CN103343283A - Production method of super-thick steel S355NL for low-temperature wind tower - Google Patents

Abstract

The invention relates to the field of steel products, in particular to a production method of steel S355NL for an extra-thick low-temperature wind tower, which comprises the following process flows of smelting, continuous casting, heating, dephosphorization, controlled rolling, controlled cooling, hot straightening, stacking and slow cooling and fire cutting to obtain a finished product, and is characterized in that the production method is controlled aiming at three key points: designing smelting components, controlling rolling at low temperature and cooling after finish rolling. The thickness of the continuous casting billet is 250-300 mm. Compared with the prior art, the invention has the beneficial effects that: by reasonably controlling the proportion of each alloy component in the steel and the subsequent heating and rolling process and by field practice, the mechanical property and the impact property of the S355NL with the thickness of 80-100 mm produced by the method can meet the standard requirements, and a good effect is obtained.

Description

A production method of extra-thick steel S355NL for low-temperature wind towers

technical field

The invention relates to the field of iron and steel products, in particular to a production method of S355NL steel for extra-thick low-temperature wind towers.

Background technique

S355NL belongs to the European standard range of weldable fine-grained structural steel. The particularity of this steel type is that the steel is required to maintain good impact performance at a low temperature of -50°C. It is mainly used for structural parts such as wind power towers in extremely cold environments.

In recent years, with the international wind power industry's demand for 80~100mm extra-thick plate S355NL, domestic thick plate manufacturers are also accelerating research and development. However, due to the harsh requirements of this steel type on the impact performance and the limitation of the technical level of each manufacturer, the extension of this steel type to the thickness is restricted.

There are domestic precedents for heavy plate manufacturers to produce 80~100mm extra-thick S355NL, but they all use continuous casting slabs with a thickness of more than 300mm. At present, there are no relevant records at home and abroad on the use of 250mm thick continuous casting slabs for 80~100mm extra thick S355NL production.

Contents of the invention

The purpose of the present invention is to provide a production method of extra-thick low-temperature wind tower steel S355NL, which is produced by continuous casting slab with a thickness of 250 mm, which meets the standard requirements of the steel type for mechanical properties and impact properties, shortens the production process, and reduces production costs.

For realizing the above object, technical scheme of the present invention is:

A production method of extra-thick low-temperature wind tower steel S355NL, the technological process includes smelting, continuous casting, heating, phosphorus removal, controlled rolling, controlled cooling, hot straightening, stacking and slow cooling, and fire cutting to finished products. Control of two key points: smelting composition design, low temperature controlled rolling and controlled cooling after final rolling, the specific control is as follows:

1) Smelting composition design: C 0.15~0.18%, Si 0.25~0.45%, Mn 1.4~1.8%, Nb 0.20~0.40%, V 0.30~0.50%, Ti 0.40~0.50%, Ni 0.20~0.40%, P≤ 0.02%, S≤0.004%, the rest is iron and unavoidable trace impurities;

2) Low temperature controlled rolling: the continuous casting slab is heated to 1200~1280°C in the heating furnace, so that the alloying elements are fully dissolved in the austenite crystal; the rolling is controlled in two stages, the first stage adopts high pressure, and each pass The primary deformation is controlled at 15~25%, and the final rolling temperature is controlled above 950°C; the second stage rolling temperature is controlled at 900~940°C, the final rolling temperature is 890~850°C, and the deformation rate is controlled at 50%-60%;

3) Controlled cooling after final rolling: After rolling, it directly enters the accelerated cooling device, cools to 500-550°C at a speed of 15-20°C/S, and stacks slowly after entering the hot straightening machine. Below 400°C, the time is not less than 16 hours, and the microstructure of the final product is a mixed structure of ferrite and pearlite.

The thickness of the continuous casting slab is 250-300mm, which can shorten the production process and reduce the production cost.

0.25%~0.45% Si is added to this steel, one of the purposes is to improve the hardenability of the steel together with Mn, so that the ability of the steel to obtain pearlite is enhanced. At the same time, Si has a large solid solubility in ferrite, which can significantly strengthen ferrite, and its solid solution strengthening effect is stronger than that of Mn. Adding a certain amount of Si to non-quenched and tempered steel can also improve the toughness of steel.

The main function of Mn in steel is solid solution strengthening, improving strength and hardenability. However, the increase of Mn content will reduce the toughness of the steel, and it will also be detrimental to the welding performance of the material. Based on the above two points, the range of Mn is set at 1.4%~1.8%.

Ni can significantly improve the toughness of steel, especially the low temperature toughness. At the same time, the combination with Si, Mn and other elements can delay the pearlite transformation, making it easier for steel to obtain pearlite.

