JPS60181229A - Production of low-yield ratio high-tension thick steel plate - Google Patents

Abstract

PURPOSE:To obtain the titled steel plate as rolled and to determine rationally a min. required alloy component system by controlling the specific parameter to decide a low yield ratio in material design of a high-tensile steel contg. component elements increasing the hardenability at the specific compsn. CONSTITUTION:A steel contg. 0.01-0.20% C, 0.07-2.0% Si, 0.8-2.5% Mn and 0.005-0.08% SolAl, contg. further 1 kinds among 0.10-0.50% Cu, 0.10-5.0% Ni, 0.05-2.0% Cr, 0.01-0.20% V, Nb, Ti, 0.0005-0.0030% B and 0.05-1.0% Mo if necessary, having the parameter P by the formula II satisfying the formula I and consisting of the balance Fe is prepd. Such steel is heated to 850-1,200 deg.C and is rolled down at >=50% cumulative draft in at least the unrecrystallized austenite region and is further rolled down at >=15% in the two-phase region of austenite + ferrite if necessary and thereafter the steel is air-cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a method for producing a high tensile strength steel plate with a low yield ratio of 80% or less, which is mainly composed of ferrite and martensite (partially fine paynite). The present invention relates to a method for manufacturing a high tensile strength steel plate by air cooling without any special cooling treatment after fully controlled pressure treatment.

Background technology In recent years, the following two points have become a problem with high-strength steel plates.

(1) High-strength steel plates with a high yield ratio generally have a low uniform elongation value and a low tensile elongation at break, so steel materials with a low yield ratio are desirable from the viewpoint of safety as steel plates for structures.

(2) The increase in yield stress is caused by the difficulty of cold working such as U-0 pressing of line pipe materials during cold-open forming, and low yield ratio steels have low strength during forming but become high strength after forming. is desirable.

Against this background, demand for high tensile strength steel plates with a low yield ratio is increasing.

On the other hand, in order to obtain a high tensile strength steel with a low yield ratio using control rolled steel, if the microstructure has a composite structure consisting of two phases of ferrite and martensite, the ferrite surrounding the second phase transformed into martensite during the cooling process. It is thought that a large number of mobile dislocations are introduced into the material, which move with a low yield stress, resulting in a low yield ratio, and it is expected that the more the second phase becomes martensitic, the lower the yield ratio will be.

In addition, hot-rolled thin steel sheets produced in a hot strip mill can be cooled at a high cooling rate after rolling, making it easy to form a complex structure and relatively easily achieve a low yield ratio.

However, as the thickness of the steel plate increases, it is not possible to increase the cooling rate and noise, and thick steel plates have a smaller total reduction ratio and longer pass intervals than thin steel plates, so the effect of refining austenite grains is small and ferrite grains are reduced. Nucleation also decreases, making it easier to form a coarse bainite structure, which lowers the yield ratio but deteriorates low-temperature toughness. For this reason, conventionally, it has been difficult to manufacture thick steel plates with a composite structure consisting of ferrite and martensite, and in particular, it has been difficult to produce steel with a composite structure consisting of ferrite and martensite, and in particular, it has been difficult to produce steel sheets that have a composite structure consisting of ferrite and martensite. It has been extremely difficult to manufacture economical, low yield ratio, high tensile strength steel plates with such alloy components. Manganese with slightly lower carbon,
Although acicular ferrite steel with increased molybdenum content is known, there is no way to confirm in material design how much expensive alloying elements can be reduced, and the only method is trial and error.

Summary of the Invention Therefore, the present invention mainly uses ferrite and martensite (
The object of the present invention is to provide a method for producing a high-strength thick steel plate having a low yield ratio of 80% or less, which is made up of (partially fine paynite), by air-cooling it without any special cooling treatment after controlled rolling. In particular, the present invention relates to the invention of an economically superior manufacturing technique that rationally determines the minimum alloy composition system necessary for low yield ratio steel at the material design stage.

