JPS6167717A - Manufacture of high tension steel plate having superior strength and toughness in its weld heat-affected zone - Google Patents

Abstract

PURPOSE:To manufacture a high tension steel plate having superior strength and toughness in its weld heat-affected zone by subjecting a low carbon steel contg. a prescribed amount of Nb to heating, rolling and cooling under specified conditions. CONSTITUTION:The composition of a steel slab is composed of, by weight, 0.005-0.05% C, 0.05-0.75% Si, 0.2-2.2% Mn, 0.005-0.1% Al, 0.01-0.1% Nb and the balance Fe with inevitable impurities. The steel slab is heated to 1,000-1,250 deg.C and rolled. The rolling is finished at >=900 deg.C in the austenite recrystallization temp. range. The resulting steel plate is cooled at once at 1-50 deg.C/sec average cooling rate in the temp. range of 800 deg.C to 300 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-strength steel plate that is sensitive to the strength and toughness of a welded heat affected zone.

In recent years, from the viewpoints of reducing heat treatment costs by omitting reheating, making the hot rolling-heat treatment process continuous, improving hardenability, and increasing strength, a direct quenching method has been developed in which steel is hot rolled in the austenite region and then rapidly cooled immediately after rolling. A method for manufacturing tempered steel has been proposed.

For example, Japanese Patent Application Laid-Open No. 50-115611 discloses that the amount of B and N is adjusted to exhibit the effect of improving the hardenability of B, and by quenching immediately after rolling and tempering as necessary, the base material is improved. A method for increasing strength and toughness is disclosed.

Furthermore, in JP-A-54-68719, an austenite recrystallization inhibiting element such as Nb is added, and during hot rolling, rolling is performed from the recrystallized region to the unrecrystallized region of the austenite low temperature region, and the austenite recrystallization suppressing element is added. 41 grains are refined, many lattice defects are introduced, and the number of nucleation sites for ferrite is increased to suppress hardenability. As a result, coarse low-temperature transformation products such as martensite and upper bainii 1~ are formed. A method is disclosed for strengthening the base material by suppressing the appearance of ferrite and creating a structure consisting mainly of fine ferrite.

However, according to the former method, the strength and toughness of the base metal are improved by effectively utilizing B and N, but the weld cracking resistance, weld Il1 hand strength and toughness are not necessarily good. In addition, in the latter method, since Nb13<tJb(C,N) precipitates during low-temperature rolling, the strength and toughness of the base metal are reduced as in the former method.
However, since rolling involves rolling in a non-recrystallized austenite region at a low temperature, rolling efficiency inevitably decreases.

Furthermore, there is no improvement in the strength and toughness of the weld.

Therefore, when using the fgI plate by the conventional method as described above, there are restrictions on preheating and heat input, so welding execution management is more complicated than with mild steel or the like.

In order to solve the problems in these conventional methods, the present inventors have investigated the effects of adding various alloying elements on the strength and toughness of the base metal and weld heat-affected zone of directly quenched steel sheets, and manufacturing processes such as heating, rolling, and cooling. As a result of detailed and extensive research on the conditions, we have found that not only the strength and toughness of the base metal but also the strength and toughness of the welded joint can be improved without special low-temperature rolling or without the need for special tempering treatment after direct quenching. We have discovered that it is possible to obtain high-strength steel sheets with improved properties.

That is, the present inventors significantly reduced the amount of C added to the steel, added an appropriate amount of Nb, and furthermore, in the manufacturing process, by limiting the heating, rolling, and subsequent cooling conditions, the A high tensile strength steel plate with excellent strength and toughness in the weld heat affected zone is obtained by direct quenching.

That is, the method for producing a high-strength steel plate with poor strength and toughness in the weld heat affected zone according to the present invention has a method of manufacturing a high-strength steel plate with poor strength and toughness in the welded heat affected zone.
Steel slab consisting of 0.05%, Si 0.05~0.75%, Mn 0.2~2.2%, Aρ 0.005~01%, Nb 0101~0.1%, balance iron and inevitable impurities. 1000
Heating to a temperature in the range of ~1250°C, completing rolling in the austenite recrystallization zone above 900°C, then immediately starting cooling, from 800°C to 300°C at an average cooling rate of 1 to 50°C/sec. It is characterized by cooling.

