JPS6220822A - Manufacture of non-heat treated high tensile steel sheet superior in weldability and low temperature toughness - Google Patents

Abstract

PURPOSE:To provide both superior weldability and low temp. toughness, by rolling a steel contg. decreased S, N quantities and specified quantities of C, Si, Mn, Al in two steps under a specified condition, immediately cooling the sheet in two steps while specifying cooling stopping temp. CONSTITUTION:The steel contg. by weight 0.005-0.20% C, 0.05-0.5% Si, 0.5-2.0% Mn, 0.005-0.08% Al, <=0.01% S, <=0.008% N is refined. The sheet is rolled at (Ar3+70 deg.C)-Ar3 range by >=30% draft, further rolled at Ar3-(Ar3-80 deg.C) range by 10-60% draft. Thereafter, immediately the sheet is cooled from 700 deg.C to 500 deg.C at 4-30 deg.C/sec rate, successively cooled at 500-200 deg.C range by 1-3 deg.C/sec rate, successively air cooled or cooled slowly. By the controlled rolling and cooling, it is highly strengthened and variance in material quality and inferior shape are avoided.

Description

[Detailed Description of the Invention] (Industrial Application Field) This specification describes a method for manufacturing a 50-60 kg class high-tensile steel plate with excellent weldability and low-temperature toughness by a combination of controlled rolling and accelerated cooling. By taking measures to achieve unprecedented high strength, we are meeting the requirements for thick steel plates for shipbuilding, steel plates for pressure vessels such as tanks, steel plates for line pipes in cold regions, steel plates for offshore structures, etc. The following describes the results of development research on this subject.

(Prior technology) In order to provide thick steel plates with excellent weldability and low-temperature toughness, alloy components are reduced, and in order to compensate for the resulting decrease in strength, controlled rolling (abbreviated as C1?) and accelerated cooling (abbreviated as ACC) are used. ) is known as a method of applying online processing heat treatment. For example, Japanese Patent Application Laid-open No. 52-123921,
Or, in Japanese Patent Application Laid-open No. 55-115924, A after CR
3 to 5 kgf/n+m compared to as-rolled material subjected to CC
A method for increasing the strength of 2 or more has been proposed. In all of these, the cooling stop strength is set higher than 500°C, so there is a limit to the increase in strength.

On the other hand, it is disclosed in JP-A-57-143431 or JP-A-58-61224 that by setting the cooling stop temperature to 500°C or less, it is possible to achieve a higher strength of 10 to 20 kgf/mm'' than as-rolled material. However, if the cooling stop temperature is set to 500°C or lower, a transition phenomenon from film boiling to nucleate boiling, which is an inevitable phenomenon during monotonous water cooling, will occur, causing distortion of the steel plate and material variation, resulting in poor product quality. It was difficult to

(Problems to be Solved by the Invention) In order to make the most of the high-strength mechanism by ACC, it is necessary to set the cooling stop temperature of ACC to 500°C or less, and to reduce strain and material variation in that temperature range. is necessary.

Therefore, it is an object of the present invention to effectively realize an increase in strength by ACC while advantageously suppressing the conventionally unavoidable distortion and material variation when performing ACC at temperatures below 500°C.

(Means for solving the problem) The present invention has C: 0.005 to 0.20 wt%, Si
:0.05~0.5wt%, Mn:0.5~2.0wt
%, A1. : Contains 0.005-0.08wtχ,
S: 0.01 wt% or less, N: 0.008 wt% or less.
0°C) to Ar3 with a reduction of at least 30%, and further rolled from Ar to (Ar3- sooC
) in a temperature range of 10% to 60% Φ reduction, then immediately cooled in a temperature range of 700°C to 500°C at a cooling rate of 4 to 30”c/s, and further from that temperature range. A non-heat treated high tensile strength steel plate with excellent weldability and low temperature toughness is obtained by cooling at 1 to 2 degrees Celsius to a temperature range of 500 to 200 degrees Celsius, followed by air cooling or slow cooling.

The inventors conducted various studies on methods for ensuring a cooling stop temperature lower than 500°C in ACC evenly within the steel plate surface, and the effects of rolling below Ar in CR on strength and toughness. The rolling reduction rate in the temperature range from (Ar*-80℃) to 10
% or more and 60% or less, and the cooling rate in the temperature range of less than 500℃ is 1 to b. High strength of 10 kgf/mm2 or more (high TS) without deterioration of toughness causes unevenness in the in-plane cooling stop temperature. We discovered something new that can be achieved without having to do so.

