Low-carbon chalcogenide free-cutting narrow strip steel and preparation method thereof

Abstract

The invention discloses low-carbon sulfur free-cutting narrow strip steel and a preparation method thereof. The low-carbon sulfur free-cutting narrow strip steel comprises the following chemical components in percentage by mass: not less than 0.05% and not more than 0.09% of C, not more than 0.07% of Si, not less than 0.90% and not more than 1.50% of Mn, not less than 0.04% and not more than 0.09% of P, not less than 0.25% and not more than 0.35% of S, not more than 0.008% of Al and the balance of Fe and inevitable impurities. According to the method, firstly, through strict control over the tapping condition and the charging sequence, it is guaranteed that the initial oxygen potential of molten steel is stable, removal of an initial deoxidation product is promoted, the content of O, S, Mn, P and Al in the LF refining process and the content of O in the molten steel are further controlled, the ideal MnS inclusion morphology is obtained, and generation of oxides containing Al and Si is inhibited; and then through the high-temperature heating and high-temperature rolling processes, curing of the long-strip-shaped MnS inclusions is promoted, and the phenomena of rolling cracking, hot brittleness and cold brittleness are avoided.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to a low-carbon sulfur-based free-cutting narrow strip steel and a preparation method thereof. Background Art

[0002] Free-cutting steel is a type of steel commonly used in mechanical processing. It is mainly used to make standard parts used in instruments, watch parts, automobiles, machine tools and other machines that are subject to less force but have strict requirements on size and finish, such as gears, shafts, bolts, valves, bushings, pins, pipe fittings, spring seats and machine tool screws, etc. It has a wide range of application scenarios and is one of the basic parts in the mechanical field.

[0003] Free-cutting steel should have excellent processing performance, electroplating performance, surface finish and environmental performance. According to the different free-cutting elements, free-cutting steel can be divided into sulfur free-cutting steel, lead free-cutting steel, calcium free-cutting steel, titanium free-cutting steel and composite free-cutting steel, etc. Influenced by environmental protection and cost factors, sulfur-based free-cutting steel is the largest type of free-cutting steel, accounting for more than 90% of the free-cutting steel output.

[0004] The control of sulfide morphology in sulfur-based free-cutting steel is the core of determining its cutting performance. Sulfides mainly exist in the form of manganese sulfide (MnS) in steel. MnS inclusions are easy to break chips as internal stress concentration sources during steel cutting and form lubrication between the tool and the workpiece, reducing tool wear and thus changing the cutting performance of the steel. The morphology of MnS inclusions in sulfur-based free-cutting steel includes type I, type II, and type III. Type I MnS inclusions are not easy to extend and deform when heated, and are spindle-shaped or elliptical in steel, with good cutting performance; type II MnS inclusions are easy to deform during processing, and the cutting performance and mechanical properties are very poor; type III MnS inclusions are more easily deformed, and the cutting performance and mechanical properties are worse. Therefore, how to effectively control the morphology of MnS inclusions is the key.

[0005] At present, the main supply forms of free-cutting steel include wire and bar. Wire is one of the common supply forms of free-cutting steel. Wire usually has a small cross-sectional size and is suitable for making various small mechanical parts, fasteners, springs, etc. Bar is another common supply form of free-cutting steel. The cross-sectional size of the bar is larger and is suitable for making large mechanical parts, shafts, gears, etc. For narrow strip steel, strip steel is a less common supply form of free-cutting steel, and free-cutting strip steel has its unique uses in the field of machining, so currently very few people are involved in the production process research of free-cutting narrow strip steel. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the disadvantages of the prior art and provide a low-carbon sulfur-based free-cutting narrow strip steel and a preparation method thereof.

[0007] In order to solve the above technical problems, the technical solution of the present invention is as follows: A low-carbon sulfur-based free-cutting narrow strip steel, whose chemical composition and mass percentage are as follows: 0.05%≤C≤0.09%, Si≤0.07%, 0.90%≤Mn≤1.50%, 0.04%≤P≤0.09%, 0.25%≤S≤0.35%, Al≤0.008%, and the balance is Fe and unavoidable impurities.

