JPH04314823A - Production of high tensile strength steel excellent in plastic deformability and having high young's modulus - Google Patents

Abstract

PURPOSE:To produce a steel having high Young's modulus, low yield ratio, and large uniform elongation, excellent in plastic deformability, and having >=70kgf/mm<2> strength. CONSTITUTION:A slab of a steel having a composition containing, by weight, 0.04-0.155% C, <=0.6% Si, 0.6-2.0% Mn, 0.3-0.7% Mo, <=0.01% N, and <=0.10% Al is heated to 750-850 deg.C. Rolling is done at >=2 reduction ratio and finished at 650-800 deg.C, and the steel is successively subjected, if necessary, to heating to 400-650 deg.C and then to air cooling, by which the high tensile strength steel excellent in plastic deformability and having high Young's modulus can be produced.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention has a tensile strength of 70 kgf/mm2 or more, a high Young's modulus in the direction perpendicular to the rolling direction (hereinafter referred to as the C direction), and a low plastic deformability (yield ratio). This relates to a method for manufacturing high-strength steel with excellent uniform elongation (large uniform elongation). Steel produced by this method can be used in steel structures such as buildings.

0002

[Prior Art] In recent years, steel materials (
With regard to structural steel materials such as steel plates, steel pipes, steel sections, etc., the weight of structures is becoming lighter due to larger structures and associated energy savings. As a means for reducing the weight, it is effective to use high tensile strength steel of 60 kgf/mm2 or more and reduce the plate thickness, for example. However, as the plate thickness decreases, buckling problems arise, so it is necessary to increase the rigidity of the steel plate. Since the rigidity of a steel plate is proportional to Young's modulus if the shape is the same, high-strength steel with a high Young's modulus is desired. As a means to increase Young's modulus,
According to Japanese Unexamined Patent Publication No. 56-23223, steel having a specific composition is rolled in a two-phase region, the cooling rate is specified to 300°C after rolling, and then the steel is tempered at a temperature of 700°C or less to improve the young strength in the C direction. modulus of 10% to 15% (Young's modulus of steel is E
= 21000 kgf/mm2) has been disclosed. Also, JP-A-59-83721
The publication describes that when hot rolling steel having a specific composition in which C is limited to a low composition (less than 0.03%)
A method for manufacturing a high-rigidity hot-rolled steel sheet is disclosed in which the Young's modulus in the C direction is improved by about 8 to 15% by rolling with a cumulative reduction rate of 10% or more at a temperature below and then rolling at a temperature below 720°C. .

On the other hand, with the application of seismic design methods to building structures, high tensile strength steel with a low yield ratio and high uniform elongation is required from the viewpoint of seismic resistance. As a manufacturing method of low yield ratio high tensile strength steel, for example, Japanese Patent Application Laid-Open No. 55-4
No. 1927 or Japanese Unexamined Patent Publication No. 55-97425 have been proposed. The former method, JP-A-55-41927, uses a combination of controlled rolling and controlled cooling, while the latter, JP-A-55-97425, uses a so-called heat treatment (QT) method. In either case, the steel has a good low yield ratio for a 60 kgf/mm2 class steel, but a sufficiently low yield ratio (yield ratio less than 70%) cannot be obtained for a high tensile strength of 70 kgf/mm2 or more. Further, the uniform elongation is extremely small at about 8 to 12%. In any case, there is currently no technology for producing high tensile strength steel of 70 kgf/mm2 or more, which has a Young's modulus improved by 15% or more, a low yield ratio of less than 70%, and a uniform elongation of more than 12%.

0004

[Problems to be Solved by the Invention] The purpose of the present invention is to solve the problems of the conventional method and clarify the optimal components and manufacturing conditions, thereby achieving a high Young's modulus in the C direction and compositional deformation. High tensile strength steel (70kgf/mm2
The present invention provides a manufacturing method for the above).