Now, Figure 1 shows C: 0.07wt%, Si: 0.25%
:, Mn:1.4wtχ, 8(1:0.020wt
1, P: 0.015wt, S: 0.003wt
! , N: 0.003wt! Steel with the chemical composition (
This figure shows the influence of the cooling stop temperature on the strength and toughness of a steel plate (thickness: 16 mm) when the following treatments were applied to the steel plate (Arff=787°C).

i) (Ar++ 70℃) - Ar, rolling reduction rate 50
ACC and air cooling at C12-10°C/S (marks in Figure 1).

ii) CR at a reduction rate of 5oχ between (Ar: + +70℃) ~ ^r, 1 → CR at a reduction rate of 4oχ between (Ar: + - 30℃) → ACC and air cooling at 10℃/s (
Figure 1 (△ mark).

iii) CR at a reduction rate of 5oz between (Arz + 70℃) ~ Ar, CR = CR at a reduction rate of 4oχ between Arl ~ (Ar3-30℃) → 10℃/Ste ACC (stop temperature 4
50-710'c) → 8CC at 2.5"C/S (stopping temperature 400°C) (○ mark in Figure 1).

The plot marked with ◯ in iii) is the strength increasing method of this invention.

From i) and ii), it can be seen that in order to impart a TS of 50 kg to a steel that exhibits a TS of 40 kg as rolled, it is necessary to set the cooling stop temperature to less than 500°C, but this temperature range is As such, uneven cooling tends to occur, making stable production difficult.

The final cooling stop temperature in iii) is 400°C,
In the middle of cooling the ACC, the cooling rate is slowed down to 2.5°C/S. Nevertheless, TS is 500 in i) and ii)
A TS equivalent to that of steel with a cooling stop temperature of less than 0.degree. C. can be obtained.

Next, in Figure 2, t 20 x W2O0x LlooO
O) Steel plate rolled in an experimental rolling mill and then cooled at 10°C/S from 750°C to 400°C (comparative method), and 2. Steel plate cooled from 750°C to 600°C by LO'C/S and then cooled to 400°C. The temperature distribution in the longitudinal direction of the steel sheet after cooling is completed after cooling at 2° C./S (inventive method) is shown. In this way, by slowing down the cooling rate during cooling, temperature unevenness is significantly reduced even when cooling is stopped at less than 500°C.

The above-mentioned high strength is achieved by the formation of processed ferrite by rolling in the γ+α region and the subsequent formation of martensite by ACC. Normally, if martensite is mixed in, the toughness deteriorates, but the present invention When manufacturing through such a process, the ferrite grains that transform from untransformed T during rolling become finer than before, and martensite is finely dispersed and mixed into them, so the deterioration in toughness is extremely small. During ACC, if continuous cooling is continued for m to a temperature range of less than 500℃ with a constant amount of water, the temperature will shift from film boiling to nucleate boiling and the cooling stop temperature will become unstable; It is a noteworthy finding that film boiling is maintained up to a temperature range below 500°C by slowing the cooling rate at a temperature of , and the cooling stop temperature is stabilized in this temperature range.

This invention was achieved as a result of various studies based on the above two findings.

The composition of the hot rolled material to which the method of the present invention is applied is limited due to the following reasons.

C: If C is less than 0.00!5wtχ, the steel plate strength is insufficient,
In addition, the weld heat affected zone (hereinafter referred to as AZ) will soften, and if it exceeds 0.20wtχ, the toughness of the base metal will deteriorate, the weld will harden, and the cracking resistance will deteriorate significantly. , C must be within the range of 0.005 to 0.20wtχ.

Si: Si is an element that is inevitably included for deoxidation during steel refining, but if it is less than 0.05χ, the base metal toughness will be insufficient;
If it exceeds 5 wt.chi., the cleanliness of the steel deteriorates and causes a tough paper, so Si must be within the range of 0.05 to 0.5 int.chi.

Mn: If Mn is less than 0.5 wt
Since the toughness of HAZ deteriorates when exceeding wtχ,
is 0.5-2. It is necessary to keep it within the range of Owtχ.

^ 1; In order to deoxidize steel, it is necessary to contain at least 0.005wtχ of i in solid solution, while 0.08wt
If it exceeds
It is necessary to keep it within the range of wtχ.

S; If S exceeds 0.01wtχ, the absorbed energy in the direction perpendicular to rolling will drop significantly, so it must be limited to 0.01wt% or less.

N: N needs to be limited in order to prevent deterioration of the welded part toughness. In other words, for HAZ toughness, it is better to have less solid solution N, and since N flows into the weld metal during welding and deteriorates the toughness of the weld metal, 0.008 wt.
% or less.

In the above component composition, the method of this invention achieves the desired effect, but in addition, even if the following components of each group are contained for the purpose of their addition, the method of this invention can achieve the desired effect. It does not prevent the achievement of the effect.

