JPH02133520A - Manufacturing method for welded structural steel plates with excellent toughness - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention applies to base metal parts and weld heat affected zones (hereinafter simply referred to as HAZ).
The present invention relates to a method for manufacturing saki timber for zeta-contact structures with excellent toughness.

<Conventional technology> In recent years, demands on the material properties of large welded structural steels for offshore structures, ships, storage tanks, etc. have become more severe, and studies have been ongoing to fundamentally improve the low-temperature toughness of welded parts in particular. However, as the demand increases, it becomes desirable to increase the supply and reduce the supply price, and it has become necessary to improve the productivity and economic efficiency when manufacturing steel materials for large welded structures.

Generally, when steel materials are subjected to automatic thermal welding such as submerged arc welding, electrogas welding, and electroslag welding, the toughness of 14AZ is significantly reduced due to coarsening of austenite crystal grains.

The most typical proposal for preventing a decrease in IAZ toughness is Japanese Patent Application Laid-Open No. 117213/1983.

This proposal consists of two requirements.

One of the requirements is that steel with limited ingredients should be cooled in the temperature range of 950°C to 700°C during solidification.
Slow cooling is performed at a cooling rate of 2°C/sec or less, and Ti, Zr, Ta nitrides (core) with a diameter of 0.1 μI or more are used as cores to generate intragranular ferrite (hereinafter referred to as IFP) in I (AZ) + MnS (outer shell) complex.

The remaining requirements are to heat the above-mentioned solidified steel piece to a temperature of 1150°C or higher and hold it for 5 hours or more to actively generate IFP from the micro-segregation zone during welding by the action of the above-mentioned IFP generation nuclei. In contrast, the purpose is to diffuse as much as possible the alloying elements in the trees where C5Mn, P, and other alloying elements that hinder the formation of IFP are concentrated, thereby lowering the alloy concentration.

With these two requirements, the above proposal allows more than 90 IFPs/ma+'' to be generated within the steel plate.

As a result, the obtained base material is YS (kgf/m+a”
) is 30-50, TS (kgf/IIIB”) is 44
~64, El (Z) is 37-42, vTrs (℃) is -
80--100, the HAZ toughness at a heat input of 100kj/cm is VE-6+1"C (kgf m) is 16-20
The IIAZ toughness at a heat input of 200 kj/cm is V
The E-ho''C (kgf, at) is 17 to 21, and the toughness of the HAZ portion has been improved to meet the requirements.

Also, the proposal in Japanese Patent Application Laid-open No. 169516/1983 was proposed by WA
In order to manufacture steel sheets with excellent not only X-toughness but also low-temperature toughness of the base metal, steel with limited ingredients is heated at a temperature of 1250 to 1350°C.
After heating for 60 minutes or more, cooling or rolling to a temperature below the Ar3 transformation point, reheating to a temperature of 900 to 1150'C and rolling at a reduction rate of 30% or more at 800 °C or less,
This method performs controlled cooling and tempering.

As a result, the low temperature toughness of the base material is vTrs -90 to -110℃
It is an excellent steel that exhibits 1 (Azure resistance of -48 to -60°C).

<Problems to be Solved by the Invention> However, as proposed in JP-A-61-117213, it is not possible to require slow cooling of 2°/sec in the 950-700°C region during the solidification process of steel. , it means that it is necessary to lower the temperature of the solidified steel slab to at least 700℃, and sometimes it is possible to lower the temperature to room temperature, so the next step ≧l+
To maintain heating for 50'c, ≧5 hr, it is necessary to newly input a huge amount of energy over a long period of time.

Meanwhile f! A large amount of thermal energy possessed by solidified steel slabs is simply dissipated and remains unused, resulting in not only a decrease in productivity but also a significant deterioration in economic efficiency based on thermoeconomic efficiency.

In addition, requiring the solidified steel billet to be heated at 21,150°C for 1≧5 hours means that the solidified steel billet remains in the high-temperature manufacturing process for at least 5 hours. This means exclusively providing a long and high temperature manufacturing process, which further reduces productivity and worsens economic efficiency.

Hey,,! In order to solve the problem, the inventors filed a patent application in 1983.
3-1 (Compensation for casting temperature and solidification shrinkage at the end of solidification according to No. i5389 (after rolling the steel slab subjected to step i at a reduction rate of 30% or more, at 1250°C or more for 2 hours or more (5 hours or less) By holding the l+AZ toughness vE-16'c (kgf-
HAZu property VE-60℃ (kgf-m) when e+) is 16 to 22 and heat input is 230kj/am (single side-layer welding)
10 to 14, solving the problem proposed in the above-mentioned Japanese Patent Application Laid-Open No. 117213/1982, and making it economically possible
We have proposed a first invention that meets the demands for toughness in the AZ section.

