JPS61159554A - High tensile steel less in softening of weld heateffected-zone and excellent in weldability and its production - Google Patents

Abstract

PURPOSE:To obtain the high tensile steel which is less in the softening of a heateffected-zone and excellent in the weldability by blending C, Si, Mn, Al, N, Nb and B of the specified quantity to steel and making the carbon equivalent a specified range. CONSTITUTION:The composition of steel is made to 0.07-0.23 C by wt%, 0.05-0.35 Si, 0.2-1.3 Mn, 0.01-0.08 Al, <0.004 N, 0.004-0.04 Nb and 0.0005-0.002 B and the carbon equivalent Ceq shown in a formula is 0.22-0.34 and the balance is Fe and the inevitable impurities. Billet is produced by casting continuously this steel and the rolling of billet is started from >=900 deg.C temp. or when it is cooled at <=900 deg.C, the rolling is started after reheating it >=1,050 deg.C. The rolling is performed so that at least >=20% cumulative rolling reduction is secured at 800-900 deg.C temp. range and the billet is acceleratedly cooled in >=3 deg.C/sec average cooling velocity after finishing the rolling at >=800 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is characterized in that the hardenability and weld cracking susceptibility of the bath heat affected zone are extremely small, and even when high heat input welding is performed, there is no thermal influence due to welding heat. The present invention relates to a high tensile strength steel having a tensile strength of 48 to 65'qf/lj with little softening of parts, and a method for manufacturing the same.

(Prior art) In recent years, various high-strength steels with good weldability have been developed through controlled rolling and subsequent accelerated cooling technology, and are being used in various structures including shipbuilding materials. Compared to conventional steel, carbon equivalent Ceq=O4Mn/6 + (Or+Mo-hV)15
+ (N j + ou )/15 is significantly reduced to 0.36 inches or less in many cases. For example, JP-A-55-1
According to Publication No. 13834, 0.24 to 0.35% O
A technique for manufacturing high tensile strength steel with a tensile strength of 50 kff 1 wIj class using O-Mr+#4 or 0-Mn-Nb steel having eq.

However, due to the low Ceq effect of these high-strength steels, the hardenability of the weld heat-affected zone and adjacent crack susceptibility are extremely low.
Although it has the feature of high notch toughness, so-called excellent weldability, welding becomes difficult when welding with high heat input of 1 nozzle on one side or 1 nozzle on both sides by multi-electrode submerged arc welding or electrogas arc welding. There is a problem in that the softening of the heat affected zone is large, causing a decrease in the strength of the welded joint.

If we look at this softened weld heat-affected zone in detail, as shown in Fig.
Low Oeq manufactured by controlled rolling + accelerated cooling method, roughly divided into a part heated to an intermediate temperature range of ~Ac5 point, ■ a part heated to a temperature between KC5 point and above and the melting point of steel.
What causes softening when high tensile strength steel is welded in a large heat input bath is ■
In addition to the areas 2 and 3, the area 2 is heated to 400°C or higher. In other words ■.

This is because in the region (2), due to insufficient hardenability due to the low Ceq composition, hardened structures of bainite and martensite appear only slightly under the cooling conditions obtained during high heat input welding.
．． This is because the portion (2) that has been heated to 400° C. to AC1 point undergoes tempering treatment during the bath heat cycle.

In the softened state shown in No. 1 and 1, as a result, parts with lower strength than the base metal or weld metal are included, and JIS Z
When a tensile test is carried out using a test piece such as 3121, the fractured portion will be within the heat affected zone, and the tensile strength will be lower than that of the base material. However, if the degree of softening is within a certain level, the tensile strength of such welded joints will decrease as the width of the test specimen increases due to the restraint of plastic deformation in the softened part and work hardening of that part. It is known that the damage can be recovered to the level of 100%, so there is no problem in terms of the strength of the actual structure.

Regarding the above-mentioned measures to soften the heat affected zone of low Oeq high tensile strength steel, JP-A-59-6355 discloses the addition of trace amounts of Nb and
It provides a technology that reduces softening under welding conditions where the cooling time from 0°C to 500°C is 30 seconds or more. however,
This technology simply adds ζb and V, but does not specify the manufacturing method, especially the conditions for using these elements to improve hardenability. Considering the case where the lower limit is 35 kJ/crn at a relatively small 1 t=15+m+, this is often not effective under the single-sided, 1-pass welding (approximately 90 kJ/cm at t=15 cm) condition targeted by the present invention. It is estimated to be.

