Hot-rolled steel sheet and method for producing same

Abstract

The purpose is to provide a hot-rolled steel sheet excellent in workability and a production method for producing the same at lower cost.　The hot-rolled steel sheet contains, in mass%, C: 0.06- 0.15%, Si: 0.50-1.50%, Mn: 1.0-2.0%, S: less than 0.0025%, P: 0.10% or less, sol. Al: 0.10% or less, N: 0.015% or less, also contains 0.005-0.050% in total of one or more selected from Ti, Nb, V, and Zr, also contains 0.0010-0.0060% in total of one or two selected from B and Ca, has a component composition in which the remainder comprises Fe and unavoidable impurities, has a tensile strength of 590 MPa or more, and has a hole expansion ratio of 80% or more.

Description

Hot-rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a hot-rolled steel sheet that is mainly used as an automobile part after being subjected to processing such as press forming, etc. In particular, the present invention relates to a hot-rolled steel sheet that has excellent workability and is suitable for use in structural members such as automobile body members and frames, or suspension parts such as suspensions, and a method for manufacturing the same.

[0002] The application of high-tensile steel sheets is one of the most effective methods for reducing the weight of automobile bodies and improving their fuel economy. In particular, hot-rolled steel sheets, which are more economical than cold-rolled steel sheets, are often used in areas where superior surface quality is not required, such as the body's strength members or suspension. This trend is driving the expansion of applications for hot-rolled high-tensile steel sheets. Traditionally, two methods have been used to strengthen high-tensile hot-rolled steel sheets with tensile strengths of approximately 490 to 780 MPa: 1) transformation strengthening, in which martensite, pearlite, or bainite phases are precipitated in the ferrite phase; and 2) precipitation strengthening, using carbonitrides of Ti, Nb, and V. These methods have been selected based on the formability of the steel sheet or the properties required for the component. For example, for inexpensive, general-purpose high-tensile hot-rolled steel sheets, precipitation-strengthened ferrite and pearlite or bainite phases (HSLA) have been selected. Furthermore, when ductility is required, dual-phase steels having a ferrite phase and a martensite phase have been selected. Furthermore, when stretch flangeability is required, precipitation-strengthened dual-phase steels have been selected. Furthermore, as manufacturing techniques for these steel sheets, for example, Patent Documents 1 to 3 propose a method for improving stretch flangeability by coiling a Si-added steel sheet at a low temperature. Furthermore, Patent Document 4 proposes a manufacturing method for a high-tensile strength hot-rolled steel sheet having excellent workability, which is composed of a ferrite phase and a low-temperature transformation phase consisting mainly of a bainite phase as the balance.

[0003] JP 62-37089, JP 58-27328, JP 62-39230, JP 11-117039

[0004] High-tensile steel sheets with excellent formability are required to reduce vehicle weight in order to improve fuel efficiency. However, steel sheets strengthened by conventional strengthening methods have demonstrated good ductility or stretch flangeability, but poor performance in the other properties. Furthermore, the level of stretch flangeability previously considered satisfactory is often insufficient to withstand today's rigorous forming processes. On the other hand, a known method for improving stretch flangeability is to strictly control the S content to 0.0014% or less, thereby minimizing nonmetallic inclusions such as MnS and improving hole expansion. However, strictly controlling the S content to 0.0014% or less at the process production level requires steelmaking to undergo a refining process such as a ladle furnace (LF), which results in excessive manufacturing costs.

[0005] The present invention has been made in view of the above circumstances, and has an object to provide a hot-rolled steel sheet having excellent workability and a manufacturing method for manufacturing the same at lower cost.

[0006] In the present invention, excellent workability means a tensile strength of 590 MPa or more and a hole expansion ratio of 80% or more.

[0007] The present inventors have conducted extensive research to achieve the above object and have found the following.

