JPH0971813A - Production of b-added high tensile strength steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably and economically produce a B-added low yield ratio high tensile strength steel by subjecting a steel contg. specified amounts of C, Si, Mn, Al, B, N, Ti, Cu, Ni, Cr and Mo to hot rolling and thereafter executing specified cooling. SOLUTION: A steel contg., by weight, 0.05 to 0.30% C, 0.02 to 0.80% Si, 0.50 to 2.50% Mn, 0.005 to 0.100%. Al, 0.0005 to 0.0025% B, <=0.0050% N and Ti; (47.9/14)N to (47.9/14)N+0.01}, furthermore contg. one or more kinds among <1.5% Cu, <2.0%. Ni, <1.0% Cr and <1.0%. Mo, moreover contg., at need, one or two kinds of <0.05% Nb and <0.20% V, and the balance Fe with inevitable impurities is subjected to hot rolling. Immediately after that, this hot rolled sheet is water-cooled, is next heated to an austenitic region and is cooled at a rate more than the air cooling rate. Thereafter, the steel sheet is heated at a two phase temp. of the Ac1 transformation point to the Ac3 transformation point, is cooled at a rate more than the air cooling rate and is furthermore tempered at the Ac1 transformation point or below.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel containing B, in particular, a B-added high-strength steel which is advantageously applied to the production of a low-yield ratio high-strength steel used as a thick steel plate for construction. The present invention relates to a method for manufacturing steel.

[0002]

In the field of steel structures such as buildings,
Recently, from the viewpoint of economy and earthquake resistance, high-strength steel with a low yield ratio tends to be applied. By the way, in general, as a method of effectively increasing the tensile strength of steel without adding a large amount of expensive alloy elements, a method of utilizing the hardenability improving effect of B on low alloy steel is known. . For example, in "Iron and Steel" 1974 (1988) No. 5 P910-917 and No. 12 P2337-2344, B-added steel is hardened from the austenite (γ) single-phase region. Studies have been reported on the mechanism for improving the hardenability by the method, the amount of B necessary to maximize the effect, the heat treatment conditions, and the like. On the other hand, as a technique for reducing the yield strength (YS) / tensile strength (TS) in high-strength steel, that is, a so-called low yield ratio (low YR), quenching from the two-phase region is effective. Japanese Unexamined Patent Publication No. 5-171263 discloses a method in which B-added steel is subjected to primary quenching and then secondary quenching from the (α + γ) 2 phase region.

[0003]

However, the technique disclosed in JP-A-5-171263 has a problem that the strength level is not always sufficient and the total balance including strength, yield ratio and toughness is poor. It was

Therefore, an object of the present invention is to propose a method for producing a high-strength steel having a low yield ratio, which solves the problems of the above-mentioned known techniques. Another object of the present invention is to have a tensile strength of 80 kg / mm ² or more and a yield ratio of 85% or less,
And tensile strength (TS (kg / mm ² )), yield ratio (YR
(%)) And toughness (fracture transition temperature vTrs (° C.)) total balance parameter (TS-80) x (-v
This is to propose a method for producing a low-yield ratio high-strength steel in which Trs) × (85−YR) / 100 is 10 or more.

[0005]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors of the present invention carried out primary hardening of the B-added steel (α
In the case of performing the double quenching treatment consisting of the secondary quenching of quenching from the two-phase region of + γ), especially B in the secondary quenching
The present inventors have conducted intensive studies on the component system and the pretreatment method that can effectively improve the hardenability. As a result, B does not segregate at the grain boundaries between α particles and α particles (α-α grain boundaries), but segregates only at the grain boundaries between γ particles and γ particles (γ-γ grain boundaries). Only B segregated at the γ-γ grain boundary improves the secondary hardenability. Therefore, it is effective to increase the γ-γ grain boundary area in order to improve the secondary hardenability. For that purpose, it is necessary to properly control the γ grain size and the structure by the pretreatment of the secondary quenching. Is. Further, if BN is generated during the secondary quenching, the hardenability deteriorates. Therefore, in order to prevent the hardenability from decreasing, it is necessary to properly control the relationship between the N content and the Ti content in the steel components. is necessary. Based on this finding, the present invention has been completed.