[0008] As a preferred solution of the low carbon sulfur free-cutting narrow strip steel of the present invention, wherein: 3.2≤Mn / S≤4.5.

[0009] The present invention also provides a method for preparing low-carbon sulfur-based free-cutting narrow strip steel, which comprises the following steps in sequence: converter, LF refining furnace refining, continuous casting, heating, rolling and cooling.

[0010] As a preferred solution of the method for preparing the low-carbon sulfur free-cutting narrow strip steel of the present invention, the steel tapping conditions of the converter process are C≤0.05%, 0.03%≤P, and temperature≥1600°C.

[0011] As a preferred scheme of the preparation method of the low-carbon sulfur-based free-cutting narrow strip steel described in the present invention, the charging order of the converter process is deoxidizer, alloy, and slag, the deoxidizer includes an aluminum-containing alloy, the alloy includes one or more of low-carbon ferromanganese, electrolytic manganese, and ferrosulphur, and the slag includes lime.

[0012] As a preferred scheme for the preparation method of the low-carbon sulfur-based free-cutting narrow strip steel described in the present invention, the chemical composition and mass percentage of the refined slag after the LF refining furnace refining process are as follows: 30%≤CaO≤40%, 10%≤Al2O3≤25%, 2%≤FeO≤6%, 10%≤MnO≤25%, 5%≤MgO≤8%, 10%≤SiO2≤20%.

[0013] As a preferred scheme of the preparation method of the low-carbon sulfur free-cutting narrow strip steel described in the present invention, wherein: the heating process adopts a walking-beam heating furnace, and the walking-beam heating furnace includes a preheating section, a first heating section, a second heating section, a third heating section and a soaking section. The furnace temperature of the preheating section is less than or equal to 850°C, the furnace temperature of the first heating section is 750-1160°C, the furnace temperature of the second heating section is 1050-1220°C, the furnace temperature of the third heating section is 1100-1260°C, the furnace temperature of the soaking section is 1150-1260°C, and the time in the furnace is ≥110min.

[0014] As a preferred solution of the method for preparing the low-carbon sulfur-based free-cutting narrow strip steel of the present invention, the starting rolling temperature of the rolling process is 1050-1220°C, the final rolling temperature is 950-1050°C, and the curling temperature is ≥600°C.

[0015] As a preferred solution of the method for preparing the low-carbon sulfur-based free-cutting narrow strip steel of the present invention, the cooling process comprises: placing the rolled workpiece into a slow cooling chamber for slow cooling.

[0016] The beneficial effects of the present invention are: The present invention firstly ensures the stability of the initial oxygen potential of the molten steel and promotes the removal of the initial deoxidation products by strictly controlling the steel-tapping conditions and the feeding sequence, further controls the O content of the molten steel by controlling the O, S, Mn, P, Al contents and the terminal refining slag system in the LF refining process, obtains the ideal MnS inclusion morphology, and inhibits the generation of Al- and Si-containing oxides; subsequently, through high-temperature heating and high-temperature rolling processes, the ripening of the long strip MnS inclusions is promoted, rolling cracking, hot brittleness and cold brittleness are avoided, and the synergistic effect between various processes on the entire production line of a low-carbon sulfur-based free-cutting narrow strip steel is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0018] Figure 1 This is a morphology of MnS after rolling in the hot-rolled narrow strip steel produced in the embodiment of the present application. DETAILED DESCRIPTION

[0019] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific implementation modes and in combination with the accompanying drawings.

[0020] The present application provides a low-carbon sulfur-based free-cutting narrow strip steel, whose chemical composition and mass percentage are as follows: 0.05%≤C≤0.09%, Si≤0.07%, 0.90%≤Mn≤1.50%, 0.04%≤P≤0.09%, 0.25%≤S≤0.35%, Al≤0.008%, and the balance is Fe and unavoidable impurities.