[0008] Generally, the hole expansion ratio of high-strength steel tends to be lower as the tensile strength increases, and tends to be higher as the amount of S decreases. In the prior art, in the region of 0.002% or less S, the hole expansion ratio was low, and there was a problem that the hole expansion ratio was particularly low in high-strength 590 MPa steel with high workability.

[0009] The present inventors have strengthened the structure while promoting solid solution strengthening by optimizing the chemical composition of a low-carbon steel containing Si and Mn. Furthermore, they have confirmed that by adjusting the finishing temperature of hot rolling, a high-tensile hot-rolled steel sheet with excellent hole expansion property can be obtained without reducing the S content to 0.0014% or less, within the specified range of S being less than 0.0025%, and have devised and implemented a manufacturing method. The present invention has been made based on the above findings and is summarized as follows: [1] In mass%, C: 0.06% to 0.15%, Si: 0.50% to 1.50%, Mn: 1.0% to 2.0%, S: less than 0.0025%, P: 0.10% or less, sol. [2] A hot-rolled steel sheet according to [1], which has a chemical composition comprising, by mass%, one or more of Cu: 0.05% or less, Ni: 0.05% or less, Cr: 0.05% or less, Sn: 0.05% or less, and Mo: 0.05% or less. [3] The hot-rolled steel sheet according to [1] or [2], wherein the steel sheet structure has a total area ratio of polygonal ferrite and bainite of 85% or more. [4] A method for producing a hot-rolled steel sheet according to any of [1] to [3], wherein a slab having the chemical composition is heated to 1100°C or more and 1300°C or less, subjected to rough rolling, and then finish rolling is performed at a finish rolling entry temperature of 1000°C or more and 1080°C or less, and a finish rolling end temperature of 900°C or more, and then the hot-rolled steel sheet is wound into a coil at a coiling temperature of 400°C or more and 600°C or less. [5] The method for producing a hot-rolled steel sheet according to [4], wherein the finish rolling end temperature is 40°C or more higher than the Ar3 transformation point, and the total rolling reduction in the last three stands including the final stand in the finish rolling is 50% or more and 80% or less.

[0010] According to the hot-rolled steel sheet having excellent workability and the manufacturing method thereof of the present invention, it is possible to manufacture a steel sheet having a tensile strength of 590 MPa or more and a hole expansion ratio of 80% or more, even without strictly controlling the S content to 0.0014% or less. Therefore, the industrial value of the steel sheet is extremely great.

[0011] Hereinafter, an embodiment of the present invention will be described.

[0012] 1. Regarding the composition of components The reasons for limiting the components in the present invention will be explained below. Note that the percentage of each element means mass % unless otherwise specified.

[0013] C: 0.06% or more and 0.15% or less C is an element that increases the strength of steel. In the present invention, 0.06% or more is added to obtain the desired strength: a tensile strength of 590 MPa or more. On the other hand, addition of more than 0.15% inhibits the formation of polygonal ferrite and bainite, resulting in the formation of a hard pearlite structure, which reduces the hole expansion test (λ value). Therefore, in the present invention, C is set to a range of 0.06% or more and 0.15% or less. The preferred lower limit is 0.07% or more. The preferred upper limit is 0.10% or less.

[0014] Si: 0.50% or more and 1.50% or less Si is an element effective in improving strength through solid solution strengthening. If its content is less than 0.50%, it is not possible to maintain uniform microstructural hardness, resulting in poor hole expansion properties. Similarly, if its content exceeds 1.50%, it not only leads to poor weldability, particularly poor fatigue properties due to increased hardness in the weld heat-affected zone, but also to poor hole expansion properties due to poor uniform hardness. It also forms fayalite on the steel sheet surface, which reduces descaling properties and worsens surface properties. For this reason, the Si content is limited to a range of 0.50% or more and 1.50% or less. The preferred lower limit is 0.60% or more. The preferred upper limit is 1.00% or less.