1st group components: Nb+Cr+Mo, Ti, V, Cu, N
iNb is effective in refining ferrite grains when it is about 0.005wtχ or more, but if it exceeds 0.1wtχ it will diffuse into the weld metal and reduce the toughness of the weld metal. Aiming for finer grains within the range of 1wtX.

Since Ti forms TiN precipitates and has the effect of refining γ grains and refining ferrite and bainite grains, O,
At 0Q3tvt% or more, TiN precipitates can be contained to advantageously bring about a fine grain effect without running out of TiN precipitates. On the other hand, if TiN precipitates exceed 0.04wtX, TiN precipitates become excessive and the toughness deteriorates. is 0.04wt%
The following are preferred.

In order to improve the strength and toughness of the base material of the steel plate and ensure the strength of the joint, V may be contained in an amount of 0.01 wt% or more, but 0.01 wt% or more is allowed.
If it exceeds 10wtχ, the toughness of the base material and 11^Z deteriorates, so V is preferably within the range of 0.10wt% or less.

Cu has almost the same effect as Ni, which will be discussed next, and also contributes to improving corrosion resistance, but if it exceeds 0.5wtχ, cranks are likely to occur during hot rolling and the surface quality of the steel sheet deteriorates. The content is preferably 0.5 Wt% or less.

Ni is advantageous in improving the strength and toughness of the base material without adversely affecting the hardenability and toughness of FIAZ, but 1. If the content exceeds Owtχ, it will increase the manufacturing cost, so the content is preferably 1.0wt% or less.

Cr can be included to ensure the strength of the base material of the steel plate and the strength of the joint, but if it exceeds 0.5wtχ, it will adversely affect not only the toughness of the base metal but also the toughness of the force-receptive joint.
% or less to achieve even higher strength.

Mo is advantageous in improving strength and toughness because it makes each grain uniform during rolling and also produces fine bainite, and as long as that is the case, it is not necessary to exceed 0.5 wtχ.

2nd group component: Ca, REM Ca is effective in controlling the morphology of MnS in a small amount of about 0.002wtχ, and is effective in improving the toughness of the steel plate in the direction perpendicular to the rolling direction. o, oi, as it will deteriorate the cleanliness and cause internal defects.

A range of wt% or less is more suitable for improving living conditions.

REM, that is, rare earth elements, can control the morphology of MnS in trace amounts of about 0.005 wtχ, and is advantageous for rolling and perpendicular toughness of steel sheets, but if it exceeds 0.010 wtχ, the cleanliness of the steel deteriorates. Since it is also disadvantageous in terms of arc welding, a range of 0.010 wt % or less is more suitable for improving air pollution.

As is clear from the above reasons, the first group of ingredients mainly relates to strength enhancement, and the second group of ingredients exclusively relates to improvement of health and well-being, and they are considered to have the same effect.

(Function) Rolling is carried out at a reduction rate of at least 30χ between (Ar3+ 70°C) and Ar1, but in the temperature range exceeding the upper limit and rolling at a reduction rate below the lower limit, deformation bands may be introduced into the austenite grains. Insufficient ferrite grains cannot be refined after transformation.

On the other hand, rolling is carried out at a rolling reduction rate of 10% to 60% in the temperature range from Ar= to (Arl-80°C), but if this rolling is excluded, the TS increase due to ACC will reach the target of 10 kg.
f/mm" or more. This is because the processing of the ferrite is insufficient. Ar+~(Ar+-80
When the rolling reduction rate between (℃) is less than 10χ, the degree of ferrite work is small, so the amount of TS increase is small.On the other hand, when rolling is performed at a temperature exceeding 60χ or lower than (Ar3-80℃), the degree of ferrite work increases. If it becomes too large, not only will it cause deterioration in toughness, but also separation will increase unnecessarily and the properties in the thickness direction will also deteriorate.

Immediately after rolling, heat in the temperature range of 700℃ to 500℃ for 4~
b Yes, but 4~b When stopped, TS hardly increases. Also 4 to 30"
When the C/s ACC is stopped at a temperature lower than 500° C., the cooling unevenness increases rapidly.

On the other hand, if the cooling rate between 700°C and 500°C is less than 4'c/S, the TS increasing effect cannot be reversed, and on the other hand, if it exceeds 30°C/S, lumpy martensite, bainite, or a mixed structure thereof increases, and the toughness increases. deteriorates.

From 700 to 500°C to less than 500°C to 200°C, cooling rate 1 to 3°C/s, less than 500°C to 200°C
ACC (referred to as post-cooling) is performed between It varies within the plane.