3. Detailed Description of the Invention In the present invention, the first important point is considered to be an appropriate alloy design technique for obtaining a high tensile strength steel having a low yield ratio during rolling. Generally O, Mn, Or
It is possible to increase component elements that increase hardenability, such as , Ni, and Mo, but from the viewpoint of economy and weldability, it is recommended to limit the addition of alloying elements to the minimum necessary to produce steel with a low yield ratio. It is desirable to do so. Therefore, the boundary conditions can only be determined through trial-and-error experience and testing, and quantitative determination is difficult. From this point of view, the present invention has been developed based on an unprecedented and unique idea to identify and measure the reduction of yield ratio at the stage of material design. A method for producing high-strength arsenide steel was developed.

The low yield ratio high tensile strength steel of the present invention also has a microstructure in which a small amount of bainite or martensite is formed on a fine-grained ferrite base to form a two-phase structure. The second phase other than ferrite becomes pearlite, or whether it becomes bainite or martensite, which is the object of the present invention, depends on the concentration of solute elements in the untransformed 7 parts at the final stage of r→α transformation, and 0. The above-mentioned solute elements are concentrated throughout the body, but zero is a necessary condition but not a sufficient condition in the material design of low yield ratio steel. That is, in the present invention, carbon plays a role as an element that adjusts strength and low-temperature toughness. This is because increasing the amount of carbon in carbon steel increases the hardenability of hardened steel, but in air-cooled materials such as controlled rolled materials, the effect is only to increase the volume ratio of pearlite. The desired two-phase structure of martensite in ferrite will not be created. In order to prevent pearlite transformation of the untransformed 7 parts and cause martensitic transformation, it is necessary to improve hardenability with an alloying element other than carbon. Conversely, even in ultra-low carbon steels such as [101], which have extremely low hardenability from the concept of heat treatment, it is possible to create a martensitic structure in ferrite by adding alloying elements. In other words, it is possible to achieve a low yield ratio regardless of the carbon content, and we have focused on the fact that it is more appropriate to judge alloy parameters based on component elements other than C. Conventionally, this type of alloy parameter was generally calculated by converting all other alloying elements into the equivalent amount of carbon, such as the carbon roaring method, but in this general method, as mentioned above, the main component Since it includes all zeros, it cannot be used as a guide for a low yield ratio. In addition, the present inventors have discovered that unavoidable impurity light such as p, s, etc.
1) Alloying elements (81, Mn, Ou, Ni.

Or, Mo, V, B, Nb, Ti
), and as a result of examining the effects on the yield ratio, it was found that all of these elements affect the hardenability of the second phase, and in order to accurately predict the yield ratio of rolled materials, it is necessary to consider all the effects of each element. I have confirmed everything that needs to be done.

As a result of various studies conducted by the present inventors from this point of view, P (%): 81150+Mn/20+Ou/20+Ni
/60+Or/20+Mo/15 + V/10 +
It has been found that the alloy parameter 5B allows the most accurate estimation of a duplex steel containing bainite and martensite formation in the following component composition. Figure 1 shows the relationship between the yield ratio and alloy parameter (P) of steel with the following chemical composition, but it is clear from this that the chemical composition and alloy parameter (P) as shown below This is a book characterized by having the following.

That is, Oα01-0.20%; S1Q, 07-2.0%:
Mn 0.8-2.5%: sol At α0
Including α08% to 05, and further adding 0u (L10
~α50%; N1a10~5.0%; Or α05
~2.0%: V, Wb, Ti G, 01-120%; B
0.0005~α0050%: 1.0% to Mo a05
and has P (%)≧110, with the remainder consisting of F'e and inevitable impurity elements. In this case, P (%) = 81150 +Mn/20 + Ou/2
0 + Ni/60 + Or/20 + Mo/15
+ V/10 + 5 B Next, the reason for limiting the components of the high tensile strength steel plate in the present invention will be explained.

