JPH04314825A - Production of 80kgf/mm2 class high tensile strength steel excellent in weldability - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel excellent in weldability by subjecting a slab of a steel containing specific trace amounts of alloy components and practically free from B to rolling, hardening, and tempering under respectively specified conditions. CONSTITUTION:A slab of a steel having a composition which contains, by weight, 0.03-0.06% 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, 0.4-0.7% Mo, 0.005-0.040% 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 0.0005-0.005% Ca and in which the sum of 59.3C(%) and Cu(%) is regulated to <=42% is reheated to 950-1150 deg.C, rolled so that cumulative reduction of area at <=1000 deg.C becomes >=40%, hardened from >=850 deg.C, and reheated to <=700 deg.C to undergo tempering treatment.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a 80 kg machine with excellent weldability.
The present invention relates to a method for manufacturing f/mm2 class high tensile strength steel.

0002

BACKGROUND OF THE INVENTION Most conventional 80 kgf/mm2 class high tensile strength steels (hereinafter referred to as HT80) have been produced by quenching and tempering B-added steel. However, the weldability of B-added HT80 was significantly inferior to that of HT60. 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. In order to improve the drawbacks of B-added HT80, recently, JP-A-2
B-free HT80 as disclosed in Japanese Patent No. 129317 has been invented. However, with this manufacturing method, hardening due to welding was unavoidable under severe welding conditions. In other words, if the cooling rate after welding is fast (welding heat input is small and the steel plate to be welded is thick), hardening due to welding is unavoidable, and a high preheating temperature is essential to prevent weld cracking. Ta. For this reason, research and development of HT80, which has excellent weldability under severe welding conditions, has been strongly desired.

0003

SUMMARY OF THE INVENTION The present invention provides a manufacturing technique for HT80 having excellent weldability. HT80 manufactured based on the method of the present invention has less hardening of the weld heat affected zone (HAZ) even under severe welding conditions where the cooling rate after welding is fast, and it is possible to reduce preheating during welding work.

0004

[Means for Solving the Problems] The gist of the present invention is that C: 0.03 to 0.05%, Si: 0.5% or less, Mn
: 0.8-1.5%, P: 0.02% or less, S: 0.0
08% or less, Cu: 0.9 to 1.8%, Ni: 0.3 to
2.0%, Mo: 0.4-0.7%, Nb: 0.005
~0.040%, V:0.02~0.08%, Ti:0
．． 005-0.020%, Al: 0.06% or less, N:
0.0015-0.0060%, further Cr as necessary
:0.05~0.4%, Ca:0.0005~0.00
Contains 5% of one or two types, 59.3C (%) + C
A steel that satisfies a sum of u (%) of 4.2 (%) or less and the balance is iron and inevitable impurities and is substantially free of B is reheated to a temperature range of 950 ° C. to 1150 ° C., 100
After rolling so that the cumulative reduction at 0°C or lower is 40% or more, it is quenched at a temperature of 850°C or higher, and then reheated to a temperature of 700°C or lower for tempering. This is a method for manufacturing 80kgf/mm2 class high tensile strength steel with excellent weldability.

[0005]

[Operation] The present invention will be explained in detail below. According to the inventors' research, in order to fundamentally solve the weldability of conventional HT80, it was essential to reduce the amount of C in the steel and eliminate the addition of B. However, reducing the amount of C and not adding B were both important alloying elements to ensure the strength of the base material. For this reason, we conducted extensive research into ways to satisfy base metal strength and good weldability, and were able to invent a new steel that is completely different from conventional ideas.

In the present invention, 1) Precipitation hardening of Cu, Nb, and V is used to reduce the amount of C and reduce the strength of the base material by not adding B, and 2) In addition, hardness has traditionally had a large influence on the occurrence of weld cracking. However, the hardness limit required to suppress cracking is Hv350.
It was known that the extent of Therefore, it is necessary to control the composition so that the hardness of the HAZ is at most Hv350 or less even under severe welding conditions. It has been found that in this class of HT80, under severe welding conditions, the HAZ microstructure becomes mostly martensite, and its hardness is determined by the sum of the C content and the Cu content.

According to conventional knowledge, it has been known that the hardness of most martensite is approximately Hv=800C(%)+293. The inventors found that in steels with a Cu content of 0.8% or more, the hardness of martensite is mostly due to C content other than the C content.
It was found that it also depends on the amount of u added. Expressing this result mathematically, Hv (hardness of almost martensite) = 80
0C (%) + 13.5Cu (%) + 293.

From these studies, it has been found that under severe welding conditions, the hardness value can be kept below a certain level (Hv 350 or below) by setting the sum of 59.3C+Cu to 4.2 or below. 3) Furthermore, the strength of the base material is increased by Cu
, Nb, V, etc., it has been difficult to ensure the toughness of the base metal because the hardenability of the steel components is low. In order to solve this problem, the manufacturing method must be appropriate as well as restricting the additive elements. Therefore, it is necessary to limit the conditions for reheating, rolling, and cooling the steel (slab). The reheating temperature must be regulated from both the viewpoints of solid solution of Nb, V, etc. and coarsening of austenite grains during heating. 950℃
If it is less than 950° C., almost no Nb will be dissolved in solid solution, V will not be sufficiently dissolved, and the strength of the base material will be insufficient, so the lower limit was set at 950°C. Further, at temperatures exceeding 1150°C, Nb and V are sufficiently dissolved and the strength of the base material is sufficient, but the austenite grains become coarse and the toughness of the base material deteriorates significantly, so the upper limit was set at 1150°C.

