JP2000008135A - High tensile strength steel and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high tensile strength steel having >=70 kgf/mm2 tensile strength and high in productivity. SOLUTION: This steel has a compsn. contg., by weight, 0.20 to 0.30% C, 0.01 to 0.50% Si, 1.00 to 3.00% Mn, <=0.030% P, 0.001 to 0.020% S, 0.03 to 0.20% V, 0.005 to 0.017% Ti, 0.005 to 0.070% Al, 0.007 to 0.030% N, and the balance Fe with inevitable impurities and has <=70 kgf/mm2 tensile strength. If required, the steel furthermore contains one or more kinds among <=1.5% Cu, <=3.0% Ni, <=1.0% Cr and <=1.0% Mo in the range satisfying Cu+Ni+3×Cr+3 Mo<=3.0%. The steel moreover contains <=0.10% Nb and 0.0005 to 0.010% Mg if necessary as well. The steel is subjected to hot rolling preferably at 1150 to 1300 deg.C heating temp. and also at >=900 deg.C finishing temp.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a tensile strength of 70 kgf /
The present invention relates to a high-tensile steel having a high productivity of not less than mm ² (≒ 700 MPa) and a method for producing the same.

[0002]

^2. Description of the Related Art In general, structural steel, especially steel (steel plate, mold steel, etc.) having a tensile strength of 70 kgf / mm ² or more for various applications in the fields of bridges, construction, tanks, etc., is required to maintain weldability. , By reducing the amount of C and adding various alloying elements,
The hardening property is improved, and at the same time, it is manufactured by quenching, tempering treatment or working heat treatment [direct quenching, tempering]. Steel produced by these production methods has strength toughness,
Although excellent in weldability, not only is it necessary to add heat treatment to a normal hot rolling process, but also, in some cases, strict control of rolling conditions is required, resulting in low productivity.

[0003] From such a viewpoint, production without heat treatment without heat treatment has been studied. Although it is a thin material, it is described in, for example, Kawatetsu Technical Report 21 (1989) 1, 19-25. As described above, 80 kgf / mm ² steel is manufactured by rolling at a high reduction rate (strong CR) with an extremely low rolling finish temperature. However, this method does not require heat treatment,
There is a disadvantage that the rolling efficiency is poor and the productivity is low.

[0004] On the other hand, a method of producing high-strength steel as-rolled by utilizing precipitation hardening of VN (vanadium nitride) by adding high NV has been studied.
No. 84959 discloses a method for producing high-strength steel using VN, and a steel sheet having a tensile strength of 60 kgf / mm ² class has been invented. Further, JP-A-3-22981
No. 7 describes the production of 80 kgf / mm ² steel by VN, but in order to satisfy the strength of 80 kgf / mm ² , production by direct quenching and reheating methods is carried out. However, even with this method, a significant improvement in productivity cannot be expected.

[0005]

A technique for improving the strength toughness of steel by adding N at the same time as V has been known. For example, in Japanese Patent Publication No. 39-2368,
"Contains at least 0.02% vanadium, 0.08% to 0.24% nitrogen, 0.15% to 0.50% carbon, 0.40% to 2.00% manganese and is not suitable for producing killed steel. A high-strength semi-killed steel characterized by being treated with a sufficient amount of a deoxidizing agent ”, and JP-B-48-1380.
No. 3 discloses "C: 0.08 to 0.20%, Si:
0.20 to 0.40%, Mn: 1.0 to 1.6%, C
r: 0.05 to 0.35, Mo: 0.10 to 0.60
%, V: 0.005 to 0.1%, N2: 0.004 to
0.02%, and 890 ° C.-10
A low-alloy steel for high temperature characterized by normalizing at a cooling rate of 1000 ° C. to 4500 ° C./h from a heating temperature of 00 ° C. ” : 0.05 to 0.20%, Si: 0.01 to
0.6%, Mn: 1.10 to 1.6%, V: 0.03 to
Forging a steel containing 0.2% and N: 0.009 to 0.02% into a flange shape, then cooking and tempering the steel, and the like ". Have been. In any of these inventions, the feature is the addition of V and N, thereby precipitating a large amount of VN, and achieving the refinement and strengthening of crystal grains. Since VN has a low melting temperature, it melts during heating by ordinary standard processing and quenching, and then precipitates in a cooling process by air cooling or precipitates in a tempering process after quenching to strengthen the steel base material.

