JPH03207814A - Manufacture of low yield ratio high tensile strength steel plate - Google Patents

Abstract

PURPOSE:To manufacture the low yield ratio high tensile strength steel plate by subjecting a slab having a specified compsn. constituted of C, Si, Mn, Cr, Mo, Nb, Ti, B, Al, N and Fe to specified hot rolling, cooling, quenching and tempering. CONSTITUTION:A slab contg., by weight, 0.08 to 0.18% C, <=0.6% Si, 1.5 to 1.6% Mn, 0.4 to 1.5% Cr, 0.2 to 0.5% Mo, 0.005 to 0.05% Nb, 0.005 to 0.03% Ti, 0.0005 to 0.003% B, <=0.10% Al and <=0.006% N, furthermore contg., at need, one or more kinds among <=1.0% Ni, <=1.0% Cu, <=0.1% V and 0.001 to 0.005% Ca and the balance Fe with inevitable impurities is heated to 1100 to 1250 deg.C and is thereafter rolled. After finishing the rolling at 800 to 950 deg.C, the steel plate is cooled by air-cooling to <=300 deg.C. After that, the steel plate is heated to the Ac1 to the Ac3, is thereafter rapidly cooled, is tempered at 400 to 580 deg.C and is thereafter air-cooled. In this way, the low yield ratio high tensile strength steel plate having about >=70kg f/mm<2> tensile strength can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a low yield ratio high tensile strength steel plate having a tensile strength of 70 kgf/1 or more.

(Prior art) In recent years, there has been a demand for low-yield-ratio, high-strength steel plates with excellent earthquake resistance for structural steel materials, including steel materials (steel plates, steel pipes, steel shapes, etc.) used in architectural structures. .

As a method for manufacturing a low yield ratio high tensile strength steel to meet such demands, the method described in, for example, Japanese Patent Laid-Open No. 55-41927 or No. 55-97425 has been proposed. The former method utilizes a combination of controlled rolling and controlled cooling methods, and the latter method uses a so-called tempering treatment (QT) method, but in both cases, 60kgf/1-grade steel has a good low yield. High tensile strength steel with a yield ratio of 70 kgf/1 or more, especially 80 kgf
/Sufficiently low yield ratio for first class steel (yield ratio 80% or less)
There was a problem in that it was not possible to obtain

(Problems to be Solved by the Invention) The purpose of the present invention is to solve the problems of the conventional method,
By clarifying the optimal portion and manufacturing conditions,
An object of the present invention is to provide a method for producing a high tensile strength steel plate (70 kgf/1 or more) having an excellent low yield ratio.

(Means for solving problems) The gist of the present invention is as follows.

(1) In weight%, c: o. o s~0. 1 8%, Si: 0.6
% or less, Mn: 0.5 to 1.6%, Cr: 0. 4 ~1. 5%, Mo: 0. 2 ~ 0. 5%, Nb: 0.005-0.05%, Ti: 0.005-0. 0 3%, B: 0.0005 to 0.003%, 7V: 0. 10% or less, N70. 0 0 6% or less, the balance being Fe and unavoidable impurities.
0 ~! After heating to 250℃ (7) temperature, 800 to 95
After finishing the rolling at a temperature of 0 °C (7), the steel plate was cooled by air cooling to a temperature of 300 °C or less, and then A c , ~
A method for producing a low yield ratio high tensile strength steel sheet, which comprises heating to a temperature of Ac3, rapidly cooling, tempering at a temperature of 400 to 580°C, and then air cooling.

(2) In weight %, c: o. os~0.18%, Si: 0.5% or less, Mn: 0. 5 ~1. 6%, Cr: 0. 4 ~1. 5%, Mo=0.2-0.5%, Nb: 0.005-0.05%, Ti: 0.005-0.03%, B: 0. 0 0 0 5~O. O O 3%
, A7: 0. 10% or less, N: 0. 0 0 6% or less, and Ni: 1. 0% or less, Cu: 1. 0% or less, V: 0. 1% or less, Ca: 0.001 to 0.005%, or two or more of them, and the balance is Fe and unavoidable impurities.
After heating to a temperature of 800 to 950 °C, the steel plate is cooled to a temperature of 300 °C or less by air cooling, then heated to a temperature of Ac+ to Acs, and then rapidly cooled to a temperature of 400 °C.
A method for producing a low yield ratio high tensile strength steel sheet, which comprises tempering at a temperature of ~580°C and then air cooling.

