, when the Young's modulus of a single crystal is examined, the <111> crystal axis direction shows the highest value and the <100> crystal axis direction shows the lowest value. For iron, the <111> axis direction is 290
The highest value is shown at 00 kgf / mm ² , and the <100> crystal axis direction is the smallest at 13500 kgf / mm ² . [0004] A steel material is usually made of fine metal crystals, and furthermore, in a general manufacturing method, the direction of each crystal is randomized, so that the steel material as a whole has a substantially constant averaged Young's modulus. Show. Considering the improvement of the in-plane Young's modulus from such a crystal orientation, if the <111> axis of each crystal constituting the steel plate can be made to have a texture with a preferred orientation such that it is parallel to the plate surface , The steel sheet may have a high Young's modulus. [0005] However, in a normal hot or cold rolling method of a steel sheet rolled using a flat roll,
It is not easy to form a texture with the <111> axis parallel to the plate surface, and the only possible preferred orientation is
There are 112 <110> directions. If this orientation can be developed, it is considered that the number of crystal grains whose <111> axis is parallel to the width direction perpendicular to the rolling direction increases, and the Young's modulus can be improved only in the width direction. Several inventions have been proposed for a method of producing a steel sheet having a high Young's modulus only in a specific direction. For example, Japanese Patent Publication No. 58-14849 discloses that C: 0.
20% or less, Si: 0.01 to 1.0% or less, Mn: 0.3 to 2.0
%, Al: 0.001 to 0.20% steel, hot rolling Ar
5 in ferrite + austenite two-phase temperature range of ₃ points or less
% Rolling, cooling at a rate of 15 ° C./s or less after rolling, and then tempering in a temperature range of 700 ° C. or less. Japanese Patent Application Laid-Open No. 57-2837 discloses a method. Ar ₃ points 780
° C. to less steel, at 780 ° C. or less Ar ₃ point or more than 5%, and the hot rolling of less than Ar ₃ point or less 5%, 2% after cooling
An invention of a method combining the above-mentioned cold rolling is shown. As a method of performing rolling in the ferrite phase temperature range equivalent to cold rolling in the hot rolling step, Japanese Patent Publication No. Sho 62-4448 discloses that the cumulative rolling reduction in the temperature range of 600 ° C. or lower at an Ar _three- point temperature or lower is 10%. An invention of a method of winding at 450 to 720 ° C. with a temperature of up to 60% is proposed. In each of these inventions, the Young's modulus in the direction perpendicular to the rolling direction is increased by rolling the ferrite phase, and the preferred orientation of the texture in the ferrite region rolling is formed. However, since cold rolling, which is rolling in the normal ferrite region, hardens when it is performed, annealing is necessary, and a favorable texture for improving Young's modulus due to recrystallization or grain growth during softening often disappears. . In addition, in order to perform the ferrite region rolling in the hot rolling step, a special treatment is required in manufacturing such as lubricating hot rolling because deformation resistance increases. On the other hand, the invention presented in Japanese Patent Application Laid-Open No. 5-247530 discloses that C: 0.05% or less and Mn: 0.5%.
As described above, in steel containing 0.01 to 0.07% of Nb, the heating temperature of hot rolling is 1100 ° C. or more, the finish rolling start temperature is 950 ° C. or less, and the finish rolling end temperature is (Ar ₃ +100 to Ar ₃ −5).