(Problems to be Solved by the Invention) The present invention has significantly reduced the softening of the weld heat affected zone during high heat input welding as described above, without impairing bath weldability, that is, while maintaining a low carbon content. The present invention provides a high tensile strength steel and a method for manufacturing the same.

(Means for solving the problem) The present inventors have discovered that softening of the weld heat affected zone (hereinafter referred to as HAZ softening) can be prevented by controlling the presence and form of trace elements in steel. By combining this with conventional controlled rolling and accelerated cooling steel, they succeeded in producing high-strength steel with low HAZ softening.

However, the gist of the present invention is that the gist of the present invention is 0:0.07 to 0.
23, Si: 0.05-0.35, Mn:
0.2-1.3, At: 0.01-0.08,
N: 0.004 or less, Nb: 0.004 to 0
．． 040, B: 0.0005 to 0.0020 as essential elements, and optionally Ou: 0.2 or less, N
i: 0.4 or less, Or: 0.1 or less, Mo: 0
．． 1 or less, V: 0.05 or less, Ti: 0.03 or less, and carbon equivalent (Oeq=0+Mn/fi+ (
Cu+Ni)/15+ (Or4M.

+V)/5} is 0.22 to 0.34, and the remainder is Fe
and unavoidable impurities, and a high-strength steel with good weldability characterized by less softening of the weld heat-affected zone, and a weight i%
So, O: 0.07-0.23. Si: 0.05-0.
35, Mn: 0.2-1.3, At: 0.01
~0.08, N: 0.004 or less, Nb: 0.
004-0.040. R: 0.0005-0.0020
is an essential element, and if necessary, Ou: 0.2 or less,
Ni: 0.4 or less.

Or: 0.1 or less, Mo: 0.1 or less, V: 0.0
5 or less, T contains 0.03 or less as appropriate, carbon equivalent {C
eq=C+Mn/6+(Cu+Ni)/15+Tor+
Mo+V)/5} is 0.22 to 0.34, the balance is Fe and unavoidable impurities, and a slab is produced by continuous casting, and rolling of the slab is started at a temperature of 900 ° C. or higher. Or, if it has been cooled to 900°C or lower, it must be reheated to a temperature of 1050°C or higher before rolling is started, and the rolling is carried out to ensure a cumulative reduction of at least 20% in the temperature range of 800 to 900°C. This is a method for producing high-strength steel with good weldability, which reduces softening of the weld heat-affected zone and is characterized by accelerated cooling at an average cooling rate of 3° C./See or higher after completion of rolling.

Below, the reasons for limiting the chemical components and manufacturing conditions of the steel of the present invention will be described in detail.

0 is KHA at the same time as increasing the strength of the base material in the steel of the present invention.
It is an important element that influences the degree of Z soft f-hi. However, if added in large quantities, the weldability will deteriorate rapidly and the toughness of the base metal will also be reduced, so the upper and lower limits are set as O, O'7 and 0.23, respectively, as necessary limits within the Oeq range of the present invention. do.

8i and At are used as deoxidizing elements, but if they coexist with less than 0.05% and 0.01%, respectively, the deoxidation of the steel will be insufficient and bubbles will be formed in the continuous steel billet. These values should be set as the lower limits for the amount of Si added, since Si may reduce the elongation and toughness of the steel.On the other hand, if the amount added is too large, there is a risk of increasing oxide-based inclusions, so Si should be set at 0. 35%, and the upper limit was 0.08% for At. In addition, Siu also has the secondary effect of slightly increasing the strength of the base material within this range: Bamboo, and At within this range, part of it becomes AtN and has the effect of fixing N.

Mn is an essential element in the steel of the present invention because it increases the strength without reducing the toughness of the base metal, and is also effective in preventing HAZ corrosion, but it harms economic efficiency and is harmful to the weldability. . Therefore, 0.2
% as the lower limit, and the upper limit as 1. considering economic efficiency and board connectability.
3%.