[0015] Mn: 1.0% or more and 2.0% or less Like Si, Mn is an element effective in improving strength through solid solution strengthening. If its content is less than 1.0%, it is not possible to maintain uniform hardness of the structure, and hole expansion properties are reduced. On the other hand, if its content exceeds 2.0%, it leads to deterioration of weldability, particularly a decrease in fatigue properties due to an increase in hardness of the weld heat affected zone, making it impossible to maintain uniform hardness of the structure, and hole expansion properties are reduced. Therefore, the Mn content is set to a range of 1.0% or more and 2.0% or less. The preferred lower limit is 1.10% or more. The preferred upper limit is 1.50% or less.

[0016] S: Less than 0.0025% S is known to form MnS, a nonmetallic inclusion, which deteriorates the hole expansion characteristic (λ). Therefore, a low S content is desirable. Reducing the S content to 0.0014% or less requires the addition of a ladle furnace (LF) in the steelmaking process, but this additional processing increases production costs. Therefore, the preferred lower limit is 0.0010% or more, more preferably 0.0014% or more, and most preferably 0.0018% or more. On the other hand, in this steel type, a correlation with the finish rolling temperature has been observed in the range of S: less than 0.0025%, and it has been confirmed that hole expansion characteristic can be ensured by controlling the structure. On the other hand, if the S content is 0.0025% or more, the effect of MnS formation cannot be ignored, and the desired λ may not be obtained. Therefore, the specified S content is set to less than 0.0025%.

[0017] P: 0.10% or less P is a harmful element that segregates at grain boundaries and reduces the toughness of steel, and since a content exceeding 0.10% in particular significantly reduces toughness, the P content is set to 0.10% or less. It is desirable to reduce the P content as much as possible, but reducing it to less than 0.005% increases the manufacturing cost, so the lower limit of P is preferably set to about 0.005% or more.

[0018] Sol. Al: 0.10% or less Al is a component contained as a deoxidizer, and the lower limit may be zero, but when Al is contained as sol. Al, it is preferably 0.005% or more. On the other hand, if it is contained in excess of 0.10%, the toughness of the steel decreases. Therefore, Al is set to a range of 0.10% or less as sol. Al. For the same reason, the lower limit of total Al is preferably 0.005% or more, and the upper limit is preferably 0.15% or less.

[0019] N: 0.015% or less N is an unavoidable impurity that deteriorates the toughness of steel in a solid solution state, and the lower the content, the better. From the viewpoint of ensuring toughness, a content of 0.015% or less is acceptable. However, it is technically difficult to completely remove N, and reducing it more than necessary only increases the steelmaking cost, so the lower limit of N is preferably set to 0.001% or more.

[0020] Ti, Nb, V, Zr: One or more of them, in total, 0.005% or more and 0.050% or less. Ti, Nb, V, and Zr all form carbonitrides, suppressing the coarsening of austenite grains during slab heating and forming fine polygonal ferrite. If the total content of these elements is less than 0.005%, the above-mentioned effect is not fully exhibited. On the other hand, if the content exceeds 0.050%, the amount of carbonitrides precipitated becomes excessive, which actually reduces stretch flangeability (hole widening characteristics) and λ. Therefore, the total content of Ti, Nb, V, and Zr is set to 0.005% or more and 0.050% or less. The preferred lower limit is 0.010% or more.

[0021] B, Ca: one or both of them, total 0.0010% or more and 0.0060% or less B and Ca are elements that change sulfides into ones that are difficult to stretch, improving stretch flangeability. In order to significantly improve stretch flangeability, it is necessary to contain these elements within a predetermined range, along with limiting the amount of S. If the total content of B and Ca is less than 0.0010%, the above effect is poor, while if it exceeds 0.0060%, the corrosion resistance of the hot-rolled steel sheet is reduced. Therefore, the total content of B and Ca is set to a range of 0.0010% or more and 0.0060% or less.