Nb, Ti, and V are the most commonly used microalloying elements, and the effects of the above three elements on the grain boundaries are sequentially reduced. In steel, the effect of composite microalloying is greater than the sum of some elements added alone. The three elements Nb, Ti, and V can all precipitate in austenite or ferrite, because the solubility in austenite is large and the diffusivity is small, so the precipitation in austenite is slower than that in ferrite. Deformation can accelerate the precipitation process. Generally, precipitation in austenite should be minimized, and more alloying elements should be kept in solid solution to be precipitated in ferrite, which can increase the solubility of microalloying elements in austenite by alloying.

Since the price of Ti is the lowest among the three alloying elements of Nb, V, and Ti, the production cost of microalloying with high Ti and low Nb is lower, which can not only achieve the effect of fine grain strengthening, but also achieve the purpose of reducing costs. Taking all the above factors into consideration. Control the range of Nb, V, Ti three alloying elements within a reasonable range.

Compared with prior art, the beneficial effect of the present invention is:

Through reasonable control of the ratio of each alloy component in the steel and the subsequent heating and rolling process, through field practice, the mechanical properties and impact properties of the 80-100mm thick S355NL produced by the method of the present invention can meet the standard requirements, and good results have been achieved. Moreover, the invention has the advantages of short production process, low production cost, suitable for mass production, etc., and can be widely applied to various thick plate manufacturers.

Description of drawings

Fig. 1 is a metallographic structure diagram of a product of Example 1 of the present invention.

Detailed ways

The present invention will be further described below in conjunction with specific embodiment:

Example 1

A production method of extra-thick low-temperature wind tower steel S355NL, the technological process includes smelting, continuous casting billet, heating, dephosphorization, controlled rolling, controlled cooling, hot straightening, stacking and slow cooling, and thermal cutting to finished products. The specific process control is as follows:

1) Smelting composition design (wt%): C: 0.17%, Si: 0.30%, Mn: 1.60%, Nb: 0.3%, V: 0.40%, Ti: 0.45%, Ni: 0.30%, P≤0.01%, S≤0.005%, the rest is iron and unavoidable trace impurities;

During smelting, the molten steel is first smelted in an electric furnace, and then sent to an LF refining furnace for refining. During the refining process, white slag formation is strengthened, and Ar gas blowing is strengthened to promote the mixing and stirring of molten steel and slag. After VD treatment, S is controlled below 0.005%. Ca treatment to improve the shape of inclusions;

2) Casting: After the vacuum is broken, the continuous casting billet is cast in the temperature range of 1545~1550 °C, and the thickness of the continuous casting billet is 250mm;

3) Heating, considering that the complete solution temperature of Nb(CN) is 1100~1200°C, and at the same time, in order to reduce the deformation resistance and ensure the effect of subsequent controlled rolling, the billet heating temperature is determined at 1150~1250°C;

4) For phosphorus removal, the pressure of high-pressure water is controlled at 18-22Mpa, and the surface oxide scale can be removed by one descaling;

5) Low-temperature controlled rolling: the continuous casting slab is heated to 1250°C in the heating furnace, and the thickness of the rolled product is 80mm. The rolling is controlled in two stages. The first stage adopts large reduction, and the deformation of each pass is controlled at 15~25 %, the finish rolling temperature is controlled at 960°C. In the second stage, the starting rolling temperature is controlled at 920°C, the final rolling temperature is 870°C, and the deformation rate is controlled at 60%;

6) Cooling after final rolling: After rolling, directly enter the accelerated cooling device, cool down to 550°C at 18°C/S, and then stack slowly after entering the hot straightening machine. The stacking temperature is 420°C for 16 hours, and finally The microstructure of the obtained product is a mixed structure of ferrite + pearlite, as shown in Figure 1;

7) Flame cutting to finished product.

Example 2

A production method of extra-thick low-temperature wind tower steel S355NL, the technological process includes smelting, continuous casting billet, heating, dephosphorization, controlled rolling, controlled cooling, hot straightening, stacking and slow cooling, and thermal cutting to finished products. The specific process control is as follows:

1) Smelting composition design (wt%): C: 0.15%, Si: 0.45%, Mn: 1.40%, Nb: 0.3%, V: 0.40%, Ti: 0.45%, Ni: 0.30%, P≤0.01%, S≤0.003%, the rest is iron and unavoidable trace impurities;

During smelting, the molten steel is first smelted in an electric furnace, then sent to the LF refining furnace for refining, and then sent to the LF refining furnace for refining. The refining process strengthens the formation of white slag, strengthens the blowing of Ar gas to promote the mixing of molten steel and slag, and after VD treatment, the S is controlled. Below 0.003%, Ca treatment is carried out at the same time to improve the shape of inclusions;

2) Casting: After the vacuum is broken, the continuous casting slab is cast in the temperature range of 1545~1550 ℃, and the thickness of the continuous casting slab is 255mm;

3) Heating, considering that the complete solution temperature of Nb(CN) is 1100~1200°C, and at the same time, in order to reduce the deformation resistance and ensure the effect of subsequent controlled rolling, the billet heating temperature is determined at 1150~1250°C;

4) For phosphorus removal, the water pressure of high-pressure water is controlled at 18~22Mpa, and the surface oxide scale can be removed by one descaling;