(Function) It is known that the yield ratio of steel generally increases as its strength increases. In other words, the higher the strength, the more difficult it is to achieve a low yield ratio. Therefore, in order to obtain a high-strength steel plate with an excellent low yield ratio, it is essential to select appropriate precepts and manufacturing conditions. Become.

In the present invention, we have clarified the precepts and manufacturing conditions as necessary conditions for ensuring the strength and toughness of the base material as a high tensile strength steel plate of 70 kgf/1 or more and achieving a low yield ratio.

According to the research of the present inventors, in order to realize a high-strength steel sheet with an excellent low yield ratio, the microstructure should be made with a homogeneous upper bainite (bainitic ferrite) phase, It is necessary to have a mixed structure containing a finely dispersed tempered martensite phase of a certain size. In general, it is known that it is effective to quench and temper steel from a temperature between A C H point and A C point in order to obtain a mixed structure containing a martensitic phase. In order to obtain a martensitic phase having a state, it is essential to appropriately control the structure before quenching. To this end, it is essential to optimize the chemical properties of the steel and the rolling conditions, and it is also necessary to pay attention to the tempering temperature.

The reasons for limiting the chemical precepts in the present invention will be explained below.

C is 0.08 to ensure hardenability in the hardening treatment at temperatures between Ac and Ac3 and to obtain martensitic phase.
It is necessary to add more than %. However, excessive addition of C causes deterioration of weldability, so the upper limit has been set to 0.
．． It was set at 18%.

St is an element contained in deoxidized steel, but its excessive addition inhibits weldability and HAZ toughness. Therefore, the upper limit is set to 0. It is necessary to set it to 6%.

Ni is a useful element for ensuring strength, toughness, and hardenability, and needs to be added in an amount of 0.5% or more. But M
If the amount of n is too large, it will cause deterioration of weldability, HAZ toughness, or temper embrittlement, so the upper limit was set at 1.6%.

Cr is an element that increases the strength of the base material, and needs to be added in an amount of 0.4% or more. However, if the Cr content exceeds 1.5%, weldability and HAZ toughness deteriorate, so the upper limit was set to 1.5%.
．． It was set at 5%.

Mo is an element that improves both the strength and toughness of the base material,
There is no effect unless 0.2% or more is added.

However, if it exceeds 0.5%, the weld toughness and weldability will deteriorate, which is undesirable, so the upper limit should be set at 0.5%. It was limited to 5%.

Nb is an effective element for improving the strength and toughness of the base metal, and in particular, in the present invention, rolling in the non-recrystallized region is used to obtain a martensitic phase of an appropriate size while cooling in air after rolling, and to improve the strength and toughness of the base metal. It is an essential element to ensure toughness. The amount needs to be 0.005% or more, but since excessive addition of Nb deteriorates the HAZ toughness, the upper limit should be set at 0.05%.
It needs to be %.

Ti is useful in suppressing the coarsening of austenite grains during heating and ensuring the toughness of the base material, and its addition in small amounts is
It is also effective in improving AZ toughness. However, o. oos%
Addition without grooves has no effect, and addition of more than 0.03% leads to deterioration of HAZ toughness due to precipitation hardening of TiC, so the addition amount is reduced to 0.03%. 005~0. It was limited to 0.3%.

B is an important element for improving hardenability and ensuring the strength and toughness of the base metal. In particular, in the present invention, it is necessary to obtain a homogeneous upper bainite (bainity fukuferrite) phase containing an appropriately sized martensite phase while air-cooling after rolling to ensure strength, and the addition of B is essential. . Addition of less than 0.0005% of B has no effect, and addition of more than 0.003% significantly deteriorates hardenability, so the amount added is limited to 0.0005 to 0.003%.