That is, the gist of the present invention is as follows. (1) C: 0.05 to 0.30 wt%, Si: 0.02 to 0.80 wt%, Mn:
0.50 to 2.50 wt%, Al: 0.005 to 0.100 wt%, B: 0.00
05-0.0025wt%, including N ≤ 0.0050wt%, Ti content is N
Ti: (47.9 / 14) N to {(47.9 / 14) N + 0.
01} wt% range and Cu: less than 1.5 wt%, N
i: less than 2.0 wt%, Cr: less than 1.0 wt%, Mo: 1.0 wt%
Contains one or more selected from less than%,
The balance consists of hot-rolled steel consisting of Fe and unavoidable impurities, immediately water-cooled, then heated to the austenite region and cooled at a speed higher than the air-cooling rate, and then (Ac ₁ transformation point).
Manufacturing of B-added high-strength steel characterized by heating to a two-phase region temperature of (Ac ₃ transformation point), cooling at a rate of air cooling rate or higher, and tempering at a temperature range of Ac ₁ transformation point or lower. Method.

(2) C: 0.05 to 0.30 wt%, Si: 0.02 to 0.
80 wt%, Mn: 0.50 to 2.50 wt%, Al: 0.005 to 0.100 wt
%, B: 0.0005 to 0.0025 wt%, N ≦ 0.0050 wt%, and the Ti content is Ti: (47.9 / 14) N
Included in the range of {(47.9 / 14) N + 0.01} wt% and Cu:
Less than 1.5 wt%, Ni: less than 2.0 wt%, Cr: less than 1.0 wt%, Mo: less than 1.0 wt%, containing one or more selected from, further Nb: less than 0.05 wt%,
V: Steel containing 1 or 2 of less than 0.20 wt% and the balance being Fe and inevitable impurities is hot-rolled, immediately water-cooled, and then heated to an austenite region at an air-cooling rate of at least Cool at a high speed and then (Ac ₁ transformation point) ~
A method for producing a high-strength steel containing B, characterized in that it is heated to a two-phase region temperature (Ac ₃ transformation point), cooled at a rate higher than the air cooling rate, and tempered in a temperature range lower than the Ac ₁ transformation point. .

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of steel in the present invention will be described below. C: 0.05 to 0.30 wt% C must be contained in an amount of 0.05 wt% or more in order to secure the strength of the steel, but if it exceeds 0.30 wt%, the low temperature toughness decreases and weld cracking occurs. Therefore, the content of C is
0.05 to 0.30 wt%, preferably 0.05 to 0.20 wt%.

Si: 0.02 to 0.80 wt% Si must be contained in an amount of 0.02 wt% or more for deoxidation and securing of strength, but if added in excess of 0.80 wt%, low temperature toughness decreases. Therefore, the Si content is 0.80 wt%, preferably 0.02 to 0.40 wt%.

Mn: 0.50 to 2.50 wt% Mn must be contained in an amount of 0.50 wt% or more to secure the strength and toughness, but if it is added in excess of 2.50 wt%, the weldability deteriorates. Therefore, the content of Mn is 0.50 ~ 2.50wt
%, Preferably 0.50 to 1.5 wt%.

Al: 0.005 to 0.100 wt% Al requires 0.005 wt% or more for deoxidation, but
When added in excess of 100 wt%, the alumina inclusions increase and the toughness decreases, so 0.005 to 0.100 wt
%, Preferably 0.015 to 0.08 wt%.