The present invention will be explained in detail below.

During hot rolling, Nb has the effect of suppressing recrystallization of austenite and refining the J1 weave after cooling. However, the most important effect of adding Nb in the present invention is This is due to the effect of improving hardenability due to Nb, and the prevention of softening of the welded joint by redecipitating the solid solution Nb in fine particles during welding.

That is, Nb existing in austenite in a solid solution state exhibits extremely high hardenability, and this effect increases significantly as the amount of solid solution Nb increases. As a result, during the quenching process after rolling, the formation of pro-eutectoid ferrite is greatly suppressed, and a fine-grained structure is obtained, making it possible to obtain a steel plate with excellent strength and toughness. Furthermore, Nb brought to room temperature in a solid solution state precipitates finely as Nb (C, N) in the weld heat affected zone during welding, significantly narrowing the softening width of the joint.

On the other hand, according to the conventional method that requires processing in the austenite non-recrystallized region, precipitation of Nb carbonitrides progresses during rolling, and the hardenability improvement effect of solid solution Nb not only disappears, but also Precipitation hardening of Nb carbonitrides during welding cannot be expected. The method according to the invention therefore differs from conventional methods requiring low temperature rolling.

Another major feature of the method of the present invention is that the C content in the steel is significantly reduced. The reduction of C content is
It not only suppresses hardening of the weld zone and significantly improves weld cracking resistance, but also suppresses the formation of island martensite that impairs the toughness of the weld heat affected zone. The above effect is remarkable especially when the C content is set to 0.03% or less according to the method of the present invention. As a result, the steel plate produced by the method of the present invention eliminates the need for preheating during welding, and at the same time, it is possible to stably maintain the toughness of the weld heat-affected zone over a wide range of welding heat input. .

Furthermore, by reducing the C content in this way, the so-called mass effect is reduced, and despite rapid cooling after rolling, it is possible to significantly reduce the strength difference in the material within the steel sheet, especially in the thickness direction. can. In addition, generally, when rapidly cooled to low temperatures after rolling, the warpage of the steel sheet becomes large due to martensitic transformation, and straightening after quenching is required. It is effective in reducing the warping of steel sheets caused by the oxidation process, and at the same time, it is extremely effective in improving stress corrosion cracking, where these hard low-temperature transformation products become the starting point for cracking.

Therefore, according to the method of the present invention, it is possible to obtain a low C Heinai 1 steel that has superior strength and toughness compared to ordinary quenched and tempered steel, and it is possible to significantly reduce the carbon equivalent, thereby improving weldability. It can be improved dramatically.

Next, the reasons for limiting the chemical composition of the steel used in the present invention will be explained.

In the present invention, the upper limit of the amount of C added is set at 0.05% in order to increase the susceptibility to weld cracking and suppress the formation of island-like martensite that deteriorates the toughness of the weld heat affected zone.
shall be. However, since it is difficult in actual operation to stably obtain steel with a C content lower than 0.0 O5%, the lower limit is set to 0.0 O5%.

Si promotes deoxidation of steel and improves its strength.
Requires addition of at least 0.05%.

However, if added in excess, weldability will be affected, so the northern limit is set at 0.75%.

Mn needs to be added in an amount of at least 0.2% to increase the strength and toughness of steel, but if it exceeds 2.2%, weld cracking susceptibility increases significantly, so the upper limit should be set at 2.2%.
shall be.

Al is essential as a deoxidizing element, and also forms nitrides to refine the structure. For this reason, in the present invention, at least 0.005% of Al is added, but
When adding too much, inclusions will increase, so add 0.1
% as "-eye".

Nb improves the hardenability of steel in a solid solution state and precipitates finely as Nb (C; N) in the weld heat affected zone during welding, so it has a significant effect on preventing softening of the welded joint. Therefore, in the present invention, at least 0.01% of Nb is added in order to effectively exhibit the effect of Nb under the cooling conditions described below.