However, since a holding temperature of 1250° C. or higher is an essential requirement, the grain size during heating becomes coarse, and when rolling is performed immediately after heating, the base material toughness is not high and does not meet all requirements.

For this reason, when it is necessary to guarantee the low-temperature toughness of the base metal in addition to l4AZ, as described in the above-mentioned Japanese Patent Application Laid-Open No. 60-169516, it is necessary to cool the base metal to below the Arx transformation point temperature, A method is adopted in which the material is heated again to 900-1150'c and then rolled at 800°C or less with a rolling reduction of 30% or more.

In this method, a steel billet heated to a -degree high temperature is once cooled and then heated again, which not only leads to a decrease in productivity but also to a deterioration in economic efficiency based on thermoeconomic efficiency and equipment costs, which inevitably increases manufacturing costs. do.

The present invention is based on the first proposed invention of the present inventors, and in addition to the HAZ toughness guaranteed under the good productivity and economic efficiency brought about by the first proposed invention, Similarly, we have developed a second manufacturing method for welded structural steel plates that guarantees toughness without using reheating, which deteriorates productivity and economic efficiency.
The objective is to provide the invention as a proposed invention.

Means for Solving the Problems> In order to achieve the problems mentioned above, the present invention (1) starts casting solidification by injecting a welded structural mesh into a mold at a temperature of freezing point temperature + 5 °C or higher, and The solidified end portion of the solidified steel piece is solidified while being rolled down in the thickness direction of the solidified piece using a member that substantially forms a surface according to the amount of solidification shrinkage, and the solidified steel piece is rolled down 4-30 mm. The basic first method is to hold the steel at 1250°C or higher for 2 hours or more after rolling to a temperature of 1250°C or higher. Solidification is started, and the solidified end portion of the solidified steel slab is solidified 1i! Solidify the solidified steel piece while rolling it down in the thickness direction using a member that substantially forms a surface according to it, and after primary rolling the solidified steel piece at a reduction rate of 30% or more,
The second method is to maintain the temperature at 1250° C. or higher for 2 hours or more and then perform secondary rolling at 800° C. or lower with a reduction rate of 60% or higher.

Normally, welded structural steel plates are manufactured in order to obtain the required material quality based on the relationship between functions and effects that have been confirmed in the past in the field of art.
As described in Japanese Patent No. 117213, each element is added in an additional amount to iron and other unavoidable components based on the reasons described below.

In other words, generally the basic components are: C: 0.02~0.18Z Al: 0.00
7-0. lXSi: ≦0.5ZS:
0.001~0.005zOn: 0.4~1.8
ZB: 0.0002-0.0032P:
≦0.015t N: ≦0.004Z, Ti: 0.003 to 0.02Z Ta: 0.
003-0.02ZZr: Contains one or more of 0.003-0.02Z, Ni: ≦2.0Z No: ≦0.
5ZCr;≦0, 5ZCeq≦0.
45Z) R is added.

Also, the reason for adding these ingredients and the reason for limiting the amount added are as follows:-
C is as follows.

C is used to improve the strength of steel, and the lower limit is set at 0.02Z in view of the required strength for the purpose, and the upper limit is set at 0.18Z in view of deterioration in weld cracking resistance.

In addition, S1 is necessary for maintaining the strength of the base metal and preliminary deoxidation of molten steel, but I There is.

14rl is M that serves as the outer shell in addition to ensuring base material strength and toughness.
For the formation of nS, the lower limit is 0.4z, and the toughness of HAZ,
The upper limit is set at 1.87 in order to prevent deterioration of the weld cracking resistance of HAZ.

The upper limit of P is 0.0i5Z to prevent deterioration of IAZ toughness (due to micro-segregation) and I (AZ cracking resistance).

AI is used in an amount of 0.007% or more for deoxidation, grain refinement of the matrix structure, fixation of solid solution N, etc., but the upper limit is set at O and Iz to prevent deterioration of the cleanliness of the steel.