[0022] In addition to the above-mentioned chemical composition, the hot-rolled steel sheet of the present invention may further contain, in mass%, one or more elements selected from Cu: 0.05% or less, Ni: 0.05% or less, Cr: 0.05% or less, Sn: 0.05% or less, and Mo: 0.05% or less, either alone or in combination.

[0023] Cu: 0.05% or less Cu is an element that improves hardenability, so it can be added as needed. When Cu is added, 0.01% or more is preferable. If the upper limit exceeds 0.05%, there is a risk of surface cracking during hot rolling, and an increase in inclusions, etc., can cause defects on the surface or inside of the steel sheet, resulting in a significant decrease in ductility. Therefore, when Cu is added, it is set to 0.05% or less.

[0024] Ni: 0.05% or less Ni is also an element that improves hardenability and is effective in ensuring better ductility. It is also an element that contributes to high strength through solid solution strengthening and transformation strengthening. Therefore, when added, 0.01% or more is preferable. On the other hand, excessive addition of Ni may cause surface cracking during hot rolling, and may also cause an increase in inclusions, which may cause defects on the surface or inside of the steel sheet, significantly reducing ductility. It may also affect hole expansion. Therefore, when Ni is added, it should be 0.05% or less.

[0025] Cr: 0.05% or less Like Ni, Cr is an element that improves hardenability and contributes to high strength as a solid solution strengthening element. It also has the effect of improving the balance between strength and ductility. Therefore, it can be added as needed. If added, 0.01% or more is preferable. On the other hand, excessive Cr addition may cause surface cracking during hot rolling, and may also cause defects on the surface or inside of the steel sheet due to an increase in inclusions, etc., which may significantly reduce ductility. Therefore, if Cr is added, it should be 0.05% or less.

[0026] Sn: 0.05% or less Sn suppresses decarburization in a region of several tens of micrometers in the surface layer of the steel sheet, which occurs due to nitriding or oxidation of the steel sheet surface. Sn is also an element that improves hardenability and may be added as needed. To achieve these effects, it is preferable to add 0.001% or more. Excessive addition of Sn, exceeding 0.05%, may result in a decrease in toughness. Therefore, when Sn is added, its content should be 0.05% or less.

[0027] Mo: 0.05% or less Like Cr and Ni, Mo is an element that improves hardenability and also has the effect of improving the balance between strength and ductility. Therefore, it can be added as needed. When added, 0.01% or more is preferable. On the other hand, if the upper limit of Mo is 0.05% or less, the amount of coarse precipitates and inclusions does not increase, and the properties of the steel sheet do not deteriorate. Therefore, when Mo is added, its content is set to 0.05% or less.

[0028] The balance of the heat-rolled steel sheet of the present invention other than the above-mentioned components is Fe and unavoidable impurities. Note that the inclusion of other components as unavoidable impurities is not prohibited as long as the effects of the present invention are not impaired.

[0029] The thickness of the hot rolled steel sheet of the present invention is in the range of 1.0 to 10.0 mm.

[0030] Tensile strength: 590 MPa or more Tensile strength is evaluated by the tensile test described in the Examples as an index for evaluating the strength of steel. Higher tensile strength means higher strength and enables weight reduction in the automobile manufacturing process, so in the present invention, the tensile strength is set to 590 MPa or more.

[0031] Hole expansion ratio: λ≧80% The hole expansion characteristic is used as an index for evaluating workability. The higher λ, the better the workability. In the present invention, the hole expansion characteristic is set to 80% or more, but λ≧90% is preferable, and λ≧100% is more preferable. λ is evaluated by the test described in the examples.

[0032] Steel sheet structure: total area ratio of polygonal ferrite and bainite is 85% or more. Since the above-mentioned tensile strength and hole expansion ratio can be obtained with the chemical composition of the present invention and the manufacturing conditions described below, there are no particular limitations on the steel structure. However, a total area ratio of polygonal ferrite and bainite of 85% or more is preferred because it further improves hole expansion properties. The steel sheet structure refers to the structure observed at a quarter position in the sheet width direction and a quarter position in the sheet thickness direction, and the structure at these positions is considered to represent the structure of the entire steel sheet. The remaining structure other than polygonal ferrite and bainite is mainly composed of a known structure including pearlite and martensite.