However, if added in excess, the toughness of the welded part will be reduced, so the upper limit is set at 0.1%.

In the high-strength steel sheet according to the present invention, in addition to the elements listed below, at least one hardenability-improving element selected from the group consisting of CuXNi, Cr, Mo, and B can be added.

Cu is effective in improving the corrosion resistance and hardenability of steel, but when added in excess, it impedes weld cracking resistance, so the upper limit of its addition amount is set at 0.5%.

On the other hand, Ni is effective in improving the toughness of steel without impairing weldability, but its upper limit is set at 4.0% in consideration of economic efficiency.

Cr is added to improve the hardenability of steel. However, if added in excess, weldability will be significantly degraded, so the upper limit is set at 2.0%.

MO improves steel strength and is effective in improving low-temperature toughness, but if added in excess, the strength-T increasing effect will be saturated, and it will actually deteriorate toughness and weldability, so the upper limit should be set at 1.
．． Set to 0%.

When added in a small amount, B improves hardenability and is effective in ensuring the strength and toughness of extremely thick plates, but the effect is saturated even if added in excess, so the upper limit is set at 0.002%.

Furthermore, according to the present invention, precipitation hardening elements such as (1) and/or Ti can be added to the steel to further increase the steel strength. However, these elements should not be added in too large a quantity.
The upper limit of each element is set at 0.1% because it impairs the toughness of the base metal and welded part during welding.

Further, according to the present invention, at least one element selected from the group consisting of Ca and REM can be further added as necessary. These elements have the effect of spheroidizing sulfide-based nonmetallic inclusions and improving their anisotropy.

In order to effectively exhibit this effect, it is necessary to add at least 0.001% of each. However, even if added in excess, the effect will be saturated, so the upper limit is set to 0.01% for each element.

According to the method of the present invention, by heating, rolling, and cooling steel having the above chemical composition under predetermined conditions,
A steel plate with excellent strength and toughness can be obtained.

That is, according to the method of the present invention, the heating temperature prior to hot rolling varies somewhat depending on the amount of C and N in the steel, but in order to sufficiently dissolve Nb carbonitride in austenite, At least 1000 as shown in FIG.
°C is required. Next, regarding the subsequent hot rolling,
In order to suppress Nb carbonitrides precipitated during rolling as much as possible and to ensure sufficient hardenability of solid solute Nb, rolling should be completed in the austenite recrystallization region at 900°C or higher, as shown in Figure 2. Therefore, in the method of the present invention, the finishing rolling temperature is set to 900°C or higher.

In the method of the present invention, in order to obtain a fine heinite structure after cooling after rolling, it is necessary to freeze the solid solution Nb in a solid solution state until the completion of the heinite transformation, and for this purpose, the cooling rate is Although it varies somewhat depending on the composition of the steel, as shown in Fig. 3, the average cooling rate is 1~b from 800°C to 300°C. If the cooling rate is slower than 1°C/sec, it is difficult to obtain fine bainite. Although the upper limit of the cooling rate is not particularly limited, it is set to 50° C./second from a practical standpoint.

In the present invention, it goes without saying that there is no need to forcibly cool a thin material that satisfies the above cooling rate during air cooling after rolling.

As described above, according to the method of the present invention, a special low-temperature rolling process is performed by using steel having a predetermined chemical composition, especially a steel with low carbon content and a predetermined amount of Nb added. It is possible to obtain a high-strength steel plate that has not only the strength and toughness of the base material but also the strength and toughness of the welded joint without requiring any particular tempering treatment after direct quenching.

However, in the present invention, there is no problem in performing tempering after cooling if necessary, and in this case, as long as the tempering temperature is Ac or less, the characteristics of the steel of the present invention will not be impaired.

The present invention will be explained below with reference to Examples.

Invention steels A to ■ having the chemical compositions shown in Table 1] and comparative steels are heated with -N to the temperatures shown in Table 2, and finish rolled.
Cooling was performed from 800 degrees to 300 degrees at the average cooling rate shown in Table 2 to obtain hot-rolled steel sheets. The mechanical properties of these are shown in Table 2.

