JPS5845318A - Production of high tensile steel having weldability and >=50kg/mm2 strength - Google Patents

Abstract

PURPOSE:To obtain high tensile steel having high strength and weldability by melting specific low alloy steel consisting basically of C, Si, Mn, solAl to ingot, and hot rolling and acceleratively cooling the same under specific conditions. CONSTITUTION:The steel consisting 0.03-0.18% C, <=0.6% Si, 0.5-2.0% Mn, 0.01-0.08% solAl, and if necessary, 0.01-0.1% Nb, 0.01-0.15% V, 0.01-0.15% Ti, <=1.0% Cu, <=1.0% Mo, <=1.0% Ni (except copresence of Cu, Cr and copresence of Ni, Mo, Nb, V, Ti) and the balance iron and satisfying <=0.20% low- temp. cracking sensitive compsn. and >=(25.8-0.18Vc)/78 carbon equiv. of accelerative cooling material is melted to ingot. The molten metal is hot rolled at >=20% cumulative draft below the Ar3 point and 650-(Ar3-30) deg.C finishing temp. and is cooled acceleratively at 4-25 deg.C/sec for at least 80 deg.C temp. width down to >=500 deg.C; thereafter the steel is allowed to cool. Thus the high tensile steel having excellent weldability and >=50kg/mm.<2> strength is obtained.

Description

[Detailed description of the invention] The present invention relates to accelerated cooling of thick steel plates after rolling,
When hot rolling, the reduction rate is 20- below the Mrs transformation point.
Taking advantage of the fact that steel sheets can be made to have high strength by applying the above reduction and immediately accelerating cooling at a cooling rate of 4 to 25 degrees until reaching a temperature of 500°C or more, TS (tensile strength)
The present invention relates to a method for producing a thick steel plate having a value of 501- or higher and also having excellent weldability. In other words, the present invention aims to improve weldability by reducing the alloy components, particularly carbon content, that are strengthened by accelerated cooling, but specifically, the relationship between the strength of the accelerated cooling material and the alloy components is By finding that there is a good correlation between The aim is to cover this issue with technology and manufacturing technology, improve the quality stability of products, and also make it possible to automate the manufacturing line.

In general, a technology company that controls the transformation structure and improves mechanical properties by accelerating cooling of thick steel plates after hot rolling, is called controlled rolling.
ing (hereinafter abbreviated as CR), a type of processing heat treatment method is used. There has been little systematic research on this controlled cooling, but recently there has been research into so-called interrupted cooling, which accelerates cooling only in a specific temperature range after the completion of rolling.
The lng method is attracting attention. This method does not require tempering treatment unlike the direct quenching method after rolling, and can be applied to general non-quality steel sheets, such as To, and when combined with CR, it maintains toughness. It is possible to achieve even higher tensile strength, and it is also expected to improve weldability and various other steel material properties.

Although such a cooling method has already been operationally used for run-out cooling in hot strip mills, it has not been put to practical use in plate rolling.

Although many proposals have become public,
At present, no proposal has been made to improve weldability. One of the reasons for this is that the effect of accelerated cooling in general low-alloy steel is to improve the transformed structure, increase strength, and prevent toughness deterioration. This is because the effect can be seen as indirect and self-evident at first glance, since it can be obtained by reducing the alloy components.

Another important reason is that when producing high-strength copper plates with excellent bath weldability by accelerated cooling, it is necessary to set manufacturing conditions that metallurgically satisfy both strength and weldability. This is because the specific settings are difficult due to lack of data accumulation and analysis, and were not previously known.

The present invention was developed in view of the above-mentioned problems and to solve them, and the gist thereof is: CQ,
03~α18%, Siα6% or less, MnO3~2.0-
1 sot AJ! The basic composition is α01~α08-, Nbα01~α196, VQ, 01~α155 as necessary.
6, T1α01-α15, Ca 1.0% or more), C
r1.0% or less, N11.0% or less, Me 1.0%
Nt, Mat Nb + V in one or more of the following (however, the coexistence of Cu and Cr, and the coexistence of these)
, Tt (excluding the coexistence of one or more types), and the remainder is free from F and unavoidable impurities.
w <0-2096 (However, PCM company low temperature cracking sensitive composition), and 0LACCaqn (25,8-Q, 18Ve
)/7B (However, 0LACC@q is the basic equivalent of the accelerated coolant leg, Vc is the cooling rate during accelerated cooling, and the steel that satisfies 4-25℃i) is melted, Cumulative rolling reduction rate of 20- or more, finishing temperature of 650℃ ~ (
Rolling is carried out to bring the Arm point to -60°C, and after the rolling is completed, the
A high-strength steel having a strength of 950 VwJ or more and having weldability characterized by accelerated cooling at the above-mentioned cooling rate ve over a temperature range of at least 80°C to a temperature of 0°C or more, and then allowing it to cool. This is the manufacturing method.

Next, the manufacturing method of the present invention will be explained in detail.

