JPS6350427A - Manufacture of thick high-tensile steel plate excellent in toughness at low temperature and weldability - Google Patents

Abstract

PURPOSE:To obtain the titled steel plate in which security of high strength under low carbon equivalent, security of superior toughness at low temp. in the steel including the center of plate thickness, and improvement in weldability are simultaneously enabled, by specifying steel components and also by limiting the heating, rolling, and cooling conditions of a slab. CONSTITUTION:A steel consisting of, by weight, 0.03-0.20% C, 0.05-0.60% Si, 0.50-2.50% Mn, 0.001-0.10% Nb, 0.005-0.1% Al, and the balance Fe with inevitable impurities is heated to 900-1,150 deg.C. After heating, the steel is cooled after rolled to a thickness of intermediate stage or is cooled in a state of a slab without rolling, and the steel then is reheated before the temp. in the center of plate thickness drops below the Ar3 point, so that surface temp. of <=Ar3 point is subjected to temp. raise to Ar3 point - 1,150 deg.C. After that, rolling is resumed and rolling is applied to the whole plate thickness at a temp. between Ar3 point and (Ar3+100) deg.C at 50-70% draft. After rolling, the plate is cooled at 1-10 deg.C/sec cooling rate down to 250-600 deg.C and successively air-cooled, so that above-mentioned steel plate can be manufactured.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing thick high-strength steel plates with excellent low-temperature toughness and weldability, and in particular, a method for manufacturing thick high-strength steel plates with excellent low-temperature toughness and weldability. The present invention relates to a method for producing a thick high-strength steel plate that is uniform in the thickness direction and has excellent low-temperature toughness.

[Conventional technology]

In recent years, energy development has become more polarized and deeper into the ocean, and the marine structures used are becoming significantly larger year by year, and the use of ice-breaking tankers and the like is required for efficient energy transport. The steel materials used for these are required to be thick and have extremely good low-temperature toughness. However, as the plate thickness increases, the difference in material properties in the thickness direction increases, and the mechanical properties of the central part of the plate are inferior to other parts. In particular, the deterioration of low-temperature toughness is significant. Furthermore, since the center of the plate thickness has the maximum confining stress and tends to become the starting point of fracture, it is necessary to have excellent low-temperature toughness from the surface to the center of the plate thickness.

Furthermore, ensuring the safety of these huge structures is an important issue, and it is necessary to keep the carbon equivalent low in order to improve weld cracking resistance, weld joint toughness, etc.

In recent years, various material improvement techniques that combine controlled rolling and controlled cooling have been studied and proposed as a means to reduce carbon equivalent and obtain high strength and toughness. The one described in Publication No. 169019 is known. However, the technology described in the above publication targets line pipes and general shipbuilding materials, as well as relatively thin plates with a thickness of 50 m1 or less. The material is originally relatively uniform.

[Problem that the invention seeks to solve]

However, when the plate thickness increases to 50 Tsuru or more, the difference in material properties increases in the thickness direction, and the toughness particularly at the center of the plate thickness decreases significantly. One of the reasons for this is that in conventional heating and rolling methods, as shown in Figure 2, the steel plate temperature decreases over time after being heated to 900 to 1150°C in a heating furnace, through rough rolling, and then to finish rolling. , there is a large temperature difference between the center of the plate thickness (+At) and just below the surface, and especially when starting rolling in the non-recrystallized area, the temperature difference between the surface and the center of the plate becomes large, and the center of the plate is recrystallized. Otherwise, rolling may occur on the high temperature side of the non-recrystallized region. For this reason, the toughness at the center of the sheet thickness is lower than that at the % where the best non-recrystallized region low-temperature rolling has been achieved. On the other hand, it is possible to improve the low-temperature toughness at the center of the sheet thickness by lowering the rolling temperature, but if the temperature at the surface side drops too much, the ferrite that has transformed and precipitated will have to be processed, and the low-temperature toughness at the surface side will decrease. descend. Therefore, it has been desired to develop a technology that provides excellent low-temperature toughness over the entire thickness direction.