S usually requires a concentration of 0.001Z or higher to form MnS, which forms the outer shell of the composite that is the core of IFP formation, but it forms coarse A-based inclusions and deteriorates the toughness and anisotropy of the base material. In order to prevent deterioration, the upper limit is set at 0.005Z.

B is generally at least O, 0 due to the suppression of grain boundary ferrite, ferrite side plates that are harmful to IAZ toughness (during high heat input welding), and fixation of HAZ solid solution N due to precipitation of 8N.
O02X is added, but a large amount of Fe2x (C
B) Decrease in toughness due to precipitation of h and free B) I
Since this leads to an increase in the hardenability of AZ, the upper limit is set at 0.003Z to prevent this.

Like S and B, N is also T1 and Zr, which are the core of the complex.

It is added to precipitate nitrides such as Ta, but the upper limit is set at 0.004z to prevent deterioration of matrix toughness and promotion of high carbon martensite formation in IIAZ.

One or more of Ti, Zr, and Ta are selectively added to form a nitriding anastomosis that becomes the core of the composite described above, and form an intragranular fluoride. 0.0 in order to act as a generation nucleus of U-ite.
Although it is necessary to add more than 03Z, the upper limit is set to o and o2x to prevent a decrease in the cleanliness of the steel.

The following are the elements used as basic components of welded structural steel in the field, the amount of each element added, and the reasons for addition.

In this field of the art, ■ Increase in base material strength and base material, HA
For the purpose of improving the toughness of Z, Ni, Cu, Cr, No
, Nb, one or more of ■, or ■+iAZ for the purpose of preventing crystal grain coarsening and reducing the anisotropy of the base material.
One or both of REH, Ca, and Hg are added.

However, N1 of group 0 depends on the strength and toughness of the base material and tlA.
Although it is added to simultaneously improve the hardenability, the formation of IFP in IAZ may be suppressed due to the increase in hardenability.2. The amount of Oz added is limited to 1.

Further, although Cu increases the strength of the base material, the increase in HAZ hardness is small, but stress relief annealing increases the hardenability of AZ, so the upper limit is set at 1.Ol.

Nb, ■, Cr, and Mo are known to increase the strength of the base metal by improving hardenability and precipitation hardening, and improve the low-temperature toughness of the base metal, but they have an adverse effect on HAZ toughness and hardenability. 0.05Z, 0.1Z and 1. Ol, 0.5z is the northern limit of each.

In addition, as the components of the group (2), in order to prevent coarsening of austenite crystal grains in HAZ, as mentioned above, lanthanoid elements with atomic numbers 57 to 71, which are oxide and sulfide forming elements, and Y
Rare earth elements (REM) selected from 1 or 2 species of
One or more of the following three types are added: Ca, and Hg.

These elements form oxides, sulfides, or oxysulfides, and are added for the purpose of increasing the crystal grain size of l (AZ) and reducing the anisotropy of mother (A). The core M
In order to prevent the formation of nS from becoming difficult, the upper limit is set at 0.005Z for the sum of one or more of these elements, and when each element is added individually, the upper limit is set at 0.0032. It is said that

In addition, Ceq is generally set to 0.45X or less, and the reason for this is said to be that the increase in hardenability makes it extremely difficult to generate intragranular ferrite, thereby lowering the HAZ toughness.

Usually, the Ceq is a value calculated by the following formula.

Ceq=C% +SiZ/24+Hrb%/6 ToN
i:W/40+CrZ15 10 No/4 + V2:/
14 The welded structural steel to which the present invention is directed is the same as the ordinary welded structural steel described in 2. It uses the above elements in the same manner as described in 7. Ori,
It does not have a component structure unique to the present invention.

In order to solve the problems of the prior art, the present inventors formed IFP generation nuclei without cooling the solidified pieces below the solidification temperature, particularly to about room temperature, and omitted heating. However, as a result of repeated experimental studies of IFP generation pretreatment that can generate the required amount of [FP] even if the retention time is shortened, we found that (1) The toughness of the HAZ area increases as the area ratio of IFP increases, as shown in Figure 2. This improves the area ratio, and if the area ratio is about 30%, the above requirements can be met.

(2) The size of the island-like martensia, which is a brittle structure, increases as the amount of segregation increases, and when the size increases to 6 μm or more, the toughness of the HAZ decreases significantly, as shown in Figure 3. case.

I found out.

Therefore, the present inventors have solved the problems inherent in the prior art.
IF
We carried out individual exploration experiments on means for increasing the area ratio of P, (1) means for reducing the size of island-shaped martensite, and (2) experiments on a method in which each of the means in conjunction with each other was combined.