5) Low-temperature controlled rolling: the continuous casting slab is heated to 1260°C in the heating furnace, and the thickness of the rolled product is 90mm. The rolling is controlled in two stages. The first stage adopts large reduction, and the deformation of each pass is controlled at 15~25 %, the finish rolling temperature is controlled above 960°C; the second stage start rolling temperature is controlled at 940°C, the finish rolling temperature is 850°C, and the deformation rate is controlled at 55%;

6) Cooling after final rolling: After rolling, directly enter the accelerated cooling device, cool to 530°C at 20°C/S, and then stack slowly after entering the hot leveling machine. The stacking temperature is 450°C for 17 hours, and finally The microstructure of the obtained product is a mixed structure of ferrite + pearlite;

7) Flame cutting to finished product.

Example 3

A production method of extra-thick low-temperature wind tower steel S355NL, the technological process includes smelting, continuous casting billet, heating, dephosphorization, controlled rolling, controlled cooling, hot straightening, stacking and slow cooling, and thermal cutting to finished products. The specific process control is as follows:

1) Smelting composition design (wt%): C: 0.18%, Si: 0.40%, Mn: 1.80%, Nb: 0.4%, V: 0.30%, Ti: 0.40%, Ni: 0.40%, P≤0.01%, S≤0.003%, the rest is iron and unavoidable trace impurities;

During smelting, the molten steel is first smelted in an electric furnace, and then sent to an LF refining furnace for refining. During the refining process, white slag is strengthened, and Ar gas blowing is strengthened to promote the mixing and stirring of molten steel and slag. After VD treatment, S is controlled below 0.003%. Ca treatment improves inclusion morphology.

2) Casting: After the vacuum is broken, the continuous casting slab is cast in the temperature range of 1545~1550 ℃, and the thickness of the continuous casting slab is 300mm;

3) Heating, considering that the complete solution temperature of Nb(CN) is 1100~1200°C, and at the same time, in order to reduce the deformation resistance and ensure the effect of subsequent controlled rolling, the billet heating temperature is determined at 1150~1250°C;

4) For phosphorus removal, the pressure of high-pressure water is controlled at 18~22Mpa, and the surface oxide scale can be removed by one descaling.

5) Low-temperature controlled rolling: the continuous casting slab is heated to 1280°C in the heating furnace, and the thickness of the rolled product is 100mm. The rolling is controlled in two stages. The first stage adopts large reduction, and the deformation of each pass is controlled at 15~25 %, the finish rolling temperature is controlled above 955°C; the second stage start rolling temperature is controlled at 930°C, the finish rolling temperature is 860°C, and the deformation rate is controlled at 50%;

6) Cooling after final rolling: After rolling, directly enter the accelerated cooling device, cool to 520°C at 15°C/S, and then stack slowly after entering the hot straightening machine. The stacking temperature is 400°C for 16 hours, and finally The microstructure of the obtained product is a mixed structure of ferrite + pearlite;

7) Flame cutting to finished product.

The physical properties of the products produced by the above three embodiments are shown in Table 1 below:

Table 1

Claims (2)

1. the thick low-temperature wind tower of a spy is with the production method of steel S355NL, its technical process comprises that smelting, continuous casting, heating, dephosphorization, controlled rolling, control cooling, heat are rectified, stacking slow cooling, fire are cut to finished product, it is characterized in that, controlled at three key points: the control cooling after the design of smelting composition, low temperature rolling and the finish to gauge, it specifically controls as follows:

1) smelt the composition design: C 0.15 ~ 0.18%, Si 0.25 ~ 0.45%, Mn 1.4 ~ 1.8%, Nb 0.20 ~ 0.40%, V 0.30 ~ 0.50%, Ti 0.40 ~ 0.50%, Ni 0.20 ~ 0.40%, P≤0.02%, S≤0.004%, all the other are iron and inevitable trace impurity;

2) low temperature rolling: continuously cast bloom is heated to 1200 ~ 1280 ℃ at process furnace; By the controlled rolling of two stages, the fs employing is depressed greatly, and each pass deformation control is 15 ~ 25%, and finishing temperature is controlled more than 950 ℃; Subordinate phase start rolling temperature control is controlled at 50%-60% at 900 ~ 940 ℃, 890 ~ 850 ℃ of finishing temperatures, deformation rate;

3) cooling of the control after the finish to gauge: after the rolling end, directly enter accelerated cooling device, speed with 15 ~ 20 ℃/S is cooled to 500 ~ 550 ℃, enter stacking slow cooling behind the heat rectification machine, the stacking temperature is not less than 400 ℃, time is not less than 16 hours, and the product microtexture that finally obtains is ferrite and pearlitic mixed structure.

2. the thick low-temperature wind tower of a kind of spy according to claim 1 is characterized in that with the production method of steel S355NL the thickness of described continuously cast bloom is 250 ~ 300mm.

Images