[Means for solving problems]

The present invention advantageously solves the above-mentioned problems, and even if 50t
The above aims to provide a manufacturing method capable of homogenizing and improving toughness over the entire thickness direction of a thick high-tensile steel plate having a tensile strength of 50 kgf/fin 2 or more.

In order to achieve the above object, the present invention provides (11 weight ratio of CF 0.03 to 0.20%, Si: 0.05 to 0.6
0%, Mn: 0.50-2.50%, Nb:
0.001-0.10%, AA: 0.005-0
．． Steel containing 1% Fe and unavoidable impurities is heated to 900-1150°C and rolled to an intermediate thickness, then the rolling is stopped and cooled, or the steel is left in a slab state without being rolled. Before the central part of the plate thickness becomes Ar= or less, reheat it, and the surface part where Ar is below Ar3 is heated to Ac3.
The temperature was raised to ~1150°C, then rolling was restarted, and rolling was performed at an Ar3+b ratio of 50 to 70% over the entire plate thickness during rolling, and after rolling, the cooling rate was 1 to 1.
A method for producing a thick high-strength steel plate with excellent low-temperature toughness and weldability, characterized by cooling to 250 to 600°C at a rate of 0°C/sec and then air cooling.

(2) C: 0.03-0.20%, Si: 0.05-0.6 in weight ratio
0%, Mn: 0.50-2.50%, Nb:
0.001-0.10%, Af: 0.005-0.1%
is the basic component, furthermore, Cr: 1.0% or less, Mo
Contains one or more of the following: 1.0% or less, V: O, t% or less, Cu: 2.0% or less, and further Ni: 4.0%.
% or less, Ti: 0.15% or less, Ca: 0.01% or less, and the balance consists of Fe and unavoidable impurities. After rolling until the point, the rolling is stopped and cooled, or the slab is left unrolled and reheated before the central part of the plate becomes Ar3 or less, and the surface area where Ar3 or less is Ac = ~ The stomach was heated to 1150°C, then rolling was restarted, and Ar, +100°C to A was applied over the entire plate thickness during rolling.
A thick high-strength steel plate with excellent low-temperature toughness and weldability characterized by rolling at a temperature of r3 with a reduction rate of 50 to 70%, and after rolling, cooling to a cooling rate of 1 to b °C and then air cooling. Production method.

The main points are as follows.

As mentioned above, conventionally, in thick conditioned plates with a thickness exceeding 50 nm, differences in material properties, especially differences in low-temperature toughness, occur in the thickness direction because it involves a controlled rolling process that controls the rolling temperature. This has been considered an unavoidable phenomenon.

However, the inventors conducted various studies on the factors behind such differences in toughness in the sheet thickness direction, and found that the surface layer where the center of the sheet thickness was close to Ar3 (or below Ar3) before or during rolling. It has been discovered that by forcibly heating the material to Ac3 or higher and rolling it, even after the subsequent controlled rolling and controlled cooling, uniform and excellent mechanical properties in the thickness direction, especially low temperature toughness, can be obtained, and the present invention has been made. It was constructed based on this knowledge.

Next, the reason for limiting the components in the present invention will be described.

C is an element that increases strength at a low cost, and is required in an amount of 0.03% or more to ensure strength, but since adding a large amount impairs the toughness and weldability of the steel, the upper limit is set at 0.20%.

0.05% or more of Si is necessary for deoxidizing the steel, but if it increases, weldability will be impaired, so the upper limit is set at 0.60%.

Mn is required at 0.50% or more to ensure strength, but
The upper limit is set at 2.5 because if it increases, the weldability and toughness will deteriorate.
Set to 0%.

Nb is required in an amount of 0.001% or more to prevent coarsening of austenite grains, suppress recrystallization, and ensure strength.
If the amount increases, weldability will be impaired, so the upper limit is set at 0.10%.

Ar is required to be 5% or more of O and OO for deoxidation, but if the amount is too large, the toughness will drop significantly, so the upper limit is set at 0.1%.

