JPH04314824A - Production of 70kgf/mm2 class high tensile strength steel excelent in weldability and having low yield ratio - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel excellent in weldability and having low yield ratio by subjecting a slab of a low carbon steel containing specific trace amounts of alloy components to reheating, rolling, hardening, and tempering under respectively specified conditions. CONSTITUTION:A slab of a steel having a composition which contains, by weight, 0.04-0.08% C, <0.5% Si, 0.8-1.5% Mn, <0.02% P, <0.008% S, 0.9-1.8% Cu, 0.3-2.0% Ni, 0.3-0.7% Mo, 0.005-0.030% Nb, 0.02-0.08% V, 0.005-0.020% Ti, <0.06% Al, and 0.0015-0.0060% N or further contains 0.05-0.4% Cr and/or 0.0005-0.0050% Ca and in which the value of Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B(%) is regulated to <=0.28 is reheated to 1000-1250 deg.C and rolled at >20% cumulative reduction of area at <=1050 deg.C. The rolled plate is subjected to air cooling down to ordinary temp. or to hardening from >=800 deg.C to ordinary temp. Then, the steel plate is reheated to 700-850 deg.C, hardened down to ordinary temp., reheated to <=700 deg.C, and tempered.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing high tensile strength steel (hereinafter referred to as HT70) having a low yield ratio of 70 kgf/mm2 and excellent weldability.

0002

BACKGROUND OF THE INVENTION Most of the conventional high tensile strength steels exceeding 60 kgf/mm2 class have been manufactured by quenching and tempering B-added steel. However, B-added high-strength steel has weldability of H
It was significantly inferior to T60. Therefore, during welding, preheating to about 200° C. (maintaining the temperature of the steel plate at a constant temperature during welding) is required to prevent weld cracking, resulting in a significant decrease in construction efficiency.

On the other hand, as the strength of 60 kgf/mm2 ultra-high tensile steel increases, the yield ratio (ratio of yield strength to tensile strength) inevitably increases. Regarding the weldability among the above-mentioned problems, B-free HT80 has been invented as disclosed in JP-A-02-129317.

However, although this manufacturing method can improve weldability, it cannot be applied to buildings that require a high yield ratio and earthquake resistance. In other words, the feature of the manufacturing method of the steel of this invention is that after rolling, it is reheated to the austenite region, quenched, and tempered, so that the carbides become fine and the yield strength inevitably increases, leading to an increase in the yield ratio. was. In recent years, buildings have become increasingly large in size, and there has been a strong desire for research and development of high-strength, high-strength steel plates with good weldability and low yield ratios.

[0005]

SUMMARY OF THE INVENTION The present invention provides a manufacturing technology for a low yield ratio HT70 with excellent weldability. HT70 manufactured based on the method of the present invention has a low yield ratio of the base metal, excellent strength and toughness, and moreover, under normal welding conditions,
There is less hardening of the weld heat affected zone (HAZ), making it possible to reduce preheating during welding.

[0006]

[Means for Solving the Problems] The gist of the present invention is (1) in terms of weight ratio: C: 0.04 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.02% or less, S:
0.008% or less, Cu: 0.9-1.8%, Ni: 0
．． 3-2.0%, Mo: 0.3-0.7%, Nb: 0.
005-0.030%, V: 0.02-0.08%, T
i: 0.005-0.020%, Al: 0.06% or less, N: 0.0015%-0.0060%, Pcm
=C+Si/30+Mn/20+Cu/20+Ni/6
0+Cr/20+Mo/15+V/10+5B (%) satisfies 0.28% or less, and the balance is iron and inevitable impurities, making the steel substantially free of B at 1000℃~12
Reheat to a temperature range of 50°C, roll so that the cumulative reduction below 1050°C is 20% or more, and then air cool to room temperature, or quench from a temperature of 800°C or above to room temperature after rolling. The cooled steel plate is heated to 70% in a heat treatment furnace.
A low yield ratio of 70 kgf with excellent weldability, characterized by being reheated to 0°C to 850°C, then immediately quenched from this temperature to room temperature, and then reheated to a temperature of 700°C or less and tempered. /mm2 class high tensile strength steel manufacturing method. (2) C: 0.04-0.08%, Si: 0.5% or less, Mn: 0.8-1.5%, P: 0.02% or less, S:
0.008% or less, Cu: 0.9-1.8%, Ni: 0
．． 3-2.0%, Mo: 0.3-0.7%, Nb: 0.
005-0.030%, V: 0.02-0.08%, T
i: 0.005-0.020%, Al: 0.06% or less, N: 0.0015%-0.0060%, and Cr: 0
．． 05-0.4%, Ca: 0.0005-0.0050
%, Pcm=C+Si/30
+Mn/20+Cu/20+Ni/60+Cr/20+
Mo/15+V/10+5B (%) satisfies 0.28% or less, the balance is iron and inevitable impurities, and substantially B-free steel is reheated to a temperature range of 1000 ° C. to 1250 ° C., A steel plate that is rolled so that the cumulative reduction at 1050°C or less is 20% or more, and then air cooled to room temperature, or quenched from a temperature of 800°C or more to room temperature after rolling, is heated to 700°C or more in a heat treatment furnace. A low yield ratio of 70kgf/mm2 class high with excellent weldability, characterized by being reheated to 850℃, then quenched from this temperature to room temperature, and then reheated to a temperature of 700℃ or less and tempered. Method of manufacturing tension steel.