[0005]

[Means for Solving the Problems] The gist of the present invention is, in weight percent, C: 0.04 to 0.15%, Si: 0.6% or less,
Mn: 1.2 to 2.0%, Mo: more than 0.25 to 0.7%
, N: 0.01% or less, Al: 0.10% or less is heated to a temperature range of 750°C or more and 850°C or less, and then rolled at a reduction ratio of 2 or more to achieve a temperature of 650°C or more and 85°C or more.
After finishing rolling at a temperature range of 0°C or lower, if necessary, continue heating to a temperature of 400°C or higher and 650°C or lower, and then air cooling, and C: 0.04 to 0.15%, S.
i: 0.6% or less, Mn: 1.2-2.0%, Mo: 0
．． More than 25 to 0.7%, N: 0.01% or less, Al: 0.
10% or less, Nb: 0.005 to 0.06 as necessary
0%, Ti: 0.005-0.030%, Ni: 1% or less, Cu: 1% or less, Cr: 0.05-1.00%, V
B: 0.1% or less, B: 0.0003 to 0.0020% or more at 750°C or higher 85
After heating to a temperature range of 0°C or less, rolling is performed at a reduction ratio of 2 or more, and after finishing rolling in a temperature range of 650°C or more and 800°C or less, continue to heat from 400°C to 650°C as necessary.
It is heated to a temperature below ℃ and then air cooled.

[0006]

[Function] In order to increase the Young's modulus of steel, the inventors proposed in Japanese Patent Application Laid-Open No. 2-269414 that steel pieces with 750 to 8
After heating to a temperature range of 50°C, rolling is performed at a reduction ratio of 2 or more, and rolling is finished in a temperature range of 650 to 800°C, thereby demonstrating a method for producing a thick steel plate with a high Young's modulus in the C direction. . It is generally known that as the strength increases, the yield ratio of the steel material increases and the uniform elongation decreases. That is, the higher the strength, the worse the plastic deformability becomes.

However, the steel manufactured in Japanese Patent Application No. 2-269414 has a problem that even though the strength is about 50 to 60 kgf/mm 2 , the yield ratio is extremely high at about 83 to 88%, and the plastic deformability is poor. Therefore, in order to produce steel with a high Young's modulus, excellent plastic deformability, and a strength of 70 kgf/mm 2 or more, we conducted intensive studies on optimal components and rolling conditions, and arrived at the present invention.

[0008] In the present invention, 70 kgf/mm2
As a high-strength steel, it is necessary to ensure the strength and toughness of the base material, as well as to achieve a high Young's modulus of over 15%, a low yield ratio of less than 70%, and a uniform elongation of over 12%. The ingredients and rolling conditions were clarified. The present invention will be explained below. As shown in Japanese Patent Application No. 2-269414, Young's modulus can be increased by heating and rolling to the α-γ coexistence region. However, since the yield ratio becomes extremely high and the plastic deformability deteriorates, it is not preferred as a steel material for construction. Therefore, without increasing the yield ratio,
As a result of intensive study on the components and manufacturing process conditions for increasing Young's modulus, we found that a high Young's modulus can be achieved by containing more than 0.26% Mo in the process of heating and rolling to the α-γ coexistence region. At the same time, they have found that they have a low yield ratio, a large uniform elongation, and extremely excellent plastic deformability, leading to the present invention.

That is, the feature of the present invention is that the steel containing a certain amount of Mo is heated to the α-γ coexistence region, and then, by appropriate rolling and subsequent heat treatment, a high Young's modulus, a low yield ratio, and a large uniformity can be obtained. The objective is to obtain steel with elongation.

[0010] The lower yield ratio and increase in uniform elongation due to the addition of Mo in this manufacturing process are due to the addition of Mo over 0.25%.
This can be achieved by adding o. Addition of Mo of 0.25% or less has no effect on low yield ratio and increase in uniform elongation. However, since adding more than 0.7% of Mo is undesirable as it leads to deterioration of weld zone toughness and weldability, the upper limit was limited to 0.7%. When rolling steel containing Mo, it is first necessary to limit the heating temperature of the steel piece to 750 to 850°C.