M is an element that is generally included in deoxidized steel, but deoxidation is also performed by St, Mn, or Ti.
In the present invention, there is no lower limit for M. However, M
If the amount increases, the cleanliness of the steel will deteriorate and the HAZ toughness will deteriorate, so the upper limit was set at 0.10%.

N is generally included in steel as an unavoidable impurity, but N
Since excessive addition of C leads to deterioration of HAZ toughness, the upper limit was set at 0.006%.

Note that P and S are included in steel as unavoidable impurities. In the present invention, the amount is not limited to 3%, but since these deteriorate the toughness of the base metal and welded part, it is preferable that the amount is as small as possible, and 0.03% or less and 0.03% or less, respectively.
．． It is desirable that the content be 01% or less.

In the steel sheet of the present invention, if necessary, Ni: 1.0
% or less, Cu: 1. 0% or less, V: 0. 1% or less, Ca: 0.001 to 0.005%, and one or more of them is contained.

The main purpose of containing these elements is to improve the strength and toughness of the steel sheet of the present invention and to increase the thickness of the manufactured sheet without impairing the characteristics of the steel sheet, and the amount of addition is determined based on aspects such as weldability and HAZ toughness. Therefore, it is a property that should naturally be restricted.

Ni has the property of improving the strength and toughness of the base material without adversely affecting the hardenability and toughness of HAZ.
If it exceeds 0%, it is unfavorable in terms of hardenability and toughness of the HAZ, so the upper limit was set at 1.0%.

Cu has almost the same effect as Ni, and also has corrosion resistance,
It is also effective against hydrogen-induced cracking resistance. However, 1.0%
If it exceeds this amount, Cu-cracks will occur during rolling, making manufacturing difficult. Therefore, the upper limit was set at 1.0%.

(2) has the effect of improving strength due to the form of fine carbonitrides, but addition of more than 0.1% causes deterioration of toughness, so the upper limit was set at 0.1%.

Ca controls the morphology of sulfides and increases the Charby absorption energy, resulting in low, 1! In addition to improving toughness, it is also effective in improving hydrogen-induced cracking resistance. However, if the amount of Ca is less than 0.001%, there is no practical effect;
If it exceeds 0.05%, Cab. Since a large amount of CaO is present and becomes large inclusions, which impairs not only the toughness but also the cleanliness of the steel, and also has a negative effect on the weldability, the range of the amount of Ca added is set at 0.001 to 0.005%. .

Next, a method for producing a steel plate having the above-mentioned fractional system will be described.

First, the heating temperature of the steel piece needs to be 1100 to 1250°C. If the heating temperature is less than 1100° C., solid solution of Nb becomes insufficient and strength cannot be ensured. Further, the upper limit of the heating temperature needs to be 1250° C. or lower in order to prevent coarsening of austenite and ensure base material toughness. In addition, after casting the steel strip, rolling is performed without reheating after cooling to a temperature lower than the Ar= point, that is, immediately after casting the steel strip, or immediately after passing through a resting process at a temperature of 3 points or higher after casting the steel strip. You can.

Next, the rolling end temperature needs to be 800 to 950°C. If the rolling is completed at a temperature lower than 800°C, the austenite becomes excessively fine-grained or elongated, resulting in decreased hardenability and strength cannot be ensured. Further, when rolling is finished at a temperature exceeding 950° C., the austenite grains are insufficiently refined and the toughness of the base material deteriorates. Furthermore, air cooling is essential for cooling after rolling. When water cooling (quenching) is applied after rolling, a martensite or lower bainite structure is obtained, but in such a uniform structure, carbides become extremely fine. According to the research of the present inventors, A c
．． If quenching is performed at a temperature of ~Ac3, the size of martensite in the finally obtained structure will not be large enough, making it impossible to obtain a low yield ratio. In order to produce a steel material with high strength and a low yield ratio, this as-rolled structure, i.e., before quenching from A C 1 to A C 3 temperature, must contain a martensitic phase of an appropriate size. It is necessary to form an upper bainite (peinitic ferrite) structure, and for this purpose, it is essential to cool the steel to 300''C or less by air cooling after rolling.