C: It is the component most involved in the strength level, as well as the amount of second phase produced, and is an important element for lowering the yield ratio.
If it is less than α01, the second phase fraction is too low and becomes almost a single ferrite phase, resulting in a decrease in strength. On the other hand, if α exceeds 20%, the second phase fraction becomes excessive and adversely affects toughness, workability, and weldability.

81:81 promotes the precipitation of ferrite, promotes the concentration of alloying elements in untransformed γ, and contributes to the formation of a composite structure. Moreover, the bath can be strengthened to a large extent without impairing the strength-ductility balance. These effects are effective at a content of 11oy%, but if the content exceeds 2.0%'(i), the toughness and weldability will deteriorate significantly, so α07A-2.0qb is used.

Mn: A relatively inexpensive element that increases the strength and hardenability of steel, and is effective in achieving a low yield ratio and high tensile strength. Hardenability 5
It is necessary to contain α8 or more to ensure strength and strength, but if it exceeds 2.5%, it will deteriorate the bath weldability or cause the entire surface to turn into batonite, resulting in deterioration of ductility (18
-2.5%.

sob kl: In addition to improving the cleanliness of steel as a deoxidizing agent, AA also has the effect of promoting the r→α transformation like Si, and this effect is effective at α005%, but when sol
If Aj exceeds [108%, cleanliness may deteriorate,
Since it has an adverse effect on weldability and completely destroys the weldability, the upper limit is set to 108%.

The above-mentioned 0, Si, Mn, and sol kl unavoidable impurity elements and the remaining Fe are the basic components of the present invention, and if necessary, Nl.

Ou, Or, Mo, B, Ti, N
The present invention can be more effectively achieved even in high-strength steel sheets containing b, Vt-.

The reasons for these limitations are shown below.

N1: An element that has the effect of improving strength without adversely affecting weldability and toughness, and is also useful for increasing hardenability. If it is less than 110%, hardly any of these effects can be expected, and since it is an expensive element, addition of more than 11% will increase manufacturing costs, and the purpose and effects of the present invention do not require addition of more than 5%. The upper limit is set at 5%.

Ou: Ou not only has the same effects as N1 and tty, but also has the effect of further improving corrosion resistance; however, if it exceeds 15%, cranking is likely to occur during hot rolling, and the surface properties of the steel sheet It completely degrades, so the upper limit is '
i[15%.

Or: It is an extremely effective element in increasing hardenability and generating the entire composite structure, and its effect is effective at Q, 05%, but when it exceeds 2.0%, it causes damage to the weld heat affected zone. The upper limit is set at 2.0 inches because it has an adverse effect on bath weldability, such as total deterioration of toughness.

Similar to Motor, it improves hardenability, promotes martensitic formation of the second phase, and improves strength, and its effect is α05%.
However, if the content exceeds 1.0%, weldability deteriorates and manufacturing costs significantly increase.

B: When α is less than 5%, the strength increasing effect and yield ratio decreasing effect (due to martensite formation of the second phase) are small (, [
If it exceeds 1,0030%, these effects will be fully saturated and deterioration of ductility and toughness will result [10005~[100!
10% IC [determine.

'ri:'['i precipitates as TiN, refines the γ grains, promotes the γ→α transformation, and promotes the effect of B mentioned above, so it is an advantageous element in producing the entire composite structure. The effect is 101% effective, but [1.2%
If the addition exceeds 12%, a large amount of Tie will precipitate, causing an increase in the yield ratio, so the upper limit is set at 12%.

Ml), V: Promotes microstructural refinement and improves strength and toughness, and is effective at 101%, but a2%
Exceeding this may cause a severe deterioration of the toughness of the heat-affected zone of the bath.
Since it increases the yield ratio, α01% to α20 is selected.