[0011] By heating to this temperature and then rolling, it becomes easy to develop a texture that is effective for improving Young's modulus even in the production of thick steel plates in which a large cumulative reduction ratio in the α region cannot be achieved. When the heating temperature is lower than 750°C, rolling will result in (100) [011] orientation or (1-11) [011]
Since the orientation also develops, the effect of improving Young's modulus in the C direction is small. Furthermore, it becomes necessary to heat the steel billet for a long time to uniformly heat it, furthermore it becomes necessary to heat the steel billet for a long time to uniformly heat it, and furthermore, the deformation resistance during rolling increases. , the energy cost increases, which is undesirable. Furthermore, when the heating temperature exceeds 860°C, the percentage of ferrite (α) during heating decreases,
As the texture develops less, the effect of improving Young's modulus becomes smaller. When rolling is performed next, the rolling reduction ratio needs to be 2 or more. If the reduction ratio is less than 2, the texture will not be sufficiently developed, and no improvement in Young's modulus will be observed.Furthermore, the pores in the steel plate will remain unpressed, resulting in a significant deterioration in toughness. be.

Furthermore, the rolling end temperature is 650 to 800°C.
It is necessary to do so. When rolling is finished at a temperature lower than 650° C., the (100) [011] direction and (111) [011] direction also develop, so the effect of improving the Young's modulus in the C direction is small. On the other hand, when the rolling end temperature exceeds 800° C., the texture is not sufficiently developed and an improvement in Young's modulus cannot be expected. Furthermore, regarding cooling after rolling, there is no problem in either air cooling or accelerated cooling.

[0013] If necessary, the temperature may be continued at 400°C or higher6.
Tempering treatment at a temperature below 50°C. This is to improve toughness by tempering when the processed ferrite introduced by rolling deteriorates the toughness. 400℃
If it is less than 650°C, the tempering will be insufficient, and if it exceeds 650°C, the strength will decrease. For this reason, the tempering temperature should be set at 400°C or higher and 650°C.
The temperature was below ℃. In addition, in the present invention, a thick steel plate has a plate thickness of 6
Indicates a steel plate of mm or more. Next, the reasons for limiting other components will be described. C is 0.0 to obtain the necessary tensile strength.
It is necessary to add 0.4% or more. However, since excessive addition of C causes deterioration of weldability, the upper limit is set at 0.15%. Si is an element contained in deoxidized steel, but excessive addition of Si can lead to poor weldability and weld heat-affected zone (HAZ).
) inhibits toughness. Therefore, it is necessary to set the upper limit to 0.6% or less. Mn is a useful element for ensuring strength, toughness, and hardenability, and must be added in an amount of 0.6% or more. However, if the amount of Mn is too large, weldability and H
Since it causes deterioration of AZ toughness, the upper limit is set to 2.0%.

[0014] N is generally contained in steel as an unavoidable impurity, but since excessive addition of N causes deterioration of HAZ toughness, the upper limit is set at 0.01%.

[0015] Al is generally an element contained in deoxidized steel, but since deoxidation is also performed by Si, Mn, or Ti, the present invention does not limit the lower limit of Al. However, if the amount of Al increases, the cleanliness of the steel will deteriorate and the HAZ toughness will deteriorate, so the upper limit is set at 0.1%. Note that P and S are contained in steel as inevitable impurities. In the present invention, the amount is not particularly limited, but since these deteriorate the toughness of the base metal and the welded part, the amount is preferably as small as possible, and is 0.03% and 0.01%, respectively.
The following is desirable.

[0016] In the steel of the present invention, if necessary, N
b: 0.003-0.060%, Ti: 0.005-0
．． 030%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 0.05 to 1.00%, V: 0.1% or less, B
: Any one of 0.0003 to 0.0030%,
Or contain two or more types. The main purpose of containing these elements is to improve the strength and toughness of the steel of the present invention and to increase the thickness of the manufactured plate without impairing the characteristics of the steel.The amount of addition should be determined depending on aspects such as weldability and HAZ toughness. Therefore, it is a property that should naturally be restricted.