Next, the reason for specifying the heat treatment method after rolling will be described. After a steel plate having the above-mentioned appropriate pre-structure is obtained by rolling, quenching from a temperature of A c 1 to A c 3 and subsequent tempering are performed in order to obtain a good low yield ratio. A c
The purpose of quenching from 1 to A c 3 temperature is to reversely transform the martensitic phase obtained by rolling into γ phase and
This is to achieve a low yield ratio by further concentrating the ferrite and providing a greater strength difference with the base structure, upper bainite (bainitic ferrite). Heating at a temperature lower than A c H does not cause reverse transformation, and heating at a temperature higher than Ac3 causes the entire structure to become gamma, making it impossible to take advantage of the previous structure imparted by rolling. It was set as A c 1 to A C 3. In addition, in this process, rapid cooling (quenching) is essential because it aims to create a two-phase payne-martensite structure. The martensite in the structure thus obtained is extremely brittle and has problems with low-temperature toughness, so it is tempered. Tempering is insufficient below 400°C;
If the temperature exceeds 580°C, the strength will decrease. Therefore, the tempering temperature was set at 400 to 580°C.

Although the present invention can be applied to various diarrheal materials, it is preferable to apply it mainly to thick steel plates with a tensile strength of 70 kgf/1 or more and a plate thickness of 80 mm or less, and steel plates for steel pipes manufactured by winding these. .

(Example) Next, examples of the present invention will be described.

Table 1 shows the chemical properties of the test steels, and Table 1 (continued) shows the manufacturing conditions and mechanical properties of each steel. Table 1, Table 1 (continued)
, symbols A to E indicate examples of the present invention, and symbols F to G indicate comparative examples. Examples A to E of the present invention have a high strength of 70 kgf/1 or more and an extremely low yield ratio of 80% or less. On the other hand, in Comparative Example F, water cooling was performed immediately after rolling, so the structure before quenching from the temperature of Ac+ to AC3 had a uniform structure of martensite or lower paynite. A sufficiently low yield ratio of 80% or less cannot be obtained with the final material obtained by quenching from a temperature of 1 to A c s and subsequent tempering.

In addition, Comparative Example G does not contain Nb or B, so it contains ferrite in the as-rolled structure, and has a strength of 70 kgf/
Less than 1 is considered poor.

(Effects of the Invention) The present invention has been described in detail above, and the present invention provides a means for manufacturing an innovative high-strength steel plate that has both an excellent low yield ratio and a high strength of 70 kgf/1 or more. be.

Claims (2)

[Claims] (1) In weight%, C: 0.08 to 0.18%, Si: 0.6% or less, Mn: 0.5 to 1.6%, Cr: 0.4 to 1.5%, Mo: 0.2-0.5%, Nb: 0.005-0.05%, Ti: 0.005-0.03%, B: 0.0005-0.003%, Al: 0.10% or less, N: 0.006% or less, the balance being Fe and unavoidable impurities.
After heating to a temperature of 0 to 1250°C, and finishing rolling at a temperature of 800 to 950°C, the steel plate is cooled by air cooling to a temperature of 300°C or less, then heated to a temperature of Ac_1 to Ac_3, and then rapidly cooled to 400°C. A method for producing a low yield ratio high tensile strength steel sheet, which comprises tempering at a temperature of ~580°C and then air cooling. (2) In weight%, C: 0.08 to 0.18%, Si: 0.6% or less, Mn: 0.5 to 1.6%, Cr: 0.4 to 1.5%, Mo: 0.2-0.5%, Nb: 0.005-0.05%, Ti: 0.005-0.03%, B: 0.0005-0.003%, Al: 0.10% or less, N: 0.006% or less, plus any one of Ni: 1.0% or less, Cu: 1.0% or less, V: 0.1% or less, Ca: 0.001 to 0.005%. , or a steel piece further containing two or more kinds, with the balance consisting of Fe and unavoidable impurities.
After heating to a temperature of 1250°C and finishing rolling at a temperature of 800 to 950°C, the steel plate is cooled by air cooling to a temperature of 300°C or less, then heated to a temperature of Ac_1 to Ac_3, and then rapidly cooled to a temperature of 400 to 580°C. A method for producing a low yield ratio high tensile strength steel sheet, which comprises tempering at a temperature of 10°C and then air cooling.