In the present invention, a steel plate having a low yield ratio and high tensile strength is obtained by further subjecting an alloy steel having the above-mentioned composition to the following controlled rolling and air cooling. That is, rolling heating temperature total AC
, above, after suppressing coarsening of γ grains as much as possible before rolling at 1200°C or lower,
The cumulative rolling reduction rate at 00°C) is at least 50%, and if necessary, rolling is performed by 15% or more in the (γ + α) two-phase region in order to obtain strength, but the reasons for the limitations on the above rolling conditions are as follows. Explain.

(a) 7111 thermal temperature: When heated above 1200 t4-, the γ grains become too coarse and are not sufficiently refined in the subsequent rolling process, making coarse bainite more likely to form, which adversely affects toughness.

Moreover, such coarse γ grains retard the γ→α transformation and reduce the concentration of alloying elements in the untransformed 7 parts, which may hinder a low yield ratio. More N.J.V.

When containing precipitate forming elements such as T1, 1200
Excessive Calorie heat in excess of 10°C increases strength, but significantly impairs toughness and ductility, and also causes a total increase in yield ratio due to the binning effect on mobile dislocations. For the above reasons, the upper limit of the heating temperature was set to 1200°C. In particular, heating at 1050° C. or lower is desirable from the viewpoint of utilizing the concentration of alloying elements to 7 parts untransformed by activating the γ→α transformation of the fine particles.

(1)) Cumulative reduction rate in the unrecrystallized γ region: Reduction in the unrecrystallized γ region flattens the γ grains, introduces a deformation zone within one grain, and greatly increases the location where Yα nuclei are generated. let This not only generates fine grains α and greatly improves strength and toughness, but also activates the r→α transformation and promotes the concentration of alloying elements in untransformed γ. These effects are effective under a pressure of 50 inches or more.

(0) (γ+α) Rolling in the two-phase region: Rolling in the two-phase region is effective in increasing strength and lowering the Charpy fracture transition temperature, but these effects cannot be expected at a reduction of less than 15%.

Next, the present invention will be explained by showing examples.

Example +11 A sample material having the chemical composition shown in Table 1 below was manufactured in a vacuum bath in a 150 mm ingot, which was hot forged into a 150 m thick slab and then cut into pieces.
℃, and after rough rolling, the rolled material was subjected to a cumulative reduction of 75% in the unrecrystallized γ range of 930 to 780℃, and then air-cooled (plate thickness: Table 2 Table 2 (Continued) Example (2) Table 3 below shows changes in mechanical properties of steels No., B, and Q (all within the composition range of the present invention) shown in Table 1 due to changes in rolling conditions.

As is clear from the above embodiments, the present invention is capable of achieving a low yield ratio of 80 inches or less during rolling by simply cooling the steel by limiting the chemical composition, heating temperature, and cumulative reduction rate during hot rolling. High tensile strength steel plates can be manufactured.

[Brief explanation of the drawing]

The attached drawing, Figure 1, is a chart showing the relationship between yield ratio and alloy parameter (P).

Claims (1)

[Claims] 0[α20% to LOl; 5ia07~2.0%; Mn1
11.8--λ5%; aoIAt[1005~α
Includes all 08qb, plus 0ua10 to a50 if necessary
%: Ni Q, 10-5.0%; Or a05-2.0
%;v, Nb,'ri Q, ol-12o%: B C
LOOO5,0,00! to%; Mo aos-1,0
Steel 'i, 850, which contains all of %01 or 2 or more species and satisfies the following formula, with the remainder consisting of Fθ and unavoidable impurities.
Heating to ~1200°C, rolling is carried out at least in the unrecrystallized austenite region with a cumulative reduction rate of 50% or more, and if necessary, in the austenite + ferrite dual phase region.
A method for manufacturing a steel plate with a low yield ratio and high tensile strength, which is characterized by performing all rolling steps of 1 or more, and air cooling after the above-mentioned rolling.
Ou/20 + Ni/60+Or/20 + Mo/
15 + V/10 + 5B