As a result, when molten steel is injected into a mold at a temperature higher than the freezing point 1°C → -5°C and cooling solidification is started, the solidified structure forms columnar crystals, and during the casting solidification process, the above-mentioned Ti and Z
r, IFP generation nuclei consisting of a composite of Ta nitride (core) + HnS (outer shell), etc. are generated within the columnar crystal structure of the solidified steel slab, and furthermore, when the solidification is completed, the unsolidified end portions are substantially removed. Members constituting the surface, such as the endless belt described in Japanese Patent Publication No. 44-2441, a large press, and
9-163064, JP-A-59202145, and JP-A-61-49761, for example, as shown in the front view in FIG. Solidification control based on the solidification using a surface member such as a device that is installed at
By applying a reduction in the thickness direction of the solidified steel slab according to the amount of jh to complete the solidification of Φ1, rolling with a reduction ratio of at least 30%, and holding at at least 1250"C for 2 hours, as shown in Fig. 4. As shown in Figure 5, the micro-segregation formed between the dendrites forming columnar crystals is reduced, and the
At least 30% of the P structure is generated, and the amount of central macro-segregation formed in the solidified center is further effectively reduced, and not only the amount but also the size of island martensite, which is harmful to toughness, is reduced. It was discovered that the decline in 11^February sexiness can be even more effectively prevented.

Surface In this experiment, if the width of the surface member was made to correspond to the unsolidified area excluding the solidified area at both widthwise ends of the thickness direction reduction rope piece, the required reduction edge could be left lightly and the surface reduction efficiency was clearly developed. I learned something good about being able to make a lotus.

The steel slabs obtained in this manner were subjected to controlled rolling under various conditions. The results are shown in FIG.

As is clear from the figure, when controlled rolling is performed at a rolling reduction of 60% at 800°C, compared to that performed at a rolling reduction of 50%,
YP and TS remain the same, but vTrs of the base material is approximately -3
It was found that the temperature improved by 0°C.

This is due to the fact that the ferrite grain size after transformation is approximately 15 μm when the rolling reduction is 50%, whereas the grain size is refined to approximately 107711 when the rolling reduction is 60%. It seems that there are. The present invention is based on these findings.

<Example> (1) Steel composition Any combination of the above-mentioned general welded structural steel elements and the amount of each element may be used, but in the field of welded structural steel, HAZ Q-woven grain refining elements are particularly suitable. Ti, Ta, which is also emphasized in the present invention for the same reason
The chemical components of typical examples with different contents of Zr, REH, Ca, and BSN are shown in Table 1 along with comparative examples.

Tables 2 and 3 show the casting conditions, hot rolling conditions, soaking diffusion conditions, and obtained materials.

(2) Casting conditions ■Good temperature Freezing point 1 degree 10 to 30℃ - (Inventive example + Comparative example) ■Solidified steel billet dimensions Thickness 250/300 mix width 1800mm ■Cooling rate 1.0 ~2.0℃, 'akin■ Surface reduction device at the end of solidification (device shown in Figure 6) Model Walking barker type configuration Inner bar (1) 3 Outer bar 2) 4 Shift amount 100III11 ・Length of slab under reduction 2500 Country m/Thickness of the slab on the side of the rolling part 284 n+m/Thickness of the slab on the side of the rolling part 2
81.501111 Pressure gradient 1.00a
+a+#a (3) Primary rolling conditions of the first invention (shown in Table 2) (4) Retention conditions of the first invention (shown in Table 2) (5) HAZ toughness of the first invention (Table 2 )
(6) Primary rolling conditions of the second invention (shown in Table 3) (7
) Retention conditions of the second invention (shown in Table 3) (8) HAZ toughness of the second invention (shown in Table 3) (9) Second invention
Base material toughness of the invention (shown in Table 3) Plate thickness 30111I11 obtained from invention examples A-1 to A-16 not shown in Table 2
Double-sided multilayer welding (heat input 75 kJ/cm) was performed on the steel plate, and a Charpy test was conducted. As a result, the minimum value of the Charpy value in the Q embrittlement area is extremely high for all steel plates, and
A value of 16.3 kgf-ra or more was obtained for E-, o'c.

Also, vE-6 for single-sided welding (heat input 230 kgf-m)
A high value of 9.8 kgf-01 or higher was obtained for °C.