The present invention will be explained in detail below. According to the research conducted by the inventors, conventional steel compositions of HT60 or higher grade have basically been B-added or relatively high C-added systems due to the need to ensure the strength and toughness of the base metal. However, B addition and high C content have extremely poor weldability, and for this reason, HT6
Compared to grade 0 steel, welding efficiency was significantly reduced, and improvements were needed. Also, JP-A-02-12931
The invention steel of Publication No. 7 has a strength of HT80 and has good weldability. However, since the steel of this invention is hardened from the austenitic region to ensure strength, it has been difficult to apply it to architectural structures that require high yield strength and earthquake resistance.

[0008] The present inventors have intensively studied the possibility of developing HT60 grade steel that has both good weldability and low yield ratio, and found that the low yield ratio steel with good weldability has a strength of 70 kgf/mm2.
We found that this can be achieved by doing the following. In the present invention, 1) the amount of C is reduced and no B is added to ensure good weldability, and the Pcm value of the steel components is suppressed to 0.28% or less. 2)
Utilizes precipitation hardening of Cu, Nb, and V to ensure base material strength. 3) Achieving a low yield ratio was achieved by reheating and quenching to the austenite-ferrite two-phase coexistence region. Above 3
This point is the gist of the present invention.

[0009] This will be explained in detail below. In order to ensure good weldability, it is extremely important not to add B and to reduce the amount of C. In addition to this, the total of the component elements needs to be 0.28% or less in Pcm. Within this limited range, under normal welding conditions, the weld heat affected zone (HAZ) will be less hardened, and preheating during welding can be reduced.

Furthermore, in order to reduce the amount of C without adding B, it is essential to add Cu, Nb, and V to improve the strength of the base material. These are known as precipitation hardening elements and exhibit their effects under appropriate manufacturing conditions. In the steel of the present invention, steel with predetermined components is reheated to 1000°C to 1250°C, Cu, Nb
, V are completely dissolved in solution, and in order to prevent precipitation as much as possible during subsequent rolling, it is necessary to avoid rolling in a low temperature range (desirably, rolling is completed at 780° C. or higher). However, in order to ensure the toughness of the base metal, it is necessary to refine the grains by rolling to a certain extent.
The reduction amount below 050°C is required to be 20% or more (preferably range 30 to 70%).

Furthermore, in order to obtain a low yield ratio, it is essential to harden from the austenite-ferrite two-phase coexistence region. Although this treatment yields lower strength than the generally practiced hardening directly above the austenite, a low yield ratio can be obtained. That is, in the two-phase coexistence state of reheating, C concentration occurs from ferrite to austenite,
This results in an austenite phase with enriched C and a ferrite phase with reduced C. By quenching from this state, a two-phase mixed structure consisting of a phase with extremely fine carbides and a phase with coarse carbides is created. A reduction in yield ratio is achieved by this two-phase mixed structure.