[0021] Among them, in the above chemical composition, the ratio of Mn element to S element is greater than or equal to 3.2 and less than or equal to 4.5, that is, 3.2≤Mn / S≤4.5.

[0022] The S content and cutting performance index show an obvious positive correlation. The addition of a small amount of S content can significantly improve the material's cutting performance and significantly extend the service life of the tool. When the S content is too high, the effect of improving the cutting performance is limited, so Si≤0.07%.

[0023] Considering that the S element easily combines with Fe to form a low-melting point (985℃) eutectic, which leads to cracking during the rolling process, in order to inhibit the formation of low-melting point eutectic, a sufficient amount of Mn must be added. At the same time, in order to ensure the precipitation of sufficient MnS inclusions, strict requirements are made on the Mn content and Mn / S, that is, 0.90%≤Mn≤1.50% and 3.2≤Mn / S≤4.5.

[0024] The P element is a free-cutting element. The addition of appropriate P content can miniaturize the chip edge during cutting and make the tool tip profile uniform, thereby improving the surface finish of the part. However, too high P content will increase the cold brittleness of the steel, so 0.04%≤P≤0.09%.

[0025] The present application also provides a method for preparing low-carbon sulfur-based free-cutting narrow strip steel, which comprises refining in a converter and an LF refining furnace, continuous casting, heating, rolling and cooling in sequence.

[0026] Among them, in the converter process, the tapping conditions are C≤0.05%, 0.03%≤P, and temperature≥1600℃. The order of charging for tapping is: deoxidizer→alloy→slag, deoxidizer 0.2-0.5kg / t, alloy 12-18kg / t, slag 3-5kg / t, deoxidizer includes aluminum-containing shot and its alloy, alloy includes low-carbon ferromanganese or electrolytic manganese, ferrosulphur, and slag includes lime.

[0027] The control of the C content in steel is mainly to prevent the C element from exceeding the upper limit. The lower limit requirement of P content is to reduce the amount of alloy added after the furnace and LF refining process, accelerate the melting of alloy, and improve the cleanliness of molten steel.

[0028] During the steel-making process, a certain amount of aluminum is first added for precipitation deoxidation. The main purpose is to stabilize the initial oxygen potential of the molten steel. Subsequently, ferrosulfide, electrolytic manganese or low-carbon ferromanganese is added to increase the alloy yield. Finally, lime slag is added to reduce the risk of slag involvement on the one hand, and to promote the absorption and removal of initial deoxidation products on the other hand, and reduce the residue of oxides such as aluminum oxide and silicon oxide, because these oxides affect the morphology of MnS and also deteriorate the cutting performance and shorten the service life of the tool.

[0029] The LF refining process is to adjust the P, S, and Mn components and control the LF end-point refining slag composition: 30%≤CaO≤40%, 10%≤Al2O3≤25%, 2%≤FeO≤6%, 10%≤MnO≤25%, 5%≤MgO≤8%, 10%≤SiO2≤20%.

[0030] The O content at the end of LF is controlled to be 100-200ppm. The high or low O content is the main reason affecting the morphology of MnS. High O content promotes the formation of type I MnS, but too high O content will reduce the performance of the material and even cause defects such as pores. The addition of deoxidizing elements such as Al and Si will inevitably generate corresponding deoxidation products. Such deoxidation products reduce the cutting performance, so it is strictly forbidden to use deoxidation treatment containing Al or Si elements in the refining process. The control of the end refining slag is, on the one hand, to promote the effective removal of oxides containing Al and Si elements, and on the other hand, to achieve steel-slag balance and improve the cleanliness of molten steel.