It is known that the strength of thick steel plates varies greatly depending on the alloy components, and that the contribution of the alloy components to the strength is greatly influenced by manufacturing conditions. Therefore, the inventors of the present invention sought to understand the influence of alloy components on the strength of accelerated cooling materials after rolling in the γ10α2 phase region, which had not been known until now (C,
Using laboratory cracked steel in which various components such as Si and Mn were varied widely, rolling was carried out at a cumulative pressure of 20 or more with an unevenness of 1 point or less, and the finishing temperature was (Arm point -6).
0℃) or lower and 650℃ or higher, and after the completion of this rolling, 10
At least 80°C to a temperature of 500°C or higher at a cooling rate of
Experiments were repeated in which accelerated cooling was performed over a temperature range of ℃, and the contribution of each component to the strength of the steel sheet was determined using multiple regression analysis. As a result, the following relational expression was obtained.

TS (Kr/j) = 26+78・0LACC・q・
...(1) Here is the 0LACC@qd accelerated coolant phosphorus equivalent equation, which is expressed by the following equation.

0LACC*qe) = C+ Mn15+ c”/2
9 +N'/15,6 + ”/7.4+ 4+ to Me
"10.97+"/15 (2) Furthermore, since the cooling rate is an important factor in accelerated cooling materials, in order to investigate the effect on strength, steel 2 in Table 1 shown below was used. We conducted experiments in which the cooling rate was varied over a wide range. As a result, it was found that an increase in the accelerated cooling rate increases the strength independently of 0LACCeq, and when the accelerated cooling rate is in the range of 4 to 25℃, the increase in T8 per roe is about 018 (-). FIG. 1 is a graph showing the relationship between the accelerated cooling rate and ΔTB (where ΔT8 is the value obtained by subtracting the TS of the rolled material under the same rolling conditions from the TS of the accelerated cooling material) in this case.

From the above results, if the rolling and accelerated cooling conditions of the accelerated cooling material after two-phase regional compaction are limited as described above, its TS will be 0LACCeQ and the cooling rate Vc (C/5
sc), it can be determined uniquely from the following equation.

T8=26+78・0LACC@qtenα18(%-10
) ......(3) From this formula (3), TS is 50
(To produce υ of high-strength steel plates of
It can be seen that LACCsq and ■ should satisfy the following formula.

50<26+78@OLmuCC@q+α18(Vc-1
0) ...-(4) That is, 0LACCeq 2
(25,8-α18vc)/78 -- (s) Regarding weldability, basically the welfare alloy component is a big factor;
If the components of the accelerated coolant and the conventional material are the same, it is considered that there is no difference. Therefore, the low temperature cracking susceptibility composition may be regulated in the same manner as conventional materials using the PCM value. It is strongly desired that high-strength steel for 50 kg class structures or ship hulls, which is the object of the present invention, do not require preheating to prevent cold cracking, and therefore rounded PCM to prevent cold cracking at room temperature is highly desirable. Price increase Pji
Since the value is known to be 0.20, this was set as the regulation value for imparting weldability.

Next, the reason for limiting the ingredients will be explained. The lower limit of C is 1031i
This is because it is necessary to impart strength to this type of steel effectively at the lowest cost, and the upper limit is set at 018- because weldability will be significantly impaired if it exceeds this. Sl
is effective in increasing strength through hardening Sm, but the upper limit is 0.6% since addition of a large amount impairs US adhesion. Corrosion is added as a basic element of steel that is effective in improving the strength and toughness of steel, but if it is less than α5, the effect will be small, so n, s%
was set as the lower limit. Moreover, if it exceeds 2.0%, weldability will be significantly impaired, so 2.0% is set as the upper limit. Since a minimum of α01 -o1kl is required to deoxidize steel, At is set as the lower limit, and since this effect is saturated when aotAA exceeds α08-, the upper limit is set at 008%.

By adding Cu, Cr + NL Mo Id, strengthening can be achieved without impairing toughness through solid solution strengthening and structural changes based on increased hardenability of the steel, but from the viewpoint of weldability and economic efficiency, each The upper limit was .0-. Nb.

V and TI have a remarkable effect on increasing strength through bending reinforcement and improving low temperature toughness, and are added as necessary, but in order to exhibit this effect, 0.01% of either type is required.
It is necessary to add more than 100% of the total amount, and this is set as the lower limit. Since any of these impairs II contact when added in large amounts, the upper limit for Nb was set at 0.10%, and the upper limit for V and Ti was set at 0.15% each.

Next, the reason for limiting the manufacturing conditions will be described. First, the reason why the cumulative reduction rate below the Arm point in hot rolling is set to 20 - or more is because if it is less than this, substructures such as the introduction of dislocations due to processing of pro-eutectoid ferrite will not develop sufficiently, and accelerated cooling will not be necessary. This is because even if it is carried out, it cannot be expected to effectively increase the tension. Figure 2 shows the relationship between the rolling reduction below the Ars transformation point and ΔTS (where ΔTS is the TS of the 0LAC material after rolling in the two-phase region). That is, the effect of accelerated cooling after rolling in the two-phase region is that accelerated cooling suppresses the recovery of the substructure (due to rolling) that occurs when the conventional material is air-cooled after rolling. This is because it is necessary to form and develop a high dislocation density structure immediately before accelerated cooling to form this substructure.