0) It is stated that rolling can be completed at a high temperature at which deformation resistance does not increase by winding at a temperature of ° C. [0008] As described above, the above inventions are all methods for producing a thin steel plate by a hot strip mill. Although hot rolling is performed, strong working in a low temperature range of ₃ points or less of Ar or ₃ points of Ar is performed. Because it is a cold working process in addition to working in the following low temperature range, it is difficult to apply it to large structural materials,
Further, if the rolling conditions are relaxed, an improvement in the Young's modulus cannot be expected. An object of the present invention is to increase the Young's modulus of a thick steel plate or a hot-rolled steel plate particularly used for a large-sized structure or a welded steel pipe, and has an Ar of ₃ or less. The present invention relates to a method for producing a high Young's modulus steel sheet with almost no low-temperature rolling or cold rolling. [0010] Rolling in the ferrite + austenite two-phase region or the ferrite single-phase region certainly improves the Young's modulus in the direction perpendicular to the rolling, but when the plate thickness is increased, it is insufficient in the low-temperature region. Rolling becomes extremely difficult. In view of the above, the present inventors have conducted various studies on the effect of the steel composition and rolling conditions on the Young's modulus of the steel sheet, with respect to relatively thick steel sheets, and in particular, investigated the feasibility of realizing a manufacturing method in which rolling in a low-temperature region was avoided as much as possible. . As a result, it was found that Nb, B, Mo and the like are effective in improving the Young's modulus. Then, these elements when added alone, are effective to increase the Young's modulus if high deformation at Ar ₃ point near or below the temperature range, but a large reduction ratio in the temperature range higher than the Ar ₃ point Even so, the effect has been reduced. However, taking a sufficient amount, be further Nb, if combined addition of two or more kinds of Mo and B, the direction of rolling at Ar ₃ point or more than Ar less than _three points may more to further improve the Young's modulus It was revealed. Examination of the texture of the steel sheet having an improved Young's modulus in the direction perpendicular to the rolling direction reveals that a preferred orientation of 112 <110> has been developed and the <111> axis has been oriented in the direction perpendicular to the rolling direction of the steel sheet. It was found that many crystal grains were generated. This is considered to be because the rolling texture in the austenite region at the Ar ₃ point or higher or the transformation texture after that was changed by the combined addition of Nb, Mo, and B. An example of the sufficient content of these elements and the effect of the composite addition is shown in FIG. 1 of the embodiment described below. Compared to the case where only Nb is used and the content is insufficient, Mo and B are not so effective. Nb
If the rolling finish temperature is Ar
_It can be seen that the Young's modulus is better when the temperature is higher than ₃ points. From such knowledge, the inventors of the present invention have come to the invention of the present production method after confirming the limits of the amounts of components and rolling conditions, and the effects of the addition of other elements. The gist of the present invention is that, by weight, C: 0.02 to 0.15%, Mn: 0.4 to 2.0%, Si: 0.80%
Hereinafter, Al: 0.001 to 0.06%, Cu: 1.5% or less, Ni:
3.0% or less, Cr: 0.60% or less, V: 0.10% or less, T
i: 0.10% or less, Ca: 0.0050% or less, Nb: 0.005 to 0.10%, Mo: 0.05 to 0.80%, and B:
A steel slab which contains two or more of 0.0003 to 0.0030%, and which satisfies 0.15 ≦ 5Nb + Mo + 250B + 3Ti + 1.5V ≦ 1.2 (1) is made of a slab consisting of unavoidable impurities and Fe, and hot-rolled into a steel sheet. In this case, there is provided a method for producing a steel sheet having a high Young's modulus, wherein the cumulative draft between 950 ° C. and _three points of Ar is 50% or more, and the cumulative draft of less than _three points of Ar is 5% or less. The steel sheet thus obtained is applied to a target structure by cutting, deforming, welding, or the like. If used, the rigidity of the product can be improved. In the case of steel pipes in particular, the bending stiffness can be increased regardless of the original plate direction and the axial direction of the steel pipe for structures where bending stress is mainly applied. What is necessary is just to make it a direction be the axial direction of a steel pipe. The reasons for limiting each component and the manufacturing conditions are as follows. (1) C C is contained in a required amount for the purpose of securing the strength of the steel sheet and the steel pipe. However, if the content is less than 0.02%, it is difficult to secure the strength required as a structural material, while the content exceeds 0.15%. Is
Not only does it become difficult to obtain a high Young's modulus, but also the toughness of the base metal and the welded part decreases. Therefore, the C content is from 0.02 to
0.15%, but to obtain a high Young's modulus stably, the C content is
0.06% or less is desirable. (2) Si Si is added for deoxidation and strength enhancement. However,
It may not be necessary to add Al sufficiently to be deoxidized. On the other hand, if the content is too large, the toughness of the welded portion in the case of welding is deteriorated. (3) Mn Mn is a component contained for the purpose of preventing hot brittleness by S and ensuring strength, and the amount of addition is controlled while maintaining a balance with other strength improving components. Further, in the present invention, the cumulative draft between 950 ° C. and the Ar ₃ point is set to 50% or more as described later. However, when the content of Mn is increased, the Ar ₃ point is decreased.