[0009] Next, the cumulative reduction amount below 1000°C is 40
% or more. For refinement of austenite grains by rolling, a cumulative reduction of 1000° C. or less is important, and the lower limit thereof is set to 40% (preferably range 45 to 70%). Furthermore, in order to obtain the strength and toughness of HT80, quenching is performed at a temperature of 850°C or higher.
It is necessary to perform the tempering treatment at a temperature of 00°C or lower.

Although the regulation of some important components and the appropriate manufacturing method have been described, in order to produce HT80 with excellent weldability, it is necessary to regulate the basic components within appropriate ranges. This point will be explained below.

The lower limit of 0.03% of C is the minimum amount in order to ensure the strength of the base metal and the welded part and to exhibit the effects of Nb, V, etc. However, if the amount of C is too large, weldability deteriorates, so the upper limit was set at 0.06%. Moreover, as mentioned above, the upper limit of the C content needs to be regulated in relation to the Cu content, and the sum of 59.3C (%) + Cu (%) is 4.2 (%).
The following shall apply. From the viewpoint of weldability, the amount of C is desirably 0.05% or less.

[0012] Adding a large amount of Si deteriorates weldability and HAZ toughness, so the upper limit was set at 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%.

[0013] 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 Z 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.

[0014] Cu is an important element in order to suppress the deterioration of weldability to a minimum and ensure the strength of the base metal. However, 1
．． If the amount added exceeds 8%, HAZ toughness will be impaired, so 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. Furthermore, in order not to deteriorate weldability, the upper limit of the Cu content must be regulated in relation to the C content, and Cu (%) +
The sum of 59.3C (%) shall be 4.2 (%) or less.

[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%.

[0016] Mo is an element that improves both the strength and toughness of the base material, and its content of 0.4% or more is essential. However, since too much content deteriorates weldability, the upper limit was set at 0.7%. Nb is an important element for ensuring the strength and toughness of the base material, and its lower limit is 0.005%. In addition, if the amount added is too large, the HAZ toughness will deteriorate, so the upper limit is set to 0.040.
%. V is important to ensure the strength of the base material,
0.02% is the lower limit. Further, if it exceeds 0.08%, HAZ toughness is impaired, so 0.08% is set as the upper limit.

[0017] When the amount of Al is small, Ti combines with O and becomes T.
Forms an oxide mainly composed of i2O3 to improve HAZ toughness. 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. A minimum content of 0.005% is required to obtain these effects. However, if there is too much, TiC will form and will harm the base material toughness and HAZ toughness.
The upper limit was set at 0.02%.

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 increase strength, and also forms TiN to form HT8 as mentioned above.
Enhance the nature of 0. However, for this reason, the minimum
% addition is required. However, the increase in the amount of N
Since it is harmful to AZ 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 components 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. For this reason, the upper and lower limits are each 0.05%,
It was set at 0.4%. Ca controls the morphology of sulfides and improves the toughness of the base metal. 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%.

[0021]

[Example] Various types of steel were manufactured using a converter-continuous casting-thick plate process, and investigations were conducted to measure the strength, toughness, and HAZ hardness of the base metal under severe welding conditions. Table 1 shows the chemical composition of the inventive steel and comparative steel, and Table 2 shows the manufacturing process of the steel plate and the strength of the base material.
The measurement results of toughness and HAZ hardness under severe welding conditions are shown.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[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 severe welding conditions are shown for Steels 1 to 10 of the present invention steels and Steels 11 to 20 of comparative steels.

[0027] The steel of the present invention has controlled amounts of C and Cu, and
The sum of 9.3C+Cu is less than 4.2. Therefore, even under the severe conditions of welding heat input of 10 kJ/cm, the maximum value of HAZ hardness was less than 350. Furthermore, both the strength and toughness of the base metal had sufficient properties as HT80.

On the other hand, comparative steels 11 to 17 have sufficient base metal properties, but have a high C content in their chemical components, and
Since the sum of C+Cu was high, the HAZ hardness far exceeded Hv350 and weldability was insufficient. Also, comparative steel 18-2
0, the HAZ hardness was less than 350, but steel 18 had a HAZ hardness of less than 350.
The strength of the base material did not reach 80 kg due to insufficient amounts of n and Mo. Steel 19 has a base metal strength of 8 due to the small amount of Cu.
It didn't reach 0km. Furthermore, since Nb was not added to Steel 20, the base metal strength did not reach 80 kg.

[0029]

Effect of the invention: The present invention provides HT80 with excellent weldability.
became possible to manufacture. Compared to the conventional HT80, it can be expected to significantly improve welding efficiency and the safety of structures. Therefore, the thick steel plate produced by the method of the present invention can be used for welded structures used in harsh environments such as building structures, pressure vessels, and marine structures.

Claims (2)

[Claims] [Claim 1] In weight ratio: C: 0.03 to 0.06%, 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: 0.4-0.7%, Nb: 0.005-0.040%, V : 0.02 to 0.08%, Ti: 0.005 to 0.020%, Al: 0.06% or less, N: 0.0015 to 0.0060%, 59.3C (%) + Cu (%) The sum of 4.2 (%) or less, the balance being iron and unavoidable impurities, and substantially B-free steel is reheated to a temperature range of 950°C to 1150°C to achieve a cumulative reduction of 1000°C or less. The 80kgf/80kgf/100% weldable steel sheet with excellent weldability is characterized by being rolled so that the mm2
Manufacturing method for grade high tensile strength steel. 2. The 80 kgf excellent weldability according to claim 1, which contains one or two of Cr: 0.05 to 0.4% and Ca: 0.0005 to 0.005% in weight ratio. /mm2 class high tensile strength steel manufacturing method.