[0031] The heating process adopts a walking-beam heating furnace, which includes a preheating section, a first heating section, a second heating section, a third heating section and a soaking section. The furnace temperature of the preheating section is less than or equal to 850°C, the furnace temperature of the first heating section is 750-1160°C, the furnace temperature of the second heating section is 1050-1220°C, the furnace temperature of the third heating section is 1100-1260°C, the furnace temperature of the soaking section is 1150-1260°C, and the time in the furnace is ≥110min.

[0032] The preheating stage heats up slowly to prevent thermal stress cracks. The first heating stage heats up quickly to quickly pass through the low melting point eutectic temperature range. The high temperature treatment in the soaking stage provides conditions for subsequent high temperature rolling, and the high temperature treatment can further promote the ripening of MnS.

[0033] The starting temperature of the rolling process is 1050-1220°C, the final rolling temperature is 950-1050°C, and the coiling temperature is ≥600°C. High temperature rolling is mainly to improve the deformation capacity of austenite, prevent cracking during the rolling process, and inhibit the occurrence of hot brittleness.

[0034] The cooling process is to place the rolled workpiece in a slow cooling chamber for slow cooling, mainly to prevent the formation of brittle phase iron phosphide (Fe3P) and the occurrence of cold brittleness.

[0035] The product obtained by the above scheme is hot-rolled narrow strip steel, and the thickness of the product is 1.5-12mm and the width is 160-450mm.

[0036] The above scheme is further described below through specific embodiments.

[0037] The present embodiment provides a method for preparing low-carbon sulfur-based free-cutting narrow strip steel, the preparation process includes: converter → LF refining furnace → continuous casting → heating → rolling → cooling. The required composition is obtained by controlling the steelmaking conditions and the refining process, combined with strict control of the temperature and cooling of the heating and rolling processes, to achieve the purpose of successfully developing low-carbon sulfur-based free-cutting narrow strip steel.

[0038] The mass fractions of each element in the low carbon sulfur free-cutting narrow strip steel are as follows: 0.05%≤C≤0.09%, Si≤0.07%, 0.90%≤Mn≤1.50%, 0.04%≤P≤0.09%, 0.25%≤S≤0.35%, Al≤0.008%, among which 3.2≤Mn / S≤4.5.

[0039] In this embodiment, SUM23 grade (US grade: 1215) is selected as a typical steel grade as an example, and the chemical composition is shown in Table 1.

[0040]

[0041] Table 1 The conditions for steel tapping from the converter are: C=0.045%, 0.03%P=0.035%, temperature=1623℃, and the order of charging for steel tapping is: deoxidizer→alloy→slag, with deoxidizer Al shot 0.3kg / t, alloy ferrosulfur+low carbon ferromanganese 16kg / t, and slag 4.2kg / t.

[0042] During the LF refining process, the P, S, and Mn components are adjusted. It is strictly forbidden to use Al wire, SiC, and other diffusion deoxidation methods. The LF end-point refining slag composition is controlled as follows: 30%≤CaO≤40%, 10%≤Al2O3≤25%, 2%≤FeO≤6%, 10%≤MnO≤25%, 5%≤MgO≤8%, 10%≤SiO2≤20%. The O content at the LF end point is controlled to be 100-200ppm. The tundish O content is 150ppm, and the refining slag composition is: 36%CaO-15%Al2O3-5%MgO-12%SiO2-3%FeO-21%MnO.

[0043] Heating process: The heating furnace is a walking-beam heating furnace, which is divided into preheating section, heating section one, heating section two, heating section three and soaking section. The furnace temperature of each heating section is controlled. The furnace temperature of the preheating section is 486°C, the furnace temperature of the heating section one is 862°C, the furnace temperature of the heating section two is 1126°C, the furnace temperature of the heating section three is 1191°C, the furnace temperature of the soaking section is 1210°C, and the time in the furnace is 180 minutes.

[0044] Rolling process: starting rolling temperature is 1099℃, final rolling temperature is 978℃, coiling temperature is 625℃; Cooling process: after rolling, enter the slow cooling room for slow cooling.