In the present invention, in addition to the above-mentioned basic components, one or more of the following elements can be selectively added depending on the required properties of the steel.

Cr is an element that is useful for improving hardenability and increasing strength, but if it increases, it impedes toughness and weldability, so it is limited to 1.0% or less.

Mo is an element that is useful for improving hardenability and increasing strength, but if it increases, it decreases weldability and toughness, so 1.0%
The following shall apply.

2. Cu is an element useful for increasing strength, but if it increases, it can cause cracks during hot working and impair weldability.
0% or less.

(2) is an element useful for increasing strength through precipitation hardening, but if it increases, it inhibits weldability, so it should be kept at 0.1% or less.

Ni is an element useful for improving toughness, but since it is an expensive element, its content is set to 4.0% or less.

Ti is an element that is useful for preventing coarsening of austenite grains and ensuring toughness, and is also useful for increasing strength through precipitation hardening, but since Ti is an element that is useful in increasing strength by precipitation hardening, it is limited to 0.1% or less since it inhibits weldability when it is in a large amount.

Ca is effective in improving the material quality in the Z direction by controlling the morphology of sulfides in steel, but when the amount increases, inclusions in the steel increase and the toughness and
Because it impairs weldability, the content should be 0.01% or less.

Among these additive elements, V, Cu, Cr, Mo
is an element that is mainly useful for increasing strength, and one or more of these elements may be added as necessary. Further, Ti, Ni, and Ca are elements mainly useful for improving toughness, and one or more of them may be added as necessary.

Next, the reasons for limiting the heating, rolling, and cooling conditions will be described.

The heating temperature is 1150℃ to refine the austenite grains.
The following low-temperature heating is preferred; however, if the temperature is too low, the precipitation hardening elements will not form a solid solution, so the heating temperature should be 900°C or higher, but from the viewpoint of strength and toughness, a temperature in the range of 950 to 1050°C is most preferable.

After heating at these temperatures, the sheet is rolled to an intermediate thickness, and then the rolling is stopped and cooled, or the sheet is cooled in the slab state without rolling, and then reheated before the center thickness of the sheet reaches Ar3 or less. That is, as shown in Fig. 1, reheating is carried out after extraction in a heating furnace to reach an intermediate stage thickness, or after cooling in the slab state without rolling, the surface layer cooled to Ar3 or less is reheated to Ac=~1150°C. It raises the temperature to .

The reason why the upper limit of reheating is set at 1150° C. is that if the temperature is higher than this, the austenite becomes coarse.

With this heating method, the temperature during rolling can be maintained at A for the entire thickness.
Ar
, ~At3+50°C, and the next rolling in the non-recrystallized area can be performed. As a result, the surface grain refinement effect is achieved by raising and lowering the transformation temperature (A,), while the center part of the sheet thickness is affected by lowering the rolling temperature. Compared to this, the toughness of the entire plate thickness can be significantly improved.

Cooling may be performed by either water cooling or air cooling, and reheating may be performed by returning the material to a reheating furnace attached to the rolling line or charging it into a slab heating furnace and reheating it.

The reason why the rolling temperature is limited to these temperatures is that if the rolling temperature is too high, the grains will not be refined sufficiently, and if the rolling temperature is lower than Ar3, sufficient hardening will not occur during the subsequent controlled cooling and the required strength will not be achieved. This is because they cannot be obtained.

The reduction rate at these temperatures is set to 50% or more because
This is because if it is less than this, the grain size is not sufficiently refined and the toughness is poor. The upper limit is preferably 70% at which the effect of controlled rolling begins to be saturated.

Next, the reason why the cooling rate after hot rolling is set to 1° C./sec or more is to obtain a hardened structure up to the center of the plate thickness and to ensure a predetermined strength. If the cooling rate is less than this, the strength will be insufficient.

On the other hand, the upper limit is preferably 10° C./sec, which suppresses a sudden increase in surface hardness and does not exhibit an intermediate structure with poor toughness.