In addition, the reason why the finishing temperature of rolling was limited to below (Ars point -60°C) was because as shown in the examples, sufficient recovery of the substructure was required before starting accelerated cooling, since it would be too high if the temperature was higher than this temperature. The reason why the lower limit was set at 650°C was because the effect of suppressing recovery by accelerated cooling would not be exhibited if
If it is less than this, the difficulties in rolling such as increased mill load will significantly increase, making it unproductive, and the amount of recovery of machining fequit will decrease even with air cooling, so the effect of suppressing recovery by accelerated cooling after rolling will be lost. This is because the essential effects of the accelerated cooling of the present invention are no longer efficiently exhibited if the temperature is too high.

Furthermore, the accelerated cooling rate was set in the range H of 4 to 25 rays.
If it is less than 4.41, as is clear from FIG. 1, sufficient high tension cannot be expected due to the above-mentioned suppression of sub-tissue recovery due to accelerated cooling. ji), if it exceeds 251, this effect will be saturated, and the strain of the steel plate will increase, making it difficult to manufacture a thick steel plate with a shape with good at.

In addition, the reason why the cooling stop temperature is set to 500°C or higher is because if it is lower than this, the distortion of the steel plate becomes large and this is not preferable in terms of the manufacturing process. The reason why the medium temperature was chosen is that if the temperature interval is made smaller than this, the substructure will recover and high tension will not be achieved sufficiently.

1st! ! indicates the chemical composition of the test steel;
The table shows the manufacturing conditions and the mechanical properties and 0' weldability of the steel manufactured under the manufacturing conditions.

Rattan Table 2 Each of the steels in the above first cost was melted in a 250-ton converter. Steels 2 to 4 have PC1il values of α1
20~α149.0LACCeq is 0.290~α61
6, all of which satisfy the component ranges defined by the present invention. Steel 1 has a Pcw value of less than 020, and the 0LACCeq of 4 is lower than the lower limit of the present invention. This steel has a PCM value exceeding 0.20 because the steel sFi alloy component is higher than the upper limit of the present invention.

As is clear from Table 2, as for steel 1, 0LACC
For steels where @q is lower than the limiting value of the present invention, the cooling rate is 25
Even if the (25,8-α18 V)/7a value exceeds the 0LACCeq value, the intensity cannot reach 50v. Steels 2 to 4 whose components satisfy the range of the present invention
In this case, the strength is 50 (-) when air-cooled after rolling in the two-phase region or at a low cooling rate (- cannot be seen, but if the high cooling rate specified in the present invention is used, the strength is (25,8-α18 V)/7
The 0LACCeq value is higher than the 0LACCeq value, and the strength exceeds 50V.For steels 2 to 6, as-rolled companies that do not perform rolling in the two-phase region have a strength of only slightly over 40V. , Even if accelerated cooling is performed after two-phase region pressure (steel 2
(g) When the accelerated cooling temperature is 80° C. or lower, the strength is still only 50 degrees or lower. Furthermore, steel 3-
When the pressure finishing temperature exceeds (Ar1 point -60°C) as shown in (g), the strength of the inside is only jp50v- or less. All of the above Steels 2 to 4 have PCM values of less than α20, and no cracks are observed in the ν groove restraint cracking test at room temperature. In Steel 5, the strength exceeds 50 V- due to accelerated cooling after rolling in the two-phase region, but the PCM value is high and cracking occurs in the ν groove restraint cracking test at room temperature.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between changes in ΔTB as the accelerated cooling rate go increases, and Figure 2 is a graph showing the relationship between the ratio of subordinates below the metamorphosis point and ΔTB.
It is a graph figure showing the relationship of T8. Agency Masunori Sato Samura Sasaki Sasaki Sasaki Muneharu

Claims (1)

[Claims] Cα06~α1896, siα6ts or less, Mnα5~
The basic composition is ZO%, molALO, 01~0.08%, and Nbα01~α1%, Vo, 0 as necessary.
1~α15-1Ti [101~α151! ,Cu1.0
% or less, Cr1.0- or less, Ni1.0% or less"F, MO
l, 0- or less (However, Cu(!:C
Coexistence of r and Ni+ Mo+ under these coexistence
(excluding the coexistence of one or more of Nb, V, and Tl), the remainder is free from iFs+ and unavoidable impurities, and PCM<α20% (however, PCM has a composition susceptible to cold cracking), and 0LACCeq2 (2 !i8-
G, 18 Vc)/78 (However, 0LACCe
q is the carbon equivalent of 1 material after accelerated cooling, and vc is the cooling rate during accelerated cooling, which satisfies the 4-25℃ layer), and the cumulative reduction rate below the Arm point in hot rolling is 2096 or more, and the finish is Rolling is carried out at a temperature of 650° C. to (KRS point -60° C.), and accelerated cooling is performed at the above-mentioned cooling rate horse over a temperature of at least 80° C. up to a piezoelectric finish vk of 500° C. or higher;
50(
- A method for manufacturing high-tensile steel having a strength of or above.