[0017] Nb is an element effective in improving the strength and toughness of the base metal, and its amount needs to be 0.003% or more.
Since excessive addition of Nb deteriorates HAZ toughness, the upper limit needs to be set to 0.06%. Ti is useful in suppressing coarsening of austenite grains during welding and ensuring HAZ toughness. However, addition of less than 0.005% has no effect, and addition of 0.03% or more results in deterioration of HAZ toughness due to precipitation hardening of TiC, so the amount added is reduced to 0.005 to 0.03%. limited to. Ni is HA
It has the property of improving the strength and toughness of the base material without adversely affecting the hardenability and toughness of Z, but if it exceeds 1.0%, it is unfavorable for the hardenability and toughness of HAZ, so the upper limit is set at 1.0%. shall be.

[0018] Cu has almost the same effect as Ni, and is also effective in corrosion resistance and hydrogen-induced cracking resistance. However, if it exceeds 1.0%, Cu-cracks will occur during rolling, making manufacturing difficult. Therefore, the upper limit is set to 1.0%. Cr is an element that increases the strength of the base material, and needs to be added in an amount of 0.05% or more. However, if the Cr content exceeds 1.0%, weldability and HAZ toughness deteriorate, so the upper limit is set to 1.0%. Although V improves strength by forming fine carbonitrides, addition of 0.1% or more causes deterioration of toughness, so the upper limit is set at 0.1%. B segregates as solid solution B at the γ grain boundaries and increases the strength. In order to obtain this effect, a B content of at least 0.0003% is required. However, if excessive B is added, coarse precipitates such as Fe23(CB)6 will precipitate and the toughness will deteriorate, so the upper limit of the B amount must be set to 0.0020%.

[0019]

[Example] Table 1 shows the chemical composition, manufacturing conditions, and mechanical properties of the test steel. Steel plates of various thicknesses were manufactured and their mechanical properties and Young's modulus were measured. Young's modulus was measured by taking a 3 mm thick test piece from the 1/2t section of the steel plate and using the resonance method.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

In Table 1, steels A to G are steels of the present invention, H to
M indicates comparative steel. Invention steels A to G are 70 kgf/mm
It has a tensile strength of 2 or more and a Young's modulus in the C direction of 241.
50kgf/mm2 (Young's modulus of normal steel 21000k
15% increase in gf/mm2) and 7
It exhibits a low yield ratio of less than 0% and a uniform elongation of 12% or more. On the other hand, Comparative Steel H has a low Mo content, so it has a high yield ratio and low uniform elongation. Comparative Steel I has too much Mo content, so HAZ toughness deteriorates. Comparative steel J has a low Young's modulus because the heating temperature is too high. Comparative steel K has a rolling reduction ratio of less than 2, so its Young's modulus is low and the base material toughness deteriorates. Comparative steel L has a low Young's modulus because the rolling end temperature is high. Comparative steel M has a low Young's modulus because the rolling end temperature is too low.

[0024]

Effects of the Invention The present invention has a high Young's modulus in the C direction, an excellent low yield ratio, a large uniform elongation, and 70 kgf/
It provides a means to manufacture revolutionary high-strength steel that has a high strength of 2 mm2 or more, making it possible to reduce the weight of building structures and improve safety against earthquakes.

Claims (2)

[Claims] Claim 1: In weight%, C: 0.04 to 0.015%, Si: 0.6% or less, Mn: 1.2 to 2.0%, Mo: more than 0.25 to 0.7%. After heating a steel piece containing N: 0.01% or less and Al: 0.10% or less to a temperature range of 750°C or more and 850°C or less, rolling is performed at a reduction ratio of 2 or more, and the steel piece is heated to 650°C.
High Young's modulus and high tensile strength with excellent plastic deformability, characterized in that after rolling is completed at a temperature range of 800°C or higher, heating is continued to a temperature of 400°C or higher and 650°C or lower, as required, and then air-cooled. Steel manufacturing method. Claim 2: In weight%, Nb: 0.005 to 0.060%, Ti: 0.005 to 0.030%, Ni: 1% or less, Cu: 1% or less, Cr: 0.05 to 1 .00%, V: 0.1% or less, and B: 0.0003 to 0.0020%. Law.