However, from the viewpoint of effective use of VN, the above-mentioned prior art is premised on not adding Ti, which forms a nitride, and improves the mechanical properties of the base material. Since Ti is not added to reduce the crystal grain size, improvement in toughness poses a problem.
As an exception, JP-A-56-127750 discloses that
"C: 0.12-0.25%, Mn: 0.5-2.9.
%, V: 0.05-0.20%, Cu: 0.05-0.
4%, Ni: 0.1-0.5%, Cr: 0.05-0.
4%, Ti: 0.005 to 0.03%, N: 0.01 to
0.03%, Ca: 0.0005 to 0.0070%,
"Structural high tensile strength steel containing at least one or two kinds of Mg: 0.0005 to 0.0070% and having little strain aging embrittlement" describes that Ti is added simultaneously with the addition of V. However, as described in the text, the addition of Ti in this case is added for the purpose of improving the strength due to the precipitation strengthening of TiC, and is added in a larger amount than the amount of Ti required for grain refinement by TiN. It is necessary to add one or more kinds of Ca or Mg in order to prevent strain aging embrittlement which is a use different from the present invention.

As described above, conventionally, a high-tensile steel having a high level of tensile strength and high productivity has not been obtained. Therefore, an object of the present invention is to provide a high-tensile steel having a tensile strength of 70 kgf / mm ² or more and having high productivity and a method for producing the same.

[0008]

Means for Solving the Problems The present inventors conducted various experiments to produce a steel having a tensile strength of 70 kgf / mm ² or more as rolled and not reducing the toughness. The following findings have been found. When C is 0.2% or less, when various alloying elements are added to impart high strength, a mixed structure containing coarse ferrite and bainite and further partially martensite is obtained,
The toughness decreases.

It is effective to increase the ferrite fraction to simultaneously secure high strength and high toughness, and to increase the ferrite fraction, it is effective to add a C amount of 0.2% or more. It is effective to reduce the addition amount of alloying elements having high hardenability such as Cu, Ni, Cr, and Mo to a certain amount or less. In order to improve the toughness, it is important to reduce the crystal grains in the austenite state, and fine dispersion of TiN in steel is effective. However, in this case T
Since N is consumed by i, it is necessary to further add N in order to secure a suitable N for obtaining a large amount of VN.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) By weight%, C: 0.20 to 0.30%, S
i: 0.01 to 0.50%, Mn: 1.00 to 3.00
%, P: 0.030% or less, S: 0.001-0.
020%, V: 0.03 to 0.20%, Ti: 0.0
05-0.017%, Al: 0.005-0.070
%, N: 0.007 to 0.030%, the balance consisting of Fe and unavoidable impurities, and a tensile strength of 70 kg
High tensile strength steel having f / mm ² or more.

(2) In addition to the chemical components described in the above (1),
Cu: 1.5% or less, Ni: 3.0% or less, Cr: 1.
A high-strength steel further comprising one or more of 0% or less and Mo: 1.0% or less in a range satisfying the following formula: Cu + Ni + 3 × Cr + 3Mo ≦ 3.0 (%).

(3) A high-tensile steel characterized by further containing Nb: 0.10% or less in addition to the chemical components described in the above (1) or (2). (4) A high-strength steel characterized by further containing 0.0005 to 0.010% of Mg in addition to the chemical components described in any of (1) to (3) above.

(5) The steel having the chemical composition described in any of (1) to (4) above is heated to a heating temperature of 1150 to 1150.
A method for producing high-strength steel, comprising hot rolling at 1300 ° C. and a finishing temperature of 900 ° C. or higher.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. First, the reason for limiting the steel components in the present invention as described above will be described. C: An element necessary for securing the strength, it is necessary to add 0.20% or more. However, if it exceeds 0.30%, the toughness is reduced, so that the content range is 0.20 to 0.30. %.