Next, the lower limit of the cooling stop temperature is set at 250°C to prevent a decrease in toughness due to an excessive increase in strength, and the upper limit is set at 600°C because the specified strength cannot be obtained at higher temperatures. This is because grain refinement also becomes insufficient.

Note that the air cooling after the cooling is stopped is to prevent an excessive increase in strength and a decrease in toughness due to the auto-tempering effect during air cooling.

(Example) Next, examples of the present invention and comparative examples will be given.

The chemical composition of the sample material is shown in Table 1, the manufacturing conditions are shown in Table 2, and the mechanical properties of the obtained thick steel plate are shown in Table 3.

Table 3 As shown above, thick steel plates AI and Bl obtained through the method of the present invention
, CI, DI, E1. Fl, Gl.

Hl has a small difference in toughness in the thickness direction, just below the surface,
Both At1%L exhibit good toughness. On the other hand,
Comparative example A2 has a low water-cooling stop temperature, so it has high strength but low toughness. B2 has poor surface toughness because the surface has not been reheated to Ac3 or higher.

C2, B2. Since H2 was not reheated, the rolling temperature at the center of the plate thickness was high and the toughness was poor. B2 is controlled rolling 38
%, the toughness of the entire plate thickness is poor. F2
The central part of the plate thickness is lower than Ar3, and the surface is also AC3.
Since the heat has not been recovered to the above level, the toughness of the central part of the plate thickness and the surface is poor. Since G2 has a high heating temperature, the toughness of the entire plate thickness is poor.

(Effects of the Invention) As described above, the present invention achieves grain refinement at the center of the thickness of a steel plate having a thickness of 50 kg or more and a tensile strength of 50 kgf or more per 1 mm by controlling heating, rolling, and cooling. This achievement has made it possible to simultaneously ensure good low-temperature toughness up to the center of the plate thickness and to ensure high strength at low carbon equivalents by appropriately limiting the component composition and content. This is a highly effective invention.

[Brief explanation of the drawing]

Figure 1 shows the area just below the surface and the area when reheated according to the present invention.
FIG. 2 is an explanatory diagram showing the temperature history of a conventional method without reheating. Figure 2 □ Time

Claims (2)

[Claims] (1) C: 0.03-0.20%, Si: 0.05-0.60 in weight ratio
%, Mn: 0.50-2.50%, Nb: 0.001-
0.10%, Al: 0.005 to 0.1%, and the balance is Fe and inevitable impurities.
After heating to 0°C and rolling to an intermediate stage thickness, the rolling is temporarily interrupted and cooled, or the slab is not rolled and is reheated before the center thickness becomes Ar_3 or less,
Ac_3~1150℃ of the surface area where Ar_3 or less
The temperature was raised to
0% rolling, cooling rate 1~10℃/se after rolling
A method for producing a thick high-strength steel plate having excellent low-temperature toughness and weldability, the method comprising cooling the steel plate to 250 to 600° C. at c. and then air cooling. (2) C: 0.03-0.20%, Si: 0.05-0.60 in weight ratio
%, Mn: 0.50-2.50%, Nb: 0.001-
0.10%, Al: 0.005 to 0.1% as basic components, further Cr: 1.0% or less, Mo: 1.0% or less,
Contains one or more of V: 0.1% or less, Cu: 2.0% or less, further Ni: 4.0% or less, Ti: 0
．． 15% or less, Ca: 1 or 2 of 0.01% or less
After heating the steel to 900 to 1150°C and rolling it to an intermediate thickness, the rolling is stopped and cooled, or the steel is left in a slab state without being rolled. Before the central part of the plate thickness reaches Ar_3 or less, reheat it, and heat the surface part where Ar_3 or less becomes
The temperature is raised to c_3~1150°C, then rolling is restarted, and rolling is performed at a temperature of Ar_3+100°C to Ar_3 with a reduction rate of 50~70% over the entire plate thickness during rolling, and after rolling, the cooling rate is 1~10°C/ A method for producing a thick high-strength steel plate having excellent low-temperature toughness and weldability, which comprises cooling the steel plate to 250 to 600°C for more than sec and then air cooling.