However, the purpose cannot be achieved unless the reheating temperature and quenching temperature are appropriate. 850
If the steel is quenched at a temperature exceeding ℃, the proportion of ferrite phase will be small, and no effect of reducing the yield ratio can be expected. Also, 700℃
If the hardening is done from below, the strength will decrease and the purpose cannot be achieved. Furthermore, re-tempering is necessary to decompose martensite and restore toughness as stable carbide, but 7
At 00°C, carbides become abnormally coarse and strength cannot be ensured, so the temperature was set at 700°C or lower.

The main points of the present invention have been described, but in order to obtain a low yield ratio HT70 with excellent weldability, it is necessary to control the basic components within appropriate ranges. This point will be explained below. The lower limit of 0.04% of C is the minimum amount to ensure the strength of the base metal and welded part and to exhibit the effect of V. However, if the amount of C is too large, weldability deteriorates, so the upper limit was set at 0.08%. Since adding a large amount of Si deteriorates weldability and HAZ toughness, the upper limit was set to 0.5%. Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.8%. However, if the amount of Mn is too large, hardenability increases and weldability and HAZ toughness deteriorate, so the upper limit was set at 1.5%.

[0014] The reason why the upper limits of impurities P and S in the steel of the present invention are set to 0.02% and 0.008% is that
This is to further improve HAZ toughness. The purpose of reducing the amount of P is to prevent grain boundary fracture during tempering, and the purpose of reducing the amount of S is to prevent deterioration of toughness due to MnS. Cu is an important element in order to suppress deterioration of weldability and ensure base metal strength. However, if the amount added exceeds 1.8%, H
Since it impairs AZ toughness, the upper limit was set at 1.8%. Furthermore, since the amount of C in the components is kept low, the lower limit of the amount of Cu was set at 0.9% to ensure strength.

[0015] Ni has little adverse effect on weldability, improves strength and toughness, and is also effective in preventing Cu cracks. However, if it exceeds 2.0%, it is not favorable for weldability, so the upper limit was set at 2.0%. Moreover, if it is less than 0.3%, the effect is small, so the lower limit was set at 0.3%. Mo is an element that improves both the strength and toughness of the base material, and 0.3% or more is essential. However, too much will deteriorate weldability, so
The upper limit was set at 0.7%. Nb is an important element for ensuring the toughness of the base material, and its lower limit is 0.005%. In addition, if the amount added is too large, it will not only deteriorate the toughness of the base material but also the HAZ toughness, so the upper limit should be set at 0.030.
%.

V is important for ensuring the strength of the base material, and its lower limit is 0.02%. Further, if it exceeds 0.08%, HAZ toughness is impaired, so 0.08% is set as the upper limit. When the amount of Al is small, Ti combines with O to form Ti2 O3
The HAZ toughness is improved by forming an oxide whose main component is It also combines with N to form TiN, which suppresses coarsening of austenite grains during reheating and is effective in refining the structure after rolling. Minimum 0.0 to obtain these effects
05% is required. However, if the amount is too large, TiC will form and damage the base material toughness and HAZ toughness, so the upper limit should be set at 0.02.
%.

[0017] Al is an element generally contained in deoxidized steel, but Si or Ti alone is sufficient for deoxidation, and the lower limit is not limited. However, when the amount of Al increases, not only does the cleanliness of the steel deteriorate, but also the toughness of the weld metal welded using this steel deteriorates, so the upper limit was set at 0.06%. N is contained in steel as an unavoidable impurity, but it combines with Nb to form carbonitrides to improve toughness, and also forms TiN to improve the properties of HT70 as described above. Therefore, it is necessary to add at least 0.0015%. However, since an increase in the amount of N is harmful to HAZ toughness, the upper limit was set to 0.0060%.

Next, the reason for adding Cr and Ca will be explained. The purpose of adding these elements to the basic ingredients is to improve the strength and toughness of the steel of the present invention without impairing its characteristics. Cr increases the strength of the base metal and the welded part, but if it is present too much, it significantly deteriorates weldability and HAZ toughness. Therefore, the upper and lower limits are 0.05% and 0.05%, respectively.
It was set at 0.4%. Ca controls the morphology of sulfides and improves the toughness of the base material. However, if the amount of Ca is less than 0.0005%, it has no practical effect, and if it exceeds 0.005%, C
A large amount of aO and CaS are generated and become large inclusions, which reduce toughness. For this reason, the upper and lower limits of the addition amount are set to 0.
005% and 0.005%. As mentioned before, steel containing Cu, Nb, and V without B additive was heated at 700°C to 850°C.
By reheating to ℃ and quenching, followed by tempering, it became possible to manufacture a steel plate with a low yield ratio of HT70 kgf/mm2 and excellent weldability.