It is also effective in expanding the temperature range during this period. If the content is less than 0.4%, these effects are not fully exhibited, and 2.0%
If Mn is contained in excess of Mn, the toughness of both the base material and the joint is reduced, so the Mn content is set to 0.8 to 2.0%. (4) Al Al is an element essential for sufficient deoxidation to obtain a sound slab. If the content is less than 0.001%, deoxidation becomes insufficient, and if the content exceeds 0.08%, the toughness of the joint is deteriorated during welding, which is not preferable. Therefore, its content is restricted to 0.001 to 0.08%. (5) Nb, Mo and B, and if necessary, Ti and V These elements not only improve the strength of the base material but also realize the improvement of the Young's modulus which is the object of the present invention. Is an important element. In particular, it is necessary to add at least two of the three elements Nb, Mo and B. The content ranges in which the effects of adding these elements are exhibited are different from each other, with Nb being 0.005 to 0.10%, Mo being 0.05 to 0.80%, and B being 0.0003 to 0.0030%. If the content of each element is less than these regulated content ranges, the effect of addition will be ineffective. If the content exceeds these ranges, not only the effect will be saturated, but also adverse effects such as a decrease in toughness will appear. [0023] Ti and V also have the effect of increasing the strength of steel, and are said to have a precipitation hardening effect by bonding with C and N. Therefore, if the strength is not required, it is not necessary to add it, but if necessary, it is added according to the strength of the steel. Further, these elements have an effect of supplementing the effect of improving the Young's modulus of the composite addition of Nb, Mo and B. Desirable contents of Ti and V for exhibiting the effect of addition are 0.005% for Ti, respectively.
As described above, V is 0.01% or more. However, if both elements are contained in excess of 0.10%, the toughness of the steel sheet and its welded part deteriorates.
Content. The respective content ranges of the five elements effective for improving the Young's modulus are as described above, and the composite is contained and hot-rolled under the rolling conditions described below to obtain a Young's material in a direction perpendicular to the rolling direction. The rate can be dramatically improved. However, the total content of each element must be within the range of the following equation (1). 0.15 ≦ 5Nb + Mo + 250B + 3Ti + 1.5V ≦ 1.2 (1) If the content is less than the range regulated by this formula, the effect of addition is insufficient, and if it exceeds this range, the effect is saturated. In addition, adverse effects such as a decrease in toughness appear. (6) Cu, Ni and Cr These elements do not need to be added when strength is not required, but by adding an appropriate amount, a steel material having a good balance between strength and toughness is manufactured. It becomes possible.
However, when added, the content is 1.50% by Cu, respectively.
Hereinafter, the content of Cr should be 0.60% or less and the content of Ni should be 3.00% or less. Exceeding these values results in excessively increasing the strength of the steel or impairing the toughness. (7) Ca Ca may not be added, but if Ca is contained in a small amount, the form of oxide / sulfide inclusions in the steel is changed, and the toughness and corrosion resistance of the base material are improved. Add by In order to achieve this object, the content must be 0.0002% or more. Desirable content when added is 0.0002%
As described above, an excessive addition causes a decrease in cleanliness and a large amount of coarse inclusions formed, which significantly deteriorates not only toughness but also corrosion resistance. Therefore, the content is at most 0.0050% or less. (8) P, S and N These elements are all inevitable impurities in steel, and degrade the toughness of the base metal and the welded portion. P is preferably 0.03% or less and S is 0.02% as a range that does not have a noticeable adverse effect on the properties of the steel.
And N is less than 0.01%. (9) Hot Rolling Conditions The cumulative rolling reduction in the temperature range from 950 ° C. to the _three Ar points is extremely important for improving the Young's modulus of steel sheet products having the above-defined chemical composition. In the area
50% or more is required. In particular, it is more effective if the processing can be sufficiently performed in a temperature range from 850 ° C. to _three Ar points. The upper limit of the cumulative rolling reduction is limited by the temperature drop and the increase in deformation resistance, but is not particularly limited. However, when the cumulative draft is less than 50%, a sufficiently high Young's modulus cannot be obtained. The reason why the Young's modulus cannot be improved even when the cumulative reduction is 50% or more at the rolling temperature exceeding 950 ° C. is that at a high temperature, the release of the processing strain and the recrystallization which starts immediately after leaving the rolling roll are difficult. This is considered to be due to the rapid progress and the cumulative effect of the rolled structure being lost. The heating temperature of the steel slab before rolling is not particularly limited as long as the condition that the cumulative draft in the temperature range from 950 ° C. to the _three Ar points is 50% or more can be achieved. In the steel having the chemical composition specified in the present invention,