B: 0.0005 to 0.0025 wt% B is 0.0005% in order to exert its hardenability improving effect.
The above addition is necessary. However, even if added in an amount of 0.0025% or more, the effect of improving the hardenability saturates, rather lowering the low temperature toughness. Therefore, the amount of B added is 0.0005
To 0.0025 wt%, preferably 0.0005 to 0.0020 wt%.

N ≦ 0.0050 wt% N forms BN and deteriorates the hardenability of B, so it is desirable to eliminate N. However, if a small amount of N,
By fixing with Ti, the harmful effect of N can be avoided. However, if the amount of N exceeds 0.0050 wt%, a Ti amount of more than 0.017 wt% is required, and TiN produced in a large amount causes deterioration of toughness. Therefore, the amount of N is
Limited to 0.005 wt% or less.

Ti: (47.9 / 14) N to {(47.9 / 14) N +
0.01} wt% When both B and N are present to form BN, this BN acts as a nucleus for transformation, and thus hardenability of steel is deteriorated.
Therefore, in order to eliminate the influence of N contained inevitably, it is necessary to add Ti to fix N. Therefore, the Ti amount for fixing the N contained inevitably needs to be (47.9 / 14) Nwt% or more. On the other hand, when the Ti content exceeds {(47.9 / 14) N + 0.01} wt%, the low temperature toughness decreases due to the precipitation of carbides. Therefore,
The amount of Ti added is (47.9 / 14) N to {(47.9 / 14) N + 0.0
It must be in the range of 1} wt%.

Cu: less than 1.5 wt%, Ni: less than 2.0 wt%, C
r: less than 1.0 wt%, Mo: less than 1.0 wt% Cu, Ni, Cr and Mo are elements that improve the strength and toughness of steel, but even if added in excess, the effect saturates and weldability is improved. Cu: less than 1.5 wt%, N
At least one of these elements should be added in the range of i: less than 2.0 wt%, Cr: less than 1.0 wt%, Mo: less than 1.0 wt%. The preferable addition amount is Cu: 0.5 wt.
% Or less, Ni: 1.2 wt% or less, Cr: 0.6 wt% or less, Mo: 0.
It is 7 wt% or less.

Nb: less than 0.05 wt%, V: less than 0.20 wt% Nb and V are elements that improve the strength and toughness of steel, but if added in excess, the effect saturates and weldability is increased. Nb: less than 0.05 wt%, V: 0.
At least one of these elements in the range of less than 20 wt%
Seeds shall be added. The preferable addition amount is N
b: 0.03 wt% or less, V: 0.1 wt% or less.

After the steel consisting of the above-mentioned component system is made into a slab by the usual ingot casting or continuous casting method, this slab is hot-rolled into a steel plate having a predetermined plate thickness, and the following heat treatment peculiar to the present invention is carried out. It is necessary to apply. That is, immediately after hot rolling, water cooling is performed, followed by heating to an austenite region and cooling (primary quenching) at a speed equal to or higher than the air cooling rate, and thereafter (Ac ₁ transformation point) to (Ac ₃ transformation point) It is heated to the phase region temperature and cooled (secondary quenching) at a speed higher than the air cooling speed. The reason why such heat treatment is performed in the present invention will be described below.

FIG. 1 shows that a steel slab having the composition shown in Table 1 is hot-rolled into a steel plate having a plate thickness of 40 mm, immediately cooled by air cooling or water cooling, and the steel plate is heated to 920 ° C. to perform air cooling or water cooling. YS and TS when cooled at 550 ° C (primary quenching), further heated to 780 ° C for two-phase region quenching (secondary quenching), and tempered at 550 ° C
It indicates the value of. As you can see from this figure, YS
Both the values of TS and TS are high when water cooling is performed immediately after rolling, and decrease when air cooling is performed later than this. Therefore, in order to obtain high-strength steel, it can be said that cooling with water immediately after rolling is effective. The rolling end temperature at this time is a temperature above the Ar ₃ transformation point in order to suppress the formation of ferrite and increase the martensite ratio of the structure after water cooling (primary quenching) to secure the strength of the final product. It is desirable to set the range.