Si: an element necessary as a deoxidizing element in steel making, contained in steel in an amount of 0.01% or more.
If it exceeds 50%, the base material and the HAZ toughness decrease. Therefore, the range is set to 0.01 to 0.50%. Mn: an element necessary for securing strength and toughness. However, if it exceeds 3.0%, the toughness is significantly reduced, and conversely,
If it is less than 1.0%, it becomes difficult to secure the strength of the base material, so the range is limited to 1.00 to 3.00%. P: It is a grain boundary embrittlement element and it is desirable to reduce it as much as possible.
Since the degree of embrittlement is small at 0.030% or less, the upper limit is set to 0.030%.

S: In the present invention, S is an element indispensable for the production of MnS acting as a precipitation nucleus of VN.
% Or more is required, but a large amount of MnS causes coarsening of MnS and a decrease in toughness.
%. V: an element that plays a central role in the present invention,
As described above, 0.03% or more must be added because precipitation can significantly increase the strength. However, if added in a large amount, carbides are precipitated and brittleness is reduced. Limit to 20% or less.

Ti: An element indispensable for refining crystal grains by TiN. Addition of 0.005% or more is necessary. However, excessive addition may cause carbides to be formed, leading to a decrease in toughness. Therefore, the upper limit is limited to 0.017%. Al: an element necessary as a deoxidizing agent, it needs to be added in an amount of 0.005% or more. However, if added excessively, a large amount of AlN is generated, and there is a possibility that N effective for generating VN may be reduced. Therefore, the upper limit is set to 0.070%.

N: Necessary for the production of TiN and VN, 0.007% or more must be added. However, excessive addition causes a decrease in toughness.
And The steel of the present invention has the above basic composition, and can further contain one or more of the following four components as necessary in terms of strength properties.

Cu: Effective as an element for improving toughness and strength, its effect is effective when added at 0.2% or more, but excessively over 1.5% causes the toughness to decrease. The upper limit is 1.5%. Ni: an element effective for improving toughness, it is necessary to add 0.2% or more. However, if the addition exceeds 3.0%, the effect is saturated, so the upper limit is 3.0%.

Cr and Mo: For increasing the strength, it is necessary to add 0.05% or more, respectively. However, since excessive addition impairs the toughness, the range of each is 1.0%.
Limited to When these elements are added, it is necessary that the respective addition amounts satisfy the range of the following formula: Cu + Ni + 3 · Cr + 3 · Mo ≦ 3.0 (%). The reason for this is
As described above, this is a constraint for making the as-rolled microstructure mainly composed of ferrite. If this value exceeds 3.0, the formation of bainite and martensite structures becomes remarkable, and the toughness is secured. It becomes difficult.

The steel of the present invention may contain Nb in the following range in addition to the above basic composition, or in addition to one or more of the above basic composition and the above four components. Nb: Effective as an element for improving toughness and strength, its effect is effective when added in an amount of 0.005% or more, but when added in excess of 0.1%, on the contrary, the toughness is reduced. Is set to 0.1%.

The steel of the present invention may contain, in addition to the above basic composition, one or more of the above basic composition and the above four components, or one or more of the above basic composition and the above four components and the above Nb. In addition to the above, Mg can be further contained in the following range. Through the fine dispersion of Mg: TiN, there is an effect on the refinement of the crystal grains. The effect can be obtained by adding 0.0005% or more. However, when adding more than 0.010%, the toughness is reduced.

In producing the steel of the present invention, a steel having any one of the above chemical compositions is melted in a converter, an electric furnace, or the like, and is subjected to continuous casting or a slab or billet method by a grinding method. Casting a billet such as. Thereafter, the slab is heated and subjected to hot rolling to obtain a steel plate and a mold steel having a predetermined thickness. At this time, as a manufacturing method that does not reduce the productivity, normal heating and normal rolling are performed, and the heating temperature is preferably 1200 ° C. or more, which is the best in heating efficiency, but 1150 ° C.
If it is less than 1300 ° C., the heating efficiency is poor, and if it exceeds 1300 ° C., the amount of fuel used increases, so it is limited to 1150 to 1300 ° C. The rolling finish temperature is preferably 9 after heating to the above-mentioned heating temperature and in a method of high rolling efficiency without waiting for the temperature between rolling passes.
900 ° C because it should be rolled to be over 00 ° C
Is the lower limit.