[0033] 2. Manufacturing Conditions Steel having the above-mentioned composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and formed into a slab by a conventional method such as a continuous casting method or an ingot making and blooming method. Thereafter, the slab is heated and rolled, and after cooling after rolling, the mechanical properties are measured. Note that the melting method and casting method are not limited to those described above.

[0034] Heating temperature: 1100°C or higher and 1300°C or lower If the heating temperature is lower than 1100°C, the carbides do not dissolve completely, resulting in a shortage of solute C, and the strength is likely to decrease. On the other hand, if the heating temperature exceeds 1300°C, the structure becomes coarse and the toughness of the steel plate decreases. For this reason, the heating temperature of the steel material is set to a range of 1100°C or higher and 1300°C or lower. In order to maintain the finishing temperature, the preferred lower limit is 1200°C or higher. The preferred upper limit is 1250°C or lower.

[0035] Finish rolling entry temperature: 1000°C or higher and 1080°C or lower Finish rolling is performed after rough rolling. The higher the finish rolling entry temperature, the more the recrystallization of the structure is promoted, and λ tends to increase. On the other hand, if the temperature exceeds 1080°C, scale defects increase, so the temperature is set to 1080°C or lower. Preferably, the temperature is 1060°C or lower.

[0036] Finish rolling end temperature (finish rolling delivery temperature): 900°C or higher. Finish rolling is performed after rough rolling. The finish rolling end temperature must be higher than the Ar3 transformation point. On the other hand, if rolling is performed just above the Ar3 transformation point, the phase transformation occurs using the strain energy generated during rolling as the driving force, resulting in the precipitation of relatively soft bainite at high temperatures and the local precipitation of hard pearlite. When relatively soft and hard structures are mixed, cold strain application causes differences in elongation, which can lead to cracks and deteriorate hole expansion characteristics. On the other hand, by increasing the finish rolling end temperature, strain introduced during rolling is released, resulting in transformation to bainite at a relatively low temperature range, resulting in a structure with uniform hardness. A structure with uniform hardness is less likely to cause the above problems. This effect is more pronounced as the finish rolling end temperature is increased, and this effect can be achieved at 900°C or higher. Preferably, the temperature is equal to or higher than the calculated Ar3 point according to the following formula +40°C. Here, the Ar3 transformation point is defined by the following formula (1), with each component being expressed in mass %, and zero if the component is not contained. Ar3 (°C) = 910 - 230√C + 45Si - 23Mn + 95Al - 15.2Ni + 32Mo ... (1) Note that if the finish rolling end temperature is raised too much, the heating temperature will increase, significantly worsening the rolling efficiency and may also generate powder scale on the surface, making it more susceptible to peeling, so the temperature is preferably 1000°C or lower.

[0037] Coiling temperature: 400°C or higher and 600°C or lower. To achieve the strength, elongation, and hole expansion ratio (stretch flangeability), the steel is coiled at a temperature range of 400°C or higher and 600°C or lower. While no particular conditions are set for the cooling from the end of hot rolling to coiling, it is preferable to perform the cooling at a rate of 20 to 80°C / s from above the Ar3 transformation point. An example of a cooling method at this time is water cooling, but this is not limiting. The coiling temperature (CT) affects the tensile strength (TS), elongation (El), and hole expansion ratio (λ) when a steel slab having a chemical composition within the range of the present invention is hot rolled. It has been found that coiling at a temperature range of 400°C or higher and 600°C or lower satisfies the target values ​​for all of these properties. The lower limit of the coiling temperature is preferably 460°C or higher. The upper limit of the coiling temperature is preferably 570°C or lower.