Nb and 8 are characteristic of the present invention IA'5r, and both have a remarkable effect on the HAZ softening adhesive VC when added in small amounts without impairing bath contact properties. In other words, by adding these in combination, the solid solution state in the areas ■ and ■ of the heat shadow area increases the hardenability, and the large heat input f
# While reducing the moderation decline in the sound part, Nb precipitates as carbonitrides on dislocations due to strain in the regions of ■ and ■, and reduces the softening of this part through precipitation hardening (7 The above effect becomes more pronounced for Nb when the coefficient exceeds 0.004%, but when it exceeds 0.04%, it becomes saturated and at the same time the base metal and the thermal form I!
Since this will damage the toughness of the i part, these are set as the lower limit and upper limit, and 8
Regarding (the effect appears from 1,0005%, 0.0
If the ratio exceeds 02%, Fe--R compounds will be formed, which will actually reduce the hardenability and the toughness of the base material, so the lower limit is set to 0.00054 and the upper limit is set to 0.002%.

N is an element that combines with B to form 8N and eliminates the hardenability improvement effect of 8 above, so it is desirable that it be as low as possible.
If it coexists with A/- and B in the above range, the hardenability of B will hardly decrease if it is 0.0040% or less, so this value is set as the upper limit. Further, by reducing N to 0.004% or less in this way, the toughness of the weld heat affected zone is also improved secondarily.

Other elements are 0.22 to 0.3 in addition to the above essential elements.
There are supplementary additions within the range of 4% Oeq, O
u, Ni, Or, Mo have the same effect as O and Mn, V has the same effect as Nb, and Ti has the effect of becoming TiN to completely fix N and achieve a substantial reduction in N. Each of these additives is added, but their upper limit is limited to the above-mentioned range in consideration of economic efficiency. The ratio of 0.22 was set as the upper limit value that does not impair the electrical connection property, and 0.34% was set as the limit value.

Next, the reasons for limiting the manufacturing conditions will be described.

The steel of the present invention is produced by casting slabs produced by continuous casting.
Rolling is started without cooling to below 900°C, or by reheating to 1050°C or above if the temperature drops below 900°C. This is to dissolve a certain amount or more of Nb and 8 necessary for improving hardness and preventing hardening due to precipitation hardening. After that, rolling is continued, and rolling is completed at a temperature of 800°C or higher. This is because, in the case of the steel of the present invention, if the temperature is lower than this, the temperature will drop below the transformation point, making it difficult to ensure the strength of the base material. The reason why the cumulative reduction rate in the low-temperature austenite region of 900°C to 800°C is set to 20% is to increase the toughness of the base material at least to the level required for room-temperature structural steel (e.g., through grain refinement of austenite). This is a necessary condition for maintaining the 2 mV Notch Charpy value at 0°C at 3.5 kg/m or more. The average cooling rate is 3°C/S, and 3°C/S is the required lower limit average cooling rate because slow cooling at a cooling rate of less than 3°C/S will cause Nb in the solid solution state to precipitate. .

Example C) Examples carried out to confirm the effects of the present invention will be described below. Steel slabs having chemical compositions as shown in Table 1 were produced by continuous casting, and rolled and accelerated cooling were performed under the process conditions shown in Table 2.

Table 4 shows the results of the mechanical tests of the base metal investigated for these K, the J-S, diagonal y-groove restraint cracking test, and the tensile test of welded joints welded under the conditions shown in Table 3.

From the above results, the steel of the present invention can provide a high tensile strength steel with a strength level in the range of 48 to 65 kpf/M depending on the combination of ingredients and process conditions, and has sufficient notch toughness. . The weldability is also good, and the tensile strength of the large heat input joint is almost equal to or higher than that of the base metal. However, although the comparison steel is approximately the same as the invention steel in terms of mechanical properties and weldability of the base metal, the strength of the high heat input welded joint is greatly reduced.

Fig. 2 shows the weld hardness distribution for the comparative steel of the present invention @a1, (■). Figure 2 shows c heat input Ji 137 in which each of the above steels is welded with three electrodes and one pass on one side by submerged arc.
kJ/1yn) shows the relationship between the distance from the center of the weld metal and the hardness. As can be seen from the figure,
The hardness of the inventive steel is not as significantly reduced as that of the comparative steel.

(Effects of the Invention) As has been made clear above, the present invention provides a high-strength steel that exhibits excellent weldability and less HAZ softening of high-heat-input welded joints, and a method for manufacturing the same.