[0019]

[Table 1]

As a result of microscopic observation of the structure, when the cooling immediately after rolling is water cooling, the γ grain size is small and the quenched structure becomes martensite, so that the γ grain size is heated during heating to the next austenite temperature range. Becomes finer and becomes finer during heating in the two-phase region. As a result, γ-γ grain boundaries increase, and the effect of improving the hardenability of B is effectively exhibited. On the other hand, when cooling immediately after rolling is air cooling, either the γ grains are coarse grains, the bainite structure is generated, or both of them occur at the same time, and as a result, the subsequent structure becomes coarse. And γ
-This is because the γ grain boundaries are reduced and the hardenability improving effect of B is lowered. Since the cooling rate after the primary quenching can be obtained with either water cooling or air cooling, the cooling rate is equal to or higher than the air cooling, and the heating temperature during the primary quenching may be in the austenite range.

Next, regarding the two-phase region quenching (secondary quenching) conditions, if the cooling after rolling and the primary quenching conditions are as described above, two phases of the Ac ₁ transformation point to the Ac ₃ transformation point are obtained. When quenching from the zone temperature at a speed higher than the air cooling rate, the effect of improving the hardenability of B is recognized. That is, in this secondary quenching condition, the reason why the cooling rate is set to be higher than the air cooling rate is that, for example, for a thick material with a plate thickness of about 100 mm, if the cooling rate is slower than the air cooling rate, the hardenability decreases and Since the transformation structure of the γ phase due to the area quenching changes from martensite to ferrite + bainite structure, the strength decreases, so the cooling rate is higher than air cooling. Also, the secondary quenching temperature is set to A
The reason for setting the range of c ₁ transformation point to Ac ₃ transformation point is that high strength after tempering cannot be obtained below the Ac ₁ transformation point,
On the other hand, when quenching from the γ single-phase region exceeding the Ac ₃ transformation point, a martensite or bainite structure is formed and high strength is obtained, but YR is increased and a steel with a low yield ratio of YR ≦ 85% cannot be obtained. .

When the tempering temperature exceeds the Ac ₁ transformation point, the hard phase disappears due to the secondary quenching and the strength decreases, so the tempering temperature is set to the Ac ₁ transformation point or lower. Also,
The lower limit of the tempering temperature varies depending on the target strength and toughness, but it is preferably 500 ° C. or higher to secure the toughness. The appropriate tempering time differs depending on the plate thickness, and it is desirable to set the tempering time to about 1 hr / 25 mm in order to make the material uniform.

For the above reason, in the present invention, T
S ≧ 80 kg / mm ² or more, YR ≦ 85%, and TS, Y
The total balance of R and toughness (vTrs (° C)) is (T
S-80) × (−vTrs) × (85−YR) / 100 ≧
In order to achieve 10, after hot rolling, immediately water cooling, followed by heating to an austenite region and cooling at a speed higher than the air cooling rate, and then (Ac ₁ transformation point) to (Ac ₃ transformation point)
It is necessary to heat it to the temperature of the two-phase region and to cool it at a speed higher than the air cooling speed. Then, by undergoing such a heat treatment step, a composite structure of fine soft ferrite and a hard phase is formed, whereby it becomes possible to manufacture a low yield ratio high tensile steel satisfying all these characteristics.

[0024]

[Examples] Table 2 shows the chemical composition of the steel (test material) used. Specimens A to C are steels having a composition that is compatible with the method of the present invention, and steels D to H are comparative steels that deviate from the conditions of the present invention. These steels were melted and cast into slabs, subjected to hot rolling, cooled, subjected to primary quenching treatment, secondary quenching and tempering treatment, and examined for mechanical properties. Table 3 shows the manufacturing conditions and the results of material inspection.