At a cooling stop temperature exceeding 500° C., the amount of increase in TS becomes insufficient, because the amount of martensite that increases strength becomes insufficient. However, if ACC is continued to a temperature below 200° C., hydrogen will not be removed sufficiently and hydrogen defects will occur. Therefore, it is necessary to perform subsequent cooling at a temperature of 200''C or higher, and then air cooling or slow cooling may be used.

Examples) Example 1 The steel symbols (S) and (C) in Table 1 were processed to have a thickness of 16 mm as shown in Table 2. Their mechanical properties are shown in Table 3.

In Tables 2 and 3, test lights 1 to 10 are comparative examples for the steel symbol (S), 1t14.15 is a reference example for the steel symbols (S) and (C), respectively, and the remaining N[111 -13 are application examples of this invention, Table 2,
The following can be seen from the data in 3.

tlh I Ar:++ The rolling reduction ratio between 70'C and Ar:+ is too low, resulting in poor toughness.

N112 The rolling reduction between Ar3 and Ar3-80°C is too large, resulting in poor toughness.

11h3 The cooling rate of the first stage cooling is too slow and the TS is low.

11h4 The cooling rate of the first stage cooling is too fast and the toughness has deteriorated.

Circle 5: The stop temperature of the first stage cooling is too high and the TS is low.

Problem 6: The stopping temperature of the first stage cooling was too low, causing distortion and material variations in the steel plate.

IVh7 The cooling rate of the latter stage cooling is too slow, resulting in insufficient generation of multi-sites and low TS.

Problem 8: The cooling rate of the latter stage cooling was too fast, causing distortion in the steel plate and variations in material quality.

Problem 9: The cooling stop temperature of the latter stage cooling is too high and the TS is low.

11hlo The cooling stop temperature of the latter stage cooling was too low and hydrogen cracking occurred.

Compared to the above comparative examples, all of 11k111 to 11h13 have good balance of strength and toughness according to the present invention.
50 has been obtained.

Note that No. 14 is an air-cooled material after rolling that is not subjected to accelerated cooling, and in comparison, No. 11 to No. 13 according to the present invention are T
S increases by more than 10 kgf/mm'', and there is almost no deterioration.

Further, No. 15 is a conventional normalized material, and compared to the composition of this type of HT50, the carbon equivalent of the invented steel is significantly lower and has excellent weldability.

Example 2 The steel code (NV) in Table 1 was again subjected to the treatments shown in Table 4 to produce a steel plate with a t of 16 mm. Its mechanical properties are also shown in Table 4.

Test m16 is the result when no rolling was carried out between Ar+ and Arz-80°C, and test light 17 was the result when rolling was not carried out between Ar+ and Arz-80°C.
Compared to
Satisfies mm2 or more. Furthermore, the DWTT characteristics are also significantly improved.

Example 3 Steels (A) and (B) in Table 1 were again subjected to the treatments shown in Table 5 to produce steel plates with t20 mm and t40 mm, respectively. Mechanical properties are also shown in Table 5.

Test Th18.20 is the result of conventional γ+α2 phase region rolling followed by air cooling, and test 11h19.21 is the result of the comparison steel N11LL obtained by performing accelerated cooling according to the present invention.
Compared to 8°20, in addition to the higher strength of 10 kgf/mn+'' in TS, the toughness is also improved by more than 10°C in vTrs.

(Effect of the invention) Better weldability than normal CR and 10kgf/m
A high strength (high TS) of more than m'' can be obtained by mild (Ar3 direct rolling) and front and rear accelerated cooling processes, and material variations and poor shape of the steel plate, which are harmful effects of accelerated cooling, are avoided.

Therefore, the steel according to the present invention can be efficiently and inexpensively supplied as TS 0.601i with a good balance of strength and toughness for applications such as shipbuilding materials, vibrator materials, tank materials, and land machinery materials.

[Brief explanation of drawings]

FIG. 1 is a graph showing the dependence of strength and toughness on cooling stop temperature, and FIG. 2 is a comparison graph of steel sheet surface temperature distribution at cooling stop. Figure 1 Stop temperature A'('(:)

Claims (1)

[Claims] 1. C: 0.005 to 0.20 wt%, Si: 0.05
~0.5wt%, Mn0.5~2.0wt%, Al: 0
．． 005 to 0.08 wt%, reduced to S: 0.01 wt% or less and N: 0.008 wt% or less. % rolling reduction and further 10% or more in the temperature range from Ar_3 to (Ar_3-80℃) 60
% or less, and then immediately rolled at a temperature range of 700℃ to 500℃ for 4 to 50℃.
Cooled at a cooling rate of 30°C/s, and further cooled from this temperature range to a temperature range of 500 to 200°C.
A method for producing a non-temperature high tensile strength steel sheet with excellent weldability and low-temperature toughness, characterized by cooling at a cooling rate of ~3° C./s, followed by air cooling or slow cooling.