Therefore, it can be said that the conflicting objectives of ensuring weldability and preventing HAZ softening have been achieved at the same time, and that it is possible to provide an easy-to-use and safe high-strength steel to the industrial world.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the general hardness distribution of the weld when high-strength steel with low carbon equivalent is welded with high heat input, and ■ is 400.
.degree. C. to AC1 point, ■ indicates a region heated to C1 to Ac3, and ■ indicates a region heated to Ac3 to melting point. Figure 2 shows a comparison of the actual weld hardness distribution of the comparative steel and the steel of the present invention, and is indicated by ■ in the figure. Areas ① and ② are the same as those in FIG. FIGS. 3(1) to 3(4) are diagrams showing the groove shapes shown in Table 3. Agent: Patent attorney Masamitsu Akizawa and 2 other people, including 5th grade and a half, 1st grade and a half age, 1st and 3rd grade, Rano fIsho 71'3 illustration

Claims (4)

[Claims] (1) In weight %, C: 0.07 to 0.23, Si: 0.
05-0.35, Mn: 0.2-1.3, Al: 0.01-0.08, N: 0.004 or less, Nb: 0.004-0.040, B: 0.0005-0 .0020, including carbon equivalent {Ceq=C+Mn/6+(Cu+N
i)/15+(Cr+Mo+V)/5} is 0.22 to 0
．． 34, and the balance is Fe and unavoidable impurities. A high-strength steel with good weldability and less softening of the weld heat-affected zone. (2) In weight %, C: 0.07 to 0.23, Si: 0.
05-0.350 Mn: 0.2-1.3, Al: 0.01-0.080 N: 0.004 or less, Nb: 0.004-0.040, B: 0.0005-0.0020 furthermore, one of Cu: 0.2 or less, Ni: 0.4 or less, Cr: 0.1 or less, Mo: 0.1 or less, V: 0.05 or less, Ti: 0.03 or less, or carbon equivalent {Ceq=C+Mn/6+(Cu+Ni)/15+(C
r+Mo+V)/5} is 0.22 to 0.34, the balance being Fe and unavoidable impurities. A high-strength steel with good weldability and less softening of the weld heat-affected zone. (3) In weight %, C: 0.07 to 0.23, Si: 0.
05-0.35, Mn: 0.2-1.3, Al: 0.04-0.08, N: 0.004 or less, Nb: 0.004-0.040, B: 0.0005-0 .0020, carbon equivalent {Ceq=C+Mn/6+(Cu+N
i)/15+(Cr+Mo+V)/5} is 0.22 to 0
．． 34, and the balance is Fe and unavoidable impurities, if steel is continuously cast to produce a slab, and the slab is started rolling at a temperature of 900°C or higher, or cooled to 900°C or lower, the temperature is 1050°C. Rolling is started after reheating to the above temperature, and rolling is performed so as to ensure a cumulative reduction of at least 20% in the temperature range of 800 to 900°C, and after finishing rolling at 800°C or higher, rolling is performed at 3°C/ A method for producing high-strength steel with good weldability, characterized by performing accelerated cooling at an average cooling rate of sec or more, with little softening of the weld heat-affected zone. (4) In weight %, C: 0.07 to 0.23, Si: 0.
05-0.35, Mn: 0.2-1.3, Al: 0.01-0.08, N: 0.004 or less, Nb: 0.004-0.040, B: 0.0005-0 .0020, furthermore, Cu: 0.2 or less, Ni: 0.4 or less, Cr: 0.1 or less, Mo: 0.1 or less, V: 0.05 or less, Ti: 0.03 or less. carbon equivalent {Ceq=C+Mn/6+(Cu+Ni)/15+(C
r+Mo+V)/5} is 0.22 to 0.34, the balance is Fe and inevitable impurities, and the steel is continuously cast to produce a slab, and the slab is rolled at a temperature of 900°C or higher. , or if it has been cooled to 900°C or lower, reheat it to a temperature of 1050°C or higher before starting rolling.
Rolling is performed to ensure a cumulative reduction of at least 20% in the temperature range of 800°C or higher, and after rolling is completed at 800°C
A method for producing high-strength steel with good weldability, characterized by performing accelerated cooling at an average cooling rate of ℃/sec or more, with little softening of the weld heat affected zone.