[0038] Total reduction in the last three stands including the final stand in finish rolling: 50% or more and 80% or less If the reduction is less than 50%, warping may occur during rolling, making it impossible to perform rolling. On the other hand, if the reduction is more than 80%, strain is large and remains after rolling, which may cause a large amount of soft upper bainite structure to form during the cooling process, resulting in a low λ. Therefore, the reduction is preferably 50% or more and 80% or less. The lower limit of the reduction is more preferably 60% or more, and most preferably 70% or more. The upper limit of the reduction is more preferably 78% or less, and most preferably 76% or less.

[0039] Examples of the present invention will be described below.

[0040] Steels having various chemical compositions shown in Table 1 were melted and formed into continuous cast slabs. In Table 1, the notation "Tr" means that the element in question was not intentionally added, and even if it was contained, it means that the amount was at the level of an unavoidable impurity. These slabs were heated, hot-rolled, and cooled under the production conditions shown in Table 2, and then wound into coils to produce hot-rolled steel sheets having a thickness of 2.0 mm.

[0041]

[0042]

[0043] Metallographic observation, tensile test, and hole widening test were performed at the 1 / 4W (midway between the edge and the center in the width direction) and 1 / 4t (midway between the surface and the center in the thickness direction) positions of the coil of the obtained hot-rolled steel sheet. JIS No. 5 test pieces were used for the tensile test. The stretch flange test (hole widening test) was performed according to the method specified in JIS Z 2256, Hole Widening Test Method for Metallic Materials, by cutting a 10 mm (d ０ Next, this hole was enlarged using a conical punch with a tip angle of 60°, and the limit hole diameter (d) at which cracks occur on the edge of the hole was calculated using the following formula: λ = (dd - d ０ ) × 100 / d ０ The hole expansion ratio λ was calculated and evaluated by the above method. The results are shown in Table 2. From these results, it can be seen that the inventive examples have good stretch flangeability and strength properties, and are excellent in workability.

Claims

1. In mass%, C: 0.06% or more and 0.15% or less, Si: 0.50% or more and 1.50% or less, Mn: 1.0% or more and 2.0% or less, S: less than 0.0025% P: 0.10% or less, sol. Al: 0.10% or less, N: 0.015% or less, and containing one or more selected from Ti, Nb, V, and Zr in a total content of 0.005% or more and 0.050% or less; Further, one or two selected from B and Ca are contained in a total amount of 0.0010% or more and 0.0060% or less, and The remainder is Fe and unavoidable impurities, A hot-rolled steel sheet having a tensile strength of 590 MPa or more and a hole expansion ratio of 80% or more.

2. The component composition is, in mass%, Cu: 0.05% or less, Ni: 0.05% or less, Cr: 0.05% or less, Sn: 0.05% or less, and The hot-rolled steel sheet according to claim 1, further containing one or more selected from the group consisting of Mo: 0.05% or less.

3. The hot-rolled steel sheet according to claim 1, wherein the steel sheet structure has a total area ratio of polygonal ferrite and bainite of 85% or more.

4. A hot-rolled steel sheet as described in claim 2, wherein the steel sheet structure has a total area ratio of polygonal ferrite and bainite of 85% or more.

5. 5. A method for producing a hot-rolled steel sheet according to claim 1, wherein a slab having the component composition is heated to 1100°C or more and 1300°C or less, and subjected to rough rolling. Thereafter, finish rolling is performed at a finish rolling entry temperature of 1000°C or more and 1080°C or less and a finish rolling end temperature of 900°C or more, and then the slab is wound into a coil at a coiling temperature of 400°C or more and 600°C or less.

6. 6. The method for producing a hot-rolled steel sheet according to claim 5, wherein the finish rolling end temperature is 40°C or more higher than the Ar3 transformation point, and the total rolling reduction in three rear-stage stands including a final stand in the finish rolling is 50% or more and 80% or less.