[0045] Furthermore, the product is in the form of hot-rolled narrow strip steel, the thickness of the product is 2.0-4.0 mm, and the width is 160-190 mm.

[0046] The morphology of MnS after rolling is as follows: Figure 1 As shown, it belongs to the typical type I MnS morphology.

[0047] Therefore, the technical solution of the present application firstly ensures the stability of the initial oxygen potential of the molten steel and promotes the removal of the initial deoxidation products by strictly controlling the steel-making conditions and the charging sequence, and further controls the O content of the molten steel by controlling the O, S, Mn, P, Al contents and the terminal refining slag system during the LF refining process, thereby obtaining the ideal MnS inclusion morphology and inhibiting the formation of Al- and Si-containing oxides; subsequently, through high-temperature heating and high-temperature rolling processes, the ripening of the long strip MnS inclusions is promoted, rolling cracking, hot brittleness and cold brittleness are avoided, and the synergistic effect between the various processes on the entire production line of a low-carbon sulfur-based free-cutting narrow strip steel is achieved.

[0048] In addition to the above embodiments, the present invention may also have other implementation modes; any technical solutions formed by equivalent replacement or equivalent transformation shall fall within the protection scope required by the present invention.

Claims

1. A low carbon sulfur free-cutting narrow strip steel, characterized in that: Its chemical composition and mass percentage are as follows: 0.05%≤C≤0.09%, Si≤0.07%, 0.90%≤Mn≤1.50%, 0.04%≤P≤0.09%, 0.25%≤S≤0.35%, Al≤0.008%, and the balance is Fe and unavoidable impurities.

2. The low carbon sulfur free-cutting narrow strip steel according to claim 1, characterized in that: 3.2≤Mn / S≤4.

5.

3. A method for preparing the low-carbon sulfur-based free-cutting narrow strip steel according to claim 2, characterized in that: It includes converter, LF refining furnace refining, continuous casting, heating, rolling and cooling in sequence.

4. The method for preparing low carbon sulfur free-cutting narrow strip steel according to claim 3, characterized in that: The steel tapping conditions of the converter process are C≤0.05%, 0.03%≤P, and temperature≥1600°C.

5. The method for preparing low carbon sulfur free-cutting narrow strip steel according to claim 3, characterized in that: The order of adding materials in the converter process is deoxidizer, alloy, and slag. The deoxidizer includes an aluminum-containing alloy, the alloy includes one or more of low-carbon ferromanganese, electrolytic manganese, and ferrosulphide, and the slag includes lime.

6. The method for preparing low carbon sulfur free-cutting narrow strip steel according to claim 3, characterized in that: The chemical composition and mass percentage of the refined slag after the LF refining furnace refining process are as follows: 30%≤CaO≤40%, 10%≤Al2O3≤25%, 2%≤FeO≤6%, 10%≤MnO≤25%, 5%≤MgO≤8%, 10%≤SiO2≤20%.

7. The method for preparing low carbon sulfur free-cutting narrow strip steel according to claim 3, characterized in that: The heating process adopts a walking-beam heating furnace, which includes a preheating section, a first heating section, a second heating section, a third heating section and a soaking section. The furnace temperature of the preheating section is less than or equal to 850°C, the furnace temperature of the first heating section is 750-1160°C, the furnace temperature of the second heating section is 1050-1220°C, the furnace temperature of the third heating section is 1100-1260°C, the furnace temperature of the soaking section is 1150-1260°C, and the time in the furnace is ≥110min.

8. The method for preparing low carbon sulfur free-cutting narrow strip steel according to claim 3, characterized in that: The rolling process has a starting rolling temperature of 1050-1220°C, a finishing rolling temperature of 950-1050°C, and a curling temperature of ≥600°C.

9. The method for preparing low carbon sulfur free-cutting narrow strip steel according to claim 3, characterized in that: The cooling process comprises: placing the rolled workpiece into a slow cooling chamber for slow cooling.

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