Regarding the manufacturing conditions after the heating of the thick plate, even if various techniques of the currently known techniques are applied, the properties of the steel sheet are not affected at all. Hereinafter, the present invention will be described in more detail with reference to examples.

[0025]

EXAMPLE A steel slab having the composition shown in Table 1 was heated as shown in Table 2 and then hot-rolled to a predetermined thickness and rolled to produce a thick plate or die steel. Specimens were sampled from the center of the sheet thickness and from the center of the flange in the case of a mold steel, and the tensile strength was determined by a tensile test, and the absorbed energy at 0 ° C. was determined by a Charpy test. Table 2 shows the results.

In the table, steels A to H of the present invention are all steels within the scope of the present invention and have a tensile strength of not less than 700 MPa (70 kgf / mm ² ) and an excellent toughness of not less than 80 J. I have. On the other hand, the comparative steels I to R have compositions deviating from the scope of the present invention, and have a tensile strength of 700M.
Pa (70 kgf / mm ² ) or toughness of 60 J
Below and low.

That is, in Comparative Steels I and J, the C content deviated from the range of the present invention, Comparative Steel I was deviated as low as 0.18%, and Comparative Steel J was deviated as high as 0.32%. ing. In this case, the comparative steel I has a low tensile strength and 700 MPa
a is not satisfied, and the comparative steel J has extremely low toughness. Comparative steel K has a Si content of 0.88%, which is higher than the range of the present invention, and has a reduced toughness. Comparative steel L, M, N
Are those in which Mn, V, and Ti deviate from the range of the present invention, respectively. Comparative steel L has a low tensile strength, and comparative steel M,
N has low toughness.

The comparative steel O contains Nb in a large amount of 0.108%, which is beyond the range of the present invention, and a remarkable decrease in toughness is observed. Comparative steels P and Q are examples in which the amount of N is out of the range of the present invention. Comparative steel P is low, and comparative steel Q is high. Therefore, the tensile strength and toughness of the comparative steel P are reduced, and the toughness of the comparative steel Q is low. Finally, Comparative Steel R is an example in which the total amount of Cu, Ni, Cr, and Mo does not satisfy the expression Cu + Ni + 3 × Cr + 3Mo ≦ 3.0, and in this case, the toughness is extremely low.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

As described above, by producing a steel having the chemical composition within the range of the present invention as it is without any temperature regulation, the rolling efficiency is reduced by the conventional temperature regulation during rolling, and the production by heat treatment is reduced. A steel with a tensile strength of 70 kgf / mm ² or more, which has high productivity for various applications and can realize a significant cost reduction as compared with a production method having a reduced resistance, is provided.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Toshinaga Hasegawa 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division of Nippon Steel Corporation (reference) 4K032 AA00 AA01 AA05 AA11 AA14 AA15 AA16 AA17 AA19 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 BA00 BA01 CA02 CA03 CC04

Claims (5)

[Claims] 1. In weight%, C: 0.20 to 0.30%, Si: 0.01 to 0.50%, Mn: 1.00 to 3.00%, P: 0.030% or less , S: 0.001 to 0.020%, V: 0.03 to 0.20%, Ti: 0.005 to 0.017%, Al: 0.005 to 0.070%, N: 0.007 ~ 0.030%, with the balance being Fe and unavoidable impurities,
A high-strength steel having a tensile strength of 70 kgf / mm ² or more. 2. In addition to the chemical components described in claim 1, one of the following: Cu: 1.5% or less, Ni: 3.0% or less, Cr: 1.0% or less, Mo: 1.0% or less Alternatively, a high-strength steel further comprising two or more kinds in a range satisfying the following formula: Cu + Ni + 3 × Cr + 3Mo ≦ 3.0 (%). 3. A high-strength steel further comprising Nb: 0.10% or less in addition to the chemical components according to claim 1. 4. A high-strength steel, further comprising Mg: 0.0005 to 0.010%, in addition to the chemical composition according to claim 1. 5. A steel having the chemical composition according to claim 1, wherein the steel is heated to a heating temperature of 1150 to 130.
A method for producing high-strength steel, comprising hot rolling at 0 ° C. and a finishing temperature of 900 ° C. or higher.