On the other hand, the plate thickness 30 mm obtained from Comparative Examples B-1 to B-16
double-sided multilayer welding (heat input 7
5kJ/cm) and single-side-to-side welding (heat input 230kgf
-m) was carried out to conduct a Charpy test.

The maximum Charpy value at the kneeling part is low for all steel plates;
The former's vE,.s6°C was 2.0 to 3.8 kgf-m, and the latter's vE,.°C was 1.4 kgf-m or less, which was not practical.

Examples A17 to A32 of the present invention shown in Table 3 are examples of the second invention of the present invention.

A17 to A32 are steel pieces of fragrances 1 to 16 subjected to secondary rolling at a temperature of 800°C or less and a rolling reduction of 60% or more.

As a result, the vTrs of the base material showed an excellent value of -85 to -107°C.

On the other hand, B17 to B32 of comparative examples are 8
It has been subjected to secondary rolling at a rolling reduction of less than 60% at a temperature of 00"C or less.

The vTrs of the base material was -55 to -73°C, which was much lower than that of the inventive example.

Also double-sided multilayer welding to the steel materials listed in Table 3 (heat input 75kJ/cm)
A Charpy test was carried out. As a result, the minimum value of the Charpy value at the most brittle part was high for all steel plates, and a value of less than 16.0 kgf-m was obtained for the steel E-s. So vE -66
A value of 9.1 kgf-m or more was obtained, and the example of the present invention showed excellent toughness that satisfied the requirements for both the base material and the HAZ.
In the comparative example, satisfactory base metal toughness was not obtained. <Effects of the Invention> The present invention makes molten steel into a copper piece having a columnar crystal structure, and generates IFP generation nuclei during solidification of the steel piece, During the solidification, the unsolidified end portion is subjected to surface pressure according to the amount of solidification shrinkage to reduce or substantially eliminate the amount of segregation in the segregation zone, and further reduce the reduction rate to 3.
By rolling at 0% or more and holding at 1250°C or more for 2 hours or more, IFP is generated more actively, the amount of island martensite generated is further reduced, and the size is reduced to 5 μm or less. After that, a steel material with improved HAZ toughness was obtained, and then a reduction rate of 60% or more was applied at 800"C or less to reduce the ferrite grain size and significantly improve and stabilize the toughness of the base metal. By combining the comprehensive actions and effects of each requirement, it has become possible to manufacture steel plates for welding construction with excellent toughness, with good productivity and economy, and the effects it brings to the industry. is extremely large.

[Brief explanation of the drawing]

Figure 1 shows the secondary rolling start temperature and the YPST of the base metal steel plate.
A relationship diagram of S and vTrs is shown. FIG. 2 shows the relationship between IFP area ratio and IIAZ toughness. Figure 3 shows the relationship between the size of island-like martensite and 1 (AZ toughness). Figure 4 shows the relationship between the rolling reduction of the primary rolling of a solidified steel slab and the HAZ toughness. Figure 5 shows the holding temperature of the rolled material. and the relationship between retention time and ILAZ toughness. Fig. 6 is a front view of the surface pressure reduction device used in the embodiment of the present invention. Patent applicant: Nippon Steel Corporation Agent: Masu Kodo (and two others) ) No. [ Fig. 1 S I Fig. Fig. a Fig. ■ ■ ■ ■ ■: Outer bar 2: Inner bar S; Slab 3: Not yet

Claims (2)

[Claims] (1) Casting and solidifying are started by injecting welded structural steel into a mold at a temperature higher than the freezing point temperature +5°C, and the solidified end portion of the steel slab during solidification substantially forms a surface according to the amount of solidification shrinkage. Excellent toughness, characterized in that the solidified steel slab is solidified while being rolled down in the thickness direction using a member, and the solidified steel slab is held at 1250°C or higher for 2 hours or more after rolling at a reduction rate of 30% or more. A method for producing welded structural steel plates. (2) Casting and solidifying are started by injecting welded structural steel into a mold at a temperature higher than the freezing point temperature +5°C, and the solidified end portion of the steel billet during solidification substantially forms a surface according to the amount of solidification shrinkage. The solidified steel slab is solidified while being rolled down in the thickness direction using a member, and the solidified steel slab is subjected to primary rolling at a reduction rate of 30% or more, and then 1250
Reduction rate 6 below 800℃ after holding at temperature above ℃ for 2 hours or more
A method for producing a welded structural steel plate with excellent toughness, characterized by performing secondary rolling of 0% or more.