Since rolling in a two-phase region or a ferrite single-phase region having less than _three Ar points has no effect on improving the Young's modulus, it is not necessary to perform rolling in this temperature region. Rolling in this temperature range may increase the deformation resistance and increase the rolling load, and may cause a decrease in Young's modulus. Up to 5%. The cooling after rolling is not particularly limited. However, in order to increase the Young's modulus even slightly, it is preferable that the cooling rate at the center of the sheet thickness be 15 ° C./s or more, rather than air cooling or slow cooling after rolling. Is to accelerate cooling. If post-heat treatment is performed after rolling and cooling,
Preferably between 300 and 500 ° C. EXAMPLES Example 1 Using slabs of steel numbers A, B and O whose chemical compositions are shown in Table 1, the cumulative rolling reduction in a temperature range of 950 ° C. or less was 75%, and the finishing temperature was changed. In particular, rolling was performed with a large reduction ratio in a temperature range close to the finishing temperature, and finished to a thickness of 10 mm. The rolled steel sheet was immediately forcibly cooled at a cooling rate of about 20 ° C./s. In this case, the Ar ₃ point was about 760 ° C. for all steels estimated from the deformation resistance of rolling. A test piece having a width of 10 mm and a length of 60 mm was cut out from the obtained steel sheet, and the Young's modulus at room temperature was measured by a transverse vibration method. [Table 1] FIG. 1 shows the change in the Young's modulus in the direction perpendicular to the rolling direction of the steel sheet with respect to the finishing temperature. Steel number A (0.03Nb−0.25
Mo) and steel numbers B (0.03Nb- 0.15Mo - 0.0012 B) is
The steel having a component and its amount effective for improving the Young's modulus represented by the formula (1) is within the range defined by the present invention, and steel No. O (0.02Nb) has only Nb as the active ingredient, and its amount is Insufficient. The Young's modulus of steel O of 0.02 Nb is improved by rolling at a temperature lower than the Ar ₃ point of about 760 ° C. In contrast, it can be seen that steel A and steel B show higher Young's modulus when finished at a temperature higher than the Ar ₃ point, and show the highest value at a temperature around 800 ° C. Example 2 A steel sheet having a chemical composition shown in Table 1 was subjected to hot rolling under the conditions shown in Table 2 by using steel slabs of steel numbers C to L and P to T to produce a steel sheet having a thickness of 10 mm. . With respect to the obtained steel sheet, the Young's modulus in the direction perpendicular to the rolling direction was measured in the same manner as in Example 1. Table 2 also shows the results. [Table 2] Prototype Nos. 16 to 18 are made of the same steel as Prototype Nos. 1 to 4 whose components fall within the range defined by the present invention.
Since the rolling reduction in the temperature range from 950 ° C. to the Ar ₃ point is insufficient, or the rolling reduction in the temperature range below the Ar ₃ point is too large, the Young's modulus is not sufficiently improved. Also, the prototype number
Rolling conditions 19 to 23 fall within the range defined by the present invention, but the steel composition is outside the present invention. As described above, the Young's modulus in the direction perpendicular to the rolling direction of the prototypes Nos. 1 to 15 manufactured under the components and rolling conditions specified in the present invention is 15% as compared with the case of manufacturing by the ordinary method.
It turns out that it has improved above. According to the production method of the present invention, the Young's modulus in the direction perpendicular to the rolling direction of the steel sheet can be greatly improved without performing strong working in a low temperature region at _three or less Ar. Therefore, by using the method of the present invention, it becomes possible to efficiently produce a thick steel plate or a hot-rolled steel plate having a high Young's modulus.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a relationship between a rolling finish temperature of hot rolling and a Young's modulus in a direction perpendicular to the rolling direction of an obtained steel sheet.

   ────────────────────────────────────────────────── ─── Continuation of front page       (56) References JP-A-4-314823 (JP, A)                 JP-A-4-147915 (JP, A)                 JP-A-57-2837 (JP, A)

Claims (1)

(57) [Claims] [Claim 1] C: 0.02-0.15%, Mn:
0.4 to 2.0%, Si: 0.80% or less, Al: 0.001 to 0.06
%, Cr: 0.60% or less, Cu: 1.5% or less, Ni: 3.0
%, V: 0.10% or less, Ti: 0.10% or less, and C
a: 0.0050% or less, Nb: 0.005 to 0.10%,
Mo: 0.05 to 0.80% and B: 0.0003 to 0.0030%, two or more of which are contained, and 0.15 ≦ 5Nb + Mo + 250B + 3Ti + 1.5V ≦ 1.2 (1) The balance satisfying (1) is from unavoidable impurities and Fe. When the steel slab is hot rolled into a steel sheet, the cumulative draft between 950 ° C. and the _three Ar points is 50% or more, and the cumulative draft below the _three Ar points is 5% or less. Manufacturing method of high Young's modulus steel sheet.