[0025]

[Table 2]

[0026]

[Table 3]

As can be seen from the results shown in Table 3, among the comparative examples, A6 to A9, E and H had TS: 59 to 78 kg.
Only f / mm ² can be obtained, and TS for D, F, G is 80-
Although 83kgf / mm ² can be obtained, vTrs is -45 to -4
Inferior to 6 ° C. Further, in the comparative example, the total balance of mechanical properties, (TS-80) × (−vTrs) × (85−YR) /
100 also stops in the range of -101-1. On the other hand, in the methods A1 to A5, B, and C according to the present invention, the effect of B is sufficiently exhibited even in the two-phase region quenching, and TS: 80 kg
/ Mm ² or more, YR: 85% or less, (TS-80) x (-
vTrs) × (85−YR) / 100: 10 to 27, and it can be said to have excellent mechanical properties.

[0028]

As described above, according to the method of the present invention, the effect of B can be sufficiently exerted even in the two-phase region quenching method in which the improvement of the hardenability of the B-added steel was not sufficiently exerted. Therefore, it becomes possible to stably manufacture the low yield ratio high tensile steel excellent in the overall balance of strength, yield ratio and toughness and economically.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between cooling after rolling and cooling during primary quenching and tensile properties.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kiyoshi Uchida 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (without street number) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (72) Akihiro Matsuzaki 1-shima-shima Kawasaki, Kurashiki-shi, Okayama Prefecture Chome (without street number) Inside Kawashima Steel Co., Ltd. Mizushima Steel Works (72) Inventor Shinichi Amano 1-chome, Kawashima-dori Mizushima Kawasaki City, Okayama Prefecture (No street) Inside Kawashima Steel Co., Ltd. Mizushima Steel Works

Claims (2)

[Claims] 1. C: 0.05 to 0.30 wt%, Si: 0.02 to 0.80 wt
%, Mn: 0.50 to 2.50 wt%, Al: 0.005 to 0.100 wt%, B: 0.0005 to 0.0025 wt%, N ≤ 0.0050 wt%, Ti
Content of Ti: (47.9 / 14) N to {(47.9 / 14) N + 0.01} wt% depending on the amount of N, and Cu: less than 1.5 wt%, Ni: less than 2.0 wt%, Cr : Less than 1.0 wt%, Mo: less than 1.0 wt%, containing one or more selected from the rest, the balance consisting of Fe and unavoidable impurities hot-rolled steel, then immediately water-cooled, then austenite Heating to the region and cooling at a speed higher than the air cooling speed, and then (Ac ₁ transformation point) to (Ac ₃ transformation point)
It is heated to the temperature of the two-phase region and cooled at a speed higher than the air cooling speed,
A method for producing a B-added high-strength steel, which further comprises tempering in a temperature range not higher than the Ac ₁ transformation point. 2. C: 0.05 to 0.30 wt%, Si: 0.02 to 0.80 wt
%, Mn: 0.50 to 2.50 wt%, Al: 0.005 to 0.100 wt%, B: 0.0005 to 0.0025 wt%, N ≤ 0.0050 wt%, and the Ti content depends on the N content: Ti: (47.9 / 14 ) N to {(47.9 / 14) N + 0.01} wt%, and Cu: less than 1.5 wt%, Ni: less than 2.0 wt%, Cr: less than 1.0 wt%, Mo: less than 1.0 wt%. Steel containing one or more selected from the above, Nb: less than 0.05 wt%, V: less than 0.20 wt%, one or two, with the balance Fe and unavoidable impurities After hot rolling, immediately water-cooled, then heated in the austenite region and cooled at a rate of air cooling speed or more,
After that, it is characterized in that it is heated to a two-phase region temperature of (Ac ₁ transformation point) to (Ac ₃ transformation point), cooled at a speed higher than the air cooling rate, and further tempered in a temperature range lower than the Ac ₁ transformation point. Manufacturing method of B-added high-strength steel.