[0019]

[Example] Manufacturing various steel plates through converter, continuous casting, plate and heat treatment processes, and measuring the strength, toughness, and HAZ hardness of the base metal under welding conditions of low heat input (standard conditions for manual welding) We conducted a survey on the following. Table 1 shows the chemical composition of the invention steel and comparison steel, and Table 2 shows the manufacturing process of the steel plate, the strength and toughness of the base metal, and the measurement results of HAZ hardness under standard welding conditions.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

Steels 1 to 10 in Table 1 show the chemical compositions of the steels of the present invention, and Steels 10 to 20 show the chemical compositions of comparative steels. Also, Table 2
The measurement results of base metal strength, toughness, and HAZ hardness under standard welding conditions are shown for Steels 1 to 10 of the invention steels and Steels 11 to 20 of the comparative steels. The Pcm value of the invented steel is controlled to 0.28% or less, so the standard welding heat input is 17 kJ/cm.
Under these conditions, the maximum value of HAZ hardness was 313 or less. Furthermore, both the strength and toughness of the base material were sufficient for HT70, and the yield ratio was a low value of 83% or less.

On the other hand, the comparative steel Steel 11 has an inappropriate manufacturing method, and although the strength and toughness of the base metal are sufficient, the yield ratio exceeds 85%, and the C content and Pcm are high. The HAZ hardness was as high as Hv386, and the weldability was also insufficient. Similarly, Comparative Steels 12, 13, 14, 15, and 16 had inappropriate manufacturing methods, high C content and high Pcm, so the yield ratio of the base metal was high, and the HAZ hardness was 354 or higher, resulting in insufficient weldability. there were. Furthermore, in Comparative Steel 17, although the manufacturing method was appropriate, the amount of Cu was insufficient, so the yield strength was insufficient. In Comparative Steel 18, although the manufacturing method was appropriate, the yield strength was insufficient due to the insufficient amount of Mo. Comparative Steel 19 was manufactured using an appropriate manufacturing method, but had insufficient yield strength due to insufficient C content. Furthermore, although the manufacturing method for Comparative Steel 20 was appropriate, the yield strength was insufficient because no V was added.

[0026]

[Effects of the Invention] According to the present invention, it has become possible to manufacture a low yield ratio HT70 with excellent base material strength, toughness, and weldability.

[0027] Compared with conventional HT60 super grade steel, it is expected that welding efficiency will be significantly improved and the safety of structures will be significantly improved.

The thick steel plate produced by this method can be used in welded structures such as building structures that require earthquake resistance (prevention of collapse of buildings due to earthquakes) and good weldability.

Claims (2)

[Claims] Claim 1: In weight ratio, C: 0.04 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.02% or less, S: 0. 008% or less, Cu: 0.9-1.8%, Ni: 0.3-2.0%, Mo: 0.3-0.7%, Nb: 0.005-0.030%, V: 0.02-0.08%, Ti: 0.005-0.020%, Al: 0.06% or less, N: 0.0015%-0.0060%, Pcm=C+S
i/30+Mn/20+Cu/20+Ni/60+Cr
/20+Mo/15+V/10+5B (%) is 0.28
% or less, the balance is iron and unavoidable impurities, and the substantially B-free steel is reheated to a temperature range of 1000°C to 1250°C so that the cumulative reduction at 1050°C or less is 20% or more. The steel plate that has been rolled and then air-cooled to room temperature, or quenched from a temperature of 800°C or higher to room temperature after rolling, is reheated to 700°C to 850°C in a heat treatment furnace, and then immediately heated from this temperature to room temperature. A low yield ratio of 70 kgf/mm2 with excellent weldability characterized by quenching, then reheating to a temperature of 700°C or less and tempering.
Manufacturing method for grade high tensile strength steel. 2. The low yield ratio with excellent weldability according to claim 1, which contains one or both of Cr: 0.05 to 0.4% and Ca: 0.0005 to 0.0050% in weight ratio. 70
Production method of kgf/mm2 class high tensile strength steel.