The Nb-added steels A to H of the present invention have a very fine haynite structure due to the Nb addition, so they have extremely poor toughness and strength compared to -N comparative steels without Nb addition.

Figure 4 shows a comparison with the invention steel A with heat input of 56 KJ/c.
This shows the hardness distribution of the joint when submerged arc welding was performed at m. It is clear that the softening width of the weld heat-affected zone in the present invention @A is extremely small compared to the specific axis steel. In addition, the figure also shows A steel in the austenite non-recrystallized region.
The results when applying % pressure are also shown. It is understood that rolling in the austenite non-recrystallized region increases the degree of softening of the weld heat affected zone.

FIG. 1 and FIG. 2 show the relationship between the heating temperature and finish rolling temperature and the strength and toughness of the obtained steel plate for steel B according to the present invention. It is understood that the higher the heating temperature and finishing temperature are, the higher the amount of Nb in solid solution is increased, and the tougher the steel is achieved.

FIG. 3 shows the influence of the average cooling rate during direct quenching on the strength and toughness of Steel C, which is the steel of the present invention. It is shown that excellent strength and toughness can be obtained by cooling at a cooling rate of approximately 1°C/second or higher.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between billet heating temperature and the mechanical properties and impact properties of the obtained steel sheets for the invention steel and comparative steel. A graph showing the relationship between temperature and the mechanical properties and impact properties of the obtained steel plate, Figure 3 is a graph showing the relationship between the average cooling rate after rolling and the mechanical properties and impact properties of the obtained steel plate, and Figure 4 The heat input is M56K to the steel of the present invention @A and the comparison steel.
It is a graph showing the hardness distribution of a joint when submerged arc welding is performed at J/cm. Figure 1 Surada kIM! ;Ro degree (°Koji Fig. 3 1-1 1-1 error”, 7, song, + to 7 kyu 10 s fη Fig. 2

Claims (3)

[Claims] (1) C 0.005-0.05%, Si 0.05-0.75%, Mn 0.2-2.2%, Al 0.005-0.1%, Nb 0.01 in weight% ~0.1%, balance iron and unavoidable impurities
Heating to a temperature in the range of ~1250°C, completing rolling in the austenite recrystallization zone above 900°C, then immediately starting cooling, from 800°C to 300°C at an average cooling rate of 1 to 50°C/sec. A method for producing a high-strength steel plate with excellent strength and toughness in a welded heat-affected zone, which is characterized by cooling. (2) In weight% (a) C 0.005-0.05%, Si 0.05-0.75%, Mn 0.2-2.2%, Al 0.005-0.1%, Nb 0.01 to 0.1%, and (b) Cu 0.5% or less, Ni 4.0% or less, Cr 2.0% or less, Mo 1.0% or less, V 0.1% or less, Ti 0 1% or less of B, and at least one element selected from the group consisting of 0.002% or less of B, the balance being iron and unavoidable impurities.
Heating to a temperature in the range of ~1250°C, completing rolling in the austenite recrystallization zone above 900°C, then immediately starting cooling, from 800°C to 300°C at an average cooling rate of 1 to 50°C/sec. A method for producing a high-strength steel plate with excellent strength and toughness in a welded heat-affected zone, which is characterized by cooling. (3) In weight% (a) C 0.005-0.05%, Si 0.05-0.75%, Mn 0.2-2.2%, Al 0.005-0.1%, Nb 0.01 to 0.1%, and (b) Cu 0.5% or less, Ni 4.0% or less, Cr 2.0% or less, Mo 1.0% or less, V 0.1% or less, Ti 0 (c) at least one element selected from the group consisting of Ca and REM 0.001 to 0.01%; A steel slab consisting of the balance iron and unavoidable impurities is heated to a temperature in the range of 1000 to 1250°C, rolling is completed in the austenite recrystallization zone of 900°C or higher, and cooling is immediately started thereafter.
Average cooling rate 1~500℃/from 00℃ to 300℃
A method for producing a high-tensile steel plate with excellent strength and toughness in a welded heat-affected zone, characterized by cooling in seconds.